320 4 Carbohydrates
Fig. 4.29.Potato starch gelatinization curve (according
toBanksandMuir, 1980)
Fig. 4.30.Gelatinization properties of various starches
(according toBanksandMuir, 1980). Brabender visco-
amylograph. 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 de-
notes the largely irreversible transition from the
solubilized or highly swollen state to an insolu-
ble, shrunken, microcrystalline state (Fig. 4.31).
This state can also be directly achieved by slowly
cooling a starch paste. The tendency towards ret-
rogradation is enhanced at low temperatures, es-
Fig. 4.31.Behavior of amylose molecules during cool-
ing of a concentrated aqueous solutionpecially near 0◦C, neutral pH values, high con-
centration, 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 de-
scribed 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 con-
formational changes. At present, these changes
cannot be fully described because they greatly de-
pend 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 wa-
ter activity, which is lowered by the dissolved
substances (aw, cf. 0.3.1). Apart from the activ-
ity of the solvent water, if its volume fraction
(v 1 ), which changes in reverse order to the vol-
ume fraction of the solute, is considered and if
the gelatinization temperature is plotted against
ln aw/V 1 , instead of aw, the effect of the differ-
ent dissolved substances is unified (Fig. 4.32, bot-
tom). The reason is that polyhydroxy compounds
cause a large change in veand a small change in
aw, while a small change in veis linked to a large
change in awin the case of the salts.
Lipids also influence the properties of starch. Like
free amino acids, monoglycerides or fatty acid es-
ters of hydroxy acids, lipids form inclusion com-
pounds with amylose (cf. 4.4.4.14.3). Like di- and
triglycerides, they also reduce the swelling capac-
ity and solubility by inhibiting water diffusion.
Therefore, both degreasing as well as lipid add-
ition are of importance as physical modification
methods of starches.