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. 2021 Oct 31;59(7):2784–2793. doi: 10.1007/s13197-021-05301-3

Inhibition of browning via aqueous gel solution of Aloe vera: a new method for preserving fresh fruits as a case study on fresh kernels of Persian walnut

Asaad Habibi 1, Navid Yazdani 1,✉, Najme Chatrabnous 1,2, Mahmoud Koushesh Saba 3, Kourosh Vahdati 1,✉
PMCID: PMC9206972  PMID: 35734107

Abstract

Aloe Vera (AV) gel is commonly used as a natural, inexpensive, edible coating that can improve the quality and shelf life of fruits. The objective of this study was to evaluate how two methods of applying AV, i.e. as an edible coating (dry environment) and as a gel solution (aqueous environment: a new method), prevent browning and maintain quality characteristics of fresh kernels of Persian walnut for 60 days during cold storage. Distilled water was used as a control group for both environments. In general, AV caused a reduction in the peroxide value (POV) of kernels, while preserving Total Phenolic Compound and Total Antioxidant Activity (TAA). The AV treatment slowed down the process of color change and maintained sensory properties during storage, compared to the control groups of both methods. The AV gel solution performed better than the AV edible coating in terms of POV, color (L* and h°) and microbial growth. In contrast, the AV edible coating was more effective in preserving TPC and TAA. Also, TAA was found to have a significant, positive correlation with L* and, simultaneously, a negative correlation with POV. As far as we know, this is the first instance that the AV gel was used as a formulated solution and as an edible coating on fresh fruits. This innovative method can be used in commercial practice, while being ecofriendly and non-chemical as a treatment for the maintenance of postharvest quality in fruits.

Keywords: Antioxidant, Aqueous environment, Color, Edible coating, Peroxide value, Sensory properties

Introduction

Persian walnut (Juglans regia L.) is the most widespread nut crop in the world (Arab et al. 2019). It is an excellent source of protein, omega-3 and omega-6 fatty acids, phenolic compounds, melatonin and several antioxidants, which frequently occur in human consumption can reduce risk of coronary heart diseases, diabetes and cancer (Askari et al. 2013; Jahanbani et al. 2016). Walnut can be consumed either as fresh or as dried. Fresh walnut contains abundant amounts of fatty acid, protein and vitamin E (Chatrabnous et al. 2018b; Ma et al. 2013). The TPC concentration in fresh kernels is 1.2-times higher than in dried kernels (Christopoulos and Tsantili 2012). Fresh walnuts are much less consumed than dried walnuts, however, while their unique flavor puts them in high demand in some countries and cultures (Habibie et al. 2019; Christopoulos and Tsantili 2012). Fresh walnut has a very short shelf life and is only available for a short period after harvest. Several factors can reduce the walnut shelf life, including mechanical damage, moisture loss, fungal infection, lipid peroxidation and oxidative stress, as well as reactions between polyphenol oxidase and phenolic compounds (Christopoulos and Tsantili 2012; Chatrabnous et al. 2018a; Habibie et al. 2019). Lipid oxidation and browning could affect sensory traits, visual quality of kernels, while also negatively affecting consumer decision on purchase and eventually leading to significant economic loss (Chatrabnous et al. 2018a; Habibie et al. 2019). Since fresh walnut is usually characterized by a short shelf life, a safe treatment for prolonging its storage would seem apt to maintain its naturalness and quality. The safety of the treatment would ideally consider the consumer and the packaging people in the industry.

The traditional way to store fresh walnuts in Iran is to keep them in a salt solution or in water for up to 3 days, if there is a lack of proper packaging at ambient temperature, which is the usual case in traditional packaging and retail (Vahdati et al. 2014). However, this method entails several problems, including a short shelf-life, unassured hygiene, less popularity and more browning (Chatrabnous et al. 2018a). Browning is a prominent feature that can shorten the shelf-life of fresh fruits. It strongly affects the consumers’ decision on purchase. A low storage temperature is usually applied to postpone the postharvest deterioration of products, although in some cases this treatment is not enough to preserve fruit quality during storage (Habibie et al. 2019). Thus, supplementary treatments can be regarded as indispensable. Edible coatings can perform as a barrier to gases and water vapor, as they reduce the respiration rate of viable cells in fruits (Martínez-Romero et al. 2013), postpone dehydration, preserve color and maintain texture quality (Hassanpour 2015; Benítez et al. 2013; Sheikhi et al. 2020).

