Effect of Changing Temperature on the Deterioration of Soya Beans

ARTICLE IN PRESS Biosystems Engineering (2004) 87 (4), 453–462 doi:10.1016/j.biosystemseng.2003.12.005 PH}Postharvest Technology

Available online at www.sciencedirect.com

Effect of Changing Temperature on the Deterioration of Soya Beans Hussain Sorour; Toshitaka Uchino Laboratory of Postharvest Science, Department of Bioproduction Environmental Science, Faculty of Agriculture, Kyushu University, Fukuoka, 812-8581, Japan; e-mail of corresponding author: [email protected] (Received 2 June 2003; accepted in revised form 6 December 2003; published online 3 March 2004)

Deterioration rates as indicated by carbon dioxide evolution for soya bean (Glycine max L. Merr.) stored under changing temperature conditions were quantified and compared with those predicted using equations. Experiments included soya bean moisture contents of 18, 22, and 26% (wet basis), constant storage temperatures of 15, 20, 25, and 308C, and cyclical storage temperatures that changed between 15 and 258C and between 20 and 308C on a 24 h basis. Also, the growth of micro-organisms was identified after 10 days from the treatments by using the pour plate method. The results indicated an increase in deterioration by increasing storage temperature and moisture content of soya bean. Equations of carbon dioxide weight versus time for each moisture content and storage temperature were fitted. The longest allowable storage time to reach 0
5% dry matter loss (1132 h) occurred at lower moisture content and lower constant storage temperature, while the shortest allowable storage time (170 h) occurred at higher moisture content and higher constant storage temperature. The allowable storage times for soya bean stored under cyclical temperatures were close to the allowable storage time for soya bean stored at a constant temperature equal to the average cyclical temperature. Microbial infection levels increased with increasing storage temperature and moisture content. The increasing rate of micro-organism growth decreased by increasing the storage temperature over 258C. However, this increasing rate of micro-organism growth for soya bean exposed to a cyclical storage temperature was usually lower than that for soya bean held at constant storage temperatures of about 208C (the average of 15 and 258C) and 258C (the average of 20 and 308C). # 2003 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd

1. Introduction Reduction in the quality of soya bean seed associated with disease infections resulting from field weathering before harvest and with adverse conditions during storage can unfavourably affect market value of the crop. McNeal (1966) investigated the effect of different temperatures on the storage life of soya bean seed. He found that soya bean can be kept for 12 months without an excessive decline in germination or rise in free fatty acid content if the temperature is kept below 168C and the moisture content is not higher than 16
2% dry basis. Byrd and Delouche (1971) found that the vigour of soya bean seed measured as speed of growth, declines in storage before there is any appreciable loss in seed viability. Although these studies indicate that soya bean seed is susceptible to damage, a large variation in quality 1537-5110/$30.00

loss, with cultivar, is reported during conditioning and storage (Wein & Kueneman, 1981). The two main factors affecting the rate of CO2 production (or of O2 consumption in normal aerobic conditions) are temperature and water activity of the grain (Fleurat-Lessard, 2002). Other factors, such as the time spent by the grain in store after the harvest, the degree of initial contamination of the grain by moulds at the start of the storage period, and the degree of hidden insect infestation, also have some influence but are usually sufficiently small to be ignored in modelling and predicting global quality changes using only the rate of CO2 production as a ‘storability risk index’ (Milner et al., 1947; Frazer & Muir, 1981; White et al., 1982; Muir et al., 1985; Wrigley et al., 1994). Stiles and Leach (1960) indicated that under aerobic conditions, carbohydrates are the principal group of compounds utilised in respiration, the final step in the 453

# 2003 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd

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process being the complete oxidation of a hexose sugar, i.e.: C6 H12 O6 þ 6O2 ! 6CO2 þ 6H2 O

