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Quality improvement of Pleurotus mushrooms by modified atmosphere packaging and moisture absorbers
Postharvest Biology and Technology 28 (2003) 169 /179 www.elsevier.com/locate/postharvbio
Quality improvement of Pleurotus mushrooms by modified atmosphere packaging and moisture absorbers R. Villaescusa, M.I. Gil * Research Group on Quality, Safety and Bioactivity of Plant Foods, Food Science and Technology Department, CEBAS-CSIC, P.O. Box 4195, E-30080 Murcia, Spain Received 19 February 2002; accepted 25 July 2002
Abstract Quality of Pleurotus ostreatus mushrooms was compared during cold storage under various temperatures and modified atmospheres with and without moisture absorbers. CO2 production followed a non-climacteric pattern and was about 0.50, 0.99 and 1.23 mmol CO2 kg 1 s 1 at 0, 4 and 7 8C, respectively, after 12 h. At the end of 11 days of storage, the respiration rates were very similar for all the assayed temperatures (9/0.37 mmol CO2 kg 1 s 1). The best mushroom appearance was found at 0 8C, although for optimizing modified atmosphere packaging (MAP), 4 8C was selected to minimize low-temperature damage. When prolonging the storage beyond 7 days, quality characteristics dropped sharply and the mushrooms were not marketable, except when kept at 0 8C. Therefore, 7 days was the maximum recommended storage period for Pleurotus mushrooms. MAP at 4 8C for 7 days was created using microperforated polypropylenes (MPP1 and MPP2), low-density polyethylene (LDPE) and polyvinyl chloride (PVC) films. Results were compared with macroperforated polypropylene (PP) control packages. In order to avoid condensation, sorbitol and silica gel were used to modify the in-package relative humidity. Moisture absorbers did not modify the gas composition of the packages. Sorbitol promoted tissue leakage and cannot be recommended. Lower relative humidity was observed in packages containing silica gel and this did not affect the quality of the mushrooms. However, increasing amounts of silica gel increased weight loss in Pleurotus mushrooms, and high weight loss detected in PP packages made these unacceptable. MAP (15 kPa O2/5 kPa CO2) was found beneficial for maintaining acceptable quality of Pleurotus for 7 days at 4 8C. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Oyster mushrooms; Respiration rate; Storage; Sorbitol; Silica gel
1. Introduction
* Corresponding author. Tel.: /34-968-396-315; fax: /34968-396-213 E-mail address: [email protected] (M.I. Gil).
Pleurotus mushrooms are a delicate variety of mushroom, requiring a growing temperature range between 5 and 22 8C, depending on the cultivar, as well as good ventilation and high relative humidity. To obtain the highest quality for con-
0925-5214/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 5 2 1 4 ( 0 2 ) 0 0 1 4 0 - 0
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sumers, it is necessary to optimize postharvest storage. Lowering the temperature of mushrooms reduces respiration and transpiration, delaying senescence, preventing wilting and shriveling and thus extending shelf life (Burton and Twyning, 1989; Beit-Halachmy and Mannheim, 1992). Much has been published about Agaricus bisporus , however very little is known about the storage characteristics of Pleurotus mushrooms. Some authors have reported an optimum inpackage O2 concentration of 6 kPa without creating anaerobic conditions for A. bisporus mushrooms (Roy et al., 1995). According to LopezBriones et al. (1992), fermentation did not occur at a minimum O2 concentration of 1 /2 kPa, although Burton et al. (1987) found that the O2 concentration must not fall below the 3 /4 kPa range. Excessive accumulation of CO2 (/12 kPa) inside the mushroom package can also prevent the opening of the cap, but results in severe browning (Nichols and Hammond, 1973; Lopez-Briones et al., 1992). Atmospheres within 5/10 kPa O2 and 2.5 /5 kPa CO2 were established by Lopez-Briones et al. (1992) for modified atmosphere packaging (MAP) of mushrooms. However, other authors found that MAP could have a damaging effect, causing anaerobic respiration as well as potential growth of anaerobic pathogens (Beit-Halachmy and Mannheim, 1992; Varoquaux et al., 1999). Commonly, prepacked mushrooms are overwrapped with stretchable polyethylene or polyvinyl chloride (PVC), partially heat-sealed at the bottom of the package, and stored at room temperature. To improve mushroom packaging further, the relative humidity inside the package can be controlled to minimize moisture loss from the produce. Loss of 3 /6% of fresh weight is usually enough to cause a marked deterioration of quality for most kinds of products and for mushrooms the acceptable weight loss is about 2% (Sveine et al., 1967; Beit-Halachmy and Mannheim, 1992). The low water transmission rate of the films commonly used in combination with the high transpiration rate of mushroom results in rapid saturation of the package atmosphere (Roy et al., 1996). These conditions may cause the growth of Pseudomonas tolasii, responsible for browning or yellowing of
the sporophore surface, known as bacterial blotch, also making the package unattractive (Jin et al., 1994). The use of moisture absorbers such as sorbitol, sodium chloride, propylene glycol and polyvinyl alcohol have resulted in better color of the mushrooms (Roy et al., 1995; Anantheswaran and Sunkara, 1996; Roy et al., 1996). Little information has been published on the gas composition recommended for storing Pleurotus mushrooms. The most efficient MAP composition for storage of Pleurotus mushrooms was determined as 1 kPa O2/5 kPa CO2 at 4 8C for 14 days by Popa et al. (1999), while according to Henze (1989) it was 1 kPa O2/30 kPa CO2 at 1 8C for 10 days. The optimum MAP for stored Pleurotus is therefore still to be determined. The aim of our work was to select the storage temperature and MAP most appropriate for maintaining Pleurotus quality and prolonging shelf life. Moisture absorbers were also studied in combination with MAP.
2. Materials and methods 2.1. Plant material Cultivated Pleurotus ostreatus mushrooms strain K-15 was commercially produced by Micelios Fungisem S.A. in Autol (La Rioja, Spain). Mushrooms from the first flush (or production interval) were used. Pleurotus bears the sporophyte or mushroom in bunches. Immediately after picking, bunches were cut and sorted for uniformity of appearance, except for the mushrooms for the first experiment which were cut at the laboratory. Freshly harvested mushrooms were transported in a refrigerated container (8 /10 8C) to the laboratory in Murcia within 24 h of harvest. Mushrooms were promptly placed in cold storage at 10 8C and sorted to eliminate defects. 2.2. Modified atmosphere packaging Three replicates of approximately 150 g of mushrooms were packaged in 1000 ml polypropylene trays (17.5 length /12 width /5.2 depth; cm3) (Borden, S.A., Alicante, Spain). MAP at
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4 8C for 7 days was created using PVC (Borden S.A.), low-density polyethylene (LDPE) (Filmwrap Plastic S.A.L., Murcia, Spain) and two microperforated polypropylenes (MPP1 and MPP2) (Danisco Flexible, Bristol, UK) films. Macroperforated polypropylene (PP) (33 holes of 2 mm/dm2 and 35 mm thickness, Pla´sticos del Segura, Murcia, Spain) was used as the control in all the MAP experiments. This film did not modify the atmosphere within the package, only limiting dehydration. Depending on the film characteristics, the trays were overwrapped (for PVC and LDPE films), heat-sealed (La Barket model BP 40E, Befor, Chassieu, France) (for MPP1) or packed in bags (for PP and MPP2) with similar dimensions to the trays and heat-sealed (Packer model IS/300H, Pla´sticos del Segura, Murcia, Spain). The effective surface area for the PVC, LDPE and MPP1 packages was 0.2 m2 and for MPP2 was 1.0 m2. The gas permeabilities of PVC and LDPE films were determined using a test cell by the method of Moyls et al. (1992) at 4 8C, and for MPP1 and MPP2 were provided by the manufacturer’s data at 25 8C (Table 1). In order to avoid condensation, sorbitol (Sigma/Aldrich, St. Louis, MO) and silica gel (Panreac, Barcelona, Spain) were used as moisture absorbers to modify the in-package relative humidity in combination with the MPP2 film. Different amounts of either sorbitol (10, 15 and 20 g) or silica gel (3, 5, 7 and 15 g) were placed in paper sacks (8 /5 cm2) under or on top of the mushrooms. Sorbitol absorbs moisture very slowly and holds about seven times less moisture than other moisture absorbers such as NaCl (Roy et al., 1996). Neither washing nor postharvest chemical treatments were applied to the mushrooms.
