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Journal of Food, Agriculture & Environment Vol.10 (1): 78-84. 2012

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Extending the shelf-life of straw mushroom with high carbon dioxide treatmentSoravit Jamjumroon 1, Chalermchai Wongs-Aree 1, 4*, William B. McGlasson 2, Varit Srilaong Piya Chalermklin 3 and Sirichai Kanlayanarat 1, 41, 4

,

1 Postharvest Technology Program, School of Bioresources and Technology, King Mongkuts University of Technology Thonburi, 126 Prachauthit Rd., Bangkok 10140, Thailand. 2 School of Natural Sciences, University of Western Sydney, Locked Bag 1797, Penrith South DC NSW 2751, Australia. 3 Agricultural Technology Department, Thailand Institute of Scientific and Technological Research, Pathum Thani 10120, Thailand. 4 Postharvest Technology Innovation Centre, Commission on Higher Education, Bangkok 10400, Thailand. *e-mail: chalermchai.won@kmutt.ac.th

Received 14 September 2011, accepted 8 January 2012.

AbstractTropical straw mushrooms (Volvariella volvacea) are important ingredients in many Asian dishes, but their rapid browning and weight loss immediately after harvest are the main factors limiting their shelf life to 1-2 days under ambient conditions. In the present study, browning and several physiological changes of straw mushrooms were investigated under various storage temperatures and under high CO2 atmospheric conditions. The browning symptoms initially appeared at the middle of the mushroom cap and at the cut surface 6 h after harvest under ambient conditions (25-34C, 60-70% relative humidity (RH)) and progressively increased with advancing storage time in parallel with an increase in weight loss. In the browning tissues, the mycelium of the mushroom cap turned brown and collapsed. However, during modified atmosphere (MA) storage with a polyvinyl chloride (PVC) film overwrapping, the browning symptoms of the stored mushrooms still occurred even when the water loss was dramatically reduced. Tyrosine and pyrocatechol were found to be the preferred substrates for the browning reaction. Storage at temperatures below the optimum of 15C induced more severe browning symptoms due to chilling injury. Malondialdehyde (MDA), a product of lipid oxidation, increased during the first day of storage at ambient temperatures and at 4C but decreased at 8, 12 and 15C. Applications of CO2 concentrations of 10 or 20% combined with 15% O2 during storage effectively decreased browning due to the inhibition of polyphenol oxidase (PPO) activity. Furthermore, exposure to 40% CO2 for 4-6 h prior to MA packing tended to reduce mushroom browning during storage, whereas a 12-h incubation in high CO2 at either 40 or 60% revealed an increase in browning symptoms. Key words: Volvariella volvacea, browning, high CO2 shock, controlled atmosphere, modified atmosphere packaging.

Introduction Straw mushrooms (Volvariella volvacea (Bull ex Fr.) Sing) are an edible tropical fungus and a popular ingredient in various East Asian dishes. The straw mushroom fruiting body is white with a short stipe and large cap, and it is cultivated mainly in Southeast Asian countries 1. Thailand produced approximately 66,000 tons of straw mushrooms worth 3,630 million Baht in 2009 2. Straw mushrooms require hot, humid growing conditions and are widely grown in central Thailand provinces such as Saraburi, Angthong, Ratchaburi and Nakhon Ratchasima. These mushrooms typically contain 85-90% moisture and rapidly transpire 3. The mushrooms are harvested at the button stage when they are considered to have the best flavour and texture. Commercial production methods yield a fruiting body with a light-coloured cap, which ages rapidly after harvest. The major postharvest problem that limits the shelf life of straw mushroom at ambient temperatures (25-34C and 6070% RH) is browning of the cap/cut surface and shrivelling related to water loss. As a result, most of the fresh straw mushroom market is domestic because of its postharvest life of only 1-2 days. Browning disorders in agricultural products are normally induced by dehydration, mechanical injury or wound deterioration4. Polyphenol oxidase (PPO) is the major enzyme involved in browning reactions. PPO catalyses the oxidation of phenolic compounds to78