In recent years, food technology has made possible a safe circulation of edible coatings and films in food production lines. Coatings and films have proven their role as promising while they preserve food and fruit quality, extend shelf life and increase the safety of food and fruits in protecting them from environmental factors (Habibie et al. 2019). Films and edible coatings have been investigated to improve the properties of films with a hydrocolloid nature, derived from lipids, whey protein, polysaccharides and plant material such as Aloe vera (AV) gel (Hassanpour 2015; Ali et al. 2019). AV gel can be used as a natural and inexpensive edible coating to improve the quality and shelf life of fruits (Sogvar et al. 2016).

Since biblical times, AV has been applied for healing skin disorders and for supplementing beauty treatments (Sánchez-Machado et al. 2017; Cardarelli et al. 2017). AV is a traditional medicinal plant with a polysaccharide matrix (glucomannan and glucose). It is rich in bioactive secondary metabolites and is one of the most biologically active plants because of its antioxidant content which comprises polyphenols, flavonoids, flavonols and antimicrobial chemicals. These usually exist in its leaf exudates and are widely used in food and pharmaceutical industries (Sánchez-Machado et al. 2017; Cardarelli et al. 2017). AV gel has already been tested as a new edible coating on kiwifruit (Benitez et al. 2013), peach (Guillén et al. 2013), raspberry (Hassanpour et al. 2015), strawberry (Sogvar et al. 2016), litchi fruit (Ali et al. 2019) and apple (Ergun and Satici 2012) with the aim of preserving fruit quality and shelf life. In these studies, AV gel was applied as an edible coating and as an edible film. The fruits were treated with the AV gel via immersion or dipping methods. In the latter, for example, the fruits were dipped in the AV gel for a few minutes and then were left to dry at room temperature before being packed. In a few of these studies, the authors concluded that AV can serve successfully as an edible coating for the maintenance of fruit quality and shelf life (Benitez et al. 2013; Hassanpour et al. 2015; Sogvar et al. 2016).

To the best of our knowledge, there is no information available on the application of AV gel in an aqueous environment and its effectiveness in maintaining fresh fruit quality. In this study, two methods were evaluated for the application of the AV gel. Here, an edible coating was used as a preserving factor in distilled water for the first time and was applied on fresh kernels of Persian walnut cv. ‘Chandler’ for the purpose of prolonging storage. The evaluations involved measuring the ability of the AV gel to inhibit browning and lipid oxidation, while preserving fruit quality, for a total period of 60 days in refrigerated storage.

Materials and methods

Aloe vera (AV) gel preparation

Mature leaves of AV plants were purchased from a local market and were washed with distilled water. The AV matrix was then separated from the outer cortex of leaves and then the colorless hydro parenchyma was uniformly mixed in a blender. The mixture was filtered to remove the fibers. The filtered gel was kept constant for two days at low temperature (4 °C) and the gel remained without puree to make its usage suitable.

