The Technology of Extrusion Cooking

Abstract

Extrusion cooking is a specialist area of food technology because of the complexity of the interactive effects which are inherent in the system. General predictive modelling is very difficult because ingredients are diverse and can vary considerably. Modelling tends to be product specific - new product development tends to be by experimental designs and good fortune. The emphasis of this book is on the latest and potential applications of twin screw extrusion in food production, specifically co-rotating inter- meshing screw extruders. Of course, in order to develop products and maximise the extruder potential in terms of energy, product quality and output, an overall understanding of the material flow mechanism, barrel fill length and rheology is essential. The book aims to give explanations and general guidance with examples of screw design, configuration and operat- ing parameters for a variety of product categories. It is also intended to help production operators diagnose the symptoms of particular problems such as temperature control, quality variation, raw material inconsistency, etc. For the product development technologist there is more than one way to make a similar product. For example, equipment manufacturers recom- mend difficult methods for producing flaked corn. In addition, their machines may differ from each other in terms of screw design, power/ volume ratio, screw tip/barrel clearance, etc., making scale-up more prob- lematic. Extruders generate mixing or shear forces in relatively short residence times which make them highly energy efficient in producing, at high outputs, the direct expanded products found in snack and RTE breakfast cereal categories. Unfortunately these shear forces can generate significant heat and physical degradation of ingredients which may be detrimental to quality parameters such as palatability, water absorption, texture or chemi- cal reaction. These factors are especially important to petfood and brewers' hop extrusion. The advantages and limitations of the twin screw extruder are clearly outlined for different categories and the examples provide the necessary information on the practicalities of setting up a new process or widening a current product range.