Evaluation of biodegradable film packaging to improve the shelf-life of Boletus edulis wild edible mushrooms

Innovative Food Science and Emerging Technologies 29 (2015) 288–294

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Evaluation of biodegradable film packaging to improve the shelf-life of Boletus edulis wild edible mushrooms Linlei Han a,1, Yuyue Qin b,1, Dong Liu a, Haiyun Chen a, Hongli Li a, Minglong Yuan a,⁎ a b

Engineering Research Center of Biopolymer Functional Materials of Yunnan, Yunnan Minzu University, 650500 Kunming, China Institute of Yunnan Food Safety, Kunming University of Science and Technology, 650500 Kunming, China

a r t i c l e

i n f o

Article history: Received 1 December 2014 Received in revised form 14 April 2015 Accepted 17 April 2015 Available online 2 May 2015 Keywords: Poly(lactic acid) film Boletus edulis Water vapor permeability Physicochemical quality Microbial quality

a b s t r a c t The objective of this work was to evaluate the effect of poly(lactic acid) (PLA) based biodegradable film packaging combining 0.5% nisin antimicrobial polypeptide on the physicochemical and microbial quality of Boletus edulis wild edible mushrooms stored at 4 ± 1 °C. The experiment was set up by packaging mushrooms with extruded PLA films containing 0, 7.5, and 15 wt.% triethyl citrate plasticizer. The low-density polyethylene (LDPE) film was used as the control. Mushrooms stored in PLA films containing 7.5 and 15 wt.% plasticizer provided better retention of quality characteristics and received higher sensory ratings compared to mushrooms stored in pure PLA film and LDPE film. Samples with these two treatments underwent minimal changes in texture, PPO activity, total bacteria count, and sensory attributes. Results suggest that nisin in combination with plasticized PLA film has the potential to maintain B. edulis wild edible mushroom quality and extend its postharvest life to 18 days. Industrial relevance: B. edulis is one of the most commercialized mushrooms worldwide. However, as with all fresh mushrooms, there are severe preservation problems. Extruded PLA films containing triethyl citrate plasticizer plus antimicrobial agent nisin proved to be a suitable technology for mushroom conservation. This material exhibits an environmental-friendliness potential and a high versatility in food packaging. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, there has been an increase in the worldwide consumption of fresh wild mushrooms (e.g. Lactarius deliciosus, Boletus edulis, Cantharellus spp., Hygrophorus spp., and Tricholoma spp.), due to their delicate flavor, trace minerals, and texture (Venturini, Reyes, Rivera, Oria, & Blanco, 2011; Fernandes et al., 2014). Mushrooms perish rapidly and have a limited shelf-life, frequently for only 1 to 3 days at room temperature (Fernandes et al., 2013). The high respiration rate, the lack of physical protection to avoid water loss and changes due to microbial attack are often associated with mushroom loss of quality. A general trend in mushroom preservation research is towards the development of preservation techniques that are less severe and therefore less damaging to food products (Fernandes et al., 2012). According to several authors, packaging films could delay mushroom spoilage with minimum changes in physiochemical and sensory quality of mushrooms (Barron, Varoquaux, Guilbert, Gontard, & Gouble, 2002; Ares, Parentelli, Gámbaro, Lareo, & Lema, 2006; Xing, Wang, Feng, & Tan, 2008; Taghizadeh, Gowen, Ward, & O'Donnell, ⁎ Corresponding author. Tel.: +86 87165913288. E-mail address: [email protected] (M. Yuan). 1 These authors contributed equally to this work.

http://dx.doi.org/10.1016/j.ifset.2015.04.008 1466-8564/© 2015 Elsevier Ltd. All rights reserved.

