Interactions of sulfur dioxide, lactic acid, and temperature during simulated corn wet milling

Cereal Chem. 72(4):371-378 Response surface methodology was used to investigate the interactions models of starch, germ, fines, gluten, and steep solids yields were predicted of lactic acid, sulfur dioxide (SO2 ), and steeping temperature and their as functions with nonlinear influences of lactic acid and SO2 . Higher effects on corn wet milling yields. A regular dent corn hybrid and a starch yields were obtained when steeping at 431C than at 570C; higher more vitreous dent corn hybrid were laboratory batch steeped. Kernel yields were predicted with moderate lactic acid and high SO2 concentraabsorption of SO 2 was higher for the more vitreous dent corn hybrid. tions. Steeping temperature interactions with lactic acid and SO 2 during Absorption also increased with lactic acid use and at lower steeping the steeping period limited its integration as a second-order modeling temperatures. Lactic acid concentrations in steepwater remained constant factor for starch, germ, and fines fractions. For all treatments, gluten over time, but kernels absorbed more steepwater at higher temperatures. recovery and steepwater solubles responses were predicted by lactic acid When wet milled on a laboratory scale, vitreous corn was more resistant concentrations. Lactic acid, which influenced all significant fraction yield to grinding and less millable. Significant first-order response surface models, affected SO2 absorption. Steeping corn (Zea mays, L.) for starch production softens and degrades kernel structure, thus aiding kernel component separation during the physical stages of the wet milling process. Sulfur dioxide (SO2 ) and elevated steeping temperatures (45-550C) are used to control the growth of putrefactive microorganisms within the steeps, as well as to aid in kernel degradation. Lactic acid is usually formed by bacterial fermentation in commercial steeps and is often added to steepwater used in laboratory batch steeping. Cox et al (1944) identified SO2 as an important steeping agent that peptidizes protein matrices enveloping endosperm starch granules. The degree of protein peptization in whole kernels increased over the 24-hr steeping period with increasing SO 2 concentrations (up to 0.4% tested) and higher steeping temperatures (up to 550C tested). When steeping horny endosperm sections (10,.m thick, unlimited steepwater diffusion), Watson and Sanders (1961) observed increased starch granule release from the surrounding protein matrix with increased SO2 concentrations. In commercial steeping, kernel degradation for starch release does not occur until kernels are exposed to SO2 (Wagoner 1948). Bisulfite ions, a form of aqueous SO2, reduce and peptidize native kernel proteins and form sulfo-protein complexes (Boundy et al 1967). Steepwater pH affects bisulfite ion formation (King et al 1981). Eckhoff and Okos (1990) showed that gaseous SO 2 penetrates corn kernels 100 times faster than the steepwater diffusion rate calculated by Fan et al (1965). Also, Eckhoff and Okos (1990) observed a higher net absorption of gaseous SO2 at temperatures lower than those typically used for steeping (30 C). Steeping times have been decreased and starch yields have been increased by mechanically (Hassanean et al 1986, Roushdi et al 1979) and enzymatically (Caransa et al 1988, Du Ling and Jackson 1991, Steinke and Johnson 1991) increasing steepwater and SO2 penetration. Cox et al (1944) reported that lactic acid softened the kernel and increased the effectiveness of SO2, but acetic and hydrochloric acids did not have softening or degrading effects. Watson and Sanders (1961) reported that lactic acid alone did not influence