Evaluation of Pectin-Based Coatings, Olive Leaf Extract, and Chitosan Nanoparticles for Acrylamide and Hydroxymethylfurfural Mitigation in French Fries: A Comparative Study of the Deep Frying and Air Frying Methods

Abstract

This study evaluated the potential of pectin (PEC)-based coatings, enhanced with olive leaf extract (OLE) and chitosan nanoparticles (CH-NPs), to mitigate the formation of harmful compounds during the frying of French fries. The research compared deep fat and hot air frying methods. Initial characterization of the coating solutions included assessing zeta potential, Z-Average, polydispersity index, and antioxidant capacity. The inclusion of OLE and CH-NPs significantly boosted antioxidant activity, reaching 78.9%, without substantially altering zeta potential or Z-Average characteristics. Notably, hot air frying induced significantly higher levels of acrylamide (ACR) and hydroxymethylfurfural (HMF) compared to deep fat frying. However, the application of the developed coatings demonstrated a marked reduction in both ACR and HMF across both frying techniques. Furthermore, the coatings, particularly PEC with OLE and CH-NPs, effectively decreased oil absorption by 55% while simultaneously increasing moisture content. The sensory evaluation indicated that the panelists liked the deep-fat-fried fries significantly more compared to hot air frying. Deep-fat frying led to an elevated browning index, which was significantly counteracted by the application of the coatings. These findings underscore the importance of informing consumers about the potential for increased ACR and HMF formation during hot air frying, despite its advantages in reducing fat content, to ensure informed dietary choices.

1. Introduction

The application of edible coatings in food systems has garnered significant attention due to their potential to enhance food quality, safety, and shelf life. Hydrocolloid-based edible coatings, derived from polysaccharides and proteins, serve as primary packaging materials that are biodegradable, environmentally friendly, and capable of modifying food properties such as water retention, texture, and oil uptake [1,2,3]. Pectin-based coatings have shown promising effects in extending the shelf life and maintaining quality of various fruits and meats during storage. These coatings can reduce weight loss, maintain color, and inhibit lipid oxidation in beef steaks [4]. When combined with nanochitosan, pectin coatings significantly extended the shelf life of mango fruits, especially at lower storage temperatures [5]. Across studies, pectin coatings demonstrated effectiveness in preserving various quality parameters of fruits and meats, including color, texture, and nutritional content, during storage under different conditions [5,6,7].

During thermal processes, compounds such as furan, methylfurans, acrylamide (ACR) and 5-hydroxymethylfurfural (5-HMF) are formed [8]. ACR forms primarily when asparagine (an amino acid) reacts with reducing sugars (like glucose or fructose) during high-temperature cooking processes like frying or baking. This reaction starts with the interaction of these components, forming an intermediate compound known as a Schiff base. Over subsequent steps, this compound undergoes decarboxylation to produce another intermediate called an azomethine ylide, from here, ACR can form through multiple pathways, such as β-elimination of a decarboxylated sugar derivative or the breakdown of 3-aminopropionamide [9].

Studies in animals show that ACR can trigger tumors in various organs, leading the International Agency for Research on Cancer (IARC) to classify it as a “probable human carcinogen” [10]. When ingested, acrylamide is quickly absorbed by the body and metabolized into glycidamide—a compound that interacts more aggressively with DNA and proteins than acrylamide itself, raising concerns about its potential health impacts [11]. Beyond its cancer risks, acrylamide is a neurotoxin. It disrupts nerve function by impairing cellular transport systems and reducing levels of key signaling molecules in the brain, which can interfere with normal nerve communication [12]. Alarmingly, even low levels of long-term exposure may cause these neurological effects, suggesting that everyday dietary acrylamide—common in foods like fried potatoes, bread, and coffee—could pose health risks, particularly for children whose developing systems may be more vulnerable [12].

The HMF is a chemical compound that forms in foods during high-heat cooking or processing, such as baking, roasting, or caramelizing. It is created through a series of reactions—starting with sugars breaking down or reacting with other compounds during the Maillard reaction (the same process that gives browned foods their flavor). HMF can also form directly when sugars are heated in acidic conditions, like in jams or fruit juices [13,14]. The amount of HMF in food often reflects how much heat the product has been exposed to, making it a useful marker for assessing cooking intensity, food freshness, and even flavor quality. Moreover, it was previously reported that HMF reacted with asparagine to form ACR during heating [14]

Several strategies can be employed to mitigate the formation of ACR and HMF in foods. Controlling time and temperature during thermal processing is a key approach, as these parameters directly influence Maillard reaction pathways. Pre-treating foods—such as washing, blanching, soaking, or enzymatic treatments—has also demonstrated potential in reducing ACR and HMF formation. Additionally, edible coatings have been previously validated as a critical strategy for minimizing ACR and HMF generation during frying. These coatings act as physical barriers, limiting heat transfer and reactive precursor availability, thereby suppressing undesirable chemical reactions. Among these, pectin-based coatings have emerged as a promising solution for mitigating the formation of heat-induced toxicants, such as ACR and HMF, in fried foods. Studies have demonstrated that PEC-based coatings significantly reduce ACR and HMF levels in French fries and fried kobbah, while also decreasing oil absorption [15,16,17,18,19]. However, challenges remain, including the potential for undesirable color changes and the need for high concentrations of coating materials, which may limit consumer acceptance.

Recent studies have demonstrated the effectiveness of pectin (PEC) and other polysaccharide coatings in reducing ACR formation in fried foods. PEC coating can inhibit ACR formation by over 90% in potato chips [20], and up to 48% in French fries [19]. Hydrolyzed PEC shows even stronger inhibitory effects than regular pectin [20]. Alginate and PEC solutions also effectively inhibited ACR formation in both chemical and food model systems under conventional and microwave heating [21]. These coatings work by increasing water retention, thereby reducing the Maillard reaction responsible for ACR formation [19]. Additionally, polysaccharide coatings can affect other quality characteristics such as browning, crispness, and oil content [20].

