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Traditionally, moisture content (MC) is measured against the total
amount of dry matter. For potato-based snacks the dry matter is
partially crystalline, and will not absorb much water. Hence, the
thermodynamic driving force for expansion (water activity aw) is
a function of moisture content and the amount of crystalline starch
present. The degree of gelatinization must be viewed as a generalisation
of its usual definition, and is defined as the amount of
amorphous starch relative to the total amount of starch. In
potato-based snacks the amorphous starch can be in the form of
cooked potato-dehydrates, and pre-gelatinized starch gel formers.
The ratio of amylose/amylopectin will be discussed separately for
potato-dehydrates, as for the pre-gelatinized starch. Viscoelastic
properties of the expanding pellet will depend significantly on
the amount of gel former, which is added directly in the form of
pre-gelatinized starch – but may also originate from potato-dehydrates
in the form of leached amylose. Hence, also the pre-processing
of potato-dehydrates is of importance to the final structure of
potato-based snacks, and this will be discussed in a separate section.
The potato-cells, and swollen starch granules will act as filler
particles in the viscoelastic dough, and will influence these properties.
Viscoelastic properties will also be influenced by the amount
of starch degradation in the mixing phase. If the mixing is
performed via extrusion at relatively high energy input, the starch
molecules are broken down (degraded). So, the pre-processing of
the dough will also be discussed separately, prior to the final intensive
heating step. The snack formulation may contain various
minor components, like salt, maltodextrins, fibres, surfactants
and oil. Solutes like salt and maltodextrins will influence the thermodynamic
driving force, while the other minor components influence
more the visco-elastic properties of the pellet. These aspects
will be discussed in detail, when we address the final heating step
inducing the expansion. As a summary, we have indicated the link
of the material properties of the snack (as listed above) with the
physical phenomenon involved in the expansion in Fig. 2.
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