Changes in microbial and postharvest quality of shiitake mushroom (Lentinus edodes) treated with chitosan–glucose complex coating under cold storage Tianjia Jiang, Lifang Feng, Jianrong Li ⇑ College of Food Science and Biotechnology, Zhejiang Gongshang University, Food Safety Key Lab of Zhejiang Province, Hangzhou 310035, PR China article info Article history: Received 13 July 2011 Received in revised form 23 August 2011 Accepted 23 August 2011 Available online 19 September 2011 Keywords: Chitosan-glucose complex Shiitake mushroom Microbiological quality Sensory evaluation Storage life abstract The effect of chitosan, glucose and chitosan–glucose complex (CGC) on the microbial and postharvest quality of shiitake (Lentinus edodes) mushroom stored at 4 ± 1 °C for 16 days was investigated. Mushroom weight loss, respiration rate, firmness, ascorbic acid, total soluble solids, microbial and sensory quality were measured. The results indicate that treatment with CGC coating maintained tissue firmness, inhib- ited increase of respiration rate, reduced microorganism counts, e.g., pseudomonads, yeasts and moulds, compared to uncoated control mushroom. The efficiency was better than that of chitosan or glucose coat- 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 shiitake 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. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Shiitake (Lentinus edodes) mushroom is highly perishable and tends to lose quality immediately after harvest. Its shelf life is short because of its high respiration rate, tendency to turn brown and 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 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, 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 and 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- 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 0308-8146/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.08.087 ⇑ Corresponding author. Tel./fax: +86 571 88071024. E-mail address: li58516@sohu.com (J. Li). Food Chemistry 131 (2012) 780–786 Contents lists available at SciVerse ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem 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- ing. Then a tissue paper was used to absorb excess solution from the 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  10 À18 mol m À1 s À1 Pa À1 for O 2 , 4134  10 À18 mol m À1 s À1 Pa À1 for CO 2 (both at 20 °C and 100% RH) and 2.8  10 À5 –6.5  10 À5 gm À2 s À1 for H 2 O (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- mal 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 M oxalate. Measurements were replicated three times. Respiration rates of samples were (expressed as CO 2 production rate) calcu- lated with the following formula: RI ¼ ðV 1 À V 2 ÞÂc  44 W  t In the formula, V 1 is the volume of oxalate titrated for the control (mL); V 2 is the volume of oxalate titrated for the samples (mL); c is the concentration of oxalate (M); 44 is the molecular weight of CO 2 ; 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 s À1 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 (10 À1 –10 v9 ) 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; BD, 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 T. Jiang et al. /Food Chemistry 131 (2012) 780–786 781 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- tured 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 CO 2 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 O 2 and CO 2 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 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 O 2 and a rise in CO 2 , 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- 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 CO 2 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 60 80 100 120 140 160 180 200 0481216 Stora g e time (da y s) Respiration rate (mg CO 2 kg 1 h 1 ) Control Glucose Chitosan CGC Fig. 