To address these limitations, researchers have explored the incorporation of nanoparticles into hydrocolloid coatings. Chitosan nanoparticles (CH-NPs), in particular, have shown remarkable potential due to their unique properties, such as biocompatibility, biodegradability, and antimicrobial activity. When combined with PEC, CH-NPs enhance the mechanical and barrier properties of coatings, allowing for effective ACR and HMF reduction at lower concentrations [22]. For instance, studies have reported a 78.0% reduction in ACR content in kobbah coated with PEC-chitosan solutions, attributed to improved water retention and reduced oil uptake [15]. Furthermore, low molecular weight chitosan has been shown to inhibit ACR formation by up to 46.7% in model systems, highlighting its potential as a functional additive in food coatings [23].

Natural antioxidants, such as olive leaf extract (OLE), have also gained attention for their ability to inhibit heat-induced toxicants and enhance the functional properties of edible coatings. Recent studies have explored the use of OLE in edible coatings and films for food preservation. OLE incorporation into PEC films improved their antioxidant activity, opacity, and water vapor barrier properties [24]. When applied to sweet cherries, chitosan and alginate coatings enriched with OLE effectively delayed ripening, preserved ascorbic acid and phenolic content, and maintained antioxidant activity during storage [25]. These findings suggest that OLE is a promising natural additive for enhancing the functionality of edible coatings and films, potentially extending the shelf life of various food products [26]. The synergistic effects of combining hydrocolloids, nanoparticles, and natural antioxidants offer a sustainable and effective approach to mitigating toxic compound formation in fried foods. For example, catechins and curcumin, when combined with chitosan and PEC, have been shown to significantly inhibit the formation of advanced glycation end products and HMF in baked goods, although sensory acceptability remains a challenge [27].

Deep-frying, a widely used cooking method, is known for its ability to impart desirable sensory attributes to foods, such as texture, flavor, and appearance. However, the high temperatures involved in deep-frying can lead to the formation of harmful compounds, including ACR and HMF, which are associated with carcinogenic and genotoxic effects [28,29]. In response, alternative frying technologies, such as air frying, have been developed to reduce the levels of these toxicants while maintaining food quality [28]. Despite these advancements, the role of edible coatings in enhancing the safety of fried foods remains underexplored, particularly in the context of emerging frying technologies.

In a domestic air fryer, powerful hot air circulates around the food at high temperatures ranging from 140 to 200 °C. This process creates a consistent and intense temperature gradient, which helps to quickly transfer heat throughout the food. As a result, moisture is removed from the food while a crispy outer crust forms, similar to what one would expect from deep frying. Occasionally, to enhance flavor and texture—especially in foods like potatoes or other starchy items—a small amount of oil is added during the cooking process [30]. Hot air frying requires much longer processing times, typically 21 min in relation to 9 min in the case of deep fat frying [31]. A recent review by Téllez-Morales et al. [30] highlighted two contrasting trends in research findings on the impact of air-frying methods on ACR formation in food. The first trend demonstrated that air frying reduces ACR levels, while the second concluded that ACR formation during air frying is comparable to that of deep frying. In contrast, according to the European Food Safety Authority [32], air fryers generate 30–40% more acrylamide than traditional deep fryers.

Given the health risks associated with ACR and HMF, as well as the lack of stringent regulations in many regions, there is an urgent need for innovative strategies to mitigate their formation in thermally processed foods. Moreover, HPLC and LC/MS-MS are used to determine the ACR or HMF formation during thermal treatments, and recently, spectrophotometric approaches have been demonstrated to efficiently determine HMF in complex food matrices [33].

This study aims to evaluate the effectiveness of PEC-based coatings reinforced with OLE in the presence or absence of CH-NPs in mitigating the formation of ACR and HMF in freshly prepared French fries. The study will compare the performance of these coatings in domestic deep fat frying and hot air frying, aiming to provide insights into the potential of edible coatings as a strategy for reducing heat-induced toxicants in fried food.

2. Materials and Methods

2.1. Materials

Pectin (PEC) of Citrus peel low-methylated (7%) was obtained from Silva Extracts srl (Gorle, Italy). Chitosan nanoparticles (CH-NPs)-based solutions (0.6% w/v) were prepared from a CH stock solution (2% w/v of hydrochloric acid 0.1 N stirred overnight). Acrylamide (ACR) and HMF standards ≥ 99.8% were obtained from the Sigma–Aldrich Chemical Company (St. Louis, MO, USA). Acetonitrile and methanol HPLC (high-pressure liquid chromatography) analytical grade, n-hexane, and formic acid were obtained from the Sigma–Aldrich Chemical Company (St. Louis, MO, USA). Food-grade glycerol (GLY) was obtained from the Merck Chemical Company (Darmstadt, Germany). Water purified by a Milli-Q system was used to provide ultra-pure water with resistivity > 18 MΩ cm⁻¹ at 25 °C and total organic carbon (TOC) < 5 ppb. Whereas Oasis HLB (Hydrophilic-Lipophilic-Balanced) 200 mg, 6 mL solid phase extraction (SPE) cartridges will be used for ACR and HMF extraction. The syringe filters (0.45 μm, 0.22 μm PVDF. Potassium hexacyanoferrate (Carrez I), and zinc sulfate (Carrez II). 2, 2-Diphenyl-1-picrylhydrazyl (DPPH) from Sigma Aldrich, USA.

2.2. Film Forming Solution Preparation

According to Sabbah et al. [24], pectin films containing olive leaf extract (OLE) were reinforced with CH-NPs at varying concentrations. A 2% pectin (PEC) stock solution was prepared, and OLE (w/v) was added to it. The mixture was continuously stirred for 30 min at room temperature. Next, the film-forming solution (FFS) was reinforced with CH-NPs, and glycerol (GLY) was added as a plasticizer at 30% (w/v relative to PEC). The pH of the solution was adjusted to 6.5. The concentrations of PEC and CH-NPs were chosen based on previous studies, with OLE concentrations ranging between 0.05%, 0.1%, and 0.2% (w/v), and CH-NPs concentrations set at 1%, 3%, and 5% (w/v). After testing, the films with the optimal concentrations of 0.2% OLE and 1% CH-NPs were selected for further study [15,24].

