2009_Book_FoodChemistry

4.4 Polysaccharides 327
adipic or citric acids, are obtained in reactions
with the reactive derivatives (e. g., vinyl acetate)
or by heating the starch with free acids or their
salts. The thickening and paste clarity properties
of the esterified starch are better than in the corresponding
native starch.
In addition, esterified starch has an improved
freeze–thaw stability. These starches are utilized
as thickeners and stabilizers in bakery products,
soup powders, sauces, puddings, refrigerated
food, heat-sterilized canned food and in margarines.
The starch esters are also suitable as
protective coatings, e. g., for dehydrated fruits
or for aroma trapping or encapsulation (cf.
Table 4.26).
4.4.4.15.8 Cross-Linked Starches
Cross-linked starches are obtained by the reaction
of starch (R OH) with bi- or polyfunctional
reagents, such as sodium trimetaphosphate, phosphorus
oxychloride, epichlorohydrin or mixed anhydrides
of acetic and dicarboxylic acids (e. g.,
adipic acid):
(4.154)
Fig. 4.38. Corn starch viscosity curves as affected
by crosslinking degree. Instruments: Brabender amylograph;
a control, b crosslinked with 0.05%, c 0.10%,
d 0.15% epichlorohydrin (according to Pigman, 1970)
4.4.4.15.9 Oxidized Starches
Starch hydrolysis and oxidation occur when aqueous
starch suspensions are treated with sodium
hypochlorite at a temperature below the starch
gelatinization temperature range. The products
obtained have an average of one carboxyl group
per 25–50 glucose residues:
(4.157)
Oxidized starch is used as a lower-viscosity filler
for salad dressings and mayonnaise. Unlike thinboiling
starch, oxidized starch does not retrograde
nor does it set to an opaque gel.
(4.155)
(4.156)
The starch granule gelatinization temperature
increases in proportion to the extent of crosslinking,
while the swelling power decreases
(Fig. 4.38). Starch stability remains high at
extreme pH values (as in the presence of food
acids) and under conditions of shear force.
Cross-linked starch derivatives are generally used
when high starch stability is demanded.
4.4.4.16 Cellulose
4.4.4.16.1 Occurrence, Isolation
Cellulose is the main constituent of plant cell
walls, where it usually occurs together with hemicelluloses,
pectin and lignin. Since cellulase enzymes
are absent in the human digestive tract, cellulose,
together with some other inert polysaccharides,
constitute the indigestible carbohydrate of
plant food (vegetables, fruits or cereals), referred
to as dietary fiber. Cellulases are also absent in
the digestive tract of animals, but herbivorous an-