o-quinones, resulting in a large molecule of brown pigment 5. Most research on mushroom browning of white button mushroom (Agaricus bisporus) associates the browning effect with PPO, especially tyrosinase, an enzyme belonging to the PPO family 6-8. Cool storage conditions slow the rate of deterioration, but temperatures that are too low may cause chilling injury. The optimum storage temperatures are 8C for oyster mushrooms 9, 5C for white button mushrooms 10, and as low as 1C for shiitake mushrooms 11. There is no official report regarding the optimum storage temperature for straw mushrooms. Storage temperature, relative humidity(RH) and air movement affect the rate of water loss in harvested mushrooms 12. The rate of water loss is high during the button stage and decreases as the mushroom matures. Similarly, the respiration rate increases with increasing temperature. Straw mushrooms were found to have high respiration rates of 245-280 mL CO2 kg-1 h-1 at 20C and 100-200 mL CO2 kg-1 h-1 at 15C 13. Rapid forced-air cooling at a non-chilling temperature will therefore help retain the quality of fresh straw mushrooms 14. High CO2 in the storage atmosphere is widely used to extend the shelf life of a number of perishable berry fruits 15, 16. High CO2 maintains a number of important quality characteristics, such as firmness, soluble solids, acidity and freshness, and it may alsoJournal of Food, Agriculture & Environment, Vol.10 (1), January 2012

reduce fungal decay 17. This method has been successfully used to maintain the quality of fresh-cut produce by slowing browning reactions at the cut surfaces and by retarding respiration rates 18. High CO2 can also inhibit the growth and metabolism of microorganisms 19. The aim of this study was to examine the effects of water loss and storage temperatures in modified atmosphere packaging on the shelf life of straw mushrooms. The possible benefits of brief exposures to high concentration of CO2 before low temperature storage were also examined. Materials and Methods Plant sample preparation: Straw mushrooms develop rapidly from the pin stage to the open stage at tropical temperatures, but the buttons (unopened caps) are preferred by the market because of their flavour (stage 3 in Fig. 1) 20. Straw mushrooms at the button stage were harvested from a commercial farm in the Saraburi Province (1432N 10053E / 14.53N 100.88E) in Central Thailand between March and June 2009. The mushrooms were packed in foam boxes and transported to the Thailand Institute of Scientific and Technological Research (TISTR) laboratory, Bangkok (a trip of approximately 1 h). The mushrooms (approximately 25 g per replicate) were selected for uniformity in colour and size.

to 15 cm 15 cm polystyrene foam trays overwrapped with PVC film. The packs of mushrooms were stored at 15C and 90-95% RH. The experiment was performed in CRD with 5 replications (one pack/replication). Visual assessment: Browning of the mushroom cap was scored based on the estimated brown surface area: 1 no browning, 2 browning 50%. In addition, the lightness (L *Hunter scales) of the cap was measured at the middle of a button on both sides with a chromameter (Model CR-300, Minolta, Japan). The buttons were photographed under standard laboratory lighting with a digital camera (Sony model DSC-W 150, Super Steady Shot). Scanning electron microscopy (SEM) photographs of the buttons were also taken after 1 and 24 h of storage under simulated ambient conditions of 25-34C and 60-70% RH. The cap tissue samples were fixed in 2.5% glutaraldehyde in 0.1 M phosphate buffer, pH 7.2, for 2 h. The fixed tissues were rinsed twice in phosphate buffer at pH 7.2 for 20 min followed by distilled water for 20 min. The tissues were dehydrated in a graded alcohol series, dried in a critical point dryer, and sputtered with platinum/ palladium. The samples were examined by an SEM (JEOL, model JSM-5410LV, Tokyo, Japan). Determination of pH: Fifty grams of straw mushroom were homogenised with a blender and centrifuged at 8000xg for 5 min at 4C. The pH values in the supernatant were measured using a pH meter (Lab Analyser Model 440, Australia).

Pinhead Tiny button Button stage stage stage

Egg stage

Elongation stage

Mature stage

Figure 1. Straw mushroom developmental stages: 1 Pinhead stage, 2 Tiny stage, 3 Button stage, 4 Egg stage, 5 Elongation stage, 6 Mature stage.