Treatments, packaging and storage conditions

Self-rooted clonally propagated walnut trees cv. ‘Chandler’ were located in Shahmirzad’s walnut orchard, Iran. Shahmirzad’s walnut orchard has an area of 700 ha and is considered by FAO, UN, as the largest of its kind in the world. The average amount of annual rainfall on Shahmirzad is 129 mm. The region has a warm climate. Uniform fruit samples were collected of ten self-rooted, clonally propagated walnut trees cv. ‘Chandler’ that had grown under normal condition. The fruits were harvested when the green husk had cracked by about 50% and the internal packing tissue had turned brown. Fruits were hand-picked and transferred directly to the laboratory. The husk and shell of the nuts were manually removed and non-damaged kernels were used. This experiment was performed using two methods of AV application, i.e. edible coating (dry environment) and gel solute (Zapata et al. 2013) ion (aqueous environment). In the gel solution, fresh walnut kernels were stored in 0.5 L aqueous solution content (AV gel 20% (v/v)) or in distilled water (control) as aqueous environments (AEs). The other two groups of kernels were dipped in an aqueous solution containing AV gel (20%) or in distilled water (control) for 15 min, and then were dried at 25 °C for half an hour, thereby representing the treatment method of dry environments (DEs). Accordingly, the treatments included AV (20%) aqueous environment (AAE), the control aqueous environment (CAE), AV (20%) dry environment (ADE) and the control dry environment (CDE). After receiving the treatments, the kernels were packed into polyethylene/polyester bags. Three replicates for each treatment were considered. The fresh walnut kernels were stored at 4 ± 1 °C for 60 days. Then, fresh kernels (FKs) were divided randomly into 51 replicates of 15 kernels. Three replicates were sampled immediately to assess fruit characteristics at harvest time (day 0). The replicates were then divided into 4 treatment groups of 12 replicates (i.e. 3 replicates each) at for each of the four storage periods.

Color parameter

Color coordinates of fresh kernels were measured using the color meter (TES 135 A, Taiwan) with 6 replications. The hue angle (H) as hº = tan−1 (b*)/(a*)] and total color differences (TCD) between the initial and final readings [TCD = (L*)2 + (a*)2 + (b*)2)½] were calculated. The lightness index (L*) indicates the color between black (0) and white (100).

Oil extraction

Extraction of kernel oil was performed according to the method described by Chatrabnous et al. (2018a), with some modifications. Briefly, 10 g of each replication was frozen in liquid nitrogen and homogenized with 40 ml n-hexane. Then, the samples were placed in a dark place at cold storage for 2 days. The mixture was filtered and then centrifuged at 2655 g for 10 min. Oils containing hexane were eventually purified using a rotary vacuum evaporator (Heidolph, Germany) at 40 °C. The lipid content was gravimetrically determined, and the sample was stored at −80 °C in darkness until a desired time of usage.

Fatty acid composition

Walnut oils were extracted by n-hexane at 4 °C. To evaluate fatty acid composition of walnut oil, three replications were used as in three injections. To do this, a fused-silica capillary column (HP-5) was connected to a gas chromatograph (PerkinElmer Clarus 500, USA), equipped with a flame ionization detector (FID) and split/split less injector. N2 was used as the carrier gas. A temperature program of 90 °C for 12 min was set to rise to 180 °C at 15 °C min−1 and then it was programmed to increase to 230 °C at 25 °C min−1. The fatty acid methyl ester (FAME) was dissolved in hexane and injected (0.5 μL) in a split mode of injection at a split ratio of 1:20. The injector and detector temperatures were 250 °C. Total Chrom workstation software was used for recording the peak areas.

The unsaturated/saturated fatty acid (USFA/SFA) ratio was calculated by the formula (18:1 + 18:2 + 18:3) / (16:0 + 18:0) where 16:0 is palmitic acid, 18:0 is stearic acid, 18:1 is oleic acid, 18:2 is linoleic acid and 18:3 is linolenic acid.

Peroxide value (POV)

The POV was evaluated using a acetic acid-chloroform method (Shahidi and Zhong 2005). The POV was expressed as meq of O2 per kg−1 oil of sample.

Total phenolic compounds (TPC) and total antioxidant capacity (TAA)

The TPC and TAA concentrations were measured by the Folin-Ciocalteu colorimetric assay and the radical scavenging capacity (DPPH) assay, respectively (Christopoulos and Tsantili 2012). The TPC was expressed as milligram of gallic acid equivalents per dry matter (g kg−1 GAE) and the TAA was expressed as percentage (%).

