Synergistic Effects of Freeze–Thaw and Osmoconvective Treatments on the Physicochemical Quality, Bioaccessibility, and Consumer Acceptability of Dehydrated Spondias tuberosa Arr. Câm. (Umbu) Slices

Abstract

This study evaluated the combined effects of freeze–thaw and osmotic dehydration (OD) pretreatments on the physicochemical, functional, and sensory qualities of umbu slices. Fresh and thawed umbu slices (thawed at 26 ± 3 °C for approximately 1 h after being frozen for at least 8 days) were submitted to OD or directly processed. All slices were then dehydrated by convective drying. Treatments varied by drying temperature (50 and 60 °C) and sucrose concentration (40, 50, and 60 °Brix), resulting in sixteen conditions, including four without OD. Freeze–thaw pretreatment significantly enhanced sucrose uptake (24.11–49.89%) during OD, affecting the slices’ physicochemical and functional attributes. It also improved appearance, color, and texture, leading to a higher sensory acceptance. Among OD treatments, experiment 2 (non-pre-frozen, 50 °Brix at 50 °C) exhibited the highest total phenolics (71.95 mg/100 g) and lowest phenolic losses during in vitro digestion. Experiment 1 showed the highest flavonoids (3.94 mg/100 g), anthocyanins (0.62 mg/100 g), and chlorophylls (0.78 mg/100 g). Phenolic bioaccessibility ranged from 10.88% (experiment 14) to 52,90% (experiment 16). Experiment 13 (pre-frozen, 40 °Brix at 60 °C) had the highest antioxidant activity among freeze–thawed samples and was notable for its greater perceived sweetness. Therefore, frozen storage combined with osmoconvective dehydration is a promising strategy for conserving and adding value to umbu fruit.

1. Introduction

Umbu (Spondias tuberosa Arr. Câm.) is a fruit native to Brazil’s semi-arid region, traditionally harvested from the wild and widely consumed due to its distinctive aroma and balanced sweet-tart flavor [1]. Beyond its cultural importance, umbu holds significant socio-economic value [2]. The fruit’s pulp, peel, and seed are rich in antioxidants, associated with bioactive compounds such as anthocyanins, flavonoids, carotenoids, and chlorophyll [3,4,5].

Notably, umbu presents a diverse phenolic profile, including phenolic acids, flavonoids, and tannins, such as ellagic acid, quercetin, p-coumaric acid, and rutin [6,7]. The concentration and bioaccessibility of these compounds vary according to the anatomical part of the fruit and its ripening stage, with the peel being especially rich in tannins and non-extractable phenolics [6]. Despite its nutritional and functional potential, the fruit’s high perishability and short harvest season pose major challenges to its commercialization [1]. Therefore, strategies that enhance preservation, add value, and expand market opportunities are essential.

Among preservation techniques, dehydration is commonly used to reduce the water content in fruits, extending their shelf life by minimizing spoilage and optimizing commercialization. Traditional methods include solar drying and hot air convective drying [8]. Recent studies, however, have focused on combining various drying methods to improve cost-effectiveness, energy efficiency, and the sensory and nutritional quality of the final product [9].

In this context, osmotic dehydration has gained attention as an effective pre-treatment method, where fruits are immersed in hypertonic solutions (such as sugar or salt solutions) to facilitate water removal while preserving nutrients and enhancing sensory qualities [10,11]. Various studies have applied osmotic dehydration to different fruits, such as apple [12,13], papaya [14], and pineapple [15,16], often combined with other pre-treatment techniques like sonication or freezing to improve the efficiency of mass transfer and drying rate. For instance, sonication has been shown to enhance the osmotic process by increasing the permeability of fruit cell membranes, facilitating better water removal and reducing drying time [13,16].

Given the need to optimize the drying process, freezing-thawing presents a promising strategy for umbu, particularly in ensuring the availability of raw materials during off-seasons. Freezing causes ice crystals to form, rupturing vacuoles and cell membranes, which aids in the removal of free water from the fruit [8], thus potentially accelerating the drying rate [17,18,19,20].

Convective drying with hot air, one of the oldest and most widely used methods in the industry, accounts for over 85% of dryers in use. However, this method is known for its high energy consumption [21]. A well-established alternative in the industry is pre-osmotic treatment before convective drying, where food is soaked in sugar solutions to enhance sensory qualities [12].

This study aims to evaluate the effects of pre-freezing and osmo-convective drying on the physicochemical, functional, and sensory qualities of umbu slices. Different drying temperatures and sucrose concentrations were tested to determine optimal conditions for producing umbu raisins, adding value to the fruit. The findings can also be applied to other seasonal fruits with similar characteristics.

2. Materials and Methods

2.1. Fruit Harvesting and Processing

To ensure uniformity, the fruits were harvested in the morning from a single plantation in Campina Grande, Paraíba, Brazil (coordinates: −7.275879° S, −35.972929° W). The fruits were collected at their physiological maturity stage, with an initial moisture content of 89.00 ± 0.15% (wet basis). After harvesting, the fruits were transported to the laboratory, where they were selected, sanitized according to [22] in chlorinated water containing 200 mg/L of sodium dichloroisocyanurate dihydrate (Sumaveg^®, Guarulhos, SP, Brazil), peeled, and sliced into 5 mm thick sections using a stainless-steel knife and caliper. The slices were then divided into two batches: one batch was kept fresh and immediately subjected to the dehydration process, while the other batch was frozen at −18 °C in a Consul^® freezer (model 534 L-CHB53 EB, São Paulo, SP, Brazil).

