Agriculture Reference
In-Depth Information
7.3 Starch composition
Starch granules usually contain 10-20% moisture and small amounts of proteins,
lipids and traces of inorganic materials, in addition to the carbohydrate content.
7.3.1 Moisture
The moisture content of starch products depends on the relative humidity (RH) of the
atmosphere in which they have been stored. If this humidity decreases, the starches
will give up moisture; if the RH increases, they will absorb moisture. The equilibrium
moisture content of starch is also dependent on the type of starch product. Under
normal atmospheric conditions, most commercial native starches contain 10-20%
moisture. The equilibrium moisture content of all starches is low at a low RH of the
atmosphere. At an RH of zero, the moisture content of the starches approaches zero.
At a RH of 20%, the moisture content of all starches is about 5-6%.
7.3.2 Lipids (fatty substances)
Tuber (potato) and root (tapioca) starches contain only a very small percentage of
lipids (about 0.1%), compared with the common cereal starches (maize, wheat,
rice, sorghum), which contain 0.8-1.0% lipids. The fatty substances in the cereal
starches are predominantly free fatty acids (in maize and waxy maize starch) or
phospholipids (in wheat starch). The free fatty acids consist mainly of palmitic,
linoleic and oleic acids.
The presence of lipids in the common cereal starches has a profound effect on the
physical properties of these starches. The lipids exist as an amylose-lipid inclusion
complex in the granules. The linear fraction of the starch molecules (amylose) forms
helical clathrates with polar fatty substances such as the higher fatty acids. The
amylose-lipid complexes are insoluble, but dissociate when heated in water above
a given temperature. The dissociation temperature is indicative of the strength of
bonding and depends on the type of complexing agent. The amylose-lipid complexes
tend to repress the swelling and solubilisation of the cereal starch granules. Elevated
temperatures (above 125°C) are required to disrupt the organised native amylose-
lipid structure in the cereal starch granules and to solubilise the amylose fraction.
The presence of fatty substances can create problems in the use of maize and wheat
starch products because of the tendency to become rancid on storage.
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7.3.3 Proteins
References to protein in starches include macromolecular proteins, but also
indicate the peptides, amino acids, nucleic acids and enzymes that may be present
in the starch granules. Tuber (potato) and root (tapioca) starches contain only a
small amount of proteins (about 0.1%) compared with the cereal starches (maize,
wheat, waxy maize), which contain 0.2-0.4% proteins. Because of the residual
protein, the cereal starches may have a mealy fl avour and odour, and also a
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