Emerging Drying Technologies and Their Impact on Bioactive Compounds: A Systematic and Bibliometric Review

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This systematic and bibliometric review identifies emerging drying technologies such as microwave, radiofrequency, infrared, ultrasound, cold plasma, and freeze-drying as promising strategies to improve the retention of bioactive compounds in dehydrated plant-based foods. The findings support the development of more efficient, functional, and sustainable drying processes for the food industry.

Abstract

Drying is a key method for food preservation; however, conventional techniques often lead to the degradation of bioactive compounds, compromising nutritional quality. This systematic review, following the PRISMA protocol, examines emerging food drying technologies designed to enhance process efficiency while preserving nutritional and functional properties. A bibliometric analysis was conducted to identify research trends from 2014 to 2024. Searches were performed in the Scopus and Web of Science databases accessed on 17 January 2025, including only original research articles published in English focusing on food drying applications. Reviews, editorials, and studies unrelated to the food sector were excluded. Due to the technological nature of the outcomes, a formal risk of bias assessment was not applicable. This review highlights several emerging drying technologies, such as microwave, radiofrequency, infrared, ultrasound, freeze-drying, and cold plasma. The qualitative synthesis indicates these technologies improve the retention of phenolics, flavonoids, and vitamins, thus enhancing nutritional stability. Nevertheless, challenges remain in industrial-scale implementation, particularly regarding the economic feasibility and optimization of operational parameters. This review received no funding and was not registered in any public database. The findings underscore the need for continued research to develop sustainable and functional dried food products that meet current market demands and consumer expectations.

1. Introduction

Food security is a global concern, especially due to the rapid growth of the world population and uncertainties surrounding the future food supply. In addition to population increases, issues such as the growing consumer awareness regarding nutritional quality and food safety, climate change, and the depletion of natural resources further complicate the challenge of ensuring food availability [1,2].

Drying is crucial for extending the shelf life of food, aiming to reduce the moisture content to a safe level where both microbial growth and moisture-related deterioration reactions are minimized [3]. It is a key step for food storage, and it also contributes to reducing transportation and storage costs. Examples include spray drying, spray chilling, freeze-drying, and vacuum drying [4].

Additionally, hot-air or convective drying is widely used due to its low cost and simple operation [5,6]. Despite its extensive use, this technique is associated with the degradation of physical, structural, and chemical characteristics of food products, including the appearance, porosity, texture, microstructure, color, and the presence of bioactive compounds. It also presents challenges related to energy consumption due to long processing times [3,7,8,9].

Therefore, strategies to mitigate these effects are essential to maintain product quality. The development of more efficient drying technologies is important to ensure the production of safe foods with a high nutritional and sensory quality, an increased production capacity, reduced drying times, the establishment of economically and environmentally sustainable processes, the minimization of environmental pollution, waste reduction, lower energy costs, operational control feasibility, safety considerations, and the use of combined drying techniques [4].

It is crucial to develop alternative drying methods that not only improve process efficiency but also preserve bioactive compounds and the structure of foods, meeting market demands for high-quality products [8,10]. Bioactive compounds present in foods play an essential role in health, helping to maintain the body balance and prevent diseases. The main ones include phenolic compounds, such as flavonoids, anthocyanins, and tannins, as well as organic acids and vitamins. Their consumption may provide health benefits due to antioxidant, cytotoxic, and disease-preventive effects against cardiovascular diseases, diabetes, and cancer [11].

The integration of new technologies in food processing aims to improve both sustainability and efficiency while ensuring microbiological safety and extending product shelf life. These technologies also seek to minimize changes in nutritional quality and flavor [12]. Such innovations focus on reducing the degradation of thermosensitive components, preserving the sensory and nutritional characteristics of foods compared to conventional methods [10].

This study aims to identify, through a bibliometric analysis and systematic review, the most promising emerging technologies for food drying developed over the last ten years, as well as their specific impacts on thermosensitive bioactive compounds. This study evaluates the effectiveness of these technologies, their impact on the preservation of bioactive compounds, and the final quality of dried products. In addition, it identifies challenges and opportunities for sustainable industrial implementation and outlines directions for future research in this innovative field.

2. Methodology

2.1. Bibliometric Study Design

The Scopus and Web of Science databases were used to retrieve publications related to emerging technologies in food drying accessed on 17 January 2025. The search was conducted using the terms “Emerging Technologies” AND “Food” AND “Drying” in the fields “title”, “keywords”, and “abstract”. The search was limited to the period from 2014 to 2024. To further refine the results to relevant thematic areas, only research articles were selected. The Scopus database returned 34 articles, while Web of Science returned 29. After merging the records and removing duplicates, a total of 52 documents were retained.

The study selection process was conducted independently by four reviewers. Each reviewer screened the titles and abstracts of all retrieved records to identify potentially relevant studies. Any disagreements among the reviewers were resolved through discussion until consensus was reached. Full-text articles were then independently assessed by the same reviewers according to predefined inclusion and exclusion criteria.

This systematic review was conducted in accordance with the principles established by the PRISMA protocol (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) as illustrated in Figure 1 [13]. This protocol provides a checklist of essential items designed to ensure transparency and accuracy in the reporting of systematic reviews.

2.2. Data Analysis

To process the metadata from the publications exported from Scopus and Web of Science, the RStudio software (version 2024.12.0-467) was used with the bibliometrix package [14]. Analyses included country collaboration networks, scientific production by country, author–country correspondence, keyword co-occurrence analysis, and multiple correspondence analysis (MCA).

2.3. Literature Review Data Analysis

Initially, a bibliometric analysis was conducted to identify research trends and determine the most frequently studied emerging drying technologies. Subsequently, separate literature searches were performed for each selected technology. The included studies were then grouped according to the type of drying technology to guide the synthesis process. Only studies reporting outcomes related to the retention of bioactive compounds were considered for qualitative synthesis. Data preparation involved extracting results when available; missing data were not imputed. Results were organized in tables using Excel. No formal heterogeneity analysis was conducted due to the qualitative nature of this review, and sensitivity analyses to assess the robustness of the synthesized results were not performed due to the characteristics of the data and the scope of this study.

The Scopus database was used to retrieve publications related to emerging technologies in food drying and their implications for bioactive compounds. The search was conducted from 17 January to 2 April 2025, using the terms “Food” AND “Drying” AND “microwaves” OR “radio frequency” OR “ultrasound” OR “infrared” OR “freeze-drying” OR “cold plasma” AND “bioactive compounds” in the fields “title”, “keywords”, and “abstract”. The search was limited to the period from 2014 to 2024. To further narrow the scope to relevant scientific areas, the following subject categories were selected: Agricultural and Biological Sciences, Chemistry, Chemical Engineering, and Engineering. The search returned 111 results for microwaves, 5 for radiofrequency, 76 for ultrasound, 93 for infrared, 297 for freeze-drying, and 8 for cold plasma. However, for comparative analysis, only the 10 most relevant articles were selected for microwave, ultrasound, infrared, and freeze-drying. The total number of articles found was retained for radiofrequency and cold plasma.

The primary outcomes sought in this review included measures related to the preservation of bioactive compounds, physical and sensory quality attributes, and drying process parameters, when available. Additional variables collected comprised the type of drying technology and characteristics of the food matrices.

As this review focused primarily on technological applications and bibliometric data rather than clinical outcomes, a formal risk of bias assessment was not performed. As this review did not perform a meta-analysis or quantitative synthesis, no formal effect measures were calculated. Results are presented qualitatively based on reported outcomes.

3. Bibliometric Analysis

A Bibliometric analysis is a widely used quantitative tool for assessing the impact and relevance of scientific publications. Its use has grown due to its ability to identify emerging trends, map the structure of a research field, and understand collaboration networks among researchers [15].

Table 1. Overview of bibliometric statistics for studies on emerging technologies in food drying.

Description	Results
Documents	52
Time span	2014–2024
Authors	251
Co-authors per document	5.23
International co-authorship (%)	15.38
Average document age (years)	4.25
Average citations per document	24.02
References	3349

Metadata obtained from the combined results of documents retrieved from Scopus and Web of Science databases.

presents a bibliographic overview of the topic of emerging technologies in food drying, based on 52 research articles published between 2014 and 2024. Regarding authorship, 251 authors contributed to this topic, with an average of 5.23 authors per document and an international co-authorship rate of 15.38%. These data highlight the level of collaboration both among authors from the same country and across different countries, all focused on the application of emerging technologies in food drying. A noteworthy point is the high average number of citations per document, calculated at 24.02, which demonstrates the significant impact of these studies within the academic community, often serving as reference works for future research in this field.

Among the 52 publications, Figure 2 shows the annual distribution. In 2024, eleven papers on the topic of emerging technologies in food drying were published, followed by nine papers in 2023 and eight in 2021. This upward trend in the past two years underscores the growing importance and research interest in this area.

