Potential for binding nutrients

The phytic acid molecule has a high P content (28.2%) and chelating potential (Fig. 10.1) to form a wide variety of insoluble salts with di- and trivalent cations at neutral pH (Vohra etal., 1965; Oberleas, 1973; Cheryan, 1980). One mole of phytic acid can bind an average of 3—6 mol of Ca to form insoluble phytates at the pH of the small intestine. Formation of insoluble phytate makes both Ca and P unavailable. Zn, Cu, Co, Mn, Fe and Mg can also be complexed, but Zn and Cu have the strongest binding affinity (Maddaiah etal., 1964; Vohra etal., 1965). This binding potentially renders these minerals unavailable for intestinal absorption. Zinc may be the trace element whose bioavailability is most influenced by phytate (Pallauf and Rimbach, 1995). Phytic acid may have a negative influence on dietary protein and amino acids (O'Dell and de Borland, 1976; Knuckles etal., 1985) and inhibits proteolytic enzymes such as pepsin and trypsin under gastrointestinal conditions

Table 10.2. Bioavailability of phosphorus for pigs and nonphytate phosphorus for poultry.

Bioavailability of P for pigsa b Nonphytate-P for poultryc Feedstuff (%) (% of total)

Cereal grains

Maize

12

28

Oats

23

33

Barley

31

36

Triticale

46

33

Wheat

50

31

Maize, high moisture

53

-

High protein meals - plant origin

Groundnut meal

12

21

Canola meal

21

26

Soybean meal, dehulled

25

35

Soybean meal, 44% protein

35

40

aAdapted from Cromwell (1992).

bRelative to the availability of phosphorus in monosodium phosphate, which is given a value of 100. cNRC (1994) - poultry.

Phytate Protein
Fig. 10.1. Structure of phytate and possible bonds (after Thompson, 1988).

(Singh and Krikorian, 1982). Under acidic conditions, the basic phosphate groups of phytic acid may complex with amino groups such as lysyl, histidyl and arginine (De Rham and Jost, 1979; Fretzdorff et al., 1995). Under neutral conditions, the carboxyl groups of some amino acids may bind to phytate through a divalent or trivalent mineral. Phytate—protein or phytate—mineral—protein complexes may reduce the utilization of protein.

Starch is also known to be complexed by phytate. The in vitro hydrolysis of either wheat or bean starch incubated with human saliva was retarded when Na phytate was included in the mixture, but digestion was restored when Ca was added with the Na phytate (Yoon et al., 1983; Thompson, 1986; Thompson et al., 1987). As will be reviewed later, Ravindran (1999) reported studies with broilers that clearly show that apparent metabolizable energy is improved when high-phytate diets are supplemented with microbial phytase.

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  • edilio
    How do phytase to hydrolysis the phytic acid?
    8 years ago

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