2.2. Description of Osmotic Treatments

Fresh and thawed umbu slices (thawed at ambient temperature, 26 ± 3 °C, for approximately 1 h after being frozen for at least 8 days) were initially subjected to osmotic dehydration at temperatures of 50 and 60 °C. Osmotic solutions with sucrose concentrations of 40, 50, and 60 °Brix were used, based on preliminary tests. The solutions were prepared with distilled water and commercial granulated sugar, with the concentration adjusted using a digital refractometer (0–90 °Brix).

The dehydration process was carried out in a refrigerated orbital incubator (Tecnal^®, model TE-421, Piracicaba, SP, Brazil). The fruit slices were placed in Erlenmeyer flasks at a fruit-to-solution ratio of 1:10, with agitation at 100 rpm. The temperatures used were 50 and 60 °C.

Based on preliminary tests, fresh slices remained in the osmotic solution for 230 min and previously frozen slices for 150 min—durations determined as the minimum exposure times required to achieve mass transfer equilibrium and reduce nutrient loss by leaching. The percentages of sucrose gain and water loss for the different experiments are shown in Figure 1.

2.3. Description of Convective Drying

After osmotic treatment, the slices underwent convective drying at the same temperatures used during osmotic dehydration (50 and 60 °C). The samples were placed in an oven with mechanical air circulation at a velocity of 1 m/s (MOD 320 E, FANEM, SP, Brazil). Control experiments were also conducted with fresh and pre-frozen slices that did not undergo osmotic dehydration, at both evaluated temperatures. This resulted in a total of 16 convective drying trials, as detailed in

Table 1. Convective drying experiments of umbu slices (Spondias Tuberosa Arr. Câm.) subjected to osmotic dehydration (or not).

Experiments	Fruit	Temperature (°C)	Osmotic Solution (°Brix)	Drying Time (min)	Dehydrated Umbu
1	Fresh	50	40	170
2			50	170
3			60	130
4			-	450
5		60	40	170
6			50	130
7			60	150
8			-	300
9	Pre-frozen	50	40	240
10			50	210
11			60	210
12			-	210
13		60	40	130
14			50	130
15			60	210
16			-	170

.

The drying time for each experiment was determined using a kinetic approach, in compliance with regulation RDC n° 272 (September 2005), which states that the moisture content of dried or dehydrated fruits, such as raisins, must not exceed 25% [23]. After drying, the slices were placed in a desiccator to cool for 1 h, then stored in laminated packaging and refrigerated at 5 °C.

2.4. Characterization of Dehydrated Umbu (Spondias Tuberosa Arr. Câm.) Slices

2.4.1. pH and Titratable Acidity

The pH and the titratable acidity were measured according to AOAC [24] standards. A 5 g homogenized sample was mixed with 50 mL of distilled water, and the pH was measured using a digital pH meter (Tecnal^®, model TEC-2, Piracicaba, SP, Brazil). The sample was then titrated with a standardized 0.1 N sodium hydroxide (NaOH) solution until a light pink color appeared, indicating the titratable acidity. The acidity was expressed as a percentage of citric acid.

2.4.2. Soluble Solids and Ratio (SS/TA)

The soluble solids content was determined using a digital bench refractometer (range 0–90 °Brix), following the AOAC [24] protocol. The results were expressed as a percentage of °Brix. The ratio of total soluble solids to titratable acidity was then calculated.

2.4.3. Sugars

Total sugars were quantified using the method described by Yemm and Willis [25], while reducing sugars were determined according to the procedure described by Miller [26]. Absorbance readings for both were taken at 620 nm using a spectrophotometer (Agilent Technologies, model Agilent Cary 60, Santa Clara, CA, USA). Non-reducing sugars were calculated as the difference between the total sugars and reducing sugars.

2.4.4. Total Flavonoids and Anthocyanins

Flavonoids and anthocyanins were quantified using the method by Francis [27]. Approximately 1 g of dried umbu fruit was weighed and macerated for 1 min in 10 mL of an ethanol solution (85:15, v/v). The mixture was left to stand for 24 h, followed by centrifugation at 3000 rpm for 5 min. After filtration, the supernatant was collected for analysis using a UV-visible spectrophotometer (Agilent Technologies, model Agilent Cary 60, Santa Clara, CA, USA), with absorbance readings taken at 374 nm and 535 nm.

2.4.5. Chlorophylls and Carotenoids

The determination of chlorophylls and carotenoids was carried out following the method described by Lichtenthaler [28], with adaptations. One gram of each sample was weighed and placed in test tubes along with 0.2 g of calcium carbonate (CaCO₃) and 5 mL of 80% acetone. The samples were then sonicated in an ultrasonic bath at a frequency of 42 kHz and power of 132 W for 5 min (Cristófoli^®, São Paulo, SP, Brazil) before being centrifuged at 3000 rpm and 10 °C for 10 min. The extract was collected, and absorbance readings were measured using a UV-visible spectrophotometer at wavelengths of 470 nm, 646 nm, and 663 nm.

To account for the overlapping UV-Vis absorbance spectra of the pigments, the concentrations of chlorophyll a, chlorophyll b, and total carotenoids (expressed in mg/100 g) were calculated using Lichtenthaler’s equations, which are based on specific extinction coefficients:

Chlorophyll a = 12.25 × A663 − 2.79 × A646

(1)

Chlorophyll b = 21.50 × A646 − 5.10 × A663

(2)

Carotenoids = (1000 × A470 − 1.82 × Ca − 85.02 × Cb)/198

(3)

where A663, A646, and A470 are the absorbance values at their respective wavelengths, and Ca and Cb are the calculated concentrations of chlorophyll a and b. These equations correct for the spectral overlap between pigments and are well established for use in plant material analyses.

