1 Introduction

Edible coatings on fruits are reported to be safe and eco-friendly alternative methods to delay fruit spoilage that aims in delaying the metabolic processes without impairing the internal fruit quality [1]. Polymer based coatings are known for their stability, antimicrobial and antioxidant properties [2]. However, they possess weak adhesion on waxy hydrophobic surfaces of fruits thereby restricting their applications in food preservation [3]. This necessitates the need to incorporate essential oils in the formulation of effective coating solution. Essential oils enhance the adhesion of the coating to the fruit surface. This property is attributed to their hydrophobic nature, which allows them to interact with the waxy surface of fruits even more effectively than the polymer alone [4]. In addition, they are also well-known for having antioxidant qualities, which can help fruits last longer on the shelf by preventing the growth of bacteria that cause spoiling and lowering oxidative damage [3]. Therefore, when incorporated into polymer-based coatings, essential oils can improve their adhesion to hydrophobic surfaces, enhance their antimicrobial and antioxidant properties and thereby improve the overall effectiveness of the coating in preserving the fruits quality [4]. The application of edible coatings has been extensively researched and recognized for its ability to maintain the quality of fresh fruits and extend their post-harvest lifespan [5,6,7,8]. The coatings are reported to decrease fruit respiration and weight loss by serving as a semi-permeable barrier to gases and water vapour, thereby help maintaining fruit firmness and impart gloss to the coated product [9].

Several polysaccharide-based edible coatings, primarily derived from chitosan [9] and pectin derived from orange peels [6], have been documented for their effective preservation of fresh grapes quality throughout the post-harvest period. Chitosan based coatings were able to preserve the phenolic content and fruit’s antioxidant capacity during shelf life [10]. Furthermore, they also exhibit anti-bacterial effects against gram-positive and gram-negative bacteria, mold, and yeast [11]. Pectin, another widely utilized polysaccharide in edible coating formulations, has been extensively studied but its application in grapes coating is limited. [6] and [12] conducted studies evaluating the efficacy of orange peel pectin and high methoxyl pectin-based coatings in extending the shelf life of grapes. Orange peel pectin and high methoxyl pectin are types of pectin known for their ability to form effective barriers against moisture loss and microbial spoilage on fruit surfaces. Building upon the above quoted work, who explored the efficacy of pectin coatings in fruit preservation, we investigated the potential of coffee husk-derived pectin (CHP) for this application. This approach aligns with the sustainable utilization of agricultural by-products and contributes to the development of eco-friendly strategies for prolonging fruit freshness and quality. Earlier, we optimized the pectin extraction protocol from coffee husk and evaluated its antibacterial activity in vitro, along with its anti-inflammatory properties using a cell line model [13]. The aim of this study was to evaluate the effectiveness of CHP supplemented with essential oil (Clove oil; CO) in extending the shelf life of grapes stored under ambient and cold conditions. Clove essential oil, derived from dried flower buds, finds wide application in fragrance, flavouring, and medical industries, known for its pain-relieving and healing properties when applied topically [14]. Eugenol, constituting over 70 % of clove oil, is its major component. Classified as "Generally Regarded As Safe" by the US FDA, clove oil exhibits diverse pharmacological benefits, including antioxidant, antifungal, anticarcinogenic, anesthetic, and antiprotozoal actions. Research highlights its antibacterial properties against various food-borne pathogens [15]. Incorporating essential oils into polysaccharide coatings is reported to enhance fruit preservation by providing natural antioxidant and antimicrobial properties, which help them to protect from spoilage and extend shelf life [16]. Strong antimicrobial properties against common pathogens were demonstrated by microbial cellulose-based coatings and films incorporating clove extract [17]. Furthermore, the combination of pectin and clove oil exhibits a synergistic effect, enhancing antimicrobial treatments by boosting their antibacterial and antifungal properties. Essential oils, with their active ingredients, enhance the bioactive qualities of pectin, potentially increasing its efficacy against bacteria. This combination holds promise for applications in food preservation, pharmaceuticals, and cosmetics, where antimicrobial and gelling properties are beneficial [18]. The need for sustainable and efficient postharvest preservation techniques for fresh fruits, such as grapes, which are particularly vulnerable to microbiological deterioration and quality loss during storage, is the rationale behind this research. Grapes are a rich source of phenolic compounds, which possess potent antioxidant properties. These compounds play an active role in mitigating the risks associated with cardiovascular and neurodegenerative diseases [19]. However, during postharvest storage, they are susceptible to infestation by various microorganisms which significantly deteriorate the fruit quality [20]. Coating grapes with edible coating materials can be viewed as an effective strategy to protect them from fruit deterioration. Synthetic chemicals used in conventional preservation techniques raise questions regarding food safety and environmental impact. Through the use of CO and CHP, an agricultural by-product, the research aims to develop a natural, environmentally acceptable edible coating that extends grape shelf life without compromising grape nutritional value or sensory appeal. In addition to addressing food waste, this strategy supports the sustainable use of agricultural wastes. The objectives of this research was to improve the shelf life of grapes by developing and optimizing an edible coating formulation that contained CO and CHP; and to compare the physicochemical, bioactive, antioxidant, and antimicrobial properties of coated versus uncoated grapes during ambient and cold storage conditions. Furthermore, in comparison to non-freeze-dried coatings, the study evaluated the effectiveness of the freeze-dried edible coating (FD-CHP-CO) in enhancing the barrier qualities and prolonging the shelf life of grapes, emphasizing its possible use in sustainable postharvest methods.

2 Materials and methods

2.1 Materials

The plant materials used in this study included coffee husks and grapes, both selected for their relevance to the research objectives. These materials were collected in full compliance with local and national regulations, with all required permits secured beforehand. Additionally, careful measures were taken to ensure the sustainable and responsible utilization of plant resources. The coffee husks, derived from Robusta coffee beans, were obtained from coffee processing plants in Chikmagalur, Karnataka, India (13.3143° N, 75.7710° E). The grapes were procured from a local supermarket, ensuring they were readily available and sourced from a supplier adhering to standard agricultural practices. Clove bud oil (98 % eugenol) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) was procured from Sigma-Aldrich (USA). Citric acid, phenolphthalein and ethanol were obtained from Sisco Research Laboratories (India). Other reagents and chemicals that were used in the study were of analytical grade.

2.2 Pectin extraction from coffee husk

Pectin from coffee husk was extracted under optimized conditions using microwave assisted reactor as published in our previous work [13]. Brief, coffee husk (2.5 g) suspended in citric acid solution (0.2 M, pH 1.5, 50 ml) was subjected to microwave assisted extraction for 15 min (microwave power of 450 W and temperature of 75 ºC). Later, pectin in the supernatant was precipitated using equal volume of 95 % chilled ethanol, which was further recovered and dried using hot air oven (50 ºC) until it attains a constant weight. The coffee husk pectin (CHP) yield of 40.2 % was obtained in the study.

