RA 00110.pdf - OAR@ICRISAT

properties for pearl millets. It is based on general
information and must be refined as progress occurs
in evaluating quality of pearl millet for specific products.
The production of fermented or low-pH products
apparently produces lighter-colored products
because the pearl millet pigments of some cultivars
are converted to colorless forms by acid.
Factors Affecting Food Quality
Kernel Structure
The kernel of pearl millet is a caryopsis similar in
structural components to sorghum (Badi et al. 1976,
Sullins and Rooney 1977, Zelaznek and Varriano-
Marston 1982). Kernel shape, size, and appearance
(color) vary significantly among pearl millet varieties,
and within a sample, kernels vary significantly
in size and shape. A drawing of the pearl millet
caryopsis is presented in Figure 8a to enable readers
to interpret the photomicrographs that follow. Scanning
electron photomicrographs (Figs. 8b, 9a and
9b) show the structure of the pericarp, seed coat or
testa, the aleurone cells, and the starchy endosperm.
The endosperm contains starch granules surrounded
by a protein matrix containing protein bodies. The
center of the kernel has a soft floury endosperm
surrounded by a flinty or translucent endosperm.
The proportion of floury to hard endosperm varies
among varieties, and among kernels within a variety.
The protein content of the endosperm within a kernel
decreases gradually from the aleurone layer to
the starchy endosperm.
The pericarp is composed of three layers of tissue:
the epicarp, mesocarp, and endocarp (Figs. 8a, 8b,
9a, 9b, and 10). The term bran refers to the pericarp,
seed coat, and aleurone layers of the kernel. The
outer layer (epicarp) has 1-2 cell layers of thickwalled,
blocky cells that contain concentric layers of
cell tissue surrounding pigments in the center (Figs.
10a, 10b, and 10c). There is a thin, waxy cutin layer
on the outer surface of the kernel that could, in
combination with the epicarp layer, help decrease
the effects of weathering on the kernel by acting as a
barrier between the environment and the internal
portions of the kernel (Sullins and Rooney 1977).
The mesocarp layer varies in thickness and is
composed of several tiers of collapsed cell walls
(Figs. 9a and 9b). This layer has no apparent function
in the mature seed, but the variability of its
thickness determines whether the cultivar is classified
as a thin or thick pericarp variety (Sullins and
Rooney 1977). The thick pericarp is preferred by
those who use the traditional mortar and pestle
method of milling because the pericarp flakes off the
kernel more easily (Kante et al. 1984, Scheuring et al.
1983).
The endocarp, located beneath the mesocarp, is
composed of two types of cells: cross cells and tube
cells (Fig. 10d). The cross cells are arranged longitudinally
across the kernel and the tube cells lie
perpendicular to the cross cells. The endocarp layer
probably functions in the transport of water and
nutrients around the kernel. During decortication,
the pericarp splits away from the kernel beneath the
aleurone layer (DeFrancisco et al. 1982) or the endocarp
(Sullins and Rooney 1977, McDonough 1986).
A partial or total seed coat is present in all varieties,
the partial seed coat predominates in the slatecolored
varieties (McDonough 1986) (Figs. lOd and
10e). The seed coat is pigmented, but it is also very
thin (0.4 ). It can contribute to the overall color
perceived in kernels with a thin colorless pericarp
(Rachie and Majmudar 1980).
The aleurone is one layer thick, with uniform cell
sizes and variable cell walls (Figs. lOd and lOe). The
cells range from 16-30 m long and 14-33 m wide.
Fluorescence microscopy reveals that aleurone cells
contain a large amount of protein and lipid bodies
(McDonough 1986). Many cereals contain phytin
(phosphorus) and nicotinic acid in the aleurone, but
pearl millet appears to contain these materials only
in the germ. The aleurone cells of some cultivars
contain pigments that can produce unacceptable
color in food products (McDonough and Rooney
1984, Rachie and Majmudar 1980).
The starchy endosperm is the part of the kernel
that comprises the bulk of the flour (Fig. 11). The
endosperm contains simple starch granules and protein
in the form of matrix and bodies. Three distinct
endosperm areas are visible from the outside to the
inside of the kernel:
• the peripheral region that contains a large amount
of protein bodies and matrix, surrounding small
starch granules (Figs. 10f and 11a),
• the corneous area with large, uniform-sized,
polygonal starch granules embedded in protein
matrix with a small amount of protein bodies
(Fig. Mb), and
• the floury endosperm area with large, round,
starch granules, loosely packed in a small amount
of thin protein matrix and a small number of
protein bodies (Fig. 11c) (McDonough 1986).
The flour fraction is composed of the free starch
granules and protein released from the floury
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