2.4.6. Phenolic Compounds

The total phenolic content in each sample was determined using the Folin–Ciocalteu colorimetric method, as described by Waterhouse [29]. Aqueous extracts were prepared at a 1:10 ratio (sample: water) and subjected to ultrasonic treatment at 40 kHz and 132 W for 5 min. The absorbance was measured at 750 nm using a UV-visible spectrophotometer. The results were expressed in milligrams of gallic acid equivalents (GAE) per 100 g of the sample, based on calibration curves created with gallic acid concentrations ranging from 0 to 100 µg/mL.

2.4.7. Bioaccessibility of Phenolic Compounds

The bioaccessibility of phenolic compounds was assessed using the methodologies proposed by Gawlik-Dziki et al. [30] and Santos et al. [31]. Approximately 1 g of the sample was treated with oral juice containing amylase (75 U/mL) at pH 7, followed by incubation at 37 °C for 2 min at 180 rpm in a shaking water bath. A pepsin solution (2000 U/mL) at pH 3 was then added and allowed to react for 2 min. Next, duodenal juice containing bile salts (4.4 mg/mL) and pancreatin (100 U/mL) at pH 6.5–7 was introduced for an additional 2 min. The total reaction time was 6 min. The enzymatic reactions were halted by cooling the samples on ice. The total phenolic content was measured both before and after the gastric and intestinal digestion phases. Bioaccessibility was calculated using the following Equation (4):

Bioaccessibility (%) = (B/A) × 100

(4)

where

• A = Phenolic content before in vitro gastrointestinal digestion;
• B = Phenolic content after the intestinal phase.

2.4.8. Antioxidant Activity

DPPH Method: Antioxidant activity was determined using the DPPH (2,2-Diphenyl-1-picrylhydrazyl) method, following Rufino et al. [32]. Aliquots of 0.1 mL of extract were mixed with 3.9 mL of a 0.06 mM DPPH solution. Absorbance was measured at 515 nm every minute until stabilization. Results were expressed in µM Trolox per gram of sample.

ABTS Method: The antioxidant activity was also determined using the ABTS radical cation (2,2’-AZINO-BIS (3-ethylbenzothiazoline-6-sulfonic acid) method, as described by Rufino et al. [33]. Aliquots of 30 µL of extract were mixed with 3.0 mL of ABTS•⁺ radical solution. Absorbance was measured at 734 nm after 6 min, with ethanol as a blank. Results were expressed in µM Trolox per gram of sample.

FRAP Method: The antioxidant capacity was further assessed using the Ferric Reducing Antioxidant Power (FRAP) method, following Rufino et al. [34]. Aliquots of 90 µL of extract were mixed with 270 µL of deionized water and 2.7 mL of FRAP reagent. After homogenization, the solutions were incubated at 37 °C for 30 min. Absorbance was measured at 595 nm, with FRAP reagent serving as the blank. Results were expressed in µM ferrous sulfate (FeSO₄) per gram of sample, based on calibration curves constructed with ferrous sulfate concentrations ranging from 100 to 2000 µmol/L.

2.4.9. Mineral Profile

The mineral profile was evaluated for five selected experiments based on the best combinations of osmotic solution and temperature, considering their functional characteristics. This process aimed to select slices both with and without pre-freezing, along with an experiment involving osmotic treatment, to assess the primary effects of the different factors evaluated. The samples were incinerated, and the resulting ashes were analyzed using an Energy Dispersive X-ray Fluorescence Spectrometer (Shimadzu, model EDX720, Japan) to quantify the levels of Fe, Cu, Mn, Zn, Ca, Mg, K, P, and Na, expressed in mg per 100 g of sample.

2.4.10. Microbiological, Sensory, and Purchase Intent Analysis

Following the mineral profile evaluation, three experiments were selected for sensory analysis: one control (without osmotic treatment) and two involving osmoconvective drying of the slices, with and without pre-freezing.

Prior to the sensory evaluation, microbiological analyses were conducted to ensure the safety of the panelists. These tests included assessments of total coliforms at 35 °C, thermotolerant coliforms at 45 °C, and the presence of Salmonella, following the APHA [35] guidelines.

Approval from the Brazilian ethics committee was obtained (CAAE: 77041123.9.0000.5182, ethics approval no. 6.700.920). For the sensory analysis, 100 untrained panelists (60 women and 40 men) were recruited to evaluate the dehydrated umbu slices. The evaluation criteria included appearance, aroma, color, flavor, texture, sweetness, and overall acceptance, using a nine-point hedonic scale (1 = extremely dislike, 9 = extremely like) [28]. Additionally, a purchase intent test was conducted using a five-point scale (1 = definitely would not buy, 5 = definitely would buy) [36].

2.5. Statistical Analysis

The experiments were conducted in triplicate, with the results presented as mean ± standard deviation. Data were analyzed using one-way analysis of variance (ANOVA) and mean comparison tests, conducted using RStudio software version 3 (2009–2019 RStudio, Inc., Boston, MA, USA). Furthermore, multivariate principal component analysis (PCA) was performed using OriginPro 2024 software. Sensory parameter results were visualized using spider web diagrams, and purchase intent was evaluated through mean comparisons.

3. Results and Discussion

3.1. Physicochemical Properties

By analyzing the various factors affecting the production of umbu slices in raisin form, their influence on the final product’s quality attributes becomes evident. Fresh umbu is primarily characterized by its distinctive sweet and acidic flavor [2], typically accompanied by a low pH. As shown in

Table 2. Physicochemical characterization of pre-frozen and fresh umbu slices (5 mm) subjected to osmotic-convective drying.