Microbial analysis

The total yeast and mold were counted using the surface plate method on a PDA (potato dextrose agar) by the method described by Habibie et al. (2019).

Sensory analysis of fresh kernels

Several sensory traits were evaluated, including pellicle color, flesh color, flavour, fragility and bitterness. Each quality attribute was scored from 1 (lowest) to 9 (highest). Higher scores showed better quality kernels. The volunteers who tested the sensory traits were 8 females and 7 males, with an age range of 24–35 years. The tasters were untrained and non-smokers. They tested the sensory traits among the treated samples on 20, 40 and 60 day intervals (Colarič et al. 2006).

Statistical analysis

SAS software version 9.00 was used for statistical analysis. A symmetric factorial experiment was carried out as in a completely randomized design (CRD). Data were expressed as the mean ± standard error. The statistical significance of differences between mean values was calculated with the LSD new multiple range test in the general model. Panel test analysis was performed using the Kruskal–Wallis Test.

Results and discussion

Color parameters

Total color difference (TCD) correlated negatively and strongly (P = 0.001) with L* and hº values and, therefore, the results on TCD were not presented here. Instead, the values of L* and hº were reported, regarding the pellicle of kernels (Fig. 1a and b). Meanwhile, L* and hº in AE were higher than DE after 15 and 30 days. Thereafter, L* and hº decreased as a result of the CWE treatment. L* and hº acquired the lowest values in response to CDE treatments and had significant differences with other treatments during storage (P = 0.01). Also, AAE functioned effectively in inhibiting surface browning during storage, while ADE ended up second in effectiveness among the treatments. CWE performed better as a treatment, compared to the CDE treatment, in terms of the ability to preserve kernel color during storage. The L* and hº values were higher in response to AAE, compared to ADE, although their difference was not significant.

Fig. 1.

Fig. 1

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Color L* (a) and h* (b) in fresh kernels of Persian walnut after receiving treatments of Aloe vera as aqueous and dry media after 60 days of storage at 4 ± 1 °C. Treatments included CAE (control aqueous environment), AAE (Aloe vera aqueous environment), CDE (control dry environment) and ADE (Aloe vera dry environment). Data are mean values ± SE. Each time point indicates a mean value. The numbers on each time point are LSD values (p < 0.01) (A and B n = 6)

Color preservation was considered as one of the main focuses of this study because it directly affects the consumers’ intention and attention in purchase. Kernel color is an indicator of freshness and quality. AV gel treatments significantly reduced the browning degree, compared to the control treatments in both environments (Fig. 2). In previous studies, several cases indicated that the AV gel can be used as an antioxidant treatment and could create a protective layer that minimizes moisture loss, suppresses oxidation reactions and inhibits browning (Nicolau-Lapeña et al. 2021; Supapvanich et al. 2012). Our findings suggested that using the AV gel in an aqueous environment could slow down the browning during storage (Figs. 1 and 2). Similarly, Chatrabnous et al. (2018a) showed that antioxidant treatments such as Thymus vulgaris and walnut green husk extract can delay the oxidation and browning of fresh walnuts in an aqueous environment. Previous studies have reported that the AV gel contains a number of beneficial antioxidant compounds, i.e. aloe-emodin, anthraquinones, acemannan and glucomannan, which could contribute to the preservation of color attributes in food (Nicolau-Lapeña et al. 2021; Sánchez-Machado et al. 2017). Other reports mentioned this effect on fresh apple fruit (Ergun and Satici 2012), fresh-cut apple (Nicolau-Lapeña et al. 2021), kiwifruit (Benitez et al. 2013), peach (Guillén et al. 2013) and fresh cut rose apple (Supapvanich et al. 2012).

Fig. 2.

