1.1 Introduction
1.1.1 Soy as the most important source of plant protein ingredients
Soybeans and soybean products have long been used by millions of people in the East as their chief source of protein and as medicine. First cultivated in Southeast Asia, soybean is now present all around the world. Soy is an abundant source of dietary protein, containing 40% protein on average, and is available at relatively low cost. In 2011, soy accounted for approximately 68% of world protein meal consumption, over other protein sources such as rapeseed (13%), cottonseed (6%), and sunflower seed (5%) [1]. From a nutritional point of view, purified soy proteins in the form of concentrates and isolates can be considered similar to animal proteins [2, 3]. Nutritional and health benefits for humans have been attributed for a long time to the consumption of soy foods, especially soy proteins. These benefits include hypocholesterolemic effects [4] and the prevention of heart disease [5] and breast cancer [6]. In 1999, the US Food and Drug Administration approved a health claim for soy proteins stating that 25 g of soy proteins each day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease [7, 8]. This recognition is a good indication of the additional benefits, beyond basic nutrition, associated with the consumption of soy proteins. Soy protein products offer more than just nutritional and health benefits for humans. Advances in soy ingredient technology have resulted in products that are industrially produced for a variety of purposes. Soy products can be found as emulsifiers, texture enhancers, and ingredients to increase or replace protein content in food products such as bread, pastries, beverages, and meat [9].
1.1.2 Production of soy protein isolates by isoelectric precipitation
Isolates are the most highly refined soy protein products that are commercially available, containing at least 90% protein on a dry basis. Commercial soy protein isolates are usually prepared from dehulled and defatted soybeans by isoelectric precipitation [10, 11]. The proteins are extracted from soybean flakes or flours with water adjusted to pH 8 to 11 using a base, at a solids/solvent ratio of 1:10 to 1:20 and at a temperature of 30 °C to 50 °C. The insoluble fibrous residue is then removed by a centrifugation step, and the pH of the resulting soy protein extract is adjusted to pH 4.2 to 4.5 (i.e., the isoelectric point of the proteins) using a mineral acid, such as HCl, to precipitate the proteins. The proteins are then recuperated by a second centrifugation step, which is followed by multiple washings to remove minerals and sugars, in order to increase the protein content, and neutralization of the proteins to pH 7 with a dilute base, such as NaOH. The resulting soy protein dispersion is fed to a spray-dryer to produce an isolate.
Although soy protein isolates produced by isoelectric precipitation usually have superior functional properties compared to protein isolates from other plant sources, soy protein isolates may still have limited functional properties due to protein denaturation [12, 13]. Soy protein isolates produced by isoelectric precipitation also have a high phytic acid content (1%â3% w/w), which alters the solubility of the isolates, especially at low pH [14, 15]. In addition, from an environmental point of view, the isoelectric precipitation process requires a large amount of water (for the extraction, precipitate washing, and neutralization steps) and generates a large volume of effluents (in the isoelectric precipitation and washing steps). With a high biochemical oxygen demand because it contains whey-like proteins that remain soluble in the pH range of 4.2 to 4.5, the effluent generated following the isoelectric precipitation step is especially problematic and constitutes a serious water pollution threat unless properly processed. The whey-like proteins found in this effluent are difficult to recover because the low solids concentration, varying from 1% to 3% [16], makes isolation of those proteins not economically viable.
1.1.3 Soy bioactive peptides
It has been well known for several decades that bioactive peptides can be derived from dietary proteins. Bioactive peptides may be present as independent entities or encrypted in the parent protein. During food processing or gastrointestinal digestion, these peptides are released from the parent protein and act as compounds with hormone-like activities [17]. In general, bioactive peptides derived from food contain two to nine amino acids [18], although this range may be extended to 20 or more amino acid units [17]. As an important protein source, soybean is also a potential source of bioactive peptides. As of 2005, ExPASy databases listed a total of 1411 protein entries (266 Swiss-Prot entries and 1145 TrEMBL entries) for soybean [19].
Acidic hydrolysis and enzymatic hydrolysis are the two main methods to generate soybean peptides. The acidic hydrolysis method is simple and less expensive but may result in amino acid damage. Enzymatic methods, in contrast, are easier to control, use mild conditions, and do not cause amino acid damage. Therefore, enzymatic hydrolysis is the most commonly used method to produce food-grade protein hydrolysate and release bioactive peptides from their parent protein. The type of enzyme used for hydrolysis is very important, given that it will impact the biological activities of the generated peptides. Proteinases (endopeptidases) such as trypsin, subtilisin, chymotrypsin, thermolysin, pepsin, proteinase K, papain, and plasmin are commonly used for the proteolysis of food proteins [20]. Enzymes are also often combined to produce bioactive peptides. Heat treatment can be applied to proteins prior to enzymatic hydrolysis, because such treatment will impact the generation of the bioactive peptides. For example, it was observed that soybean meals that were heat-treated at high humidity had higher levels of aggregated peptides [21]. Fermentation has also been considered for producing bioactive peptides. Bioactive peptides can be released by the microbial activity of fermented foods or through enzymes derived from microorganisms [17]. Inte...
