Extruded foods are generally low in lipid content, but fat is often added post-extrusion by frying or spraying of lipids to hold seasonings. Generally, foods containing less than 10% lipids are extruded because greater quantities of lipids reduce slip within the extruder barrel, making extrusion difficult, particularly for expanded products. Single screw extruders can process lipid levels of 12-17%, while twin screw extruders with proper screw configurations can handle feed lipid contents as high as 22% (Riaz, 2001). Extruders are used in oilseed extraction because the heat and shear disrupt cellular tissue and free oil (Nelson et al, 1987).

An early problem in extrusion was the apparent disappearance of lipids during processing. Starch-lipid complexes formed during extrusion are resistant to some lipid extraction procedures. Lipid recovery is higher when extruded foods are first digested with acid or amylase, then extracted with ether or another organic solvent. While total fat was not significantly changed in extruded whole wheat, just half of ether-extractable lipids were detected (Wang, W-M et al, 1993). In the same study, wheat bran had more free lipids after extrusion. Cornmeal extruded at lower barrel temperatures (50-60 °C or 85-90 °C) had greater than 75% of its lipids bound, but extrusion at 120-125 °C bound 70% of the lipids (Guzman et al, 1992).

Other nutritional aspects of lipids before and after extrusion have been studied very little. Both docosahexaenoic (DHA) and eicosapentaenoic (EPA) acids were retained in extruded chum salmon muscle with 10% wheat flour (Suzuki et al, 1988). Unlike other processing methods, extrusion cooking does not promote significant cis-trans isomerisation of unsaturated lipids. Corn and soy blends had 1.5% more trans-fatty acids after extrusion (Maga, 1978). Formation of free radicals and subsequent lipid oxidation could have nutritional implications. Artz et al (1992) reviewed extrusion factors affecting lipid oxidation. Screw and barrel wear raise levels of pro-oxidant minerals in extruded foods. For example, iron and peroxide values were higher in extruded rice and dhal compared to dried products (Semwal et al, 1994). Increased surface area in expanded products is another factor increasing oxidation. Factors that retard oxidation in extruded foods include denaturation of lipolytic enzymes, formation of starch-lipid complexes, and creation of antioxidant Maillard compounds.

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