2010; Qin et al., 2013; Li et al., 2014). Xing et al. (2008) studied the shelf-life of mushrooms wrapped by different polymer films (polyvinyl chloride film, biaxially oriented polypropylene film, and polyethylene film). They found that the biaxially oriented polypropylene film maintained the postharvest appearance of the mushrooms most effectively. Ares et al. (2006) evaluated the effect of low-density polyethylene film, polypropylene film, and polypropylene macroperforated film integrated with passive modified atmosphere packaging on the sensory characteristics and shelf-life of shiitake mushrooms. The results showed that polypropylene macroperforated film could extend the shelf-life of freshly harvested mushrooms from 5 to 12 days. Most films, used to preserve vegetables and fruits, have been produced from synthetic polymers. However, for environmental reasons, attention has recently been focused on biodegradable polymers for the preparation of food packaging films (Guillaume, Schwab, Gastaldi, & Gontard, 2010; Pantani, Gorrasi, Vigliotta, Murariu, & Dubois, 2013). Poly(lactic acid) (PLA) is a compostable polymer derived from renewable sources, in particular from starch and sugar (Molinaro et al., 2013). There is increasing interest in using PLA as disposable packaging material. In many food-packaging applications, polymerfilm materials are prepared by extrusion methods. However, its brittleness prevents PLA film from being applied for practical food-packaging purposes (Wang et al., 2014). Tributyl citrate, a nontoxic citrate ester, is

L. Han et al. / Innovative Food Science and Emerging Technologies 29 (2015) 288–294

effective in reducing the glass transition temperature and improving the elongation at break of PLA (Labrecque, Kumar, Dave, Gross, & McCarthy, 1997). To control postharvest diseases of fruits and vegetables, antimicrobial compounds are often used. Nisin is produced by the food-grade Lactococcus lactis subsp. lactis. Nisin effectively inhibits Gram-positive bacteria and outgrowth spores of Bacillus and Clostridium. It has been increasingly used as a natural antimicrobial agent for direct incorporation in food. This antimicrobial peptide has been approved as a food preservative in more than 50 countries worldwide and shows high efficiency at nanomolar levels (La Storia, Mauriello, Villani, & Ercolini, 2013; Imran, El-Fahmy, Revol-Junelles, & Desobry, 2010; Willey & van Der Donk, 2007). In the light of these considerations, the present study was undertaken to investigate the effect of PLA films in combination with nisin and low temperature storage on the physicochemical and microbial quality of B. edulis wild edible mushrooms. In order to accomplish this objective, several variables including the tissue firmness, weight loss, total soluble solids, package atmosphere composition, microbiological quality, and sensory attributes of the mushrooms, packed by PLA films with 0, 7.5, and 15 wt.% tributyl citrate, and low-density polyethylene (LDPE) film, were evaluated.

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2.3. Package atmosphere composition The O2 and CO2 concentrations in the package were determined by an O2 and CO2 analyzer (SCY-2A, Xinrui Instrument Co., Ltd, Shanghai, China). Gas sample was taken from the packages with a 20-ml syringe. 2.4. Weight loss The weight of five mushrooms from each package was determined at the initial time and each sampling time. Weight loss was determined gravimetrically. The results were expressed as the percentage loss of initial weight. 2.5. Firmness measurement The firmness of mushrooms was measured with a texture analyzer (TA-XT, Stable Micro System Ltd., London, UK) equipped with a cylindrical probe of 2-mm diameter. Due to the hardness of the stem, it was removed from each piece, measuring only the firmness of the cap. The sample was penetrated 5 mm in depth. The speed of the probe was fixed at 2 mm s− 1. From the force versus time curves, firmness was defined as the maximum force (Newton, N). An average of ten mushrooms in each package was calculated.

2. Materials and methods

2.6. Total soluble solids

2.1. Materials

B. edulis wild edible mushrooms were ground in a mortar and squeezed with a hand-press. Total soluble solids (TSSs) were measured from the juice using a digital refractometer (MZB 92, Shanghai Mi Qingke Industrial Co., Ltd., Shanghai, China) (Jiang, Feng, Zheng, & Li, 2013).

B. edulis wild edible mushrooms were harvested from a local grove in Kunming, Yunnan Province, China. The mushrooms were transported to a laboratory immediately after harvest. PLA (poly(L -lactic acid), Mw = 280 kDa, Mw/Mn = 1.98) was purchased from Natureworks LLC (Nebraska, USA), tributyl citrate from Sigma (St. Louis, MO, USA), and nisin from Yinxiang Biotechnology Co., Ltd (Taizhou, China). PLA films were prepared by an extrusion method. Prior to blending, the PLA was dried in vacuum at 80 °C for 24 h to eliminate any water that may have absorbed on the surface of the polymer particles. PLA was blended with tributyl citrate by adding tributyl citrate to the PLA matrix in different amounts (0, 7.5, and 15 wt.%) and PLA films were prepared using a film-blowing machine (LSC-120, Kechuang Rubber Plastics Machinery Set, Shanghai, China). The temperature of blowing head was 160 °C and 165 °C, and the temperature of cooling air was 20 °C. The blow-up ratio was 2:1 and the draw-down ratio was held at 7:1 (Wang et al., 2014). Commercial low-density polyethylene (LDPE) film was purchased from Xingnong Co., Ltd, Zhongshan, China. The thickness of all the films was 50 ± 2 μm. Water vapor permeability of films was determined gravimetrically according to the standard method E96 (ASTM 1995).

