ABSTRACT
Soybean and soybean products, which are important protein sources, are known as one of the major allergenic foodstuffs, in common with other protein-rich foods such as milk and eggs (1). In recent years, soybean protein isolate (SPI) prepared from defatted soybean meal has been increasingly employed in food processing as an ingredient in minced meat products, stuffing, pasta flour, yogurts, sauces, and flavor enhancers because of its high nutritional quality, processing functionality, and low cost. Accordingly, it has been difficult for patients allergic to soybean to get allergen-free products from commercially available processed foods; also, it has become necessary for these patients to have wide knowledge about the ingredients in the processed foods in which allergenic food residues are incorporated as processing aids. At present, strict elimination of offending foodstuffs is generally recommended as a conventional and effective treatment of food allergies. Long termavoidance of nutritionally fundamental or
essential foods, however, may lead to malnutrition in young patients. There is, therefore, an urgent need to identify the protein components eliciting allergic manifestations for the reduction of the allergenicity of soybean products. The allergenicity of soybean is known to reside in the protein fractions, not in the soybean oil itself (2), whereas oxidized soybean oil has been shown to enhance the immunoglobulin E (IgE)-binding ability against soybean or other food proteins (3). In 1934, Duke (4) pointed out soybean as a possible important source of food allergy among people taking soymilk formula as milk substitutes. In 1980, a soybean allergen was first reported by Moroz and Yang (5); using the serum of a laboratory worker, who might have been sensitized through the airways, the allergen was isolated and identified as Kunitz type soybean trypsin inhibitor (KSTI). Shibasaki et al. (6) also reported that various allergenic protein components occurred in soybean protein fractions, and the IgE antibodies in the sera of the soybean allergic patients showed cross-reactivity among the 2S-, 7S-, and 11S-globulin fractions by radioallergosorbent test (RAST)-inhibition analyses. However, this method could not characterize the individual protein component responsible for cross-reactivity. They demonstrated that the most allergenic fraction was the 2S-globulin fraction, and then the 7S-and 11S-, in a decreasing order. Several investigators described the features of soybean allergens, but no detailed information was presented. Burks et al. (7) showed that allergenic proteins in soybean predominated in the 7S-or 11S-globulin fractions rather than in the 2S-globulin fraction, as a result of immunochemical analysis using the sera of soybean-sensitive patients with atopic dermatitis. Recently, Herian et al. (8) reported that the sera of patients sensitive to both peanuts and soybean bound to several protein components with molecular weights ranging from 50,000 to 60,000 Da (probably subunits of β-conglycinin) and also to the component with a molecular weight of about 20,000 Da, not identical with KSTI, which was strongly recognized by the IgE from patients allergic only to soybean. Rodrigo et al. (9) reported that the accidental inhalation of soybean dust caused asthma in individuals in Barcelona. Asthma patients inhaling soybean dust had specific IgE antibodies for the glycoproteins with molecular weights lower than 14,000 Da, which were assumed to be degradation products of β-conglycinin or unique protein species occurring in soybean hull, designated to be Gly m 1 and 2. Herian et al. (8) described that different soybean-allergic subjects were sensitive to quite different proteins that could be classified into three categories according to immunoblotting patterns. Results also showed that the IgE-binding proteins varied among patients, but the patients could not be classified into distinct groups according to their immunoblotting patterns (1). Recently, the occurrence of about 15 protein components binding with IgE antibodies in the sera of soybean-sensitive patients was demonstrated, three of which, named as Gly m Bd 30K, Gly m Bd 28K, and Gly m Bd 60K, were shown to be major allergenic proteins (1). Based on this information about allergenic proteins in soybean as the target to be removed, many approaches to reduce the allergenicity of soybean and soybean products have been proposed: (a) physicochemical procedures such as heat denaturation and precipitation, (b) destruction
and modification of allergenic structures such as an introduction of polysaccharide moieties and enzymatic digestion, (c) breeding (selection of allergen-deficient varieties or induction of mutant), (d) genetic engineering, and (e) fabrication of nonallergenic constituents. Furthermore, more selective and sensitive methods for evaluation of the allergenicity of soybean products have concurrently been developed, which are applicable during the course of processing. The convenient methods to detect and determine the major allergens by immunoblotting and enzyme-linked immunosorbent assay (ELISA or sandwich ELISA) have been established using allergen-specific monoclonal antibodies. This chapter describes recent knowledge about soybean allergens and development of hypoallergenic products.