Experiments	pH	Titratable Acidity (% Citric Acid)	Soluble Solids (°Brix)	Ratio
1	3.01 ± 0.04 c	1.56 ± 0.02 e	23.66 ± 0.58 f	15.18 ± 0.42 g
2	2.95 ± 0.03 c	1.52 ± 0.05 e	25.50 ± 0.50 e	16.76 ± 0.80 g
3	3.03 ± 0.07 c	1.44 ± 0.17 e	24.50 ± 0.50 f	17.18 ± 2.44 g
4	2.49 ± 0.03 f	12.46 ± 0.28 a	20.05 ± 0.50 g	1.65 ± 0.08 h
5	3.19 ± 0.12 b	1.01 ± 0.07 f	23.33 ± 0.58 f	23.24 ± 2.34 f
6	3.21 ± 0.03 b	0.79 ± 0.04 f	24.50 ± 0.50 f	30.88 ± 1.39 e
7	3.38 ± 0.11 a	0.62 ± 0.06 f	32.00 ± 1.00 c	52.15 ± 5.39 b
8	2.83 ± 0.14 d	9.79 ± 0.71 b	17.00 ± 1.00 g	1.75 ± 0.23 h
9	3.50 ± 0.04 a	0.49 ± 0.03 f	26.50 ± 0.50 e	53.95 ± 3.70 b
10	3.44 ± 0.03 a	0.71 ± 0.01 f	27.50 ± 0.50 d	38.66 ± 1.26 d
11	3.18 ± 0.10 b	0.56 ± 0.03 f	32.50 ± 0.50 c	58.27 ± 2.89 a
12	2.60 ± 0.01 e	7.64 ± 0.29 d	14.83 ± 0.29 i	1.94 ± 0.11 h
13	3.15 ± 0.03 b	0.57 ± 0.06 f	36.00 ± 1.00 a	63.03 ± 7.53 a
14	3.10 ± 0.01 b	0.75 ± 0.06 f	33.50 ± 0.50 b	45.00 ± 2.88 c
15	3.05 ± 0.02 c	0.85 ± 0.05 f	34.00 ± 1.00 b	40.03 ± 3.55 d
16	2.42 ± 0.02 f	8.43 ± 0.10 c	15.00 ± 1.00 i	1.78 ± 0.10 h

Note: Different letters in the same column indicate significant statistical differences based on the Scott-Knott test (p < 0.05).

, the experiments that did not involve osmotic dehydration exhibited the greatest variations in pH, likely due to an increased concentration of organic acids from water content reduction. However, the incorporation of sugar during the osmotic treatment resulted in pH values more closely resembling those of fresh fruit. For slices that were not pre-frozen, the experiments conducted at 60 °C had the highest pH values. In contrast, pre-frozen samples showed the opposite trend, with treatments at 60 °C resulting in lower pH values compared to those at 50 °C.

Titratable acidity, which reflects the citric acid content in the samples, is presented in Table 2. The incorporation of solids significantly reduced the titratable acidity in the umbu raisin slices compared to the control experiments, which exhibited the highest citric acid levels (7.64–12.46), particularly for slices that were not pre-frozen. Among the experiments subjected to osmotic dehydration, with or without prior freezing, two distinct groups emerged. Notably, significant differences were only observed for the fresh slices dehydrated at 50 °C (experiments 1, 2, and 3), which had higher acidity values than the others. This behavior suggests that increasing the temperature of the osmotic solution, along with prior freezing, may have reduced these compounds, possibly due to degradation, leaching during the process, or a dilution effect from sugar incorporation.

For the fruits that were solely dehydrated using convective drying, the pH and titratable acidity values were similar to those reported by Barroso et al. [37] and Araújo et al. [38] for freeze-dried umbu powder, which showed pH values between 2.55 and 2.74, and titratable acidity ranging from 8.17 to 12.00. In contrast, slices subjected to osmotic treatment exhibited higher pH values, similar to those observed by Alfaro et al. [39] for osmotically dehydrated blueberries in a 60 °Brix sucrose solution at 40 °C. Except for experiments 1, 2, and 3, the titratable acidity values were comparable to those reported by Hossain et al. [40] for ‘taikor’ fruit slices pre-treated in a 10% sucrose osmotic solution and dehydrated at 45, 50, and 55 °C, where titratable acidity decreased as temperature increased, ranging from 0.70 to 0.95.

The soluble solid content in foods is a crucial indicator of quality, especially in fruits, where the soluble solids are primarily composed of sugars. Given that this study focuses on sugar incorporation, this parameter is particularly relevant. The soluble solids values reported across the various experiments (Table 2) show that slices without osmotic dehydration had the lowest levels due to the absence of the osmotic treatment.

Among the experiments involving osmotic dehydration without prior freezing, Experiment 7 stands out with the highest value (32.00 °Brix). This outcome is likely due to the combination of higher sucrose concentration (60 °Brix) and increased temperature (60 °C). In contrast, the other experiments (1 to 6) exhibited only minor variations in soluble solids, with levels ranging from 23.33 to 25.50 °Brix.

For the experiments involving pre-freezing, a clear effect of temperature was observed, with an increase in soluble solids at 60 °C. The sucrose solution also influenced the results, with the umbu slices resembling raisins showing an increase in soluble solids at 50 °C and a decrease at 60 °C.

The ratio between soluble solids content and titratable acidity, known as the sweetness ratio, reflects the perceived sweetness of the food. As shown in Table 2, apart from the control experiments (4, 8, 12, and 16), the umbu slices resembling raisins with the lowest sweetness were those not subjected to pre-freezing and dehydrated at 50 °C. This was likely due to higher citric acid levels. At 60 °C, the trend was more influenced by the soluble solids content, particularly in experiment 7, which exhibited the highest value.

Furthermore, pre-freezing resulted in the highest sweetness ratios, with experiment 13 showing the highest ratio, likely due to its elevated soluble solids content. Similarly to the experiments without pre-freezing, the titratable acidity had a greater impact at 50 °C, while soluble solids were more influential at 60 °C. Since the sweetness ratio is directly linked to sensory attributes (such as taste), the importance of osmotic treatment becomes clear, as it creates a significant distinction between products with and without prior osmotic dehydration before convective drying.