Storage conditions: The storage experiments were conducted under the following conditions: 1) Mushroom buttons were placed on 15 cm x15 cm polystyrene foam trays and held at 25-34C and 60-70% RH for 24 h to simulate ambient storage conditions. Browning and weight loss were evaluated every hour. Twenty replicates were used. 2) Four to five buttons (approximately 100 g) were placed on a 15 cm x 15 cm polystyrene foam tray overwrapped with polyvinyl chloride (PVC) film and stored at 4, 8, 12 or 15C and 90-95% RH. The experiment was arranged in a completely randomised design (CRD) with five replications (each pack represented a replication). 3) Twelve mushroom buttons (approximately 300 g) were enclosed in a 7.5 cm 10 cm 6.5 cm PVC plastic box and flushed continuously with saturated normal air or saturated gas mixtures comprised of 15%O2 + 10%CO2 + 75% N2 and 15%O2 + 20%CO2 + 65%N2. The controlled atmosphere (CA) of the boxes of stored mushrooms was maintained at 15C. The experiment was performed in CRD with 5 replications (one box/replication). 4) Mushroom buttons were enclosed in 20 cm 30 cm 11.5 cm PVC plastic boxes, flushed with normal air or a saturated gas mixture comprised of 40%CO2 + 60%N2 and 60%CO2 + 40%N2 and incubated for 4, 6 and 12 h at 15C. Four buttons (approximately 100 g) of each pre-treatment were then transferredJournal of Food, Agriculture & Environment, Vol.10 (1), January 2012

Determination of malondialdehyde (MDA) content: The extraction and detection of MDA in the straw mushrooms followed the protocol of Wang et al. 21.Ten grams of mushroom cap tissue were collected from each treatment and homogenised in 25 mL of ice-cold extraction buffer [(100 mM sodium phosphate buffer, pH 6.4, containing 0.5 g polyvinyl polypyrrolidone (PVPP)]. The homogenate was centrifuged at 27,000xg for 50 min at 4C, and the resulting supernatants were used directly for the assay. The MDA content was determined by adding 2 mL of 0.5% thiobarbituric acid (TBA) in 15% trichloroacetic acid (TCA) to a 1-mL sample. The solution was heated at 95C for 20 min, quickly cooled in an ice-bath for 5 min, and then centrifuged at 12,000 xg for 10 min to clarify the solution. The absorbance at 532 nm was measured and subtracted from the absorbance at 600 nm. The amount of MDA was calculated with an extinction coefficient of 155 mm/cm. Determination of polyphenoloxidase (PPO) activity: Samples (10 g) of straw mushroom cap were extracted and assayed for PPO activity using the method of Luh and Phithakpol 22. The assay medium contained 0.1 mL of enzyme extract and 1 mL of 40 mM catechol. PPO activity was determined by measuring the absorbance at 410 nm. One unit of PPO activity was defined as the change in absorbance after 1 min of measurement per g fresh weight. Determination of protein content: The proteins of the mushroom caps were measured following the protocol of Akoum et al. 23 Encapsulated mushroom tissue (2 g) was placed into the loading head of the Nitrogen/Protein Model-FP-528, where it was sealed and purged of any atmospheric gases that had entered during79

sample loading. The sample was then dropped into a hot furnace and flushed with pure oxygen for extremely rapid combustion. The by-products of the combustion CO, H, O, NO and N passed through the furnace filter and a thermoelectric cooler for subsequent collection in a ballast apparatus. The collected gases in the ballast were mixed, and a small aliquot was used for further conversion of the gases. The remaining aliquot that had been reduced was measured by the thermal conductivity cell for nitrogen. The system was controlled by an external PC using Windows-based operating software. The protein content was reported as a percentage of the fresh weight (FW).

Statistical analysis: The data were subjected to analysis of variance (ANOVA), and the means were compared with Duncans New Multiple Range Test (DMRT) using SPSS software (SPSS version 17.0 for Windows, SPSS Inc., Chicago, IL, USA).