2.3 Preparation of edible coating solution via antibacterial activity

The antibacterial activity of varying concentrations of CHP and CO against S. aureus MTCC 96 was evaluated using the disc diffusion method on Mueller–Hinton (MH) agar plates. CHP concentrations ranging from 0.5 % to 10 % and CO concentrations from 0.2 % to 5 % were tested [21]. A suspension of S. aureus MTCC 96 (10⁷ cells/ml) was evenly spread onto the MH agar. Wells (6 mm in diameter) were created using a sterile cork borer after the agar solidified, and 100 μl of sample were added to the respective wells. The plates were refrigerated for 30 min to allow the diffusion of the coating solutions into the agar [22], followed by incubation at 37 °C for 24 h. The antibacterial activity was assessed by measuring the zones of inhibition around each well, recorded in mm as mean ± SD. To evaluate synergistic effects, a combination of 2.5 % CHP and 2 % CO was selected based on the significant enhancement of the inhibition zone to 15 mm, indicating optimized antibacterial performance. The resulting solution was then homogenized for 5 min to ensure a well-mixed and uniform suspension of CHP and CO [23], providing a stable and effective formulation for further evaluation. To further assess the stability and functionality of the optimized edible coating solution, a portion of it was subjected to freeze-drying process using TFD5503 bench-top freeze-dryer (IlShin BioBase Co. Ltd., Korea).

2.4 Characterization of edible coating solution using Fourier transform infrared spectroscopy (FT-IR)

The characterization of the edible coating solutions was carried out using Fourier Transform Infrared Spectroscopy (FT-IR; BRUKER, Germany, Model Alpha) to analyze the functional groups present in CHP and FD-CHP-CO. The FT-IR analysis was conducted to assess the structural integrity of CHP and to detect any interactions between CHP and CO in the FD-CHP-CO formulation as described by [24]. The transmittance spectra were recorded within the wavenumber range of 4000 to 500 cm⁻1 with a resolution of 4 cm⁻1.

2.5 Application of Edible Coating Solution to Grapes

Grapes were coated with the edible coating solution by immersion technique. They were coated by dipping in solution for 5 min. Grapes dipped in distilled water were considered control and the control sample is designated as uncoated as it is coated solely with water. Uncoated and coated grapes were stored in sanitised perforated bags, kept at ambient (25 ± 1 °C) and under cold (4 ± 1 °C) storage conditions up to 14th d.

2.6 Evaluation of quality changes of coated grapes during storage

2.6.1 Weight loss

Weight loss in the grapes was determined by subtracting the final fruit weight from the initial weight, a method previously employed by [25]. The calculation was performed using the below formula.

$$\text{Weight loss }\left(\text{\%}\right)= \frac{\text{Initial weight}-\text{ Final weight }}{\text{Initial weight}}* 100$$

2.6.2 Grapes decay

The grapes decay was determined by counting the number of decayed fruits and dividing this by the total number of fruits. The result was then expressed as a percentage.

$$\text{Grapes Decay}(\text{\%})= \frac{\text{Total decayed grapes}}{\text{Total number of grapes}}*100$$

2.6.3 Colour

The L*, a*, and b* values of the fruit peel were measured during storage using a Hunter Colorimeter. The L* values indicate lightness, a* values represent the degree of redness or greenness, and b* values denote the degree of blueness or yellowness. ΔE values, representing the total colour difference, were also calculated to assess the overall colour change during storage [26].

2.6.4 pH and titratable acidity

The pH of the samples was measured using a digital pH meter (VSI-01, VSI Electronics Pvt. Ltd., Chandigarh, India) after proper calibration. Titratable acidity was determined by the titration method using 0.1 N NaOH solution and calculated using the below formula. The TSS/acid ratio of grapes was determined by dividing the value of the TSS by its titratable acidity.

$$\text{Titratable acidity }\left(\text{\%}\right)= \frac{\text{Titre value }*\text{ Normality of NaOH }*\text{ Total volume }*\text{ Acid factor }}{\text{Weight of the sample }*\text{ Volume of the filtrate taken}}*100$$

2.6.5 Total soluble solids

The total soluble solids (TSS) content of grapes was determined using a hand refractometer (Bombay Scientific ERMA Make, India). One drop of grapes extract was placed on the prism of the refractometer and the refractive index was expressed in °Brix.

2.6.6 Total phenol content

The total phenolic content was measured using the Folin-Ciocalteu method as described previously [27]. Briefly, 0.5 ml of the grape sample was mixed with 0.5 ml of Folin-Ciocalteu reagent (diluted 1:1 with distilled water) and incubated at 25 °C for 5 min. Briefly, 0.5 ml of;‘the grape sample was mixed with 0.5 ml of Folin-Ciocalteu reagent (diluted 1:1 with distilled water) and incubated at 25 °C for 5 min. Following incubation, 1 ml of 7.5 % Na₂CO₃ was added, and the mixture was adjusted to a final volume of 5 ml. The reaction mixture was then incubated for an additional 30 min at 25 °C. The absorbance of the resulting solution was measured at 765 nm. The total phenolic content (µg of gallic acid equivalents (GAE)/ml sample) was quantified using a gallic acid standard curve, represented by the equation: Y = 0.0095X—0.0315; R2 = 0.986, where Y denotes the absorbance at 765 nm, and X corresponds to the concentration of phenolics in the sample.

2.6.7 Total flavonoid content

The total flavonoid content was assessed using the aluminium chloride colorimetric method as described. Grapes (0.5 ml) was diluted using distilled water (0.5 ml), was treated with 0.3 ml of 5 % (m/v) sodium nitrite. After a 10 min of incubation, aluminium chloride (7.5 % w/v; 0.3 ml) was added to the mixture and further incubated for another 5 min at RT. The reaction was terminated with the addition of sodium hydroxide (4 % w/v; 2 ml) solution. The absorbance of the resulting solution was measured using UV-1800 Vis Spectrophotometer (Shimadzu, India) at 510 nm. A calibration curve was prepared using quercetin as a standard and the total flavonoid content was calculated as µg of quercetin equivalents (QE) per ml of sample, represented by the equation Y = 0.104X + 0.015; R2 = 0.992 [27].

2.6.8 In vitro antioxidant activity: 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical scavenging and potassium ferricyanide reducing power (PFRAP) assays

The method previously described by our research group was employed to determine DPPH radical scavenging activity and PFRAP of the samples [13, 28].

2.6.9 Microbiological population studies

The food safety indicators evaluated in the study were mesophilic aerobes, yeasts and molds. In brief, approximately 25 g of coated and uncoated grapes were aggregated in sterile bags using sterile physiological water at a 1:10 ratio and homogenized. Thereafter, the resulting solution (1 ml) was transferred to respective culture media on a petri plate [6]. The total bacterial count was determined on nutrient agar using the pour plate method [29]. Yeast and mold was enumerated using 2 % Malt Extract Agar [30]. The plates were then inverted and incubated at 37 °C for 48 h to allow bacterial colonies to develop while yeasts and molds populations were incubated at 25 °C for 5 d [31]. Results were expressed as log CFU/g for each treatment.