Since sucrose incorporation is central to osmotic dehydration, sugar content analysis is a critical tool for assessing the effect of this treatment on the final product’s quality (

Table 3. Physicochemical characterization (g. (100 g)⁻¹ dry matter) of pre-frozen and fresh slices of umbu (5 mm) subjected to osmoconvective drying.

Experiments	Reducing Sugar	Non-Reducing Sugars	Total Sugar
1	5.67 ± 0.05 g	23.05 ± 1.02 b	28.72 ± 1.07 c
2	6.07 ± 0.35 e	22.40 ± 0.58 c	28.47 ± 0.34 c
3	6.34 ± 0.15 d	23.03 ± 1.18 b	29.38 ± 1.34 c
4	3.82 ± 0.05 j	20.37 ± 0.27 d	24.19 ± 0.32 e
5	6.11 ± 0.08 e	22.26 ± 0.24 c	28.37 ± 0.31 c
6	5.92 ± 0.02 f	24.32 ± 0.20 a	30.25 ± 0.19 b
7	5.23 ± 0.06 h	23.36 ± 0.14 b	28.58 ± 0.89 c
8	4.42 ± 0.06 i	19.64 ± 0.29 d	24.06 ± 0.35 e
9	6.86 ± 0.03 b	22.13 ± 0.16 c	28.99 ± 0.19 c
10	7.26 ± 0.04 a	21.91 ± 1.11 c	29.17 ± 1.08 c
11	7.23 ± 0.07 a	22.83 ± 0.20 b	30.06 ± 0.27 b
12	4.28 ± 0.12 i	21.05 ± 0.60 d	25.34 ± 0.72 d
13	6.64 ± 0.08 c	23.35 ± 0.28 b	29.99 ± 0.37 b
14	6.66 ± 0.06 c	24.71 ± 0.30 a	31.36 ± 0.36 a
15	6.93 ± 0.03 b	24.88 ± 0.13 a	31.81 ± 0.17 a
16	4.51 ± 0.02 i	19.99 ± 0.73 d	24.50 ± 0.71 e

Note: Different letters in the same column indicate significant statistical differences based on the Scott-Knott test (p < 0.05).

). It was observed that reducing sugar content increased with rising sucrose concentrations, except for fresh slices subjected to osmotic dehydration at 60 °C. Additionally, freezing led to higher sugar content, which is directly related to greater sugar absorption during osmotic treatment.

In experiments without osmotic dehydration, only the fresh slices subjected to 50 °C (experiment 4) exhibited a statistically significant difference from the others. This may be attributed to a greater degradation of certain compounds due to the longer convective drying time. Regarding non-reducing sugars, higher levels were observed compared to reducing sugars, ranging from 19.64 to 24.88 g per 100 g. The highest values were found in the experiments conducted at 60 °C using osmotic solutions with concentrations of 50 and 60 °Brix, particularly in the pre-frozen slices.

Overall, the total sugar content followed a similar trend to the individual sugars, with factors favoring greater sucrose absorption during osmotic treatment having a significant direct influence. For instance, the lowest total sugar content was found in the treatment without osmotic dehydration (24.04 g per 100 g, experiment 8), while the highest total sugar content was found in the treatment with osmotic dehydration (31.81 g per 100 g, experiment 15). This represents an increase of up to 32.21% in total sugar content.

3.2. Bioactive Compounds

Bioactive compounds, which are present in small amounts in whole grains, fruits, and vegetables, provide various health benefits beyond basic nutrition. These compounds have therapeutic potential for mitigating pro-inflammatory conditions, reducing oxidative stress, and improving metabolic disorders [41].

Regarding chlorophyll content (

Table 4. Bioactive compounds (mg.(100 g)⁻¹ of dry matter) of pre-frozen and fresh umbu slices (5 mm), subjected to osmo-convective drying.