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. 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0481216 Stora g e time (da y s) Weight loss (%) Control Glucose Chitosan CGC Fig. 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. 782 T. Jiang et al. /Food Chemistry 131 (2012) 780–786 glucose 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 matrix and reducing the central vacuole, resulting in partially collapsed cells and a loss of turgor. This kind of bacterial-induced softening 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 mushroom at end of the storage. Tao, Zhang, Yu, and Sun (2006) have reported an increase in SSC in button mushrooms stored at 4 ± 1 °C and 75% RH. The effect of chitosan in reducing the increase in SSC during storage of shiitake mushroom was probably due to the slowing down of respiration and metabolic activity, hence retarding the senescence process. Indeed, the greater changes in SSC occurred in those mushrooms which suffered the greatest water loss. The solubilisation of the cell wall polysaccharides and hemicelluloses in senescent mushroom might also contribute to the increase in SSC. It is well documented that the filmogenic property of chitosan results in an excellent semi-permeable film around the vegetable and fruit, modifying the internal atmosphere by reducing O 2 and/ or elevating CO 2 , and suppressing ethylene evolution (Dong, Cheng, Tan, Zheng, & Jiang, 2004). A suppressed respiration rate also slows down the synthesis and the use of metabolites, resulting in lower SSC, due to the slower hydrolysis of carbohydrates to sugars (Roh- ani, Zaipun, & Norhayati, 1997). Our results are in line with those of Kittur, Saroja, Habibunnisa, and Tharanathan (2001), where a slow rise in SSC was recorded in mango and banana treated with chito- san. However, other studies have indicated that the SSC of chitosan dipped papaya and zucchini were the same as in the untreated 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- creased throughout storage, the use of CGC coating significantly re- duced the loss of ascorbic acid in mushroom samples. After 16 days of storage, ascorbic acid retention of mushroom treated with glu- cose, chitosan and CGC coating was 19.3, 23.5 and 25.9 mg/kg, respectively, whereas control samples maintained 17.7 mg/kg of initial ascorbic acid content. Since ascorbic acid loss can be greatly favoured by the presence of O 2 , the incorporation of chitosan to coating formulations may reduce O 2 diffusion, slow down the res- piration rate and consequently better preserve ascorbic acid con- tent and delay senescence of shiitake mushroom. Similar results were obtained by Ayranci and Tunc (2003), who reported that methylcellulose-based edible coating reduced ascorbic acid loss in both button mushrooms and cauliflower. The ascorbic acid con- tent in the CGC coated samples was higher than that in the samples coated with chitosan. It has been suggested that edible coatings containing chitosan promote ascorbic acid loss by acting as an abi- otic elicitor, generating reactive oxygen species (ROS), which are scavenged by antioxidant compounds, such as ascorbic acid. CGC coating could inhibit ascorbic acid loss, due to the protection ef- fected by its superior antioxidant activity, as compared to chitosan or glucose (Kanatt et al., 2008). 0 0.5 1 1.5 2 2.5 3 3.5 4 Stora g e time (da y s) Firmness (N) Control Glucose Chitosan CGC Fig. 3. Effect of CGC coating on firmness 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. 