Mineral content and bioavailability are generally retained well during extrusion. Abrasive foods, such as brans rich in dietary fiber, or with low lipid and moisture content, gradually wear away metal from the extruder screws and barrel. The equipment must be replaced or refurbished periodically due to this wear, as the metal accumulates in the extruded food. As barrel temperature increased during single screw extrusion of potato flakes, iron content also increased (Maga and Sizer, 1978). Total iron increased by as much as 38% due to extrusion (Camire et al, 1993). On the other hand, cornmeal, which has a low dietary fiber content, had no changes in total, elemental, or soluble iron after twin screw extrusion (Camire and Dougherty, 1998).

Although iron from screw wear is typically in the elemental form, the bioavail-ability appears adequate as long as excessive amounts of iron and related metals are not present. Rats fed extruded corn and potato absorbed iron well (Fairweather-Tait et al, 1987). Utilisation of iron and zinc from wheat bran and wheat in adult human volunteers was not affected by extrusion (Fairweather-Tait et al, 1989). Extrusion slightly increased iron availability in corn snacks under in vitro digestion followed by dialysis (Hazell and Johnson, 1989). Low-shear extrusion retained dialysable iron in navy beans, lentils, chickpeas and cowpeas better than did high-shear extrusion (Ummadi et al, 1995). Weaning food blends of pearl millet, cowpea and peanut had greater iron availability and protein digestibility compared to similar foods processed by roasting (Cisse et al, 1998). None of the processed blends provided adequate iron to meet infant needs, however. Zinc bioavailability of semolina and soy protein concentrate blends (85:15) (Kang, 1996) was unaffected by extrusion.

Mineral bioavailability may be improved in extruded foods if mineral-binding phytate is reduced during processing. Published research has had mixed results. Extrusion reduced phytate levels in wheat flour (Fairweather-Tait et al, 1989), possibly due to inactivation of phytases during extrusion. Although phytic acid was reduced under all processing conditions, total phytate was not affected. Legume phytate was not affected by extrusion (Lombardi-Boccia et al, 1991; Ummadi et al,1995).

While screw speed had no effect on phytate in wheat, rice and oat brans, insoluble fiber decreased in all but wheat bran after extrusion (Gualberto et al, 1997). After phytate was removed, extruded rice and oat brans bound more calcium and zinc, but not copper, in vitro (Bergman et al, 1997). Comparable results were found with a high-fiber cereal fed to seven persons with ileostomies (Sandberg et al, 1986; Kivisto et al, 1986). Dietary fiber and phytate values in the cereals were not affected by extrusion, but nonetheless mineral availability was reduced. Despite the formation of phytate complexes with protein and starch in rice bran, over 90% of the phytate could still be extracted (Fuh and Chiang, 2001).

Mineral fortification has become common, especially in ready-to-eat breakfast cereals. Calcium hydroxide added at levels of 0.15-0.35% to corn reduced expansion and increased lightness in color (Martínez-Bustos, et al, 1998); bioavailability was not determined. Snacks made from blue maize on a small single screw extruder had acceptable textural characteristics with added calcium hydroxide levels of 0.02-0.078% (Zazueta-Morales et al, 2001). Since dark color can result when some iron salts react with phenolics, Kapanidis and Lee (1996) recommended ferrous sulfate heptahydrate as an added iron source in a simulated rice product.

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