storage. Kassinee et al. (2004) reported that acid invertase activity in soluble and cell wall-bound fractions continuously increased in vegetable soybean during storage at 5 _C. The acid invertase mainly functions in breaking down sucrose to glucose and fructose. The loss of these soluble sugars during storage was probably due to their transformation to cell wall materials and/or their

translocation迁移 to floral sections 花切片to help maintain the floral sugar pool and support the respiratory 呼吸demand of floral sections (King & Morris, 1994).

Table 3 shows the changes of sugar content during fresh soybean storage in open air at 25 _C for 12 days. Glucose increased slightly, whereas fructose and sucrose decreased rapidly in the first 24 h and then gradually increased thereafter in V95-7456. Sucrose content declined from 52.8 to 19.8 mg/g in 24 h, and then increased to 30.9 mg/g at the 12th day. In contrast to mono- and di-saccharides, oligosaccharides寡聚糖, which were present in minimal amounts in freshly

harvested vegetable soybean, accumulated significantly during storage at 25 _C. In V95-7456, raffinose increased from 1.0 to 4.0 mg/g, and stachyose that was not found in freshly harvested soybean accumulated to 39.2 mg/g at 12th day, which was even higher than the level of sucrose content at the same day.

Sucrose, raffinose and stachyose in 03-CB14 during 25 _C open air storage showed similar trends as those in V95-7456 (Table 3). Sucrose decreased from 67.7 to 46.4 mg/g and then increased to 86.9 mg/g, which was higher than the value at harvest. The oligosaccharides also increased significantly during storage, but were present at a much lower level than those in V95-7456. Raffinose increased from 0.1 to 3.6 mg/g, and stachyose accumulated to 4.5 mg/g during storage at 25 _C for 12 days. No change was noted in glucose, while fructose decreased from 0.9 to 0.2 mg/g in 48 hand remained unchanged.

The sugar composition of vegetable soybean after storage at 25 _C for 12 days was similar to that of field mature soybean for both genotypes (Table 1 and 3). Sucrose decreased and oligosaccharides increased significantly from vegetable soybean stage (R6) to maturation 成熟(R8) of V95-7456, and the same trend was noted for the vegetable soybean stored at 25 _C in open air. In 03-CB14, a significant increase in sucrose and a slow increase of raffinose and stachyose was observed during soybean maturation and 25 _C open air storage. The changes in both appearance and sugar profile suggest that vegetable soybean stored at 25 _C experienced a physiological change similar to natural seed maturation. The increase of soluble sugars may be due to the degradation降解 of storage starch in vegetable soybean seeds. It is believed that soybean accumulates raffinose and stachyose in mature seeds as an energy source for future seed germination发芽 (Dey, 1985). 3.2. Fresh storage in nitrogen

Soybean pods stored in plastic bags flushed with nitrogen at 4 _C exhibited similar color change and water loss as those stored in air at 4 _C. However, a strong alcoholic odor酒精香味was detected after 48 h, indicating that anaerobic respiration 无氧呼吸occurred in vegetable soybean under nitrogen atmosphere.

The sugar compositions of fresh vegetable soybean stored in nitrogen at 4 _C for 28 days are listed in Table 4. Glucose, fructose and sucrose decreased, with

03-CB14 displaying greater reduction of these sugars than did V95-7456. Sucrose decreased from 52.8 to 44.5 mg/g in V95-7456, and from 67.7 to 51.4 mg/g in 03-CB14. Raffinose in 03-CB14 did not change, while raffinose in V95-7456 decreased from 1.0 to 0.2 mg/g in 48 h, and then accumulated gradually to 0.6 mg/g during storage. Stachyose was not detected in both genotypes during storage.

Both genotypes preserved slightly more mono- and di-saccharides when stored in nitrogen than when stored in air at 4 _C. The nitrogen-rich atmosphere probably resulted in elevated CO2 and reduced oxygen, thus reducing respiration呼吸 and delaying compositional changes (Kader, Zagory,&Kerbel,1989).Koseki and Itoh (2002) reported that the gas in enclosed packages of cut vegetables (lettuce and cabbage) filled with 100% N2 consisted of 1.2-5.0% O2 and 0.5-3.5% CO2 after 5 days of storage. Degradation of cut vegetables was delayed byan atmosphere of lowO2 and high CO2 concentration, which occurred in the packages filled with nitrogen gas.