ð1Þ

with 2835 kJ generated by the exothermic reaction. Thus, when the respiratory quotient (i.e. the ratio of oxygen absorbed to carbon dioxide evolved) is unity, measurement of the carbon dioxide output will enable an estimate to be made of the dry matter utilised, by multiplying the weight of carbon dioxide evolved by 0
68. Although fat as mentioned by Mclennan (1926) and proteins as mentioned by Eberhardt (1960) can be utilised for respiration in grass, they are only used when carbohydrates are not readily available and laboratory experiments studied by Greenhill (1959), and Wilkinson and Hall (1966) have confirmed that cut forage utilises carbohydrates primarily so that measuring the carbon dioxide output is a suitable method for estimating the weight of material metabolised. Saul and Steele (1966), Steele (1967), and Steele et al. (1969) studied maize deterioration rates by capturing carbon dioxide produced by storage fungi and seed embryos in adsorption tubes. They used a glucose respiration model (glucose+oxygen ¼ carbon dioxide+water+energy) to calculate dry matter loss (14
7 g carbon dioxide per kilogramme maize dry matter is equivalent to a 1
0% dry matter loss). It was found that carbon dioxide production by the seed was small compared to carbon dioxide production by actively growing fungi. Combine-harvested maize fell one commercial grade (from US no. 2 to no. 3) by the time 0
5% the original maize dry matter was consumed. All the allowable storage time data collected by Steele (1967), Steele et al. (1969), and Saul (1970) were for maize held at constant temperature and moisture throughout the storage period. If shelled maize is aerated with outdoor air during storage, maize temperature and moisture are likely to change over time and there is a need to adjust the prediction of allowable storage time for changing conditions. Thompson (1972) used results from Steele (1967), Steele et al. (1969), and Saul (1970) to develop a procedure that can be used to predict maize dry matter loss under varying conditions. Thompson (1972) calculated the amount of carbon dioxide produced y in g kg1 [dry matter] during a given storage period t, in h under the reference storage conditions as follows: y ¼ 1
3 ½expð0
006tÞ  1 þ 0
015t

ð2Þ

Since consumption of 1
0% dry matter results in the production of carbon dioxide (Saul & Lind, 1958), Thompson (1972) calculated the percentage dry matter loss (LDM in %) by dividing the amount of carbon dioxide produced during storage under reference condi-

tions by 14
7: LDM ¼ y=14
7 ¼ 0
0883 ½exp ð0
006tÞ  1 þ 0
00102t ð3Þ The objectives of this study were to collect new data to be used in quantifying the effect of storage temperature on the deterioration of soya bean and micro-organism growth and to define equations of CO2 production versus time for dry matter loss of soya bean under different storage conditions.

2. Experimental apparatus and procedures 2.1. Dry matter loss deterioration The deterioration of soya bean in storage was monitored by measuring carbon dioxide produced by soya bean samples held under controlled moisture and temperature conditions and using carbon dioxide data to calculate dry matter loss. Soya bean samples (250 g) were held in 500 mL glass bottles in an incubator that could be independently cooled or heated to the desired temperature. When it was time to determine dry matter loss for a particular sample, a carbon dioxide measurement for each sample bottle was taken by drawing 1 mL from the exhausted air by a syringe (once every 6 h for 10 days and after that once every 12 h) and injecting it into the gas chromatograph (GL Sciences GC-390). During ventilation, air passed through a flow meter, copper tube, bubbled through a sodium hydroxide solution (NaOH), bubbled through a glycerol–water solution, entered near the bottom of the soya bean sample bottle and finally vented to the atmosphere. The copper tube was used to adjust the air temperature to the determined temperature. Sodium hydroxide solution was used at a concentration of about 25% to adsorb the carbon dioxide from the air inlet. However, the glycerol solution was used to adjust the humidity of the air intake to maintain constant soya bean moisture content. Deterioration measurement was based on the cumulative weight of carbon dioxide produced by each soya bean sample. The apparatus shown in Fig. 1 was used to measure this weight. It incorporates conditioning air stream temperature, carbon dioxide removal, humidity conditioning and sample storage. Supply air, from a compressed-air source at a rate of 0
1 L min1, was first cleansed of dust and other particles. It was then conditioned for temperature by passing the air through the spiral copper tube. The CO2 removal stage was used to adsorb carbon dioxide from the air. The humidity conditioning was used to maintain soya bean moisture levels of approximately 26, 22 and 18%, respectively.

