Nutritional value

During the second century bc, Wu mentions that vegetable soybeans enhance the yang principle and have medicinal value (Shurtleff and Aoyagi, 1994). In the 1950s in the USA, vegetable soybeans were considered distinctly superior to grain soybeans for human consumption (Weber, 1956). Compared to sweet green peas (Pisum sativum L.) vegetable soybeans are rich in protein, fat (cholesterol-free), phosphorus, calcium, iron, thiamine, riboflavin, vitamins A, B1, E and C, folic acid, isoflavones and dietary fibre (Table 19.1). Vegetable soybean has a lower percentage of flatulence-producing starches compared to grain soybean. Like the grain soybean, vegetable soybean also has anti-nutritional factors. Trypsin inhibitor (TI) activity is low in vegetable soybean compared to grain soybean. One third of the TI activity remains in vegetable soybean after boiling for 5 min. Vegetable soybean is highly nutritious, yet the nutritional value is not a major factor determining its market value. Pod and green bean appearance, taste, flavour, texture and nutritional value, in that order, are the five most important quality requirements for vegetable soybean (Masuda, 1991).

Soy isoflavones, genistein, daidzein, and to a limited extent glycitein and their ^-glycosides, have been reported to have an anti-carcinogenic effect on breast cancer in premenopausal women (Messina, 2004) and androgen-sensitive and -insensitive prostate cancer in men (Kucuk, 2004; Zhou, 2004).

At 25 and 40 days after flowering (DAF), the a, p, y, and 8 tocopherol content is 32, trace, 1038, 148 ig g-1 lipid and 44, 2, 1124, and 306 ig g-1 lipid, respectively. At the mature grain stage, 75 DAF, the four tocopherol contents are 109, 62, 1109, and 402 ig g-1 lipid, respectively. At the vegetable

Table 19.1. Nutritional composition (values 100 g-1) of vegetable soybean, grain soybean and green pea.

Nutrient composition

Vegetable soybean

Grain soybean

Green

pea

Raw

Cooked

Raw

Cooked

Raw

Cooked

Energy (kcal)

147

141

446

173

81

84

Moisture (g)

67.5

68.8

8.54

62.55

78.86

77.87

Protein (g)

12.95

12.35

36.49

16.64

5.42

5.36

Fat (g)

6.8

6.4

19.94

8.97

0.4

0.22

Total carbohydrate (g)

11.05

11.05

30.16

9.93

14.45

15.63

Crude fibre (g)

4.2

4.2

9.3

6

5.1

5.5

Ash (g)

1.7

1.6

4.87

1.91

0.87

0.92

Phosphorus (mg)

194

158

704

245

108

117

Calcium (mg)

197

145

277

102

25

27

Iron (mg)

3.55

2.5

15.7

5.14

1.47

1.54

Vitamin A (|jg RAE)

9

8

1

0

38

40

Vitamin B1 (mg)

0.435

0.26

0.874

0.155

0.266

0.259

Vitamin B2 (mg)

0.175

0.155

0.87

0.285

0.132

0.149

Vitamin C (mg)

29

17

6

1.7

40

14.2

Vitamin E (mg)

(1476)a

-

0.85

0.35

0.13

0.14

Folate (jg)

165

111

375

54

65

63

Isoflavones (mg)

20.42

13.79

128.35

54.66

-

-

RAE, retinol activity equivalent.

Values of isoflavones obtained from US Department of Agriculture (USDA)-Iowa State University Database on the Isoflavone Content of Foods (1999). Other values obtained from USDA National Nutrient Database for Standard Reference, Release 20 (2007). -: data not available in the USDA database. aValue in parenthesis is total tocopherol content (|jg g-1 lipid) at 40 days after flowering (Masuda, 1991).

RAE, retinol activity equivalent.

Values of isoflavones obtained from US Department of Agriculture (USDA)-Iowa State University Database on the Isoflavone Content of Foods (1999). Other values obtained from USDA National Nutrient Database for Standard Reference, Release 20 (2007). -: data not available in the USDA database. aValue in parenthesis is total tocopherol content (|jg g-1 lipid) at 40 days after flowering (Masuda, 1991).

soybean stage, the total tocopherol content is around 12 )jg per seed (40 DAF). At 75 DAF, the total tocopherol content is 83 )ig per seed (Masuda, 1991). Thus, tocopherol, protein and TI activity increases with maturity. However, sucrose increases up to the vegetable soybean stage and then starts to decrease. Therefore, vegetable soybean should be harvested at the optimum stage to get the sweet taste. Frozen soybean has more sucrose (e.g. 1.7% versus 1.1%) and amino acids (e.g. alanine at 30 mg 100 g-1 fresh weight versus 16 mg 100 g-1 fresh weight) than fresh. Delaying the harvest, after the appropriate stage, results in low sucrose content.

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