Comparison Between Alkali and Conventional Corn Wet-Milling: 100-g Procedures

Cereal Chem. 76(1):96-99 A corn wet-milling process in which alkali was used was studied as an alternative to the conventional corn wet-milling procedure. In the alkali wet-milling process, corn was soaked in 2% NaOH at 85°C for 5 min and then debranned mechanically to obtain pericarp as a coproduct. Debranned corn was cracked in a roller mill, and the cracked corn was steeped with agitation for 1 hr in 0.5% NaOH at 45°C. The cracked and steeped corn was then processed to separate germ, fiber, and gluten by steps similar to those in conventional wet-milling. Alkali wet-milling yielded soakwater solids, pericarp, germ, starch, gluten, and fine fiber. The protein content of the starch and the starch content of the fiber from the alkali process were lower than those from the conventional process. Corn wet-milling produces starch for the food, pharmaceutical, paper, chemical, textile, cosmetic, and energy industries. The corn wet-milling industry has increased production from 3.8 million metric tons (149 million bushels) during 1957–1959 (Moore and Dwoskin 1970) to 43.2 million metric tons (1,700 million bushels) during 1996–1997 (Anonymous 1997). Conventional corn wet-milling, which uses sulfur dioxide (SO2), is capitaland energy-intensive. Among the steps in conventional wet-milling, steeping and steepwater evaporation are the most costly. It is estimated that these two operations use 21% of the total energy and capital (unpublished data). The great diversity of industrial starch applications has increased the demand for corn wet-milling products. However, the high capital and energy costs are barriers to new construction or production expansion. Steeping, the most important step in conventional wet-milling, is a process during which SO2 and water penetrate corn. The absorbed SO2 cleaves the disulfide bonds in the protein matrix that encapsulates the starch granules (Watson 1984), dispersing the protein matrix, and enhancing starch release (Watson and Sanders 1961). Because the pericarp creates a barrier to the diffusion of water and SO2 (Syarief et al 1987, Eckhoff and Okos 1989, Ruan et al 1992), steeping for 24–40 hr at 50–52°C is required to achieve sufficient release of starch granules from the protein matrix. However, Watson and Sanders (1961) showed that starch could be released from the endosperm protein matrix in <2 hr when the pericarp layer was cut open. Alkali has been used for removing pericarp by several researchers (Hansen 1949, Weinecke 1962, Morgan et al 1966, Freeman and Watson 1969, Blessin et al 1970, Mistry and Eckhoff 1992a). Alkali can also be used to extract starch from corn kernels and corn flour. In fact, alkali was used to separate starch from corn kernels before the SO2 wet-milling process was developed (Bensing et al 1972). The original alkali wet-milling process was not environmentally benign because residues and solubles, amounting to ≈40% of corn, were dumped into a river. Dimler et al (1944) used an alkali solution to extract starch from corn flour but obtained a relatively low starch yield (67% on a dry-flour basis, 75% extraction rate). Mistry et al (1992) steeped corn flour in 0.1% NaOH at 55°C for 30 min and obtained a 73.8% starch yield (dry flour basis). The protein and sodium contents of the starch were 0.19% and 889 ppm, respectively. Alkali can effectively extract starch from corn because glutelin, the major protein encapsulating the starch, is alkali soluble (Lasztity 1996). A previous study showed that starch obtained by alkali extraction from corn flour was different from commercial starch. The protein content of the alkali-extracted starch was higher or lower than that of commercial starch, depending on extraction conditions, including alkali concentration, temperature, and time (Mistry and Eckhoff 1992b). The sodium content of alkali-extracted starch was higher than that of commercial starch. Alkali-extracted starch had a lower pasting temperature and higher viscosity after 15 min of holding at 95°C (Mistry and Eckhoff 1992b). Alkali-extracted starch absorbed more water than the commercial starch at a given water activity (Mistry and Eckhoff 1992b). The X-ray diffraction pattern, birefringence, morphology, and enzymatic hydrolysis characteristics of the starch extracted by alkali from corn flour were similar to those of the commercial starch (Mistry and Eckhoff 1992b). The objectives of this study were to: 1) set up an alkali wetmilling procedure to separate starch from whole corn kernels and to determine the need for each process step, and 2) compare the yields of the products from alkali wet-milling to products from conventional wet-milling. MATERIALS AND METHODS Materials A yellow dent corn hybrid, FR1064 × LH59, was harvested during 1994 and dried with ambient air to ≈14% (wb) moisture content. Dried corn was divided into 100-g subsamples, sealed in plastic bags, and refrigerated until processed. The chemical composition of the corn was analyzed by near-infrared transmittance (GrainSpec, Foss Electric, Minneapolis, MN) by the Identity Preserved Grain Laboratory, Champaign, IL. Transmittance readings of 250 g were taken over a wavelength range of 800–1,100 nm. Sodium hydroxide (NaOH, ≥97.0%, reagent-grade pellets) was used to prepare a 20% alkali solution by dissolving 80 g of NaOH in 400 mL of water. This solution was diluted with water to the desired concentration for soaking and steeping. Sodium meta-bisulfite and lactic acid (Fisher Scientific, Pittsburgh, PA) were used to prepare steep solutions for conventional wet-milling. Alkali Wet-Milling Procedure The initial laboratory procedure involved soaking 100-g samples of corn in 200 mL of 2% NaOH (pH 13) at 85°C for 5 min and then draining the solution, or soakwater (pH 13), into a graduated cylinder through a screened funnel. The total volume of soakwater was recorded, and three subsamples (25 mL each) were taken to determine the solids content. The soaked corn was debranned with an apparatus designed by Mistry and Eckhoff (1992a) with modifications that included the use of a different sieve size (a U.S. no. 5 sieve instead of a no. 4 sieve) and an increased brush rotating speed 1 Professor, former graduate research assistant, graduate research assistant, visiting assistant professor, and former research technician, respectively, Department of Agricultural Engineering, University of Illinois, Urbana, IL 61801. 2 Corresponding author. Fax: 217/244-0323. E-mail: [email protected] 3 Current address: Corn Products International, 6500 Archer Avenue, Bedford Park, IL 60501-1933. 4 Professor, Department of Veterinary Biosciences, College of Veterinary Medicine, University of Illinois, Urbana, IL 61801. Publication no. C-1999-0106-06R. © 1999 American Association of Cereal Chemists, Inc.