Postharvest Treatment Effects on ‘Somerset Seedless’ Cold-Hardy Table Grapes

ABSTRACT

Limited amount of information is available for cold-hardy table grape postharvest storage and strategies to extend the storage time. ‘Somerset Seedless’ is a cold-hardy table grape with a potential market for the Upper Midwest and Northern Great Plains. Postharvest treatments were assessed as a possible route to increase cold-hardy table grape shelf-life. In this study, a 1.4% chitosan postharvest treatment was tested on ‘Somerset Seedless’ grapes through 7 weeks of storage with temperature 1–4°C, humidity ≥90%. The effects were compared to two controls: one was diH₂O, and the other one was acetic acid. The assessment included grape appearance traits (rachis, decay, mold, scattering, and splitting), physiochemical properties (TSS, pH and total acidity), antioxidant activity, as well as total phenolics and flavonoid content. In general, 1.4% chitosan, 1% acetic acid, and diH₂O had a large impact on grape appearance during storage with limited impact on chemistry. Throughout 7 weeks of storage, no significant differences were observed in grape physiochemical and phytochemical changes. Chitosan performed the best for ‘Somerset Seedless’ postharvest storage in regard to the low rate of visible damage. Although acetic acid had similarly positive effects on mold control as chitosan treatment, it caused the highest split rate after 1 week and the highest shatter rate after 5 weeks of storage. Chitosan treated ‘Somerset Seedless’ still met the USDA standard of table grapes after 5 weeks. This study suggests chitosan postharvest treatments may have applications for enhancing the shelf-life of cold-hardy table grapes.

KEYWORDS:

• ‘Somerset seedless’
• chitosan
• acetic acid
• postharvest storage
• cold-hardy grape

Introduction

‘Somerset Seedless’ (ES 12–7) was developed by Elmer Swenson in Wisconsin and is a popular seedless table grape for cold climates. Due to its advantages (firm texture, sweet-tasty flavor, decent size of clusters, and acceptable production), it has the potential to be a cold-hardy table grape cultivar (Treiber et al., 2022). A key question for local producers is how to maintain fruit quality in storage and increase shelf-life. The common problems for table grapes during storage include weight loss, berry drop from clusters, decay, mold growth, split berries, and discoloration of the rachis (Raquel et al., 2016). Minnesota researchers recently evaluated multiple cold-hardy table grapes for postharvest storage with sulfur dioxide (SO₂) pads based on the evaluation of fruit quality during storage (Treiber et al., 2022). Beyond this research, minimal efforts have focused on cold-hardy table grape postharvest treatments.

For fruits, many strategies have been developed to maintain fruit postharvest quality, such as cold storage, controlled atmosphere, ethanol treatment, sulfur dioxide (SO₂) fumigation, hypobaric treatment, and more. Each strategy for postharvest storage enhancement has inherent issues. Fumigation via SO₂ is the standard method to control mold during postharvest handling and storage; however, this can cause quality issues and irritation problems for consumers. Both fungicides and active ingredients, such as SO₂, are not allowed in certain countries or based on certain food requirements. Controlled atmosphere storage has certain limitations, due to the contribution of physiological disorders, irregular ripening, development of off flavors or off odors at low oxygen concentration, and high cost. Ethanol treatment usually effectively reduces some fruit and vegetable decays but may cause certain fruit color changes and ethanol residues (FM et al., 2005). Hypobaric treatment as an option accelerated the outward diffusion of gasses in fruit tissues by increasing the external and internal differential pressures, but this method must be optimized for treatment time length and frequency (Huan et al., 2021). Beyond these techniques, a low-cost method for local storage of table grapes could expand market potential and sales for small farmers.

