2009_Book_FoodChemistry

320 4 Carbohydrates
Fig. 4.31. Behavior of amylose molecules during cooling
of a concentrated aqueous solution
Fig. 4.29. Potato starch gelatinization curve (according
to Banks and Muir, 1980)
Fig. 4.30. Gelatinization properties of various starches
(according to Banks and Muir, 1980). Brabender viscoamylograph.
40 g starch/460 ml water, temperature
programming: start at 50 ◦ C, heated to 95 ◦ Catarate
of 1.5 ◦ C/min. Held at 95 ◦ C for 30 min — potato, - - -
waxy corn, −−− normal corn, and ••• amylomaize
starch
the amylose goes into solution. The viscosity
of a starch paste generally increases on rapid
cooling with mixing, while a starch gel is formed
on rapid cooling without mixing.
Amylose gels tend to retrograde. This term denotes
the largely irreversible transition from the
solubilized or highly swollen state to an insoluble,
shrunken, microcrystalline state (Fig. 4.31).
This state can also be directly achieved by slowly
cooling a starch paste. The tendency towards retrogradation
is enhanced at low temperatures, es-
pecially near 0 ◦ C, neutral pH values, high concentration,
and by the absence of surface active
agents. It also depends on the molecular weight
and on the type of starch, e. g., it increases in the
series potato < corn < wheat. The transitions described
from very water-deficient starting states
via very highly swollen states or solutions to more
or less shrunken states are linked to changes in
the interactions between the glucans and to conformational
changes. At present, these changes
cannot be fully described because they greatly depend
on the conditions in each case, e. g., even on
the presence of low molecular compounds.
It is known that the gelatinization temperature is
increased by polyhydroxy compounds (glycerol,
sugar) and decreased by salts (NaCl, CaCl 2 ), as
presented in Fig. 4.32 (top) as a function of water
activity, which is lowered by the dissolved
substances (a w , cf. 0.3.1). Apart from the activity
of the solvent water, if its volume fraction
(v 1 ), which changes in reverse order to the volume
fraction of the solute, is considered and if
the gelatinization temperature is plotted against
ln a w /V 1 , instead of a w , the effect of the different
dissolved substances is unified (Fig. 4.32, bottom).
The reason is that polyhydroxy compounds
cause a large change in v e and a small change in
a w , while a small change in v e is linked to a large
change in a w in the case of the salts.
Lipids also influence the properties of starch. Like
free amino acids, monoglycerides or fatty acid esters
of hydroxy acids, lipids form inclusion compounds
with amylose (cf. 4.4.4.14.3). Like di- and
triglycerides, they also reduce the swelling capacity
and solubility by inhibiting water diffusion.
Therefore, both degreasing as well as lipid addition
are of importance as physical modification
methods of starches.