Study On Oil Body Behaviors During Soymilk Processing And Its Effects On Properties Of Tofu Curd

Lipids and proteins are respectively stored in oil bodies(OBs) and protein storage vacuoles(PSVs) in soybean cell. OBs have a triacylglycerols matrix core, which is coated by oleosin-phospholipid membrane. Oleosins are the intrinsic proteins of OBs. By soaking and grinding, soybean cells and PSVs are disrupted, and proteins(β-conglycinin, glycinin, and P34 probable thiol protease; OB extrinsic proteins) in them are released and bound to OBs, and the homogenate is filtered to get raw soymilk. By heating, extrinsic proteins release from OBs, and different thermal treatment would produce oil bodies with different properties, which would affect the properties of soymilk and its related products. In this study, effects of heat temperature and time(70~100°C; 0~30 min) on particle size and bound proteins of OBs were systematically examined in soymilk and OB emulsion(OBs from raw soymilk into deionized water). Based the results above, effects of soybean OBs on the properties of tofu curd were examined. At last, effects of soybean OBs and oil on properties of tofu curd were compared.Soymilk is a very complicated system, so OB emulsion was firstly used to examine the effects of heat temperature and time on particle size and bound proteins of OBs. It was found that mass ratio of extrinsic proteins/oleosins(EP/O) of OBs in unheated emulsion was about 1.1. By heating, oleosins were affected little, but extrinsic proteins released from OBs with different rates in the beginning, and plateaued afterwards. It was calculated that EP/O was respectively 0.72(70°C), 0.43(80°C), 0.34(90°C), and 0.29(100°C) at equilibrium. Totally speaking, P34 and α’ and α subunits of β-conglycinin released from OBs the most quickly, followed by acidic polypeptides of glycinin(A), and basic polypeptides of glycinin(B) and β subunit of β-conglycinin released the most slowly. Z-average size of OBs firstly increased and then plateaued with heat time at all temperatures. The amount of residual proteins at equilibrium was negatively correlated with Z-average size of OBs in emulsion(70~100°C, R2 = 0.797; 80~100°C, R2 = 0.996).Based on the results above, the effects of heat temperature and time on particle size and bound proteins of OBs were examined in soymilk. It was found that EP/O of OBs in raw soymilk was about 2.5, which was larger than that in OB emulsion. By heating, the results in soymilk were similar to those in OB emulsion: in the beginning, P34 was the most easily removed from OBs, followed by acidic polypeptides of glycinin and α’ and α subunits of β-conglycinin, and basic polypeptides of glycinin and β subunit of β-conglycinin were the most difficult to release; afterwards, equilibrium was reached, and EP/O was respectively1.60(70°C), 0.92(80°C), 0.81(90°C), and 0.74(100°C), indicating that considerable extrinsic proteins were remained in heated OBs; Z-average size of OBs slightly increased in the beginning, and reached equilibrium afterwards. The only difference was found at 70°C: some acidic and basic polypeptides of glycinin were bound back to OBs after 8 min with similar rates. In addition, the amount of residual proteins was also negatively correlated with Z-average size of OBs in soymilk(70~100°C, R2 = 0.534; 80~100°C, R2 = 0.890), but the slope value(0.824) was obviously smaller than that(3.550) in OB emulsion, indicating that non-OB components(such as free proteins and carbohydrates) in soymilk could inhibit the heat-induced coaleasence of OBs.Finally, this study examined the effects of soybean OBs on the properties of tofu curd. It was found that hardness and storage modulus of tofu curd increased and its water loss rate decreased when OBs were heated for a long time. When the total solid content was constant, there existed an optimal mass ratio(0.143/1) between OB and non-OB components, and properties of tofu curd were the best at this time. When raw soymilk was exposed to room temperature for different time(0~60 min), it was found that hardness and storage modulus of tofu curd slightly decreased with prolonged time, but its water loss rate almost did not change. Some volatile compounds were greatly changed: aldehydes decreased, alcohols and alkenes increased, and ketones and furan almost did not change. Compared with soybean oil, OBs were better for tofu curd formation.This study clarified the behaviors of OBs during soymilk processing and its effects on tofu curd properties, so it provides some fundamental information for the production of soymilk and its related products.