2.2.1. Determination of Zeta Potential and Z-Average and Polydispersity Index (PDI) for Coating Solutions

Zeta potential, (Z-Average) and PDI was measured by using Brookhaven Instruments basis on dynamic light scattering.

2.2.2. Determination of Antioxidant Activity for Coating Solutions

To evaluate antioxidant activity, a DPPH (2,2-diphenyl-1-picrylhydrazyl) stock solution with a concentration of 0.05 mg/mL (soluble in methanol) was used. The analysis was conducted using spectroscopy (Thermo Scientific Vision Lite software Version 4) at a wavelength of 517 nm. For the test, 0.5 mL of the coating sample was mixed with 4.5 mL of the DPPH solution.

2.3. Dipping Method

Four treatments were applied to the potato samples: water (used as the control) and three coating solutions (PEC alone, PEC + 0.2% OLE, and PEC + 0.2% OLE + 1% CH-NPs). The Spunta potato variety, fresh, was used for the experiment. The potatoes were cut into slices (8–9 cm in length) and then dipped into 500 mL of either distilled water (for the control group) or one of the coating solutions. After dipping, each slice was allowed to drip for 30 s and rinsed for 1.5 min before frying.

2.4. Frying Method

Two frying methods were employed: deep frying and air frying, using a deep fryer and an air fryer, respectively. The frying conditions were set as follows: 1.5 L of sunflower oil was used with the deep fryer, and the controlled temperature was 190 °C. However, the hot air fryer temperatures were controlled at 200 °C without using oil. Fresh French fries (200 g) were fried for 9 min in the deep fryer and 18 min in the hot air fryer, with the air-fried sticks flipped halfway through to ensure even cooking. The cooking times were selected based on the primary study where French fries achieved the acceptable golden color [19]. In deep fat frying, the oil was replaced with fresh oil for each batch of differently coated samples.

2.5. Acrylamide and HMF Standard Preparation

Following Al-Asmar et al. [19] for ACR standard preparation and Jozinović et al. [34] for HMF standard preparation the stock solution (1.0 mg/mL) for both ACR and HMF was prepared. From the stock solution of ACR, the working calibration standards (100, 250, 500, 1000, 2000, 3000, 4000, 5000, and 10,000 µg/L), were prepared. Whereas, from the stock solution of HMF, the calibration standards (100, 250, 500, 1000, 2000, 3000, 4000, 5000, 10,000, and 15,000 µg/L), were prepared. Then, the working standards were measured at 210 nm for ACR and 284 nm for HMF.

2.6. Extraction of ACR and HMF from the French Fries

ACR and HMF extraction was performed as described in Al-Asmar et al. [19] and Huang et al. [35], with some modifications. About 100 g of french-fried potato for each treatment individually, accurately weighed after cooling, were immersed in n-hexane for 30 min to remove the oil from their surfaces. The fries were then ground, then two different Falcon tubes were set up for each sample, one for detecting ACR formed in the sample and the second one for detecting HMF. In both tubes, 1.0 g of sample, accurately weighed, 50 µL of Carrez reagent potassium salt and 50 µL of Carrez reagent zinc salts were added to each sample and, 9.9 mL of Milli-Q water were added. The samples were extracted in an incubated shaker for 30 min at 25 °C, then followed by centrifugation at 7879× g for 10 min at 4 °C. The supernatant was filtered through a 0.45 µm syringe filter for the clean-up of the Oasis HLB SPE cartridges. The SPE cartridge was preventively conditioned with 2.0 mL of methanol followed by washing with 2 mL of water before loading 2.0 mL of the filtered supernatant, the first 0.5 mL was discarded and the remaining elute collected (≈1.5 mL; exact volume was measured by weight and converted by means of density). All extracts were kept in dark glass vials at 4 °C before analysis. The clean sample extracts were further filtered through 0.2 µm nylon syringe filters before HPLC-UV (ultra violet) analysis. Each analysis was performed in triplicate.

2.7. Determination of ACR by HPLC Method

The analysis of HPLC-UV was conducted using the reverse phase HPLC (RP-HPLC) technique on a Waters 5695 series HPLC system, which included an degasser, a dual pump, and a photodiode array detector 2996. A Synergi™ 4 µm Hydro-RP 80 Å HPLC column measuring 250 × 4.6 mm was utilized for the separation (Phenomenex, Torrance, CA, USA). The experimental parameters were set as follows: detection was performed at a wavelength of 210 nm, and an isocratic elution was achieved with 0.1% formic acid (v/v) in a mixture of water and acetonitrile (97:3, v/v), with a flow rate of 1.0 mL/min. The injection volume used was 20 µL. The entire chromatographic analysis lasted 10 min per sample, with the temperature maintained at 30 °C using a “HPLC column heater”. In both the ACR standard and samples derived from fried potatoes, the retention time for ACR was consistently found to be 4.9 min. The method demonstrated a relative standard deviation of less than 5% across three repeated trials. The equation was obtained by applying the linear regression of y = 113.01x − 9.6054, with R² equal to 0.9993; this equation was used to calculate the amount of ACR in all analyzed samples. The acrylamide recovery test was between 103% and 87%. The limit of detection (LOD) and limit of quantification (LOQ) were calculated by injecting lower concentrations of standards. A concentration with a signal-to-noise ratio of 3 was assigned to LOD (35.22 µg/L), and a signal-to-noise ratio of 10 was assigned to LOQ (117.42 µg/L). All analyses were performed in triplicate.

