Sulfur Dioxide in Acid Environment Facilitates Corn Steeping

Cereal Chem. 73(1):40-44 An effective steeping procedure for corn is proposed, using sulfur corn kernel and the steepwater increased markedly with decreasing the dioxide under acidic conditions. Corn samples were steeped at 50'C in pH of the latter. Apparent partition coefficients of S02 (mean ± standard pH 3.0-5.0 citric acid and sodium citrate buffer solutions containing deviation) of 2.23 ± 0.09, 1.64 ± 0.05, 1.16 ± 0.12, and 0.78 ± 0.06 were 2,200 ppm S02 and 2% sodium chloride. The steeping solutions were obtained for steeping solutions at pH 3.0, 3.5, 4.0, and 5.0, respectively. refreshed every 5 hr. The highest level of solubilization of corn insoluble A mechanism is proposed to explain the effective cleavage of the disulproteins was achieved in the steepwater at pH 3.0 and 3.5, with protein fide bonds of the insoluble glutelin matrix, with the subsequent release degradation ceasing after 20-25 hr. The partition of SO 2 between the of the starch granules by the S02 under acidic conditions. The major objectives of corn steeping are to induce chemical and physical changes in the kernel that will result in leaching of soluble components, in effective separation of the endosperm from the germ and hulls, and in quantitative separation of starch and protein during the wet-milling and fractionation steps (Watson 1984). The separation of starch and protein is the most difficult step to achieve and constitutes the bottleneck of the entire process. Conventional corn steeping occurs over a period of 24-40 hr and consists of a lactic fermentation phase followed by a sulfur dioxide treatment phase. Inherent in the process is an interrelationship between S02 levels and development of the lactic fermentation and total titratable acidity. When high levels (>2,000 ppm) of S02 are introduced into the process water, fermentation and acidity develop rather slowly and at the later stages of the process. Characteristic of the conventional countercurrent process is an intermediate period of =15-20 hr, during which the rate of lactic fermentation slows due to slowly increasing S02 levels. Because lactic fermentation cannot occur before the S02 concentration drops to low levels, and because the concentration of S02 decreases only by oxidation and diffusion into the kernel, little can be done to control and to shorten this intermediate, rather ineffective period. It is generally accepted that lactic acid is beneficial for corn steeping, due to its softening action on cell walls (Watson 1984) and possibly due to induction of some proteolytic activity which enhances protein degradation (Watson et al 1955, Roushdi et al 1981). The major role of sulfur dioxide in steeping is to cleave disulfide linkages, thereby loosening the protein matrix that encapsulates the starch granules (Watson 1984). The introduction of SO2 into the process is accompanied by a rapid increase of the soluble proteins as a result of an accelerated degradation of the corn insoluble proteins (Biss and Cogan 1988). In conventional steeping however, the S02 action exerted on the corn kernels following the lactic fermentation step may not be optimal. The objectives of this study were to evaluate the action of sulfur dioxide during steeping at low pH on the degradation of the corn insoluble protein matrix, and to quantify the pH dependence of the distribution of sulfur dioxide between the kernel and the steeping solution. A mechanism is proposed to explain the mode of action of S02 under acidic conditions.