2.6.10 Antibacterial activity

The ability of coated and uncoated grapes extract to inhibit the growth of S. aureus MTCC 96 was evaluated as described. The LB broth supplemented with 10 % grapes extract was cultured with S. aureus MTCC 96, by incubating at 37 °C for 24 h. After the incubation period, the growth was measured using a spectrophotometer (UV-1800, Shimadzu, India) at 600 nm. The percentage of growth inhibition was calculated using the following formula.

$$\text{Percentage of Growth Inhibition }=1- \frac{\text{OD of Control }}{\text{OD of Sample}}*100$$

2.6.11 Statistical analysis

All experiments were conducted in triplicate. Data were reported as the mean ± standard deviation (SD). To determine statistical significance, we used one-way ANOVA followed by Dunnett's post-hoc test. The statistical analyses were conducted using InStat3 software, version 3.36 (GraphPad Software, Inc.).

3 Results and discussion

3.1 Preparation of edible coating solution via antibacterial activity

In vitro antibacterial activity of CHP and CO was performed using disc diffusion method against S. aureus MTCC 96 on Mueller–Hinton agar plates (Fig. 1a and b). The zones of inhibition on each plate with various concentrations of CHP and CO were examined for the inhibitory effect on growth of S. aureus MTCC 96. Antibacterial activity was evaluated against S. aureus as it is reported to be commonly found on the fruit surface and can further cause foodborne illness when contaminated grapes are consumed [32]. The diameter of zone of inhibition is presented in mm and represented as mean ± SD of duplicates. It can be clearly observed from the Fig. 1a and b that S. aureus MTCC 96 was susceptible to both CHP and CO in a very appreciable manner. Similar results are obtained by [33] and [34] for pectin and CO, respectively. From the results, it can be concluded that the antibacterial activity of these CHP and CO is dose dependent. Concentrations of CHP above 2.5 % and CO above 2 % exhibited a visible zone of inhibition. Specifically, a zone of inhibition with a diameter of 11 mm was observed for CHP at a concentration of 2.5 % (Fig. 1a), while a stringent zone was observed for CO at a concentration of 2 % (Fig. 1b). Despite observing an increase in the zone of inhibition with varying concentrations of CHP and CO, the concentration of 2.5 % CHP and 2 % CO was selected to test the synergistic improvement in the zone of inhibition. At the concentration of 2.5 % CHP and 2 % CO, the enhancement in the zone of inhibition was observed from an initial value of 11 to 15 mm. A part of the coating solution, comprising, 2.5 % CHP and 2 % CO, was freeze-dried to evaluate the effectiveness of the coating solution after the process. The CHP in the solution acts as a stabilizing and encapsulating agent for the CO, allowing the mixture to undergo lyophilization effectively [35]. Notably, the study by [36] showed that pectin extracted via hydrodynamic cavitation from waste lemon peel exhibited superior antibacterial activity against S. aureus, significantly outperforming commercial citrus pectin. This superior activity is attributed to the unique extraction and processing methods, which preserve bioactive compounds that enhance the pectin’s antibacterial properties. Similar to this findings, the antibacterial effect of CHP, particularly at concentrations above 2.5 %, showed a notable zone of inhibition, confirming the efficacy of natural pectin extracted under specialized conditions. The synergistic effect observed with 2.5 % CHP and 2 % CO, which increased the inhibition zone from 11 to 15 mm, further supports the enhanced antibacterial potential when pectin-based coatings are combined with essential oils. The ability of CHP to stabilize and encapsulate CO, thereby enhancing the antibacterial activity of the coating solution, further parallels the functionalization observed in IntegroPectin, which was enhanced with citrus-derived compounds [21].

Fig. 1
figure 1

a and b: In vitro Antibacterial Activity of (a) Coffee Husk Pectin (CHP) and (b) Clove Oil (CO)

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3.2 Characterization of edible coating solution using Fourier transform infrared spectroscopy (FT-IR)

The FTIR analysis for CHP and FD-CHP-CO solution reveals notable differences in their spectral profiles (Fig. 2), indicating successful encapsulation and interaction between the CHP and CO components. The broad peak around 3540 cm−1 can be attributed to the O–H stretching vibrations, indicating the presence of hydroxyl groups in the CHP [37]. The peak shifts slightly to 3454 cm−1 and 3541 cm−1, suggesting hydrogen bonding changes due to interactions between CHP and CO. The peak at 1678 cm−1 in CHP corresponds to C = O stretching of ester/carboxyl groups in CHP, which shifts to 1633 cm−1 and 1678 cm−1 in FD-CHP-CO, suggesting interactions between the two components. Peaks at 1518 cm−1 (CHP) and 1524 cm−1 (FD-CHP-CO) correspond to C = C stretching, indicative of aromatic rings in CO [38], confirming successful encapsulation. Peaks around 1164 cm−1 and 1169 cm−1 correspond to C-O stretching and O–H bending vibrations, confirming the polysaccharide nature of CHP in both spectra, indicating structural preservation post freeze-drying. Within the fingerprint region (1200–700 cm⁻1), characteristic peaks such as those at 943 cm⁻1 and 922 cm⁻1, corresponding to the deformation of glycosidic linkages, exhibit slight shifts, suggesting that the encapsulation process maintains the integrity of glycosidic bonds while allowing for interactions with CO components. Additionally, the peak at 773 cm⁻1, associated with C-H bending vibrations, shows minor shifts in FD-CHP-CO, reflecting subtle changes in the polysaccharide ring structures due to encapsulation. The peak at 746 cm⁻1, representing ring vibrations, further confirms the successful integration of CO, as indicated by its shift, signifying alterations in the pectin matrix during the freeze-drying process [39]. The shifts and intensity changes in these peaks suggest successful encapsulation and interaction between CHP and CO, highlighting the efficacy of CHP as a stabilizing agent in the freeze-drying process [35, 40]. These observations confirm that CHP not only serves as an effective encapsulation medium but also maintains its structural integrity while interacting with CO, ensuring the stability and functionality of the composite material.

Fig. 2
figure 2

FT-IR Analysis of Freeze-dried CHP and CO solution (FD-CHP-CO)

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3.3 Evaluation of quality changes of coated grapes during storage

In this study, grapes were coated using a dipping technique, where they were immersed in the edible coating solution for 5 min and then dried at RT. The visual assessments indicated that grapes coated with CHP-CO and FD-CHP-CO were better maintained than the uncoated during the storage period (Fig. 3a & b). It is clearly visible that the coated grapes looked better after 15 days of storage compared to the grapes stored at ambient (Fig. 3a) and at cold storage conditions, the improvement was even more pronounced (Fig. 3b). Similar results were observed for coating grapes using edible biodegradable protein films [5]. This preservation was observed in terms of retained colour, surface texture, and overall freshness of the grapes. Coating treatment viz., FD-CHP-CO, appeared to effectively mitigate visual deterioration that typically occurs during storage, suggesting enhanced preservation of fruit quality attributes over time. Our results are consistent with [41], who found that grapes coated with a combination of cassava starch, chitosan, and fermented extract residue exhibited superior preservation of visual appearance compared to other treatments by the 14th d of storage. The application of edible coating had a significant impact on the weight loss of coated fruits, effectively reducing the rate at which they lose moisture and thereby enhancing their overall shelf life and quality.