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2.3. Gas sampling Six replicates (about 150 g each) were placed in 500 ml glass jars at 0, 4 and 7 8C for 11 days to measure respiration rates. An open system with a continuous flow of 60 ml min 1 of humidified air was pumped into the jars to avoid dehydration and excessive CO2 accumulation. Carbon dioxide production was monitored daily during the storage period using a gas chromatograph (Shimadzu GC14, Kyoto, Japan), equipped with a thermal conductivity detector (TCD). The initial measurement was taken after 12 h of storage. Ethylene production was measured using a closed system (Kader, 1992). Glass jars of 500 ml (six replicates) were filled with 150 g of Pleurotus and stored in the same conditions as mentioned above. The increase in C2H4 concentration was measured by keeping the jars closed for 2 h and taking 5 ml samples from the headspace. Samples were analyzed using a gas chromatograph (model 5370 A, Hewlett Packard, Avondale, PA) equipped with a flame ionization detector (FID). Changes in O2 and CO2 concentrations in the MA packages were monitored daily using a gas chromatograph (Perkin Elmer autosystem, CT) equipped with a TCD. 2.4. Quality characteristics A panel of three trained judges evaluated the quality characteristics of all the mushrooms from each tray. The trays were left at room temperature for 2 h to warm the product. After storage, the weight of the moisture absorber was determined. Mushrooms were removed from the trays and weight loss was immediately determined, expressed as a percentage on a fresh weight basis. The overall
Table 1 Gas permeabilities of PVC and LDPE films were calculated using a test cell at 4 8C and 90% RH (Moyls et al., 1992) Films
Thickness (mm)
O2 permeance (10 10 mol s 1 m 2 Pa 1)
Ratio, CO2:O2
PVC LDPE MPP1 MPP2
11 11 35 35
0.28 0.48 9.34 3.36
9:1 5:1 1:1 1:1
Gas permeabilities of MPP1 and MPP2 films were according to the manufacturer’s data at 25 8C.
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visual quality of the intact mushrooms was evaluated on a scale of 9 /1, where 9 /excellent, 7 / very good, 5 /good and limit of marketability, 3 /fair and limit of usability, and 1 /poor and inedible. Aroma was determined after breaking the mushrooms and evaluating on a scale of 9 /1, where 9 /full typical aroma or flavor, 7 /moderately full, 5 /moderate, 3 /slight, and 1 / none. Texture was evaluated when the mushroom was pressed between the thumb and index finger, on a scale of 9 /1, where 9 /very firm and turgid, 7 /firm, 5 /moderately firm, 3 /soft, and 1 / very soft. Visual color was evaluated based on a scale of 9 /1, where 9 /typical uniformity and intensity color, 7 /moderately intense, 5 /moderate, 3 /slight, and 1 /poor. Spoilage, mycelium growth, wilting (as withered deterioration) and off-odors (mainly fermentation) were evaluated on scales of 1 /5, where 1 /none, 2 /slight, 3 / moderate, 4 /moderately severe, and 5 /severe. Surface color was measured at three equidistant points of the cap of the mushroom using a Chroma Meter (model CR 300 Minolta, Ramsey, NJ). The values were expressed as CIELAB color space units (Little, 1975) and mean values for lightness (L *), red-greenness (a*) and blue-yellowness (b*) parameters were calculated for each batch of mushrooms. Mushroom firmness was determined on each Pleurotus cap using a texturometer (model LR10K, Lloyd instrument, Fareham, Hants, UK) equipped with a 10.5 mm diameter probe. The maximum force required to deform the mushrooms 6 mm at a speed of 10 mm min 1 was recorded. Titratable acidity (TA) was determined by titrating 10 ml of juice with 0.1 mol l 1 NaOH to pH 8.1 (AOAC, 1984). The pHvalues were measured using a pH meter (Crison model 501, Barcelona, Spain) and soluble solids content (SSC) with a handheld refractometer (Atago N1, Tokyo, Japan). Three experiments were conducted. The first experiment was to select the best storage temperature and shelf life for maintaining Pleurotus quality under normal atmosphere conditions. Respiration rate and ethylene production were measured daily and quality characteristics were evaluated after 7 and 11 days storage at 0, 4 and 7 8C. The second experiment was performed to
select the most appropriate MAP for the temperature selected in experiment one, using PVC, LDPE and MPP1 films. In the third experiment, moisture absorbers (sorbitol and silica gel) were studied in combination with MPP2 film. Changes in gas composition were checked daily. and quality characteristics and weight loss determined after 7 days at 4 8C. 2.5. Statistical analysis All data were analyzed by analysis of variance (ANOVA) using SPSS (Windows 2000) from Statistical Analysis. To determine differences between treatments, Duncan tests were applied and significant differences were established at P 5/ 0.05.
3. Results 3.1. Selection of temperature and storage time The initial respiration rate of Pleurotus K-15 after 12 h of storage was 0.50, 0.99 and 1.23 mmol CO2 kg 1 s 1 at 0, 4 and 7 8C, respectively (Fig. 1). After this time, the respiration rate dropped at all temperatures by about 50% (including at 0 8C) for the first 2 storage days. These initial high respiration rates were associated with harvest stress caused by the cutting process, which in this trial was carried out in the laboratory just before storage. After the decline, the respiration rate remained constant throughout 7 days of storage, but decreased a second time at 7 8C between days 7 and 10. At the end of the storage time, the respiration rates were very similar for all the assayed temperatures (9/0.37 mmol CO2 kg 1 s 1). The respiration rate of Pleurotus reported by Hammond (1980) at 18 8C was 10-fold that detected in our experiments. There was no detectable C2H4 production at any temperature throughout the storage period. Significant differences in quality characteristics of mushrooms were observed at 0, 4 and 7 8C and between 7 and 11 days of storage (Table 2). Comparing temperatures, Pleurotus mushrooms kept at 0 8C had the best visual quality and no
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Fig. 1. Respiration rate of Pleurotus K-15 mushrooms at 0, 4 and 7 8C after the first 12 h storage and during 11 days storage. Each point represents the mean of six replicates of 150 g. Vertical lines represent S.D.
evidence of chilling injury was found. After 7 days of storage, the visual quality of Pleurotus was very good at 0 and good at 4 8C, but only fair and under the limit of usability at 7 8C. When
prolonging the storage time from 7 to 11 days, visual quality was considered good at 0 8C but was unacceptable at 4 and 7 8C. Aroma decreased from full typical to moderate aroma after 11 days
Table 2 Visual quality, aroma, texture, visual color, spoilage and wilting of Pleurotus K-15 mushrooms initially and after 7 and 11 days of storage at 0, 4 and 7 8C Time
Temperature (8C)
Visual quality 8.0 a
9.0 a
After 7 days
0 4 7
7.8 a 5.0 c 2.5 d
6.8 b 6.7 bc 5.7 c
After 11 days
0 4 7
6.0 b 1.8 d 1.8 d
4.5 d 4.0 d 4.3 d
*** *** ***
NS *** NS
Initial
Temperature Time Temperature /time
Aroma
Texture
Visual color
Spoilage
Wilting
8.6 a
8.8a
1.0 b
1.0 e
8.0 a 5.0 b 3.3 c
7.7 a 5.8 b 4.2 c
1.0 b 3.0 a 3.7 a
1.0 e 2.0 d 2.8 b
5.3 b 3.8 c 2.0 d
7.5 a 4.5 bc 3.8 c
1.0 b 3.0 a 3.8 a
3.0 b 2.5 c 3.5 a
*** *** ***
*** *** **
*** *** **
*** *** ***
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. NS, not significant. *, P 5/0.05; **, P 5/0.01; and ***, P 5/0.001. Visual quality evaluated on a 9 /1 scale (9 /excellent; 7 /very good; 5 /good, limit of marketability; 3 /fair, limit of usability; and 1 /poor, inedible). Aroma scored on a 9 /1 scale (9 /full typical aroma or flavor; 7 /moderately full; 5 /moderate; 3 /slight; and 1 /none). Texture scored on a 9 /1 scale (9 /very firm and turgid; 7 /firm; 5 /moderately firm; 3 /soft; and 1 /very soft). Visual color scored on a 9 /1 scale (9 /typical uniformity and intensity color; 7 /moderately intense; 5 /moderate; 3 /slight; and 1 /poor). Spoilage and wilting scored on 1 /5 scales (1 /none; 2 /slight; 3 /moderate; 4 /moderately severe; and 5 /severe).