Results and Discussion Effects of water loss and low temperature storage on straw mushroom browning: Straw mushroom buttons develop browning rapidly after harvest. The browning symptoms are initially generated around the cap and cut surface within 6 h of storage at ambient conditions (Figs. 2A and B). The browning and shrivelling symptoms began at the middle part of the cap and Determination of possible browning substrates: Straw mushroom became progressively more severe with increasing time in storage. buttons were immersed in solutions of 3 mM L-tyrosine, LThe development of browning increased in parallel with the increase in weight loss (Fig. 3A). The decrease in L Hunter scales phenylalanine, cinnamic acid, pyrocatechol or distilled water for was highly correlated with the increase in browning severity (r2 = 30 min at ambient conditions The colour changes of the buttons were monitored with a digital camera (Sony model DSC-W 150, 0.9803) (Fig. 3B). It was apparent that browning was dependent Super Steady Shot). on water loss. The critical point was reached after 6 h of storage (Fig. 2B), when weight loss increased to above 12% (Fig. 3B). A scanning electron micrograph (SEM) showed A B collapsed mycelium after 24 h of ambient storage (Fig. 2D) in contrast to the turgid appearance of the mycelium before storage (Fig. 2C). After 24 h storage, weight loss further increased to 40% (Fig. 3B), and the mycelium of the cap flattened and turned brown (Fig. 2D). The hypothetical mechanism of browning in straw mushrooms associated with a water0 hour 6 hours loss dependent reaction was apparent in a subsequent experiment of MA storage at C D various temperatures. Although the browning was delayed under the MA conditions with PVC film overwrapping, the scores reached 4 (30-50% browning) after 2 days of storage at 15C (Fig. 4A). The browning symptoms increased despite a low weight loss of only 3.6% (Fig. 4A) on day 3 of storage at 15C and 90-95% RH. This result suggests that the browning of straw mushrooms is not completely dictated by the rate of water loss. 24 hours The optimum storage temperature was 15C, 0 hour Figure 2. Visual appearance of browning in straw mushrooms at 0 (A) and 6 h (B) and which kept the mushrooms for approximately scanning electron microscope pictures of fruit body of straw mushroom at 0 (C) and 24 h (D) 3 days. The straw mushrooms kept at 12Cin storage at ambient conditions (25-34C, 60-70% RH).45 45

A

5 5

79 79 78 78 77 77

B

45 4540 40 35 35

40 40Browning scores Score brow ning

35 35Weight loss % W eight loss30 30

4 4

LLValues * Value

25 2520 20 15 15 10 10 5 5 0 0

3 3

L value76 76 75 75 74 74

W eight los s

25 25 20 20

2 2Weight lossloss Weight Score browning Browning

1 173 73

* L Values Weight loss

15 1510 10 5 5

0 0

3 3

6 6

9 9

12 12

15 15

18 18

21 21

0 0 24 24

72 72

0 0

2 2

4

6

8 8

10

12 12

14 16 14 16

18

20 22 20 22

0 0 24 24

Storage time(hr.) Storage time (hours)

Storage time (hours)

Storage time (hr.)

Figure 3. Relationship of weight loss and browning scores (A) and L* values and weight loss (B) of straw mushrooms during storage at ambient conditions (25-34C, 60-70% RH).

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Journal of Food, Agriculture & Environment, Vol.10 (1), January 2012

W eight loss Weight loss %

30 30

5Browning scores B ro w n in g S co re

A

6 5Browing score Browning scores

5 4Weighthloss % W eig t lo o s

B

Ambient temp.

4 3 2 1 0 0 1Days of storage

4 3Browning Weight loss

A

4 CB BC AB

3 2 1 0 0AB

BAB

8 CB

CBB

12 C

C

2 1 0

15 C

2

3

Days after storageC6

1 2 Days of storage Days after storageD

3

2.5MMDA(n m o lM(nmol/gFW) D A content D A /g F W

Ambient temp.

2 1.5 1 0.5 0 0 1 2Days of storage

Ambient temp. ambient temp.4 C 4 C

4 C4

8 C

Protein % %Protein

8 C

8 C

12 C

12 C

12 C 15 C2

15 C

15 C

0

3

0

1

2

3

Figure 4. Browning scores and weight loss of straw mushrooms stored under MA at 15C (A) and browning scores (B), MDA content (C) and protein content of MA-stored straw mushrooms under various temperatures. Vertical bars indicate the standard error of the mean (n = 5). Means with different letters are significantly different based on DMRT, 5%.

