Phytate from edible beans: chemistry, processing and health benefits

Demand for bean products is growing because of the presence of several health-promoting components in edible bean products such as phytates. Phytates are a naturally occurring compound, which is distributed widely in all cells of legume plants. Clinical studies have suggested that these health-promoting components, phytates, affect the immune system to protect the human body against cancers, and to reduce cholesterol level. Phytates decrease blood lipids, lowers cancer risks and lower blood glucose response. A high phytate diet is used in the inhibition of dental caries and platelet aggregation, in the treatment of hypercalciuria in humans, and in the antidote activity against acute lead poisoning. In epidemiological studies, phytates have shown an inverse relationship with the incidence of renal stones. Thermal processing such as canning is the typical method to process beans. The study is to review the effect of thermal processing on the characteristics and stability of phytates in canned bean products. Phytates are less thermal sensitive. During soaking and blanching, portions of phytates were dissolved in water and lost in soaking, washing and blanching liquors. An optimum thermal process can increase the stability and maintain the phytates in canned bean products, which is useful for food industry to improve thermal processing technology and enhance bean product quality. Ke y words: Edible bean, thermal processing, health benefits, phytate. Phytate Distribution in Edible Beans Phytate is a naturally occurring compound, which is found widely in all cells of legume plants. In legumes, phytate is predominantly concentrated as crystalline globoid inside protein bodies in the cotyledons 1. However, phytate in soybeans is distributed throughout the kernel instead of cotyledons 2. The resason is that the protein bodies of soybeans do not possess globoids. Phytate makes up 60-80% of the dry weight of globoids in legumes 1. It primarily occurs as a salt of mono and divalent cations such as Ca2+, Mg2+ and K+. Reddy et al. 1 reported that the amount of phytate ranges from 0.40 to 2.06% in legumes. On a dry weight basis, there is approximately 5% in legumes 3. Presence of phytate content in legumes varies in different species and even within the legumes of the same species. For instance, phytate content of 15 varieties of soybeans ranges from 1.0 to 1.47% on a dry weight basis. A similar trend is observed among moth bean cultivars ranging from 0.85 to 0.90% on a dry weight basis 6. Generally, the difference in phytate content may be variable due to variations in legume species, cultivators, varieties, locations, irrigation condition, type of soil, climate and the period of the year during they are grown and harvested 4-5. The time of harvest and its effect on maturity also plays a role in causing variations among the same type of beans. For instance, mature dry pinto beans contain approximately 1.0% phytate whereas immature beans contain approximately 0.13% 7. The phytate content of various legumes is listed in Table 1. Lima beans have the highest phytate content in comparison to other bean varieties reported in Table 1. Chemical and Physical Properties of Phytate Physical and chemical properties: Phytate consists of a myoinositol ring and six symmetrically distributed phosphate moieties. In dry mature legumes, 99% of phytate exists in watersoluble form 2,11. It is inert, unreactive and a very stable antinutrient component. It serves as a chelating agent, which binds nutritionally important monoand divalent minerals (i. e., calcium, magnesium, cobalt, manganese, zinc, copper, and iron) to form complex phytate 12. All monovalent phytate complexes and Ca2+ phytate complexes are soluble except higher metal ion phytate complexes 13. It means that diand trivalent cations (Cu2+, Zn2+, Fe3+ and Ca2+) are insoluble at neutral pH14. The solubility of these complexes depends upon the ratio of metal to phytate. Calcium, aluminum and magnesium phytates are soluble at lower pH and insoluble at higher pH 2. In contrast, ferric phytates are soluble at higher pH and insoluble at lower pH. Therefore, the tendency of the metal ions to complex with phytic acid increases with decreasing pH. Phytate exhibits a Bean Variety Phytate (%) Sources Black-eyed beans 1.148 8 Red kidney beans 1.170 Mung beans 0.205 Pink beans 0.503 Soybeans 1.4 2 Lima beans 2.52 Navy beans 1.78 Cowpeas 1.37 9 Faba beans 0.98 10 Table 1. Phytate content of dry beans (on dry weight basis). www.world-food.net Food, Agriculture & Environment Vol.2 (1) : 49-58. 2004 WFL Publisher Science and Technology