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at the assayed temperatures (Table 2). No significant differences were observed between the initial texture and that at the end of the 7 days at 0 8C. However, texture losses occurred by 11 days storage from an initial firm texture to soft texture at 4 and 7 8C. In addition, Pleurotus stored at 0 8C maintained the initial intense color, whereas a moderate discoloration to yellowish hues was observed at 4 and 7 8C. No spoilage was found at 0 8C after 7 and 11 days (Table 2). Pleurotus mushrooms at 0 8C did not show significant differences as regards wilting after 7 days, compared with the initial samples. Storage time significantly affected the color parameters (Table 3). A significant decrease between the initial L *-value and those values after 7 and 11 days is shown in Table 3. In addition, no significant differences were observed between initial a*-values and those after 7 days at any of the studied temperatures, while there was an increase in a * (red coloration) after 11 days compared with the initial one (Table 3). Both temperature and time affected the color measured as b *. No changes were detected in b *-values at 0 and 4 8C after 7 days. But after 11 days of storage, considerable yellowing of Pleurotus was observed (Table 3). The storage temperature 0 8C was considered the best one for keeping the initial Pleurotus color. Mushroom aging was characterized by a soft and spongy texture, reported by Lopez-Briones et
al. (1992) to be due to cell growth and water migration. Firmness of Pleurotus K-15 was influenced by temperature and storage time. A significant reduction in firmness was detected after 7 and 11 days of storage compared with the initial values, in agreement with a report by Murr and Morris (1975) on Agaricus (data not shown). SSC and pH decreased after 11 days of storage although no significant differences were observed among mushrooms kept at 0, 4 or 7 8C (Table 4). No significant changes in TA were detected in Pleurotus mushroom storage for 11 days when compared with the values at harvest as well as among the assayed temperatures. As expected, weight loss increased with temperature and was accentuated by storage time, in accordance with the previous results for other mushroom types (Burton and Noble, 1993; Kang et al., 2000). The lowest weight loss was registered after 7 days of storage with significant differences between temperatures (0.6% weight loss at 0 and 4 8C, and 1.6% at 7 8C). After 11 days of storage, weight loss reached 2% at both 0 and 4 8C and 2.5% at 7 8C. Although the circulating air was highly humidified, weight loss was higher than expected due to the high water content of mushrooms and their large surface area. Since most of the quality characteristics were judged acceptable after 11 days at 0 8C, the recommended storage temperature for Pleurotus mushrooms should be 0 8C.
Table 3 Changes in L *-, a *- and b *-values of Pleurotus K-15 mushrooms initially and after 7 and 11 days of storage at 0, 4 and 7 8C Time
Temperature (8C)
Initial
L*
a*
b*
96.9 a
0.2 b
5.1 c
After 7 days
0 4 7
85.8 b 86.3 b 87.0 b
0.8 b 0.8 b 0.2 b
3.7 c 4.6 c 6.6 b
After 11 days
0 4 7
61.9 c 63.3 c 61.7 c
2.3 a 2.4 a 2.2 a
18.5 a 19.8 a 19.0 a
NS *** NS
NS *** NS
*** *** ***
Temperature Time Temperature /time
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. NS, not significant. *, P 5/0.05; **, P 5/0.01; and ***, P 5/0.001.
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Table 4 SSC, pH and TA of Pleurotus K-15 mushrooms initially and after 11 days of storage at 0, 4 and 7 8C Time
Temperature (8C)
SSC (8Brix) 5.1 a
6.4 a
0.10
0 4 7
4.2 bc 4.0 c 4.4 b
6.2 b 6.3 ab 6.2 b
0.09 0.08 0.09
NS *** NS
NS ** NS
NS NS NS
Initial After 11 days
Temperature Time Temperature /time
pH
TA (g citric acid/100 ml)
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. NS, not significant. *, P 5/0.05; **, P 5/0.01; and ***, P 5/0.001.
3.2. Modified atmospheric packaging Although 0 8C was the best storage temperature for maintaining Pleurotus quality, 4 8C was used to optimize MAP, so as to reduce lowtemperature injury. Stretchable PVC film, currently used to overwrap mushrooms, was used and compared with LDPE and MPP1 films to create the MA conditions reported in the literature for Pleurotus mushrooms (Henze, 1989; Popa et al., 1999). The initial air composition within the packages was passively modified, according to the permeances of the films used. A decrease in O2 and an increase in CO2 levels were detected after 1 day of storage at 4 8C for PVC, LDPE and MPP1 packages (Fig. 2). When the mushroom trays were wrapped with PVC and LDPE, O2 declined and CO2 accumulated (Fig. 2). After 1 day, O2 and CO2 levels were very similar for both LDPE and PVC packages. However, CO2 decreased within PVC packages and was maintained throughout the storage within the LDPE ones (Fig. 2). Less modification of O2 and CO2 was detected within the MPP1 packages compared with the other films. Steady-state MAP conditions were reached after 2 days with concentrations for O2/CO2 of 1.5/8 kPa for PVC, 2/12 kPa for LDPE and 12/7 kPa for MPP1 packages (Fig. 2). Significant differences in visual quality, aroma, off-odors and mycelium growth were observed between initial Pleurotus mushrooms and those stored for 7 days in PP, PVC, LDPE and MPP1 (Table 5). Mushrooms kept in PVC films resulted in better visual quality and were not significantly
different from the initial mushrooms (Table 5). Samples stored in LDPE maintained good visual quality while they were under the limit of marketability in PP and MPP1 packages. Pleurotus mushrooms packaged in MPP1 film were scored with the worst visual quality for spongy appearance due to the high condensation created in the package. The visual appearance of these mushrooms was dramatically affected by the high water content of the tissue. The characteristic aroma of the mushrooms decreased with storage time in all conditions. In addition, moderately severe offodors were detected for samples stored in PVC and LDPE packages. These off-odors were induced by the intense modified atmosphere created in PVC and LDPE packages with O2 levels of 2 kPa, which probably promoted fermentation (ethanol-like odors) (Table 5). Mushroom firmness decreased after storage although it was judged acceptable in all the treatments (data not shown). After 7 days of storage at 4 8C, mycelium growth was observed only in Pleurotus kept in PP and MPP1 (Table 5). No evidence of spoilage and wilting was detected in any stored sample. The most representative parameter to detect differences in the color of Pleurotus was the b *value, which showed that all mushrooms after storage underwent yellowing (Table 6). In contrast, no differences were found between treatments with respect to a*, although L * increased for mushrooms stored in MAP (data not shown). At the end of 7 days at 4 8C, SSC and TA decreased for mushrooms kept in PVC, LDPE and
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3.3. Effect of MAP and modified humidity packaging
Fig. 2. Changes in MAP O2 and CO2 concentrations of Pleurotus K-15 mushrooms at 4 8C during 7 days storage in PVC, LDPE and MPP1 films. Each point represents the mean of three replicates of 100 g. Vertical lines represent S.D.
MPP1 packages and pH increased after storage as compared with the initial values (Table 6). The tested MA packages resulted in either anoxia (PVC, LDPE) or CO2 stress (LDPE) or condensation (MPP1). Other films need to be tested to develop MAP in order to obtain the required gas conditions in combination with the reduced weight loss and prevention of condensation.
In order to avoid fermentation and prevent condensation, we increased the surface area of the package fivefold (from 0.2 m2 for the trays to 1.0 m2 for the bags). In addition, different amounts of sorbitol and silica gel were incorporated inside the packages to control condensation. MPP2 packages with no moisture absorbers were also used. No differences were observed in MAP generated with or without moisture absorbers. The changes in O2 and CO2 concentration within MPP2 packages without moisture absorbers are shown in Fig. 3. The steady-state atmosphere was reached within 1 day with average values of 15 kPa O2 and 4 kPa CO2 (Fig. 3). Sorbitol did not improve Pleurotus quality. On the contrary, it promoted deterioration, in disagreement with previous results by Roy et al. (1996) for Agaricus mushrooms. In our case, sorbitol was responsible for mushroom texture deterioration and the exudation of tissue fluids, which accumulated in the trays making mushrooms unacceptable in MPP2 packages (data not shown). On the other hand, silica gel reduced condensation in those package areas of the film where the sacks were located, particularly when it was placed on the top of the mushrooms. Although, silica gel provided a clear view through the package, no improvements in quality were found when comparing samples with or without silica gel (Table 7). This finding is in accordance with the results described by Roy et al. (1996) for Agaricus mushrooms. The visual quality of Pleurotus mushrooms after 7 days at 4 8C in MPP2 with and without silica gel was very good and no significant differences were observed with respect to the initial samples (Table 7). However, significant losses in visual quality and texture were found when mushrooms were kept in PP packages, although they were always above the threshold of marketability (Table 7). The aroma of mushrooms after storage was no different from that at harvest and no off-odors were noticed for all samples (data not shown). The color of Pleurotus changed after storage, a white discoloration was particularly observed, although no other differences among samples were detected.