The global geographical distribution of authors indicates that Brazil has the highest number of contributors (fourteen) (Figure 3), followed by China, France, Greece, and Italy with five authors each, Mexico and Poland with four each, and Canada, India, Spain, the United States, and Vietnam with three each.

Analyzing the country of the corresponding author in relation to the co-authors (Figure 4), it is observed that Brazilian corresponding authors tend to collaborate predominantly with researchers from the same country (five papers), and in two papers there was international collaboration. A similar pattern is seen in publications from Mexico, with three national-only collaborations and one involving foreign authors.

Chinese papers show a balance between publications authored solely by nationals (two) and those involving international collaboration (two). Publications from Canada, Greece, Argentina, Chile, India, Iran, Italy, the Netherlands, Poland, the United States, and Belgium included only authors from their respective countries.

Analyzing the most relevant keywords in the publications (Figure 5), the term “emerging technologies” has the highest frequency and is therefore highlighted at the center of the network. The term “drying” shows the second highest frequency. The red cluster encompasses topics related to processing and extraction technologies, as indicated by keywords such as osmotic dehydration, microwaves, heating, and assisted extraction. The blue cluster is centered around drying and related analyses, including terms like freeze-drying, moisture, and economic analysis. In the purple cluster, terms reflect the interest in physical processes applied to drying, such as ultrasonics, hydrostatic pressure, temperature, and desiccation chemistry. The green cluster includes keywords related to moisture control and food preservation. The orange cluster presents two isolated terms, technology and kinetics, representing a more general area of study.

A multiple correspondence analysis (MCA) was used to explore associations among keywords grouped into dimensions (Figure 6). The MCA revealed three distinct clusters, which correlate well with the keyword co-occurrence analysis shown in Figure 4. Dimension 1 (30.18%) shows the greatest variance, separating thermal processing and drying techniques (right side) from extraction and optimization methods (left side). Dimension 2 (18.13%) shows less contrast between groups, distinguishing between physicochemical and bioactive properties (upper part) and emerging technologies and dehydration techniques (lower part). The green cluster is associated with optimization and extraction, with terms such as optimization, assisted extraction, recovery, spray drying conditions, and impact. It includes studies focusing on food processing and improving techniques to optimize product quality. The red cluster is centered on physicochemical properties and food storage, with keywords like bioactive compounds, temperature, physicochemical properties, food storage, food handling, fruits, and vegetables. This cluster connects studies exploring the chemical and physical characteristics of food, as well as the effects of storage and handling, and aligns with the co-occurrence network where emerging technologies and phenolics are central terms. The blue cluster groups keywords indicating physical and technological processing methods, such as drying, air dehumidification, food products, thermal processing, ultrasonics, osmotic dehydration, and microwaves. It aligns with connections observed in the collaboration network, where microwaves and osmotic dehydration are closely linked.

4. Comprehensive Review

Based on the systematic review, 28 research articles out of the initial 52 were selected, specifically addressing the topic of emerging technologies, as presented in

Table 2. A summary of studies addressing emerging drying technologies in the literature.

No.	Drying Technology	Sample	Reference
1	Closed-loop spray drying	Milk	[16]
2	Hot air-assisted radiofrequency	Corn kernels (Zhengdan)	[17]
3	Heat pump	Lavender flowers (Lavandula angustifolia)	[18]
4	Spray drying	Essential oils of oregano (Origanum vulgare), rosemary (Rosmarinus officinalis), chamomile (Chamaemelum nobile), and hypericum (Hypericum perforatum)	[19]
5	Freeze-drying	Belimbing Buluh (Averrhoa bilimbi)	[20]
6	Freeze-drying, microwave, infrared, high-frequency drying	Yellow mealworm (Tenebrio molitor)	[21]
7	Instant controlled pressure drop	Green lentils (Lens Culinaris)	[22]
8	Ultrasound	Guavas (Psidium Guajava)	[12]
9	Microwave and vacuum microwave	Barley malt (Hordeum vulgare)	[23]
10	Cold plasma	Animal feed pellets	[24]
11	Rotating tray dryer	Tomato seeds (Solanum lycopersicum L.)	[25]
12	Microwave	Strawberries (Fragaria x ananassa)	[26]
13	High-voltage electrical discharge	Blueberries (Vaccinium sect. Cyanococcus)	[27]
14	Moderate electric field	Apples (Malus domestica)	[28]
15	Radiofrequency	Canola seeds (Brassica Napus. L.)	[29]
16	Pulsed electric field	Spinach (Spinacia oleracea)	[30]
17	Vacuum impregnation and ohmic heating	Apple (Malus domestica)	[31]
18	Micronization	Spinach (Spinacia oleracea L.)	[32]
19	Ultrasound	Wheat starch (Triticum aestivum)	[33]
20	Ultrasound	Potatoes (Solanum tuberosum)	[34]
21	Ultrasound	Wheat (Triticum aestivum) and barley (Hordeum vulgare)	[35]
22	Infrared	Bocaiúva (Acrocomia Aculeata)	[36]
23	Ultrasound	Barley and barley malt (Hordeum vulgare)	[37]
24	Spray drying	Pomegranate seeds (Punica granatum L.)	[38]
25	Spray drying	Cocoa shells (Theobroma cacao L.)	[39]
26	Spray drying	Sage (Salvia officinalis L.)	[40]
27	Spray drying	L. nobilis L. leaves (Laurus nobilis L.)	[41]
28	Freeze-drying	Kernels pulp (Prunus armeniaca) and peach pulp (Prunus persica L.)	[42]

. Review articles and those unrelated to the food sector were excluded. From Table 2, several emerging drying technologies can be highlighted, including microwave, radiofrequency, infrared, ultrasound, freeze-drying, and cold plasma.

4.1. A Comprehensive Overview of Bioactive Compounds in Different Food Drying Systems

4.1.1. Microwave

Microwaves are electromagnetic radiations with long wavelengths (1 mm to 1 m) and high frequencies (300 MHz to 300 GHz), which interact with water molecules in materials, generating volumetric heating through dipolar and ionic mechanisms [43]. Microwave heating involves the conversion of electromagnetic energy into heat through dipolar reorientation and ionic conduction. Fruits and vegetables, for instance, contain approximately 80% water, whose molecules are dipolar and capable of aligning with the electric field of the waves. Since the electric field oscillates at a very high frequency, the dipolar molecules continuously attempt to realign themselves, leading to intense molecular movement [43,44].

Thus, the rotation of dipolar molecules, especially water, creates friction and heat within the food matrix. In the case of ionic conduction, ions migrate under the influence of the electric field, and molecular friction also generates heat. Due to the electromagnetic field and the friction-induced internal heating, microwave radiation results in rapid temperature increases within the sample, with molecular expansion and porosity increases facilitating mass transfer [43,44,45].

In conventional thermal processing, energy is transferred to the food surface by convection and then conducted inward. However, due to the low thermal conductivity of food, this heating process is slow. In contrast, microwave processing delivers energy directly to the food through interactions with the electromagnetic field, generating heat throughout the volume without relying on surface diffusion. This mechanism results in faster, more uniform heating and is more energy-efficient while better preserving the sensory, nutritional, and functional properties of food [43,46,47].

In microwave drying, this principle results in more efficient moisture removal compared to conventional hot air drying, which is slower due to the limited heat transfer. Microwaves penetrate the product and generate internal heating that creates pressure gradients, accelerating the moisture migration to the surface. Additionally, due to its high dielectric loss factor, water efficiently absorbs microwave energy, enabling a selective drying that minimizes the degradation of certain bioactive compounds [47].

Microwave drying thus emerges as a promising alternative technology for thermal processing due to its volumetric heating nature [46]. Particularly during the falling-rate period, microwave drying proves useful because this phase is limited by diffusion, which typically leads to structural shrinkage and reduced surface moisture. Microwave heating, by generating internal vapor pressure, facilitates water migration and helps avoid material shrinkage [48].

Microwave drying is considered an interesting approach for food processing due to its efficient drying rate, lower energy consumption, and capacity to preserve heat-sensitive compounds. However, prolonged microwave exposure may cause overheating, as reported by Huang et al. [49] and Tepe [9]. A solution is intermittent microwave drying, in which the magnetron is turned on and off periodically, resulting in a discontinuous process. This approach allows for a more uniform temperature and moisture distribution throughout the drying cycle, reducing the risk of overheating [50]. Intermittent drying is a promising alternative to continuous heating, as it reduces the processing time, optimizes energy use, and preserves the product quality. Unlike continuous drying, which may cause thermal damage and energy waste, heat modulation in intermittent drying allows for controlled moisture migration, minimizing negative effects [43].

