2009_Book_FoodChemistry

710 15 Cereals and Cereal Products
The commercial product semolina (“griess”)
is made from endosperm cells of hard durum
wheats. Semolina keeps its integrity during
cooking and is used mostly for pasta production.
Since semolina is a milled flour of low
extraction rate, it contains few minerals and
vitamins.
Table 15.38. Vitamin content of raw, white and parboiled
rice
B-vitamins (mg/kg)
Thiamine Riboflavin Niacin
Raw rice 3.4 0.55 54.1
White rice 0.5 0.19 16.4
Parboiled rice 2.5 0.38 32.2
15.3.2 Other Cereals
15.3.2.1 Corn
Corn endosperm, with the germ removed, is
ground to grist for corn porridge (Polenta) and
into corn flour for flat cakes (tortillas). Corn
flakes are made from cooked and sweetened corn
slurry, by drying, flaking and toasting. Similar
products are made from millet, rice and oats.
15.3.2.2 Hull Cereals
Dehulling of rice, oats and barley requires special
processes (cf. 15.1.4).
15.3.2.2.1 Rice
Rice milling involves the following processing
steps: rough rice (paddy rice) → hull removal →
brown rice → polishing to remove the bran coats
(fruit and seed coats), the silvery cuticle, the germ
and the aleurone layer → rubbing-off or rice polishing
to obtain the end-product, white rice. Undamaged
rice (45–55%), broken kernels or flour
(20–35%) and a husk/hull fraction (20–24%) are
obtained.
Polished white rice is made from this cleaned rice
by additional treatment of the kernels with talc
(a magnesium silicate) and 50% glucose solution.
This imparts a glossy, transparent coating to the
kernels.
White rice, in comparison to rough or brown rice,
is low in vitamin content (cf. Table 15.38) and in
minerals. A nutritionally improved product may
be obtained by a parboiling process, orginally
developed to facilitate seed coat removal.
About 25% of the world’s rice harvest is treated
by the following process: raw rice → steeping in
hot water, steaming in autoclaves, followed by
drying and polishing → parboiled rice.
This treatment causes the following changes:
the starch gelatinizes, but partly retrogrades
again during drying. Enzymes are inactivated
by the heat, causing inhibition of the enzymatic
hydrolysis of lipids during storage of rice. The
oil droplets (cf. 3.3.1.5) are broken and lipids
partly migrate from the endosperm to the outer
layers of the rice kernels. Since antioxidants
are simultaneously destroyed, parboiled rice
is more susceptible to lipid peroxidation. In
contrast, minerals and vitamins diffuse from the
outer layers to the inner endosperm and remain
there after the separation of the aleurone layer
(Table 15.38). The changes in starch mentioned
above result in reduced cooking time.
Unlike in Europe and USA, some rice varieties
popular in Asia develop a popcorn-like aroma on
cooking. This is due to the formation of 2-acetyl-
1-pyrroline, which is present in concentrations
of 550–750 µg/kg in aromatic varieties of rice
(cooked) and <8µg/kg in lowaroma varieties.
15.3.2.2.2 Oats
Oat flakes are produced by the following processing
steps: the kernels (12–16% water content)
are steamed and then the moisture content
is decreased to 7–10% in 2–3 h by heating
at 90–100 ◦ C. The hull (fruit and seed coats)
is removed, i. e., the kernel is polished. This
is followed by repeated steaming, squeezing
between drum rollers, and drying of the moist
flakes till the water content is 10–11%. The yield
is 55–65%. This hydrothermic process also
inactivates the oat enzymes involved in offflavor
development. (E,E,Z)-2,4,6-Nonadienal produces
the cereal-like, sweet aroma of oat flakes. It has