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Flow meter Valve

Flow meter Air outlet

Air inlet Spiral copper tube

Sodium hydroxide solution

Glycerol and water solution

Conditioning

CO 2

Humidity

airstream

removal

conditioning

Soya bean sample

Storage

temperature Fig. 1. Apparatus for measuring carbon dioxide produced by aerated stored soya bean

It has been assumed that all CO2 was trapped by the solution of sodium hydroxide. This assumption was verified by gas chromatograph analysis of gas samples drawn from the air before admission to the sample bottles. The solution of sodium hydroxide was changed every week. Humidity levels could be changed by adjusting the glycerol and water ratio. The actual humidity levels were not measured in the ventilating air but the sample bottles were weighed every 24 h and the weight changes were used as an indication of moisture changes. Glycerol and water ratios were adjusted as necessary to increase or decrease humidity to counteract sample moisture changes. More details about the system used to adjust humidity levels can be found in Wilcke et al. (1993, 1998, 2000, 2001), Ng et al. (1998), and Ileleji et al. (2003).

2.2. Storage temperature and moisture Deterioration of 18, 22 and 26% moisture content [wet basis (w.b.)] soya bean samples was monitored during storage period until they reached 0
5% dry matter loss under the following temperature conditions: (1) constant 308C; (2) constant 258C; (3) constant 208C; (4) constant 158C; (5) 24 h cycle with the temperature held at 208C for 12 h and then at 308C for 12 h during every cycle; and (6) 24 h cycle with the temperature held at 158C for 12 h and then at 258C for 12 h during every cycle. Conditions from (1) to (4) were selected to simulate the situation where soya bean is aerated occasionally

during storage, while conditions (5) and (6) were selected to simulate the diurnal temperature fluctuation that soya bean experiences during continuous aeration or during in-storage drying with ambient air. 2.3. Sample preparation All samples were treated in a similar manner by drying them to a low moisture content of about 9
5% (w.b.) and holding them in a refrigerator at 108C for storage. When it was time to start a test, the amount of soya bean needed for tests was removed from the refrigerator and then rewetted to the test moisture content. After rewetting, the soya bean sample was held in a 58C refrigerator for 72 h to allow for moisture equilibration and then placed in sample bottles in the carbon dioxide measuring system. Three replicates were used for each moisture and temperature condition. The average level of mechanical damage for soya bean used in this study was 1
0%. 2.4. Measuring micro-organism Soya bean samples that had been stored for 10 days during the carbon dioxide tests were checked for total damage, which for our samples, was primarily microorganism damaged. To determine the microbial count found in stored grains, 10 g of the tested grains were stirred in 100 mL of sterile saline solution (0
85% NaCl) and then the supernatant was serially diluted in the same solution. One millilitre of each dilution was inoculated

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in soya bean agar plates, using the pour plate method. The colony-forming units (cfu) of the total microorganisms found in 10 g grains was easily determined. 2.5. Dry matter loss prediction Equations and procedures given by Thompson (1972) were used to predict the dry matter loss for each storage condition tested.

3. Results and discussion 3.1. The effect of constant storage temperature on dry matter loss Figures 2–5 show plots of dry matter loss for soya bean samples held at constant temperatures of 15, 20, 25 and 308C throughout the storage period. Data points in the figures are the average dry matter loss values at each measurement time for three replicates, and the tables list average values for three replicates too. In addition to plotting the data, a curve was included for the dry matter loss predicted by using the following equation: y ¼ A ½exp ðBtÞ  1 þ Ct

ð4Þ

where: y is the CO2 produced in g kg1 [dry matter]; t is the storage time in h; and A, B and C are the coefficients specific for each of the storage conditions as shown in Table 1. It was observed from the curves in Figs 2–5 and Table 2 that dry matter loss increased with the storage time.

The longest allowable storage time of 1132 h to reach 0
5% dry matter occurred at the lowest moisture content of 18% and lowest constant storage temperature of 158C, while the shortest allowable storage time of 170 h occurred at the highest moisture content of 26% and the highest constant storage temperature of 308C. Predicted storage times to reach 0
5% dry matter loss were within 2
67 to 3
81% of the average measured values at 18% moisture, within 3
64 to 4
74% at 22% moisture, and within 5
0 to 5
84% at 26% moisture for the constant storage temperatures. The allowable storage times varied according to slight changes in soya bean moisture content from the target during experiments. However, the nominal moisture content was used in the prediction equation which is slightly different from the actual moisture content.