Days after storage

Days of storage Dayafter storage

exhibited slight chilling injury (CI) (Fig. 4B). At 8C and 12C, the shelf life was less than 2 days. Decreasing the temperature to 4C further decreased the shelf life mainly due to more severe CI. The CI of the straw mushrooms exhibited water-soaking of tissues and browning. The shelf life was shortest at ambient conditions and at 4C (less than 1 day). The MDA content, a measure of lipid peroxidation, was highest at ambient conditions and at 4C on day 1 (Fig. 4C), indicating a rapid rate of cellular deterioration resulting in browning. Furthermore, the total protein contents in the mushrooms stored at high temperatures of 15C and ambient conditions (Fig. 4D) slightly increased from initial values (4.1%) during storage. As a tropical fungus, straw mushroom is highly sensitive to low storage temperatures compared with mushrooms grown in other zones, such as button mushrooms (5C) 10 and shiitake mushrooms (1C) 11, causing the storage life of straw mushrooms to be much shorter.5 5 4 4 3 3A

Effects of high CO2 treatments on alleviation of straw mushroom browning: The controlled atmospheres with high CO 2 concentrations of 10% or 20% combined with 15% O2 effectively retarded the browning of the straw mushrooms during storage at 15C with 90-95% RH (Fig. 5A). This effect was attributed to the inhibition of PPO activity, particularly after 2-4 days of storage (Fig. 5B). High CO2 treatments have been reported to control browning generation by inhibiting PPOs in fresh produce 24-26. Furthermore, tyrosine was the preferred precursor for the browning reactions (Fig. 6A) compared with phenylalanine (Fig. 6B), cinnamic acid (Fig. 6C) and the control in distilled water (Fig. 6E), which was similar to the browning reported in Agaricus bisporus68 . Incubation with phenylalanine, a product of the shikimic pathway (the same as tyrosine), did not show much browning, suggesting that melanin polymerisation from L-dopa oxidation 27, 28 from tyrosine may be the main pathway of browning in straw mushrooms. However, the browning evidence in the pyrocatecholB

A

PPO activity PPO activity (Abs min-1gFW-1) ( A/min/g F.W.)

Browning scores Browning score

A Normal air Normalair 15%O2+10%CO2 15%O2+10CO2 15%O2+20%CO2 15%O2+20CO2

0.30

A

0.25 0.20 0.15 0.10 0.05 0.00 0 2 4 6 Days after storage

B B A A B B

2 2 1 1 0 0

Normal air 15%O2+10%CO2 15%O2+20%CO2

0

2

4

6

0

2

4

6

Days after storage Figure 5. Browning scores (A) and PPO activity (B) of straw mushrooms stored in air or 10-20% CO2 with 15% O2. Means with different letters are significantly different based on DMRT, 5%.

Days of storage

Days of storage

Journal of Food, Agriculture & Environment, Vol.10 (1), January 2012

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Figure 6. Browning appearance of straw mushroom button incubated with L-tyrosine (A), L-phenylalanine (B), cinnamic acid (C), pyrocatechol (D) and distilled water (E) for 30 min.

treatment (Fig. 6D) implies that there could be other contributions by various PPOs to browning in straw mushrooms. High CO2 was applied in short periods prior to storage to investigate browning inhibition. Similarly, a high CO2 content of 40% as a pre-storage treatment reduced browning, resulting in lower browning scores and higher L* values than that of the air pre-treatment (Tables 1and 2). The high CO2 pre-treatment for 6 h was more effective than either the 4 or 12 h pre-treatments. There was no interaction between the CO2 concentrations in the microatmosphere and the incubation times of the browning generation. The pH of the mushrooms also decreased in response to high

CO2 treatment (Table 3), which was related to the reduction of browning symptoms. The lower pH in cells could reduce the activity of PPOs, which work optimally at approximately pH 7.0 25, 27, 29 . Browning is related to the enzymatic oxidation of phenolic compounds catalysed by PPO 27. The results of the present study agree with earlier findings on the inhibitory effect of high CO2 storage 18, 26 and high CO2 pre-treatment 29 on PPO activity and browning. An extremely high CO2 treatment could induce more rapid deterioration 30, as also obtained in the present study in the 12-h high CO2 pre-treatment.