Chitosan is a natural biopolymer composed of randomly distributed β-(1→4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). It is highly valued by academia and industry in food application due to its special properties, such as biodegradable, nontoxic, biocompatible and film-forming (Chen et al., 2023). It has also been used for tissue culture to increase the primary and secondary metabolite production (Badiali et al., 2018). As a natural coating material, chitosan biopolymer, through chitin deacetylation, can be used for both preharvest and postharvest application. It has been reported that chitosan has multiple benefits for postharvest storage. As a plant defense elicitor, it had been used before as well as after harvest to induce fruit resistance of pathogen attack and to accumulate defense gene products (Lucini et al., 2018). Chitosan also played an important role in antimicrobial activity, changing the hyphal morphology of some fungal species and cause cellular leakage in Botrytis cinerea and Rhizopus stolonifer, which might explain some disease control by chitosan (Junior et al., 2012). Chitosan disrupts the cell wall and plasma membrane of Penicillium expansum, which might explain some reasons for reducing blue mold (Wang et al., 2014). The function of preventing water loss stems from chitosan biofilm formation (Gianfranco Romanazzi et al., 2018). Chitosan has also been observed to reduce senescence and cell wall modification in citruses (Chen et al., 2021).

For table grapes, water loss and gray mold are the main problems during postharvest handling and storage. Gray mold (Botrytis cinerea) is the most aggressive postharvest disease even at low temperatures. Chitosan treatment elicited defense mechanisms, indicated by pentacyclic triterpenoids and stilbene accumulation in grape (Vitis vinifera L.) bunches (Lucini et al., 2018). Research on the table grape ‘El-Bayadi’ with different concentrations of chitosan indicated weight loss reduction at 1% chitosan compared to the control treatment (Al-Qurashi and Awad, 2015). ‘Sagrantino’ wine grapes were examined under partially dehydrated conditions and the activity of antioxidant enzymes was enhanced by chitosan treatment (Petriccione et al., 2018). Chitosan-treated fruits may also have some changes related to slow ripening, such as increased titratable acidity, delayed changes in pH and soluble solids, reduced ethylene, changed enzymatic activities and non-enzymatic activities (such as antioxidants levels). Postharvest chitosan treatment was used to manage decay and basic fruit physiochemical quality maintenance of table grapes (Meng et al., 2010).

Prior research noted chitosan treatments can influence table grape antioxidant capacity, antioxidant compounds, and enzyme activities (Al-Qurashi and Awad, 2015). Chitosan also can be combined with other treatments, such as ethanol, to prolong cold storage of table grapes, including ‘Autumn Seedless’ and ‘Thompson Seedless’ (Gianfranco Romanazzi et al., 2018). Chitosan glucose complex, a modified form of chitosan with heating chitosan with glucose, had been tested on ‘Muscat Hamburg,’ maintaining its quality and extending its shelf-life (Gao et al., 2013). Integrated with new technology, chitosan has been tested in the form of nanoparticles to delay table grape ripening processes (Melo et al., 2018).

Since cold-hardy grapes are mainly interspecific hybrids with variation genetic contributions from V. vinifera with V. labrusca and V. riparia, and other Vitis spp., their chemical compositions are typically distinctly different from common table grapes cultivars (predominantly V. vinifera) which may contribute to differences in their storage and processing procedures (Pedneault et al., 2013). The postharvest strategies for table grapes have yet to be optimized for cold-hardy grapes. Research on postharvest evaluation of cold-hardy table grapes is limited and new methods have not been explored in cold-hardy table grapes (Treiber et al., 2022). Most cold-hardy grape production is conducted on a relatively small scale when compared to traditional table grape production in the United States. Research is necessary to define and refine postharvest treatments that are applicable and accessible for producers working toward filling local market needs. In this research, the exploration of chitosan postharvest treatment on ‘Somerset Seedless’ table grapes was compared with a diH₂O and acetic acid treatment through 7 weeks of postharvest storage. This research aimed to test the effects of a chitosan coating, an accessible technique for local producers, for its capacity to extend the shelf-life of cold-hardy grapes. The primary goal of this work was to determine chitosan coating effects on ‘Somerset Seedless’ grape shelf life and qualities. The findings not only provide the potential applicability of chitosan coating on ‘Somerset Seedless’ grape postharvest storage but also presents important information that can be used in the future for the investigation of cold-hardy grape postharvest storage.