2.8. Determination of HMF by HPLC Method

The analysis of HPLC-UV was conducted using the reverse phase HPLC (RP-HPLC) technique on a Waters 5695 series HPLC system, which included an degasser, a dual pump, and a photodiode array detector 2996. Atlantis dC18 Column, 100 Å, 5 µm, 4.6 mm × 250 mm was utilized for the separation (Waters, Torrance, CA, USA). The experimental parameters were set as follows: detection was performed at a wavelength of 284 nm, and an isocratic elution was achieved with 0.1% formic acid (v/v) in a mixture of water and acetonitrile (97:3, v/v), with a flow rate of 1.0 mL/min. The injection volume used was 20 µL. The entire chromatographic analysis lasted 25 min per sample, with the temperature maintained at 30 °C using a “HPLC column heater”. In both the HMF standard and samples derived from fried potatoes, the retention time for HMF was consistently found to be 19.8 min. The method demonstrated a relative standard deviation of less than 5% across three repeated trials. The equation was obtained by applying the linear regression of y = 154.317x + 4.0282, with R² equal to 0.9996; this equation was used to calculate the amount of HMF in all analyzed samples. The HMF recovery test achieved values between 101% and 85%. The limit of detection (LOD) and limit of quantification (LOQ) were calculated by injecting lower concentrations of standards. A concentration with a signal-to-noise ratio of 3 was assigned to LOD (16.4 µg/L), and a signal-to-noise ratio of 10 was assigned to LOQ (54.66 µg/L). All analyses were performed in triplicate.

2.9. Determination of Water Content

The water content of the French fries was determined using the gravimetric analysis method as reported by AOAC [36]. After frying, the potato sticks coated with different solutions (including the control group, which was treated with water) were dried in an oven (Drying Oven SLN 115 STD, POL-EKO-APARATURA SP.J) at 105 °C overnight until a constant weight was achieved. The water content in all samples were calculated using the following formula:

$$\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}\left(\mathrm{\%}\right)={\displaystyle \frac{\mathrm{w}\mathrm{e}\mathrm{t} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}-\mathrm{d}\mathrm{r}\mathrm{y} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}{\mathrm{w}\mathrm{e}\mathrm{t} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}}\times 100$$

(1)

2.10. Determination of Oil Content

The oil uptake content of fried potato samples was determined according to AOAC [36] by Soxhlet extraction, and the oil reduction due to coating was calculated as a percentage from the following equation:

$$\mathrm{O}\mathrm{i}\mathrm{l} \mathrm{r}\mathrm{e}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{d}\mathrm{u}\mathrm{e} \mathrm{t}\mathrm{o} \mathrm{c}\mathrm{o}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}(\mathrm{\%})={\displaystyle \frac{\mathrm{o}\mathrm{i}\mathrm{l} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}\left(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}\right)-\mathrm{o}\mathrm{i}\mathrm{l} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t} \left(\mathrm{c}\mathrm{o}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\right)}{\mathrm{o}\mathrm{i}\mathrm{l} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t} \left(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}\right)}}$$

(2)

2.11. Determination of Color

Color measurement in food products is often used as an indirect indicator of other quality attributes, such as flavor and pigment content [37]. The color of French fry samples was measured using a Chroma Meter Konica Minolta CR-400 (Tokyo, Japan), which determines the L*, a*, and b* values. For each treatment, ten French fry samples were randomly selected, and their color was tested at least at three points along each strip. The average color value was then calculated. The Lab* color system is an international standard for color measurement established by the Commission Internationale d’Eclairage (CIE) in 1976. In this system, L* represents lightness and ranges from 0 (black) to 100 (white). The parameters a* (ranging from green to red) and b* (ranging from blue to yellow) are chromatic components, each ranging from −120 to +120 [36]. The total color difference (ΔE) between the coated French fries and the uncoated control sample was calculated to quantify the magnitude of the color difference. This value was determined using the following equation [35,36]:

$$\mathrm{DE}=\sqrt{{\left(\mathrm{L}\ast -\mathrm{L}\prime \ast \right)}^2+{\left(\mathrm{a}\ast -\mathrm{a}\prime \ast \right)}^2+(\mathrm{b}\ast -\mathrm{b}\prime \ast )²}$$

(3)

where L′*, a′* and b′* are the parameters of treated French fries fried and L*, a* and b* the ones of the control (uncoated).

The browning index (BI) was used to define the overall changes in browning color [38]. BI of the French fries fried was calculated by the following equation:

$$\mathrm{BI}={\displaystyle \frac{100(\mathrm{X}-0.31)}{0.17}}$$

(4)

where

$$\mathrm{X}={\displaystyle \frac{\mathrm{a}\ast +1.75\mathrm{L}\ast }{5.645\mathrm{L}\ast +\mathrm{a}\ast -3.012\mathrm{b}\ast }}$$

(5)

2.12. Sensory Evaluation

The treatments were evaluated for their sensory properties by using 9-point hedonic scale methods. The 30 panelists aged 20–30 years old from the Department of Nutrition and Food Technology at An-Najah National University in Palestine were randomly selected and informed about the evaluation method before the test. Samples were served randomly on plastic plates with three-digit codes. The appearance, odor, texture, flavor, and overall acceptability, with one denoting “extremely dislike” and nine denoting “extremely like”. The analysis was conducted in individual booths containing the coded samples and instructions for the evaluation procedure. The tasting room for sensory evaluation was air-conditioned and free of disturbing factors. Water was given to the panelists for mouth rinsing between samples [31].

2.13. Statistical Analysis

The statistical analysis was performed by means of JMP software 10.0 (SAS Institute, Cary, NC, USA), Two-way ANOVA and the t-student test for mean comparisons were used. Differences were considered significant at p < 0.05.

