Milling and extrusion characteristics of New Zealand corn : development of a hardness test and an on-line extruder viscometer : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University
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Ready to Eat (RTE) snack foods are commonly manufactured using single and twin screw extruders and com grits as raw materials. Variations in product quality caused by grits from different hybrids and grain hardness have not been investigated. Furthermore, the relationship between rheological properties of the extrudate melt and the operating conditions in the extruder is not fully understood. Appropriate methods to determine com grain hardness to characterise com hybrids and the on-line viscosity of the extrudate melts have not yet been developed. These methods could provide sound and appropriate techniques to investigate the areas of milling and extrusion of com based food products. In this study, milling characteristics of 38 com hybrids from the 92-93 season and 12 com hybrids from the 94-95 season produced in New Zealand were studied. A modified Stenvert Hardness Test (SHT) using new parameters including milling energy and milling resistance time was developed. It was found that the modified SHT was simple and easy to use with low variability. The SHT milling energy can be used as an effective hardness index. It increased with grain bulk density and the ratio of hard to soft endosperm. All measured properties were highly dependent on the moisture content. For the same hybrid, SHT hardness increased and the grain bulk density decreased when the moisture content of the com grains increased. A roller-milling test was also developed to study the dry milling characteristics of these com hybrids. During milling, the breaking force measured in the roller-milling test increased with grain hardness. Analysis of particle size distributions in the ground samples after the modified Stenvert Hardness tests and the roller-milling tests showed that grit recovery rate increased with the grain hardness. Grits produced from hybrids harvested in the 92-93 and the 94-95 seasons, along with other grits and starches commercially manufactured in New Zealand, were used for the extrusion experiments. A new Slit-Die-Viscometer (SDV) was developed to measure the viscosity of extrudate melts on-line. Unlike many other viscometers used on-line, the operation of the new SDV did not interfere with the operating conditions of the extruder. The rheological properties and the degree of starch gelatinisation were affected by the operating conditions of the extruder and the characteristics of the raw materials: It was found that the melt viscosity decreased as moisture content increased. The apparent viscosity had a maximum value at barrel temperature of about 130DC, changed very little when screw speed increased at constant feed, and decreased slightly when the feed increased at constant screw speed. The grits were less gelatinised at high moisture content. The degree of starch gelatinisation increased slightly with screw speed and linearly with barrel temperature between 90DC and 130DC. At barrel temperatures higher than 130DC, the extrudate was almost fully gelatinised. Melts produced with starch of high amylopectin content had an overall lower viscosity with less shear thinning and a higher degree of starch gelatinisation than that produced with starch of high amylose content. Grit size affected the rheological properties and the degree of starch gelatinisation. Melts produced from medium and coarse grits had a lower viscosity and a lower degree of starch gelatinisation than that produced with fine grits. The effect of different hybrids of the same season on the rheological properties of the melt was negligible. However, the rheological properties were affected by the methods used to produce the grits. Grits from degermed grains had less oil and produced melts with lower viscosity and less shear thinning than grits from whole grains (higher oil content).
Snack foods, Corn properties, Corn testing