Nevertheless, a key limitation of microwave drying is uneven heating caused by temperature differentials between cold and hot spots. Due to the dielectric nature of microwave interactions, heating concentrates in regions with higher water contents, forming hot spots. To mitigate this, it is essential to improve the radiation uniformity and control the hot spot temperature. Strategies include using modulated stirrers, hot-air-assisted agitation, the precise control of the surface temperature and microwave power, and rotating trays to promote homogeneous heating [46,51].

Table 3. Applications of microwave drying in food matrices.

No.	Drying Method	Material	Main Findings	Reference
1	Microwave	Onion (Allium cepa) and ginger (Zingiber officinale)	Higher antioxidant activity content (9% and 6% for onion and ginger, respectively) compared to conventional drying (2% and 4%). A similar pattern was observed for total phenolics: microwave (18% and 12% for onion and ginger, respectively) vs. conventional (15% and 11%).	[52]
2	Oven, microwave, and freeze-drying	Turmeric (Curcuma longa L.)	Microwave preserved phenolic compounds better, resulting in ~60% degradation. It also caused less degradation of tannins and antioxidant activity (ascorbic acid equivalent).	[11]
3	Microwave followed by vacuum pump, tray dryer, and freeze-drying	Thunbergia laurifolia leaves	The combination of microwave and heat pump resulted in a 67.5% drying time reduction and higher levels of caffeic acid (increased by 51.24%) and quercetin (increased by 60.89%) compared to other methods.	[53]
4	Rotating pulsed fluidized bed assisted by microwave	Okara	Microwave application reduced total phenolic compounds and isoflavones.	[47]
5	Microwave at reduced power	Golden Berry (Physalis Peruviana L.)	At 480 W, higher levels of bioactive compounds and better antioxidant properties were observed compared to convective drying.	[54]
6	Vacuum, microwave, oven, infrared, and freeze-drying	Saffron (Crocus Sativus L.)	For crocin I, oven-dried sample (25.41%) had the highest content, close to microwave (25.26%). For crocin II, freeze-dried (6.76%) was highest; no significant difference with microwave (6.31%). Freeze-dried and microwave samples retained the most total crocin.	[55]
7	Pretreatment and hot air drying assisted by microwave	Apple (Malus domestica)	Samples pretreated with citric acid and subjected to microwave-assisted hot air drying had the lowest reduction in total phenolic content.	[9]
8	Pretreatments for vacuum-assisted conductive drying	Ginger (Zingiber officinale Roscoe)	Although microwave reduced total phenolics compared to ultrasound and ohmic heating, it effectively enhanced flavonoid availability and improved antioxidant activity due to cell wall disruption.	[56]
9	Sun, tray, and microwave drying	Catharanthus roseus leaves	Microwave-dried leaves showed the highest antioxidant activity (152 mg TE/g), phenolics (11.28 mg GAE/g), and flavonoids (59.69 mg QE/g) compared to other drying methods.	[57]
10	Infrared, microwave, and freeze-drying	Yellow passion fruit residue (Passiflora edulis)	Microwave drying at 480 W for 20 min resulted in 3.45 ± 0.09 g gallic acid/kg (total phenolics), 0.0153 ± 0.0009 g rutin/kg (total flavonoids), and 0.0417 ± 0.0043 g/kg (ascorbic acid)—increases of 188%, 226%, and 673% vs. fresh residue.	[58]

summarizes the application of microwave technology in various food matrices.

Microwave drying (MW) stands out for significantly reducing processing times compared to conventional methods. Khan et al. [52] reported that MW shortened the drying time by up to 100-fold compared to hot air drying, while the combination of MW with a heat pump also led to shorter times than conventional dryers [53]. This intensified process minimizes polyphenol oxidation [54] and enhances the moisture removal both on the surface and internally [47,59]. This enhancement is attributed to the volumetric heating and high vapor pressure gradients generated inside the product, which increase the internal water migration and reduce the resistance to mass transfer, as also confirmed in the microwave-assisted pulsed fluidized bed drying of okara [47].

Regarding bioactive compounds, the effects of MW can vary. Intermittent microwave drying has been shown to help preserve phenolic compounds, increasing their retention by up to 80%. For turmeric, MW caused as low as a 60% degradation of phenolics, which may be associated with the inactivation of polyphenol oxidase [11]. In ginger, the microwave pretreatment reduced the total phenolic content compared to ultrasound and ohmic heating, but increased their availability due to the cell membrane rupture, which promotes a greater release and conversion into smaller, more bioavailable structures [56]. Similarly, the antioxidant activity was enhanced, possibly due to the release of previously retained compounds or the formation of new bioactive molecules. Nonetheless, an excessive MW power and exposure time can lead to the thermal degradation of sensitive compounds. Lazarin et al. [47] reported significant losses of isoflavones.

The microwave drying of Catharanthus roseus leaves resulted in higher phenolic contents, due to the combination of the temperature and vapor pressure generated at 300 W, which facilitated cell wall ruptures and the release of previously vacuole-trapped compounds. Similar findings were reported by Silva et al. [58]. Compared to sun and tray drying, microwave-treated samples had higher phenolic contents. However, higher powers (450 W) led to phenolic degradation due to the temperature rise. Studies indicate that the phenolic retention is optimal at temperatures between 50 and 60 °C. The microwave drying at 450 W provided the highest flavonoid retention, while 600 W caused significant degradation though still superior to sun or tray drying. At 900 W, the degradation was lower, likely due to the rapid deterioration of bioactives at high powers. These findings are consistent with previous studies, which indicate that microwave drying can accelerate the disruption of the cellular matrix, thereby promoting a faster release of flavonoids previously bound to cellular structures [57]. However, the microwave intensity and exposure time must be carefully managed to avoid excessive compaction or the collapse of the structure, which may negatively affect the rehydration capacity and mechanical properties. Nawirska-Olszańska et al. [54] observed that Physalis fruits dried under microwaves at 480 W exhibited an increased hardness and resistance to cutting compared to convective drying.

MW also affects other compounds, such as ascorbic acid, organic acids, and pigments. The microwave technique yielded the highest increase in vitamin C contents in passion fruit residue—up to a 700% increase compared to fresh material. This was attributed to the inactivation of degrading enzymes and the release of ascorbic acid due to the cell rupture. However, a high microwave power reduced the vitamin C retention, highlighting the need to optimize the time and power levels [58]. This behavior was also observed for isoflavones in okara: while MW promoted the interconversion of conjugated forms into bioavailable aglycones, it also led to notable losses when high temperatures were reached inside the drying chamber [47]. Such dual effects underscore the importance of fine-tuning MW parameters according to the target compound’s thermal sensitivity.

The citric acid content in Spirulina decreased by up to 30%, with the lowest degradation observed at lower microwave powers [43]. Conversely, the phycocyanin retention was higher at elevated powers, suggesting that the exposure time plays a crucial role. In the case of Bletilla striata flowers, MW promoted a greater retention of polysaccharides (10.55 mg/g) [60]. These differences across compounds and matrices suggest that MW outcomes are highly specific to the chemical nature of each bioactive. As observed in different studies, the antioxidant capacity, vitamin retention, and enzyme inactivation respond variably to the MW intensity and duration. Therefore, predictive modeling and experimental validation are essential for process optimization.

Moreover, combining MW with other treatments can enhance the preservation of bioactives. Ginger samples pretreated with MW showed a higher total flavonoid content than controls, due to the cellular matrix disruption [56]. Similarly, samples pretreated with citric acid and subjected to microwave-assisted drying showed a lower degradation of total phenolics, since citric acid inhibited polyphenol oxidase and facilitated compound release [9].

In this context, hybrid systems, such as microwave-assisted fluidized beds or vacuum microwave drying, have shown promising results. Lazarin et al. [47] reported that applying MW only in the first 10 min of drying was sufficient to accelerate the drying while reducing the thermal degradation of phenolics and isoflavones in okara. Such intermittent or pulsed strategies may represent an energy-efficient and quality-preserving alternative for industrial-scale drying. Combining MW with a heat pump, as tested for Thunbergia laurifolia leaves, has been shown to result in faster drying times and a higher retention of bioactive compounds compared to single methods [53].

4.1.2. Radiofrequency

The dissipation of electromagnetic energy throughout the product reduces the need for thermal conduction or convection, resulting in shorter processing times and minimized quality deterioration [61,62]. Radiofrequency (RF) is a form of non-ionizing electromagnetic radiation with wavelengths up to 11 m and frequencies between 1 and 300 MHz. These waves can penetrate dielectric materials and generate heat through ionic polarization or dipole rotation. Compared to microwaves, RF waves have a greater penetration depth due to their longer wavelengths [51,63,64].

RF heating systems use standard oscillating circuits and include an automatic impedance matching system to maintain a stable coupling power and frequency. During RF processing, the moisture content of the material is critical, as it affects the dielectric properties and enhances the heating uniformity [51]. Dielectric properties are material characteristics that determine how a material interacts with an applied electromagnetic field and influence how quickly it heats. Thus, they are a useful way to differentiate good absorbers from poor ones [65].