2.7. PPO activity PPO activity was measured spectrophotometrically by a modified method based on those of Mohapatra, Frias, Oliveira, Bira, and Kerry (2008). Briefly, mushroom pericarps (5.0 g) were taken out using a sharp knife and were homogenized with 20 ml of 0.5 M sodium phosphate buffer (pH 6.5) containing 20 g/l of polyvinylpyrroline for restricting oxidation in the samples. The homogenate was filtered and kept at 4 °C for 1 h before being centrifuged at 7000 g for 35 min at 4 °C. Supernatant was collected and used as crude enzyme extract. Enzyme activity (PPO) was measured immediately after the extraction, to avoid degradation of enzymes. PPO activity was assessed based on oxidation of p-phenylendiamine by catechol. The absorbance was measured at 420 nm by UV–vis Spectrophotometer (T90, Beijing Purkinje general instrument Co., Ltd., Beijing, China). One unit of PPO activity was defined as the amount of enzyme causing 0.001 increase of absorbance per minute in 1 ml of reaction mixture. 2.8. Microbiological analysis

2.2. Sample preparation Each sample was sorted according to shape and maturity of mushrooms. Damaged or rotten mushrooms were removed. Then, the mushrooms were carefully cleaned by hand to remove mud and pine needles on the surface of wild mushrooms. The mushrooms were treated with 0.5% nisin solution, tied with string, hung, and dried. Quantities of approximately 250 g of mushroom were weighed and packed with rectangular bags (150 mm × 300 mm) that were constructed from the tested films and stored at 4 ± 1 °C for 18 days. Four groups of samples were prepared in total: PLA group; PLA-7.5 group; PLA-15 group; and control group (mushroom was packed in the LDPE film). The quality of mushrooms was determined initially and after 3, 6, 9, 12, 15, and 18 days. Three replicates from each group were randomly selected and sampled as described below.

All samples were analyzed for mesophilic and psychrophilic bacteria counts. Twenty-five grams of mushrooms was removed aseptically from the package, weighed, and homogenized in a sterile stomacher bag for 2 min with 225 ml of 0.1% peptone water. Further decimal dilutions were made with the same diluent. Aerobic counts were determined on plate count agar. The plates were incubated at 37 °C for 2 days for mesophilic bacteria, and at 4 °C for 7 days for psychrophilic bacteria (Simón & Gonzalez-Fandos, 2011; Gao, Feng, & Jiang, 2014). 2.9. Sensorial analysis The mushrooms were evaluated for their appearance, spoilage, and odor by a sensory panel of ten assessors, trained in descriptive analysis of mushrooms. For scoring, a ten point rating scale was used to

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3. Results and discussion

film packaging. This was probably due to the high respiration rate of mushrooms (Ares et al., 2006). The changes in CO2 concentration in the headspace of film packaging during storage are shown in Fig. 1b. The mushroom samples in all the film packaging showed a sharp increase in CO2 concentration during the early stages of storage. The changes in LDPE and PLA film packaging were faster than those in PLA-7.5 and PLA-15 film packaging. Throughout the duration of the test, CO2 concentration was significantly (p b 0.05) higher in PLA film packaging than in PLA-7.5 and PLA-15 film packaging. The modification of the CO2 package headspace concentration readily occurred according to the carbon dioxide transmission rate (CTR) of films (Kim, Ko, Lee, Park, & Hanna, 2006; Ornelas-Paz et al., 2012). The CTR was 8.1 × 104, 9.5 × 104, 6.3 × 105, and 9.9 × 105 cm3 m−2 day−1 0.1 MPa−1, for LDPE, PLA, PLA-7.5, and PLA-15 films, respectively. The differences in CO2 concentration among the film packaging could be explained by the differences in CTR mentioned above.