3. Results and Discussion

3.1. Effect of OLE and CH-NPs on Zeta Potential, Z-Average and PDI of the PEC-Based Coating Solution

The formation coating solution is the primary step in the coating process to achieve the based materials that will be used for the coating of fried potato. Understanding the effect of using OLE and CH-NPs on the physicochemical properties of the coating solution is critical for having suitable coating materials’ stability and distribution properties. The PEC-based coating solutions were evaluated for their Zeta potential, Z-Average, and PDI. The results are reported in

Table 1. Effect of OLE and, CH-NPs of zeta potential, Z-Average, polydispersity index (PDI), and antioxidant activity for the coating solutions used for dipping.

	Zeta Potential (mV)	Z-Average (d.nm)	Polydispersity Index (PDI)	Antioxidant Activity (%)
Control OLE 0.2% (w/v)	−10.31 ± 1.5	1210.1 ± 204	0.289 ± 0.01	33.68 ± 0.5
CH-NPs 1%	4.93 ± 1.1	407.9 ± 29.5	0.121 ± 0.06	4.11 ± 0.2
Coating solutions PEC 1%	−36.08 ± 2.4	3833 ± 316	0.213 ± 0.06	6.92 ± 1.1
PEC 1% + OLE 0.2%	−36.53 ± 2.8 c,d	3369± 224 c,d	0.252 ± 0.05 a,c,d	62.06 ± 1.7 a,c,d
PEC 1% + OLE 0.2% + CH-NPs 1%	−35.23 ± 1.4 c,d	3378± 173 c,d	0.290 ± 0.13 a,d	78.99 ± 1.2 a,b,c,d

Values were compared to PEC 1% sample: “a” indicates values that were significantly different to PEC 1% coating solution sample values; “b” indicates values that were significantly different from PEC1% + OLE 0.2% coating solution sample values; “c” indicates values that were significantly different from OLE 0.2% control sample values; “d” indicates values that were significantly different from CH-NPs 1% control sample values. d.nm = diameter values in nanometers.

.

Zeta potential is one of the most important values that gives the produces the possibility to understand the stability of the coating solution. When the Zeta potential has a value higher than +30 or less than −30 it indicates that the solution is stable, while an unstable solution has a Zeta potential between these values, which means it will separate into two phases during storage. The 0.2% OLE and 1% CH-NPs were prepared and tested for their Zeta potential, Z-Average, and PDI as a control to understand their effect when incorporated into the PEC-based coating solution. The results indicated that the Zeta potential of OLE alone was −10.31 and CH-NPs 4.93 mV. The Z-Average of OLE and CH-NPs were 1210.1 and 407.9 d.nm, and the PDI was 0.289 and 0.121, respectively. As previously reported at pH 7, CH-NPs typically exhibit a Zeta potential that is close to neutral or slightly positive, and their Z-Average can vary depending on several factors such as the CH source, molecular weight, degree of deacetylation, preparation method, and the presence of other substances [39]. As reported by Al-Asmar et al. [15], the highest Zeta potential was detected at a pH lower than 5.0 and decreased significantly as the pH approached 6.0 and 7.0.

Adding OLE alone or with the presence of CH-NPs to the PEC-based coating solution did not show a significant effect on the Zeta potential, and Z-Average, of the PEC-based coating solutions (Table 1). This finding is in accordance with the previously reported results by Champrasert et al. [21], and Lorevice et al. [40]. Furthermore, the PDI differed significantly between the treatments; the addition of OLE, with or without CH-NPs, into PEC-based coating solutions resulted in an increased PDI value for the coating solutions.

3.2. Effect of OLE and CH-NPs on the Antioxidant Activity of the PEC-Based Coating Solution

During this study, adding OLE and/or CH-NPs was evaluated, and the results were reported in Table 1. The results indicated that OLE alone exhibited significantly higher antioxidant activity compared to CH-NPs. When OLE was incorporated into the PEC coating solution, the antioxidant activity reached 62.06%. By adding CH-NPs to the PEC containing OLE, the antioxidant activity peaked at the highest value of 78.99%. Since oleuropein in OLE and CH-NPs both possess antioxidant properties, their combination within the PEC likely results in a broader and more effective neutralization of various reactive species present in the matrices. Furthermore, the obtained results clearly demonstrate that using OLE alone or in combination with CH-NPs significantly improved antioxidant activity compared to the coating solution prepared solely with PEC. However, Albertos et al. [41], reported that by using OLE with gelatin film the antioxidant activity was improved. They also reported that increasing OLE concentrations significantly enhanced the antioxidant activity of the gelatin film. Moreover, Mi et al. [42], reported that CH-NPs possessed good antioxidant activity, inhibiting lipid peroxidation. However, CH-NPs with different derivatives exhibited antioxidant and antibacterial properties when added to polylactic acid films [43].

3.3. Effect of Coating Solutions on ACR and HMF Formation for Both Frying Methods

The formation of ACR and HMF during frying is shown in Figure 1 and Figure 2, respectively. The results revealed that hot air frying produced significantly higher levels of ACR compared to deep fat frying, regardless of whether the French fries were coated with a PEC-based solution or not, and whether OLE or CH-NPs were added.

For uncoated French fries, the ACR content was 4874.1 µg/kg when deep-fried and 7191.3 µg/kg when air-fried. When coated with 1% PEC alone, the ACR levels dropped significantly to 2897.0 µg/kg (deep frying) and 4903.0 µg/kg (air frying). Adding OLE to the PEC-based coating further reduced ACR formation to 2169.5 µg/kg (deep frying) and 3813.0 µg/kg (air frying). Additionally, incorporating CH-NPs with OLE into the PEC coating significantly lowered ACR levels to 3056.0 µg/kg during air frying. While this reduction was not significantly different from deep frying when using the OLE-containing coating, it was notably lower compared to uncoated fries or those coated with PEC alone. The findings raised concerns with the Codex Alimentarius, which highlighted that ACR levels in chips and French fries were found to be significantly high, ranging from 59 to 5200 μg/kg [32].

