Plant or vegetable protein products have been consumed for centuries in Asia and Middle Eastern countries. It was not until the mid-20th century that the western world recognized their food value, especially that of soybeans. With the growing interest in reducing the consumption of animal products for health and economic reasons, plant proteins are making up a higher proportion of the human diet in recent years. Soybeans (18-20% oil) are the major source of edible oil in the United States and by far the most important source of vegetable protein ingredients. However, after the oil is removed, the remaining soybean meal is used mostly as animal feed.
The problem with soybeans is that it also contains some undesirable components that must be removed or reduced to increase the usefulness and functionality of the soybean protein (almost all sources of plant protein share this problem). For example, soybeans contain oligosaccharides that have been implicated with gastrointestinal stress. Lipid-lipoxygenase interactions must be avoided to prevent painty off-flavors from developing. Phytic acid forms insoluble chelates with minerals and can form complexes with proteins that reduce bioavailability of the minerals and proteins. Trypsin inhibitors are proteinaceous compounds that affect the efficiency of protein digestion.
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. A typical soy isolate process is shown in Figure 1. 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 for soy protein concentrates and isolates
As shown in Figure 2, the initial extraction steps are the same as with any conventional protein isolate process. Defatted soy flour is extracted under optimum conditions of temperature, flour-to-water ratio and pH. To produce isolates (90% protein), the fiber and insoluble components are removed by centrifugation or filtration of the extract prior to ultrafiltration. The underflow from the centrifuge (or the filter cake) can be re-extracted if necessary.
If a concentrate is required, the extract can be directly ultrafiltered to remove the oligosaccharides. The retentate's final composition will approximate a soy protein concentrate (70% protein, dry basis).
Membrane process for soymilk
The process shown in Figure 3 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. Ultrafiltration of these water extracts with 20,000 -500,000 MWCO membranes have been reported in the literature beginning in the 1970s. The composition of the soymilk we obtained with a 50,000 MWCO membrane is shown in Table 1. 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.
Table 1. Composition of soymilk (% dry basis) produced by ultrafiltration
* By difference: includes fiber ** Units: TIU/mg solids
Membrane process for 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 (Figure 4). 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 shown in Figures 2 and 3 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, as shown in Figure 4. 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 coproduct that has hitherto been wasted.
Membranes for water recovery and recycle
The whey and washings can be treated by membranes (after appropriate pretreatment) to recover a substantial portion of the water with varying degrees of purity. For example, the permeate from the membrane whey protein recovery step can be cleaned up further by nanofiltration and/or reverse osmosis to result in two streams: water for recycle and re-use, and a soy sugar concentrate containing potentially valuable nutraceuticals. At the very least, it will largely reduce or even eliminate the discharge of high-BOD waste water from the plant.
We have considerable experience in membrane processing of soy and other plant proteins. Contact us if you want to incorporate these technologies in your plant.
Cheryan,M. 1998. Ultrafiltration and Microfiltration Handbook, CRC Press, Boca Raton, FL.