Báo cáo Khoa học changes in microbial and postharvest quality of shiitake mushroom (lentinus edodes) treated with chitosan–glucose complex coating under cold storage

al c Key Chitosan-glucose complex cose ed te, fi s in rat ing treatment. In addition, CGC coating also delayed changes in the ascorbic acid and soluble solids con- centration. Sensory evaluation proved the efficacy of CGC coating by maintaining the overall quality of oom is er harv tende shelf-life of shiitake mushrooms (Ares, Parentelli, Gámbaro, Lareo, & Lema, 2006; Jiang, Wang, Xu, Jahangir, & Ying, 2010). Jiang, Luo, Chen, Shen, and Ying (2010) also reported application of gamma- irradiation in combination with MAP can extend the storage life of shiitake mushroom up to 20 days. Chitosan [b-(1,4)-2-amino-2-deoxy-D-glucopyranose], which is mainly made from crustacean shells, is the second most abundant natural polymer in nature after cellulose (Shahidi, Arachchi, & Jeon, tions. However, chemical modifications are generally not preferred for food applications because of the formation of potential detrimental products. Chitosan–lysozyme conjugates have been reported to have better emulsifying properties and bactericidal action (Song, Babiker, Usui, Saito, & Kato, 2002). The Maillard reaction, resulting from condensation between the carbonyl group of reducing sugars, aldehydes or ketones and an amine group of amino acids, proteins or any nitrogenous compound, is one of the main reactions taking place in food. Mail- lard reaction compounds contribute to flavour formation, antioxi- dative and antimicrobial effects and improvement of functional ⇑ Corresponding author. Tel./fax: +86 571 88071024. Food Chemistry 131 (2012) 780–786 Contents lists available at he lseE-mail address: li58516@sohu.com (J. Li).having no physical protection to avoid water loss or microbial at- tack (Simón, González-Fandos, & Tobar, 2005). Bacteria, moulds, enzymatic activity and biochemical changes can cause spoilage during storage. Gram-negative microorganisms, such as Pseudomo- nas tolaasii, Pseudomonas fluorescens and yeasts, such as Candida sake, have been associated with mushroom spoilage (Masson, Ainsworth, Fuller, Bozkurt, & Ibanoglu, 2002). The short shelf-life of mushroom is an impediment to the distribution and marketing of the fresh product. The use of modified atmosphere packaging as an adjunct to low temperature storage has been extensively reported to extend the antioxidant in muscle foods (Gómez-Estaca, Montero, Giménez, & Gómez-Guillén, 2007; Kim & Thomas, 2007). Furthermore, chito- san also has potential for food packaging, especially as edible films and coatings (Tual, Espuche, Escoubes, & Domard, 2000). It has been used to maintain the quality of postharvest fruits and vegetables, such as citrus (Chien, Sheu, & Lin, 2007), tomatoes (El Ghaouth, Ponnampalam, Castaigne, & Arul, 1992), apples (Ippolito, El Ghaouth, Wilson, & Wisniewski, 2000), longan fruit (Jiang & Li, 2001), peach, pear and kiwifruit (Du, Gemma, & Iwahori, 1997). Several researchers have developed methods to improve the properties of chitosan using chemical and enzymatic modifica-Shiitake mushroom Microbiological quality Sensory evaluation Storage life 1. Introduction Shiitake (Lentinus edodes) mushr tends to lose quality immediately aft because of its high respiration rate,0308-8146/$ - see front matter  2011 Elsevier Ltd. A doi:10.1016/j.foodchem.2011.08.087shiitake mushroom during the storage period. Our study suggests that CGC coating might be a promising candidate for maintaining shiitake mushroom quality and extending its postharvest life.  2011 Elsevier Ltd. All rights reserved. highly perishable and est. Its shelf life is short ncy to turn brown and 1999). Due to its non-toxic nature, antioxidative and antibacterial activity, film-forming property, biocompatibility and biodegrad- ability, chitosan has attracted much attention as a natural food additive (Majeti & Ravi, 2000). Chitosan has been used in foods, as a clarifying agent in apple juice, and antimicrobial andKeywords: compared to uncoated control mushroom. The efficiency was better than that of chitosan or glucose coat-Changes in microbial and postharvest qu mushroom (Lentinus edodes) treated with complex coating under cold storage Tianjia Jiang, Lifang Feng, Jianrong Li ⇑ College of Food Science and Biotechnology, Zhejiang Gongshang University, Food Safety a r t i c l e i n f o Article history: Received 13 July 2011 Received in revised form 23 August 2011 Accepted 23 August 2011 Available online 19 September 2011 a b s t r a c t The effect of chitosan, glu quality of shiitake (Lentinus weight loss, respiration ra were measured. The result ited increase of respiration Food C journal homepage: www.ell rights reserved.ity of shiitake hitosan–glucose Lab of Zhejiang Province, Hangzhou 310035, PR China and chitosan–glucose complex (CGC) on the microbial and postharvest