Fig. 2

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Effects of Aloe vera (AV) (20%) on color changes in fresh walnuts after 60 days, a (aqueous environment), b (dry environment). Treatments included CAE (control aqueous environment), AAE (Aloe vera aqueous environment), CDE (control dry environment) and ADE (Aloe vera dry environment)

TAA correlated significantly and positively with L* but negatively with POV (Fig. 4a and c). Also, a significant negative correlation was observed between TPC and TCD (Fig. 4b). Similarly, Habibi et al. (2019) showed a strong correlation between TAA and color, as well as a significant, negative correlation with POV in fresh walnut kernels. Furthermore, a strong, positive relationship reportedly existed between TPC and browning in walnut (Christopoulos and Tsantili 2011). The browning of a product results from phenolic oxidation by enzymes and oxidative reactions, thereby leading to dark pigments (Chen et al. 2017). Christopoulos and Tsantili (2011) stated that the loss of nutrients and antioxidants in kernels, along with higher rates of phenolic oxidation, cause kernel browning during storage, thereby affecting consumer perception.

Fig. 4.

Fig. 4

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Correlation analysis between L* (luminosity), TCD (total color difference), TPC (total phenolic compounds), TAA (total antioxidant activity) and POV (peroxide value). Significant correlation at 0.05, 0.01 levels are indicated as * and **, respectively. ( −) negative correlation and ( +) positive correlation

Fatty acids profile of fresh walnut

Extracted from treated and untreated walnuts, kernel oil contained a high degree of unsaturated fatty acids (USFA) (oleic, linoleic and linolenic acid) which comprised more than 90% of the total fatty acids before and after storage (Table 1). The results were in agreement with previous findings on dry walnuts (Jensen et al. 2003) and those that were reported about fresh walnuts (Jiang et al. 2015; Ma et al. 2013). Also, saturated fatty acids (SFA) (palmitic and stearic acid) amounted to 9.1 g 100 g−1 before storage. The level of saturated fatty acids and unsaturated fatty acids remained stable during storage and considerable changes occurred neither in treated walnuts nor in untreated ones.

Table 1.

Fatty acid composition, unsaturated fatty acid (USFA), saturated fatty acid (SFA) and unsaturated/saturated fatty acid (USFA/SFA) of fresh walnut kernels of the control and Aloe vera 20% treatments before and after storage at 4 ± 1 °C for 60 days. Treatments were including CAE (control aqueous environment), AAE (Aloe vera aqueous environment), CDE (control dry environment) and ADE (Aloe vera dry environment)

Fatty acids (g 100 g−1) Before storage Treatments
CAE AAE CDE ADE
Palmitic (C16:0) 6.46 ± 0.03ab 6.3 ± 0.05b 6.39 ± 0.01b 6.6 ± 0.05a 6.3 ± 0.03b
Stearic (C18:0) 2.64 ± 0.08a 2.64 ± 0.01a 2.67 ± 0.04a 2.6 ± 0.05a 2.57 ± 0.07a
Oleic (C18:1) 18.49 ± 0.12c 17.8 ± 0.05d 19.43 ± 0.04a 18.3 ± 0.05c 19.06 ± 0.01b
Linoleic (C18:2) 55.28 ± 0.16b 56.72 ± 0.02a 55.81 ± 0.02ab 56 ± 0.57ab 56.1 ± 0.05ab
Linolenic (C18:3) 16.56 ± 0.1a 16.04 ± 0.02b 15.2 ± 0.11d 15.89 ± 0.01b 15.49 ± 0.08c
USFA 90.33 90.56 90.44 90.27 90.65
PUFA 71.84 72.76 71.01 71.89 71.59
PUFA/SFA 7.89 8.13 7.83 7.81 8.06
USFA/SFA 9.92 10.12 9.98 9.81 10.20
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Each value represents mean ± standard deviation of three replicates, different letters in the same line mean significant difference (P < .05)

SFA: Saturated fatty acid, USFA: unsaturated fatty acid, PUFA: poly unsaturated fatty acid

In the fresh walnut oil, linoleic acid increased in kernels of the AV gel treatment. Oleic acid increased significantly in response to the AAE treatment after 60 days of storage. Ma et al. (2013) reported that oleic and linoleic acid increased after irradiation treatments in fresh kernels. Also, palmitic, stearic and linolenic acids decreased in all treatments after 60 days of storage in both environments. In a previous study, the exposure of fresh walnuts to an aqueous medium led to a decrease in the fatty acid content of the kernels (Chatrabnous et al. 2018a).