0 1 2 3 4 5 6 7 8 9 0481216 Storage time (days) Soluble solids concentration (%) Control Glu co s e Chitos an CGC 0 5 10 15 20 25 30 35 40 45 0481216 Stora g e time (da y s) Ascorbic acid (mg •kg ) Control Glu co s e Chitos an CGC A B 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. T. Jiang et al. /Food Chemistry 131 (2012) 780–786 783 3.5. Effect of CGC coating on microbiological quality Mesophilic bacteria, pseudomonads, yeasts and moulds pre- dominated during storage in all the analysed samples. It is evident from this study that CGC coating was more effective in reducing microbial counts than other treatments (Table 1). In any of the studied treatments, the psychrophilic bacteria counts increased less than two orders during the entire storage period. All samples Table 1 Effect of CGC coating on microbial counts (log 10 cfu g À1 ) change of shiitake mushrooms stored at 4 °C for 16 days. a,b,c Days at 4 °C Control Glucose Chitosan CGC Mesophilic 0 4.12 ± 0.06 aE 4.10 ± 0.08 aE 4.16 ± 0.04 aE 4.13 ± 0.11 aD 4 4.54 ± 0.12 aD 4.38 ± 0.20 aD 4.43 ± 0.02 aD 4.45 ± 0.14 aC 8 5.30 ± 0.08 aC 5.25 ± 0.17 aC 4.76 ± 0.16 bC 4.57 ± 0.08 cC 12 5.83 ± 0.20 bB 6.10 ± 0.05 aB 5.12 ± 0.07 cB 4.84 ± 0.14 dB 16 6.61 ± 0.17 bA 6.82 ± 0.13 aA 5.47 ± 0.14 cA 5.22 ± 0.24 dA Psychrophilic 0 1.64 ± 0.12 aD 1.62 ± 0.11 aD 1.60 ± 0.05 aC 1.61 ± 0.12 aD 4 2.24 ± 0.17 aC 2.12 ± 0.05 abC 2.27 ± 0.16 aBC 1.98 ± 0.18 bC 8 2.67 ± 0.06 aB 2.59 ± 0.09 aB 2.58 ± 0.12 aB 2.47 ± 0.09 aB 12 2.89 ± 0.02 aAB 2.92 ± 0.14 aAB 2.90 ± 0.09 aAB 2.87 ± 0.10 aAB 16 3.25 ± 0.24 aA 3.20 ± 0.24 aA 3.17 ± 0.14 aA 3.18 ± 0.17 aA Pseudomonad 0 5.37 ± 0.16 aE 5.35 ± 0.06 aE 5.34 ± 0.13 aE 5.38 ± 0.07 aE 4 6.46 ± 0.09 aD 6.11 ± 0.08 bD 5.75 ± 0.22 bD 5.57 ± 0.14 cD 8 6.90 ± 0.27 bC 7.21 ± 0.10 aC 6.34 ± 0.18 cC 5.74 ± 0.27 dC 12 7.87 ± 0.35 aB 7.90 ± 0.07 aB 6.68 ± 0.27 bB 6.12 ± 0.24 cB 16 8.46 ± 0.21 aA 8.55 ± 0.16 aA 7.36 ± 0.15 bA 6.35 ± 0.28 cA Yeasts and moulds 0 3.81 ± 0.11 aE 3.82 ± 0.07 aE 3.86 ± 0.05 aE 3.80 ± 0.13 aE 4 4.55 ± 0.06 aD 4.36 ± 0.13 aD 4.11 ± 0.09 bD 4.15 ± 0.06 bD 8 5.62 ± 0.04 aC 5.58 ± 0.18 aC 4.87 ± 0.16 bC 4.50 ± 0.09 cC 12 5.97 ± 0.15 aB 6.04 ± 0.27 aB 5.32 ± 0.22 bB 4.82 ± 0.18 cB 16 6.60 ± 0.22 aA 6.68 ± 0.14 aA 5.76 ± 0.21 bA 5.05 ± 0.26 cA a Mean of three replications ± standard deviation. b Means in same row with different small letters are significantly different (p < 0.05). c Means in same column with different capital letters are significantly different (p < 0.05). Table 2 Effect of CGC coating on sensory attributes change of shiitake mushrooms stored at 4 °C for 16 days. a,b,c Days at 4 °C Control Glucose Chitosan CGC Off-odour 00 0 0 0 4 1.62 ± 0.04 aD 1.51 ± 0.06 abD 1.54 ± 0.03 abD 1.41 ± 0.05 bD 8 2.51 ± 0.06 aC 2.20 ± 0.11 bC 2.16 ± 0.07 bC 2.19 ± 0.08 bC 12 5.35 ± 0.12 aB 5.05 ± 0.07 bB 4.22 ± 0.08 cB 3.83 ± 0.12 dB 16 7.12 ± 0.15 aA 6.93 ± 0.15 bA 5.82 ± 0.05 cA 5.52 ± 0.16 dA Gills colour 00 0 0 0 4 1.32 ± 0.05 abD 1.23 ± 0.05 bcD 1.10 ± 0.06 cD 1.87 ± 0.11 aD 8 2.93 ± 0.09 aC 2.81 ± 0.12 abC 2.55 ± 0.06 cC 2.63 ± 0.08 bcC 12 5.41 ± 0.24 aB 5.18 ± 0.26 bB 4.17 ± 0.12 cB 5.21 ± 0.16 bB 16 7.85 ± 0.20 aA 6.73 ± 0.14 cA 5.04 ± 0.16 dA 7.37 ± 0.19 bA Gill uniformity 010101010 4 8.47 ± 0.20 aA 8.36 ± 0.24 aA 8.12 ± 0.31 bA 8.03 ± 0.16 bA 8 6.62 ± 0.13 bB 6.53 ± 0.07 bB 7.46 ± 0.16 aB 7.34 ± 0.05 aB 12 4.95 ± 0.14 cC 5.17 ± 0.12 bC 5.72 ± 0.12 aC 5.87 ± 0.08 aC 16 4.11 ± 0.04 dD 4.32 ± 0.10 cD 5.08 ± 0.05 bD 5.32 ± 0.13 aD Cap uniformity 010101010 4 8.53 ± 0.14 bA 8.44 ± 0.26 bA 8.60 ± 0.07 bA 8.85 ± 0.30 aA 8 7.56 ± 0.17 bB 7.42 ± 0.20 bB 7.83 ± 0.21 aB 7.91 ± 0.21 aB 12 6.20 ± 0.07 cC 6.57 ± 0.27 bC 7.21 ± 0.14 aC 7.16 ± 0.17 aC 16 5.12 ± 0.27 bD 5.22 ± 0.16 bD 6.21 ± 0.15 aD 6.11 ± 0.15 aD Dark zones 00 0 0 0 4 1.25 ± 0.04 aD 1.17 ± 0.04 aD 0.88 ± 0.07 bD 1.07 ± 0.02 aD 8 2.32 ± 0.05 aC 2.22 ± 0.07 aC 1.73 ± 0.11 bC 1.53 ± 0.04 cC 12 3.59 ± 0.08 aB 3.33 ± 0.12 bB 2.55 ± 0.02 cB 2.12 ± 0.10 dB 16 5.72 ± 0.14 aA 4.36 ± 0.21 bA 3.12 ± 0.10 cA 2.87 ± 0.08 dA a Mean of three replications ± standard deviation. b Means in same row with different small letters are significantly different (p < 0.05). c Means in same column with different capital letters are significantly different (p < 0.05). 