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Table 5 Visual quality, aroma, texture, mycelium growth and off-odors of Pleurotus K-15 mushrooms initially and after 7 days of storage at 4 8C in PP, PVC, LDPE and MPP1 films Time Initial After 7 days
Films
Visual quality
Aroma
Texture
Mycelium
Off-odors
PP PVC LDPE MPP1
7.0 a 4.7 c 6.3 ab 5.3 bc 3.2 d ***
8.0 a 3.3 bc 1.3 c 3.0 bc 4.0 b ***
8.7 5.0 6.3 5.3 5.3 *
1.0 2.0 1.0 1.0 1.7 **
1.0 1.0 2.7 3.7 1.0 **
a b ab b b
b a b b a
b b a a b
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. *, P 5/0.05; **, P 5/0.01; and ***, P 5/0.001. Quality ratings are as described for Table 2.
Mycelium growth was only observed in samples stored in PP packages. MAP with silica gel increased weight loss in Pleurotus mushrooms. The weight loss was higher when the amount of silica gel increased for MPP2 (Table 7). High weight loss was also detected in PP packages (3.7%). This made that type of wrapping unacceptable. MPP2 without silica gel reduced weight loss to acceptable values (5/1.1%) (Table 7).
4. Discussion Mushrooms are a fast respiring and highly perishable product, and therefore MAP, in addition to low temperature can be beneficial to extend their shelf life. After harvest, mushrooms should be precooled at 1/4 8C and stored for a few hours
at the same temperature for the mushrooms to recover from harvest stress before packaging. When the mushrooms were cut at the laboratory, they showed a high initial respiration rate (Fig. 1). These high respiration rates could also be used by packaging mushrooms immediately. As a result, MA conditions will be established more rapidly and this could increase the effectiveness of the MAP by shortening the time at sub-optimal MA conditions. Cooling should be used in fresh mushroom storage and during distribution and sale to improve quality and extend storage life. Packaging mushrooms has been essential for reducing transpiration and maintaining quality. Pleurotus mushrooms are available in the market in Europe in a stretchable PVC package. This kind of film is subject to environmental criticism, and may be banned for food packaging in the future. The packaging film must be environmentally
Table 6 Color (b *-value), SSC, pH and TA of Pleurotus K-15 mushrooms initially and after 7 days of storage at 4 8C in PP, PVC, LDPE and MPP1 films Time Initial After 7 days
Films
b*
PP PVC LDPE MPP1
14.2 18.8 16.2 16.8 19.1 ***
c a b b a
SSC (Brix)
pH
TA (g citric acid/100 ml)
6.3 a 6.3 a 6.0 b 5.2 d 5.7 c ***
6.3 d 6.7 a 6.6 b 6.5 c 6.6 b ***
0.31 0.14 0.17 0.14 0.14 ***
a b b b b
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. *** P 5/0.001.
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Fig. 3. Changes in MAP O2 and CO2 concentrations of Pleurotus K-15 mushrooms at 4 8C during 7 days storage in MPP2 film with or without silica gel (3, 5, 7 and 15 g). Each point represents the mean of three replicates of 100 g. Vertical lines represent S.D.
friendly and adapted to the O2 requirement of the commodity, which largely depends on the storage temperature. Fermentation in Agaricus mushrooms stored in O2 concentrations between 1 and 2 kPa (depending on CO2 concentration) did not occur in previous work (Lopez-Briones et al., 1992), although other authors have suggested higher values (3 /4 kPa; Burton et al., 1987). In our case, fermentation was detected in samples kept in PVC and LDPE
packages that contained 2 kPa O2. Carbon dioxide content was important in controlling mycelium growth. Extension of Pleurotus mushroom shelf life was attainable through MAP using MPP2 bags (flow pack system). This is due to the increase in the surface area of the package as well as reduced water condensation, although the condensation could not be completely eliminated. Controlling RH within the package affects the quality of the mushrooms since sorbitol promoted deterioration
Table 7 Visual quality, texture, visual color, mycelium growth and weight loss of Pleurotus K-15 mushrooms initially and after 7 days of storage at 4 8C in PP and MPP2 films with silica gel (3, 5, 7 and 15 g) or without a moisture absorbant Time
Films
Initial After 7 days
PP MPP2 MPP2/silica MPP2/silica MPP2/silica MPP2/silica
gel gel gel gel
3 5 7 15
Visual quality
Texture
Visual color
Mycelium growth
7.9
8.9 a
8.2 a
1.0 b
5.1 7.0 7.2 6.3 7.3 6.7 NS
5.7 c 6.6 ab 6.5 ab 6.3 ab 6.5 ab 7.2 b ***
7.0 7.0 6.6 7.2 6.2 6.7 **
2.5 1.1 1.0 1.1 1.3 1.1 **
b a a a a b
a b b b b b
Weight loss (%)
3.7 b 1.1 d 2.8 c 3.3 bc 3.7 b 5.0 a ***
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. NS, not significant. *, P 5/0.05; **, P 5/0.01; and ***, P 5/0.001. Quality ratings are as described for Table 2.
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and increasing levels of silica gel increased weight loss. The RH control by silica gel resulted in a clear appearance in the package with a localized effect depending on the location of the silica gel. Addition of hygroscopic compounds in the packaging does not improve quality parameters sufficiently to justify their acceptance by the consumers, as has been shown in the case of Agaricus (Varoquaux et al., 1999). In summary, the most important parameters for extending shelf life of Pleurotus mushrooms are low temperature and proper internal humidity. MAP (12 /15 kPa O2/5 kPa CO2) was found beneficial for maintaining quality, although other combinations should be explored. High weight loss with visible evidence of deterioration and senescence were the main effects observed using perforated packages. Lowering the temperature had the dominant effect of reducing respiration and the contribution of MAP was to reduce transpiration. Further studies will be carried out in order to make a reliable prediction of MAP conditions without water condensation.
Acknowledgements The authors are grateful to UE, Project EUCRAFT QLK5-CT-1999-70010 for financial support. Thanks are due to Micelios Fungisem S.A. for providing Pleurotus and to Filmwrap for film supplied. We thank M. Pedren˜o for helping with the sensory analysis.
References AOAC, 1984. Official Methods of Analysis, 15th ed. Association of Official Analytical Chemists, VA. Anantheswaran, R.C., Sunkara, R., 1996. Use of commercial moisture absorbers to increase the shelf life of fresh mushrooms. In: Royse, D. (Ed.), Mushrooms Biology and Mushroom Products. Proceedings of the Second International Conference, Pennsylvania State University Press, PA. Beit-Halachmy, I., Mannheim, C.H., 1992. Is modified atmosphere packaging beneficial for fresh mushrooms? Lebensm.-Wiss. Technol. 25, 426 /432. Burton, K.S., Noble, R., 1993. The influence of flush number, bruising and storage temperature on mushroom quality. Postharvest Biol. Technol. 3, 39 /47.
