Effects of Steam, Moisture, and Screw Speed on Physical Properties of DDGS-Based Extrudates

Cereal Chem. 90(3):186–197 A fractional factorial design with a replicated central composite point was used to investigate the effects of extrusion processing on physical properties of distillers dried grains with solubles (DDGS) based aquafeeds using a twin-screw extruder. Extrusion cooking trials were performed with a nutritionally balanced ingredient blend for Nile tilapia, with two levels of screw speed (350 and 450 rpm), two levels of extruder water (0.236 and 0.302 kg/min), and two levels of conditioner steam (0.1 and 0.15 kg/min). The central point was 400 rpm screw speed, 0.271 kg/min extruder water, and 0.12 kg/min conditioner steam. Effects of these processing conditions on extrudate characteristics were extensively analyzed and included moisture content, water activity, thermal properties, expansion ratio, unit density, bulk density, color, water stability, sinking velocity, water absorption and solubility indices, and pellet durability index. Increasing the extruder water and conditioner steam resulted in a 5.3% decrease and nearly 8.6% rise in mass flow rate, respectively. As screw speed increased from 350 to 400 rpm, water stability and water activity increased by 13 and 58%, respectively. Increasing extruder water from 0.236 to 0.302 kg/min led to a significant increase in water stability by 12.5% and decreases in water absorption index, water activity, and expansion ratio by 13, 21, and 5.5%, respectively. As conditioner steam increased from 0.1 to 0.15 kg/min, sinking velocity and water absorption index decreased by 25 and 15%, respectively. Increasing conditioner steam from 0.1 to 0.12 kg/min resulted in 20, 5.5, 10, and 3% decreases in moisture content of the extrudates, brightness (L*), water stability, and expansion ratio, respectively. It also increased bulk density by 5.8% and unit density by 4.2%. Overall, all trials produced viable extrudates with properties appropriate for Nile tilapia feeding. The aquaculture industry consumes a large amount of wild fish to raise other fish for harvest (human food). Aquaculture farms use a considerable amount of fishmeal and fish oil (made from wild fish) to maximize fish growth and enhance feed flavor. In fact, up to 60% of total costs of farm production can be accounted for by diet costs (Tan and Dominy 1997), of which protein is the most expensive ingredient. According to Naylor and Burke (2005), there are several viable protein substitutes, including protein made from grain and other livestock by-products (Ayadi et al 2012). One way of improving plant-based ingredient digestibility is to supplement with enzymes (Cain and Garling 1995; Rodehutscord and Pfeffer 1995; Storebakken et al 1998; Vielma et al 1998, 2000; Papatryphon et al 1999; Sugiura et al 2001; Cheng and Hardy 2002). Many studies have indicated that corn-based distillers dried grains with solubles (DDGS) (a coproduct from fuel ethanol manufacturing) could be a good protein source to replace portions of fish meal in fish diets (Cheng et al 2003; Ayadi et al 2010). In recent years, interest in renewable energy has resulted from several factors, such as environmental pollution, the importance of clean air, the ban on methyl tertiary-butyl ether, and unstable oil supplies, all of which have led to tremendous growth in the biofuels industry. Although biological materials such as residue straw, corn stover, perennial grasses, and legumes can be used, corn starch is by far the most common substrate used for ethanol manufacture in the United States because of its economic viability (Rosentrater 2006). For example, in 1980, the annual production rate of ethanol fuel was about 175 million gallons in the United States. This amount increased to 900 million gallons, 1.630 billion gallons, and 13.23 billion gallons in 1990, 2000, and 2010, respectively (Renewable Fuels Association 2012). From such an increase in the production of ethanol, there has been a parallel increase in feed coproducts (primarily DDGS). Ethanol can be produced commercially via two main methods: wet-mill processing and dry-mill processing (Belyea et al 2004). In the former method, grain is steeped in water, and then the slurry is ground and the components are mechanically separated. The products of this method are corn oil, corn gluten feed, gluten meal, starch, and ethanol. In the latter process, however, the entire corn kernel is ground into flour, and then the starch is converted to simple sugars with enzymes. Thereafter, the sugars are converted into ethanol using yeast. The products of dry milling include ethanol, DDGS, and CO2. Generally, for each 1 kg of corn consumed, 1/3 kg each of ethanol, DDGS, and CO2 will be produced (Chevanan et al 2005; Rosentrater 2006; Rosentrater and Muthukumarappan 2006). DDGS has become a widespread source of protein for livestock diets globally. DDGS is composed of the nonfermentable components of the original grain (i.e., proteins, lipids, fibers, and ash). Typically, DDGS contains 27–33% protein and 5–12% fiber (Belyea et al 2004; Rosentrater 2006; Rosentrater and Muthukumarappan 2006). The total production of DDGS increased nearly 200% between 2006 and 2010 in the United States, from 15.62 million to 32.44 million metric tons, and it is expected to reach 33.5 million metric tons by 2013 (Agricultural Marketing Resource Center 2011). Considering the dramatic increase in the ethanol industry in recent years, developing alternative utilization opportunities for DDGS must be taken into consideration to avoid potential market saturation. Various potential alternatives have been proposed, including bioplastics (Schilling et al 2004; Cheesbrough et al 2008), biofillers (Tatara et al 2009), human food additives (Rosentrater 2006), and aquaculture feed ingredients (Naylor and Burke 2005; Chevanan et al 2005, 2007a, 2007b, 2007c, 2008, 2009, 2010). Generally, the majority of fish feed cost is accounted for by the cost of protein sources, because protein is the most expensive ingredient in animal diets. Belyea et al (1989) reported that variations in protein content of feeds can affect animal productivity. Because of the moderately high protein content of DDGS, it has been used extensively in various animal feeds. Although DDGS can be a valuable source of protein, it has low levels of essential amino acids, particularly lysine and methionine (Lim et al 2009). This drawback can limit its use in aquafeeds. The average protein component of the fish body is 65–75%; thus, the basic nutrient structure of fish feed is protein, and balanced amino acid 1 Graduate research assistant and professor, respectively, South Dakota State University, Brookings, SD 57007. 2 Assistant professor, Iowa State University, Ames, IA 50011. 3 Corresponding author. Phone: (515) 294-4019. Fax: (515) 294-6633. E-mail: [email protected] 4 Director, Research, Development and Innovation, Alliance Grain Traders, Regina, SK, S4N 7K9, Canada. http://dx.doi.org/10.1094 / CCHEM-08-12-0102-R © 2013 AACC International, Inc.