Abstract-- In this study the influence of chitosan coating on the physiochemical properties of button mushrooms stored at 4 ºC was investigated. The button mushrooms were coated with 1 %, 2 % and 3 % (w/v) chitosan and then stored for 20 days. The results showed that chitosan coating is influential in inhibiting the senescence of button mushrooms and preserving their nutritional qualities. The optimum coating rate for button mushrooms in which the respiratory rate (28.26 %), weight loss (54.97 %) and percentage of open caps (72.22 %) are reduced and the changes in polyphenols (42.18 %), antioxidant capacity (4.38 %), ascorbic acid (33.29 %), titratable acidity (27.56 %) and total soluble solids (40.92 %) are retarded, was determined to be 2 %. The findings of this study suggest that 2 % (w/v) chitosan can be utilized for extending the postharvest shelf life of button mushrooms by delaying the ripening process during cold storage.
Keywords-- Antioxidant capacity, ascorbic acid, edible coating, nutritional quality, respiratory rate.
I. INTRODUCTION
Consumers all around the world demand the production of more natural and environmentally friendly foods that have a longer shelf life and are of high quality without the use of any chemical preservatives. Thus, the use of edible coatings, an environmentally friendly bio-based technology used to extend the shelf life of foods, has become a popular strategy (Gol et al., 2013). Chitosan, which is an edible coating agent, is the second most abundant polysaccharide after cellulose in nature and has attracted a great deal of scientific and agricultural interest due to its unique characteristics such as biocompatibility, film-forming and antimicrobial effect (Yu et al., 2018). Many studies have been carried out on the use of chitosan as an edible coating to preserve the quality of various fruits and vegetables, however very little is known about its effects on the quality characteristics of button mushrooms.
Agaricus bisporus, known as the button mushroom, is one of the most economically considerable edible mushrooms. It is of great nutritional value with high contents of ergothioneine, polyphenols, minerals, vitamins and polysaccharides. Moreover, it has several important biological activities such as antitumor, anti-inflammatory, antimicrobial, anti-aromatase, immunomodulatory and antioxidant properties (Liu et al., 2013). Although the button mushroom is consumed worldwide, its commercial potential is limited due to its short shelf life which is around 8 days in refrigerated conditions (Gholami et al., 2017). Therefore, an economic, adequate, and efficient postharvest preservation of the mushroom to extend its shelf life is of great necessity which would be profitable for both the mushroom industry and the consumers.
Considering the increasing demand for fresh food, many preservation methods have been developed including low temperature storage, γ-irradiation, modified atmosphere packaging and chemical treatments in order to slow down the postharvest deterioration rate of fresh mushrooms (Nasiri et al., 2018). The application of a semi-permeable edible coating such as chitosan may have the same effect as storing the mushrooms in a modified atmosphere to prolong their shelf life. Chitosan coatings act as a barrier to gas exchange and water vapor transfer. They reduce the gas exchange between the mushrooms and the surrounding atmosphere by creating a modified interior atmosphere (low O2 and high CO2) and decreasing water loss (Nasiri et al., 2018). The preservation of food quality and the extension of postharvest life was achieved by using chitosan coatings in mangos (Kittur et al., 2001), sliced button mushrooms (Eissa, 2007), Eksotika II papayas (Ali et al., 2011), guavas (Hong et al., 2012; Sneha Nair et al., 2018), shiitake mushrooms (Jiang et al., 2012), bananas (Wang and Gao, 2013), asparagus (Qiu et al., 2013), Cavendish bananas (Suseno et al., 2014), red kiwifruit (Kaya et al., 2016), pomegranate arils (Ozdemir and Gokmen, 2017), plums (Kumar et al., 2017), table grapes (Gao et al., 2013; Castelo Branco Melo et al., 2018) and grass carp fillets (Yu et al., 2018). According to the literature review, the use of chitosan coating in whole fresh button mushrooms has never been studied before. Furthermore, no studies investigating the influence of 3 % (w/v) chitosan coating on the shelf life and quality of the product were found.
The aim of this study was to evaluate the effects of edible coatings using chitosan at different concentrations on the postharvest quality of button mushrooms stored at 4ºC for 20 days and to determine the formulation of the chitosan coating that extends the shelf life of the mushrooms by best preserving their postharvest qualities. By determining and using the proper formulation of chitosan coating, it was also aimed to help keep the quality characteristics of the mushrooms at maximum level during storage.