Experiments	Chlorophyll a	Chlorophyll b	Total Chlorophyll	Carotenoids	Phenolic Compound	Flavonoids	Anthocyanins
1	0.30 ± 0.01 f	0.48 ± 0.02 c	0.78 ± 0.02 e	0.20 ± 0.02 d	43.79 ± 2.14 f	3.94 ± 0.10 d	0.62 ± 0.01 d
2	0.26 ± 0.01 g	0.36 ± 0.05 d	0.62 ± 0.01 f	0.15 ± 0.01 e	71.95 ± 1.90 d	3.53 ± 0.10 e	0.47 ± 0.01 f
3	0.26 ± 0.01 g	0.30 ± 0.01 e	0.56 ± 0.01 g	0.14 ± 0.02 e	44.30 ± 1.61 f	3.93 ± 0.06 d	0.50 ± 0.01 f
4	0.53 ± 0.01 c	0.93 ± 0.02 a	1.46 ± 0.02 c	0.33 ± 0.02 a	85.80 ± 1.89 c	7.59 ± 0.17 b	1.03 ± 0.02 b
5	0.34 ± 0.01 e	0.27 ± 0.01 f	0.61 ± 0.01 f	0.12 ± 0.01 g	46.46 ± 1.00 e	3.80 ± 0.06 d	0.47 ± 0.01 f
6	0.23 ± 0.01 h	0.29 ± 0.00 f	0.52 ± 0.01 g	0.10 ± 0.02 h	38.56 ± 0.68 g	3.61 ± 0.05 e	0.58 ± 0.01 e
7	0.20 ± 0.02 i	0.28 ± 0.01 f	0.48 ± 0.03 h	0.11 ± 0.03 g	42.46 ± 0.94 f	3.39 ± 0.03 f	0.43 ± 0.05 g
8	0.84 ± 0.05 a	0.95 ± 0.01 a	1.79 ± 0.06 a	0.28 ± 0.01 c	91.19 ± 1.48 b	7.07 ± 0.18 c	0.86 ± 0.02 c
9	0.25 ± 0.00 g	0.31 ± 0.00 e	0.56 ± 0.01 g	0.09 ± 0.01 h	41.56 ± 0.59 f	2.89 ± 0.06 g	0.40 ± 0.01 h
10	0.21 ± 0.00 h	0.34 ± 0.01 d	0.55 ± 0.00 g	0.10 ± 0.01 h	38.19 ± 0.72 g	2.56 ± 0.11 h	0.45 ± 0.01 g
11	0.18 ± 0.03 i	0.22 ± 0.01 g	0.40 ± 0.01 i	0.13 ± 0.02 f	38.43 ± 1.20 g	2.30 ± 0.10 i	0.28 ± 0.01 j
12	0.44 ± 0.02 d	0.91 ± 0.06 b	1.35 ± 0.06 d	0.30 ± 0.04 b	72.74 ± 1.58 d	8.00 ± 0.07 a	1.08 ± 0.01 a
13	0.17 ± 0.01 i	0.28 ± 0.02 f	0.45 ± 0.03 h	0.12 ± 0.02 g	47.16 ± 1.23 e	2.13 ± 0.06 i	0.24 ± 0.01 k
14	0.15 ± 0.04 j	0.25 ± 0.02 g	0.39 ± 0.01 i	0.10 ± 0.03 h	33.92 ± 1.06 h	2.57 ± 0.12 h	0.33 ± 0.01 j
15	0.13 ± 0.01 j	0.25 ± 0.00 g	0.38 ± 0.01 i	0.10 ± 0.01 h	34.31 ± 0.17 h	3.32 ± 0.14 f	0.42 ± 0.01 g
16	0.72 ± 0.02 b	0.88 ± 0.03 b	1.60 ± 0.02 b	0.30 ± 0.03 b	93.88 ± 1.80 a	8.16 ± 0.31 a	1.06 ± 0.04 a

Note: Different letters in the same column indicate significant statistical differences based on the Scott-Knott test (p < 0.05).

), control experiments without osmotic dehydration (OD) generally showed higher total chlorophyll levels, with experiment 8 displaying statistically significant values. A higher prevalence of chlorophyll b was observed across all experiments. Significant reductions in chlorophyll content were associated with pre-freezing of the samples, as well as increases in temperature and osmotic solution concentration.

Mazzeo et al. [42] conducted a similar study, evaluating the effect of pre-freezing on the chlorophyll content of asparagus, zucchini, and green beans. They found that freezing primarily reduces chlorophyll a and observed greater degradation in cooked vegetables that had been previously frozen. This behavior aligns with the results seen in umbu slices subjected to osmo-convective dehydration (Table 4). The authors suggest that this degradation is related to structural damage caused by both freezing and dehydration, which facilitates pigment loss and degradation. This degradation is evidenced by the higher content of pheophytin a, a compound formed when chlorophyll loses its magnesium ions [43].

A similar trend was observed for carotenoids, with higher values in control treatments and in those dehydrated at 50 °C. The influence of increased temperature, osmotic solution concentration, and freezing followed a similar pattern to that seen with chlorophyll. The carotenoid levels observed were consistent with the findings of Gualberto et al. [44], who reported a carotenoid content of 0.16 mg per 100 g for umbu powder dried at 40 °C using convective drying.

Among bioactive compounds, phenolic compounds are some of the most prevalent in food products and by-products, primarily consumed through plant-based foods [45]. This widespread presence was also observed in the compounds evaluated in this study. Across the various experiments, the phenolic content ranged from 33.92 to 93.88 mg per 100 g (Table 4), showing a significant variation. Notably, the experiments that were not subjected to osmotic treatment exhibited higher phenolic content, likely due to reduced exposure to conditions conducive to the degradation of these compounds, considering their susceptibility to high temperatures [40] and material disintegration.

The observed changes in the slices subjected to osmotic treatment align with expectations in the methods used [39], underscoring the importance of evaluating conditions that minimize nutritional quality degradation. In this regard, the combination of freezing and osmotic dehydration had the greatest impact on reducing phenolic compound content. However, experiments subjected to a 40 °Brix osmotic solution showed lower overall losses, maintaining comparable quality between slices with and without pre-freezing. This could be linked to lower mass transfer during osmotic dehydration, as indicated by the reduced sucrose absorption (Figure 1).

This relationship has also been observed in other studies involving osmotic dehydration of fruits such as plums [10], persimmons [46], and apples [13]. In these studies, treatments with lower solid gains tended to retain higher levels of phenolic compounds. However, the influence of lower-intensity ultrasonic treatments in these studies suggests that ultrasound may have a similar effect to freezing, as both techniques tend to facilitate mass transfer by inducing cell rupture.

It is important to note that the phenolic compound values reported in this study were lower than those found by Bozkir and Ergun [46], Ma et al. [13], and Li et al. [10], but higher than the values reported by Hossain et al. [40] for slices of ‘Taikor’ fruit subjected to osmoconvective dehydration in a 10% sucrose solution at 45, 50, and 55 °C (8–12 mg/100 g). Similarly, the values observed in this study were higher than those reported by Wang et al. [12] for apple slices dehydrated in a 40% sucrose solution with vacuum assistance, followed by drying at 65 °C in an air-circulating oven (9.20–33 mg/100 g).