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Burton, K.S., Twyning, R.V., 1989. Extending mushroom storage life by combining modified atmosphere packaging and cooling. Acta Hort. 258, 265 /271. Burton, K.S., Frost, C.E., Nichols, R., 1987. A combination plastic permeable film system for controlling post-harvest mushroom quality. Biotechnol. Lett. 9, 529 /534. Hammond, J.B.W., 1980. The composition of fresh and stored oyster mushrooms (Pleurotus ostreatus ). Phytochemistry 19, 2565 /2568. Henze, J., 1989. Storage and transport of Pleurotus mushrooms in atmospheres with high CO2 concentrations. Acta Hort. 258, 579 /584. Jin, K.S., Kim, J.J., Jeon, C.S., Lee, E.J., 1994. Characteristical differentiation on isolation and identification of mushroom bacterial brown blotch pathogens Pseudomonas tolasii and Pseudomonas agarici . RDA J. Agric. Sci. Crop. Prot. 36, 343 /348. Kader, A.A., 1992. Methods of gas mixing, sampling, and analysis. In: Kader, A.A. (Ed.), Postharvest Technology of Horticultural Crops (Publ. 3311). University of California, Berkley, CA, pp. 93 /95. Kang, J., Park, W., Lee, D., 2000. Quality of enoki mushrooms as affected by packaging conditions. J. Sci. Food Agric. 81, 109 /114. Little, A.C., 1975. Off on a tangent: a research note. J. Food Sci. 40, 410 /411. Lopez-Briones, G., Varoquaux, P., Chambroy, Y., Bouquant, J., Bureau, G., Pascat, B., 1992. Storage of common mushroom under controlled atmospheres. Int. J. Food Technol. 27, 493 /505. Moyls, L., Hocking, R., Beveridge, T., Timbers, G., 1992. Exponential decay method for determining gas transmission rate of films. Trans. ASAE 35, 1259 /1265. Murr, D.P., Morris, L.L., 1975. Effect of storage atmosphere on postharvest growth of mushrooms. J. Am. Hort. Sci. 100, 298 /301. Nichols, R., Hammond, J.B.W., 1973. Storage of mushrooms in pre-packs: the effects of changes of carbon dioxide and oxygen on quality. J. Sci. Food Agric. 24, 1371 /1381. Popa, M., Stanescu, M., Ilie, A., Dumitrescu, R., Vraci, I., 1999. Some aspects regarding modified atmosphere packaging of mushrooms. In: Ha¨gg, M., Ahvenainen, R., Evers, A.M., Tiilikkala, K. (Eds.), Agri-Food Quality. II. Quality Management of Fruits and Vegetables, vol. 4, pp. 177 /181. Roy, S., Anantheswaran, R.C., Beelman, R.B., 1995. Fresh mushroom quality as affected by modified atmosphere packaging. J. Food Sci. 60, 334 /340. Roy, S., Anantheswaran, R.C., Beelman, R.B., 1996. Modified atmosphere and modified humidity packaging of fresh mushrooms packaging. J. Food Sci. 61, 391 /397. Sveine, E., Klougart, A., Rasmussen, C.R., 1967. Ways of prolonging the shelf life of fresh mushrooms. Mushrooms Sci. 6, 463 /474. Varoquaux, P., Gouble, B., Barron, C., Yildiz, F., 1999. Respiratory parameters and sugar catabolism of mushrooms (Agaricus bisporus Lange). Postharvest Biol. Technol. 16, 51 /61.
Quality improvement of Pleurotus mushrooms by modified atmosphere packaging and moisture absorbers R. Villaescusa, M.I. Gil * Research Group on Quality, Safety and Bioactivity of Plant Foods, Food Science and Technology Department, CEBAS-CSIC, P.O. Box 4195, E-30080 Murcia, Spain Received 19 February 2002; accepted 25 July 2002
Abstract Quality of Pleurotus ostreatus mushrooms was compared during cold storage under various temperatures and modified atmospheres with and without moisture absorbers. CO2 production followed a non-climacteric pattern and was about 0.50, 0.99 and 1.23 mmol CO2 kg 1 s 1 at 0, 4 and 7 8C, respectively, after 12 h. At the end of 11 days of storage, the respiration rates were very similar for all the assayed temperatures (9/0.37 mmol CO2 kg 1 s 1). The best mushroom appearance was found at 0 8C, although for optimizing modified atmosphere packaging (MAP), 4 8C was selected to minimize low-temperature damage. When prolonging the storage beyond 7 days, quality characteristics dropped sharply and the mushrooms were not marketable, except when kept at 0 8C. Therefore, 7 days was the maximum recommended storage period for Pleurotus mushrooms. MAP at 4 8C for 7 days was created using microperforated polypropylenes (MPP1 and MPP2), low-density polyethylene (LDPE) and polyvinyl chloride (PVC) films. Results were compared with macroperforated polypropylene (PP) control packages. In order to avoid condensation, sorbitol and silica gel were used to modify the in-package relative humidity. Moisture absorbers did not modify the gas composition of the packages. Sorbitol promoted tissue leakage and cannot be recommended. Lower relative humidity was observed in packages containing silica gel and this did not affect the quality of the mushrooms. However, increasing amounts of silica gel increased weight loss in Pleurotus mushrooms, and high weight loss detected in PP packages made these unacceptable. MAP (15 kPa O2/5 kPa CO2) was found beneficial for maintaining acceptable quality of Pleurotus for 7 days at 4 8C. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Oyster mushrooms; Respiration rate; Storage; Sorbitol; Silica gel
1. Introduction
* Corresponding author. Tel.: /34-968-396-315; fax: /34968-396-213 E-mail address: [email protected] (M.I. Gil).
Pleurotus mushrooms are a delicate variety of mushroom, requiring a growing temperature range between 5 and 22 8C, depending on the cultivar, as well as good ventilation and high relative humidity. To obtain the highest quality for con-
0925-5214/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 5 2 1 4 ( 0 2 ) 0 0 1 4 0 - 0
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sumers, it is necessary to optimize postharvest storage. Lowering the temperature of mushrooms reduces respiration and transpiration, delaying senescence, preventing wilting and shriveling and thus extending shelf life (Burton and Twyning, 1989; Beit-Halachmy and Mannheim, 1992). Much has been published about Agaricus bisporus , however very little is known about the storage characteristics of Pleurotus mushrooms. Some authors have reported an optimum inpackage O2 concentration of 6 kPa without creating anaerobic conditions for A. bisporus mushrooms (Roy et al., 1995). According to LopezBriones et al. (1992), fermentation did not occur at a minimum O2 concentration of 1 /2 kPa, although Burton et al. (1987) found that the O2 concentration must not fall below the 3 /4 kPa range. Excessive accumulation of CO2 (/12 kPa) inside the mushroom package can also prevent the opening of the cap, but results in severe browning (Nichols and Hammond, 1973; Lopez-Briones et al., 1992). Atmospheres within 5/10 kPa O2 and 2.5 /5 kPa CO2 were established by Lopez-Briones et al. (1992) for modified atmosphere packaging (MAP) of mushrooms. However, other authors found that MAP could have a damaging effect, causing anaerobic respiration as well as potential growth of anaerobic pathogens (Beit-Halachmy and Mannheim, 1992; Varoquaux et al., 1999). Commonly, prepacked mushrooms are overwrapped with stretchable polyethylene or polyvinyl chloride (PVC), partially heat-sealed at the bottom of the package, and stored at room temperature. To improve mushroom packaging further, the relative humidity inside the package can be controlled to minimize moisture loss from the produce. Loss of 3 /6% of fresh weight is usually enough to cause a marked deterioration of quality for most kinds of products and for mushrooms the acceptable weight loss is about 2% (Sveine et al., 1967; Beit-Halachmy and Mannheim, 1992). The low water transmission rate of the films commonly used in combination with the high transpiration rate of mushroom results in rapid saturation of the package atmosphere (Roy et al., 1996). These conditions may cause the growth of Pseudomonas tolasii, responsible for browning or yellowing of
the sporophore surface, known as bacterial blotch, also making the package unattractive (Jin et al., 1994). The use of moisture absorbers such as sorbitol, sodium chloride, propylene glycol and polyvinyl alcohol have resulted in better color of the mushrooms (Roy et al., 1995; Anantheswaran and Sunkara, 1996; Roy et al., 1996). Little information has been published on the gas composition recommended for storing Pleurotus mushrooms. The most efficient MAP composition for storage of Pleurotus mushrooms was determined as 1 kPa O2/5 kPa CO2 at 4 8C for 14 days by Popa et al. (1999), while according to Henze (1989) it was 1 kPa O2/30 kPa CO2 at 1 8C for 10 days. The optimum MAP for stored Pleurotus is therefore still to be determined. The aim of our work was to select the storage temperature and MAP most appropriate for maintaining Pleurotus quality and prolonging shelf life. Moisture absorbers were also studied in combination with MAP.