Fig. 3
figure 3

a and b: Visual Assessment: Impact of Storage Period (1st, 7th, and 14th d) on Uncoated and Coated Grapes Stored at (a) Ambient Storage Condition (b) Cold Storage Condition

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3.3.1 Weight loss

The Table 1 summarizes the weight loss of grapes under various treatments, stored at ambient and cold storage conditions over different intervals. The uncoated grapes showed significant weight loss over time, with a weight loss of 1.69 ± 0.24 % at 7th d, increasing to 3.89 ± 1.63 % ($p < 0.05) by the end of 14th d at ambient storage condition. The grapes coated with CHP-CO (**p < 0.001) and FD-CHP-CO (***p < 0.001) exhibited reduced weight loss significantly compared to the uncoated grapes, with 1.12 ± 0.12 % and 0.56 ± 0.11 % at 7th d and 1.72 ± 0.31 % and 0.92 ± 0.26 % at 14th d, respectively, at ambient condition. When stored at cold storage, the uncoated grapes exhibited lower weight loss, with 1.76 ± 0.37 % at 7th d, increasing significantly to 3.08 ± 0.35 % ($p < 0.05) at 14th d. The CHP-CO and FD-CHP-CO coated grapes showed reduced weight loss at cold storage condition, with 0.81 ± 0.16 % and 0.86 ± 0.53 % at 7th d and 2.35 ± 0.34 % (*p < 0.05) and 0.17 ± 0.14 % (***p < 0.001) at 14th d, respectively. The results indicate that both CHP-CO and FD-CHP-CO effectively reduce weight loss in fruits compared to the uncoated grapes, with the FD-CHP-CO coating being the most effective. This trend is consistent at both ambient and cold storage conditions, though weight loss is generally lower at the cooler temperature. The decrease in mass of grapes is linked to water loss through the surface of fruit slices [41]. In this process, the rate of water loss is influenced by the water pressure gradient within the fruit tissue and the surrounding atmosphere, along with the storage temperature [42]. The reduced weight loss in coated grapes can be attributed to the coatings ability to form a barrier that limits respiration and transpiration, thereby preserving the grapes moisture content [43]. Similarly, the shelf life and quality of strawberries were greatly increased by bio-nanocomposite edible coatings made with arrowroot starch, cellulose nanocrystals, carnauba wax nanoemulsion, and essential oils. These coatings prevented weight loss (2.6–3.9 %) and preserved the characteristics of the strawberries after harvesting while they were refrigerated [44].Furthermore, [42] assessed how fresh-cut oranges kept at 4 °C for 17 d would fare in terms of quality and shelf life when coated with gelatin-based edible coatings combined with Aloe vera gel and natural antioxidant extracts. In comparison to controls, the study indicated that coatings, particularly those containing 10 % green tea extract and 100 % aloe vera, considerably decreased quality parameter changes, minimized weight loss, and improved sensory qualities. These coatings prolonged the shelf life of fresh-cut oranges during cold storage by successfully delaying the growth of microorganisms. Strawberries coated with 1 % chitosan nanomaterial showed the lowest weight loss (3.85 %) compared to uncoated (13.65 %), extending shelf life from 2 to 6 d at 25 °C and 8 to 16 d at 6 °C, effectively preserving quality [45].

Table 1 Effect of Edible Coating on Weight Loss, Grapes Decay, pH, Titratable Acidity (TA) and Total Soluble Solids (TSS)
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3.3.2 Grapes decay

The decay percentage of grapes was monitored over a 14th d storage period under both storage conditions across different treatments (Uncoated, CHP-CO, and FD-CHP-CO). Under ambient storage condition, no decay was observed in any of the treatments (1st d). The decay rate for the uncoated grapes group was 15 ± 2.91 %. The CHP-CO coated grapes showed a lower decay rate of 8 ± 12.91 %, while no decay was observed in the FD-CHP-CO treatment (7th d). The trend continued to show significant differences by 14th d, with the uncoated grapes reaching a decay percentage of 52.5 ± 17.08 %. The CHP-CO coated grapes showed significantly lower decay percentage at 35 ± 4.91 % (*p < 0.05), and the FD-CHP-CO treatment exhibited the least decay at 27.5 ± 4.68 % (*p < 0.05). Similar to ambient storage condition, no decay was observed in any of the treatment groups until 7th d. At 14th d, uncoated grapes showed a decay rate of 20 ± 1.16 %. The decay was lower in the CHP-CO coated grapes at 15 ± 1.77 %, and the lowest decay was observed in the FD-CHP-CO coated grapes at 12.5 ± 2.05 % (*p < 0.05). The results indicate that the coating treatments, especially FD-CHP-CO, were effective in reducing grapes decay over time, with the most significant reductions observed at cold storage condition. [46] investigated the application of edible alginate coatings enriched with natural antimicrobials on strawberries using electrostatic spraying compared to conventional non-electrostatic spraying. Electrostatic spraying technology provided better coating efficiency, which led to significantly lower fruit decay (5.6 %) compared to non-electrostatic spraying (8.3 %) and uncoated controls (16.6 %) after 13 d of storage.

3.3.3 Colour

Colour plays a pivotal role in the perception of fruit quality. The colour parameters viz., L* (Lightness), a* (Red-Green Colour) and b* (Yellow-Blue Colour) of grapes coated with CHP-CO and FD-CHP-CO were significantly influenced by both the type of coating and storage temperature over the evaluated storage periods (Fig. 4a & b). At ambient storage condition, both CHP-CO and FD-CHP-CO coatings generally maintained higher L* values compared to the uncoated grapes, indicating better preservation of lightness (Fig. 4a). At cold storage condition, variations were observed across days, with FD-CHP-CO showing consistent or improved L* values compared to the uncoated and CHP-CO. Coatings showed varying effects on values (Fig. 4b). At ambient storage condition, both coatings showed minor shifts in a* values compared to the uncoated grapes, suggesting slight influences on red-green colour perception. Refrigerator storage generally resulted in more stable a* values, with FD-CHP-CO often showing minimal changes compared to the uncoated grapes and CHP-CO. FD-CHP-CO consistently maintained or improved b* values at both storage conditions, indicating enhanced preservation of yellow-blue colour tones compared to the uncoated grapes and CHP-CO. Both CHP-CO and FD-CHP-CO coatings demonstrated potential in preserving the grapes colour parameter during storage, with FD-CHP-CO often showing more consistent and improved results compared to CHP-CO and the uncoated grapes. The coating using FD-CHP-CO exhibited subtle shifts in colour values, suggesting a slowdown in respiration and senescence processes, thereby extending their shelf life [47]. These findings underscore the effectiveness of our freeze-dried coatings in maintaining fruit quality attributes such as colour, which are crucial for consumer acceptance and marketability. Furthermore, the total colour difference (∆E*) values highlight the extent of colour change in grapes during storage, with higher values indicating greater visual degradation. Under ambient storage, control grapes showed significant colour change (Table 2), with ∆E* increasing from 5.67 ± 0.34 on the 7th d to 8.76 ± 0.15 on the 14th d, indicating noticeable deterioration. In contrast, CHP-CO and FD-CHP-CO coatings effectively reduced colour change, with ∆E* values of 0.64 ± 0.17 and 3.42 ± 0.14 on the 7th d, and 6.26 ± 0.23 and 6.58 ± 0.36 on the 14th d, respectively, highlighting their ability to maintain the visual quality of grapes. At cold storage, the coatings were even more effective in preserving colour. Control grapes exhibited the highest ∆E* values, reaching 11.33 ± 0.24 by the 14th d, while CHP-CO and FD-CHP-CO coated grapes showed significantly lower ∆E* values of 1.29 ± 0.18 and 1.87 ± 0.30, respectively. These results demonstrate the superior protective effect of both coatings, particularly under cold conditions, in maintaining grape quality during storage. A research study by [48] found that the combined use of chitosan coating and argon-based modified atmosphere packaging effectively preserved the colour of fresh-cut cucumber by minimizing chlorophyll degradation, resulting in enhanced visual quality and extended shelf life during 12 d of storage at 5 °C. These findings support that polysaccharide-based coatings, are effective in retaining colour and overall quality in fresh produce. Further research could delve into the specific mechanisms by which these coatings interact with fruit surfaces to influence colour retention under different storage conditions.