The highest PUFA in walnut oils was linoleic acid, similar to other PUFA elements, especially linoleic acid (Venkatachalam and Sathe 2006). However, the high oil content and amount of unsaturated fatty acids ultimately induced lipid oxidation (Jensen et al. 2003) which results in rancidity and off-flavor. Linolenic acid is more likely to be oxidized compared to oleic acid (Chatrabnous et al. 2018a; Venkatachalam and Sathe 2006). In this study, the highest USFA content was observed in walnuts treated with ADE.

Peroxidase value (POV)

The effects of treatments on POV varied considerably (Fig. 3a). Despite an initial low level of POV in the kernels (0.09 ± 0.01 O2 per kg−1 oil), there was a gradual increase of POV in all samples over the 60 days of storage. The lowest peroxide level was found in AAE after 60 days of storage, whereas untreated kernels in both environments showed higher POV levels.

Fig. 3.

Fig. 3

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Peroxide value (a), total phenolic compound (b), total antioxidant activity (c) and total yeast and mold count (d) in fresh kernels treated with Aloe vera as aqueous medium and dry medium during storage at 4 ± 1 °C. Treatments included CAE (control aqueous environment), AAE (Aloe vera aqueous environment), CDE (control dry environment) and ADE (Aloe vera dry environment). Data are mean values ± SE. The numbers on each time point are LSD values (p < 0.01) (A, B C n = 3 and D n = 3)

A reason for lipid oxidation during the storage period is autoxidation which occurs spontaneously when the lipids combine and react with atmospheric oxygen (Shahidi and Zhong 2005). Here, AAE was more effective in inhibiting lipid oxidation during storage, followed by the effectiveness of ADE. While AE contained the AV gel, it served as a quenching O2 agent and controlled the permeability of gases, thereby inhibiting the autoxidation of lipids. This is in agreement with Benítez et al. (2013) who reprted that coating kiwi fruits with AV gel led to a lower rate of O2 and CO2 production. The AV gel is a rich source of antioxidants (Cardarelli et al. 2017; Hu et al. 2005). Oxidative stress occurs when the presence of O2 contributes to the formation of ROSs. In other words, ROSs reacts with lipids, proteins and DNA, thereby causing oxidation and browning. Quenching the ROSs by antioxidants can happen through trapping free radicals, decomposing peroxides, scavenging oxygen species, and establishing stability in O2 radicals (Teotia and Singh 2014). Also, TAA correlated negatively and significantly with POV (Fig. 4c). Previous studies indicated similarly that preserving the antioxidant system prevents the oxidation of lipids in fresh walnut (Jiang et al. 2015; Habibie et al. 2019). It has been reported that AV gel is able to enhance the functionality of antioxidant enzymes such as superoxide dismutase and ascorbate peroxidase (Hassanpour 2015). It can establish an antioxidant system of preservation for kernels and play a key role in scavenging oxygen (Jariteh et al. 2011). Hu et al. (2005) demonstrated that R-tocopherol or butylated hydroxyl toluene have lower antioxidant capacity than the extract of AV. Also, POV became significantly stable in response to the AAE treatment after 15 to 60 days, suggesting the strong role of the AV gel as an antioxidant in suppressing O2 inside the kernel pack.

Total phenolic compound (TPC) and Total antioxidant activity (TAA)

The initial TPC content at harvest was 0.48 g kg−1 GAE in fresh kernels, although it decreased in response to AE during storage. The decrease in TPC was more consistent by the AAE treatment, however. There was an initial increase in kernel TPC by the CDE and ADE treatments. The TPC increased up to the fifteenth day of storage and then decreased. The highest TPC was observed in walnut kernels of CDE and ADE after 15 days of storage (Fig. 3b). Also, the AV gel reduced TPC in both environments during storage.