Radiofrequency drying has been explored in various food products, such as corn kernels [17] and carrots [66]. RF technology is often combined with other novel processing approaches to enhance its effectiveness [67], such as hot air [17,66], cold plasma [68], or vacuums [65]. Although RF heating offers advantages, such as rapid heating and a deep penetration, it also presents significant challenges when applied to agricultural products. These include overheating at the edges and corners of rectangular samples, non-uniform heating, and arc phenomena due to excess moisture in the RF cavity, which can damage the quality of fruits and vegetables with high water contents, factors that may limit RF’s applicability in the food industry [66].

The vacuum-assisted radiofrequency drying (RF-vacuum) of bananas demonstrated advantages in the drying rate and uniformity compared to conventional methods and microwave drying. RF–vacuum drying resulted in a lower final moisture content after 270 min compared to the RF alone. The moisture distribution was more uniform across the banana layers, with the top layer drying faster than the middle and bottom layers, indicating the potential of RF–vacuum drying for more efficient and uniform drying [65].

The findings of Zheng et al. [66] also suggest that RFs combined with hot air may be a promising approach for the fluidized bed drying of fresh fruits and vegetables, showing a remarkable performance in improving the heating uniformity in high-moisture foods.

Table 4. Applications of radiofrequency in food matrices.

No.	Drying Method	Material	Main Findings	Reference
1	Radiofrequency	Lotus pollen (Nelumbo nucifera)	RF drying reduced lipid oxidation by 31.58% and better preserved polyunsaturated fatty acids, such as alpha-linolenic acid and docosahexaenoic acid (DHA), compared to hot air drying.	[63]
2	Ultrasound pretreatment, chemical coating, and vacuum RF drying	Cherry (Prunus avium)	Pretreatment increased neochlorogenic (241.48 ± 4.12 mg/100 g) and isochlorogenic acid (23.79 ± 1.03 mg/100 g) levels by 1.25× and 1.48× and improved catechin and kaempferol retention.	[62]
3	Ultrasound-assisted osmotic dehydration and convective-assisted RF drying	Apple (Malus domestica)	Samples pretreated and dried by this method showed higher ascorbic acid content.	[69]
4	Hot-air-assisted RF drying	Melon (Cucumis melo) and apple (Malus domestica)	RF drying preserved higher vitamin C content (8.37–8.58 mg/100 g) compared to hot air (4.22–5.07 mg/100 g) and increased flavonoid retention by 17% to 38.5%.	[64]

summarizes the application of this technology in food matrices.

Luo et al. [63] suggest that RF drying can be an efficient alternative for preserving the nutritional and sensory quality of food, minimizing the thermal and oxidative degradation compared to hot air drying. However, while RF drying offers advantages, such as reduced thermal stress, it is important to consider its cost-effectiveness and scalability, especially for large-scale industrial applications. Moreover, although RF drying reduces oxidative degradation, it does not completely eliminate the risk of compound loss during the drying process. The effects of RF on specific nutrients still require further investigation, particularly in relation to the long-term storage and transportation of RF-dried products.

Treatments that combined ultrasound (US) with carboxymethylcellulose and cellulase significantly increased the total phenolics, flavonoids, and antioxidant activity (DPPH, ABTS, and FRAP). These effects were attributed to the mechanical and cavitation effects of US, which partially ruptured plant cells and facilitated the release and extraction of these compounds during drying. Additionally, drying under low pressures reduced the boiling point of water, allowing a more efficient moisture removal at lower temperatures and preventing heat damage to sensitive compounds [62]. While low-pressure drying is a promising approach, its application in large-scale food drying systems may present challenges in terms of the equipment cost and energy consumption. Further studies should evaluate the efficiency of these methods in different food matrices and the potential for energy savings in industrial settings. However, the interplay between US and RF in drying processes remains underexplored.

The retention of compounds such as catechin, chlorogenic acid, neochlorogenic acid, and cyanidin-3-O-rutinoside was also improved by combining drying with protective coatings such as carboxymethylcellulose. These coatings formed a physical barrier that limited the oxygen exposure and oxidative degradation. Moreover, the combination of US and coatings helped preserve the cellular structure, inhibiting undesirable enzymatic reactions. The inhibition of polyphenol oxidase (PPO) and peroxidase (POD), enzymes responsible for polyphenol degradation, resulted in a higher antioxidant content and a better nutritional quality in the final product [62]. Despite these promising results, the use of protective coatings may increase production costs, and more sustainable alternatives should be explored, considering both the environmental impact and cost-effectiveness.

Jin et al. [69] explained that the effect of US on RF drying can be attributed to pretreatments increasing the ε″ (dielectric loss factor) of the samples. This indicated that apples absorbed energy more effectively during drying, accelerating the process and improving the retention of bioactive compounds, particularly ascorbic acid. RF drying has emerged as an effective alternative for preserving bioactive compounds, reducing thermal degradation, and enhancing antioxidant release [64]. However, while the effectiveness of RF drying in preserving bioactive compounds is well-documented, further studies are needed to optimize the RF parameters (such as power and frequency) to enhance both the nutrient retention and process efficiency without incurring excessive energy costs.

The studies analyzed demonstrate that radiofrequency is an effective alternative for drying processes, as it reduces operating times and, consequently, energy consumption. Moreover, RF heating better preserves the color, texture, vitamin C, phenolic compounds, and reduces lipid oxidation compared to traditional convective drying. However, the number of studies on the application of radiofrequency is still limited, and despite the promising results, industrial adoption requires further investigations that deepen the understanding of drying mechanisms and facilitate the development of optimized equipment for pilot and industrial-scale applications.

4.1.3. Freeze-Drying

Also known as lyophilization, freeze-drying (FD) is one of the most relevant dehydration methods for foods rich in bioactive compounds. This is due to the nature of the process, which involves the sublimation of water from a previously frozen product under a low pressure. It is reported that typical FD conditions involve temperatures below 0 °C and pressures lower than 2 mmHg [58,70].

According to Nowak and Jakubczyk [71], the process consists of three stages: the freezing of the product (usually under atmospheric pressure), primary drying, and secondary drying. During freezing, ice crystals form, producing benefits such as reduced physical, chemical, and microbiological changes; the immobilization of ingredients; the prevention of foaming during the next phase (caused by the reduced pressure in the drying chamber); and the structural stabilization of cells, minimizing ruptures and deformations. The freezing rate directly influences the properties of the final dried product, as it determines the morphology of the ice crystals, which in turn affects the sublimation rate in the next stage [71,72].

In the first drying stage, the frozen product is placed in the freeze-dryer, and sublimation occurs under a reduced pressure at a temperature about 2–3 °C below the product’s collapse temperature (the temperature at which physical alterations compromising quality may occur). In the second stage, most of the sublimation is complete, and the desorption of bound water molecules begins. This is the longest stage and is essential for reducing the moisture content to the desired level [71,72].

Regarding the structure of the final product, Sultana et al. [4] reported that porosity is directly influenced by the ice crystal formation during the freezing stage. Rapid freezing results in finer powders, while slow freezing produces coarser particles. In this regard, FD has been noted as a favorable option for producing highly porous powders, which improves solubility but may reduce the storage stability.

Table 5. Applications of freeze-drying in food matrices.