II. METHODS
A. Coating Treatments
The button mushrooms used in this study, all of which were similar in terms of size, shape and color, were purchased from commercial market. The purchased mushrooms had been separated from the bed one day ago. For surface-sterilization, the mushrooms were immersed in a 0.1 % (v/v) NaClO solution during 1 min and then air-dried at room temperature for 30 min. Chitosan solutions of various concentrations (1.0, 2.0, 3.0 % w/v) were prepared from hydrosoluble chitosan powder (chitosan from crab shells, 91.6% deacetylation, Qingdao Reach International Inc., Qingdao, China) in the solution containing 2 % (w/v) malic acid with a pH value of 6.0. The button mushrooms were dipped into the chitosan solutions for 1 min. A control group was prepared from the mushrooms treated only with 2 % (w/v) malic acid solution (Eissa, 2007). All coated samples were placed in macroperforated polypropylene film bag (40 μm thickness, 0.2mm2 surface, 1.3 × 104 perforations/m2) and stored at 4 °C for 20 days to evaluate both their postharvest shelf life and qualities every 5 days. All of the samples were prepared in duplicates.
B. Respiration Rate, Percentage of Open Caps and Weight Loss
Respiration rate was performed according to the method reported by Li et al. (2006) with various modifications. A closed system was used to determine the respiration rate of the mushrooms. 50 g of randomly selected mushrooms were placed in a Petri-dish filled with 10 mL of 0.4 M NaOH. Then, they were kept in a desiccator during 30 min. The residual NaOH in the Petri-dish was titrated with 0.4 M oxalic acid. The respiratory rate was calculated from the consumed oxalic acid volume.
The development of an umbrella-like shape on the cap followed by the detachment of the veil were taken as the criteria for evaluating the percentage of open caps. The percentage of open caps was expressed as the ratio of the number of open capped mushrooms to the total number of mushrooms in the package (Jiang, 2013).
Weight loss was determined in accordance with the methods of Ares et al. (2006) by weighing the packages of mushrooms before and after the storage period. Weight loss was determined as the percentage of the loss of weight concerning the initial weight.
C. Total Soluble Solids and Titratable Acidity
Twenty grams of mushrooms were homogenized and centrifuged (multispeed centrifuge IEC CL31; Thermo Fisher Scientific Inc., Wilmington, DE, USA) for 20 min at 10,000 × g. The collected supernatant was then analyzed. The percent soluble solids were determined by a digital refractometer (RFM 330; Bellingham + Stanley Ltd, Kent, UK) and expressed as Brix (Eissa, 2007). The device reads ºBrix values at corrected temperature levels. The titratable acidity was also determined according to the method described by Ali et al. (2011). Ten grams of mushrooms were homogenized with 40 mL of distilled water using a commercial blender (7010S; Waring Commercial, Torrington, Connecticut USA). The mixture was filtered and then 5 mL of the filtrate was titrated with 0.1 M NaOH in the presence of phenolphthalein as an indicator. Titratable acidity was expressed as a percentage of citric acid per 100 g fresh mushrooms.
D. Antioxidant Capacity, Total Polyphenol Content and Ascorbic Acid Content
For the preparation of the polyphenol extracts, 0.5 g of mushrooms were extracted with 10 mL of pure methanol by centrifugation (multispeed centrifuge IEC CL31; Thermo Fisher Scientific Inc., Wilmington, DE, USA) at 936 × g for 15 min at 4°C (Mami et al., 2014). The abilities of the polyphenol extracts obtained from the mushrooms to scavenge 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical were determined by modifying the methods of Cheung et al. (2003). One mililiter of extract or methanol (control sample) was mixed with 0.5 mL of 0.1 mM DPPH radical prepared in methanol. The mixture was vortexed (test tube shaker TTS 2; IKA-Werke GMBH and CO.KG, Lille, France) and the absorbance was read immediately at 520 nm with an UV-vis spectrophotometer (Optizen Pop; Mecasys Co., Ltd., Korea). The results were expressed as the percentage of inhibition. The total polyphenol contents of the mushrooms were determined using the Folin-Ciocalteu colorimetric method (Mami et al., 2014). Three hundred microliters of the extract were mixed with 200 μL of distilled water and 2.5 mL of 10 % Folin-Ciocalteu reagent. After standing for 6 min, 2 mL of 7.5 % sodium carbonate solution was added to the mixture. The mixture was kept in the dark and at room temperature for 90 min and then its absorbance was determined by a UV/vis spectrophotometer (Optizen Pop; Mecasys Co., Ltd., Korea) at 760 nm. The results were evaluated as mg gallic acid equivalent per 100 g fresh mushrooms.
