Heat Inactivation of Trypsin Inhibitors in Soymilk at Ultra-High Temperatures

Soymilk samples at pH 7.5, 6.5 and 2 were subjected to heat treatment at 93°C and indirect ultra-high temperature. When heated at 93°C, 121°C and 132°C, trypsin inhibitor activity (TIA) in soymilk was more heat-labile at high pH than at lower pH. However, the effect of pH on rate of thermal inactivation was less pronounced when the holding temperature was increased to 143°C and 154°C. The point on a curve relating holding temperature and holding time, indicating inactivation of 90% of the TIA in soymilk at pH 6.5 in the range 93–154°C, coincided with the thermal-death-time curve of the organism putrefactive anacrobe 3679 at about 125°C.     

Composition,Nutrition, and Utilization of Okara (Soybean Residue)

Okara is a by-product generated during tofu or soymilk production processes. It contains about 50% dietary fiber, 25% protein, 10% lipid, and other nutrients. The huge quantities of okara produced annually pose a significant disposal problem. Extensive studies have been done on the chemical composition, nutritional values, and biological activities of okara and on its potential utilization. Due to its high fiber content and low production costs, okara is a good raw material and rich source for preparing fiber and could also be used as a dietary supplement to prevent diabetes, obesity, and hyperlipidemia. Chemical or enzymatic treatment, fermentation, extrusion, high pressure, and micronization can increase the content of soluble fiber of okara, which improves its nutritional quality and processing properties. Fresh okara putrefies quickly due to its high moisture content, so it should be dried as early as possible. This review focuses on the application of okara in the food industry as partial replacement for wheat or soy flour to increase fiber and protein contents of foods. Okara can also be used as a fermentation substrate to produce a variety of products (natto, fibrinolytic enzymes, α-glucosidase inhibitor, β-fructofuranosidase, edible fungi, iturin A, chitosan, alcohol, etc.) for human consumption and nonfood production. In addition, the application of okara in feed and environmentally friendly material has also been documented.     

Heat Inactivation Kinetics of Trypsin Inhibitors During High Temperature-Short Time Processing of Soymilk

Heat treatment of soymilk was studied in the conventional batch boiling process and under High Temperature-Short Time (HTST) conditions. Reduction in total trypsin inhibitor was assayed enzymatically, and individual inhibitors, the Kunitz and the Bowman-Birk inhibitor, were assayed by ELISA technique. Standard first-order reaction kinetics and thermodynamics were applicable for inactivation, and results indicated that the mechanism was not protein unfolding, because entropy changes were zero or negative. The two inhibitors were inactivated at the same rate around 137°C. Therefore, a simple first-order kinetic model which gave a good, slightly conservative estimate of residual anti-trypsin activity under HTST conditions could be established.     

Isolation and characterization of proteins from soymilk residue (okara)

Protein was extracted from okara at pH 9.0 and 80 °C for 30 min, giving a recovery of 53% protein. The extracted protein was isolated by isoelectric precipitation at pH 4.5, and the dried, defatted protein isolates (prepared at 25 and 80 °C) had over 80% protein.

The okara protein isolates have essential amino acid profiles similar to the FAO scoring pattern, and high in vitro protein digestibility, with methionine and cysteine as the limiting amino acids. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate showed that okara protein isolates had a large quantity of high molecular weight components suggesting protein aggregation. Differential scanning calorimetry and hydrophobicity data suggested extensive protein unfolding in the okara products.

Okara protein isolates had lower solubility than a commercial soy protein isolate at both acidic and alkaline pH, probably due to protein aggregation. Other functional properties, including emulsifying, water and fat binding, and foaming properties, were found to be comparable to the commercial soy isolate.     

利用脱皮大豆制备高膳食纤维豆浆和豆腐的研究

传统豆制品生产过程中排出的豆渣是一种利用率低且成本低廉的膳食纤维来源。选用去皮大豆,采用保留豆渣的方式生产高膳食纤维豆浆和豆腐,研究均质条件对其理化性质和品质的影响。结果表明,高压均质豆浆(High pressure homogenized soymilk,HPHS)的总膳食纤维、脂肪和黄酮含量均高于传统过滤豆浆(Filtered soymilk,FS),蛋白质含量低于FS;与FS相比,HPHS的颗粒粒径和分布范围较大,稳定性较差,但这些指标随均质强度的提升而得到明显改善,稳定性也随之增加,而加热会增大粒径和分布范围并降低稳定性;高膳食纤维豆腐(High dietary fiber content tofu,HDFT)的硬度、弹性和咀嚼性随着均质强度提高呈先升高后下降的趋势,其中均质条件为30 MPa+35 MPa与普通豆腐(Normal tofu,NT)质构性能最接近,而加热会导致豆腐的质构参数降低;NT比HDFT拥有更为紧密有序、孔洞更少的微观三维结构,但用30 MPa+35 MPa制作的HDFT比其它HDFT拥有较好的且接近NT的微观三维结构。     

Okara flours from chickpea and soy are thickeners: increased dough viscosity and moisture content in gluten-free bread

The by-product of plant-based beverages, okara, can be dried in a nutritious flour, but it generates dense bakery products due to high water absorption. Gluten-free bread often tastes dry, so the objective of this work was evaluating okara flour as thickener for mouthfeel enhancement. Proximate analysis revealed that chickpea okara contained more starch than soy (35.3 vs. 3.41 g/100 g), less insoluble fibre (43.3 vs. 57.0 g/100 g) and protein (9.51 vs. 18.1 g/100 g). Water absorption capacity was higher in okara than flour and for soy (8.29 vs. 6.01 g g⁻¹, respectively). When added to a gluten-free batter, both okara flours significantly increased viscosity. Upon addition of either okara to gluten-free bread (2% w/w) moisture content increased from 31.6 to 33.5 and 36.5 g/100 g, while crumb hardness increased by up to 45% and specific loaf volume decreased by up to 42%. Soy okara flour enhanced moistness of gluten-free bread.     

