Towards a Better Understanding and Control of Factors Affecting the Physical Quality of Fish Feed

Abstract: As aquaculture expands, the volume of fish feed produced and transported must also increase. Therefore a better understanding and increased control of the factors that influence the physical quality of feed pellets is required. To achieve this goal, reliable methods measuring the physical quality of feed are needed. The Holmen pellet tester (NHP 100) could be adapted for this purpose, but the special characteristics of fish feed require particular care during testing. The first study showed that when the temperature and operating pressure are maintained at a standardized level, the Holmen pellet tester is a valuable tool for assessing the physical quality of fish feed providing additional information on the quality of feed pellets compared to other durability tests. In the second study, the effect of different levels of sunflower meal, starch and moisture on the stability of physical quality of feed during storage was investigated. After three weeks of storage, the durability of the diet with the elevated levels of all the variables has not changed significantly, while the others showed significant durability decrease. The potential reason for this situation can be a relationship between morphological structure and oil migration in the pellet, which can be restrained by changing diet composition. The oxidation was ruled out as a possible cause of this phenomenon. The third study investigated the effect of the composition of the feed, and the mode of handling, on the mechanical stability of both fresh and aged feed pellets. Intensive handling significantly decreased the durability of fresh pellets; however this decrease in durability was lessened by increasing the moisture content of the feed. In aged pellets, after three weeks of storage, the decrease in durability was more prominent for the feed handled intensively than for the feed handled gently. In the fourth study, the possibility of improving the physical quality of fish feed using protein crosslinking enzyme, transglutaminase, was tested. It was concluded that the transglutaminase treatment, when used at the appropriate dose and under the correct processing conditions, can improve the physical quality of feed pellets. The most important outcome of this work is the increased knowledge about the parameters that cause aging and the identification of tools to control this phenomenon. It also proved that in-line enzymatical modification of feed ingredients is possible and should be investigated further.