Twin-Screw Extrusion Processing of Feed Blends Containing Distillers Dried Grains with Solubles (DDGS)

Cereal Chem. 84(5):428–436 Extrusion trials were conducted with varying levels of distillers dried grains with solubles (DDGS) along with soy flour, corn flour, fish meal, vitamin mix, mineral mix, and net protein content adjusted to 28% using a Wenger TX-52 twin-screw extruder. The properties of extrudates were studied in experiments conducted using a full-factorial design with three levels of DDGS content, two levels of moisture content, and two levels of screw speed. Increasing the DDGS content from 20 to 60% resulted in a 36.7% decrease in the radial expansion, leading to a 159 and 61.4% increase in the unit density and bulk density of the extrudates, respectively. Increasing the DDGS content resulted in a significant increase in the water absorption index (WAI) but a significant decrease in the water solubility index (WSI) of the extrudates. Changing the screw speed and moisture content had no significant effect on the radial expansion ratio but resulted in a significant difference in the bulk density of the extrudates, which may be due to the occurrence of longitudinal expansion. Even though changing the moisture content and screw speed had no significant effect on the WSI of the extrudates, significant differences in the WAI of the extrudates were observed. The ingredient components in the blend and moisture content had an influence on the color changes of the extrudates, as did the biochemical changes occurring inside the barrel during processing. Overall, it was determined that DDGS could be included at a rate of up to 60% using twin-screw extrusion, and that viable pelleted floating feeds can be produced. Distillers dried grains with solubles (DDGS), a coproduct of ethanol production, contains high amounts of protein and fiber, and a low amount of starch. It is mostly used as animal feed. The protein conversion efficiency of fish is very high compared with other animals. Hence DDGS has great potential to be used as an alternative protein source for aquaculture feed production. Research by Wu et al (1994, 1996) indicated that tilapia fish can be grown with DDGS, which can improve the economic viability of aquaculture farms. Because starch is converted to ethanol during the fermentation process, micronutrients are available in concentrated quantities compared with whole corn. This potentially makes DDGS a better base material for aquaculture feed than whole corn. But the main problems with the production of extruded aquaculture feed with DDGS are its low starch content and high fiber content (Chin et al 1989). As the starch content is decreased, expansion obtained during extrusion is reduced, which subsequently affects the physical characteristics of the extrudates. As the fiber content is increased, extrudate mechanical strength and durability decrease due to its noninteracting nature with other components in the ingredient mix. Whey, a by-product of cheesemaking, has excellent binding properties. Whey has actually been used as a binder for production of aquaculture feed by the pelleting process (Lovell 1988). But inclusion of whey during extrusion processing of cereal foods has been found to decrease expansion, increase unit density, water absorption index, and breaking strength (Martinez-Serna and Villota 1992; Matthey and Hanna 1997). The exact mechanism of interaction of whey with other components is not well understood (Cumming et al 1973). We have found that whey can be successfully used as a binder with DDGS-based aquaculture feed blends (Chevanan et al 2005, 2006). In the aquaculture industry, depending on the fish species, floating feeds are often preferred to sinking feeds, to prevent over feeding, and to maintain the cleanliness in the pond (Lovell 1988; Chang and Wang 1999). Twin-screw extruders have various advantages over single-screw extruders for production of floating feeds. Twin-screw extruders can handle viscous, oily, sticky, or wet ingredients with varying levels of protein, starch, fat, and fiber over a wide range of particle sizes and can achieve wide variations in the extrudate properties (Riaz 2000). The main difference between single-screw and twin-screw extruders is the transport mechanism (Mercier et al 1989). Single-screw extruders transport material by friction between the screw and the product, as well as the barrel and the product. With single-screw extruders, it is very difficult to achieve the required operating conditions for the production of floating feeds from ingredient mixes containing DDGS due to the low starch content. For example, extrudates (containing up to 40% DDGS and other ingredients including soy flour, corn flour, fish meal, mineral mix, vitamin mix, and whey, with a net protein content adjusted to 28%, db) obtained using a Brabender laboratory-scale single-screw extruder did not float (Chevanan et al 2006). In twin-screw extrusion, however, material within the barrel is transported positively, irrespective of the friction between the screw, barrel surface, and material (Zuilichem and Stolp 1984). Proper operating conditions can be easily achieved by varying parameters such as screw speed, feeding rate, ingredient moisture content, and temperature. Because of differences between the single-screw and twin-screw processes, it may be possible to use twin-screw extruders for ingredient mixes containing low starch materials such as DDGS, to produce floating feeds, although no studies have yet been reported that have accomplished this. Aquaculture feeds typically require 26–50% protein, depending on the species to be fed (Lovell 1988), so the formulated ingredient mix will often contain a high amount of both starch and protein. During extrusion processing of the starch-based products, a relatively elastic melt is formed inside the barrel which results in a more expanded and crispy product (Ibanoglu et al 1996; Ilo et al 1996; Alves et al 1999; Thomas et al 1999). During extrusion processing of protein-based products, on the other hand, a plastic melt is formed inside the barrel and a more porous and textured product is formed (Gwiazda et al 1987; Harris et al 1988; Singh et al 1991; Sandra and Jose 1993). Extrudate floatability can be achieved through expansion during extrusion processing. Very little information is available on the effect of ingredient mixes containing high proportions of both starch and protein on expansion and other physical properties of extrudates by twin-screw extrusion. Thus the objective of this study was to examine the effect of varying levels of screw speed, blend moisture content, and DDGS content 1 Graduate research assistant, South Dakota State University, Brookings, SD 57007. 2 Bioprocess engineer, North Central Agricultural Research Laboratory, USDA – ARS Brookings, SD 57006. 3 Corresponding author. Phone: (605) 693-3241. Fax: (605) 693-5240. E-mail address: [email protected] 4 Professor, South Dakota State University, Brookings, SD 57007. doi:10.1094 / CCHEM-84-5-0428 © 2007 AACC International, Inc.