Table 1. Changes in browning scores of straw mushrooms during storage at 15C.Treatment 0 Atmosphere (A) Air 40% CO2 60% CO2 F-test Time (T) 4h 6h 12 h F-test A*T CV1 2

1a a

2b

Browning scores 1, 2 Storage time(days) 3 4a

5a a

F-test ** ** ** ** ** ** -

1 1 a D 1 NSa D

a C

1.7C 1.5D a 1.5C NSa

2.3B 1.8CD b 1.7BC ** 1.7B 1.7CD a 2.4C ** NS 27.02b b

a

2.6B 2.1BC b 2.1AB **b b b

a

b

3.3A 2.7A b 2.6A **

2.5AB 2.3A NS

1 1 a D 1 NS a D

a B

1.5B 1.5D a 1.7D NS NS 36.49a

1.9AB 2.1BC a 2.8B ** * 23.94

2.7A 2.6A a 3.7A ** NS 18.01b

b

2.3A 2.5AB NS NS 21.85a

a

Means (n = 5) with different lower case letters within the same column are significantly different. Means with different capital letters within the same rows are significantly different. NS Not significant, * Significant difference at P0.05, ** Significant difference at P0.01.

Table 2. Changes in L-Hunter scales of straw mushrooms during storage at 15C.Treatment 0 Atmosphere (A) Air 40% CO2 60% CO2 F-test Time (T) 4h 6h 12 h F-test A*T CV1 2

1 82.63B 82.90B a 83.44A NSa a a a b a

L- values 1, 2 Days of storage 2 3 80.9B 82.3B a 82.1B *a a a b a

4b

5 72.07C a 70.64C NSa a a

F-test ** ** ** ** ** ** -

a a

86.5 A 86.5 A a 86.5 A NSa a

76.4C 80.3B b 76.2C **a

70.7D 74.8C b 69.2D **a a

86.5 A 86.5 A a 86.5 A NS -

83.5B 82.9B a 82.4B NS NS 3.09

83.4B 82.2B b 79.7C * NS 3.99

79.4AB 80.1AB b 73.4C ** NS 4.67

74.0B 75.4B b 65.2D ** ** 4.69

a

70.76C 71.95C NS NS 3.43

Means (n = 5) with different lower case letters within the same column are significantly different. Means with different capital letters within the same rows are significantly different. NS: Not significant, * Significant difference at P0.05, ** Significant difference at P0.01.

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Journal of Food, Agriculture & Environment, Vol.10 (1), January 2012

Table 3. Changes in pH of straw mushrooms during storage at 15C.Treatment 0 Atmosphere (A) Air 40% CO2 60% CO2 F-test Time (T) 4h 6h 12 h F-test A*T CV1 2

1 6.7B 6.57A b 6.58A **b a a a

pH 1, 2 Days of storage 2 3 4 6.73A 6.70A b 6.50A ** 6.6A 6.7A a 6.7A ** ** 26.72b c a b

5 6.6A a 6.5A NSa a a

F-test ** NS NS NS NS ** -

6.65C 6.53A b 6.52A *b a

a

7.05A 6.60A c 6.50A * 6.7A 6.6A b 6.4BC ** * 2.73a a

c

a

6.72B 6.6A b 6.4A **

6.6 A 6.61 A a 6.54B NS -

a

6.6A 6.7A b 6.6AB ** ** 12.57a

c

6.6A 6.5A b 6.1C ** NS 3.15a

a

6.6A 6.6A NS NS 2.40

Means (n = 5) with different lower case letters within the same column are significantly different. Means with different capital letters within the same rows are significantly different. NS: Not significant, * Significant difference at P0.05, ** Significant difference at P0.01.

Conclusions The browning and weight loss of straw mushrooms stored at ambient conditions were strongly correlated. At low storage temperature and under MA conditions, browning was related more to PPO activity and phenolic metabolism than to weight loss. The optimum storage temperature for longer shelf life was 15C. High CO2 treatment before or during storage further reduced browning and increased the shelf life of straw mushrooms. Acknowledgements This project is part of the Doctoral degree research of Mr.Soravit Jamjumroon and is supported by the Ministry of Science and Technology of Thailand. Some equipment and facilities were supplied by the Postharvest Technology Innovation Centre, Commission on Higher Education, Bangkok 10400, Thailand. References1

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