3.1. Headspace gas composition

3.2. Weight loss

Changes in O2 package headspace concentration were presented in Fig. 1a. The patterns of changes in O2 concentration during storage were similar for the four packaging materials. The general trend was that the O2 concentration rapidly decreased during the first 12 days of storage, and then remained constant until the end of the tested time. Despite of their different permeability, no significant differences was found in O2 concentration inside the LDPE, PLA, PLA-7.5, and PLA-15

In general, the incorporation of plasticizer enhances the flexibility of the PLA films and lead to a decrease in the films' barrier properties. This is due to the decrease in the free volume for gas or water molecules to diffuse through the polymer matrix. This effect is an important aspect considered in food packaging because of the noticeable role played by water in deteriorative reactions and microbial growth (Jamshidian, Tehrany, Imran, Jacquot, & Desobry, 2010). The water vapor permeability of the LDPE, PLA, PLA-7.5, and PLA-15 film was 1.01 × 10− 13, 9.08 × 10−14, 3.15 × 10−13, and 8.44 × 10−13 kg m/m2 s Pa, respectively. The weight loss for all samples investigated in this study during refrigerated storage is shown in Fig. 2. As could be seen from Fig. 2, weight loss increased as the storage period progressed for all samples due to produce dehydration. In fact, the highest weight loss values were recorded for mushrooms packed in PLA-15 film, which had higher water vapor permeability than LDPE, PLA, and PLA-7.5 film. No statistically significant difference between mushrooms packed in LDPE and PLA film was observed. The water vapor permeability of the LDPE film (1.01 × 10 − 13 kg m/m 2 s Pa) was equal to that of PLA film (9.08 × 10− 14 kg m/m2 s Pa). PLA and other biodegradable materials had been reported to retard weight loss in produce (Almenar, Samsudin, Auras, Harte, & Rubino, 2008). Although the weight loss of mushrooms packed in PLA-7.5 film was significantly (p b 0.05) higher than that of mushrooms packed in LDPE and PLA film at day

differentiate changes in appearance, spoilage and odor of mushrooms. A score of 6 was regarded as good and limit of marketability. The visual appearance of mushrooms was evaluated taking into account the color of caps, the color of the entire mushrooms and the dark blotch incidence. Spoilage incidence was determined by the number of mushrooms affected by spoilage. The presence of off-odors when the packages were opened was evaluated (Simón, González-Fandos, & Vázquez, 2010; Simón & Gonzalez-Fandos, 2011). 2.10. Statistical analysis SPSS software (SPSS Inc., version 13.0) was utilized to calculate analysis of variance (ANOVA). Significance between mean values was determined by Duncan's multiple range tests. 0.05 was the significance limit.

Fig. 1. Effect of different packages on the (a) O2 and (b) CO2 concentration of Boletus edulis mushrooms stored at 4 ± 1 °C for 18 days. Data are presented as mean ± standard deviation (n = 3).

Fig. 2. Effect of different packages on the weight loss of Boletus edulis mushrooms stored at 4 ± 1 °C for 18 days. Data are presented as mean ± standard deviation (n = 5).

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6 of storage, a moderate weight loss (4.0%) was observed in the PLA7.5 film at the end of storage. Singh, Langowski, Wani, and Saengerlaub (2010) reported that a loss of 5–6% of harvested mushrooms would lead to a depression of its commercial value because of marked deterioration of quality. 3.3. Texture Change in texture is considered one of the chief problems in the postharvest deterioration of mushrooms (Antmann, Ares, Lema, & Lareo, 2008; Khan et al., 2014). The stems of B. edulis wild edible mushrooms were 16% harder and 33% springier than the caps (Jaworska & Bernaś, 2010). So, the caps of B. edulis were selected to measure the firmness of mushrooms. From a quality perspective, the texture of B. edulis wild edible mushroom is an important parameter judged by the consumer. At harvest, mushrooms are firm, crisp, and tender. However, they become soften during postharvest deterioration (Oliveira, Sousa-Gallagher, Mahajan, & Teixeira, 2012). Firmness of mushrooms during storage is shown in Fig. 3. In the present investigation, firmness of all samples (LDPE, PLA, PLA-7.5, and PLA-15 groups) regularly softened during the storage period. Firmness decreased from 5.27 N to a range of 2.00–3.53 N, after 18 days of storage. No significant difference (p N 0.05) in firmness was observed among all the samples on day 9, while PLA-7.5 and PLA-15 samples maintained significantly (p b 0.05) higher firmness than LDPE and PLA samples. Firmness is sometimes correlated to weight loss and the degree of injury due to decay or microbial growth (Koide & Shi, 2007). Antmann et al. (2008) reported that storage of mushrooms under atmospheres saturated in water vapor was responsible for the acceleration of mushroom softening. Packaging films with a lower water transmission rate would result in a higher relative humidity inside packages. So, mushrooms packed in PLA-7.5 and PLA-15 films with a higher water transmission rate were firmer than those packed in LDPE and PLA films. 3.4. Total soluble solids Differences in the total soluble solids (TSSs) among the various packaging during storage are shown in Fig. 4. TSS of mushrooms was found to increase with storage time. The observed increment in TSS content of the samples might be because of high respiration rate and ripening of the mushrooms during storage (Fig. 4), as reported in a previous study (Jafri, Jha, Bunkar, & Ram, 2013). However, no significant differences (p N 0.05) in the TSSs of mushrooms were observed from day 0 to day 18 at either packaging. The reduction in water content of