2. Materials and methods 2.1. Plant material Cultivated Pleurotus ostreatus mushrooms strain K-15 was commercially produced by Micelios Fungisem S.A. in Autol (La Rioja, Spain). Mushrooms from the first flush (or production interval) were used. Pleurotus bears the sporophyte or mushroom in bunches. Immediately after picking, bunches were cut and sorted for uniformity of appearance, except for the mushrooms for the first experiment which were cut at the laboratory. Freshly harvested mushrooms were transported in a refrigerated container (8 /10 8C) to the laboratory in Murcia within 24 h of harvest. Mushrooms were promptly placed in cold storage at 10 8C and sorted to eliminate defects. 2.2. Modified atmosphere packaging Three replicates of approximately 150 g of mushrooms were packaged in 1000 ml polypropylene trays (17.5 length /12 width /5.2 depth; cm3) (Borden, S.A., Alicante, Spain). MAP at
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4 8C for 7 days was created using PVC (Borden S.A.), low-density polyethylene (LDPE) (Filmwrap Plastic S.A.L., Murcia, Spain) and two microperforated polypropylenes (MPP1 and MPP2) (Danisco Flexible, Bristol, UK) films. Macroperforated polypropylene (PP) (33 holes of 2 mm/dm2 and 35 mm thickness, Pla´sticos del Segura, Murcia, Spain) was used as the control in all the MAP experiments. This film did not modify the atmosphere within the package, only limiting dehydration. Depending on the film characteristics, the trays were overwrapped (for PVC and LDPE films), heat-sealed (La Barket model BP 40E, Befor, Chassieu, France) (for MPP1) or packed in bags (for PP and MPP2) with similar dimensions to the trays and heat-sealed (Packer model IS/300H, Pla´sticos del Segura, Murcia, Spain). The effective surface area for the PVC, LDPE and MPP1 packages was 0.2 m2 and for MPP2 was 1.0 m2. The gas permeabilities of PVC and LDPE films were determined using a test cell by the method of Moyls et al. (1992) at 4 8C, and for MPP1 and MPP2 were provided by the manufacturer’s data at 25 8C (Table 1). In order to avoid condensation, sorbitol (Sigma/Aldrich, St. Louis, MO) and silica gel (Panreac, Barcelona, Spain) were used as moisture absorbers to modify the in-package relative humidity in combination with the MPP2 film. Different amounts of either sorbitol (10, 15 and 20 g) or silica gel (3, 5, 7 and 15 g) were placed in paper sacks (8 /5 cm2) under or on top of the mushrooms. Sorbitol absorbs moisture very slowly and holds about seven times less moisture than other moisture absorbers such as NaCl (Roy et al., 1996). Neither washing nor postharvest chemical treatments were applied to the mushrooms.
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2.3. Gas sampling Six replicates (about 150 g each) were placed in 500 ml glass jars at 0, 4 and 7 8C for 11 days to measure respiration rates. An open system with a continuous flow of 60 ml min 1 of humidified air was pumped into the jars to avoid dehydration and excessive CO2 accumulation. Carbon dioxide production was monitored daily during the storage period using a gas chromatograph (Shimadzu GC14, Kyoto, Japan), equipped with a thermal conductivity detector (TCD). The initial measurement was taken after 12 h of storage. Ethylene production was measured using a closed system (Kader, 1992). Glass jars of 500 ml (six replicates) were filled with 150 g of Pleurotus and stored in the same conditions as mentioned above. The increase in C2H4 concentration was measured by keeping the jars closed for 2 h and taking 5 ml samples from the headspace. Samples were analyzed using a gas chromatograph (model 5370 A, Hewlett Packard, Avondale, PA) equipped with a flame ionization detector (FID). Changes in O2 and CO2 concentrations in the MA packages were monitored daily using a gas chromatograph (Perkin Elmer autosystem, CT) equipped with a TCD. 2.4. Quality characteristics A panel of three trained judges evaluated the quality characteristics of all the mushrooms from each tray. The trays were left at room temperature for 2 h to warm the product. After storage, the weight of the moisture absorber was determined. Mushrooms were removed from the trays and weight loss was immediately determined, expressed as a percentage on a fresh weight basis. The overall
Table 1 Gas permeabilities of PVC and LDPE films were calculated using a test cell at 4 8C and 90% RH (Moyls et al., 1992) Films
Thickness (mm)
O2 permeance (10 10 mol s 1 m 2 Pa 1)
Ratio, CO2:O2
PVC LDPE MPP1 MPP2
11 11 35 35
0.28 0.48 9.34 3.36
9:1 5:1 1:1 1:1
Gas permeabilities of MPP1 and MPP2 films were according to the manufacturer’s data at 25 8C.
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visual quality of the intact mushrooms was evaluated on a scale of 9 /1, where 9 /excellent, 7 / very good, 5 /good and limit of marketability, 3 /fair and limit of usability, and 1 /poor and inedible. Aroma was determined after breaking the mushrooms and evaluating on a scale of 9 /1, where 9 /full typical aroma or flavor, 7 /moderately full, 5 /moderate, 3 /slight, and 1 / none. Texture was evaluated when the mushroom was pressed between the thumb and index finger, on a scale of 9 /1, where 9 /very firm and turgid, 7 /firm, 5 /moderately firm, 3 /soft, and 1 / very soft. Visual color was evaluated based on a scale of 9 /1, where 9 /typical uniformity and intensity color, 7 /moderately intense, 5 /moderate, 3 /slight, and 1 /poor. Spoilage, mycelium growth, wilting (as withered deterioration) and off-odors (mainly fermentation) were evaluated on scales of 1 /5, where 1 /none, 2 /slight, 3 / moderate, 4 /moderately severe, and 5 /severe. Surface color was measured at three equidistant points of the cap of the mushroom using a Chroma Meter (model CR 300 Minolta, Ramsey, NJ). The values were expressed as CIELAB color space units (Little, 1975) and mean values for lightness (L *), red-greenness (a*) and blue-yellowness (b*) parameters were calculated for each batch of mushrooms. Mushroom firmness was determined on each Pleurotus cap using a texturometer (model LR10K, Lloyd instrument, Fareham, Hants, UK) equipped with a 10.5 mm diameter probe. The maximum force required to deform the mushrooms 6 mm at a speed of 10 mm min 1 was recorded. Titratable acidity (TA) was determined by titrating 10 ml of juice with 0.1 mol l 1 NaOH to pH 8.1 (AOAC, 1984). The pHvalues were measured using a pH meter (Crison model 501, Barcelona, Spain) and soluble solids content (SSC) with a handheld refractometer (Atago N1, Tokyo, Japan). Three experiments were conducted. The first experiment was to select the best storage temperature and shelf life for maintaining Pleurotus quality under normal atmosphere conditions. Respiration rate and ethylene production were measured daily and quality characteristics were evaluated after 7 and 11 days storage at 0, 4 and 7 8C. The second experiment was performed to
select the most appropriate MAP for the temperature selected in experiment one, using PVC, LDPE and MPP1 films. In the third experiment, moisture absorbers (sorbitol and silica gel) were studied in combination with MPP2 film. Changes in gas composition were checked daily. and quality characteristics and weight loss determined after 7 days at 4 8C. 2.5. Statistical analysis All data were analyzed by analysis of variance (ANOVA) using SPSS (Windows 2000) from Statistical Analysis. To determine differences between treatments, Duncan tests were applied and significant differences were established at P 5/ 0.05.
3. Results 3.1. Selection of temperature and storage time The initial respiration rate of Pleurotus K-15 after 12 h of storage was 0.50, 0.99 and 1.23 mmol CO2 kg 1 s 1 at 0, 4 and 7 8C, respectively (Fig. 1). After this time, the respiration rate dropped at all temperatures by about 50% (including at 0 8C) for the first 2 storage days. These initial high respiration rates were associated with harvest stress caused by the cutting process, which in this trial was carried out in the laboratory just before storage. After the decline, the respiration rate remained constant throughout 7 days of storage, but decreased a second time at 7 8C between days 7 and 10. At the end of the storage time, the respiration rates were very similar for all the assayed temperatures (9/0.37 mmol CO2 kg 1 s 1). The respiration rate of Pleurotus reported by Hammond (1980) at 18 8C was 10-fold that detected in our experiments. There was no detectable C2H4 production at any temperature throughout the storage period. Significant differences in quality characteristics of mushrooms were observed at 0, 4 and 7 8C and between 7 and 11 days of storage (Table 2). Comparing temperatures, Pleurotus mushrooms kept at 0 8C had the best visual quality and no
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173
Fig. 1. Respiration rate of Pleurotus K-15 mushrooms at 0, 4 and 7 8C after the first 12 h storage and during 11 days storage. Each point represents the mean of six replicates of 150 g. Vertical lines represent S.D.
