6.6 STARCH
In this section a particular food polymer will be discussed in some detail.
Starch is a very important nutrient, providing roughly half of the edible
energy used by mankind. Moreover, large quantities are isolated and used as
functional ingredients, starch being the most important thickening agent
applied in manufacture and preparation of food.
In its native form, starch occurs as a very concentrated polymer mass,
and discussion of its properties may provide some understanding of such
systems in general. On the other hand, native starch granules provide an
example of a highly specific structure. Another example of a concentrated
system of polymers is the cell wall of plants. A cell wall contains several
different polysaccharides, and its structure is even far more intricate and
specific than that of starch. It will not be discussed here.
6.6.1 Description
Starch is a polymer ofa-D-glucose. It occurs in most higher plants species,
in the form of roughly spherical granules, ranging from 2 to 100mmin
diameter. The granules consist of about 77%starch and about 1%other dry
matter (lipids, proteins, minerals), the remainder of the mass being water.
Starch granules often are surrounded by large quantities of water, but they
do not dissolve, in accordance with their physiological function. Starch
provides energy when needed, which may be a long time after its synthesis,
for example during the germination of a seed. It is then hydrolyzed by
enzymes (amylases) to yield sugars, mostly maltose, that are well soluble and
can be metabolized.
Starch consists of two components. The one isamylose, a linear
polymer of 1!4 linked anhydroglucose monomers; see Figure 6.18. The
degree of polymerizationn ranges from several hundred to about 10^4.
Amylose mostly makes up 20 to 30%of starch. Amylose in solution readily
forms helices of a pitch of about 6 monomers and a diameter of about 1 nm.
These helices can accommodate hydrophobic molecules in their central
cavity, especially fatty acid chains; fairly stable compounds are formed.
Iodine can be included in a similar way, and the compound has an intense
blue color, which allows identification of amylose and determination of its
concentration.
Amylopectin, the main component, hasn¼ 105 –10^8 , but it should be
realized that the size of very large polymer moleculesðn> 106 Þis always
uncertain, since the solubilization procedure needed for determination
readily causes breaking of very long molecular chains. Amylopectin is highly
branched, 4–5%of the monomers also having a 1!6 linkage. Its structure