3.2. The effect of cyclical temperature changes on dry matter loss Figures 6,7 and Table 3 show results for the test where the storage temperature was varied between 15 and 258C, and between 20 and 308C on a 24 h basis. The longest allowable storage time of 996 h to reach 0
5% dry matter occurred at the lowest moisture content of 18% and lowest cyclical storage temperature of between 15 and 258C, while the shortest allowable storage time of 231 h was occurred at the highest moisture content of 26% and higher cyclical storage temperature of between 20 and 308C. The predicted allowable storage time values for cyclical storage temperature treatments were

0.9 0.8

Dry matter loss, %

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

0

192

384

576

768

960

Storage time, h Fig. 2. Dry matter loss versus storage time at constant storage temperature of about 308C: &, measured at 26% m.c.; m, measured at 22% m.c.; ^, measured at 18% m.c.; &, predicted at 26% m.c.; n, predicted at 22% m.c.; }, predicted at 18% m.c.

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0.8 0.7

Dry matter loss, %

0.6 0.5 0.4 0.3 0.2 0.1 0

0

192

384

576

768

960

1152

Storage time, h Fig. 3. Dry matter loss versus storage time at constant storage temperature of about 258C: &, measured at 26% m.c.; m, measured at 22% m.c.; ^, measured at 18% m.c.; &, predicted at 26% m.c.; n, predicted at 22% m.c.; }, predicted at 18% m.c.

0.8 0.7

Dry matter loss, %

0.6 0.5 0.4 0.3 0.2 0.1 0

0

192

384

576

768

960

1152

1344

Storage time, h Fig. 4. Dry matter loss versus storage time at constant storage temperature of about 208C: &, measured at 26% m.c.; m, measured at 22% m.c.; ^, measured at 18% m.c.; &, predicted at 26% m.c.; n, predicted at 22% m.c.; }, predicted at 18% m.c.

within 3
01 to 4
20% of the average measured values at 18% moisture content, within 4
14 to 4
47% at 22% moisture content, and 5
32 to 5
62% at 26% moisture content. The allowable storage time for soya bean stored under cyclical temperature changes between 20 and 308C was higher than it was for soya bean held at a constant storage temperature of about 258C (the average of 20 and 308C), except for the moisture content

of about 22% which is lower than that of the average cyclical temperature. The allowable storage time values for soya bean storage under cyclical temperature changes between 15 and 258C were close to, but usually less than the allowable storage time for soya bean stored at the constant storage temperature equivalent to the average of cyclical temperatures, except for the moisture content of about 22% which is higher than that of the

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0.8 0.7 Dry matter loss, %

0.6 0.5 0.4 0.3 0.2 0.1 0

0

192

384

576

768

960

1152

1344

1536

Storage time, h Fig. 5. Dry matter loss versus storage time at constant storage temperature of about 158C: &, measured at 26% m.c.; m, measured at 22% m.c.; ^, measured at 18% m.c.; &, predicted at 26% m.c.; n, predicted at 22% m.c.; }, predicted at 18% m.c. Table 1 Coefficients, residual sum of squares and coefficient of determination for Eq. (3) under different storage conditions Storage temperature, 8C

Storage moisture content, %

Coefficients A

30 25 20 15 2030 1525

26 22 18 26 22 18 26 22 18 26 22 18 26 22 18 26 22 18

1
296 1
297 1
297 1
296 1
297 1
297 1
297 1
297 1
297 1
297 1
297 1
297 1
297 1
297 1
297 1
297 1
297 1
297

B

3.3. The effect of constant storage temperature on the growth of micro-organisms Micro-organism growth results after 10 days are represented in Table 4. The results indicated increasing micro-organism activity by increasing both storage

Coeff. of determination (R2)

3
49 105 2
13 104 7
53 104 3
21 105 1
5 104 2
94 104 7
69 105 7
04 104 6
6 104 6
68 105 2
27 104 10
19 104 8
26 105 2
30 104 2
10 104 1
01 104 2
97 104 7
67 104

0
998 0
994 0
983 0
998 0
996 0
994 0
997 0
985 0
990 0
998 0
995 0
987 0
996 0
994 0
996 0
996 0
994 0
988

C 4

12
73 10 4
85 104 3
13 104 10
77 104 4
14 104 2
54 104 6
69 104 3
69 104 2
12 104 4
46 104 2
94 104 1
86 104 9
40 104 4
42 104 2
49 104 6
77 104 3
42 104 2
12 104

average cyclical temperature of 208C (the average of 158C and 258C).

