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Effects of blanching 热汤 and storage conditions on soluble sugar contents in vegetable soybean

X. Saldivar a, Y.-J. Wang a,*, P. Chen b, A. Mauromoustakos c a b s t r a c t

Vegetable soybean is becoming increasingly popular in the U.S. because of its rich source of isoflavones, folic acid, 异黄酮，叶酸and other nutrients. The objective of this study was to investigate various blanching and storage

conditions in order to identify the proper post-harvest management strategy in preserving sugar composition of vegetable soybean during storage. Fresh

soybean pods 豆荚of two vegetable soybean genotypes基因 were stored at 4 _C for 30 days or 25 _C for 8-12 days in fresh air or nitrogen atmosphere. Shelled seeds and intact pods were blanched in boiling water or steamed at 100 _C for 10 min. All blanched soybean was stored at -20 _C and sampled monthly for 6 months六个月每月采一次样 for sugar analysis. Glucose葡萄糖, fructose and sucrose蔗糖 decreased gradually in fresh soybean when stored at 4 _C in air or nitrogen atmosphere for 28 days. Soybean stored at 25 _C in open air showed a rapid decrease of sucrose in the first 24 h, and then followed by a gradual increase; whereas oligosaccharides accumulated 低聚糖聚集significantly

during storage. Significant degradation降解 in all sugars was found in soybean stored in nitrogen atmosphere at 25 _C. Soluble sugars decreased from leaching 过滤during the water blanching and cooling treatment of vegetable soybean seeds. Steam blanching and the presence of pod effectively retained soluble sugars in vegetable soybean during thermal treatment. 1. Introduction

Vegetable soybean is a specialty food-type ，soybean harvested while the seeds are still immature未成熟 at R6 (full pod) stage (McWilliams, Berglund, & Endres, 1999). It is similar to field matured 成熟soybean except for its larger seeds (Shurtleff & Aoyagim, 2001) and lighter hilum粒 color (Konovsky, Lumpkin, & McCleary, 1994). Vegetable soybean has advantages in sensory attributes over mature soybean, such as green color, soft texture, large seed size, slightly sweet taste, and less objectionable beany flavor. In addition, vegetable soybean is higher in nutritional content than mature soybean, including higher ascorbic acid and b-carotene contents,抗坏血酸和b-胡萝卜素 and lower

amounts of indigestible oligosaccharides 不消化的的低聚糖and anti-nutritional 抗营养物质substances such as trypsin-inhibitors胰蛋白酶抑制剂 and phytates 植酸盐(Rackis, 1978; Rackis, Hoing, Sessa, & Moser, 1972).

If properly stored, fresh vegetable soybean can retain its flavor and appearance for a short time period. The two most important sensorial 感官attributes for vegetable soybean are sweetness and savory 味香的taste. The sweet taste comes from its high sucrose content, and the savory taste is probably attributed to its amino acid composition (Konovsky et al.,1994). For a desired appearance,

vegetable soybean should have a bright green color without any defect and other color spot on the pods豆荚 (Konovsky et al., 1994). Kassinee, Matsui, and Okuda (2004) reported that vegetable soybean stored at 5 _C for 10 days

gradually decreased in green color, sucrose content, and seed weight.

Because of the short harvest period of vegetable soybean, proper processing is essential for year-round availability in the market.

Combination of blanching in water and freezing has long been established as an excellent way of preserving vegetables by inactivating enzymes and decreasing the rate of enzymatic deterioration酶性变质 (Williams, Lim, Chen, Pangborn, & Whitaker, 1986). However, blanching vegetables improperly will affect their quality by softening the texture, browning the color, and decreasing the nutrient content. Greaves and Boggs (1945) showed that steam blanching had advantages over water blanching because more soluble materials were lost during water blanching. Song, An, and Kim (2003) showed that vegetable soybean blanched at higher temperatures for shorter time intervals had minimal loss of nutrients, including sugars, amino acids and vitamins. Nevertheless little research has been done comparing changes in sugar composition under different processing and storage conditions.

The objectives of this work were to investigate the change in soluble sugars of fresh vegetable soybean under different storage conditions, and the effects of different blanching methods on the soluble sugar content in vegetable soybean during frozen storage.

2. Materials and methods

Vegetable soybean genotypes,遗传型 V95-7456 and 03-CB14, were planted on May 21, 2007 in Fayetteville, Arkansas, USA V95-7456 is a large seeded genotype, and 03-CB14 is high in sucrose and low in oligosaccharides寡聚糖. Plants were grown in 4-row plots4排样地 in a randomized complete block design随机化完全区组设计 with two replications复制, and only the two middle rolls滚 of a soybean block were harvested for study. Vegetable soybean V95-7456 was

harvested on August 29 and 31 for each replication, and 03-CB14 was harvested on September 5 and 7 when immature未成熟 green seeds expanded 展开fully in the pods. Each replication of harvested vegetable soybean received same treatments for fresh and frozen storage. Mature soybean of V95-7456 was harvested on September 26 and 28, and 03-CB14 was harvested on October 1 and 3 for each replication. 2.1. Fresh and frozen storage

For the fresh storage study, fresh soybean pods were stored at 4 _C in plastic bags flushed with nitrogen or air for 28 days. Soybean pods were also stored at 25 _C in nitrogen-flushed bags or in open air for 8-12 days. For the frozen

storage study, vegetable soybean was blanched in boiling water or steam prior to freezing. Both water and steam blanching methods were applied to intact pods and shelled seeds of both soybean genotypes. Half of the vegetable soybean pods were manually shelled用手带壳 immediately after harvest, and the other half of pods were used directly for treatments. Shelled vegetable soybean seeds of 180 g or intact pods of 360 g were blanched in 3.8 L of boiling water for 10 min, or steamed at 100 _C for 10 min on the same day of harvest. The water blanching condition was selected according to the results of Song et al. (2003), and the same

duration持续时间 was used in steam blanching. Steam blanching was carried out in a covered steamer (Dixie Canner Co., Athens, GA, USA) where soybean pods or seeds were placed on a perforated stainless steel tray.多孔不锈钢托盘 After blanching, soybean pods or seeds were rapidly cooled in ice water for 1 min. After the water was drained排去, all soybean pods were shelled, and the seeds were packaged in zip-lock bags拉链加锁 for frozen storage. Blanched soybean were stored at -20 _C and sampled monthly for 6 months for sugar analysis.