The TAA level in kernels decreased in response to all treatments during storage. Compared with the control samples, the AV gel caused kernels to have significantly higher TAA levels in both environments (P = 0.01), but ADE was more efficient than AAE in preserving TAA during storage and, up to the fifteenth day, TAA remained more stable in the DE treatment (Fig. 3c).

The increase in TPC and the stability of TAA during storage could be due to the low temperature in which the fresh kernels were stored by the DE. In other words, the observations could be attributed to chilling stress (Christopoulos and Tsantili, 2012; Habibie et al. 2019). Similarly, previous studies have shown that AV can cause a significant retention of antioxidant capacity and TPC in fruits, compared to untreated fruits, in raspberry (Hassanpour, 2015) and strawberry (Sogvar et al. 2016). The preservation of TPC and TAA could be due to the effect of AV gel on PAL activity which enhanced the resistant of tissues to decay via increasing antioxidant functionality and inhibiting the effect of oxidizing enzymes on phenolic and antioxidant compounds (Hassanpour 2015). In the current study, contrary to expectations, AV gel in DE was most effective for in preserving TPC and TAA, followed by the effectiveness of AV gel in AE. Many studies have considered using AV gels as edible coatings, but none have dealt with this coating in fresh crops stored in AE. Furthermore, phenolic antioxidants can protect antioxidants against oxidative decomposition, even as antioxidants can function synergistically with phenols (Miller and Rice-Evans 1997; Habibie et al. 2019). The AV gel can be regarded as an effective treatment because of its bioactive secondary metabolites (Cardarelli et al. 2017) which preserved the antioxidant system and phenolic compounds in the kernels.

Microbial analysis

The population of yeasts and mold increased in both environments during storage (Fig. 3d). Microbial growth was inhibited in AAE in fresh kernels during the storage period. In the dry environment, AD was more effective than the CD treatment, so that microbial growth increased more in the dry environment, compared to the aquous environment during storage.

The AV gel can be considered to have good potential against microbial agents in some fruits. In our study, the AV gel showed the best results in both environments and inhibited microbial growth more effectively during storage. A few studies confirmed that the AV gel can reduce populations of yeast and mold in strawberry (Sogvar et al. 2016), kiwifruit (Benítez et al. 2013), raspberry (Hassanpour 2015) and table grapes (Castillo et al. 2010) which are in agreement with our results. The AV gel is reportedly used as an insect repellent (Sánchez-Machado et al. 2017). The mechanism by which the AV gel controls decay in fruits is reportedly claimed to occur through the suppression of mycelial germination and the inhibition of mycelial diameter. The success of this mechanism is known to correlate with major components such as aloin, saponins and anthraquinones, as well as phenolic compounds and polysaccharides such as acemamman and glucomannan (Castillo et al. 2010; Zapata et al. 2013; Cardarelli et al. 2017).

Sensory evaluation

The sensory properties of fresh walnut kernels led to different results after 60 days of storage (Fig. 5). The visual aspects of kernels received the highest scores as a result of the ADE treatment, compared with the CDE. There were large differences between the kernels treated with AAE and ADE. Maximum scores were gained in the fragility, pellicle color and interior kernel color by the AAE treatment. The tasters reported that the tastiest kernels resulted from the ADE treatment, whereas the least tasty resulted from the CAE treatment.

Fig. 5.