Given that the primary types of phenolic compounds are flavonoids, among which anthocyanins are one of the most widely distributed classes of soluble pigments in plants, second only to chlorophylls [47,48], the quantification of these parameters followed a similar trend to total phenolics. Although experiment 2 showed a higher overall concentration of total phenolics, experiment 1 had higher levels of flavonoids and anthocyanins across the osmotic treatments. Similarly, experiment 1 also exhibited higher levels of total chlorophylls and carotenoids. The flavonoid values for the control treatments (without osmotic dehydration) were higher than those reported by Zielinski et al. [49] for umbu pulp (6.93 mg/100 g) and comparable to those found in cajá pulp (8.71 mg/100 g).

3.3. In Vitro Digestibility

Bioaccessibility refers to the amount of a specific component that is released from food in the gastrointestinal tract, making it available for absorption [41]. For a compound to exert its bioactivity, it must be bioavailable [50]. In this study, the bioaccessibility of phenolic compounds in different treatments was evaluated (Figure 2).

The results showed a significant decrease in phenolic compound content between the gastric and intestinal phases across all experiments. The maximum retention of phenolic compounds was 52.90% (experiment 16), while the minimum retention was 10.88% (experiment 14).

Among the experiments with osmotic pre-treatment, Treatments 2 and 13 had the lowest losses in phenolic compounds between the gastric and intestinal phases, comparable to the control experiments.

Regarding phenolic compound bioaccessibility in the intestinal phase, the highest values were observed in fresh umbu slices dehydrated at 50 °C (experiments 1, 2, and 3), and in slices subjected to a 40 °Brix sucrose solution, both without pre-freezing (experiment 5) and with pre-freezing (experiments 9 and 13). Notably, slices that were not subjected to osmotic pre-treatment but were dried at 60 °C (experiments 8 and 16) exhibited the highest bioaccessibility overall, likely due to their higher initial phenolic compound content. These results are consistent with findings by Cangussu et al. [51], who reported bioaccessibility values of 21.02% and 16.59% for seriguela peel and seed flour dried convectively at 60 °C for 24 h.

3.4. Antioxidant Capacity

In the evaluation of antioxidant activities (

Table 5. Antioxidant activity of umbu slices (5 mm), pre-frozen and fresh, subjected to osmoconvective drying.

Experiments	DPPH (µM Trolox g−1)	ABTS (µM Trolox g−1)	FRAP (µM FeSO4 g−1)
1	14.47 ± 0.62 g	10.27 ± 0.98 e	4.10 ± 0.09 f
2	19.73 ± 0.50 d	16.45 ± 0.49 c	7.30 ± 0.29 d
3	16.32 ± 0.32 f	12.36 ± 0.39 d	4.31 ± 0.22 f
4	21.90 ± 0.35 c	17.77 ± 0.87 b	8.47 ± 0.39 c
5	16.20 ± 0.18 f	12.84 ± 0.30 d	5.03 ± 0.14 e
6	12.01 ± 0.16 i	7.26 ± 0.32 g	3.75 ± 0.47 f
7	14.30 ± 0.21 g	10.14 ± 0.79 e	4.18 ± 0.13 f
8	27.58 ± 0.74 a	21.85 ± 1.29 a	12.65 ± 0.53 a
9	13.37 ± 0.45 h	9.14 ± 0.77 f	3.84 ± 0.25 f
10	11.16 ± 0.66 j	7.03 ± 0.10 g	3.00 ± 0.17 g
11	12.92 ± 0.13 h	7.63 ± 0.55 g	3.56 ± 0.34 f
12	18.38 ± 0.26 e	15.67 ± 0.43 c	6.96 ± 0.24 d
13	15.81 ± 0.40 f	11.21 ± 0.13 e	4.56 ± 0.60 e
14	10.17 ± 0.11 k	6.37 ± 0.34 g	2.12 ± 0.14 h
15	10.48 ± 0.36 k	6.67 ± 0.27 g	2.35 ± 0.18 h
16	26.16 ± 0.95 b	20.95 ± 0.37 a	11.80 ± 0.66 b

Note: Different letters in the same column indicate significant statistical differences based on the Scott-Knott test (p < 0.05).

), the DPPH radical scavenging method yielded the highest values, aligning with the findings of Xue et al. [52] for dehydrated Ziziphus jujuba var. Spinosa fruits. Notably, higher antioxidant activities were observed in control experiments 8 and 16 (both at 60 °C, with and without pre-freezing, respectively), especially in experiment 8.

Among the experiments that used osmotic pre-treatment, experiment 2 (50 °Brix at 50 °C) stood out among the non-pre-frozen slices, while experiment 13 (40 °Brix at 60 °C) demonstrated the highest antioxidant activity among the pre-frozen slices. These results are consistent with the quantification of phenolic compounds, as previously observed in other studies on the osmotic dehydration of nutmeg pericarp [53] and Adajamir fruit (Citrus assamensis) [54]. These findings underscore the significant role of phenolic compounds in contributing to antioxidant properties.

The reduction in antioxidant activity observed in experiments combining freezing with osmotic treatment could be attributed to the loss of antioxidant compounds during osmotic dehydration, which is exacerbated by the structural disruption caused by freezing. Hossain et al. [55] reported a similar effect, where the combination of ultrasound and osmotic dehydration negatively impacted antioxidant retention in the drying of Taikor fruit slices. Conversely, Nudar et al. [49] demonstrated that sucrose can promote the retention of these compounds, thereby helping to preserve antioxidant activity. This may explain the relatively high antioxidant activity observed in experiment 13 (pre-frozen, 40 °Brix, 60 °C) despite the pre-freezing step.

3.5. Experiment Selection

Given that the experiments without pre-freezing tended to exhibit the best functional characteristics (1–8), the selection process emphasized the need to also evaluate the sensory attributes of those experiments that underwent freezing prior to dehydration (9–16), as well as a control treatment without osmotic dehydration (OD). This approach was used to better assess the impact of osmotic treatment on the commercial acceptability of the developed product.