Materials and Methods

Plant Materials

Grapes ‘Somerset Seedless’ were grown at Western Agricultural Research Center (Corvallis, MT) of Montana State University. Nine grapevines in three rows were spaced 1.0 m within rows and 3.0 m between rows. ‘Somerset Seedless’ were trained on a top wire, high-cordon system during growing season. Before harvesting, the general physiochemical characteristics (Total Soluble Solids [TSS], degrees Brix >21) were measured to ascertain the grapes’ ripeness. General cluster traits were evaluated and approximately 120 clusters were hand-harvested from three consistent, evenly ripened grapevines (40 clusters per vine) on Sept. 21st, 2022. Clusters were stored overnight in a walk-in cooler with temperature 1–4°C, humidity ≥90% before postharvest treatment. HOBO temperature/RH data logger (MX2301A, HOBO, MA) was used to monitor the temperature and humidity. Postharvest treatments were applied on Sept. 22nd, 2022.

Postharvest Coating Treatment

Ninety-nine clusters were randomly selected from the harvested clusters. At the time of harvest, nine clusters were destructively sampled for fruit quality traits, including cluster weight, TSS, pH and total acidity. The remaining 90 clusters were randomly partitioned into three groups for treatment by diH₂O, 1% (w/v) acetic acid, and 1.4% chitosan solutions. Due to the chitosan viscoelastic behavior, 1.4% (w/v) of chitosan was chosen for the preparation of coating solution concentration (Patois et al. 2009). The chitosan (Sigma-Aldrich, USA) coating solution were prepared by dissolving 1.4% of chitosan in 1% (w/v) aqueous acetic acid (Sigma-Aldrich, USA) overnight at room temperature for complete dissolution with consistent stirring using a magnetic hotplate stirrer (Slender SH-2, US). Thirty grape clusters were completely immersed in the chitosan solution for 15 s, drained, and air dried at room temperature for 4 hours. One percent (w/v) acetic acid was used as the solvent for the chitosan solution and was set as one of the control treatments. Thirty clusters treated with acetic acid solution (Control 1) or diH₂O (Control 2) went through the same processes as the chitosan treatment. After air drying, clusters were marked with treatment and packed in transparent clamshells. All the clam shells were completely randomized for storage on shelves for postharvest experimental study in a walk-in cooler with temperature 1–4°C, and humidity ≥90%.

Grape Appearance Analysis

Grape clusters were stored in clamshells and removed at 1, 3, 5, 7 weeks after treatment. At each time point, six clusters were destructively sampled and evaluated for rachis index, split rate, decay rate, shatter rate, and mold rate. Rachis index values were scaled 1–5 based on rachis browning rate (Lichter et al., 2011). In brief, rachis index was evaluated by the dehydration and browning symptoms for primary/secondary branches, where 1 = absence of these symptoms, 2 = slight occurrence, 3 = moderate occurrence, 4 = severe symptoms, and 5 = extremely severe browning and dehydration. The percentage (%) of berry split rate, decay rate, shatter rate, and mold rate were calculated in each cluster when the appearance evaluation was conducted at each sampling time.

Physicochemical Property Analysis

Grapes from week 0, 1, 3, 5, 7 were collected and cluster weight loss was calculated as: Weight loss percentage were calculated as Weight loss (%) per cluster = $\frac{\mathit{Weight}\mathit{of}\mathit{fresh}\mathit{treated}\mathit{fruits}\left(\mathit{g}\right)-\mathit{Weight}\mathit{after}\mathit{storage}\left(\mathit{g}\right)}{\mathit{Weight}\mathit{of}\mathit{fresh}\mathit{treated}\mathit{fruits}\left(\mathit{g}\right)}\times 100\mathrm{\%}$.

Grapes were then crushed manually, and grape juices were collected for basic physiochemical properties, including pH, total soluble solids (TSS, °Brix) and total acidity (TA). The total soluble solid of the grapes was determined using an ATAGO PAL-1 digital pocket refractometer (ATAGO, Tokyo, Japan) and results were represented as ºBrix. Total acidity content of the grapes was assessed via dilution of juice into 50 times diH₂O and was subsequently measured using a PAL-BX|ACID2 meter (ATAGO, Tokyo, Japan) with the results as % tartaric acid. The pH of the grapes was measured by the PAL-pH meter (ATAGO, Tokyo, Japan).

