Soy protein concentrates (SPC) and soy protein isolates (SPI) were produced from hexane-defatted soy white flakes and from two extruded-expelled (EE) soy protein meals with different degrees of protein denaturation. Processing characteristics, such as yield and protein content, and the key protein functional properties of the products were investigated. Both acid-and alcohol-washed SPC from the two EE meals had higher yields but lower protein contents than that from white flakes. Generally, SPC from an acid wash had much better functional properties than those from an alcohol wash. The SPI yield was highly proportional to the protein dispersibility index (PDI) of the starting material, so the EE meal with lower PDI had lower SPI recovery. The protein content in SPI prepared from EE meals was about 80%, which was lower than from white flakes. Nevertheless, SPI from EE meals showed functional properties similar to or better than those from white flakes. The low protein contents in SPC and SPI made from EE meals were mainly due to the presence of residual oil in the final products. SPI made from EE meals had higher concentration of glycinin relative to β-conglycinin than that from white flakes.Paper no. J10816 in JAOCS 81, 713-717 (July 2004). KEY WORDS:Extruding-expelling, functional property, soy protein concentrate, soy protein isolate.Soy protein products have become increasingly popular because of their low price, high nutritional quality, and versatile functional properties. Two important soybean protein products are soy protein concentrate (SPC) and soy protein isolate (SPI). SPC is defined as an edible protein product with a protein content of at least 65% on dry weight basis (1), whereas SPI is a product with at least 90% protein on dry weight basis (2). Currently, flash-desolventized solvent-extracted white flakes (typically containing 50% protein) are generally the starting materials for SPC and SPI preparation. Other soybean meals or flours besides white flakes may also be used as starting materials provided that the final products meet protein content specifications and demonstrate desired functional properties. Soybean meals produced from the extruding-expelling (EE) processing of soybeans may be used as starting materials for SPC and SPI preparation. EE is a mechanical processing technology that allows small-scale production of protein meals having a high oil content and partial recovery of oil. Extrusion, the first step of processing, provides a heat treatment that reduces trypsin inhibitors, permitting the use of the full-fat or defatted protein meals as livestock feed. The extrudate can be pressed by an expeller to partially recover the oil. The protein in the meal typically is extensively heat-denatured by extrusion. Depending on the processing conditions, EE meals with different oil contents and protein denaturation can be achieved (3). Advantages of EE technology include process simplicity, low capital investment, no need for organic solvents, and applicability to identity-preserved (IP) process...
To improve protein digestibility of aqueously extracted soy proteins, an effective chemical treatment under mild conditions is needed. Soy proteins, including storage protein glycinin and antinutritional factors such as trypsin inhibitors, are rich in disulfide bonds. Reduction of these disulfide bonds by incubating soy proteins with sodium sulfite and sodium metabisulfite at 55°C showed no net increase of free sulfhydryl groups after dialysis to remove the residual reducing agent. However, the in vitro digestibility measured by trypsin hydrolysis using the pH-Stat method was significantly increased. Sodium metabisulfite (SMBS) was more effective in increasing in vitro digestibility than sodium sulfite at the same molar concentration. The digestibility of soy protein treated by 0.5 mmol SMBS/g soy flour at 55°C was more than doubled compared to that of the control without reduction treatment. Large-scale testing of soy proteins treated with SMBS for an in vivo animal feeding study showed similar in vitro digestibility by trypsin, e.g., the degree of hydrolysis of the treated sample was 8.5% compared to 1.6% of the control. These soy proteins were further evaluated using a chick growth model. The protein efficiency ratio (PER) increased by 57% when the chicks fed SMBStreated soy were compared to the chicks fed raw soy flour. SMBS-fed chicks did not display any pancreatic hypertrophy compared to those fed with raw soy control. These results indicate that there is great potential to use safe chemicals and mild temperature to inactivate the antinutritional factors in soybeans and thus improve digestibility of soy proteins that are extracted with low-temperature aqueous process.
The effects of hydrothermal cooking (HTC) at alkaline conditions on refunctionalization of heat-denatured protein of extruded-expelled (EE) soy meals and on preparation of soy protein isolate (SPI) from EE soy meal were determined. Two HTC setups, flashing-out HTC (without holding period) and HTC with holding for 42 s at 154°C, were evaluated. Alkali (NaOH) addition dramatically enhanced the refunctionalization of EE meal having an initial protein dispersibility index of 35. The more alkali added, the more refunctionalization occurred. Extensive refunctionalization was achieved at 0.6 mmol alkali/g EE meal, and additional improvement was small with more alkali. For both HTC setups, the solids and protein yields of SPI from alkali-HTCtreated EE meals were significantly higher than those from HTC without alkali addition. The yield of protein as SPI increased from 40 to 82% after HTC treatment at 0.6 mmol alkali/g EE meal compared with no alkali addition. The emulsification capacities of SPI after alkali-HTC were similar to those from HTC without alkali. SPI from holding-tube HTC-treated EE meals had higher emulsification capacities than those prepared by flashing-out HTC.Paper no. J11081 in JAOCS 82, 451-456 (June 2005).The use of soy protein ingredients by the food industry is increasing rapidly owing to health benefits, low cost, and versatile functionalities. Most soy protein products are manufactured from highly soluble (flash-desolventized) hexane-defatted soybean meal. Solvent extraction is capital intensive and unsuitable for processing small amounts of identity-preserved soybeans. Extrusion-expelling (EE) is a promising technique that presses the oil from the whole or cracked seeds and is suitable for processing small lots of identity-preserved seeds (1,2). EE differs from traditional screw pressing in that a dry extruder replaces steam-heated stack cookers or rotary dryers. Compared with solvent-extracted white flakes, EE protein meals that are processed into EE soy flour have much more heat exposure and protein denaturation [15-60 vs. 80-90 protein dispersibility index (PDI)], contain significantly more residual oil (6-12 vs. 1%), have lower levels of heat-sensitive antinutritional factors (trypsin inhibitors) and enzymes (such as lipoxygenase), and possess a pleasant nutty flavor. Typical EE meal contains 50% protein and 6% oil, and 90% of its trypsin inhibitors are inactivated (1). The use of protein ingredients prepared from EE meal in foods, however, is limited by their poor functional properties and low yields of soy protein isolate (SPI), a direct consequence of heat denaturation of the protein.Our previous work showed that SPI could be prepared from EE meals; however, the yields were low (the SPI and protein yields were only 25 and 40%, respectively) (6). Inspired by the work of Johnson (3) and Wang and Johnson (4), we demonstrated that hydrothermal cooking (HTC) could be used to refunctionalize heat-denatured proteins of EE meals (5). We also demonstrated the feasibility of using HTC as a pretreat...
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