The ascorbic acid contents of the mushrooms were carried out using the 2,6- dichlorophenolindophenol titration method developed by Mami et al. (2014). Six grams of mushrooms were mixed with 40 mL of 2 % (w/v) oxalic acid and then homogenized. Ten mL of supernatant was titrated by 2,6-dichlorophenolindophenol solution and the results were expressed as mg ascorbic acid per 100 g of fresh mushrooms.
E. Statistical Analysis
All experiments were replicated twice, and their results were reported as means ± standard deviations. One-way analysis of variance (ANOVA) was applied on the data sets using SPSS 20.0 statistical package program. Statistical significances were determined (p<0.05). Duncan multiple range test was used for evaluating the significant difference amongst the samples.
Figure 1: Effect of chitosan coating on respiration rates (A), percent open caps (B) and weight losses (C) of button mushrooms stored at 4 °C during 20 days. Vertical bars represent standard deviations of means.
III. RESULTS
A. Evaluation of Changes in Respiration Rate, Percentage of Open Caps and Weight Loss
The main properties of the respiration rates, percentage of open caps and weight losses of the button mushrooms coated with different ratios of chitosan are shown in Fig. 1.
During the storage period, the respiration rates of the mushrooms significantly decreased while their percentage of open caps and weight losses significantly increased (p<0.05). The respiratory rates, percentage of open caps and weight losses of the coated mushrooms were lower than those of the control group (p<0.05). At the beginning of the storage period, the respiratory rate of the control group was 78.30 % while this rate was 1.26, 1.39 and 1.34 times higher than those of the 1 % chitosan, 2 % chitosan and 3 % chitosan-coated mushrooms at the end of the storage period, respectively. Throughout the storage period, 36.91 % reduction in weight was observed in the control group while 34.63 %, 16.62 % and 26.01 % weight losses occurred in the 1 % chitosan, 2 % chitosan and 3 % chitosan-coated mushrooms, respectively. The percentage of open cap mushrooms in the control group was 50.0 % after 20 days in storage. On the other hand, the percentages of open caps in the mushrooms coated with 1 % (w/v) chitosan, 2 % (w/v) chitosan and 3 % (w/v) chitosan were in the range of 13.89 (2 % chitosan-coated mushrooms) – 32.14 % (1 % chitosan-coated mushrooms) after 20 days. The mushrooms coated with 2 % (w/v) chitosan showed the lowest respiratory rates, percentage of open caps and weight loss (p<0.05). The results of this study regarding the changes in the respiration rates, percentage of open caps and weight loss during the storage period of the mushrooms were consistent with the previous works of Ali et al. (2011), Jiang et al. (2012), Jiang (2013), Gao et al. (2013), Suseno et al. (2014), Kaya et al. (2016), Kumar et al. (2017), and Sneha Nair et al. (2018).
Chitosan coating modifies the internal atmosphere of the mushrooms. It leads to a reduction of CO2 production in the coated mushrooms and thereby, the respiration of the mushrooms slows down (Jiang et al., 2012). Gas exchange between the mushrooms and the surrounding atmosphere occurs through the open pores that exist as a result of the high permeability of the skin of the mushrooms. Owing to the partial blockage of the pores existing in mushrooms with chitosan coating, moisture loss is partially prevented and the respiratory rate is kept under control (Castelo Branco Melo et al., 2018). The cap opening of the mushrooms is a result of the mushrooms drying due to water loss during the storage period. Increased water loss during the storage period causes a reduction in the cohesive forces of water and molecules like proteins responsible for the entire position of the cap and veil of mushroom. Chitosan coating also reduces the percentage of open cap by reducing the water loss in mushrooms (Jiang, 2013). Due to the fact that the ripening process continues throughout the storage period, the percentage of open cap and water loss in mushrooms increases while the rate of respiration decreases.
B. Evaluation of Changes in Total Soluble Solids and Titratable Acidity
The data of total soluble solids content and titratable acidity, which are important quality properties related to the ripening and maturation of the button mushrooms coated with different ratios of chitosan, are presented in Table 1. Chitosan coating caused a significant increase in the total soluble solids content of the mushroom (p<0.05). The total soluble solids content exhibited an increasing trend throughout the 20 day storage period (p<0.05) while the titratable acidity did not change significantly throughout the storage period, except for in the control group, and was not affected by the coating application (p>0.05).
During the 20 day storage period, the
increase of total soluble solid content was in the range of 14.63 (2 %
chitosan-coated mushrooms) – 21.43 % (1 % chitosan-coated mushrooms) in the
coated mushrooms while it was 55.56 % in the control group. At the end of the
stor-
