Arendt und Zannini - 2013 - Cereal grains for the food and beverage industries

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Rice 129
In accordance with Mano et al. (1999), cereal lipids can be separated into
neutral lipids, glycolipids and phospholipids. Neutral lipids, principally triglycerides,
are evenly distributed amongst the different structural parts of
the grain even though the highest proportion is localized to the bran and
in the embryo. Conversly, glycolipids and phospholipids are concentrated
in the hulls and in the hulls/endosperm, respectively (Table 3.7).
Based on cellular distribution and association, rice lipids can be classified
into two basic fractions - non-starch lipids and starch lipids. Non-starch
lipids are stored in oil droplets called spherosomes, about 0.1-1 pm in size
(Bechtel and Pomeranz, 1978) and are distributed throughout the kernel
(Choudhury and Juliano, 1980). Choudhury and Juliano (1980) found that
the highest content of non-starch lipids in brown rice was concentrated in
the bran (39-41 %) followed by polish (15-21 %), embryo (14-18%), subaleurone
layer (12-14%) and the inner endosperm (12-19%). Starch
lipids typically comprise 0.5-1 % of the milled rice and include those lipids
(mainly monoacyl lipids, i.e. fatty acids and phospholipids) in complexes
with amylose in the starch granules of the rice kernels. Their content differs
between waxy and non-waxy varieties with the latter having more starch
lipids and less non-starch lipids in brown and milled rice than the former
(Choudhury and Juliano, 1980). The predominant fatty acids in rice starch
were linoleic (C18:2) and palmitic (C16:0) acids (Kitahara et al., 1997).
The existence of starch-lipid complexes has several consequences. For
example, complex development impacted on the formation of resistant
starch (RS) (Kitahara et al., 1996, 1997) and, additionally, yields of RS
(Mangala et al., 1999) were increased significantly by the removal of lipids
from rice starch. Guraya et al. (1997) reported that the amylose-lipid
complex formation with long-chain saturated monoglycerides generally
increases the resistance of the in vitro digestion over complexes with shorter
chains or more unsaturated monoglycerides. This is due to the increase of
the amylose-lipid structure stability. Amylose-lipid complexes also impact
pasting behaviour by decreasing the water-solubility in rice pastes cooked
for 30-90 min (Kaur and Singh, 2000) and increasing the pasting temperature
and peak viscosity, as determined using a Brookfield viscometer. Tocopherols
and tocotrienols, collectively called tocols or the vitamin E complex,
is an important group of nutrients associated with rice lipids due to their
non-polar solubility which will be discussed further in the following section.
3.3.3 Vitamins
Vitamins are nutritional components produced by plants. Cereal grains are
well known to be good sources of certain vitamins, particularly some of the
B-complex vitamins. The rice kernel contains little or no vitamin A, C or D.
Vitamin A deficiency (VAD) is prevalent among the poor in Asia, because
their diets are totally dependent on rice, which does not contain vitamin A