PHYSICAL QUALITY OF PELLETED ANIMAL FEED .1. CRITERIA FOR PELLET QUALITY

In a series of three articles, the physical quality of pelleted animal feeds is discussed from an engineer's point of view. In this first ma nuscript an overview is given on the binding mechanisms in pelleted an imal feeds. Principles and methods for evaluation of physical quality of pelleted animal feeds are reviewed with respect to pellet hardness and durability. Methods are outlined with respect to quality standards both from a pragmatical and scientific point of view. It is concluded that binding in pellets most probably is due to solubilisation and su bsequent crystallisation of feedstuff components e.g. starch, sugars, fats or 'liquid necking', Liquid necking is a binding mechanism which uses the surface tension of water, in a three-phase system of air, wat er and particles to maintain structural integrity of the pellet. Solub le components might be introduced in the feed mash subjected to pellet ing, They are either incorporated in the mixing phase or result from p rocessing as a function of processing variables during the subsequent stages, conditioning, pelleting and cooling/drying, of the feed manufa cturing process. To evaluate the physical quality of pelleted feeds, g enerally a subdivision is made into tests that evaluate 'hardness' and tests evaluating 'durability' of a given pellet. Several devices meas uring fragmentation strength, and devices determining abrasion strengt h of pellets are discussed. It is concluded that some tests presently available evaluate a mixture of hardness (fragmentation) and durabilit y (abrasion) effects, The feed manufacturer or feed technologist shoul d be aware of the reason for evaluating the pellet quality and subsequ ently choose the appropriate, most suitable method, since no unique te st exists that covers all parameters of interest related to physical q uality of pelleted animal feeds.