Fig. 4
figure 4

Effect of Storage Period (1st, 7th, and 14th d) on Color Parameters (L, a, b*) of Uncoated and Coated Grapes Stored under (a) Ambient Storage Condition and (b) Cold Storage Condition

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Table 2 Effect of Edible Coating on Total Colour Difference (∆E*)
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3.3.4 pH and titratable acidity

The Table 1 represents the pH values of grapes under various treatments, stored at ambient and cold storage conditions over different intervals. The pH is an important indicator of fruit quality, as it reflects changes in acidity which can affect flavour and preservation. The pH values of grapes stored at ambient storage condition showed a clear trend across different treatments. Uncoated grapes exhibited a gradual increase in pH over the storage period, starting at 3.45 ± 0.01 on 1st d, rising to 3.67 ± 0.01 on 7th d, and reaching 3.86 ± 0.01 by 14th d. This indicates a decrease in acidity, likely due to ongoing metabolic processes and microbial activity leading to the breakdown of organic acids. In contrast, grapes coated with CHP-CO displayed a more stable pH, with values of 3.48 ± 0.01, 3.56 ± 0.01, and 3.63 ± 0.10 at days 1st, 7th, and 14th, respectively. The CHO-CO appeared to slow down metabolic and microbial activities, thereby better maintaining the grapes acidity. The FD-CHP-CO coated grapes showed the most stable pH values, with 3.35 ± 0.01 at day 1, 3.49 ± 0.01 at 7th d, and 3.46 ± 0.01 at 14th d, suggesting that the freeze-dried chitosan coating is particularly effective in preserving acidity by forming a more effective barrier against moisture loss and microbial invasion. Grapes stored at cold storage condition showed a different pattern, with generally lower pH values compared to ambient storage. The uncoated grapes had a pH of 3.54 ± 0.01 at 1st d, increasing to 3.76 ± 0.01 at 7th d and 3.91 ± 0.01 at 14th d. The lower temperature slowed down the metabolic processes and microbial activity, resulting in a slower rate of acidity loss. For CHP-CO coated grapes, the pH values were 3.60 ± 0.04, 3.69 ± 0.01, and 3.73 ± 0.01 at 1st, 7th and 14th d, respectively, indicating that the CHP-CO coating helped maintain pH stability, though there was a slight increase over time. The FD-CHP-CO coated grapes demonstrated the most consistent pH values at cold storage condition, with 3.34 ± 0.01 at 1st d, 3.38 ± 0.04 at 7th d, and 3.49 ± 0.01 at 14th d. This consistency highlights the effectiveness of the freeze-drying process, particularly at lower temperatures, in providing a robust barrier against changes in acidity. Low pH of fruits during storage particularly facilitates the infestation of molds and yeasts, causing maximum spoilage, because most of the bacteria are eliminated as they prefer a near-neutral pH [49]. These results align with previous studies, such as those by [41], underscoring the significant role of edible coatings in preserving fruit quality during storage by maintaining a stable pH and thereby reducing the susceptibility to spoilage caused by molds and yeasts. Additionally, [50] demonstrated that chitosan-incorporated edible coatings containing Mentha piperita and Mentha x villosa essential oils provided significant protection for table grapes against yeast and moulds. The study found that the use of these coatings resulted in a lower infection rate, with only 32–38 % of the fruit becoming infected. This further supports the effectiveness of edible coatings in reducing spoilage and maintaining the quality of stored fruits by inhibiting the growth of harmful microorganisms.

Citric, malic, and tartaric acids are the predominant acids found in grapes, that imparts acidity, flavour, and taste to the fruit [5]. The titratable acidity of grapes coated with CHP-CO and FD-CHP-CO evaluated under both ambient and cold storage conditions is presented in Table 1. The uncoated grapes showed a significant decrease in titratable acidity from 0.91 ± 0.09 % at day 1 to 0.42 ± 0.07 % ($p < 0.05) at 14th d, indicating natural changes in acidity during storage. Grapes coated with both CHP-CO and FD-CHP-CO coatings exhibited slightly different patterns in titratable acidity compared to the uncoated grapes. While variations were observed across storage intervals, both coatings generally maintained titratable acidity levels comparable to or slightly lower than those of the uncoated ones. At cold storage condition, there was a more pronounced decrease in titratable acidity in the uncoated group, with levels decreasing from 0.90 ± 0.08 % at 1st d to 0.48 ± 0.04 % at 14th d. Coated grapes generally exhibited stable titratable acidity levels compared to the uncoated grapes at cold storage condition. CHP-CO showed slight fluctuations, while FD-CHP-CO maintained more consistent acidity levels throughout the storage period. At the end of storage, less reduction in titratable acidity value of the coated grapes was due to utilization of the acids present in the fruits as substrate during reverse glycolysis/respiration [51]. This indicates that the coatings may have slowed down the metabolic processes that lead to acid degradation, thereby preserving the fruit's acidity better compared to the untreated fruits [52].

