Submitted to: Animal Feed Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/31/2020
Publication Date: 1/13/2021
Citation: Liu, K., Frost, J.B., Welker, T.L., Barrows, F. 2021. Comparison of new and conventional processing methods for their effects on physical properties of fish feed. Animal Feed Science and Technology. 273. Article 114818. https://doi.org/10.1016/j.anifeedsci.2021.114818.
Interpretive Summary: World aquaculture has grown steadily over the past several decades, while fish feeds have transformed from locally available wet mixture to semi-moist pelleted diets and finally to dry pelleted feeds. Among several methods for making pelleted feeds, extrusion has become the primary technique, mainly because it improves physicochemical properties and nutritional value of the feed. Yet, at present, thermal extrusion for making aquafeed commonly takes place at low moisture levels (15-45%). Upon exiting an extruder, feed undergoes a sudden drop in pressure, causing rapid moisture loss and volume expansion. The porous extrudate is cut into pellets and dried. These dried pellets have poor durability and disintegrate quickly in water. Furthermore, for binding and facilitating feed expansion, starch (from grain flour) is typically added to a raw feed mash at 10-20% w/w, but the increased carbohydrate content is generally undesirable for carnivorous fish as it can suppress fish growth and performance. To effectively address these issues, we have developed a new processing method for making fish feed at our U.S. Department of Agriculture laboratories. It features high moisture extrusion (45-80%) by using a twin-screw extruder attached to a uniquely designed cooling die. The present study is the first to compare two extrusion methods for trout feed manufacture, a conventional method of low moisture extrusion and the new method of high moisture extrusion, with respect to their effects on physical properties and chemical composition of trout feed. This new method produces a soft texture and water stable aquafeed, which helps reduce water pollution issues associated with aquaculture. Furthermore, addition of carbohydrates as binders becomes unnecessary.
Technical Abstract: At present, aquafeed is made mainly by low-moisture extrusion cooking. To further improve feed quality, we developed a new process featuring high-moisture extrusion with a twin-screw extruder and a specially designed cooling die. In this study, aquafeed was made with two mash formulations (with or without wheat flour) by low moisture (conventional) or high moisture (new) extrusion under the same temperature (138 'C). Fresh and/or dried extrudates were evaluated for visual appearance, physical quality (water absorption and hardness with soaking time, bulk density, pellet durability, water stability and oil absorption capacity), chemical composition and degree of starch gelatinization. Results show that processing methods, feed formulation and post-extrusion drying all had significant effects on the visual appearance and physical quality of feed. The new feed (NF) was less expanded and less porous than conventional feed (CF), leading to significant reduction in oil absorption capacity, about 85 % of CF. Upon contact with water, fresh and dried CF absorbed water quickly and disintegrated easily, while NF absorbed water slowly and maintained a soft texture (around 50 g/mm2, comparable to fish fillet texture) even after 24 h of soaking. Dry matter loss from NF was significantly less than CF, after 6 h shaking in water. Due to its compact texture, NF had higher pellet durability than CF, with a bulk density greater than 1 g/cm3. Addition of wheat flour to the mash made CF more porous, harder in texture and less water stable but exerted a limited effect on NF. Therefore, the new process produced aquafeed that was soft in texture, durable, and extremely water-stable, while making addition of starch or other binder unnecessary. Although an increase in drying cost for NF is expected, the new method offers a good alternative to produce specialty feed and a promising strategy to reduce water pollution issues associated with aquaculture.