Fig. 3. Effect of different packages on the firmness of Boletus edulis mushrooms stored at 4 ± 1 °C for 18 days. Data are presented as mean ± standard deviation (n = 10).

Fig. 4. Effect of different packages on the total soluble solids of Boletus edulis mushrooms stored at 4 ± 1 °C for 18 days. Data are presented as mean ± standard deviation (n = 3).

mushrooms during storage could also result in the increment in TSS content of the samples. The low water vapor permeability of the LDPE and PLA-based films, combined with the high transpiration rate of B. edulis, developed a nearly saturated condition in the packaging, which was responsible for the small weight loss of mushrooms (Antmann et al., 2008). 3.5. PPO activity PPO plays an important role in browning of fruits and vegetables by catalyzing the oxidation of mono- and di-phenols to o-quinones, and these quinones polymerize to produce brown pigments (RichardForget & Gauillard, 1997). The influence of different packaging films on the activity of polyphenol oxidase is shown in Fig. 5. During the storage, PPO activity showed similar patterns in all the samples. PPO activity gradually increased from 57.7 U/g min to a range of 110.0–162.7 U/ g min, after 15 days of storage. The PLA-7.5 and PLA-15 treatments showed comparable lower enzyme activity than PLA and LDPE treatments on day 6. PPO activity of LDPE treatment increased quickly during storage, being with an approximately 184.2% increase within 15 days, PPO activity was inhibited by the PLA, PLA-7.5, and PLA-15 treatments. This indicated that the higher CO2 concentration induced a higher polyphenol oxidase activity. Ye et al. (2012) found that higher CO2 concentration would lead to enzymatic browning by PPO. The trend in PPO activity was in accordance with that of CO2 concentration in this study.

Fig. 5. Effect of different packages on the PPO activity of Boletus edulis mushrooms stored at 4 ± 1 °C for 18 days. Data are presented as mean ± standard deviation (n = 3).

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3.6. Microbiological analysis The changes of total aerobic bacteria counts of B. edulis wild edible mushrooms during storage are shown in Fig. 6a and b. In all of the groups, the total level of mesophilic and psychrophilic bacteria counts steadily increased during the whole storage period. It was found that mesophilic count in LDPE and PLA film showed significantly (p b 0.05) lower level than those in PLA-7.5 and PLA-15 film on day 9. The mesophilic count in PLA-15 film was the lowest among the four packaging treatments after 9 days of storage period. For psychrophilic bacteria count, the level of PLA-7.5 and PLA-15 film was significantly (p b 0.05) lower than that of LDPE and PLA film after 12 days of storage period. There was no significant (p N 0.05) difference between PLA-7.5 and PLA-15 treatment during the whole storage period. The initial total aerobic bacteria count (6.88 log10 CFU/g and 1.89 log10 CFU/g for mesophilic and psychrophilic bacteria count, respectively) of B. edulis wild edible mushrooms were higher than those reported by Cliffe-Byrnes and O'Beirne (2008) on water rinsed white button mushrooms and Jiang et al. (2013) on shiitake mushrooms. This was probably because that B. edulis wild edible mushrooms were harvested from a local grove and much mud and pine needles were adhered to the stems and caps of wild mushrooms. Furthermore, the wild mushrooms were not rinsed by water and only carefully cleaned by hand. Although the wild mushrooms were treated with 0.5% nisin solution, the initial total aerobic bacteria count of B. edulis wild edible mushrooms were still higher than those cultivated mushrooms.