odes) mushroom stored at 4 ± 1 C for 16 days was investigated. Mushroom rmness, ascorbic acid, total soluble solids, microbial and sensory quality dicate that treatment with CGC coating maintained tissue firmness, inhib- e, reduced microorganism counts, e.g., pseudomonads, yeasts and moulds, SciVerse ScienceDirect mistry vier .com/locate / foodchem ing. Then a tissue paper was used to absorb excess solution from mistrthe surface. The treated samples were placed and sealed in 18 cm  20 cm bags of low density polyethylene (PE) (0.04 mm thickness) in the laboratory; the PE gas transmission rates were 1078  1018 mol m1 s1 Pa1 for O2, 4134  1018 mol m1 s1 Pa1 for CO2 (both at 20 C and 100% RH) and 2.8  105–6.5  105 g m2 s1 for H2O (at 37 C and 90% RH). They were then stored for 16 days at 4 ± 1 C and 95% relative humidity (RH). Fifteen replicates were included in each treatment group, and sub- sequently every 4 days, three replicates from each treatment group were analysed. 2.3. Respiration rate Respiration rate was determined according to the method of Li, Zhang, and Yu (2006). A closed system was chosen to measure res- piration rate of the product. At each storage time, approximately 50 g of mushrooms from the four groups were placed under nor-properties (Chevalier, Chobert, Genot, & Haertle, 2001). It is desir- able to modify chitosan so that it attains excellent antioxidant activity without affecting its antimicrobial activity. Chitosan has an amino group which can react with the carbonyl group of a reducing sugar. Hence, chitosan was heated with glucose to form a Maillard reaction product. Kanatt, Chander, and Sharma (2008) found that chitosan–glucose complex (CGC), a modified form of chitosan prepared by heating chitosan with glucose, showed excel- lent antioxidant activity, while chitosan or glucose alone did not have any significant activity. On the other hand, the antimicrobial activity of CGC was similar to chitosan against Escherichia coli, Pseudomonas, Staphylococcus aureus and Bacillus cereus, and it can increase the shelf life of pork cocktail salami to 28 days. However, research on the application of CGC to fruits and veg- etables is limited. The objectives of this work were to evaluate the effect of CGC on the microbiological and postharvest quality of shiitake mushrooms during cold storage. 2. Materials and methods 2.1. Materials Shiitake (Lentinula edodes) mushrooms used in this study were harvested from a local farm in Hangzhou, China. Mushrooms were picked from the same flower and from the same area of the shed so as to reduce possible variations caused by cultivation and environ- mental conditions. The mushrooms were transported to the labo- ratory within one hour of picking, under refrigerated conditions, then stored in darkness at 1 ± 1 C and 95% relative humidity (RH). 2.2. Preparation of chitosan–glucose complex (CGC) solutions and application of treatments Chitosan (deacetylated P95%, and viscosity 630 mPa s) was purchased from Zhejiang Xuefeng Calcium Carbonate Co., Ltd. (Zhejiang, China). One percentage of chitosan was prepared in 1% glacial acetic acid. Chitosan glucose complex (CGC) was prepared by autoclaving chitosan (1%) and glucose (1%) for 15 min. Mush- rooms were divided into four samples of 60 each. Four different treatments were used: (1) control; (2) 1% glucose coating; (3) 1% chitosan coating, and (4) CGC coating. Mushrooms were dipped into the solution for 5 min. Samples dipped in distilled water were used as control. Treated samples were kept over a plastic sieve for 30 min and a fan generating low-speed air was used to hasten dry- T. Jiang et al. / Food Chemal air for 1 h. Then, mushrooms were stored at 20 C for 1 h in a closed container, which contained 15 mL 0.05 M Ba(OH)2. Then, 2 drops of phenolphthalein were added, and titrated with 1/44 Moxalate. Measurements were replicated three times. Respiration rates of samples were (expressed as CO2 production rate) calcu- lated with the following formula: RI ¼ ðV1  V2Þ  c  44 W  t In the formula, V1 is the volume of oxalate titrated for the control (mL); V2 is the volume of oxalate titrated for the samples (mL); c is the concentration of oxalate (M); 44 is the molecular weight of CO2; W is the weight of samples (g); t is the test time (h). 2.4. Weight loss Weight loss was determined by weighing the whole mushroom before and after the storage period. Weight loss was expressed as the percentage of loss of weight with respect to the initial weight. 