Preparation of Extracts

The methanol extraction method was used to extract samples with some modifications (Kim et al., 2003). All grapes from each cluster at each collection event were ground into fine powder in liquid nitrogen by IKA A11 basic analytical mill (IKA, US). One gram of each cluster powder was mixed with 5 mL ice-cold 95% methanol (v/v) and homogenized for 2 minutes by a Vortex mixer (Scientific Industries, US) at the speed of 3,400 rpm. The homogenates were kept at 4°C in the dark for 12 hours, then centrifuged at 11,337 g for 20 minutes using a microcentrifuge (Eppendorf, US). The supernatant was used for the total phenolics, antioxidant, and flavonoid analysis.

Total Phenolic Assays

The Folin–Ciocalteau (FC) method (Ainsworth and Gillespie, 2007) with slight modification was used to test the sample total phenolic content. Gallic acid was used as a standard, and methanol gallic acid solution (1 mg/1 mL) was diluted with methanol to provide the standard curve. Sample extracts (0.02 mL) were mixed with 0.2mLmethanol in 5 mL tubes. Then Folin–Ciocalteau reagent (0.2 mL, diluted 1:10) was added and allowed to react for 3 minutes. Then 0.2 mL of 1N sodium carbonate (Na₂CO₃) solution was added into the mixture, followed by distilled water to fill up the total volume to 2.5 mL. After 30 minutes at room temperature in a dark place, absorbance at 760 nm was measured using a Cary 60 UV–Vis spectrophotometer (Agilent, US). The results were expressed as mg gallic acid equivalent/100 g (mg GAE/100 g) of Full Weight (FW) of each sample.

Total Antioxidant Capacity Assessment

The antioxidant capacity was estimated by DPPH radical scavenging assay with some modifications (Kedare and Singh 2011; Sricharoen, Techawongstein, and Chanthai 2015). The basic procedures were as follows: 20 μL of plant extracts and 20 μL dimethylsulfoxide (DMSO) were mixed up with 2.96 mL DPPH (0.1 mM) solution. The mixture was then incubated in dark conditions at room temperature for 20 minutes. Afterward, the absorbance of the mixture was read at 517 nm with a SPECTROstar Nano plate reader (BMG Labtech, Ortenberg, Germany). The 3 mL of 0.1 mM DPPH was taken as the control. The % radical scavenging activity of the plant extracts was calculated using the following formula:

The Radical Scavenging Activity (% RSA) = (Abs control – Abs sample)/Abs control × 100; Abs control is the absorbance of DPPH radical + ethanol; Abs sample is the absorbance of DPPH radical + plant extract.

Total Flavonoid Content Assessment

The aluminum chloride colorimetric method was used to determine the total flavonoid content in the samples (Suman et al., 2014). Generally, 5–200 μg/mL serial methanol diluted quercetin solutions were prepared as the standard. An amount of 0.6 mL diluted standard quercetin solutions or grape sample extracts was separately mixed with 0.6mLof 2% aluminum chloride. After mixing, the solution was incubated at room temperature for 1 hour. The absorbance of each reacted mixture was measured at 420 nm with a UV spectrophotometer (Agilent, US). The total flavonoid was calculated from the calibration plot and expressed as mg quercetin equivalent (QE)/g of grape material.

Total Anthocyanin Content Measurement

The anthocyanin content assay was measured by Lee’s method with some slight modifications (Lee et al., 2005). The grape grinded sample (0.5 g) was mixed thoroughly with 9.5 mL of potassium chloride buffer (0.025 M, pH 1.0) and sodium acetate buffer (0.4 M, pH 4.5), then incubated for 20 minutes at room temperature followed by centrifugation with a speed of 4,563 g for 20 minutes (Thermo Scientific Medical Centrifuge, US). The absorbance (A) was measured at 520 nm and 700 nm. The formula used in the research was Total anthocyanin = $\frac{\mathit{AxV}}{M}$; where A = (A max – A 700 nm) _pH1.0 - (A max – A700nm) _pH4.5; V= volume of extract (mL); M = fresh mass of the sample (g).