No.	Drying Method	Material	Main Findings	Reference
1	Encapsulation with maltodextrin (different dextrose equivalents), followed by freeze-drying	Amazonian fruits (Clidemia hirta L.) D. Don and Clidemia japurensis DC.	The antioxidant activity of C. hirta and C. japurensis juice ranged from 97.04% to 99.79% (DPPH method) and from 87.80% to 99.11% (ABTS method).	[73]
2	Natural drying, microwave, hot air, infrared, vacuum, and freeze-drying	Flowers (Bletilla striata (Thunb.) Reichb.f.)	Freeze-drying was the best option for preserving active compounds, with high levels of total phenolics (31.44 mg/g) and total anthocyanins (9.94 mg/g).	[60]
3	Freeze-drying and oven drying (at 50 and 65 °C)	Fresh pumpkin (Cucurbita moschata) residues	Freeze-dried samples had higher total phenolic and β-carotene contents—294.69 ± 14.13 and 14.26 ± 0.27 mg/100 g dry weight, respectively—and higher antioxidant activity (DPPH: 131.64 ± 6.40 mg TE/100 g dry weight).	[70]
4	Freeze-drying, spray drying, and foam-mat tunnel drying	Arabica coffee husks (Coffea arabica silverskin)	Freeze-dried products had higher total phenolics and antioxidant activity values than foam-mat drying and slightly lower than spray drying.	[74]
5	Cast-tape drying (CTD) and freeze-drying	Tucumã pulp (Astrocaryum aculeatum)	Freeze-dried samples had higher total phenolic and carotenoid contents (467.87 ± 3.33 and 47.11 ± 0.34, respectively), although antioxidant activity was lower than CTD.	[75]
6	Hot air drying and freeze-drying with sugar infusion pretreatments under wet and dry conditions	Frozen raspberries (Rubus idaeus L.)	Freeze-drying caused 18% and 10.7% losses in total phenolics and anthocyanins, respectively, versus the fresh sample; hot air drying caused losses of 63% and 55.5%, respectively.	[76]
7	Spray drying and freeze-drying	Kanuka (Kunzea ericoides) wood chips	Total phenolic content was similar between methods (65.4–86.4 mg GAE/g). Flavonoid content and antioxidant activity (DPPH and FRAP) were also similar: 77.00–101.2 mg QE/g, 23.5–45.3 mg TE/g (DPPH), and 81.5–91.7 mg TE/g (FRAP).	[77]
8	Freeze-drying (fruit bars) and conventional hot air drying (fruit paste)	Discarded yellow kiwifruit (Actinidia chinensis ‘Jintao’), with added fruits and vegetables	Freeze-dried bars had higher flavonoid content (3.78 ± 0.37 to 5.83 ± 0.14 mg quercetin/100 g DW) compared to fruit pastes (0.45 ± 0.08 to 0.94 ± 0.08 mg quercetin/100 g DW). Higher polyphenol content and lower antioxidant activity were also observed in the bars.	[78]
9	Freeze-drying (slow freezing at −18 °C for 24 h and fast freezing with liquid nitrogen at 150 °C)	Yellow passion (Passiflora edulis f. flavicarpa) fruit residues	Slow freezing yielded higher total phenolics and flavonoids than fresh material (119.87 ± 4.71 mg gallic acid/100 g, 0.47 ± 0.05 mg rutin/100 g). Ascorbic and citric acid contents were slightly lower than in fresh samples. Pectin content increased, especially with rapid freezing (78.92%).	[79]
10	Spray drying (conventional and ultrasonic nozzle) and freeze-drying	Blueberries (Highbush blueberry, V. corymbosum L.)	Freeze-drying showed similar phenolic content to spray drying (1663.30 ± 82.16 mg GAE/100 g DW), antioxidant activity (21,206.96 ± 33.05 μmol Trolox/100 g DW), and higher anthocyanin content (6072.72 ± 72.62 mg C3G/100 g DW).	[80]

summarizes the application of this technology in food matrices.

When it comes to thermosensitive bioactive compounds, FD has proven to be an excellent preservation method, mainly due to its use of low temperatures that minimize thermal degradation. According to Mar et al. [73], this drying technique is essential for preserving antioxidant compounds in Amazonian fruits, such as Clidemia hirta L. and Clidemia japurensis DC. The study demonstrated that FD played a significant role in retaining the antioxidant activity in these fruits, likely due to the low temperatures used during the process. Similarly, Lu et al. [60] reported higher levels of polyphenols and anthocyanins in Bletilla striata (Thunb.) Reichb.f. flowers compared to other drying methods. However, they noted that anthocyanins tend to degrade more as a result of prolonged drying times rather than the temperature used, suggesting that while FD minimizes the heat exposure, extended drying durations can still contribute to the degradation of sensitive compounds.

Sette et al. [76] found similar results in raspberries (Rubus idaeus L.), observing that although FD reduces the thermal degradation of some bioactive compounds, polyphenols may still be exposed to oxidative conditions during the long drying time, potentially reducing their concentrations. This highlights that even though the low temperatures of FD limit heat-induced damage, the process still poses risks for oxidative degradation during prolonged drying times.

Stajčić et al. [70] conducted a study on pumpkin (Cucurbita moschata) residues and reported that FD yielded samples with a higher antioxidant activity when measured by the DPPH method compared to oven drying. However, when the reducing power method was applied, FD showed a lesser effect. The authors argued that while FD is effective in preserving antioxidant activity, it does not promote the Maillard reaction, which occurs at higher temperatures and is linked to the formation of antioxidants, thus making freeze-dried samples less antioxidant-rich compared to those dried at higher temperatures.

Some studies have also reported increased levels of bioactive compounds after FD, attributing this to the breakdown of internal structures that enhance the extraction of these compounds. For instance, Silva et al. [58] found higher levels of total phenolics, flavonoids, and pectin in pumpkin following FD. This suggests that freeze-drying can not only preserve bioactive compounds but also facilitate the release and quantification of certain nutrients by disrupting the cellular structure.

Although FD is a slow and costly process, it is consistently recognized as one of the most effective methods for preserving bioactive compounds, especially when combined with pretreatments or rapid freezing methods. While it has some limitations, particularly with the oxidative degradation of sensitive compounds and the lack of the Maillard reaction, FD remains one of the most reliable methods for preserving thermosensitive bioactive compounds, with significant potential for food and nutraceutical applications. Continued research and the optimization of FD parameters could further enhance its effectiveness and reduce some of these limitations.

4.1.4. Ultrasound

Ultrasound (US) is an emerging technology that uses mechanisms such as cavitation and the sponge effect to modify the structure of food products. This leads to an improved heat and mass transfer during processing, enabling drying at lower temperatures and/or shorter times [35,81]. This technique has been applied both as a pretreatment prior to drying and as an in-process aid [82].

US refers to sound waves with frequencies equal to or greater than 16 kHz, which is above the range of human hearing. These waves can propagate through solids, liquids, or gases depending on the wavelength and medium composition. US can induce physical, chemical, and mechanical changes in fruits and vegetables depending on the power and frequency applied. A high-power US at frequencies between 20 and 100 kHz can promote physicochemical modifications, microbial inactivation, enhance heat and mass transfer, accelerate chemical and biochemical reactions, and activate or inactivate enzymes. In contrast, low-power US at frequencies above 100 kHz is used for nondestructive and/or noninvasive analyses. It is important to note that, in addition to affecting food structures, the composition and structure of the foods themselves influence how US propagates and how ultrasonic energy is converted to modify the matrix [8,83].

Commercial US equipment includes ultrasonic baths and probes. Baths are more affordable and easier to maintain, operating with submerged piezoelectric transducers (40–130 kHz) that transmit sound waves to the material and enhance mass transfer, although at a low intensity (0.1–1 W/cm²). Probes, while more expensive, enable higher intensities (>5 W/cm²) and are used for material fragmentation and homogenization [8]. The primary effect of US in liquids is acoustic cavitation, characterized by the formation and collapse of bubbles due to pressure variations in longitudinal waves. During rarefaction, the bubbles grow past a critical threshold, and their collapse produces microjets and microstreaming that impact nearby cells and molecules, forming microchannels that increase water diffusivity and improve the drying performance [8,84].

In solids, ultrasonic waves cause rapid compressions and expansions, known as the “sponge effect”, that overcome the surface tension, releasing internal fluids and allowing osmotic solution penetration. This phenomenon facilitates mass transfer and can also structurally modify the material [85,86]. While the sponge effect is a direct mechanism, the microchannel formation from acoustic cavitation represents an indirect structural change. Therefore, ultrasound not only enhances mass transfer but also alters the physical properties of treated materials, making it a relevant technology for various industrial applications [82].

Fruit and vegetable tissues may undergo physical and chemical changes due to the interaction between sonication and drying, which can alter food textures either positively or negatively. Additionally, ultrasound may minimize color degradation. Depending on the application conditions and the type of food, ultrasound may improve not only the physical quality of fruits and vegetables but also their nutritional quality. Ultrasound has been shown to aid in the preservation of the polyphenols, flavonoids, vitamin C, and antioxidant activity in dried fruits and vegetables [8] (

Table 6. Applications of ultrasound as a pretreatment associated with drying technologies in food matrices.