2.2. Extraction, separation and quantification定量 of soluble sugars

For each soybean sample, 40 g of soybean seeds were taken and dried at 40 _C for 24 h, ground with a coffee grinder咖啡杯 for 60 s, and passed through a 150-mm sieve screen (W.S. Tyler, Nentor, OH, USA). Samples were stored in hermetic 密封bottles at 4 _C before analysis. Ground soybean (0.15 g) was extracted with 1.5 mL of double distilled water in a 2-mL centrifuge tube. The tube was shaken horizontally地平的 (Barnstead 1314, Melrose Park, IL, USA) at ambient temperature at 200 rpm for 20 min, and then centrifuged at 20,000_g for 10 min. Supernatant (500 mL) was removed and mixed with 0.7 mL

acetonitrile乙晴 in a 1.5-mL tube, and kept at room temperature for 30 min. The mixture was centrifuged at 20,000_g for 10 min, and the supernatant was filtered through a 0.2-mm membrane细胞膜. Filtrate (24 mL) was dissolved in 576 mL double distilled water prior to HPLC analysis (Giannoccaro, Wang, & Chen, 2006).

High-performance anion-exchange chromatography with pulsed-amperometric detection (HPAEC-PAD) was used for separation and quantification of soluble sugars. Samples were injected via a Dionex autosampler 自动进样器equipped with a 25-mL sample loop.

Sugars were separated on a Dionex CarboPac PA 10 pellicular anionexchange 表面阴离子交换树脂柱resin column (250 mm _ 4 mm i.d.), preceded by a Dionex CarboPac PA 10 guard column (50 mm _ 4 mm i.d.), and a Dionex AminoTrap column (30 mm _ 3 mm i.d.) at a flow rate of 1 mL/min (Dionex, Sunnyvale, CA, USA). The mobile phase consisted of 90 mmol/L NaOH solution prepared by diluting carbonatefree 19.1 mol/L NaOH solution in distilled water, which was previously filtered with a 0.45-mm membrane. The detection of sugars was accomplished by ED40 electrochemical detector. Commercial saccharides,商业糖类 including glucose, fructose果糖, sucrose, raffinose and stachyose棉籽糖水苏糖, were purchased from Sigma Co. (St. Louis, MO, USA) and used as

external standards to identify and quantify the sugars based on their retention times保质时间 and peak areas. The results were expressed as percentage on a dry-weight basis.

2.3. Statistical analysis

Analysis of variance (ANOVA) and LSD test for mean separation were used to detect significant differences among soybean soluble sugar contents under different processing and storage conditions. Data were analyzed with JMP software version 7.0.1 (SAS Software Institute, Cary, NC, USA).

3. Results and discussion

Table 1 lists the sugar composition in both soybean genotypes harvested at both vegetable and mature stages. Glucose, fructose, sucrose and raffinose 棉籽糖were present in vegetable soybean seeds of both genotypes at harvest, and

sucrose was the predominant sugar (Table 1). Stachyose 水苏糖was not found in vegetable soybean. Genotype 03-CB14 had higher sucrose content, but lower raffinose content than V95-7456 in vegetable soybean seeds. The glucose and fructose levels were similar in both genotypes. Mature soybean of both genotypes had significantly higher raffinose and stachyose, but lower fructose than vegetable soybean. The sucrose content decreased in V95-7456 during maturation but increased significantly in 03-CB14. 3.1. Fresh storage in air

Fresh soybean pods stored in plastic bags at 4 _C in air showed little change in appearance during 28 days of storage. The color of soybean pods and seeds

became slightly dark green over time. The color change may be due to the loss of water at the beginning of storage and/or the loss of magnesium ion镁离子 in the porphyrin ring of chlorophyll叶绿素卟啉环 (Chen, Guo, Lai, & Chang, 1995). The presence of water drops in plastic bags indicated that vegetable soybean still respired when stored at 4 _C.

In contrast, soybean stored at 25 _C under open air condition displayed drastic 激烈的changes in appearance. Soybean pods started turning yellow on the

second day of storage. On the 4th day, most of the soybean pods already turned yellow, nevertheless only one third of soybean seeds turned yellow. On the 6th day, soybean pods became dry and hard, and most seeds turned yellow. Many color spots appeared on soybean pods on the 8th day of storage, and soybean seeds started to shrink收缩 and became hard. All soybean pods turned brown and hard on the 12th day. This change was similar to but much faster than natural maturation of soybean seeds, which takes about 4 weeks to complete in the field.

Soluble sugar contents in fresh soybean stored in air-flushed bag at 4 _C for a period of 28 days are shown in Table 2. Glucose, fructose and sucrose gradually decreased in both genotypes during storage. Sucrose decreased from 52.8 to 41.5 mg/g in V95-7456, and from 67.7 to 47.3 mg/g in 03-CB14, respectively.

Raffinose in V95-7456 decreased from 1.0 to 0.2 mg/g during first 24 h and then remained unchanged; whereas raffinose content in 03-CB14 did not change. Stachyose was not detected in both genotypes at harvest or during

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.

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