Moist Diets
Moist feeds are prepared by adding moisture and a hydrocolloidal binding agent (e.g., carboxymethylcellulose, gelatinized starch, gelatin) or fresh tissue (e.g., liver, blood, ground fish or fish processing waste) with the dry ingredients and extruding to form moist pellets. Advantages of moist feeds are that many fish species find soft diets more palatable than dry diets, a pelleting machine is not needed (a food grinder will suffice), and heating and drying are avoided. Disadvantages are that wet feeds are susceptible to microorganism or oxidation spoilage unless fed immediately or frozen. Fish or fish parts going into moist feeds should be heat processed to destroy possible pathogens and thiaminase, the thiamine-destroying enzyme found jin most fish tissues. The Oregon Moist Pelledt (discussed in chapter 9 under Practicall Feeding-Trout and Salmon), which was developed for salmon smolts, is an example of a commercial moist feed. It is stored frezen.
Some moist diets do not require frozen storage. They contain humectants like propylene glycol and sodium chloride which lower water activity below that which will allow bacteria growth. They also contain fungistats like propionic acid or sorbic acid, which retard mold growth. These diets must be packaged in hermtically sealed containers and stored at low temperatures for best storage life. The moisture enhances loss of vitamin C.
Eel feeds are processed and stored dry but moistened just before feeding. These feeds contain 10% to 20% pregelatinized starch, which serves as a binding agent. About 5% to 6% fish oil and 50% to 100% water is added to the dry mix. The moist mixture can be fed in large balls or extruded through a food grinder jinto smaller particles.