Expansion of biopolymer matrices is the basis for the production of a wide variety of cereal foods. A limited number of manufacturing processes provide practical solutions for the development of an impressive variety of expanded products, just by changing process variables. It is therefore essential that the mechanisms involved in expansion are well known and controlled. This paper summarizes the knowledge of nucleation and expansion in extruded and microwaved products available to date. The effect of processing conditions and properties of the biopolymeric matrix on nucleation and expansion are discussed. Moisture content enables the glassy polymeric matrix to turn into rubbery state at process temperatures, which allows superheated steam bubbles to form at nuclei and then expand, expansion being governed by the biaxial extensional viscosity of the matrix. Nucleation and expansion theories are presented along with quantitative data that support them.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1541-4337.2003.tb00020.x

Nucleation and Expansion During Extrusion and Microwave Heating of Cereal Foods

Corn starch extruded at 32.9, 29.2, 24.5, 22.5, 15.9 and 14.2% moisture was analyzed for expansion ratio, enzyme susceptibility, water solubility index, water absorption index, degree of gelatinization, paste viscosity and heat of gelatinization. A model system based on the combination of raw, gelatinized and dextrinized starch was used to represent the physicochemical properties of the extrudates. Reducing extrusion moisture content resulted in a progressive change from gelatinized-like to dextrinized-like properties. Maximum gelatinization was observed at about 28-29% moisture. Below 20% moisture, dextrinization becomes predominant during high- shear cooking-extrusion. Scanning electron micrographs also validate the assumptions of the model.

A Physicochemical Model for Extrusion of Corn Starch

Effect of inlet temperature on physicochemical properties of spray-dried jamun fruit juice powder

Normal corn starch was extrusion-cooked in a C. W. Brabender Laboratory model single-screw extruder at various extruder barrel temperatures, screw speeds, feed rates, and moisture contents. The expansion ratios and shear strengths of the products were highly dependent on extrusion-cooking conditions. The maximum expansion ratio of 16.1 was obtained when 14% moisture content (d.b.) starch was fed at a rate of 60g min-1 with a screw speed of 150 rpm, a barrel temperature of 140°C and a die-nozzle L/D ratio of 3.1. The shear strengths of the extruded starch products were inversely proportional to the expansion ratios.

Optimum Extrusion-Cooking Conditions for Maximum Expansion of Corn Starch

Effects of extrusion temperature and feed composition on the functional, physical and sensory properties of chestnut and rice flour-based snack-like products

Whole ground corn was extruded at 23.7, 18.5, 15.4,13.9 and 7.6% moisture contents (EMC). Decreasing EMC resulted in increases in water solubility index (WSI), enzyme susceptibility (ES), degree of gelatinization and blue values, while water absorption index and water insoluble carbohydrates decreased. ES and WSI of several blends prepared by combining raw (R), gelatinized (G), and dextrinized (D) corn were compared to those of extruded products. Corn extrudates had properties similar to blends containing G and D corn only. The relative proportion of D corn increased from about 10 to 60%, as EMC decreased. “Dextrinization” appears to become the predominant mechanism of starch degradation during low-moisture, high-shear extrusion. Viscoamylographs, scanning electron and light photomicrographs support these findings.

M. H. Gomez

Papers

Changes in the Starch Fraction During Extrusion-cooking of Corn

A Physicochemical Model for Extrusion of Corn Starch