shaped granules are still observed, but they should not be
envisaged as having a wall or membrane: the amylopectin is
held together by covalent bonds and by entanglements. The
swelling causes a very great increase in apparent viscosity,
especially at fairly high starch content, since then the volume
will be fully taken up by swollen granules. The extent of swelling
greatly varies among starch types, as illustrated in Figure 6.24c.
For potato starch, even far stronger swelling has been reported, up
to 1 kg of water per g of starch. This will only occur at very low
ionic strength; the phosphate groups present will be ionized
(unless the pH is low), causing considerable electric repulsion
between chains (cf. Section 6.2.2).
- At still higher temperature, the swollengranules are broken upinto
far smaller fragments, especially during vigorous stirring. This
may be partly due to disentangling, partly to breaking of
amylopectin molecules. It causes the apparent viscosity to
decrease again; the explanation is disputed.
The various stages may overlap, and the amount of swelling in stage 2
as compared to that in stage 3 markedly varies among starch types; compare
wheat and maize starch in Figure 6.24. If starch granules have been
damaged (broken), as for instance during the grinding of wheat, some water
may be taken up at earlier stages. Gelatinization depends on several
conditions: amount of water present, temperature regime, intensity of
stirring, presence of other substances. For example, high concentrations of
some sugars cause the melting temperature to increase. If the pH is low, say
<3, heating may cause marked hydrolysis of the starch, leading to a far
lower apparent viscosity.
In practice, gelatinization is applied in fairly dilute solutions to achieve
binding(thickening), as in soups or gravies. Actually, gelatinized starch is
not a very efficient thickening agent; the concentration needed to obtain a
certain increase in viscosity is about 10 times higher than for several other
polysaccharides. The main reason is the highly branched character of
amylopectin, giving rise to a small value (about 0.25) for the exponent in the
Mark–Houwink equation (6.5). During the cooking of vegetables or the
baking of bread, gelatinization occurs as well, but stage 4 is generally not
reached, partly because the starch solution is not stirred. Moreover, swelling
may be restricted by the amount of water available. In bread, the ratio of
water to starch is about unity, implying that the granules cannot nearly
attain full swelling; nevertheless, they have lost most of their crystallinity. In
even dryer products, as in some biscuits, stage 2 is not reached and the
granules are much like native ones.