No.	Pretreatment	Drying Method	Material	Main Findings	Reference
1	–	Convective drying with air recirculation, airflow control, temperature control, and ultrasound-activated drying chamber	Kiwis (Actinidia deliciosa)	High-power ultrasound reduced kiwi drying time by 55–65%. Without ultrasound, samples dried at 15 °C showed greater losses of bioactive compounds (39–54%) and antioxidant activity (57–69%) compared to those dried at 5 °C (14–43% and 23–50%). Applying ultrasound at 15 °C minimized these losses to 15–47% and 47–58%, respectively.	[87]
2	Dual-frequency ultrasonic reactor	Humidity-controlled convective drying	Sweet potato (Ipomoea batatas L.)	Ultrasonic pretreatments significantly altered the phytochemistry and microstructure of dried sweet potato slices. Total phenolics, flavonoids, and carotenoids increased compared to control at 70 °C. β-carotene increased with temperature, while antioxidant property reduction was influenced by ultrasound and related to phenol characteristics.	[88]
3	Microwave, ohmic heating, and ultrasound	Vacuum-assisted conductive drying	Ginger extract (Zingiber officinale Roscoe)	Pretreatments significantly affected total phenolics and flavonoids, DPPH activity, color, and extraction time. Ultrasound and ohmic heating pretreatments yielded slightly higher phenolics and flavonoids, which correlated with increased antioxidant activity.	[56]
4	Ultrasonic bath	Convective drying of the sample and spray drying of optimal extract	Tradescantia zebrina leaves	Optimal extraction conditions (6.25 min, 60 °C, 20% amplitude) led to higher levels of phenolics and anthocyanins. Ultrasound increased the diversity of extract compounds compared to the control.	[89]
5	Ultrasonic bath	Conductive drying and vacuum drying	Honeyberry (Lonicera caerulea var. kamtschatica)	Ultrasound pretreatment and drying technique affected the final composition. The highest vitamin C content was observed in pretreated fruits, regardless of the drying method. Higher levels of phenolics, flavonoids, and anthocyanins were obtained with ultrasound followed by vacuum drying at 40 °C.	[90]
6	Freezing	Ultrasound-assisted convective drying	Beetroot (Beta vulgaris var.)	Freezing pretreatment and ultrasound reduced drying time. Freezing significantly increased beetroot bioactive compounds by releasing free forms. However, drying reduced their levels, further intensified by ultrasound. Although mass transfer was enhanced, the processes could compromise stability and availability of bioactives.	[91]
7	Ethanol, ultrasound, and/or vacuum	Convective drying	Cantaloupe melon (Cucumis melo)	The shortest drying time occurred with ultrasound pretreatment in 100% ethanol. Drying reduced bioactive compounds and changed color, but immersion in 50% ethanol minimized losses of phenolics, carotenoids, and ascorbic acid.	[92]
8	Ultrasound	Spray drying	Oregano (Origanum vulgare)	The ultrasound-extracted oregano extract was more active and selected for microencapsulation, showing higher phenolic (29%) and protein (4.7%) content. Its IC50 was 0.27 g/L, lower than that of the sequential treatment (0.49 g/L).	[93]
9	Ethanol and ultrasound	Convective drying	Banana nanica (Musa acuminata)	Ethanol and ultrasound pretreatments accelerated drying and reduced energy consumption. Bananas treated with ethanol + ultrasound showed better physical quality, attractive color, and greater preservation of bioactives due to shorter drying time.	[94]
10	Ultrasound, pulsed electric field, and sonication	Vacuum microwave drying	Cranberries (Vaccinium oxycoccus)	Vacuum microwave drying reduced drying time by up to 96.9% compared to convective drying. Although pulsed electric field and ultrasound did not accelerate drying, their combination with blanching better preserved color and enhanced antioxidant activity.	[95]

).

Several studies have extensively investigated the influence of US as a pretreatment in reducing drying times and improving process efficiency (Table 6). In kiwi samples, ultrasound-activated drying reduced the conventional drying time by up to 65% [87], a behavior also observed in beetroot [91]. The enhancement in drying efficiency is mainly attributed to the cavitation effects induced by US, which disrupt cellular structures and promote a faster water removal, minimizing the exposure to heat and preventing the degradation of heat-sensitive bioactive compounds.

The combination of US and ethanol as a pretreatment also proved effective in reducing the drying time and energy consumption, as demonstrated for cantaloupe melons [92] and bananas [94]. In these studies, ultrasound facilitated mass transfer by increasing the cell membrane permeability, which enhanced the release of soluble compounds, thereby improving the extractability of bioactive components. Moreover, vacuum-assisted conductive drying applied to ginger extract [56], and the vacuum drying of honeyberries [90], proved effective in both reducing the drying time and preserving bioactive compounds. These benefits are particularly noticeable in antioxidant compounds, which are typically vulnerable to heat stress during conventional drying.

On the other hand, ultrasound-assisted convective drying applied to beetroot [91] showed that, although it accelerates mass transfer, the method may compromise the stability of certain compounds, such as betalains. The oxidative degradation of betalains was more pronounced when the US intensity was high, indicating the importance of carefully controlling process parameters to prevent the loss of sensitive compounds.

US has shown beneficial effects in preserving bioactive compounds during the drying process. In kiwi, its application minimized the losses of bioactive compounds and antioxidant activity at higher temperatures [87], showing that US can enhance the thermal efficiency and compound retention. In cranberries, blanching combined with US better preserved color and enhanced the antioxidant activity [95], suggesting that US could act synergistically with other pretreatments to improve the final product’s quality. Additionally, the US pretreatment resulted in higher levels of phenolic compounds and flavonoids in ginger extract samples [56], Tradescantia zebrina leaves [89], and oregano [93], which is attributed to the increased solubility and extraction efficiency of these compounds.

Overall, while US offers a promising approach for improving drying efficiency and preserving bioactive compounds, the precise control of operational parameters, such as the amplitude, frequency, and treatment duration, is crucial to maximize its benefits and prevent potential losses due to excessive degradation or oxidation.

4.1.5. Cold Plasma

Cold plasma (CP) is an environmentally friendly emerging technique that has gained prominence in studies focused on food preservation and drying methods. This technique is based on the generation of non-equilibrium plasma, where a high electrical current is used to excite molecules in the surrounding atmosphere. This process leads to the formation of several reactive components, mainly ionized molecules, with gas temperatures typically ranging from 30 to 60 °C. Owing to these characteristics, cold plasma enables enzyme inactivation, cell wall disruptions, and the oxidation or alteration of compounds without the need for high temperatures, elevated pressures, or prolonged exposure times [96,97].

Some of the main methods used to generate cold plasma include dielectric barrier discharge (DBD), corona discharge, glow discharge, radiofrequency discharge, gliding arc discharge, atmospheric pressure plasma jets, and multi-pin discharge. In addition to reactor types, parameters such as the frequency, voltage, and gas composition can directly influence the effects of cold plasma on food matrices [96].

The use of cold plasma as a pretreatment for food drying is currently the focus of several studies due to its ability to induce microstructural changes on food surfaces, thereby increasing diffusion rates and reducing the drying time and energy consumption. These advantages are particularly relevant in the preservation of heat-sensitive compounds, such as phenolic compounds [2,98].

Table 7. Applications of cold plasma as a pretreatment associated with drying technologies in food matrices.

No.	Pretreatment	Drying Method	Material	Main Findings	Reference
1	Dielectric Barrier Discharge (DBD) cold plasma	Convective drying at 60 °C with air velocity of 0.5 m/s	Tucumã pulp (Astrocaryum aculeatum)	Regardless of the pretreatment, a reduction was observed in both β-carotene and total phenolic content.	[99]
2	Dielectric Barrier Discharge (DBD) cold plasma	Convective drying with air velocity of 1.5 m/s	Saffron flowers (Crocus sativu)	The compounds crocin, safranal, picrocrocin, and total phenolics were analyzed. A 30 s pretreatment yielded higher values for all compounds compared to the control.	[96]
3	Gliding Arc Discharge cold plasma	Conventional convective drying (50 °C, 1 m/s) and ultrasound-assisted convective drying	Fresh goldenberries (Physalis peruviana L.)	The combined treatment of 60 s plasma pretreatment and ultrasound-assisted convective drying resulted in the best bioactive compound retention, including vitamin C (80.87%), total phenolics (85.57 mg GAE/100 g dry matter), and antioxidant capacity (27.06%).	[2]
4	Dielectric Barrier Discharge cold plasma reactor	–	Grape (Vitis vinifera L) pomace	Cold plasma treatment increased total phenolic content by up to 22.8% and antioxidant capacity by up to 34.7%.	[97]
5	High-voltage plasma with 64-pin multi-electrode configuration	Traditional convective drying (70 °C, 2 m/s) and refractive window drying	Apple slices (Malus domestica)	Without cold plasma, total phenolics (TP) and antioxidant capacity (AC) were 174.15 mg GAE/100 g DM and 122.53 mg QE/100 g DM (convective) and 314.74 mg GAE/100 g DM and 220.94 mg QE/100 g DM (refractive window). With plasma pretreatment, these values increased to approx. 270 and 180 (convective) and 450 and 340 (refractive window), respectively.	[100]
6	Steam blanching and cold plasma-activated water (DBD type)	Vacuum drying and traditional convective drying at 40, 50, and 60 °C	Centella (Centella asiatica L)	Samples treated with plasma-activated water followed by vacuum drying had the highest total phenolics (35.049 mg GAE/g DW) and flavonoids (311.274 mg QE/g DW), followed by those dried convectively. Antioxidant capacity was similar for both pretreatments (~4021.462 μg TE/g DW) and higher than untreated samples.	[101]
7	Steam blanching and cold plasma-activated water (DBD type)	Microwave drying	Centella (Centella asiatica L)	Pretreated samples showed higher levels of total phenolics and flavonoids compared to untreated ones: 35.755 mg GAE/g DW and 306.573 mg QE/g DW (plasma water), 32.681 mg GAE/g and 297.573 mg QE/g (steam), and 31.689 mg GAE/g and 285.317 mg QE/g (untreated).	[102]
8	High-voltage atmospheric cold plasma (HVACP) with various gases (air, Ar, He, N2)	–	Tomato (Solanum lycopersicum) pomace	All gas treatments led to higher total phenolic and antioxidant values compared to control. Phenolic content (mg GAE/g DW): air (0.949), Ar (0.947), He (1.025), N2 (1.033). Antioxidant capacity (mg TE/g DW): air (0.85), Ar (0.85), He (0.85), and N2 (0.89).	[98]

summarizes the main studies on cold plasma applications. These studies primarily focused on using cold plasma as a pretreatment to optimize drying processes, reducing processing times and energy costs, minimizing the degradation of heat-sensitive compounds, and enhancing desired sensory and physicochemical properties.