Statistical Analysis

All the antioxidants, phenolics and flavonoid assessment were repeated three times. Data were submitted to analysis of variance (ANOVA) in R software version 4.0.5 (R Core Team, 2023), and the means were compared using the test of Tukey.

Results and Discussion

Grape Damages During Storage

‘Somerset Seedless’ cluster quality for cold hardy ‘Somerset Seedless’ was first estimated by ratings and enumerations based on visual appearance. Split rate, decay rate, shatter rate and mold rate were scored by percentage in grapes from each cluster. As one of the chitosan common solvent solutions (Sikorski, Gzyra-Jagieła, and Draczyński 2021), acetic acid was included in this research to compare against chitosan effects. Throughout storage, the acetic acid treatment was significantly different (p < .01) on the grape split rate compared to the chitosan and diH₂O treatment. The differences of the split rates grew as time went on. At week 7, the split rate of diH₂O treatment reached about 2%. No available publications focused on acetic acid effects on fruit skin split. The possible causes of skin splitting might be derived from the low pH of acetic acid (around 2.8). The chemical reaction between chitosan and acetic acid resulted in polycation chitosan, which became the polymer for fruit or vegetable coatings (Chan et al., 2013). Despite its use in chitosan solution preparation, acetic acid treatment on fruits lacks references.

Shatter rate, the portion of berries separating from rachises is another important storage factor for table grapes. Shatter rate (Figure 1b) in the grape clusters indicated chitosan and diH₂O treated fruit experience less shatter compared to acetic acid treatment. At week 7, chitosan treated grapes had less than 2.0% shatter rate, which was not significantly from diH₂O treatment. In acetic acid treated grape clusters, the shattering rate was above 6.0%.

Figure 1. Grape damage changes are rated through split rate (a), shatter rate (b), decay (c) rate and mold rate (d) in clusters. The clusters with acetic acid (acid), Chitosan and diH₂O treatment are in cold storage and collected for rating at 1, 3, 5, 7 weeks. The cluster damages are rated by split, shattering, decay, and mold rate. The postharvest grape appearance is evaluated by destructively sampled at each treatment. Different letters (a, b, c, d) correspond to mean values significantly different (p < .01) by analysis of variance (ANOVA) and Tukey post hoc test.

To evaluate the postharvest treatment applicability, the decay rate during grape cold storage is important to estimate (Ciccarese et al. 2013). From the decay rate of grape clusters (Figure 1c), chitosan treatment had no significant effects stemming from acetic acid treatments before week 5. Both treatments had lower decay rate compared to diH₂O at week 5. At week 7, approximate 6% decay rate was observed in the acetic acid treatment and diH₂O treatment. Chitosan treated grapes had about 3% lower decay rate. Antifungal activities were observed based on lower mold ratings in both acetic acids treated and chitosan treated grapes from week 3, but not from the grapes under diH₂O treatment (Figure 1d). Antifungal capabilities were observed in both acetic acid and chitosan treatments with higher fungi inhibition rate compared to diH2O treatment. Mold incidence was about 10% only in clusters with acetic acid and chitosan treatments, whereas clusters from diH₂O treatment had ratings of more than 20%, significantly higher than the other treatments. This indicated both acetic acid and chitosan had some antifungal activities. Chitosan, with its positive charge could interact with negatively charged phospholipid components of fungal membranes, which might explain a portion of the observed antifungal properties (Ing et al., 2012). To date, no references compared acetic acid and chitosan treatments. Resultingly, this research provides important information for postharvest storage with these two treatments.