3.3.5 Total soluble solids

The TSS in fruits is generally attributed to the presence of sucrose, fructose, glucose, as well as other dissolved substances viz., proteins, minerals and organic acids [51]. The TSS content of both coated and uncoated grapes was periodically measured using the Brix value, which was determined through a refractometer (Table 1). The uncoated grapes showed a significant increase in TSS from 4.3 ± 0.83°Brix at day 1 to 13.7 ± 0.71°Brix ($$p < 0.001) at 14th d at ambient storage condition. This steady increase in TSS indicates ongoing metabolic processes and sugar accumulation, possibly attributed to moisture loss during storage [51]. The grapes coated with CHP-CO exhibited a moderate reduction in TSS accumulation compared to the uncoated grapes. TSS levels from 4.2 ± 0.91°Brix (1st d) was significantly increased to 9.9 ± 0.57°Brix (*p < 0.05) by 14th d. This suggests that the CHP-CO coating slowed down the metabolic activities responsible for sugar formation, thereby preserving fruit quality. Similar to CHP-CO, grapes coated with FD-CHP-CO also showed reduced TSS accumulation significantly over time (from 3.9 ± 1.12°Brix to 9.4 ± 0.31°Brix; *p < 0.05). This indicates effective preservation of TSS levels throughout the storage period. Under cold storage condition, the uncoated group exhibited higher initial TSS levels, 4.2 ± 0.41°Brix at 1st d increasing to 14.5 ± 0.78°Brix by 14th d ($p < 0.05). This rapid increase underscores the necessity for preservation techniques to mitigate sugar accumulation. However, grapes coated with CHP-CO showed an initial increase in TSS from 5.1 ± 0.39°Brix at 1st d, followed by a gradual decrease to 9.1 ± 0.61°Brix by 14th d (*p < 0.05). This suggests that the CHP-CO coating effectively slowed down metabolic processes and preserved fruit quality over the storage period. Grapes coated with FD-CHP-CO displayed a similar trend to CHP-CO, starting at 4.9 ± 0.75°Brix at 1st d and stabilizing around 8.7 ± 0.31°Brix by 14th d (*p < 0.05). This indicates that FD-CHP-CO also effectively mitigated TSS accumulation and maintained fruit quality during refrigerated storage. Both CHP-CO and FD-CHP-CO coatings demonstrated significant benefits in preserving the TSS content of grapes stored at both storage conditions. Our results are in line with [53] wherein pectin based edible coating delayed the increase of TSS of mangoes. These coatings effectively slowed down metabolic activities responsible for sugar formation, thereby maintaining fruit quality and extending shelf life. The findings highlight the potential of edible coatings as practical solutions for enhancing the post-harvest preservation of fruits, contributing to reduced food waste and improved consumer satisfaction. Further studies could explore the specific mechanisms by which these coatings influence metabolic processes in fruits, facilitating optimized applications in agricultural and food industries. The findings align with previous studies on various fruits coated with edible composite coatings, as observed in research on longan fruit [54], guava [55] and strawberry [26].

3.3.6 Total phenolic content

The application of edible coatings demonstrated better preservation of TPC during both storage conditions (Fig. 5a & b). Initially, the TPC was slightly higher across all treatments and showed a steady decline over time. In ambient storage condition (Fig. 5a), the uncoated grapes showed a significant reduction in TPC from 11.34 ± 0.41 µg GAE/ml (1st d) to 6.12 ± 0.32 µg GAE/ml on 14th d ($$p < 0.001). Grapes coated with CHP-CO exhibited better TPC preservation, decreasing from 11.47 ± 0.22 µg GAE/ml to 8.83 ± 0.31 µg GAE/ml over the same period. The FD-CHP-CO coated grapes had the highest TPC retention, reducing from 11.57 ± 0.24 µg GAE/ml to 10.02 ± 0.19 µg GAE/ml. Under refrigerated condition (Fig. 5b), the uncoated grapes showed a decrease in TPC from 10.04 ± 0.12 µg GAE/ml on 1st d to 6.84 ± 0.36 µg GAE/ml on 14th d. Grapes coated with CHP-CO showed a better retention, decreasing from 9.99 ± 0.43 µg GAE/ml to 8.11 ± 0.31 µg GAE/ml. The FD-CHP-CO coated grapes again demonstrated the best preservation, with TPC decreasing from 10.14 ± 0.27 µg GAE/ml to 8.45 ± 0.19 µg GAE/ml over the 14th d. The TPC in both storage conditions indicates that the coatings effectively slow down the degradation of phenolics. Total phenols are significant secondary metabolites contributing to the antioxidant activities in fruits [56]. The observed decrease in phenol content could be attributed to the activity of polyphenol oxidase (PPO) and peroxidase (POD) enzymes, which oxidize phenolic compounds [57]. Additionally, [58] noted that total phenols degrade due to higher respiration rates, especially in climacteric fruits. The superior phenolic retention in coated fruits might be due to the protective barrier provided by the coatings, which reduces the oxidative processes and respiration rates that break down phenolics. [59] suggested that coatings might promote the accumulation of phenolic compounds through the activation of phenylalanine ammonia-lyase (PAL) enzyme. Similar findings were reported by [60] in mandarin fruits and by other studies on strawberries [46, 61], sweet cherries [62], and guava [1]. The FD-CHP-CO coating was the most effective in preserving the TPC of grapes during both storage conditions, highlighting the potential of using CHP-based coatings to extend the shelf life and maintain the nutritional quality of fresh produce. The study indicates that such coatings can effectively slow down the degradation of phenolics, contributing to the extended freshness and antioxidant properties of the grapes. The impact of carnauba and mineral oil coatings on TPC in tomatoes stored for 28 d at 10 °C was evaluated. For breaker tomatoes, TPC were 20.22, 18.22, and 18.74 mg GAE/100 g for control, mineral oil, and carnauba, respectively. Pink tomatoes showed TPC of 19.15, 18.65, and 17.37 mg GAE/100 g, with notable differences between carnauba and control. The coatings effectively preserved phenolic content and overall quality, though coated fruits showed slight reductions compared to uncoated samples [63].

Fig. 5
figure 5

a and b: Effect of Storage Period (1st, 7th, and 14th d) on Total Phenolic Content (TPC) of Uncoated and Coated Grapes Stored under (a) Ambient Storage Condition and (b) Cold Storage Condition

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3.3.7 Total flavonoid content

The Fig. 6a & b illustrates the TFC at ambient and cold storage condition for the different treatments (Uncoated, CHP-CO, FD-CHP-CO). The TFC for the uncoated grapes decreased slightly from 18.91 ± 0.92 µg QE/ml on 1st d to 6.89 ± 0.56 µg QE/ml on 14th d. This decline suggests a gradual degradation or loss of flavonoids over time at ambient storage condition (Fig. 6a). In contrast, CHP-CO coated grapes exhibited a slower decline, with flavonoid levels decreasing from 17.8 ± 0.52 µg QE/ml to 12.34 ± 0.47 µg QE /g over the same period. This suggests that the pectin-clove oil coating helped preserve flavonoid content better than the uncoated grapes. Notably, FD-CHP-CO coated grapes retained the highest levels of flavonoids, dropping only from 17.51 ± 0.89 µg QE/ml at 1st d to 14.67 ± 0.65 µg QE/ml by 14th d. The freeze-drying process appears to have further enhanced the protective effect of the coating, creating a more effective barrier against oxidative degradation and microbial activity. At cold storage (Fig. 6b), the trends were similar but less pronounced. The uncoated grapes again showed a marked reduction in flavonoids, from 15.45 ± 0.53 µg QE/ml at 1st d to 7 ± 0.47 µg QE/ml at 14th d. CHP-CO coated grapes showed a lesser decline, from 15.51 ± 0.25 µg QE/ml to 8.65 ± 0.68 µg QE/ml. FD-CHP-CO coated grapes demonstrated the best preservation, with flavonoid content decreasing only from 14.74 ± 0.34 µg QE/ml to 9.98 ± 0.31 µg QE/ml over 14th d. These results indicate that both CHP-CO and FD-CHP-CO coatings are effective in preserving the flavonoid content of grapes, with FD-CHP-CO showing superior performance. The freeze-drying process likely enhances the barrier properties of the coating, offering improved protection against factors that contribute to flavonoid degradation. This preservation of bioactive compounds is crucial for maintaining the antioxidant properties of grapes, thereby extending their shelf life and nutritional value. Similar effects have been observed in other studies involving edible coatings or instance, coatings made from gum arabic enriched with cinnamon essential oil were effective in maintaining the quality and bioactive content of guava fruits during storage [32]. Furthermore, alginate-based edible coatings enriched with eugenol and citral were effective in preserving the bioactive compounds, including flavonoids, in Arbutus unedo fruits during storage. Coatings containing 1 % alginate with eugenol (0.20 %) and citral (0.15 %) maintained higher levels of flavonoids, contributing to enhanced antioxidant activity and improved nutritional quality compared to uncoated fruits, highlighting their potential in extending the shelf life of fresh fruits by protecting key phytochemicals [64].These findings highlight the effectiveness of composite edible coatings in maintaining the nutritional quality of fruits, particularly in preserving bioactive content. The results of our study and those from similar research underscore the potential of such coatings to enhance the shelf life and quality of perishable fruits, thereby reducing postharvest losses and ensuring better consumer satisfaction.