It is important to mention that the frying time of deep frying was 9 min, while hot air frying was 18 min. This time, according to the primary study, achieved a golden color of the French fries. This may be one of the causes that lead to the higher ACR formation during hot air frying. Recent studies have concluded that the extended cooking time in air frying allows for a more complete reaction between asparagine and reducing sugars in potato strips [44]. Moreover, Dong et al. [45], concluded that the concentration of ACR generated on the edges of frozen potato strips was the highest, at least five times that of the core.

Air frying has become popular as a healthier alternative to deep frying because it uses little to no oil, reducing the fat content of foods. However, the cooking time and high temperatures used in air frying can still promote ACR formation. Air fryers work by circulating hot air around the food, creating a crispy texture while cooking the interior [46]. While this method is convenient, the high temperatures it reaches have raised concerns about ACR levels, as highlighted by numerous studies [31,45].

The results demonstrated that the coating solutions effectively reduced ACR formation. Specifically, the incorporation of OLE and CH-NPs into the PEC-based coating achieved the highest ACR reduction: 53% during deep fat frying and 58% during air frying, outperforming other coating solutions tested in the study. Notably, the reduction in ACR was significantly greater in deep fat frying compared to air frying when using this coating. These findings align with prior research by Al-Asmar et al. [19]—the authors reported that coating French fries with 1% PEC reduced ACR formation by approximately 48%. This reduction was superior to coatings containing chitosan or grass pea flour, whether prepared with or without transglutaminase enzymes. Moreover, it was proved that using plant extracts such as ginger, borage, and fennel succeeded in decreasing ACR (59.67, 67.99, and 73.36% in air frying and 21.91, 66.29 and 29.15% in deep frying of French fries, respectively [47]). Additionally, the extracts from green tea, cinnamon, and oregano reduced the ACR level by 62%, 39%, and 17% of fried potatoes, respectively [48].

The effect of different coating solutions on HMF levels in French fries was reported in Figure 2. In the control group, representing fried potatoes without coating, HMF content is measured at 10,680.0 µg/kg and 14,148.9 µg/kg for both deep and air frying methods, respectively. Similarly to ACR, the treatment of the French fries resulted in notable reductions in HMF formation.

A significantly higher formation of HMF was observed during the air frying method compared to deep frying. The highest HMF levels were detected in uncoated fries, while the application of coatings significantly reduced HMF content. These findings align with research by Verma et al. [49], who reported that control (uncoated) French fry samples had the highest HMF content, reaching 12,930 µg/kg under deep frying. However, in all cases, air frying produced markedly higher HMF concentrations than deep frying. Interestingly, the use of PEC as a coating solution alone significantly reduced HMF formation in both frying methods. For deep frying, HMF levels were 8878.3 µg/kg (a 16.9% reduction), while in air frying, levels reached 13,429.0 µg/kg (a 5.1% reduction) compared to control samples. Furthermore, the addition of 0.2% OLE strongly influenced HMF reduction in both methods. When isolating the effect of OLE (by subtracting the PEC reduction percentage), OLE alone contributed to 27.1% HMF reduction in deep frying and 15.7% in air frying.

The reduction in HMF reached 65.0% when French fries were coated with PEC containing OLE in the presence of CH-NPs during deep frying. Under the same conditions during air frying, HMF reduction was 39.5%. Moreover, many previous researchers concluded that using different extracts to treat French fries has a significant reduction in HMF formation and ACR, such as using cinnamon, clove, curry leaf, mint, and turmeric [49]. These effects are primarily linked to the bioactive compounds in plant extracts, which may act through mechanisms such as antioxidant activity, carbonyl group trapping, asparagine binding, and other proposed pathways [49]. As detected in this study, the combination of CH-NPs and OLE in PEC dipping coating solution can lead to synergistic effects, further enhancing the inhibition of both ACR and HMF formations.

3.4. Effect of Coating Solutions on Oil Uptake for French Fries in the Deep Fat Frying Method

Oil absorption in fried foods occurs through three mechanisms: (1) the water replacement mechanism, where escaping water creates pores for oil to enter [50]; (2) the condensation effect, where oil is drawn into pores as pressure drops after frying [51]; and (3) prolonged frying, which reduces interfacial tension due to oil hydrolysis, allowing easier oil penetration [50,52]. The coating method was recently used due to its significant contribution to controlling the oil absorption during frying, due to its capability to cover the product surface of the potato strips during dipping. The effect of different PEC-based coating solutions containing OLE with or without CH-NPs on fat content was evaluated and the obtained results are shown in Figure 3. The obtained results indicated a significant reduction in oil uptake through all coating solutions that were used compared to the control sample. The highest oil uptake was detected in the control sample, which was around 23%, while the lowest value was obtained after the French fries were coated with PEC containing OLE + CH-NPs, with an oil uptake of 11%. These results were in agreement with previous data, where the conclusion was that using the oil content of French fries coated by PEC 1% was 14% and the PEC was able to reduce the oil content by 29% compared to the control sample [19]. Moreover, Kizito et al. [53] studied the effect of different hydrocolloid materials on the oil uptake of potato strips, and they found that PEC achieved the most significant oil reduction of 12.93%, compared to the carboxymethylcellulose 11.71%, chitosan 8.28% and agar 5.25% at the same experimental conditions. The obtained results indicated that by using a PEC-based coating solution, and PEC in the presence of OLE or OLE + CH-NPs, the oil content was reduced to 24.5, 42, and 55%, respectively. Recently, Yang et al. [51], concluded that by coating the French fries with sodium alginate, the oil content was reduced by 52.5%. The ability of hydrocolloid-based coatings to minimize excessive oil absorption can be attributed to their capacity to lower the heat transfer coefficient, as thoroughly explored in the review by Kurek et al. [54]. The result indicated that OLE with or without CH-NPs is able to enhance the capability of the PEC coating solution to be less permeable to oil, which helps to produce healthy French fries products.