Residual sum of squares 12
78 104 4
85 104 3
12 104 10
82 104 4
15 104 2
54 104 6
71 104 3
68 104 2
12 104 4
48 104 2
94 104 1
85 104 9
42 104 4
43 104 2
49 104 6
78 104 3
43 104 2
11 104

temperature and moisture content. The rates of increases were within the range of 24
9–40% at 158C storage temperature, within 41
1–143
1% at 208C storage temperature, within 51
6–176
8% at 258C storage temperature, and within 77
1–148
1% at 308C storage temperature whereas on changing the moisture content from 18 to 26%, the rate of micro-organism growth increased by 74
9, 243
2, 319
6, and 339
9% for the storage temperatures of 15, 20, 25, and 308C, respectively. On the other hand, by changing the

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0.8 0.7

Dry matter loss, %

0.6 0.5 0.4 0.3 0.2 0.1 0 0

192

384

576

768

960

1152

Storage time, h Fig. 6. Dry matter loss versus storage time at storage temperatures cycling between 208C and 308C on a 24 h basis:&, measured at 26% m.c.; m, measured at 22% m.c.; ^, measured at 18% m.c.; &, predicted at 26% m.c.; n, predicted at 22% m.c.; }, predicted at 18% m.c.

0.8 0.7

Dry matter loss, %

0.6 0.5 0.4 0.3 0.2 0.1 0

0

192

384

576 768 Storage time, h

960

1152

1344

Fig. 7. Dry matter loss versus storage time at storage temperatures cycling between 158C and 258C on a 24 h basis:&, measured at 26% m.c.; m, measured at 22% m.c.; ^, measured at 18% m.c.;&, predicted at 26% m.c.; n, predicted at 22% m.c.; }, predicted at 18% m.c.

moisture content from 18 to 26%, the rates of dry matter loss increased to 197
3, 281
0, 395
4, and 414
0% at constant storage temperatures of about 15, 20, 25 and 308C. Maximum microbial infection was 6966
6 cfu/g (colony-forming units/gramme) for 26% moisture content at 308C and minimum infection was 66
6 cfu/g for 18% moisture content at 158C. It seems that the rate of increases for micro-organism growth decreased by increasing the storage temperature over 258C. This was

related to faster growth of micro-organisms at temperature of about 258C.

3.4. The effect of cyclical temperature changes on the growth of micro-organisms Table 5 shows the effect of micro-organism growth after 10 days at cyclical storage temperature. It indicated

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Table 2 Allowable storage time for soya bean seeds stored at different constant storage temperature and moisture content to reach 0.5% dry matter loss Storage temperature, 8C

15 20 25 30

Moisture content, % w.b.

Allowable storage time, h

Nominal

Initial

Final

Measured

Predicted

18
0 22
0 26
0 18
0 22
0 26
0 18
0 22
0 26
0 18
0 22
0 26
0

18
0 22
1 26
0 18
1 22
2 26
1 18 22
1 25
9 18
2 22
0 26
1

18
1 22
2 26
1 17
9 22
2 26
2 18
0 22
2 25
9 18
1 22
1 26
2

1132 729 484 997 577 322 838 519 200 672 443 170

1101 696 458 965 556 306 806 495 190 655 422 161

Table 3 Allowable storage time for soya bean seeds stored under cyclical changes in temperature every 24 h Cyclical changes in storage temperature, 8C 15–25 20–30

Moisture content, % w.b.