784 T. Jiang et al. /Food Chemistry 131 (2012) 780–786 had counts below 10 4 cfu g À1 . This contamination level suggests that shiitake mushrooms in the studied coating conditions did not favour the development of this type of bacteria. Mushrooms from the control treatment exhibited tiny brown spots on Day 4 that developed into dark zones, characteristic of Pseudomonas spoilage by Day 8. Mushrooms were highly decayed at this point and the end of shelf-life was due to microbial spoilage. The CGC- coated samples did not exhibit these characteristics of microbial degradation even on Day 12. The organisms usually responsible for spoilage of mushrooms are gram-negative, psychrotrophic bac- teria, particularly belonging to the Pseudomonadacae family, be- cause of contamination of the product from compost. Kanatt et al. (2008) have reported the antimicrobial activity of CGC was similar to chitosan against E. coli, Pseudomonas, S. aureus and B. cer- eus, the common food spoilage and pathogenic bacteria. However, in our experiment, we found that CGC exhibited superior antimi- crobial activity to chitosan in coating shiitake mushrooms during storage. Therefore, microbial degradation resulting in changes such as browning and softening was clearly delayed in CGC-coated samples. 3.6. Effect of CGC coating on sensory attributes As expected, mushroom off-odour, gill colour, gill uniformity, cap uniformity and dark zones significantly (p < 0.05) changed with storage time, supporting the validity of using these parameters as indicators of mushroom deterioration. Average values for the sen- sory attributes are shown in Table 2. Off-odour intensity signifi- cantly increased after 8 days of storage in control samples. The colour of mushroom gills gradually became browner and less uni- form with time for all the evaluated conditions. The gills of control mushrooms showed a colour intensity of 5.4 and uniformity near to 5.0 at the 12th day of storage. However, the gills of chitosan coated mushrooms reached these intensities after 4 days of storage. A bet- ter trend was observed for the uniformity of the cap surface and the presence of dark stains on the cap in CGC-coated samples. These re- sults suggest that CGC and chitosan coating were more effective in retarding mushroom sensory deterioration. The browning of mush- rooms is attributed to the action of polyphenol oxidase (PPO) and the action of bacteria and mould on the mushroom tissue. CGC and chitosan coating cause reduction of spoilage organisms, such as Pseudomonas, responsible for oxidation of phenolic compounds to form brown-coloured melanins, and thus prevent the formation of brown patches, hence improving the appearance and colour. Considering the development of the evaluated sensory attributes, mushrooms coated with CGC showed the lowest deterioration rate, followed by those coated with chitosan and finally those coated with glucose and the control treatment. Although CGC coating showed negative effect on gills colour because of the browning col- our itself, it could maintain sensory characteristics of shiitake mushrooms for the longest time. This could be related to the fact that CGC had superior antioxidant and antimicrobial activity as compared to chitosan or glucose. 4. Conclusions Our research showed that the senescence inhibition of cold- stored shiitake mushroom by the CGC coating treatment involved the maintenance of tissue firmness and sensory quality, inhibition of respiration rate, and reduction of microbial counts compared with the control. In addition, CGC coating also delayed changes in the ascorbic acid and soluble solids concentration during the storage period. 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Effect of CGC coating on texture The texture of shiitake mushroom is often the first of. CGC coating might be a promising candidate for maintaining shiitake mushroom quality and extending its postharvest life. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Shiitake (Lentinus