3.5. Effect of Different Coating Solutions on the Water Content of French Fries Fried at Different Frying Methods

The water content of fried food is one of the most critical factors affecting its shelf life, as higher water content promotes both chemical reactions and microbial growth. The obtained results indicated that by air frying, the water content was significantly higher compared to the deep fat frying method (Figure 4). The highest water content, 61.9%, was found in the fries that were coated by PEC with OLE and CH-NPs and fried by hot air frying. However, during the deep frying of French fries by using the same coating material, the water content was 50.7%. Recently, Li et al. [50], concluded that by using sodium alginate (0.5–2.0 g/100 g), the water content of French fries fried by deep fat frying method, was almost about 60%, and the difference concentration of sodium alginate did not have significant influence the water content. As well as the research highlights that the primary mechanism of oil absorption in French fries is water replacement.

The study indicates that PEC-based edible coatings with or without OLE or CH-NPs create a barrier on the surface of fries by modifying the surface structure. This alteration reduces the formation of water vapor channels, thereby changing the pathway and distribution of oil absorption. This finding is consistent with the previous research obtained by [15,19,50,55,56]. The results of water content of hot air frying are consistent with the previous findings by Dong et al. [45], who concluded that air-fried samples exhibited higher moisture content than deep-fried fries. Deep frying involves immersing the food in hot oil at temperatures above the boiling point of water, leading to significant heat and mass transfer. This process results in substantial water loss and higher oil absorption, which explains the lower moisture content in deep-fried products [57]. In contrast, air frying, which relies on lower heat and mass transfer, typically requires longer cooking times and often produces less crispy results [58].

3.6. Effect of Coating Solutions on Color Properties for Both Frying Methods

Heat and mass transfers during frying lead to physicochemical changes that influence the color of fried products. The loss of water and absorption of oil resulting from these transfers play a key role in the development of color, aroma, and texture. As the product gradually dehydrates, a crust forms on the surface, and browning occurs due to Maillard reactions [59]. The development of color in French fries is driven by the progression of Maillard reactions. When raw potatoes, which initially have a pale color, are cooked, they gradually turn golden-brown. This transformation is caused by the formation of melanoidins—brown-colored polymers generated through intricate chemical processes that take place during the Maillard reactions [60]. The effect of different coating solutions on the color of French fries fried using deep fat frying and hot air frying methods is summarized in

Table 2. Color properties of French fries coated with different coating solutions.

Coating Solutions	L*	a*	b*	∆E	BI
Deep Fat Frying
Control	66.6 ± 3.2 a	7.3 ± 0.4 a	40.7 ± 1.9 a	n.a	97.2 ± 1.0 a
PEC	79.0 ± 1.8 b	1.0 ± 0.6 b	37.3 ± 1.2 b	14.3 ± 1.3 a	62.5 ± 2.1 b
PEC + OLE	75.0 ± 1.3 c	2.9 ± 0.2 c	39.5 ± 0.4 a	9.6 ± 0.3 b	74.4 ± 1.2 c
PEC + OLE + CH-NPs	77.0 ± 1.4 c	1.0 ± 0.5 b	35.3 ± 0.5 c	13.3 ± 1.5 a	60.2 ± 3.1 b
Hot Air Frying
Control	86.7 ± 0.8 a*	−5.7 ± 0.6 a*	28.2 ± 0.6 a*	n.a	33.3 ± 1.1 a*
PEC	81.0 ± 0.7 b*	−3.7 ± 0.8 b*	26.3 ± 0.8 b*	6.3 ± 0.8 a*	34.6 ± 2.3 a*
PEC + OLE	82.2 ± 1.3 b*	−3.5 ± 1.0 b*	33.5 ± 1.3 c*	7.4 ± 1.6 a*	47.4 ± 3.1 b*
PEC + OLE + CH-NPs	81.0 ± 1.3 b*	−3.3 ± 0.5 b*	28.1 ± 1.5 a*	6.3 ± 1.0 a*	38.2 ± 2.9 a*

The superscript letters “a ,b, and c” indicate where there is statistical significance for each frying method in the same column, and “*” indicates the statistically significant differences between deep fat frying and hot air frying, p < 0.05. n.a: not applicable. “Control” represents a French fries sample dipped in distilled water.

. The results indicate significant differences in the color parameters (L*, a*, b*, ∆E, and BI) between deep fat frying and hot air drying. These findings are consistent with published studies by [31,54], who reported similar trends.

The L* value indicates lightness, where higher values correspond to a lighter color, and lower values indicate a darker color. As shown in Table 2, the control sample during deep frying had the lowest L* value (66.6 ± 3.2), indicating that it was darker compared to the coated French fries. All coating solutions significantly increased the L* value compared to the control, suggesting that the coated fries were lighter in color. This increase in L* is attributed to the application of coating solutions on the French fries. However, there was no significant difference in L* values between fries coated with PEC containing OLE and those coated with PEC containing OLE + CH-NPs during deep frying. This indicates that the addition of CH-NPs did not significantly influence the lightness of the fries. In contrast, during hot air frying, the L* value of the control sample was significantly higher (86.7 ± 0.8) compared to the coated samples. The application of coatings significantly reduced the L* value, indicating that the coated fries were darker than the control. However, there was no significant difference in L* values between fries coated with PEC containing OLE and those coated with PEC containing OLE + CH-NPs, suggesting that CH-NPs did not significantly affect the lightness of the fries during hot air frying. Thus, the darkening of the samples is due to the Maillard reactions, which are those presented by Iglesias-Carres et al. [61].

The a* values correspond to redness/greenness, where positive values indicate redness and negative values indicate greenness. The obtained results showed that in deep frying, all samples, including the control, exhibited positive a* values, indicating a reddish hue. However, the a* values were significantly reduced when coating solutions were applied. Among the coated samples, there was no significant difference in a* values between fries coated with PEC and those coated with PEC + OLE + CH-NPs. However, fries coated with PEC + OLE had significantly higher a* values compared to the control, indicating greater redness. In contrast, during hot air frying, all samples exhibited negative a* values, indicating a greenish hue. Moreover, the addition of OLE or CH-NPs had no significant influence on the a* values, as there were no notable differences between the coating materials. The obtained results were similar to those reported by Teruel et al. [31].