To estimate total damage during postharvest treatments on ‘Somerset Seedless’ grapes, a stacked bar graph was constructed for each treatment (Figure 2). The results indicated the main grape damage transpiring after week 3 was mold related. This suggests that mold control should be a main target for future work on ‘Somerset Seedless’ cold-hardy table grapes. Chitosan treatment had similar mold-control effects as the acetic acid control (Figure 2a,b), although acetic acid may have increasing problems which were observed after week 5, such as increased split rate and shatter rate. Following the standard of US Department of Agriculture (USDA) table grapes, diH₂O as a control would not pass the standard after week 3, whereas both acetic acid and chitosan could pass 10% damage standard for table grapes even at week 5 (Figure 2a–c). Further, chitosan had slightly lower damages compared to aceticacid treatment alone. As a chemical elicitor, chitosan was reported to cause both enzymatic and non-enzymatic metabolites of stress response changes, which might be associated with resistance against fungi diseases, such as in vegetables (Adiletta et al., 2021; Guilli et al., 2016). Meanwhile, rachis color as one of the key indicators representing grape cluster freshness was used as an additional metric during storage (Figure 3). As in Figure 3a,b, diH₂O had slightly higher rachis ratings with storage from week 3 to week 7. Chitosan and acetic acid treated clusters did not have significant differences from each other during the whole storage time, although the rachis rate was scaled higher than week 0.

Figure 2. Stacked bar chart of the total damage in average with acetic acid, chitosan and diH2O as postharvest treatments. The damages include decay, mold, shattering and split rate by percentage. Treatments include acetic acid (a), chitosan (b) and diH2O (c) treatment. The red dashed line indicates the 10% damage threshold for grape bunches to meet quality acceptance standards of the U.S. Department of Agriculture (USDA) for table grapes.

Figure 3. Grape cluster rachis rate with each treatment during storage. Each rachis from each cluster is collected at week 0, 1, 3, 5 and 7. Rachis dehydration is scored from 1 (all green rachis) to 5 (all brown rachis). (a) Rachis index changes during storage. Different letters (a, b, c) correspond to mean values significantly different (p < .01) by analysis of variance (ANOVA) and Tukey post hoc test. (b) Representative photo of rachis with different treatment.

Physiochemical Property Changes

Physicochemical property changes were the main fruit quality measurement. Weight loss in treated clusters indicated there was no significant difference among the three treatments at each sampling week (Table 1). The weight loss at week 1 was about 0.5% and continuously increased to about 2.2% at week 7 in the walk-in cooler. The weight loss varied by time (p < .0001). For TSS, none of the treatments caused significant differences from each other at each sampling time. The range of TSS was between 21 and 22.5 on average. The pH ranged from around 3.30 to 3.40 with no differences for treatments at each sampling week. Across time, pH changed, growing higher at week 5 compared with week 0. The total acidity in grapes during storage showed the trend of decreasing (p < .01) during storage, but none of the treatments significantly differed from each other within any given week. In conclusion, the postharvest treatment in our research did not make significant differences to the general physiochemical characteristics of weight loss, TSS, pH, or total acidity.

Table 1. ‘Sommerset Seedless’ postharvest physicochemical characteristics under postharvest treatment.

	Grape Physical Characteristics
Treatment	W0			W1			W3			W5			W7
Weight Loss (%)
Acetic Acid	0	±	0	.57	±	.07	.73	±	.09	1.39	±	.18	2.2	±	.64
Chitosan				.44	±	.21	.87	±	.23	1.49	±	.35	2.1	±	.39
diH2O				.5	±	.07	.84	±	.09	1.58	±	.13	2.37	±	.22
p-value	–			.22			.29			.4			.59
1.1102e-16 **
TSS (°Brix)
Acetic Acid	21.38	±	2.38	22.02	±	1.15	21.95	±	1.49	22.07	±	1.21	22.25	±	1.2
Chitosan				21.95	±	1.55	22.33	±	1.08	22.43	±	.63	21.87	±	.58
diH2O				22.13	±	1.85	21.87	±	1.44	21.88	±	1.34	22	±	1.25
p-value	–			.98			.82			.69			.82
.6191
pH
Acetic Acid	3.34	±	.12	3.1	±	.19	3.33	±	.08	3.41	±	.08	3.42	±	.11
Chitosan				3.28	±	.15	3.37	±	.14	3.46	±	.06	3.4	±	.13
diH2O				3.16	±	.21	3.37	±	.12	3.41	±	.1	3.5	±	.16
p-value	–			.25			.73			.53			.41
5.6262e-08 **
Total Acidity (g/L)
Acetic Acid	9.98	±	.41	8.97	±	.76	8.58	±	.49	8.42	±	.46	9.22	±	.7
Chitosan				8.52	±	.59	8.32	±	.34	8.42	±	.78	9.05	±	.4
diH2O				9.57	±	1.25	7.98	±	.65	8.55	±	.63	8.85	±	.7
p-value	–			.17			.15			.92			.61
	4.9660e-13 **