2.5. Texture measurement A penetration test was performed on the shiitake mushroom cap using a TA.XT Express-v3.1 texture analyser (Stable Micro Sys- tems, Godalming, UK), with a 5 mm diameter cylindrical probe. Samples were penetrated 5 mm in depth. The speed of the probe was 2.0 mm s1 during the pretest as well as during penetration. Force and time data were recorded with Texture Expert (Version 1.0) from Stable Micro Systems. From the force vs time curves, firmness was defined as the maximum force used. 2.6. Total soluble solids and ascorbic acid content Mushrooms were ground in a mortar and squeezed with a hand press, and the juice was analysed for total soluble solids (TSS). TSS was measured at 25 C with a digital refractometer (Atago, Tokyo, Japan). The determination of total ascorbic acid was carried out as described by Hanson et al. (2004), on the basis of coupling 2, 4-dinitrophenylhydrazine (DNPH) with the ketonic groups of dehy- droascorbic acid through the oxidation of ascorbic acid by 2,6- dichlorophenolindophenol (DCPIP) to give a yellow/orange colour in acidic conditions. Mushroom tissues (10 g) were blended with 80 mL of 5% metaphosphoric acid in a homogeniser and centri- fuged. After centrifuging, 2 mL of the supernatant were poured into a 20 mL test tube containing 0.1 mL of 0.2% 2,6-DCIP sodium salt in water, 2 mL of 2% thiourea in 5% metaphosphoric acid and 1 mL of 4% 2,4-DNPH in 9 N sulphuric acid. The mixtures were kept in a water bath at 37 C for 3 h followed by an ice bath for 10 min. Five millilitres of 85% sulphuric acid were added and the mixtures were kept at room temperature for 30 min before reading at 520 nm. 2.7. Microbiological analysis All samples were analysed for the mesophilic, psychrophilic, pseudomonad, and yeasts and moulds bacteria counts. Twenty-five grams of mushrooms were removed aseptically from each pack and diluted with 225 mL 0.1% peptone water. The samples were homogenised by a stomacher at high speed for 2 min. Serial dilu- tions (101–10v9) were made in serial dilution tubes by taking 1.0 mL with 9.0 mL of 0.1% peptone water. Aerobic counts were determined on plate count agar (PCA; Merck, Darmstadt, Germany) following incubation at 35 C for 2 days for mesophilic bacteria, and at 4 C for 7 days for psychrophilic bacteria. Pseudomonas was counted on cephaloridin fucidin cetrimide agar (CFC; Difco; y 131 (2012) 780–786 781BD, Franklin Lakes, NJ), with selective supplement SR 103 (Oxoid, Basingstoke, UK). The incubation temperature was 25 C and plates were examined after 48 h. Yeasts and moulds were estimated on potato dextrose agar (PDA; Merck) and incubation conditions were 28 ± 1 C for 5–7 days. 2.8. Sensory evaluation The sensory attributes that characterised mushroom deteriora- tion were determined. These attributes were: off-odour, gill colour, gill uniformity, cap surface uniformity, and presence of dark zones on the cap (Ares et al., 2006). Samples were evaluated by a sensory panel of 10 trained assessors. Mushrooms were served in closed, odourless plastic containers at room temperature. After opening polyethylene bags, mushrooms were placed in plastic containers and evaluations were performed within 2 h, in order to avoid loss of off-odours. A balanced complete block design was carried out for duplicate evaluation of the samples. For scoring, 10 cm unstruc- modify the internal atmosphere of tomatoes (El Ghaouth et al., 1992), Japanese pear (Du et al., 1997) and apples (Gemma & Du, 1998) by depletion of endogenous O2 and a rise in CO2, without achieving anaerobiosis. In our study, CGC coating is more effective in reducing the respiration rates of shiitake mushroom although the difference between the three coating treatments was not sig- nificant (p > 0.05). This could be because CGC coating is more effi- cient in restricting the gas exchange between mushroom and the atmosphere during storage. 3.2. Effect of CGC coating on weight loss Compared with the control samples, the coated mushrooms showed a significantly (p < 0.05) reduced weight loss during stor- age (Fig. 2). After 16 days of storage, the mushrooms coated with CGC and chitosan showed 2.41% and 2.71% weight loss, respec- 782 T. Jiang et al. / Food Chemistry 131 (2012) 780–786tured scales anchored with ‘‘nil’’ for zero and ‘‘high’’ for 10 were used, except for the gill colour descriptor, for which the anchors were ‘‘white’’ and ‘‘brown’’. 2.9. Statistical analysis Experiments were performed using a completely randomised design. Data were subjected to one-way analysis of variance (ANO- VA). Mean separations were performed by Tukey’s multiple range test (DPS Version 6.55). Differences at p < 0.05 were considered significant. 3. Results and discussion 3.1. Effect of CGC coating on respiration rate The main characteristics of the respiration rates of the shiitake mushrooms treated with different kinds of coatings are shown in Fig. 1. According to the results, throughout the storage period, the respiration rates of coated mushrooms significantly decreased (p < 0.05). These values were 78.2–92.6% of those of the control samples at the beginning of the cold storage period. By Day 16, the respiration rates of the control samples were 1.23–1.37 times higher than those of the coated mushrooms. Internal gas atmo- sphere modification has been suggested to be the cause of reduced CO2 production by coated fruits and vegetables. In this regard the gas barrier properties and permselectivity of the edible coating ap- plied to the skin surface and their dependence on relative humidity and temperature will play an important role in the changes in endogenous O2 and CO2 levels. It is well known that excessive restriction of gas exchange can lead to anaerobiosis and the devel- opment of off-flavour. Chitosan coating has been reported to 60 80 100 120 140 160 180 200 0 4 8 12 16 Storage time (days) R es pi ra tio n ra te (m g C O2 kg 1 h 1 ) Control Glucose Chitosan CGCFig. 