The total mesophilic and psychrophilic bacteria count in PLA films plasticized by tributyl citrate did not exceed 9.5 log10 CFU/g and 4.5 log10 CFU/g after 18 days of storage, respectively. Nisin had a marked effect on reducing mesophilic and psychrophilic bacteria counts. For example, Barbosa, Silva de Araújo, Matos, Carnelossi, and Almeida de Castro (2013) reported that nisin showed antimicrobial activity in minimally processed mangoes without interfering with the organoleptic characteristics of the fruit. Raouche, Mauricio-Iglesias, Peyron, Guillard, and Gontard (2011) also reported that biodegradable films containing either carvacrol or allyl isothiocyanate could be used efficiently to control the growth of microorganisms in model food during storage. Jin, Liu, Zhang, and Hicks (2009) observed that nisin incorporated into the pectin/PLA film was an effective approach to inhibit the microorganism growth in foods. Since the water vapor permeability of PLA-7.5 and PLA-15 film was higher than that of LDPE and PLA film, the moisture in the packaging headspace of packaging increased the water activity in the mushrooms. Srinivasa, Baskaran, Ramesh, Prashanth, and Tharanathan (2002) reported that the condensation inside the packaging under cold storage enhanced microbial spoilage. It was also reported that the storage of green peppers in modified atmosphere packaging resulted in high microbial counts due to produce transpiration and as a consequence, resulting to high relative humidity (Koide & Shi, 2007). The results could be explained by the high water vapor permeability of PLA-7.5 and PLA-15 film, which lowered the relative humidity inside the packaging.

Table 1 Effect of different packaging on the sensory attributes of Boletus edulis wild edible mushrooms stored at 4 ± 1 °C for 18 days. Treatments

Appearance

Spoilage

Odor

9.86 ± 0.31

10.0

10.0

3 days PLA-15 PLA-7.5 PLA LDPE

9.12 ± 0.21a 9.16 ± 0.54a 9.10 ± 0.13a 9.04 ± 0.02a

9.73 ± 0.14 a 9.75 ± 0.56 a 9.77 ± 0.09 a 9.74 ± 0.53 a

9.88 ± 0.09a 9.79 ± 0.32a 9.81 ± 0.47a 9.72 ± 0.50a

6 days PLA-15 PLA-7.5 PLA LDPE

8.68 ± 0.30a 8.56 ± 0.57a 7.90 ± 0.76a 7.75 ± 0.11a

9.35 ± 0.41a 9.22 ± 0.25a 9.21 ± 0.67a 9.25 ± 0.14a

9.22 ± 0.13a 9.16 ± 0.07a 9.04 ± 0.34a 9.10 ± 0.12a

9 days PLA-15 PLA-7.5 PLA LDPE

8.03 ± 0.45a 7.89 ± 0.14a 7.35 ± 0.81a 7.17 ± 0.30a

8.13 ± 0.38b 8.15 ± 0.73b 7.39 ± 0.32ab 7.07 ± 0.50a

8.76 ± 0.35a 8.81 ± 0.52a 8.53 ± 0.37a 8.42 ± 0.16a

12 days PLA-15 PLA-7.5 PLA LDPE

7.36 ± 0.04b 7.32 ± 0.02b 6.68 ± 0.34a 6.40 ± 0.57a

7.54 ± 0.12b 7.62 ± 0.41b 6.31 ± 0.59a 6.09 ± 0.12a

8.02 ± 0.09c 8.26 ± 0.26c 6.17 ± 0.18b 4.84 ± 0.27a

15 days PLA-15 PLA-7.5 PLA LDPE

6.89 ± 0.03b 6.74 ± 0.27b 5.29 ± 0.55a 5.15 ± 0.38a

7.03 ± 0.60b 6.92 ± 0.15b 5.15 ± 0.24a 4.86 ± 0.28a

7.58 ± 0.15b 7.62 ± 0.13b 4.56 ± 0.04a 4.39 ± 0.38a

18 days PLA-15 PLA-7.5 PLA LDPE

6.20 ± 0.01b 6.25 ± 0.16b 3.86 ± 0.08a 3.90 ± 0.09a

6.57 ± 0.53b 6.30 ± 0.17b 3.50 ± 0.02a 3.00 ± 0.03a

6.96 ± 0.54b 7.02 ± 0.15b 3.65 ± 0.02a 3.71 ± 0.30a

0 day

Fig. 6. Effect of different packages on the microbiological quality of button mushrooms stored at 4 ± 1 °C for 18 days. (a) Mesophilic counts and (b) psychrophilic counts. Data are presented as mean ± standard deviation (n = 3).