W0, W1, W3, W5, and W7 indicate the sampling time of Week 0, Week 1, Week 3, Week 5, and Week 7. The ** indicates the p value < .01.

Phytochemical Activity Changes

‘Somerset Seedless’ grape secondary metabolites levels were reported in Table 2. Total phenolics in each treatment were in the range of 240–425 mg gallic acid equivalent (GAE)/100 g, but none of the treatments had significant differences in each selected week during postharvest storage. With time, the phenolics had significantly decreased during storage (p < .05). The reduced total phenolic content is generally expected in fruit and vegetable storage. This phenomenon might relate to storage conditions, polyphenol reactions with sugar and sugar metabolites and oxidation of sensitive phenolic compounds (Deng et al., 2018; Eshghi et al., 2022; Farr and Monica Giusti, 2018).

Table 2. Effect of different postharvest treatments on ‘Somerset Seedless’ total phenolic, flavonoid content and antioxidant component activity levels.

Treatment	W0			W1			W3				W5			W7
Total Phenolics (mg GAE/100 g)
Acetic Acid	373.88	±	58.22	312.93	±	18.52	291.14	±	7.71	265.26		±	4.38	285.25		±	15.54
Chitosan				280.53	±	5.96	280.62	±	19.49	259.42		±	15.3	330.95		±	10.24
diH2O				279.45	±	17.03	292.5	±	1.45	314.45		±	34.9	309.91		±	9.05
p-value	–			.5			.84			.11				.23
.0244*
Flavonoid (μg QE/g)
Acetic Acid	135.46	±	35.91	92.71	±	28.23	111.98	±	45.92	131.29		±	27	114.44		±	24.68
Chitosan				125.73	±	23.44	108.04	±	32.59	111.06		±	8.68	125.71		±	30.94
diH2O				118.21	±	25.75	131.13	±	23.93	96.18		±	16.4	112.92		±	15.43
p-value	–			.33			.71			.15				.79
.1328
DPPH Scavenging Activity (%) of 4 mg FW
Acetic Acid	13.46	±	2.19	15.9	±	.94	11.81	±	5.77	12.95		±	3.65	11.65		±	2.36
Chitosan				13.71	±	2.19	11.14	±	4.22	12.63		±	3.3	11.76		±	3.87
diH2O				15.67	±	.79	15.23	±	1.85	15.56		±	.5	9.88		±	5.97
p-value	–			.21			.35			.44					.84
	0.1326

Mg GAE/100 g represents the total phenolics amount expressed as mg gallic acid equivalent/100 g full weight (FW) of grapes (mg GAE/100 g). Flavonoid amount is expressed as the μg quercetin equivalent (μg QE/g) of the sample. DPPH scavenging activity (%) is the free radical-scavenging activity (RSA) of an antioxidant compound; DPPH scavenging activity can be expressed as the percentage of DPPH reduced by antioxidants. ANOVA statistical tests (p < .05) results are shown by p values. Each sampling week of each treatment contains three different clusters. The * indicates p value < .05.

The flavonoid assays in grapes indicated this kind of bioactive molecule was not altered by treatment within sampling time or across the whole storage time. The range of total flavonoid content was approximately 92.71 to 135.46 μg quercetin equivalent (μg QE/g).

In this research, DPPH activity was in the range of 10% to 16% in our grapes. DPPH radical scavenging activity as a measurement of antioxidant activity in ‘Somerset Seedless’ grapes indicated antioxidant activity was not altered by treatments. From the results of total phenolics, flavonoids, and relative scavenging activities of grapes, this research indicated the three postharvest treatments did not have significant impacts on these phytochemical characteristics. Although reduction trends were observed in total phenolics during storage, there were no significant differences among the three treatments. The principles of detecting antioxidant and antioxidant activities were varied, which will provide differential antioxidant capacity results (Al-Qurashi and Awad, 2015). In general, the antioxidant capabilities during the 7-week storage were not affected by the treatments in this research.

