Physical Characteristics of Food Extrudates - A Review

Abstract

Extrusion is a high temperature short time cooking technique used to produce variety of products from different food ingredients. Food extruders are generally available with segmented screws and barrel section, which facilitates total control over the configuration of the machine to get a combination of various process parameters. These make possible many functional changes and inactivation of undesirable factors present in the food. Optimization of protein denaturation/texturization, starch gelatinization and fat globules modification, etc., could be achieved by selective processing treatments to proteins, carbohydrates, and fats. Extrusion cooking is a high-pressure operation that provides sudden expansion of the processed products. The physical characteristics of the extrudate reflect the effectiveness of the process and suitability of ingredients. Though, food extrusion research is being conducted over 50 years, one finds lot of variation in the methods of evaluation of the properties as well as variation in sample size for determining them. Since methods of characterizing raw materials and evaluating extrudates are not standardized, the research results are not interpreted uniformly as well as repeatability of the results for scaling up becomes difficult. In this paper measurement techniques for the raw materials and extrudate properties have been critically reviewed. The feed and/or extrudate properties considered were: Particle Size Distribution, Moisture Content , Water Activity Densities, Pore Size, Expansion Indices, Water Absorption Index, Water Solubility Index, Oil Absorption Capacity, Textural properties, Microstructure, Color, Specific Mechanical Energy , Shear Rate, Foam Capacity, and Residence Time Distribution. Standardization of some of these methods through ASAE is proposed.