evidence of chilling injury was found. After 7 days of storage, the visual quality of Pleurotus was very good at 0 and good at 4 8C, but only fair and under the limit of usability at 7 8C. When
prolonging the storage time from 7 to 11 days, visual quality was considered good at 0 8C but was unacceptable at 4 and 7 8C. Aroma decreased from full typical to moderate aroma after 11 days
Table 2 Visual quality, aroma, texture, visual color, spoilage and wilting of Pleurotus K-15 mushrooms initially and after 7 and 11 days of storage at 0, 4 and 7 8C Time
Temperature (8C)
Visual quality 8.0 a
9.0 a
After 7 days
0 4 7
7.8 a 5.0 c 2.5 d
6.8 b 6.7 bc 5.7 c
After 11 days
0 4 7
6.0 b 1.8 d 1.8 d
4.5 d 4.0 d 4.3 d
*** *** ***
NS *** NS
Initial
Temperature Time Temperature /time
Aroma
Texture
Visual color
Spoilage
Wilting
8.6 a
8.8a
1.0 b
1.0 e
8.0 a 5.0 b 3.3 c
7.7 a 5.8 b 4.2 c
1.0 b 3.0 a 3.7 a
1.0 e 2.0 d 2.8 b
5.3 b 3.8 c 2.0 d
7.5 a 4.5 bc 3.8 c
1.0 b 3.0 a 3.8 a
3.0 b 2.5 c 3.5 a
*** *** ***
*** *** **
*** *** **
*** *** ***
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. NS, not significant. *, P 5/0.05; **, P 5/0.01; and ***, P 5/0.001. Visual quality evaluated on a 9 /1 scale (9 /excellent; 7 /very good; 5 /good, limit of marketability; 3 /fair, limit of usability; and 1 /poor, inedible). Aroma scored on a 9 /1 scale (9 /full typical aroma or flavor; 7 /moderately full; 5 /moderate; 3 /slight; and 1 /none). Texture scored on a 9 /1 scale (9 /very firm and turgid; 7 /firm; 5 /moderately firm; 3 /soft; and 1 /very soft). Visual color scored on a 9 /1 scale (9 /typical uniformity and intensity color; 7 /moderately intense; 5 /moderate; 3 /slight; and 1 /poor). Spoilage and wilting scored on 1 /5 scales (1 /none; 2 /slight; 3 /moderate; 4 /moderately severe; and 5 /severe).
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at the assayed temperatures (Table 2). No significant differences were observed between the initial texture and that at the end of the 7 days at 0 8C. However, texture losses occurred by 11 days storage from an initial firm texture to soft texture at 4 and 7 8C. In addition, Pleurotus stored at 0 8C maintained the initial intense color, whereas a moderate discoloration to yellowish hues was observed at 4 and 7 8C. No spoilage was found at 0 8C after 7 and 11 days (Table 2). Pleurotus mushrooms at 0 8C did not show significant differences as regards wilting after 7 days, compared with the initial samples. Storage time significantly affected the color parameters (Table 3). A significant decrease between the initial L *-value and those values after 7 and 11 days is shown in Table 3. In addition, no significant differences were observed between initial a*-values and those after 7 days at any of the studied temperatures, while there was an increase in a * (red coloration) after 11 days compared with the initial one (Table 3). Both temperature and time affected the color measured as b *. No changes were detected in b *-values at 0 and 4 8C after 7 days. But after 11 days of storage, considerable yellowing of Pleurotus was observed (Table 3). The storage temperature 0 8C was considered the best one for keeping the initial Pleurotus color. Mushroom aging was characterized by a soft and spongy texture, reported by Lopez-Briones et
al. (1992) to be due to cell growth and water migration. Firmness of Pleurotus K-15 was influenced by temperature and storage time. A significant reduction in firmness was detected after 7 and 11 days of storage compared with the initial values, in agreement with a report by Murr and Morris (1975) on Agaricus (data not shown). SSC and pH decreased after 11 days of storage although no significant differences were observed among mushrooms kept at 0, 4 or 7 8C (Table 4). No significant changes in TA were detected in Pleurotus mushroom storage for 11 days when compared with the values at harvest as well as among the assayed temperatures. As expected, weight loss increased with temperature and was accentuated by storage time, in accordance with the previous results for other mushroom types (Burton and Noble, 1993; Kang et al., 2000). The lowest weight loss was registered after 7 days of storage with significant differences between temperatures (0.6% weight loss at 0 and 4 8C, and 1.6% at 7 8C). After 11 days of storage, weight loss reached 2% at both 0 and 4 8C and 2.5% at 7 8C. Although the circulating air was highly humidified, weight loss was higher than expected due to the high water content of mushrooms and their large surface area. Since most of the quality characteristics were judged acceptable after 11 days at 0 8C, the recommended storage temperature for Pleurotus mushrooms should be 0 8C.
Table 3 Changes in L *-, a *- and b *-values of Pleurotus K-15 mushrooms initially and after 7 and 11 days of storage at 0, 4 and 7 8C Time
Temperature (8C)
Initial
L*
a*
b*
96.9 a
0.2 b
5.1 c
After 7 days
0 4 7
85.8 b 86.3 b 87.0 b
0.8 b 0.8 b 0.2 b
3.7 c 4.6 c 6.6 b
After 11 days
0 4 7
61.9 c 63.3 c 61.7 c
2.3 a 2.4 a 2.2 a
18.5 a 19.8 a 19.0 a
NS *** NS
NS *** NS
*** *** ***
Temperature Time Temperature /time
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. NS, not significant. *, P 5/0.05; **, P 5/0.01; and ***, P 5/0.001.
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175
Table 4 SSC, pH and TA of Pleurotus K-15 mushrooms initially and after 11 days of storage at 0, 4 and 7 8C Time
Temperature (8C)
SSC (8Brix) 5.1 a
6.4 a
0.10
0 4 7
4.2 bc 4.0 c 4.4 b
6.2 b 6.3 ab 6.2 b
0.09 0.08 0.09
NS *** NS
NS ** NS
NS NS NS
Initial After 11 days
Temperature Time Temperature /time
pH
TA (g citric acid/100 ml)
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. NS, not significant. *, P 5/0.05; **, P 5/0.01; and ***, P 5/0.001.
3.2. Modified atmospheric packaging Although 0 8C was the best storage temperature for maintaining Pleurotus quality, 4 8C was used to optimize MAP, so as to reduce lowtemperature injury. Stretchable PVC film, currently used to overwrap mushrooms, was used and compared with LDPE and MPP1 films to create the MA conditions reported in the literature for Pleurotus mushrooms (Henze, 1989; Popa et al., 1999). The initial air composition within the packages was passively modified, according to the permeances of the films used. A decrease in O2 and an increase in CO2 levels were detected after 1 day of storage at 4 8C for PVC, LDPE and MPP1 packages (Fig. 2). When the mushroom trays were wrapped with PVC and LDPE, O2 declined and CO2 accumulated (Fig. 2). After 1 day, O2 and CO2 levels were very similar for both LDPE and PVC packages. However, CO2 decreased within PVC packages and was maintained throughout the storage within the LDPE ones (Fig. 2). Less modification of O2 and CO2 was detected within the MPP1 packages compared with the other films. Steady-state MAP conditions were reached after 2 days with concentrations for O2/CO2 of 1.5/8 kPa for PVC, 2/12 kPa for LDPE and 12/7 kPa for MPP1 packages (Fig. 2). Significant differences in visual quality, aroma, off-odors and mycelium growth were observed between initial Pleurotus mushrooms and those stored for 7 days in PP, PVC, LDPE and MPP1 (Table 5). Mushrooms kept in PVC films resulted in better visual quality and were not significantly
different from the initial mushrooms (Table 5). Samples stored in LDPE maintained good visual quality while they were under the limit of marketability in PP and MPP1 packages. Pleurotus mushrooms packaged in MPP1 film were scored with the worst visual quality for spongy appearance due to the high condensation created in the package. The visual appearance of these mushrooms was dramatically affected by the high water content of the tissue. The characteristic aroma of the mushrooms decreased with storage time in all conditions. In addition, moderately severe offodors were detected for samples stored in PVC and LDPE packages. These off-odors were induced by the intense modified atmosphere created in PVC and LDPE packages with O2 levels of 2 kPa, which probably promoted fermentation (ethanol-like odors) (Table 5). Mushroom firmness decreased after storage although it was judged acceptable in all the treatments (data not shown). After 7 days of storage at 4 8C, mycelium growth was observed only in Pleurotus kept in PP and MPP1 (Table 5). No evidence of spoilage and wilting was detected in any stored sample. The most representative parameter to detect differences in the color of Pleurotus was the b *value, which showed that all mushrooms after storage underwent yellowing (Table 6). In contrast, no differences were found between treatments with respect to a*, although L * increased for mushrooms stored in MAP (data not shown). At the end of 7 days at 4 8C, SSC and TA decreased for mushrooms kept in PVC, LDPE and
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3.3. Effect of MAP and modified humidity packaging
Fig. 2. Changes in MAP O2 and CO2 concentrations of Pleurotus K-15 mushrooms at 4 8C during 7 days storage in PVC, LDPE and MPP1 films. Each point represents the mean of three replicates of 100 g. Vertical lines represent S.D.
