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 manuscript an overview is given on the binding mechanisms in pelleted animal 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 subsequent 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, water and particles to maintain structural integrity of the pellet. Soluble components might be introduced in the feed mash subjected to pelleting, They are either incorporated in the mixing phase or result from processing as a function of processing variables during the subsequent stages, conditioning, pelleting and cooling/drying, of the feed manufacturing process. To evaluate the physical quality of pelleted feeds, generally a subdivision is made into tests that evaluate 'hardness' and tests evaluating 'durability' of a given pellet. Several devices measuring fragmentation strength, and devices determining abrasion strength of pellets are discussed. It is concluded that some tests presently available evaluate a mixture of hardness (fragmentation) and durability (abrasion) effects, The feed manufacturer or feed technologist should be aware of the reason for evaluating the pellet quality and subsequently choose the appropriate, most suitable method, since no unique test exists that covers all parameters of interest related to physical quality of pelleted animal feeds.