Cereal inositol phosphates

An important contemporary issue involves the impact of dietary manipulations to reduce myo-inositol hexakisphosphate in manures on the fate of manure phosphorus in the environment. Inositol phosphates form a major component of the total phosphorus in many manures (Peperzak et ah, 1959 Fig. 12.3), because although myo-inositol hexakisphosphate is the dominant phosphorus compound in most cereal grains, it cannot be digested by monogastric animals, such as poultry and swine, due to low levels of intestinal phy-... 

Phytate (myo-inositol hexaphosphate Fig. 15.3, structure 33) is found in many food species and can be considered as a phytochemical. Its role in the plant is primarily as a phosphate store in seeds, but it is found in other tissues as well, for example, tubers (Harland et al., 2004). Phytate and its hydrolysis products are anti-nutrients that chelate metal ions and thus reduce their bioavailability (Persson et al., 1998 House, 1999). This is particularly a problem with cereal grains, but pre-processing can improve mineral absorption from these foods (Agte and Joshi, 1997). There is some concern that high phytate foods could also contain higher levels of toxic heavy metals caused by natural accumulation. Plants also contain phytate-degrading enzymes that can also influence metal ion bioavailability (Viveros et al., 2000). 

Nuts, seeds, and grains are not considered as sources of organic acids. Other than amino acids and fatty acids, they contain phenolic acids and phytic acid. Phytic acid and its salts, phytats, are regarded as the primary storage form of both phosphate and inositol in seeds, mostly within the hulls. The phytic acid content of cereals (whole grain) varies from 0.5% to 2.0% [21],... 

Phytic acid, myo-inositol hexakis (dIhydrogen phosphate), as-l,2,3,S-tnns-4,6-cyclohexanehexol-hexaphosphate a major phosphate storage compound in plants, which is especially abundant in oil seeds, legumes and cereal grains. It is the hexaphosphate of A/yo-inositol (see), in which each OH-group of myoinositol is esterified with phosphoric acid. Calcium and magnesium salts of P.a. are known as phytin. The commercial preparation of myo-inositol involves extraction of P. a. from com (maize) steep liquor, hydrolysis of the P.a. to myo-inositol and inorganic phosphate, and crystallization of the myo-inositol from water. 

Certain ions, e.g., oxalate and phosphate, can bind Ca within the intestinal canal and high intake of phosphate can restrict calcium absorption. A similar effect is encountered by phytic acid (inositol hexaphosphate) that is present in high amounts in certain cereals. Disturbances in the fat absorption (dietary) decreases the calcium absorption because Ca is bound to nonabsorbable fatty acids. 

Phosphate is stored in the mature seed as phytic acid, which is the hexaphospho-ric ester of myo-inositol —see Chapter 2. This combines with potassium, calcium and magnesium to form the salt called phytin. Accumulation of phytin in cereals is almost exclusively in the aleurone grains of the aleurone layer where it is deposited as a discrete globoid in association with protein. The precise mechanism of phytin deposition and the enzymes involved therein have been little studied and will receive no more attention here. The importance of phytin as a phosphate reserve is discussed in Chapter 6. 

Certain foods such as unrefined cereals and sugar cane juice contain organic phosphates, e.g. phytate (inositol hexaphosphate), which reduce enamel solubility, apparently by reacting with calcium phosphate salts on the enamel surface. The view that this type of compound has a caries-inhibiting effect is supported by observations that dietary supplements of calcium glycerophosphate reduce caries in rats and monkeys. At the same time, the calcium-binding properties of phytate may adversely affect the absorption of calcium from the intestine (page 143). 

Phytic acid (myo-inositol-l,2,3,4,5,6-hexakisdihydrogen phosphate) occurs in a number of important crops, especially cereals, legumes and oilseeds. The main form is a mixed calcium and magnesium salt, which is called phytin. Phytate phosphorus has reduced biological utihsation and lower utihsation than other minerals (Ca, Mg and Zn and Fe in particular). The contents of phytic acid in some food materials and foods, and the ratio of phytate phosphorus to total phosphorus, are shown in Table 6.5. 

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