Fig. 6
figure 6

a and b: Effect of Storage Period (1st, 7th, and 14th d) on Total Flavonoid Content (TFC) of Uncoated and Coated Grapes Stored under (a) Ambient Storage Condition and (b) Cold Storage Condition

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3.3.8 In vitro Antioxidant activity: 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical scavenging and potassium ferricyanide reducing power (PFRAP) Assays

The analysis of DPPH inhibition over time for grapes coated with CHP-CO and FD-CHP-CO provides valuable insights into the antioxidant preservation efficacy of these coatings (Fig. 7a & b). On 1st d, all samples (Uncoated, CHP-CO, and FD-CHP-CO) exhibited high levels of DPPH inhibition, indicating strong antioxidant activity. However, as storage time increased, the antioxidant activity decreased across all samples, with the uncoated grapes experiencing the most significant decline. Under ambient storage condition (Fig. 7a), the uncoated grapes DPPH inhibition dropped from 87.23 ± 0.98 % on 1st d to 55.31 ± 1.25 % by 14th d. In contrast, the CHP-CO and FD-CHP-CO coatings significantly mitigated this decline. By 14th d, CHP-CO coated grapes retained 80.92 ± 1.37 % DPPH inhibition, while FD-CHP-CO coated grapes maintained 83.76 ± 0.86 %, demonstrating the superior antioxidant preservation of the freeze-dried coating. Under refrigerated condition (Fig. 7b), the trend was similar. The uncoated grapes DPPH inhibition decreased from 88.51 ± 0.35 % on 1st d to 62.19 ± 1.25 % by 14th d. CHP-CO and FD-CHP-CO coatings again showed better preservation, with 79.35 ± 1.06 % and 80.35 ± 0.58 % DPPH inhibition, respectively, on 14th d. This indicates that both coatings are effective in maintaining antioxidant activity, with FD-CHP-CO showing slightly better performance.

Fig. 7
figure 7

a and b: Effect of Storage Period (1st, 7th, and 14th d) on DPPH Inhibition ( %) of Uncoated and Coated Grapes Stored (a) Ambient Storage Condition and (b) Cold Storage Condition

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The PFRAP assay results demonstrate the antioxidant activity of CHP-CO and FD-CHP-CO coating on grapes compared to the uncoated across different conditions (Fig. 8a & b). The assay measures the reduction of the Fe3⁺-ferricyanide complex over time, where a lower absorbance indicates higher antioxidant capacity [13]. The results indicate that CHP-CO and FD-CHP-CO, demonstrates notable antioxidant activity as evidenced by its ability to reduce the Fe3⁺-ferricyanide complex, measured by absorbance at 700 nm (Fig. 8a). Across the storage days evaluated (1st, 7th, and 14th), all formulations-uncoated, CHP-CO, and FD-CHP-CO showed a trend of decreasing absorbance, suggesting a gradual decline in antioxidant capacity over time. This decrease is likely due to the natural degradation of antioxidants present in grapes as they age. However, the CHP-CO and FD-CHP-CO coated grapes consistently exhibit higher absorbance compared to the uncoated grapes, indicating their sustained antioxidant efficacy even after 14th d of storage (Fig. 8b). The slight variation in absorbance values over the storage period indicates a stable antioxidant activity maintained by the CHP-CO and FD-CHP-CO coatings. The absorbance values for FD-CHP-CO coated grapes were generally comparable to or slightly higher than those for CHP-CO coated ones, indicating that the freeze-drying formulations maintained the antioxidant capacity of the grapes more effectively during the storage period. The study by [65] demonstrated the effectiveness of zein nanofiber films loaded with thyme essential oil to preserve the antioxidant activity of strawberries during 15 d of storage at 4 °C. The encapsulated thyme essential oil in the nanofiber films maintained higher levels of antioxidant properties compared to the control, highlighting the potential of essential oil films to effectively reduce oxidative deterioration and enhance the antioxidant stability of strawberries during storage.

Fig. 8
figure 8

a and b: Effect of Storage Period (1st, 7th, and 14th d) on Potassium Ferricyanide Reducing Power (PFRAP) assay of Uncoated and Coated Grapes Stored under (a) Ambient Storage Condition and (b) Cold Storage Condition

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3.3.9 Microbiological population studies

Throughout the storage period under ambient and cold storage conditions, coated and uncoated grapes consistently showed no counts of bacteria. The absence of bacterial growth suggests that the initial sanitization effectively removed or inhibited bacterial contaminants, and the controlled storage conditions prevented bacterial proliferation. However, significant changes in yeast and mold were observed (Fig. 9a & b). In both storage conditions, the uncoated group exhibited a substantial increase in yeast and mold counts, rising from approximately 200 ± 13.50 CFU/ml to about 456 ± 19.27 CFU/ml in ambient storage and to 350 ± 14.05 CFU/ml in cold storage over a 14 d period. This indicates that uncoated grapes are highly susceptible to mold and yeast growth, emphasizing the necessity of effective preservation methods. Conversely, grapes coated with CHP-CO showed a significantly slower increase in yeast and mold counts in both storage conditions. This suggests that the antimicrobial properties of clove oil, combined with the stabilizing effect of coffee husk pectin, effectively inhibit microbial growth. The FD-CHP-CO coated grapes demonstrated the most effective inhibition, with yeast and mold counts after 14th d of storage in both conditions. The freeze-drying process likely enhances the stability and efficacy of the antimicrobial properties, providing a stronger barrier against microbial contamination. Our data clearly indicate that the coatings containing CHP and CO, particularly when freeze-dried, significantly reduce yeast and mold growth on grapes, thereby extending their shelf life. This efficacy is evident under both ambient and refrigerated storage conditions, highlighting the potential of these coatings for use in preserving fresh produce. Our results are line with [7] wherein edible coatings made of alginate and essential oils of lemon or orange effectively extended the shelf life of red raspberries by preventing the growth of aerobic mesophilic bacteria, mold, and yeast throughout a 14 d period of storage. The fruit's post-harvest quality was preserved by the coatings without appreciably changing important quality measures, indicating the potential of essential oils to prolong the freshness and security of raspberries. While the use of mint essential oil did not yield any further antifungal effects, [66] showed that chitosan coating was an efficient means of inhibiting the growth of Colletotrichum gloeosporioides and reducing the severity of anthracnose in papaya, hence maintaining postharvest quality. The study by [67] found that enriching the chitosan coating with poppy seed phenolic extract significantly enhanced the quality of fresh-cut fruit salads. The addition of phenolics not only improved sensory attributes like colour and taste but also helped in maintaining antioxidant properties, reducing browning, and enhancing microbial safety during storage. This suggests that phenolic-infused coatings are effective in preserving the nutritional and visual quality of fresh-cut fruits, making them more appealing and longer-lasting. Similarly, our CHP and CO based edible coating effectively retained higher phenolic content, which contributed not only to improved antioxidant capacity but also to a significant reduction in microbial load, underscoring the dual role of phenolics in preserving both the nutritional quality and microbial safety of fresh produce during storage.

