School of Graduate Study Department of Chemical Engineering

2.4.2 Role of major food components during extrusion cooking
During extrusion cooking, raw materials undergo many chemical and structural
transformations that lead to a variety of unique products. Chemical changes that occur
includes conversion (gelatinization and melting) of starch, denaturation and cross liking
of proteins, complex formation between amylose and lipids, maillard reactions in the
presence of sugars, and degradation reactions of polymers and other molecules (Llo et al.,
2000).
The structure of extruded products is created by forming a melt fluid from biopolymers
and blowing bubbles of water vapour in to the fluid to form foam. The film of
biopolymers must flow easily in the bubble walls to allow the bubbles to expand as the
superheated water is released very quickly at atmospheric pressure (Guy, 2001).
Starch
Starch polymers are very good at the above mentioned function and well expanded
cellular structure showing a continuous phase can be made from materials such as wheat,
maize, rice or potato.
Under the condition of extrusion cooking (high temperature, pressure and shear forces)
starch granules are disrupted and melted at low moisture content or swell and gelatinized
at high moisture (Harper, 1992).
Protein
Proteins may be used to form structures in extrudates at high concentration in the recipe
that is >40%w/w at moisture level of 30-40%w/w. They are globular proteins
significantly smaller than starch linked together to form larger structures as they flow
through a die channel.
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