Soybean pods stored in bags flushed with nitrogen at 25 _C deteriorated变质 rapidly, generating an off-odor 特殊臭味and becoming moldy 发霉的from the 6th day for both genotypes. At 25 _C, an elevated anaerobic提高的无氧呼吸 respiration of soybean pods under nitrogen condition degraded sugars and generated alcohol. The combination of suitable moisture, nutrients and

temperature promoted mold growth.

The results suggest that temperature control is more important than modified atmosphere in extending the shelf-life of fresh soybean (Phillips, 1996).

All soluble sugars decreased rapidly when soybean pods were stored in nitrogen atmosphere at 25 _C (Table 5). Oligosaccharides became nonexistent after one day. Sucrose decreased to 2.5 and 5.2 mg/g in V95-7456 and 03-CB14,

respectively, at the 8th day of storage. After 12 days, no sucrose was detected in V95-7456. Fructose in V95-7456 decreased from 0.8 to 0 mg/g after the 4th days, and then accumulated again to 0.6 mg/g at the 10th day, which was

probably due to the degradation of sucrose by microorganisms. Because of the high storage temperature of 25 _C, vegetable soybean experienced higher

anaerobic respiration and faster conversion of sugar to alcohol than it stored in N2 at 4 _C. The high temperature also caused increased degradation from microorganisms, which depleted most soluble sugars in soybean. 3.3. Blanching and frozen storage

Prior to frozen storage, soybean seeds and pods were blanched in boiling water or steamed at 100 _C for 10 min. Blanched vegetable soybean was stored at -20 _C and sampled monthly for 6 months for sugar analysis. Because no difference was found in sugar composition during 6 months of storage for all treatments of both genotypes, the means of sugar contents of each month’s observation were calculated to reduce the variation between samples。

Soluble sugars in vegetable soybean before and after blanching are listed in Table 6. Steam blanching significantly preserved more soluble sugars than did water blanching. Soybean seeds blanched in water had significantly lower

sugars content than those treated with other three methods. This was attributed to the water used in water blanching, which solubilized sugars, and the absence of pods for seed protection. Soybean pods prevented part of soluble sugars in seeds from leaching out 浸出during water blanching. Steamed soybean pods of

03-CB14 preserved more sucrose than steamed seeds; however, no difference was shown between steamed pods and seeds in V94-7456.

In general, there were no changes in sugar composition of steam-blanched

vegetable soybean pods relative to the raw ones for both genotypes, except that raffinose in V95-7456 decreased significantly after steam blanching. The increase in fructose after steam blanching in pods was attributed to sample variation. Both water and steam blanching at 100 _C for 10 min followed by frozen storage used in the present study were effective in inactivating enzymes that responsible for sugar changes during storage.

4. Conclusions

Fresh soybean stored at 4 _C in air or nitrogen atmosphere showed gradual decrease in soluble sugars during 28 days of storage. Soybean stored in open air at 25 _C showed significant accumulation of oligosaccharides; whereas soybean stored in nitrogen atmosphere at 25 _C showed significant degradation of all sugars. Low temperature was more important than atmospheric condition in preserving the nutritional and sensory values of fresh vegetable soybean. Soluble sugars decreased in soybean seeds during water blanching treatment as a result of leaching. The presence of pod effectively impeded阻碍 the leaching of sugars in water blanching treatment. Steam blanching preserved soluble sugars in both vegetable soybean pods and seeds than water blanching. There was no

significant change in sugar composition of blanched vegetable soybeans during 6 months of frozen storage. This research information will be useful in selecting post-harvest processing and/or storage method for vegetable soybean. Acknowledgments

The authors thank Dr. Luck Howard for his consultation and discussion in vegetable post-harvest processing, and Eddie Stiles for his help with soybean blanching.