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
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
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
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