Among the control experiments, those processed at 60 °C displayed the best functional qualities, likely due to shorter processing times and reduced degradation of compounds. In particular, experiment 8, without pre-freezing, exhibited the highest levels of chlorophylls and carotenoids. Meanwhile, experiment 16 stood out for its content of anthocyanins, phenolics, and flavonoids.

Considering the benefits of preserving umbu slices during the post-harvest period, the faster drying rate, and the similar functional performance to the non-pre-frozen treatments, experiment 16 emerges as the most promising among those without osmotic treatment. Among the experiments with OD, experiment 2, which did not undergo pre-freezing, exhibited the best antioxidant activity and phenolic compound content. At 60 °C, experiment 5 showed the least compound alteration and the highest antioxidant profile.

In the pre-frozen condition, across both evaluated temperatures, the experiments subjected to the 40 °Brix osmotic solution (9 and 13) were notable. Although experiment 13 experienced a decline in pigment levels, it had the highest levels of phenolics and antioxidant activity among the pre-frozen experiments, closely resembling experiment 5 (non-pre-frozen).

Therefore, based on these factors, experiments 2, 5, 9, 13, and 16 stand out for further mineral profile analysis and sensory evaluation.

3.6. Mineral Profile

Based on the analysis of the mineral profile from the selected experiments (Figure 3), it was observed that osmotic treatment led to losses of these compounds, as predicted by Kroehnke et al. [11]. In this context, experiment 16 (control) stood out, mainly due to the predominance of Na, P, and K, followed by Mg, Ca, and Zn. It is important to note that P, K, Mg, and Ca are considered macronutrients. Among these, K and P exhibited the highest concentrations, corroborating the findings of Ribeiro et al. [4] when evaluating the mineral content of fresh umbu pulp. Additionally, it is noteworthy that pre-freezing also caused a slight loss of these compounds, serving as a key parameter in distinguishing between experiments, as evidenced by the dendrogram, which highlights the similarities between experiments 13 and 9, as well as between 5 and 2.

3.7. Microbiological Analysis

Microbiological analyses indicated that the samples were suitable for sensory analysis, as they complied with the microbiological standards set by Normative Instruction No. 161, 1 July 2022, concerning the evaluated parameters: total coliforms (35 °C) (<1.0 × 10 MPN/g), thermotolerant coliforms (45 °C) (<1.0 × 10 MPN/g), and Salmonella (absent).

3.8. Sensory Analysis

The analysis of various sensory parameters of dehydrated umbu slices highlights the effectiveness of osmotic treatment. This is evident in the significantly lower acceptance of the control treatment compared to the others (Figure 4). Among the sensory attributes, aroma exhibited the least variation across treatments. In contrast, the combination of prior freezing with osmotic dehydration produced more favorable characteristics in the slices, leading to higher acceptance scores compared to those without prior freezing. Notably, prior freezing resulted in improved appearance, color, and texture, which directly contributed to higher overall acceptance. However, the flavor showed only a slight sweetness variation, without any significant alteration in its overall profile.

The evaluation of testers’ purchase intent is crucial for estimating the commercial potential of the product under study. As demonstrated in Figure 5, osmotic treatment plays a pivotal role in enhancing the value of dehydrated umbu by producing a product with strong public acceptance. The control slices did not receive favorable commercial feedback, unlike the other treatments. Slices that underwent prior freezing before osmotic dehydration achieved a mean purchase intent score closer to the maximum rating (4.25 out of 5.00). While the results are promising, they reflect preliminary perceptions that would benefit from validation through broader consumer panels, including participants from diverse geographic regions and cultural backgrounds, in future studies.

The evaluated parameters were grouped into two principal components (PC), which accounted for 83.16% (PC1) and 5.57% (PC2) of the study’s variability. Only the ratio of soluble solids to titratable acidity (SS/TA) and the osmotic dehydration (OD) parameters specifically, sucrose gain (% SG_OD) and water loss (% WL_OD) contributed to PC2.

As illustrated in Figure 6, the process of sucrose gain was more pronounced in the treatments that included prior freezing. This increase in soluble solids, ratio (SS/TA), and sugars was expected since these are the primary outcomes of the osmotic treatment. The point of intersection between the slices with and without prior freezing corresponds to the zone associated with these sugar and solid parameters, reinforcing the negative correlation with experiments that did not undergo osmotic dehydration. Figure 6 b highlights the more significant changes caused by freezing, as only the fresh slices showed any similarity to the control group. Furthermore, it is important to note that sucrose absorption was the most influential parameter in shaping the outcomes of pre-frozen treatments. Among the experiments, treatments 14 and 15 showed the highest similarity.

4. Conclusions

Prior freezing and thawing before drying imparted significant distinction to the umbu slices. Thus, the study concluded that in the physical–chemical, chemical, and functional contexts, the amount of sucrose absorbed during the osmotic treatment played a crucial role, which was optimized in the experiments with prior freezing. Notable results were observed for drying at 50 °C with 50 °Brix in non-frozen slices (experiment 2) which showed the highest concentration of total phenolics (71.95 mg/100 g) and the lowest losses of these compounds during in vitro digestion. Experiment 1 presented the highest levels of flavonoids (3.94 mg/100 g), anthocyanins (0.62 mg/100 g), and total chlorophylls (0.78 mg/100 g). For pre-frozen slices, the combination of 60 °C and 40 °Brix (experiment 13) stood out with the highest antioxidant activity and greater perceived sweetness, due to the favorable balance between titratable acidity and soluble solids. Moreover, the sensory analysis reinforced the relevance of osmotic treatment in obtaining dehydrated umbu, demonstrating significant acceptance of the slices subjected to prior freezing and thawing before osmotic dehydration.