MPP1 packages and pH increased after storage as compared with the initial values (Table 6). The tested MA packages resulted in either anoxia (PVC, LDPE) or CO2 stress (LDPE) or condensation (MPP1). Other films need to be tested to develop MAP in order to obtain the required gas conditions in combination with the reduced weight loss and prevention of condensation.
In order to avoid fermentation and prevent condensation, we increased the surface area of the package fivefold (from 0.2 m2 for the trays to 1.0 m2 for the bags). In addition, different amounts of sorbitol and silica gel were incorporated inside the packages to control condensation. MPP2 packages with no moisture absorbers were also used. No differences were observed in MAP generated with or without moisture absorbers. The changes in O2 and CO2 concentration within MPP2 packages without moisture absorbers are shown in Fig. 3. The steady-state atmosphere was reached within 1 day with average values of 15 kPa O2 and 4 kPa CO2 (Fig. 3). Sorbitol did not improve Pleurotus quality. On the contrary, it promoted deterioration, in disagreement with previous results by Roy et al. (1996) for Agaricus mushrooms. In our case, sorbitol was responsible for mushroom texture deterioration and the exudation of tissue fluids, which accumulated in the trays making mushrooms unacceptable in MPP2 packages (data not shown). On the other hand, silica gel reduced condensation in those package areas of the film where the sacks were located, particularly when it was placed on the top of the mushrooms. Although, silica gel provided a clear view through the package, no improvements in quality were found when comparing samples with or without silica gel (Table 7). This finding is in accordance with the results described by Roy et al. (1996) for Agaricus mushrooms. The visual quality of Pleurotus mushrooms after 7 days at 4 8C in MPP2 with and without silica gel was very good and no significant differences were observed with respect to the initial samples (Table 7). However, significant losses in visual quality and texture were found when mushrooms were kept in PP packages, although they were always above the threshold of marketability (Table 7). The aroma of mushrooms after storage was no different from that at harvest and no off-odors were noticed for all samples (data not shown). The color of Pleurotus changed after storage, a white discoloration was particularly observed, although no other differences among samples were detected.
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Table 5 Visual quality, aroma, texture, mycelium growth and off-odors of Pleurotus K-15 mushrooms initially and after 7 days of storage at 4 8C in PP, PVC, LDPE and MPP1 films Time Initial After 7 days
Films
Visual quality
Aroma
Texture
Mycelium
Off-odors
PP PVC LDPE MPP1
7.0 a 4.7 c 6.3 ab 5.3 bc 3.2 d ***
8.0 a 3.3 bc 1.3 c 3.0 bc 4.0 b ***
8.7 5.0 6.3 5.3 5.3 *
1.0 2.0 1.0 1.0 1.7 **
1.0 1.0 2.7 3.7 1.0 **
a b ab b b
b a b b a
b b a a b
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. *, P 5/0.05; **, P 5/0.01; and ***, P 5/0.001. Quality ratings are as described for Table 2.
Mycelium growth was only observed in samples stored in PP packages. MAP with silica gel increased weight loss in Pleurotus mushrooms. The weight loss was higher when the amount of silica gel increased for MPP2 (Table 7). High weight loss was also detected in PP packages (3.7%). This made that type of wrapping unacceptable. MPP2 without silica gel reduced weight loss to acceptable values (5/1.1%) (Table 7).
4. Discussion Mushrooms are a fast respiring and highly perishable product, and therefore MAP, in addition to low temperature can be beneficial to extend their shelf life. After harvest, mushrooms should be precooled at 1/4 8C and stored for a few hours
at the same temperature for the mushrooms to recover from harvest stress before packaging. When the mushrooms were cut at the laboratory, they showed a high initial respiration rate (Fig. 1). These high respiration rates could also be used by packaging mushrooms immediately. As a result, MA conditions will be established more rapidly and this could increase the effectiveness of the MAP by shortening the time at sub-optimal MA conditions. Cooling should be used in fresh mushroom storage and during distribution and sale to improve quality and extend storage life. Packaging mushrooms has been essential for reducing transpiration and maintaining quality. Pleurotus mushrooms are available in the market in Europe in a stretchable PVC package. This kind of film is subject to environmental criticism, and may be banned for food packaging in the future. The packaging film must be environmentally
Table 6 Color (b *-value), SSC, pH and TA of Pleurotus K-15 mushrooms initially and after 7 days of storage at 4 8C in PP, PVC, LDPE and MPP1 films Time Initial After 7 days
Films
b*
PP PVC LDPE MPP1
14.2 18.8 16.2 16.8 19.1 ***
c a b b a
SSC (Brix)
pH
TA (g citric acid/100 ml)
6.3 a 6.3 a 6.0 b 5.2 d 5.7 c ***
6.3 d 6.7 a 6.6 b 6.5 c 6.6 b ***
0.31 0.14 0.17 0.14 0.14 ***
a b b b b
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. *** P 5/0.001.
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Fig. 3. Changes in MAP O2 and CO2 concentrations of Pleurotus K-15 mushrooms at 4 8C during 7 days storage in MPP2 film with or without silica gel (3, 5, 7 and 15 g). Each point represents the mean of three replicates of 100 g. Vertical lines represent S.D.
friendly and adapted to the O2 requirement of the commodity, which largely depends on the storage temperature. Fermentation in Agaricus mushrooms stored in O2 concentrations between 1 and 2 kPa (depending on CO2 concentration) did not occur in previous work (Lopez-Briones et al., 1992), although other authors have suggested higher values (3 /4 kPa; Burton et al., 1987). In our case, fermentation was detected in samples kept in PVC and LDPE
packages that contained 2 kPa O2. Carbon dioxide content was important in controlling mycelium growth. Extension of Pleurotus mushroom shelf life was attainable through MAP using MPP2 bags (flow pack system). This is due to the increase in the surface area of the package as well as reduced water condensation, although the condensation could not be completely eliminated. Controlling RH within the package affects the quality of the mushrooms since sorbitol promoted deterioration
Table 7 Visual quality, texture, visual color, mycelium growth and weight loss of Pleurotus K-15 mushrooms initially and after 7 days of storage at 4 8C in PP and MPP2 films with silica gel (3, 5, 7 and 15 g) or without a moisture absorbant Time
Films
Initial After 7 days
PP MPP2 MPP2/silica MPP2/silica MPP2/silica MPP2/silica
gel gel gel gel
3 5 7 15
Visual quality
Texture
Visual color
Mycelium growth
7.9
8.9 a
8.2 a
1.0 b
5.1 7.0 7.2 6.3 7.3 6.7 NS
5.7 c 6.6 ab 6.5 ab 6.3 ab 6.5 ab 7.2 b ***
7.0 7.0 6.6 7.2 6.2 6.7 **
2.5 1.1 1.0 1.1 1.3 1.1 **
b a a a a b
a b b b b b
Weight loss (%)
3.7 b 1.1 d 2.8 c 3.3 bc 3.7 b 5.0 a ***
Values (mean of three replicates) in the same column followed by the same letter are not significantly different by Duncan’s test. NS, not significant. *, P 5/0.05; **, P 5/0.01; and ***, P 5/0.001. Quality ratings are as described for Table 2.
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and increasing levels of silica gel increased weight loss. The RH control by silica gel resulted in a clear appearance in the package with a localized effect depending on the location of the silica gel. Addition of hygroscopic compounds in the packaging does not improve quality parameters sufficiently to justify their acceptance by the consumers, as has been shown in the case of Agaricus (Varoquaux et al., 1999). In summary, the most important parameters for extending shelf life of Pleurotus mushrooms are low temperature and proper internal humidity. MAP (12 /15 kPa O2/5 kPa CO2) was found beneficial for maintaining quality, although other combinations should be explored. High weight loss with visible evidence of deterioration and senescence were the main effects observed using perforated packages. Lowering the temperature had the dominant effect of reducing respiration and the contribution of MAP was to reduce transpiration. Further studies will be carried out in order to make a reliable prediction of MAP conditions without water condensation.
Acknowledgements The authors are grateful to UE, Project EUCRAFT QLK5-CT-1999-70010 for financial support. Thanks are due to Micelios Fungisem S.A. for providing Pleurotus and to Filmwrap for film supplied. We thank M. Pedren˜o for helping with the sensory analysis.
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