Fig. 5

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Sensory attributes of the walnut kernels treated with Aloe vera (AV) as aqueous medium and dry medium during storage at 4 ± 1 °C. Treatments included CAE (control aqueous environment), AAE (Aloe vera aqueous environment), CDE (control dry environment) and ADE (Aloe vera dry environment). Data are mean values of evaluation collected from 15 tasters in 3 replicates (n = 15)

AV is gradually becoming recognized as a natural edible coating for fruits and foods. It is characterized by a high degree of effectiveness and safety, as well as a replacement for chemical postharvest treatments (Sánchez-Machado et al. 2017). The sensory analyses revealed beneficial effects of AV in terms of maintaining the walnut fruit sensory quality treated with AAE. In this regard, Martinez-Romero et al. (2013) reported high scores in arils treated with a combination of AV gels and acids. Also, AV was highly effective in maintaining the sensorial qualities of table grapes and kiwi fruit (Martínez-Romero et al. 2006; Benitez et al. 2013). In the current study, no off-flavor occurred in walnut kernels of the AV gel treatment. In particular, AAE was more effective in maintaining sensory qualities. The AV as an edible coating in DE had positive effects on sensorial traits. The positive effects of edible coatings can be attributed to their hygroscopic feature which serves as a barrier to ambient moisture.

Sánchez-Machado et al. (2017) cited several studies in which no evidence suggested photoxicity as a result of using the extracts of Aloe barbadensis. It can be postulated that the AV gel is a suitable edible coating that can be loaded with organic food additives. The presence of such natural preservatives on the fruit surface can maintain food safety and attract consumer attention. The AV gel has antioxidant, anti-inflammatory and antimicrobial effects, while having the ability to improve the immune system of the body because of its biologically active ingredients. The AV gel is also a valuable source of polysaccharides such as glucomannan (Sánchez-Machado et al. 2017; Cardarelli et al. 2017). Glucomannan is a good moisturizer that can prevent decay, preserve sensory properties and retain kernel color during storage. Also, AV acts as a hydrocolloid that can cause water absorption, preserve texture and induce lipid stability (Soltanizadeh and Ghiasi-Esfahani 2015).

Conclusion

We demonstrated in the present study for the first time that using the AV gel as an aqueous solution can inhibit browning and preserve sensory qualities in Persian walnut kernels during storage. This effective treatment can be considered as a natural alternative to chemical agents in the food industry, while having the ability to prevent color change in susceptible fresh fruits. As an edible coating, the AV gel can significantly increase the shelf life of kernels. Kernel browning was inhibited significantly by the AAE treatment. In sum, AV can serve as an optimum treatment for minimally processed products which require prolonged shelf lives for enhanced marketability.

Acknowledgements

The authors appreciate Iran National Science Foundation (INSF), University of Tehran and Center of Excellence for Walnut Improvement and Technology for their financial supports of this research.

Abbreviations

AV

Aloe vera

AEs

Aqueous environments

DEs

Dry environments

CAE

Control aqueous environment

AA

Aloe vera aqueous environment

CDE

Control dry environment

ADE

Aloe vera dry environment

POV

Peroxide value

TPC

Total phenolic compound

TAA

Total antioxidant activity

TCD

Total color difference

USFA

Unsaturated fatty acid

SFA

Saturated fatty acid

Author contribution

AH designed, performed all the experiments, compiled the data and wrote the manuscript. NY designed, directed the project and contributed to the interpretation of the results and revised the manuscript. NCH contributed to data analysis and the interpretation of the results. MKS contributed to the interpretation of the results and revised the manuscript. KV directed the project and revised manuscript. All authors listed, have made direct and intellectual contribution to the work, and approved it for publication. Also, all authors read and approved the final manuscript.

Availability of data and material

All data generated or analysed during this study are included in this published article.

Declarations

Conflict of interest

The authors declare no conflict of interest.

Ethical approval

Not applicable (include appropriate approvals or waivers).

Footnotes

Publisher's Note

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Contributor Information

Asaad Habibi, Email: asadhabibi@ut.ac.ir.

Navid Yazdani, Email: n.yazdani@ut.ac.ir.

Najme Chatrabnous, Email: n.chatrabnous@ut.ac.ir.

Mahmoud Koushesh Saba, Email: m.saba@uok.ac.ir.

Kourosh Vahdati, Email: kvahdati@ut.ac.ir.

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