Allowable storage time, h

Nominal

Initial

Final

Measured

Predicted

18
0 22
0 26
0 18
0 22
0 26
0

17
8 21
9 26
2 17
9 22
1 26
1

18
4 22
6 26
3 18
0 22
5 25
8

996 626 319 857 483 231

966 598 302 821 463 218

Table 4 Micro-organism growth and dry matter loss after 10 days for soya bean seeds stored at different constant storage temperature and moisture content Storage temperature, 8C

15 20 25 30

Nominal moisture content, %

18 22 26 18 22 26 18 22 26 18 22 26

that the rates of micro-organism growth increases within 29
6–100%, and 84
2–98
6% at storage temperatures cycled between 15 and 258C and cycled between 20 and 308C on a 24 h basis, respectively. By changing the

Micro-organism growth, cfu g1

Dry matter loss, %

Initial

Final

Measured

Predicted

133
3 216
7 416
7 116
7 200 383
3 166
7 233
3 433
3 133
3 200 416
7

200 300 533
3 850 1983
3 2900 1100 2816
7 4350 1716
7 4133
3 7383
3

0
08 0
15 0
24 0
10 0
17 0
36 0
12 0
21 0
61 0
14 0
25 0
73

0
10 0
17 0
26 0
12 0
21 0
36 0
14 0
24 0
64 0
18 0
28 0
77

moisture content from 18 to 26%, the rates of microorganism growth increases by 159
2% and 265
8% at storage temperatures cycled between 15 and 258C and cycled between 20 and 308C on a 24 h basis, respectively.

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Table 5 Micro-organism growth and dry matter loss after 10 days for soya bean seeds stored under cyclical changes in temperature every 24 h Micro-organism growth, cfu g1 Cyclical changes in storage temperature, 8C 1520 2030

Dry matter loss, %

Nominal moisture content, %

Initial

Final

Measured

Predicted

18
0 22
0 26
0 18
0 22
0 26
0

116
7 250 466
7 166
7 233
3 483
3

933
3 1883
3 2583
3 1433
3 2566
7 5116
7

0
10 0
17 0
37 0
12 0
23 0
53

0
12 0
19 0
39 0
14 0
25 0
56

The rate of micro-organism growth for soya bean exposed to temperatures that cycled between 15 and 258C, and between 20 and 308C were usually lower than that for soya bean held at constant temperatures of about 208C (the average of 15 and 258C) and 258C (the average of 20 and 308C). This was expected since fungi grow disproportionately faster during the warmer parts of a time-varying temperature cycle (Christensen & Meronuck, 1986). This result also agreed with that of Wilcke et al. (2000).

4. Conclusions The following conclusions can be drawn from this study. (1) The minimum allowable storage time for constant storage temperature was 170 h for 26% moisture content at 308C storage temperature while the maximum allowable storage time was 1132 h for moisture content of 18% at 158C storage temperature. The differences between the predicted and measured allowable storage times are related to a slight variation in soya bean moisture content from the target during experiments. (2) The predicted allowable storage time values for soya bean exposed to temperature that cycled between 15 and 258C were fairly close to allowable storage time values for soya bean stored at a constant temperature of 208C (the average of 15 and 258C). On the other hand, the predicted allowable storage time values for soya bean exposed to temperature that cycled between 20 and 308C were higher than those for soya bean held at a constant storage temperature of about 258C (the average of 20 and 308C). (3) The highest deviations of 5
84% and 5
62% between predicted and measured deterioration rates occurred at the highest storage temperature and moisture content for both constant and cyclical

temperatures, respectively. On the other hand, the lowest deviations of deterioration rates, which are 2
73% and 3
01%, occurred at the lowest storage temperature and moisture content for both constant and cyclical temperatures, respectively. (4) There was essentially no difference in microbial infection before and after 10 days of the deterioration test for low storage temperature. However, increasing storage temperature causes rapid increases in microbial infection, especially with high storage moisture content. (5) The highest growth of micro-organisms was 696
6 cfu/g for a soya bean moisture content of 26% at 308C storage temperature, while the lowest was 6
6 cfu/g for a soya bean moisture content of 18% at 158C storage temperature. (6) Micro-organism growth increased by increasing both storage temperature and moisture content. The rate of increases in micro-organism growth decreased by increasing the storage temperature over 258C.

Acknowledgements The authors thank Prof. Shun-ichiro Tanaka for his technical assistance. This research was supported by the Grant-in-Aid for Science Research (Project No. P01234) from the Japan Society for the Promotion of Science.

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