Loureiro et al. [99] observed that frequency had a direct impact on the effects caused by cold plasma. Nearly all tested frequencies (200, 500, and 800 Hz) resulted in higher levels of β-carotene, total phenolic compounds, and antioxidant capacity compared to untreated samples. At 500 Hz, the DPPH activity was similar to that of the fresh sample. The cold plasma pretreatment also promoted greater pore formation on the surface of the samples, which facilitated moisture diffusion and significantly reduced the drying time—from 270 min in untreated samples to 105 min with the most effective pretreatment (200 Hz).

Similarly, Tabibian et al. [96] studied Crocus sativus (saffron) subjected to various exposure times to cold plasma before convective drying. The plasma treatment significantly reduced the sample moisture content and shortened the drying time by 16% compared to the control. Additionally, a 30 s pretreatment resulted in higher concentrations of crocin and picrocrocin—heat-sensitive bioactive compounds—with increases of 12.28% and 19.29%, respectively, relative to untreated samples. These findings confirm that cold plasma acts not only as a physical modifier but also as a barrier against the thermal degradation of bioactive compounds, better preserving the functional properties of foods.

These positive effects of cold plasma are recurrent in the literature. For instance, Ashtiani et al. [2] reported that combining a cold plasma pretreatment and ultrasound-assisted convective drying in goldenberries led to the highest retention of vitamin C (80.87%), the total phenolics (85.57 mg GAE/100 g), and the antioxidant capacity (27.06%) compared to treatments without plasma. Tripathy et al. [101,102] also found significant increases in phenolic and flavonoid contents in Centella asiatica L. treated with plasma-activated water, with superior values regardless of the drying method applied (vacuum, convective, or microwave).

Although all studies have reported benefits in preserving bioactive compounds, few have quantified the energy consumption associated with drying operations. Tabibian et al. [96] is the only study that presents energy consumption values for both the plasma reactor and the convective dryer, reporting ranges from 15 to 60 kJ for the reactor and from 934.29 to 565.02 kJ for the dryer.

Other studies, such as Tripathy et al. [101,102], provide only the total energy consumption of the drying process with and without cold plasma pretreatments. Tripathy et al. [101] reported an energy consumption ranging from 2.25 to 4.18 kWh for the tray dryer and from 2.48 to 9.87 kWh for the vacuum dryer. In turn, Tripathy et al. [102] reported values between 0.063 and 0.105 kWh for microwave drying. Energy consumption is an important parameter for assessing the operational costs associated with the use of cold plasma reactors and, consequently, for determining the feasibility of their application.

In addition to the scarcity of energy consumption data, there is a notable lack of technical information regarding plasma reactors. Often, only the operating conditions (power, frequency, and time) are described, without detailed structural or electrical specifications.

Regarding the effects of cold plasma on foods, several studies mention physical and chemical modifications; however, only Tabibian et al. [96] directly investigated chemical transformations. They reported an increase in crocin and picrocrocin contents in saffron (Crocus sativus) following treatments, which was attributed to plasma-induced reactions.

Thus, cold plasma as a drying pretreatment emerges as a promising strategy for preserving or even enhancing the content of bioactive compounds in dehydrated foods. Nevertheless, further studies are needed to elucidate the underlying chemical mechanisms and to advance reactor design, aiming at the development of larger-scale equipment suitable for industrial adoption.

4.1.6. Infrared

Infrared (IR) radiation is a type of electromagnetic radiation that covers the wavelength range between 0.75 and 1000 μm, which lies just below the visible red light spectrum—hence the name “infrared”. IR radiation is emitted or detected by any object with a temperature above absolute zero (0 K). It can be classified into near-infrared (0.75–3.0 μm), mid-infrared (3.0–25 μm), and far-infrared (25–1000 μm) regions [43,103].

Infrared drying works by delivering intense heating to the sample. Due to the penetration capability of certain IR frequencies, the molecules within the food vibrate, generating internal heating. This mechanism enables highly efficient heating with minimal heat loss to the environment. Near-infrared, due to its lower frequency and greater penetration depth, is considered more suitable for thick food products. In contrast, far-infrared has a higher frequency and shallower penetration, making it more appropriate for thin layers [43,103].

The infrared dehydration technique offers several advantages, including its faster drying, lower energy consumption, high heat transfer efficiency, uniform heating, and reduced quality losses. It also positively influences sensory attributes such as color, flavor, and texture; preserves antioxidants and essential nutrients; allows direct heating without affecting the surroundings; ensures precise process control; and is compact and cost-effective [51,104].

Recent findings support the efficiency of infrared drying in reducing the drying time and energy consumption while preserving the physicochemical properties of fruits. For example, in the drying of Phoenix dactylifera L. (date fruit), the use of infrared radiation significantly reduced the drying time by up to 50% compared to conventional convective methods, while better retaining the color, total phenolic content, and antioxidant activity [105]. These results emphasize the potential of infrared technology not only in improving the process efficiency but also in maintaining the nutritional and functional quality of thermosensitive compounds.

Table 8. Applications of infrared drying in food matrices.

No.	Drying Method	Material	Main Findings	Reference
1	Infrared radiation and microwaves	Microalga (Spirulina platensis)	The intermittent infrared drying method ensured higher contents of phenolic compounds and flavonoids (423.67 mg/100 g and 6.08 mg rutin/100 g dry matter, respectively). Although citric acid decreased, its retention was better with continuous drying at 65 °C and intermittent drying. Phycocyanin content also declined but was better preserved under intermittent conditions (6.63 and 7.17 g/100 g dry matter).	[43]
2	Convection, freeze-drying, and infrared	Physalis (Physalis Peruviana L.)	At 80 °C, infrared drying yielded similar levels of total phenolics, flavonoids, and carotenoids compared to convective drying: 189.70 ± 5.87 mg GAE/100 g DM, 25.52 ± 1.53 mg CE/100 g DM, and 32.20 ± 0.92 μg/g DM, respectively. Antioxidant activity showed no significant difference (DPPH: 366.57 ± 21.31 μmol TE/100 g DM; ORAC: 4622.00 ± 523 μmol TE/100 g DM).	[106]
3	Freeze-drying, hot air drying, infrared + hot air, relative humidity control drying, and pulsed vacuum drying	Fresh garlic (Allium sativum L.) with peel	Infrared combined with hot air was the fastest drying method (3.8 ± 0.3 h) and resulted in the highest antioxidant activity (20.94 ± 0.11 mg/mL), total phenolics (1.89 ± 0.04 mg GAE/g), and allicin content (144.27 ± 1.06 mg/100 g).	[107]
4	Freeze-drying, freeze-drying assisted by near-IR, freeze-drying + near-IR after 20% weight loss, freeze-drying assisted by far-IR, freeze-drying + far-IR after 20% weight loss	Açaí purée (Euterpe oleracea Martius)	Samples treated with freeze-drying assisted by near-IR and freeze-drying followed by near-IR heating after 20% weight loss showed superior antioxidant activity and anthocyanin content (151.07 ± 5.92 and 148.53 ± 6.95 mg cyanidin-3-glucoside/100 g, respectively).	[103]
5	Hot air, infrared, hot air + puffing, infrared + vacuum with microwaves, and freeze-drying	Frozen raspberries (Rubus idaeus)	Infrared + microwave vacuum drying led to the highest retention of polyphenols (61.42%) and flavonoids (97.99%). It also provided the greatest antioxidant retention (74.12% and 73.49%).	[108]
6	Hot air, microwave, IR-assisted microwave, freezing, infrared, sun, and oven drying	Bergamot peels (C. bergamia Risso et Poiteau)	IR-dried samples had the highest levels of total phenolics (193.40 mg/100 g) and flavonoids (530.14 mg/100 g).	[109]
7	Freeze-drying, convective drying, vacuum drying, sun drying, and infrared	Myrtle (Ugni molinae)	IR drying negatively impacted the preservation of bioactives compared to freeze-drying and convective drying. A ~43% reduction in total phenolics and ~60% loss in antioxidant capacity was observed. However, total flavonoid levels remained satisfactory.	[110]
8	Freeze-drying, infrared, hot air, and pulsed vacuum drying	Ginkgo biloba seeds	IR drying yielded the highest total phenolics (5.34 mg GAE/g dry seed) and high flavonoid content (1.91 mg RE/g), comparable to freeze-drying and pulsed vacuum. Among five drying methods, IR led in three antioxidant assays (ABTS, CUPRAC, ORAC) and preserved terpene trilactones, unlike the others.	[111]
9	Hot air and infrared	Apple (Malus domestica)	High-intensity IR drying resulted in greater total phenolics (7.69 mg GAE/g), flavonoids (3.58 mg QE/g), antioxidant activity (DPPH: 83.27% inhibition), and energy efficiency (10.25%).	[112]
10	Hot air and infrared	Rice (Oryza sativa)	IR drying increased total phenolics by up to 18%, total flavonoids by up to 15%, quercetin by 2×, apigenin by 3×, and anthocyanins (248.76 µg/g) by more than 2× compared to hot air drying.	[113]

summarizes the applications of this technology in various food matrices.