1. Effect of CGC coating on respiration rate changes of shiitake mushrooms stored at 4 C for 16 days. Each data point is the mean of three replicate samples. Vertical bars represent standard errors of means.tively, as compared to 3.71% and 3.13% weight loss in control and glucose-coated mushroom. Mushroom weight loss is mainly cause by water transpiration and CO2 loss during respiration. The thin skin of shiitake mushrooms makes them susceptible to rapid water loss, resulting in shrivelling and deterioration. The rate at which water is lost depends on the water pressure gradient between the mushroom tissue and the surrounding atmosphere and the storage temperature. Low vapour pressure differences between the mushroom and its surroundings and low temperature are rec- ommended for the storage of mushrooms. Edible coatings act as barriers, thereby restricting water transfer and protecting mush- room epidermis from mechanical injuries, as well as sealing small wounds and thus delaying dehydration. Chitosan coatings have been effective at controlling water loss from some commodities, including cucumber, pepper and longan fruit (El Ghaouth, Arul, Ponnampalam, & Boulet, 1991; Jiang et al., 2001). Clearly, relatively lower weight loss in CGC coated mushrooms contributed to main- taining better quality of mushroom during cold storage. 3.3. Effect of CGC coating on texture The texture of shiitake mushroom is often the first of many quality attributes judged by the consumer and is, therefore, extremely important in overall product acceptance. Shiitake mush- room suffers a rapid loss of firmness during senescence which con- tributes greatly to its short postharvest life and susceptibility to fungal contamination. Fig. 3 shows that CGC and chitosan coatings significantly (p < 0.05) reduced the loss in firmness of shiitake mushroom during storage. There was no significant (p > 0.05) difference in the firmness of the control mushrooms and those 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 4 8 12 16 Storage time (days) W ei gh t l os s ( %) Control Glucose Chitosan CGCFig. 2. Effect of CGC coating on weight loss changes of shiitake mushrooms stored at 4 C for 16 days. Each data point is the mean of three replicate samples. Vertical bars represent standard errors of means. collapsed cells and a loss of turgor. This kind of bacterial-induced fruits (Bautista-Baños, Hernández-López, Bosquez-Molina, & Wil- son, 2003). Fig. 4B shows changes in ascorbic acid content of coated and un- coated shiitake mushrooms during 16 days storage. The initial ascorbic acid content of shiitake mushrooms was 41.6 mg/kg. Although ascorbic acid of both coated and uncoated samples de- 0 5 10 15 20 25 0 4 8 12 16 Storage time (days) A sc o rb ic ac id Fig. 4. Effect of CGC coating on total soluble solids (A) and ascorbic acid (B) change of shiitake mushrooms stored at 4 C for 16 days. Each data point is the mean of three replicate samples. Vertical bars represent standard errors of means. mistrsoftening was observed in control samples but was inhibited by chitosan and CGC coating treatments. The maintenance of firmness in the mushrooms treated with CGC and chitosan coatings could be due to their higher antifungal activity, and covering of the cuticle and lenticels, thereby reducing infection, respiration and other senescence processes during storage, according to previous reports in sweet cherry coated with aloe vera gel (Martínez-Romero et al., 2006). 3.4. Effect of CGC coating on total soluble solids and ascorbic acid content Changes in the soluble solids content (SSC) of shiitake mush- rooms over storage are shown in Fig. 4A. The SSC of control mush- rooms increased after 4 days of storage whilst coated mushrooms experienced a slight increase during the same period. The lowest levels of SSC were recorded in CGC and chitosan-coated mushroomglucose coated. The maximum retention in firmness was obtained by CGC and chitosan coating, with 2.80 N and 2.76 N firmness values, respectively, at the end of storage. Softening can occur be- cause of the degradation of cell walls in postharvest mushrooms by bacterial enzymes and increased activity of endogenous autolysins (Zivanovic, Buescher, & Kim, 2000). Microorganisms such as Pseudomonas degrade mushrooms by breaking down the intracel- lular ma