Steeping Maize in the Presence of Multiple Enzymes. II. Continuous Countercurrent Steeping'

Cereal Chem. 68(l):1217 A laboratory procedure for steeping maize, which closely simulates 48 and 24 hr in 0.20% sulfur dioxide alone. Reducing the steeping time commercial practice in wet milling, was developed and used to determine without adding enzymes reduced starch yield and increased the protein the effects of multiple-enzyme treatment on wet-milling performance. content of the starch. Maize steeped with multiple enzymes and sulfur Yellow dent maize was continuously and countercurrently steeped for dioxide for 24 hr gave nearly the same yields and purities of fractions 24 hr in a solution containing 1.25% of a multiple-enzyme preparation as maize steeped for 48 hr in sulfur dioxide alone. Thermal properties, and 0.20% sultur dioxide and then wet milled. Yields and protein contents pasting properties, and colors of the starch were unaffected by steeping of wet-milling fractions were compared with those of maize steeped for in multiple enzymes and sulfur dioxide. Watson (1984) and May (1987) have discussed modern industrial practices for wet milling of maize. Steeping is the first and most important step in the milling process. Improper or inadequate steeping fails to produce normal yields of starch (about 90% of the total starch) and low residual protein contents in the starch (0.40% protein or less). But steeping is, by far, the most time-consuming step in the process, typically requiring 36-52 hr. New approaches to steeping are being sought to make wet milling more cost-effective and to reduce the cost of maize starch and starch-derived products, such as sweeteners and ethanol. One approach that has been considered is the addition of 'Journal paper J-13618 of the Iowa Agriculture and Home Economics Experiment Station, Ames 50011; Project 0178. Research supported by the Center for Crops Utilization Research, the Corn Refiners' Association, the Iowa Corn Promotion Board, and the Iowa Agriculture and Home Economics Experiment Station. 2 Graduate research assistant, professor, and graduate research assistant, respectively, Department of Food Technology and Center for Crops Utilization Research, Iowa State University, Ames 5001 1. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists, Inc., 1991. 12 CEREAL CHEMISTRY enzymes, which was recently reviewed and studied by Steinke and Johnson (1991). They used a static batch-steeping procedure to evaluate the feasibility of adding multiple enzymes to steep water containing typical levels of sulfur dioxide to enhance starch separation and reduce steeping time. Steeping for 24 hr in a solution of multiple enzymes and sulfur dioxide produced milling results equivalent to those obtained by steeping for 48 hr in a 0.20% sulfur dioxide solution alone. In industry, however, steeping is accomplished in a continuous, countercurrent manner by putting maize that has been steeped the longest in contact with downstream process water to which 0.10-0.20% sulfur dioxide has been added. Each subsequent steep tank receives steep water from the previous steeping tank, until the steep water (about 20 L/ bu) finally exits the steeping tank containing maize that has been steeped for the least amount of time. Although Watson et al (1951) developed a laboratory system for continuous, static, batch-advanced steeping, most steeping studies (Cox et al 1944, Zipf et al 1950, Watson et al 1955a, Anderson 1963, Vojnovich et al 1975, Krochta et al 1981, Weller et al 1988) have used static batch procedures. These procedures do not simulate the industrial continuous, countercurrent flow of gradually declining sulfur dioxide concentration and increasing solubles concentration or the mixing brought about by countercurrent flow and recirculation of steep water within each steeping vessel. These differences may affect results and lead to erroneous conclusions. Thus, a laboratory steeping procedure that more closely simulates commercial practice would give more reliable predictions of what to expect in industry. The objectives of this study were to develop a laboratory continuous, countercurrent steeping system, which closely simulates industrial practice; to determine whether the presence of multiple enzymes and sulfur dioxide enhances starch separation and facilitates reduced steeping time in continuous, countercurrent steeping; and to evaluate the effects of such steeping on the quality of starch. MATERIALS AND METHODS Steeping Treatments Maize was given three steeping treatments. The first treatment, 48 hr of steeping with 0.20% sulfur dioxide, was used as the control. The second treatment involved the same control procedure except that steeping time was reduced to 24 hr to examine the effects of reduced steeping time. In the third treatment, maize was steeped with an experimental multiple-enzyme system in a 0.20% sulfur dioxide solution for 24 hr. The 0.20% sulfur dioxide solution was prepared by diluting a 6% sulfurous acid solution. The multiple-enzyme system (Table I) consisted of cellulase, hemicellulase, pectinase, /3-glucanase, and bromelin, as used in previous batchwise steeping trials of Steinke and Johnson (1991). All five enzymes were added in equal amounts (0.25%), regardless of their individual activities, to a 0.20% sulfur dioxide solution. The total enzyme concentration was 1.25%. Steeping System The continuous countercurrent steeping system developed in this study (Fig. 1) was based on commercial practice as described by Watson (1984) and May (1987). The steeping vessels (1-6) were 1,200-ml jacketed glass vessels arranged in a battery of six vessels (three more are needed for loading and are waiting). Each vessel was stoppered and fitted with four Teflon tubes. Three tubes were inlets (one for recirculating within the vessel, another for incoming steep water from the previous vessel, the third for inflow of fresh steeping solution), and the fourth was an outlet for overflow into the next succeeding steeping vessel. The depth of the overflow tube into each steeping vessel was set to maintain 1,050 ml. The bottom of each vessel was funnel-shaped and fitted with a Teflon stopcock. A perforated acrylic disk was placed in the bottom of the vessel to hold the maize. The temperature was maintained at 50 ± 20C by circulating water through the vessel jackets and by using a water bath coupled to a centrifugal pump. Solution was advanced through the steeping vessels by positive pressure from the smaller peristaltic pump (B in Fig. 1) and by negative pressure from a second peristaltic pump (C). Solution was recirculated within the vessels by a 10-channel peristaltic drive (E). The system for delivering fresh steeping solution was automated. Inflow of steeping solution into each vessel was actuated by a normally closed two-way solenoid valve (I1-16) controlled TABLE I Sources and Properties of Enzymes Used in Steeping Maize Optimum Optimum Temperature Activity Enzyme Source pH Range Range ( C) (units/g) Cellulasea Trichoderma 4.0-6.0 50-60 150 viride Hemicellulasea Aspergillus 3.0-6.0 30-80 25 niger f3-Glucanaseb Bacillus 4.0-7.0 40-55 300 (Ceremix 2 X 1) subtilis Pectinaseb A. niger 3.5-6.0 20-50 3,000 (Pectinex 3 X 1) Bromelina Pineapple 4.0-5.5 45-55 3,000 aPurchased from Sigma Chemical Co., St. Louis, MO. bPurchased from Novo Industri, Wilton, CT.