a–c

Values followed by different letters in the same column are significantly different (p b

0.05), where a is the lowest value. Data are presented as mean ± standard deviation (n = 3).

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3.7. Sensorial analysis Average values for the sensory attributes were listed in Table 1. All the selected sensory attributes gradually decreased as the storage period advanced. This supported the validity of the chosen descriptors as indicators of mushroom deterioration. Off-odor of mushrooms stored in PLA-7.5 and PLA-15 films was significantly (p b 0.05) lower than those stored in LDPE and PLA films after 12 days of storage. The higher CO2 concentration developed inside packaging might reduce the oxidative capacity of mitochondria, leading to anaerobic respiration (Ares et al., 2006). The result was consistent with the trend of CO2 concentration inside film packaging. Appearance of mushrooms stored in PLA-7.5 and PLA-15 films was significantly (p b 0.05) better than those stored in PLA and LDPE films after 12 days of storage. Mushrooms stored in PLA and LDPE films became unacceptable on day 15. However, mushrooms stored in PLA-7.5 and PLA-15 films were still good and limit of marketability at the end of the experiment. Nichols and Hammond (1973) reported that CO2 concentration inside Agaricus mushroom packaging higher than 12% would cause loss of firmness and an increase in the enzymatic browning due to cell membrane damage. CO2 concentration inside PLA-7.5 and PLA-15 film packaging was below 15% during the whole storage time. This indicated that B. edulis wild edible mushrooms were less susceptible to high CO2 concentration than Agaricus mushrooms. Based on judgments made by sensory panel members, mushrooms stored in PLA and LDPE films became spoiled after 12 days of storage. However, mushrooms stored in PLA-7.5 and PLA-15 films were still acceptable in a marketable condition and recorded upon 6 score at the end of storage time. This might be due to the high relative humidity inside the PLA and LDPE film packaging (Antmann et al., 2008). 4. Conclusions The biodegradable plasticized PLA film had a significant effect on the texture, PPO activity, total bacteria count, and sensory attributes of B. edulis wild edible mushrooms, although the effect on weight loss and TSS was not great. The barrier property of film packaging influenced the physicochemical and microbial quality of B. edulis wild edible mushrooms within the packages. Although PLA-15 film retarded the deterioration of mushrooms more efficiently than PLA-7.5 film, 15 wt.% plasticizer of PLA film would make the film hard to prepare and handle during processing. Furthermore, 15 wt.% triethyl citrate plasticizer sharply lowered the tensile strength of PLA film. PLA-7.5 film was preferred to the application in mushroom preservation. The results suggest that biodegradable plasticized PLA film can provide an alternative to the chemically replace LDPE film. Acknowledgments This work was supported by the National Natural Science Foundation of China (Project Nos. 31160198, 31360417, 31460247), the Graduate Student Innovation Fund Program of Yunnan Minzu University(2014YJY87), the Applied Basic Research Key Project of Yunnan (2013FA039), and the High-End Technology Professionals Introduction Plan in Yunnan province (2010CI119). References Almenar, E., Samsudin, H., Auras, R., Harte, B., & Rubino, M. (2008). Postharvest shelf life extension of blueberries using a biodegradable package. Food Chemistry, 110(1), 120–127. Antmann, G., Ares, G., Lema, P., & Lareo, C. (2008). Influence of modified atmosphere packaging on sensory quality of shiitake mushrooms. Postharvest Biology and Technology, 49(1), 164–170. Ares, G., Parentelli, C., Gámbaro, A., Lareo, C., & Lema, P. (2006). Sensory shelf life of shiitake mushrooms stored under passive modified atmosphere. Postharvest Biology and Technology, 41(2), 191–197.

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