Lectins : Lectins are carbohydrate binding proteins present in most plants, especially in bean seeds. Many lectins are toxic and/or inflammatory, resistant to cooking and digestive enzymes/stomach acids in the body. They can damage gut wall allowing other proteins to cross undigested causing allergic reactions. When lectins are in the blood stream, they can bind to cell memranes in arteries and organs such as joints, kidney, pancreas and brain, causing antigen-antibody reactions leading to autoimmune disorders. Lectins can be blocked by simple sugars and oligosacharides in the body. Lectins may be removed by soaking, sprouting, cooking or fermenting. Heating also inactivates most of lectins.
Phytates : Phytates can reduce absorption of minerals such as zinc, calcium, magnesium, and iron, because they form insoluble complexes with these minerals. Processing beans such as cooking, soaking, sprouting and fermentation all reduce phytate activity. Phytates can also bind to proteins causing reduced protein solubility. Phytates are the main reservoir for P and other minerals in the seed that are mobilized by germination. Phytates also inhibits enzymes including pepsin, trypsin and alpha-amylase. Some recent studies have indicated that phytates may have anti-cancer and cholesterol-lowering effects. Antioxidant properties of phytates help prevent free radical damage to DNA in the colon which can lead to cancer. Phytates enhance the immune system, reduce serum cholesterol and help control many types of cancer cell growth.
Trypsin inhibitor : Inhibits the function of trypsin enzyme, causes pancreatic hypertrophy and dietary loss of cysteine. Lack of proper irrigation may increase the activity of trypsin inhibitor.
Chymotrypsin inhibitor : Inhibits the function of chymotrypsin enzyme.
Alpha-amylase inhibitor : Prevents complete utilization of bean starch in the body by inhibiting the Alpha-amylase enzyme. Alpha-amylase inhibitors are also toxic to the larvae of bruchid beetles.
Subtilisin inhibitor : Inhibits Subtilisin (a peptide hydrolase) which plays role in protein hydrolysis.
Raffinose Oligosaccharides : During digestion raffinose oligosaccharides; raffinose, stachyose and verbascose, are not hydrolyzed due to the lack of alpha-1-6 galactosidase in the intestinal mucosa. As a result they are not absorbed into the blood and fermented by the microorganisms in the lower intestinal tract causing the formation of carbondioxide, hydrogen and methane. Raffinose oligosaccharides are not the only ones causing flatus in the body. Fiber components including hemicellulose also cause flatulance.
Polyphenolics : Common beans contain polyphenolic compounds such as tannins which interfere with protein digestibility and protein quality. They are found primarily in the seed coat. They can bind to proteins preventing enzymatic susceptibility of the protein as well as inhibiting the proteolytic enzymes such as trypsin and Chymotrypsin.
Other compounds : Phytoalexin are antimicrobial substances produced by a plant in response to infection by fungi or bacteria and that help to defend the plant by inhibiting the microbial growth. They are toxic interims of hemolytic effects and uncoupling of oxidative phophorylation. Phytoalexins can also be potent antioxidants (Free Radical Research, June 2002, pages 621–631; and Biochemical Pharmacology, January 2002, pages 99–104). Saponins are glycosidic and have foam forming ability. Arcelins are insecticidal storage proteins in beans.
Biotechnology employs breeding techniques to eliminate the undesirable components from the beans. Such an approach requires long-term studies depending on the type and number of the undesirable components.
Anti-nutritional compounds are removed by employing physical and chemical methods such as soaking, cooking, germination, fermentation, enzymatic removal and irradiation.
Soaking: Soaking beans and then discarding the soak medium can remove some of the unwanted components such as enzyme inhibitors and raffinose oligosaccharides. Soak temperature, medium type, bean type, length of soaking and solubility of the components are the factors affecting the extent of the removal. Salts and alkali in the solution increase the permeability of cell membrane increasing the amount of anti-nutrient leaching as well as some loss of desirable nutrients such as soluble vitamins and proteins.
Cooking : Cooking inactivates heat sensitive trypsin and chymotrypsin inhibitors but not completely. Other factors such as saponins, flatulance factors and phytates may not be affected by cooking. If the soaking and cooking media are discarded, appreciable amounts of heat stable compounds can be removed.
Germination : When the bean seeds are germinated, reserve nutrients are released. Available vitamin and mineral content increases while the phytate content and raffinose oligosaccharide contents decrease by 20-77% and over 70 %, respectively. The activities of trypsin, chymotrypsin and alpha-amylase are also reduced by germination depending on the bean type and germination conditions.
Fermentation : Fermentation helps reduce raffinose oligosaccharides due to the alpha-galactosidase found in the bean seeds. Phytic acid content is also decreased due to the action of phytase present in the bean seeds. Microorganisms which are responsible for the fermentation also play a role.
Enzymatic Methods : Enzymatic removal of certain compounds such as raffinose oligosaccharides depends on the level of endogenous alpha-galactosidase which may be different in different beans. Enzymatic removal of phytate in white beans is successful up to 80 % reduction at 60 C when soaking and heating treatment are combined.
Irradiation and Ultra-filtration: Gamma irradiation results in the destruction of trypsin and chymotrypsin inhibitors in certain bean types at different extents. Ultra-filtration could help remove low molecular weight compounds if the beans are wet-processed.
Despite the adverse effects of anti-nutritional compounds, simple food processing operations can help produce acceptable bean products.