Amylose-Lipid Complex Formation During Single-Screw Extrusion of Various Corn Starches'

Cereal Chem. 71(6):582-587 Commercial corn starches containing 0-70% amylose were extruded formation of complexes between amylose and lipids. Maximum binding with and without various lipids (myristic, stearic, and behenic acids; monowas observed for myristic acid and 70% amylose starch. No binding was glyceride Dimodan PV; and tristearin) in a single-screw laboratory observed with tristearin. Physico-chemical properties of starches (expanextruder at 1100C, 110 rpm, 19% moisture, and 4% lipid. X-ray diffractosion ratio, water-soluble carbohydrates, and water-solubility index) grams, iodine-binding capacity, and iodine spectrum of the soluble fracdecreased. Bulk density increased upon addition of complexing lipids tions of the extrudates were determined. Additions of fatty acids and before extrusion. These properties reached limiting values as the permonoglyceride resulted in V-patterns in X-ray diffractograms and centage of complexed amylose increased above 20, 15, and 10% for 25, decreased iodine-binding capacity of the extruded starches, indicating 50, and 70% amylose starches, respectively. Extrusion cooking is a popular unit operation for producing a variety of food products with numerous ingredients requiring a wide range of processing conditions. Typical feedstocks include starch, protein, lipids, water, and additives. In general, extrusion of starchy foods results in gelatinization of starch, denaturation of protein, and formation of complexes between starch and lipids and between proteins and lipids (Mercier and Feillet 1975, Mercier et al 1980, Colonna and Mercier 1983, Ho and Izzo 1992). The complexes between starches and lipids are due to the ability of the amylose fraction of starches to bind lipids such as fatty acids. This ability of amylose to form complexes with a wide range of polar and nonpolar organic compounds is currently used to fractionate amylose from amylopectin (Mercier et al 1980). Amylose complexes are generally prepared by adding a complexing agent to a hot aqueous solution of starch. With an extruder, however, this can be achieved momentarily and at low moisture contents. Complex formation during twin-screw extrusion cooking has been studied by Mercier et al (1979, 1980), Colonna and Mercier (1983), Schweizer et al (1986), Galloway et al (1989), Guzman et al (1992), Ho and Izzo (1992), and Strauss et al (1992). Mercier et al (1979, 1980) reported that twin-screw extrusion cooking of manioc, potato, and corn starches in the presence of native lipids or added saturated and unsaturated fatty acids containing 12-20 carbon atoms, glyceryl monostearate, and sodium stearoyl lactylate resulted in the formation of V-amylose complexes. Water solubility and susceptibility of the starches to a-amylase digestion both decreased upon formation of the complexes. Formation of complexes during twin-screw extrusion cooking of corn starch and glyceryl monolaurate was confirmed by Stute and Konieczny-Janda (1983) by using differential scanning calorimetry. In a similar study, Schweizer et al (1986) found that the addition of oleic acid to wheat flour, before extrusion, decreased water solubility and paste viscosity of the extrudates. There was also an inverse relationship between the degree of a-amylase digestion and the amount of complexed starch. Galloway et al (1989) studied the properties and structure of amylose-glyceryl monostearate complexes formed in solution or upon twin-screw extrusion of wheat flour and reported decreases in starch solubility, 'Journal series 10414, Agricultural Research Division, University of NebraskaLincoln. 2 Research associate and professor, respectively, Department of Biological Systems Engineering and Industrial Agricultural Products Center, University of NebraskaLincoln. © 1994 American Association of Cereal Chemists, Inc. water-holding capacity, enzyme susceptibility, and degree of expansion of the extrudates. Except for the work of Mercier et al (1980), most of the work on amylose-lipid complex formations during extrusion cooking has been reported only for a limited range of lipids and with wheat flour or manioc starch. No systematic study has been reported on the effects of different categories of lipids on properties such as expansion ratio, bulk density, water-solubility index, and water-absorption index of different qualities of starch. No attempt has been made to correlate the effect of increasing degree of complexing to physico-chemical properties of extruded starches. Therefore, this study was undertaken with the objective of studying complex formation during single-screw extrusion cooking of starches with lipids, and the effect of degree of complexing on selected physico-chemical properties of extruded starches. MATERIALS AND METHODS Materials Commercially available 0, 25, 50, and 70% amylose corn starches used in this study were received gratis from American Maize-Products Co., Hammond, IN. Their chemical compositions, as supplied by the manufacturer, are given in Table I. Five different lipids were used. Myristic (tetradecanoic), stearic (octadecanoic), and behenic (docosanoic) acids were received gratis from Humko Chemical Division of Witco Corp., Memphis, TN. Dimodan PV, a monoglyceride based on soybean oil with 70-95% C18 and 5-30% C16, was received gratis from Grindsted Products, Inc., Industrial Airport, KS. The triacylglycerol tristearin was purchased from Sigma Chemical Co., St. Louis, Mo. A lipid substitution level of 4% (dwb of starch) was used for all samples. Starches were granulated by spraying water on a inclined rotating pan before extrusion and were preblended with lipids and distilled water in a Hobart mixer for 2 min to achieve desired lipid and moisture contents. TABLE I Proximate Analyses of Various Corn Starches Starch Amylose Content (%, dwb)