Fig. 9
figure 9

a and b: Effect of Storage Period (1st, 7th, and 14th d) on Yeast and Mold Counts in Uncoated and Coated Grapes Stored under (a) Ambient Storage Condition and (b) Cold Storage Condition

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3.3.10 Antibacterial activity

The antibacterial activity of uncoated and coated (CHP-CO and FD-CHP-CO) grapes extract against S. aureus MTCC 96 was evaluated over different storage periods at ambient and cold storage conditions. The results, expressed as percentage growth inhibition, reveal the effectiveness of these coatings in inhibiting bacterial growth over time (Fig. 10a & b). At ambient storage, initial (1st d) growth inhibition for the uncoated grapes was 72.76 ± 0.89 %, while CHP-CO and FD-CHP-CO coatings showed slight improvements at 74.36 ± 1.45 % and 79.06 ± 0.56 %, respectively. These results align with previous studies indicating that edible coatings can enhance the antibacterial properties of fruit extracts by providing a protective barrier and maintaining the stability of active compounds [68]. By day 7, the uncoated grapes inhibition decreased to 42.06 ± 0.77 %, whereas CHP-CO and FD-CHP-CO exhibited higher inhibitions of 63.21 ± 1.19 % and 65.82 ± 0.67 %. At the end of the storage period, the uncoated grapes inhibition further dropped to 15.33 ± 0.67 %, while CHP-CO and FD-CHP-CO maintained substantial antibacterial activity at 43.08 ± 0.86 % and 47.69 ± 0.47 %. This sustained antibacterial activity can be attributed to the coatings’ ability to slow down the degradation of bioactive compounds, as reported in similar studies [69]. At cold storage, initial growth inhibition for the uncoated group was 70.43 ± 0.89 %, with CHP-CO and FD-CHP-CO showing similar inhibitions at 72.78 ± 1.19 % and 71.25 ± 1.25 %. By 7th d, the uncoated grapes inhibition decreased to 48.05 ± 0.77 %, while CHP-CO and FD-CHP-CO exhibited higher inhibitions of 65.52 ± 1.25 % and 67.05 ± 1.05 %. On 14th d, the uncoated grapes inhibition further dropped to 21.59 ± 0.67 %, with CHP-CO and FD-CHP-CO showing substantial long-term antibacterial activity at 50.82 ± 0.58 % and 58.45 ± 1.57 %. Both CHP-CO and FD-CHP-CO coatings significantly enhanced the antibacterial activity of grapes extracts against S. aureus MTCC 96, with FD-CHP-CO consistently showing superior long-term efficacy at both ambient and cold storage conditions, suggesting that the freeze-drying process effectively preserves and stabilizes the active antibacterial components. These findings highlight the potential of both CHP-CO and FD-CHP-CO coatings in extending the shelf life and ensuring the microbial safety of grapes and potentially other perishable food items. In line with this, soy-protein edible coatings with 3 % thyme and oregano essential oils demonstrated significant antibacterial activity against S. aureus on fresh beef, achieving reductions of 2.86 and 2.59 log CFU/g, respectively, after 14th d of refrigerated storage. This highlights the potential of essential oil-enriched coatings in effectively controlling S. aureus and improving the microbial safety of meat products [70]. The antimicrobial effects of ginger essential oil-fortified edible coatings on S. aureus in Kashar cheese. Coatings were made using whey protein isolate, alginate, sorbitol, and 1.5 % ginger essential oil. During 30 d of storage at 4 °C, S. aureus levels increased in the uncoated control samples, while a significant reduction was observed in the coated samples. This demonstrates that the ginger essential oil-fortified coating effectively inhibits the growth of S. aureus, highlighting its potential as an antimicrobial strategy for improving the microbial safety of cheese products. Further research could explore the underlying mechanisms and broaden the scope of application to different types of microorganisms and food products [71].

Fig. 10
figure 10

a and b: Growth Inhibition ( %) of S. aureus MTCC 96 by Uncoated and Coated Grapes under (a) Ambient Storage Condition and (b) Cold Storage Condition

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4 Conclusion

The study successfully demonstrated the effectiveness of CHP-CO and FD-CHP-CO edible coatings in preserving the quality and extending the shelf life of grapes. The antibacterial activity of these coatings, particularly at concentrations of 2.5 % CHP and 2 % CO, effectively inhibited the growth of S. aureus MTCC 96, with a significant inhibition zone increase from 11 to 15 mm, suggesting their potential in food safety applications. The application of CHP-CO and FD-CHP-CO coatings significantly mitigated the visual deterioration of grapes stored at both ambient and cold storage conditions. This was evident through the preservation of colour, surface texture, and overall freshness. Notably, FD-CHP-CO coating exhibited superior results, maintaining better lightness (L* values) and colour stability (a* and b* values) over time compared to uncoated and CHP-CO coated grapes.

Physicochemical parameters such as weight loss, pH, total soluble solids (TSS), and titratable acidity (TA) were better maintained in coated grapes. The FD-CHP-CO coating was particularly effective in reducing weight loss by up to 76 % at ambient storage conditions (0.92 ± 0.26 % at 14th d compared to 3.89 ± 1.63 % in uncoated grapes). It also stabilized pH, TSS, and TA levels, highlighting its efficacy in slowing down metabolic and microbial activities that lead to fruit deterioration. The study also highlighted the coatings’ role in preserving bioactive compounds, with both CHP-CO and FD-CHP-CO coatings effectively maintaining higher levels of TPC and TFC compared to the uncoated. The FD-CHP-CO coating showed the most significant retention of these compounds, with TPC retention of 86.9 % and TFC retention of 83.7 % compared to initial values, indicating its potential in preserving the nutritional and antioxidant properties of grapes during storage. Overall, our findings underscore the potential of CHP based edible coatings, especially those incorporating essential oils and freeze-drying techniques, in enhancing the post-harvest quality and shelf life of fresh produce. Future studies could explore the specific mechanisms through which these coatings influence metabolic processes and interact with fruit surfaces, aiming to optimize their applications in the agricultural and food industries.