In this research, 1.4% high molecular chitosan was used for postharvest treatment on ‘Sommerset Seedless.’ In research articles, both low and high molecular weights of chitosan are reported in use (Qiu et al., 2013). The concentration of chitosan applications by most researchers is usually 1% to 3% (Lin et al., 2019; Obianom et al., 2019). It is necessary to conduct further research with low-molecular and high-molecular chitosan postharvest treatment on cold-hardy grapes.

Chitosan application needs to be further assessed, especially regarding its film-forming properties. The length of chitosan treatment immersion time on fruits was assessed as a factor influencing the effectiveness (Guilli et al., 2016). Also, chitosan has had variable effects depending on the dissolving acids (Eshetu et al., 2019; Gianfranco et al., 2009; Romanazzi et al., 2006). Acetic acid as a control method has been noted in orange storage methods with chitosan arising as more effective on disease control in navel oranges (Zeng et al., 2010). In this research, chitosan with acetic acid showed advantages of preserving grapes based on grape appearance during storage.

Chitosan may also be applied with other materials or combined with other postharvest treatment. Chitosan nanoparticles expanded chitosan application and improved its functionalities (Saberi Riseh et al., 2023). Romanazzi et al. (2006) evaluated table grape preharvest and postharvest treatment with chitosan alone or combined with UV-C treatment. These prior results indicated the treatments were synergistic in reducing gray mold, although the concentration of catechin or resveratrol in berry skins was variably dependent on the treatments (Romanazzi et al., 2006). High-CO₂ modified atmosphere packaging plus chitosan treatment has been studied on ‘Italia’ table grapes, indicating the most effective methods were the combined treatment of modified atmosphere and chitosan. Further observations indicate chitosan may prevent some of the negative effects of high-CO₂ in packages, such as berry browning, rachis browning, and dehydration (Liguori et al., 2021). Another conjoined treatment application, hot water and chitosan treatment was reported to control postharvest decay of sweet cherries (Chailoo and Asghari). Similarly, 1-Methylcyclopropene (1-MCP) and chitosan have been combined to elongate fruit shelf-life (Ali et al., 2022). Other combination methods have been evaluated including natural wax and seed extracts in conjunction with chitosan. Beewax has combined with chitosan to elongate mango shelf-life (Eshetu et al., 2019). Grapefruit seed extract and chitosan also have had demonstrated a synergistic effect in reducing fungal rot in ‘Redglobe’ grapes (Xu et al., 2007).

The trend of large-scale chitosan applications for postharvest storage of fruit and metabolite elicitation may increase with several chitosan-based commercial products available, such as Chito Plant (ChiPro GmBH, Germany), Lytone Enterprise, Inc. (Shanghai Branch, China), OIL-YS (Venture Innovations, US), and others.

Conclusion

In conclusion, this study of chitosan and its solvent acid, acetic acid, both demonstrated a positive effect on controlling ‘Somerset Seedless’ damages during postharvest storage, especially influencing reduced decay and mold incidence rates of grapes. Acetic acid treatment alone had some negative effects on appearance of grape cluster qualities with increased split rate and shatter rate compared to diH₂O and chitosan treatment. Meanwhile, chitosan showed some potential impact on preventing the rachis dehydration, although the effects were not significantly different from other treatments during the observation time range. For both the basic physiochemical and phytochemical properties, none of the postharvest treatments had altered these properties significantly. Further research can be conducted to focus on improving and optimizing of chitosan treatment on cold-hardy table grapes, such as chitosan molecular weight, concentration, and solvent. In summary, our research is the first report of application of chitosan on cold-hardy table grape cultivar ‘Somerset Seedless,’ which can provide crucial information for chitosan application for research and industry.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Montana Department of Agriculture [AM22SCBPMT1127].