Extrusion cooking : craft or science?

It is not longer ago than the seventies when a food extruder was described as a piece of equipment, consisting out of a barrel and one or two screws. The function of the extruder was believed to receive an agricultural product, to transport it from a feed section to a screw metering section over a compression section. With the help of shear energy, exerted by the rotating screw, and additional heating by the barrel, the food material is heated to its melting point or plasticating-point. In this changed rheological status the food is conveyed under high pressure through a die or a series of dies and the product expands to its final shape.It is obvious that this more or less phenomenological definition is simple and not complete, since some important properties of the extruder as a food and feed unit operation are not yet mentioned. In the first place one should recognize the cooking extruder to belong to the family of HTST (High Temperature Short Time)-equipment, with a capability to perform cooking tasks under high pressure (van Zuilichem et al. 1975; van Zuilichem et al. 1976). This aspect may be explained for vulnerable food and feed as an advantageous process since small time span exposures to high temperatures will restrict unwanted denaturation effects on e.g. proteins, aminoacids, vitamins, starches and enzymes. However, one should realize that now physical technological aspects like heat transfer, mass transfer, momentum transfer, residence time and residence time distribution have a strong impact on the food and feed properties during extrusion cooking and can drastically influence the final product quality. secondly we should realize that a cooking extruder is a process reactor (van Zuilichem et al. 1979; Janssen et al. 1980), in which the designer has created the prerequisites in the presence of a certain screw lay-out, the use of mixing elements, the clearances in the gaps, the installed motor power and barrel heating and cooking capacity, to control a food and feed reaction. This can be a reason "in itself", when only mass is transferred in wanted and unwanted reaction products due to heating, e.g. the denaturation of proteins under presence of water and the rupture of starches, both affected by the combined effects of heat and shear. The reaction can also be provoked by the presence of a distinct biochemical or chemical component like an enzyme or a pH controlling agent. When we consider the cooking extruder to be more than was mentioned originally, a thorough investigation of the different physical technological aspects is more than desirable. This is the motive for the set up of the chapters 1 to 7 and the objective for this study. In cooking extrusion, it is obvious that the study concerns the residence time behaviour, which is done in the chapters 2 and 3 for single and twin screw extruder equipment. The transfer of heat and mass during extrusion-cooking is reported in the chapters 4 and 5. The effect of equipment design on reactor performance has always been very strong. As an example of the outcome of a certain important design feature on the degree of fill in a twin-screw extruder and the resulting unavoidable residence time distribution effects is reported in chapter 6. Finally the capability of an extruder to replace partly a conventional bioreactor, is discussed in chapter 7, in which a process is developed for starch disclosure in a cooking extruder, followed by a saccharification process in a conventional piece of equipment, leading to a calculable process with unique features in bioconversion. The above mentionedaspects are explained further in the paragraphs 1.2 to 1.7.