The b* values correspond to yellowness/blueness, where positive values indicate yellowness and negative values indicate blueness. Initially, all obtained b* values were highly positive. During deep frying, the control sample had a b* value of 40.7 ± 1.9, which was significantly reduced in all coated samples except for fries coated with PEC + OLE. This suggests that the PEC + OLE coating helped retain yellowness compared to other coatings. However, during hot air frying, the control fries had a b* value of 28.2 ± 0.6, and when coated with PEC + OLE, the b* value significantly increased, indicating enhanced yellowness compared to the control.

The ∆E values represent the total color difference compared to the control (uncoated fries). Higher ∆E values indicate more significant color changes relative to the control. In deep frying, the ∆E values of fries coated with PEC or PEC + OLE + CH-NPs were higher than those coated with PEC + OLE alone, indicating greater color changes. However, during hot air frying, there were no significant differences in ∆E values between the different coating solutions, suggesting minimal impact on color changes. A similar trend was detected by Coria-Hernández et al. [62]; they found that hot air frying resulted in ∆E values lower than deep drying.

The browning index (BI) corresponds to the degree of browning, with higher values indicating more browning. The results showed that during deep frying, the fries exhibited higher BI values compared to hot air frying, indicating more browning. The images in Figure 5 clearly show that the deep-fat-fried samples exhibited a darker brown coloration compared to the hot air dried samples or the control. The addition of OLE, either alone or with CH-NPs, significantly reduced the BI values compared to the control, meaning that uncoated fries were more browned than coated fries in deep frying. However, during hot air frying, the BI values were significantly lower compared to deep frying, with the control fries having a BI value of 33.03 ± 1.1. The addition of OLE and CH-NPs had no significant influence on the BI values in hot air frying.

3.7. Effect of Coating Solutions on Sensory Properties for Both Frying Methods

The effect of edible coatings on the sensory properties of French fries was evaluated. The results, presented in

Table 3. Effect of different coating solutions on sensory properties for both frying methods.

Coating Material	Appearance	Odor	Texture	Flavor	Overall Acceptability
		Deep Fat Frying
Control	6.0 ± 1.7 a	6.2 ± 1.5 a	5.7 ± 2.0 a	6.3 ± 1.7 a	6.1 ± 1.7 a
PEC	6.1 ± 1.5 a	6.5 ± 1.4 a	5.8 ± 1.8 a	6.1 ± 1.8 a	6.1 ± 1.6 a
PEC + OLE	6.4 ± 1.9 a	6.3 ± 1.4 a	5.6 ± 2.0 a	5.5 ± 2.1 a	5.7 ± 1.9 a
PEC + OLE + CH-NPs	6.7 ± 1.4 a	6.4 ± 1.3 a	5.7 ± 1.9 a	6.1 ± 1.9 a	6.2 ± 1.4 a
		Hot Air Frying
Control	3.0 ± 1.5 b	4.8 ± 1.8 b	3.2 ± 1.9 b	3.3 ± 2.0 b	3.1 ± 2.0 b
PEC	3.6 ± 1.9 b	4.6 ± 2.3 b	3.9 ± 2.5 b	3.3 ± 2.2 b	3.4 ± 2.0 b
PEC + OLE	3.9 ± 2.2 b	5.2 ± 2.0 b	3.8 ± 2.0 b	3.7 ± 2.3 b	3.8 ± 2.2 b
PEC + OLE + CH-NPs	4.7 ± 1.8 b	5.5 ± 2.0 b	4.3 ± 2.3 b	4.4 ± 2.6 b	4.4 ± 2.3 b

The treatments not connected by the same superscript letters “a and b” are significantly different in the same column p < 0.05.

, indicated that panelists preferred fries prepared using the deep-frying method over those prepared by air frying. Furthermore, samples coated with PEC, both with and without OLE or CH-NPs, received similar average sensory scores to the uncoated control samples. However, air-fried fries consistently received lower average scores compared to their deep-fried counterparts. Notably, French fries coated with PEC containing both OLE and CH-NPs showed a significant effect on all sensory properties, except odor, when compared to the control group and samples coated with PEC alone or with OLE. The significant difference between hot air fried and deep fat fried samples is also in agreement with instrumental color measurement [31]. The changes that are detected by panelists are due to several chemical reactions that occur during deep fat and hot air frying methods, such as water evaporation [19], Maillard reaction [60], starch gelatinization [63], and changes in the microstructure of French fries [64].

4. Conclusions

This study underscores the dual benefit of advanced coatings in mitigating health risks while enhancing the nutritional profile of fried foods. While air frying reduces oil consumption, its propensity to elevate ACR and HMF levels necessitates strategic interventions. OLE + CH-NPs emerged as the most effective coating, slashing ACR/HMF formation by up to 55% in air frying and preserving moisture content, thereby addressing the trade-off between “healthier” low-oil methods and harmful compound generation. Crucially, deep frying—though associated with darker coloration—benefited from coatings that reduced browning and oxidative damage. For consumers, these findings advocate for adopting antioxidant-rich coatings like OLE + CH-NPs to reconcile the convenience of air frying with safety, while industry applications could leverage such coatings to align with regulatory trends targeting ACR and HMF reduction. This research provides an actionable framework for minimizing dietary carcinogens without compromising culinary quality. The sensory analyses indicated that coating solutions with PEC in the presence of OLE with or without CH-NPs are accepted by the consumer mainly in deep fat frying. However, the fries that are prepared in hot air with or without coating need to improve their sensorial properties to be more accepted by the consumer. Informed by these insights, both home cooks and food manufacturers can optimize frying practices to deliver safer, healthier, and more nutritious fried products.