热处理对鲜豆奶中胰蛋白酶抑制子活性的影响

分别以95℃、120℃、140℃三种温度处理鲜豆奶,研究热处理对胰蛋白酶抑制子活性的影响。测定结果表明要达到胰蛋白酶抑制子90％的失活率,95℃时需要35min;120℃时需要7min;而140℃时只需要60s左右。胰蛋白酶抑制子的热致死时间曲线表明,加热温度增加30℃,钝化胰蛋白酶抑制子90％的热处理时间可缩短10倍。     

Chemical Characteristics of Low-Fat Soymilk Prepared by Low-Speed Centrifugal Fractionation of the Raw Soymilk

Abstract: Large oil–protein particles (2 to 60 μm) were found in raw soymilk (or water extract of soybean), which was prepared in specific conditions. The large particles could be separated by sedimentation by centrifuging raw soymilk for 5 to 30 min at a low gravitational force ranging from 96 to 2410 × g. Chemical analysis showed that 80% to 90% of the total lipids and 30% to 40% of the total proteins were located in the precipitated fraction. The supernatant fraction had a dramatically higher protein-to-lipid ratio than the whole soymilk. The ratio of 11S/7S proteins and the ratio of 11S acidic/basic subunits were significantly (P < 0.05) higher in the precipitate than that either in the whole soymilk or in the supernatant. Besides centrifuging conditions, other factors, including soymilk concentration, grinding method, soybean variety, and soybean storage, also significantly (P < 0.05) affected the centrifugal fractionation. This study showed that low-speed centrifugation facilitated the separation of oil-protein particles from raw soymilk, and can be used as an innovative method for preparing low-fat soymilk and 11S protein-enriched ingredients. The findings also increased our understanding of the association or aggregation between proteins and lipids in raw soymilk after grinding. Practical Application: Soymilk has become a popular beverage in the Western world due to its health benefits. Consumer demands for low-fat and organic foods have been increasing in the recent years. Currently, there are no alternative methods for manufacturing low-fat soymilk from whole soybeans. We found that most, if not all, of lipids in the raw soymilk were located in large particles, which could be separated by low-speed centrifugation. This centrifugal fractionation was investigated by varying processing parameters, soybean varieties, and soybean storage conditions. The approach has potential to be used for manufacturing low-fat soymilk. This study also has increased our understanding of the interactions between lipid and protein in raw soymilk.     

改性对玉米蛋白质功能性质和结构的影响(I)酶解

蛋白质的食品功能性质主要由其结构决定 ,改性是改善或加强某些食物蛋白的功能性质的有效方法。通过Asl.398枯草杆菌中性蛋白酶对玉米蛋白质的水解改性发现 ,Asl.398蛋白酶对玉米蛋白质的最大水解度为 2 9.47% ;随水解度的增大 ,水解物的表面疏水性先增大后减少 ,分子柔性不断增大 ,溶液粘度不断降低 ,这表明水解使玉米蛋白结构发生了明显变化。而随着水解度的增大 ,玉米蛋白质的水溶性大幅度提升 ,水解度为 3.0 %的水解物的乳化活性最大 ,而水解度为 9.0 %的水解物的乳化稳定性最高     

Comparative Effects of Ohmic,Induction Cooker,and Electric Stove Heating on Soymilk Trypsin Inhibitor Inactivation

During thermal treatment of soymilk, a rapid incorporation of Kunitz trypsin inhibitor (KTI) into protein aggregates by covalent (disulfide bond, SS) and/or noncovalent interactions with other proteins is responsible for its fast inactivation of trypsin inhibitor activity (TIA). In contrast, the slow cleavage of a single Bowman–Birk inhibitor (BBI) peptide bond is responsible for its slow inactivation of TIA and chymotrypsin inhibitor activity (CIA). In this study, the effects of Ohmic heating (220 V, 50 Hz) on soymilk TIA and CIA inactivation were examined and compared to induction cooker and electric stove heating with similar thermal histories. It was found that: (1) TIA and CIA inactivation was slower from 0 to 3 min, and faster after 3 min as compared to induction cooker and electric stove. (2) The thiol (SH) loss rate was slower from 0 to 3 min, and similar to induction cooker and electric stove after 3 min. (3) Ohmic heating slightly increased protein aggregate formation. (4) In addition to the cleavage of one BBI peptide bond, an additional reaction might occur to enhance BBI inactivation. (5) Ohmic heating was more energy‐efficient for TIA and CIA inactivation. (6) TIA and CIA inactivation was accelerated with increasing electric voltage (110, 165, and 220 V) of Ohmic heating. It is likely that the enhanced inactivation of TIA by Ohmic heating is due to its combined electrochemical and thermal effects.