Phosphorus Concentrations and Flow in Maize Wet-Milling Streams

Cereal Chem. 82(4):431–435 Marketing of coproducts such as corn gluten meal (CGM) and corn gluten feed (CGF) is important to the maize wet-milling industry. High phosphorus concentrations could lead to limited markets for CGF due to its potential to increase phosphorus in animal wastes. The objective was to measure the concentration and flow of phosphorus in the wet-milling process and identify streams that could be altered. Samples were taken from 21 process streams of three facilities and the phosphorus content of each was determined. Flow of phosphorus was simulated using a computer model for a 2,700 tonne/day (105,000 bu/day) wet-milling plant. Phosphorus concentrations of streams varied from <10 mg/kg to >14,000 mg/kg. Phosphorus content of many streams differed significantly among facilities. Flow of phosphorus (kg/day) varied dramatically among streams. However light steepwater, light gluten, and process water streams (5,960, 3,080, and 970 kg/day, respectively) accounted for much of the phosphorus flow. Modification of these streams could reduce phosphorus content of coproducts. The high phosphorus content of either CGF or CGM could be reduced markedly if phosphorus was reduced in the appropriate streams. Wet milling is an important maize processing technology; it accounts for the processing of ≈51 million tonnes of maize or 22% of the U.S. crop annually (ERS 2003). In wet milling, the maize kernel is steeped and fractionated; starch, fiber, protein, and oil are separated and concentrated in specific process streams. Two major coproducts are generated (corn gluten feed [CGF] and corn gluten meal [CGM]). CGF is produced from mixing heavy steepwater with maize fiber (Fig. 1); it has high fiber and protein content and is fed mainly to ruminant animals. CGM is produced from dewatering of gluten; it contains high protein concentrations and is fed mainly to nonruminant animals. Income from marketing of CGF and CGM is important to the economic viability of the wet-milling industry because it partially offsets production costs. Factors that affect quality or marketability can have marked effects on value of these coproducts. One emerging issue is the phosphorus concentration of CGF (≈6 g/kg), which is high relative to ruminant requirements (≈3 g/kg). This could affect marketing in the near future. When ruminants consume diets containing elevated concentrations of phosphorus, the amount of phosphorus excreted in wastes is increased (Morse et al 1992). This is a concern because environmental regulations for land application of animal wastes are based partly on phosphorus concentration and are becoming more restrictive. Animal producers who access land with soils with high phosphorus concentrations or who have limited cropland for waste disposal may have to minimize use of high phosphorus feed ingredients such as CGF (Tamminga 1992; Van Horn et al 1996; Dou et al 2001; Rotz et al 2002; Spears et al 2003). A substantial (i.e., 50%) reduction in phosphorus concentration would lessen the potential environmental implications of using CGF in animal diets. To reduce phosphorus content, it is necessary to know the concentration and flow of phosphorus in wetmilling process streams. This information can be used to identify which processing streams have high phosphorus loads and potential strategies for phosphorus reduction. The objective was to determine concentrations and flows of phosphorus in wet-milling process streams. MATERIALS AND METHODS Three maize wet-milling plants located in the midwestern United States collaborated in the study by providing samples and information regarding processing streams. All plants used regular dent maize obtained from commodity markets. Samples were obtained from 21 processing streams in each plant. These included maize, process water, process water after sulfur dioxide (SO2) addition, light steepwater, heavy steepwater, steepwater condensate, steeped maize, wet germ, wet fiber, pressed germ, pressed fiber, light gluten, heavy gluten, gluten cake, starch slurry, dry germ, CGF, CGM, wastewater, fresh water, and final effluent (Fig. 1). Maize was sampled as it entered the steep tanks. Samples of process water and process water with SO2 were taken before and after addition of SO2 and before addition to the steep tanks. Light steepwater was obtained from sample ports on lines leading from the steep tanks. Heavy steepwater and steepwater condensate were obtained from ports leading from the evaporators used to concentrate steepwater. Steeped maize samples were obtained from sample points located just before the first grind mills. Wet germ and wet fiber were taken from dewatering screens before pressing of germ and fiber. Pressed germ and pressed fiber samples were obtained from respective sampling ports located just downstream from the dewatering presses. Light gluten and heavy gluten were sampled near gluten-thickener centrifuges; gluten cake was sampled directly from vacuum belt filters. Starch slurry samples were taken downstream from the starch-washing hydrocyclones. Dried germ, CGF, and CGM were sampled at ports just downstream from the respective drying operations. The wastewater sample was untreated raw waste sampled at a port before the waste treatment system. Samples were taken during four sampling periods for one plant and during three periods for the other two plants. During each sampling period, three sets of samples were obtained. Each sample set consisted of 21 samples; three sample sets were taken over a seven-day period and at least 24 hr apart. Samples were taken only when operating conditions were considered to be stable. As soon as samples were obtained from process lines, they were placed in sealed containers, frozen, and shipped on dry ice to the University of Illinois for analyses. Phosphorus contents were determined using Approved Method 40-56 (AACC International 2000). Flow rates of process streams (kg/day) were estimated for a 2,700 tonne per day (105,000 bushel/day) wet-milling plant using values found in the literature (Blanchard 1999) and a simulation model developed by the University of Illinois and the Eastern Regional Research Center (USDA-ARS, Wyndmoor, PA, USA). For this simulation, all process flows were held constant and wastewater and final effluent flow rates were assumed to be equal 1 Assistant professor, former graduate assistant, and professor, respectively, Department of Agricultural and Biological Engineering, University of Illinois at UrbanaChampaign, 1304 W. Pennsylvania Ave., Urbana, IL 61801, USA. 2 Corresponding author. Phone: 217-265-0697. E-mail: [email protected] 3 Former graduate assistant and associate professor, respectively, Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign. 4 Professor, Department of Animal Sciences, University of Missouri, Columbia. 5 Professor, Department of Civil Engineering, University of Missouri, Columbia. DOI: 10.1094 / CC-82-0431 © 2005 AACC International, Inc.