Protein concentrates and isolates
Traditional processing techniques for producing soy protein concentrates and isolates partially overcome these problems. These methods involve extraction, heat treatment, precipitation by the addition of acid or alcohol, and centrifugation to separate the protein from the other components. The main objective is to remove the non-protein components in a stepwise manner. These conventional methods are time-consuming, they sometimes result in products with poor functional properties, and can generate a whey-like waste stream which contains some of the proteins and nutraceuticals.
An increasing concern these days is that large quantities of water are required, especially to “wash” the curd (the precipitated protein) to remove as much of the adhering non-protein components. Since most concentrates and isolates are dried, every kg of water added is a kg of water that must be removed, usually by energy-intensive dehydration techniques. The waste water in a soy isolate plant has a high BOD (2000-10,000 ppm) and low solids level (less than 0.5%, usually). Discharging the waste water usually results in heavy penalties. Recycling the waste water within the plant will usually require it to be cleaned up to prevent carryover of off-flavors and undesirable components, which will build up in the product.
We have developed alternate processes for purifying vegetable proteins and removing many objectionable flavor compounds using membrane technology. Since the undesirable oligosaccharides, phytic acid and some of the trypsin inhibitors are smaller in molecular size than proteins and fat components, it should be possible, by careful selection of the membrane and operating parameters, to selectively remove these undesirable components and produce a purified protein isolate, concentrate or soymilk.
Membrane process can be used to produce a soymilk with an excellent flavor profile, lower trypsin inhibitor, reduced off-flavors and low in phytic acid and oligosaccharides. Whole soybeans are soaked and then blanched to prevent lipoxygenase-induced off-flavors during grinding. The first separation step (filtration or centrifugation) serves to remove insoluble carbohydrate and fiber and ensure the particle size is appropriate to the membrane being used. Using membranes with higher MWCO changed the final product composition and yield, but resulted in higher flux. Adjusting the pH of the extracts either to the acidic or alkaline region during ultrafiltration results in a different flavor profile in the product, e.g., due to hydrolysis of the oligosaccharides (soy sugars) at low pH.
Soy whey protein
Soy “whey” is the liquid portion remaining after the proteins (and fiber) have been removed from an extract, usually by isoelectric precipitation. The whey contains water, protein fractions, sugars (oligosaccharides), phytate, salts, isoflavones, saponins and other acid-soluble minor components. Soy whey is dilute, with 0.2-1.5% total solids, depending on the process. The whey is sometimes further diluted with the washings. The whey proteins are soluble even at acidic pH and have good functional properties; it is unfortunate that they are lost in the whey during conventional isolate manufacture. The membrane methods do not result in a loss of whey proteins: they are part of the retentate and thus included with the products. However, even in the conventional soy isolate process, the whey proteins can be recovered with membranes from the whey and/or the washings. The membrane will concentrate and purify the soy whey proteins by filtering out the non-protein components, most of which fortunately are much smaller in molecular size than the proteins. The result will be a high-protein, highly functional and economically valuable co-product that has hitherto been wasted.