Infrared radiation has a high penetration capacity, allowing the volumetric heating of tissues. This heating occurs from the inside out, breaking cell walls and releasing bioactive compounds, such as phenolics and flavonoids, which are bound to polysaccharides and proteins [111]. The release of these compounds occurs mainly through the conversion of bound forms into free forms, increasing their solubility and, consequently, their bioavailability. Infrared radiation also facilitates the hydrolysis of glycosidic and ester bonds, which helps release flavonoids such as catechin and epicatechin, potentially increasing the levels of total phenolics and flavonoids [106,112].

One of the main advantages of using infrared radiation is the reduced exposure time to heat, which lowers the risk of the thermal and enzymatic oxidation of these compounds. Shorter drying times prevent the phenolic and flavonoid degradation caused by prolonged exposure to heat [111,112] and also minimize enzymatic degradation reactions that can compromise antioxidant activity. Indeed, studies have shown that infrared drying, when carefully controlled, preserves antioxidant properties by maintaining bioactive compound levels [107].

The presence of phenolic compounds and flavonoids, both with antioxidant properties, is essential to maintaining a food’s antioxidant capacity. Infrared radiation, by releasing bioactive compounds from their bound forms, increases the active fraction of these compounds in extracts, potentially improving their radical scavenging capacity [111,112]. Furthermore, the thermal process can promote the formation of pigments, such as melanoidins, via the Maillard reaction, which also contributes to antioxidant activity [109].

However, infrared radiation should be used with caution to avoid the degradation of heat-sensitive bioactive compounds. Studies indicate that, when properly applied, infrared radiation not only preserves but can even enhance antioxidant activity, particularly when drying times are minimized [108]. For instance, Ratseewo et al. [113] observed an increase in the anthocyanin content due to the breakdown of glycosidic bonds in complex anthocyanins, generating simpler and more bioactive forms. This is consistent with findings in other studies, where infrared drying positively impacted bioactive compounds such as flavonoids and anthocyanins.

On the other hand, excessive exposure may lead to the thermal degradation of these compounds, resulting in a reduced antioxidant capacity. Some studies have shown that elevated temperatures caused by infrared radiation can negatively affect certain compounds, such as anthocyanins, which are particularly sensitive to oxygen and heat [103]. The combination of high temperatures and oxygen may decrease the content of these substances, highlighting the importance of strict temperature control to preserve bioactive compounds that require more specific conditions.

Infrared drying also stands out for its energy efficiency. Since the radiation directly heats the water within the food, without needing to heat the surrounding air, the energy consumption is significantly reduced. This results in a faster and more efficient drying process [111]. Additionally, shorter drying times imply less heat loss and lower electricity consumption, making this process an attractive option for industrial applications [112].

Despite these promising results, Silva et al. [43] reported that infrared radiation caused a decrease in the bioactive compound content of the microalga Spirulina platensis, similar to what occurs with conventional drying methods. The authors attributed this to the absence of cellulose in the microalga’s cell wall, making it more heat-sensitive. Nevertheless, in the same study, higher levels of bioactives were found in samples dried under intermittent infrared radiation, although the drying time was also longer in these cases. This indicates that a balance must be struck between the drying time and energy input, as excessive heat exposure could negate the benefits of faster drying times.

Although the results are encouraging, infrared technology is still mainly limited to pilot-scale and laboratory environments. To make it viable on an industrial scale, more comprehensive evaluations of its energy efficiency and feasibility in commercial settings are needed [36]. Infrared radiation offers a promising approach for both the functional preservation of bioactive compounds and the improvement of energy efficiency in drying processes, but it must be applied judiciously to maximize its benefits and minimize its drawbacks [110,111]. Future research should focus on optimizing drying parameters for specific food types and exploring the potential for the industrial-scale adoption of infrared-based drying technologies.

5. Future Perspectives and Challenges

Emerging drying technologies represent innovative and promising alternatives for the dehydration of plant-based products, promoting the preservation of nutritional, sensory, and functional attributes. In addition to improving energy efficiency and process selectivity, these techniques play a crucial role in retaining bioactive compounds such as phenolics, flavonoids, carotenoids, and vitamins, which are highly susceptible to thermal and oxidative degradation.

Despite significant progress, technical challenges remain, especially regarding the optimization of operational conditions to ensure the maximum retention of these compounds. The precise control of parameters such as the temperature, exposure time, and thermal radiation intensity is essential to avoid significant losses. In this context, hybrid approaches that combine different technologies have proven particularly effective in maintaining the chemical and functional stability of bioactives.

Among the analyzed technologies, freeze-drying stands out as one of the most effective for preserving bioactive compounds, particularly due to the absence of heat during drying. Although this is a costly and low-throughput process, the reviewed studies highlight its high retention capacity for heat-sensitive compounds, especially when combined with pretreatments or rapid freezing techniques [73]. However, freeze-drying presents challenges such as prolonged drying times, which can extend from several hours to days, and high overall costs, making it unsuitable for low-cost food products [71].

Radiofrequency drying has shown promise due to its volumetric penetration and uniform heating, resulting in reduced thermal degradation and the enhanced release of antioxidant compounds [64], although its industrial application remains limited. Microwave technology has demonstrated a high efficiency with significantly reduced processing times, ensuring a good retention of crocin, phenolic compounds, and volatiles. This makes it a more accessible alternative compared to freeze-drying and infrared. However, challenges related to thermal uniformity and industrial scalability still limit its widespread adoption [55].

Infrared radiation can be an effective tool for preserving bioactive compounds during the drying process, especially phenolics and flavonoids, which exhibit strong antioxidant activity. The main advantage of IR lies in its ability to rapidly heat materials, releasing bioactives without prolonged heat exposure, thereby protecting these sensitive substances. Nonetheless, using IR in heat-sensitive foods presents challenges, as its high thermal efficiency and application conditions may not always be suitable. Excessive IR heating may result in quality deterioration, such as over-roasting or undesired alterations during fruit peeling, potentially affecting the texture, color, and bioactive content [110,111,114].

Ultrasound, mainly used as a pretreatment, has demonstrated beneficial effects by enhancing the drying process through microstructural changes in plant matrices that favor moisture diffusion and the preservation of bioactive metabolites [2]. Lastly, cold plasma emerges as an innovative non-thermal approach with a potential for the surface modification of foods, microbial inactivation, and possible improvements in the stability of bioactive compounds. However, it remains an exploratory technology requiring further research into its efficacy, safety, and scalability for industrial applications [101].

Continued scientific and technological investigation is essential to consolidate these techniques and enable their transition from laboratory-scale to commercial applications. Future studies should also deepen the understanding of the relationship between drying methods and the bioavailability of bioactive compounds, ensuring not only their retention during processing but also their effective functionality in the human body. In this way, emerging drying technologies may be strategically applied to promote the development of healthier, functional foods aligned with modern demands for quality and sustainability.

6. Conclusions

This systematic and bibliometric review demonstrated that emerging drying technologies, such as microwave, infrared, ultrasound, radiofrequency, cold plasma, and freeze-drying, have a significant impact on the preservation of bioactive compounds in various foods. Compared to conventional methods, these technologies offer a greater drying efficiency, an improved retention of thermosensitive compounds, and the potential to maintain or enhance nutritional and functional properties. The findings highlight the growing scientific interest in sustainable processing approaches that align with consumer demands for high-quality foods. Furthermore, the integrated analysis of the scientific trends and technological performance reinforces the relevance of these methods in advancing food drying innovation, particularly in the context of nutritional quality and environmental responsibility.