Agriculture Reference
In-Depth Information
and made it a desirable feed ingredient. Wheat can account for up to 70% of the
metabolizable energy requirements of broilers and turkeys and as much as 80% of that of the
laying hens. In addition, wheat may contribute up to 35% of the protein in broiler diets when
included at high levels.
Wheat is highly diverse with different types of grain. The major factors used to
distinguish between types of wheat are hardness and softness of the grain, whether they are
spring or winter wheats, what colour bran they produce (red or white) and protein content.
For example in the US, six distinct classes are grown: Durum, Hard Red Spring, Hard Red
Winter, Hard White, Soft Red Winter and Soft White. In Canada, eight classes are grown and
within each of these classes, there are a number of species. In terms of feed processing and
feeding value, whether a grain is hard or soft is of the greatest importance.
P
HYSICAL
S
TRUCTURE OF
G
RAIN
A mature, whole grain of wheat comprises 5% pericarp, 8% aleurone layer, 2% embryo
and 85% endosperm.
The pericarp consists mainly of cellulose and other polysaccharides,
mainly hemicelluloses and pentosans (Lasztity, 1999). The aleurone layer, which is devoid of
starch, is made up of equal amounts of oil, mineral matter and protein (Kent, and Evers,
1994). In comparison to other parts of the kernel, this layer is high in protein, ash, phosphorus
and vitamins (especially niacin). It acts as a major store of phosphate and macronutrient
mineral elements within the cereal grain. Phytate, a potassium, magnesium and calcium salt
of
myo-inositol
hexaphosphoric acid, is the storage form of phosphorus. The endosperm
consists mainly of starch granules that are surrounded by protein matrix.
C
HEMICAL
C
OMPOSITION
Starch
Starch is the most abundant carbohydrate and the main energy-yielding component in
wheat. The starch content varies from 60 to 75% and is inversely related to the protein
content of wheat. Starch in wheat occurs as discrete particles or granules within the cells of
the endosperm. These granules are relatively densely packed and are insoluble. Three types of
starch granules are present in the endosperm, namely, large (10-50 µm) lenticular shape (type
A), medium (8-11µm) spherical shape (type B) and small starch granules with a maximum
diameter of 5.3 µm (type C). In the mature grain, type A granules form 51.6%, type B 45%
and type C 3.4% of the total mass of starch.
Table 1. The types and level of NSP present in wheat (g/ 100 g dry matter)
1
Arabinoxylan β-Glucan Cellulose Total NSP
Soluble 1.7 0.4 0 2.3
Insoluble 6.6 0.5 2.3 9.8
1
Mean of six wheat cultivars (Ravindran, G. and Ravindran, V., unpublished data).
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