Starch recovery from degermed corn flour and hominy feed using power ultrasound

Power ultrasound is sound waves in the frequency range of 20– 100 kHz with a sound intensity of 10–1,000 W/cm. It has been studied for use in a number of food-processing applications. The ultrasound mode of action is attributed to a phenomenon called cavitation, which is the formation, growth, and implosion of tiny gas bubbles or cavities in a liquid. Implosion of cavities results in micro-streaming and localized high shear that can be used to perform various physical or biochemical operations or to enhance biological and biochemical separations. Mason et al (1996), Salisova et al (1997), and Vinatoru et al (1997) studied the efficacy of ultrasound on extracting low molecular weight substances from plant materials. They postulated that the enhanced extraction of organic compounds was due to disruption of cell walls and enhanced mass transfer between interfaces of biocomponents. Ebringerova et al (1998) extracted immunologically active xylan from corncobs and heteroxylan from corn hulls with ultrasound and did not observe noticeable changes in functional properties of the biopolymers. The physical effect of ultrasound is also utilized in food emulsification and homogenization applications (Mason 1998). Few studies have documented the effects of power ultrasound on starch physicochemical properties or the use of ultrasound in corn processing. Jackson et al (1988, 1989) used ultrasound to dissolve corn and sorghum starch granules after heating instead of solubilizing in alkali or dimethyl sulfoxide for the analysis of molecular structures. They reported that ultrasonic vibrations disrupt swollen granules, thereby releasing amylose and amylopectin from the granules, and resulting in an increase in water solubility of starch. Chung et al (2002) treated mung bean, potato, and rice starches with ultrasound after heating and reported that the average degree of polymerization did not change after sonication. They also proposed that the changes in starch properties were induced by disruption of swollen granules rather than a breakage of glucosidic linkages. There are no studies in the literature on the effect of sonication on native or raw corn starches. In a study on rice starch isolation, Wang et al (2003) found that power ultrasound treatment resulted in a high starch recovery but a slightly higher residual protein content. Yang et al (2002a,b) used ultrasound to treat corn kernels in steeping water. They found that the affinity between pericarp and endosperm was reduced, indicating an enhanced separation of corn components by ultrasound treatment. Corn dry milling often produces process streams, which due to market conditions go to animal feed. Hominy feed, which contains corn bran, germ, and some starchy portion of the corn kernel, is a by-product of dry milling and is sold as feed. Degermed corn flour is a prime product that has an inelastic demand. Frequently, processing conditions and product mix result in overproduction of degermed flour. Both products are reasonably high in starch and may be a source of nonsulfate-treated starch. The objectives of this study were to test power ultrasound as a means to recover starch from degermed corn flour and hominy feed, and to study the properties of the resulting starches.