Cereals processing technology

102 Cereals processing technology
Ready-to-eat rice breakfast cereals are being made in increasing amounts by
extruding super-heated and pressured doughs through an orifice into the
atmosphere. The sudden expansion of water vapor in the extrudate as the excess
pressure is released results in a volume increase of several times. The process
has several advantages such as high and continuous production rates, greater
versatility in product shape, and easier control of product density.
The rice flour mix containing a 60–75% expandable starch base is moistened
with water or steam and equilibrated to ensure a uniform supply of extrusion
material. The resultant mash is compacted by a screw revolving inside a barrel,
which may be heated by steam. The thread of the screw has a progressively
closer pitch as it approaches discharge. In some extruder designs, the rice premix
is fed directly into the extruder. The water and/or steam are injected into the
barrel and mixed with the premix. The pressurizing, sharing, and steam heating
bring the dough to a temperature of about 150–175ºC and a pressure of 5–
10 MPa at the die end. Under these conditions, the dough is quite flexible and
easily adapts to complex orifice configurations. The dough pieces expand very
rapidly as they leave the dice orifice, and the expansion may continue for a few
seconds since the dough is hot and still flexible and water continues to boil off.
The moisture content of pieces is in the order of 10–15% and is too high for
satisfactory crispness. Thus, the pieces are dried on vibrating screens in hot-air
ovens to a final moisture content of 3–4%. The product may be coated with
sugar syrup and flavoring if desired, dried again, cooled, and packaged.
Fortification of ready-to-eat rice breakfast cereals with vitamins, minerals,
and flavor compounds is now a very common practice. The usual approach is to
add the minerals and more heat-stable vitamins such as niacin, riboflavin, and
pyridoxine to the basic formula mix and then spray the more heat-labile vitamins
such as vitamin A and thiamin on the product after processing.
5.9 Shredded rice
Shredded rice is a ready-to-eat breakfast cereal. Whole kernel or broken rice is
washed and cooked in a rotary cooker with sugar, salt, and malt syrup under
100–150 kPa steam pressure for a period of 1–2 h or until the rice is uniformly
cooked. The cooked particles are then discharged at a moisture content of about
40% and partially dried to a moisture content of 25–30%. The dried kernels are
tempered to ensure a uniform moisture distribution and form a hard, glazed
surface. This process allows the rice kernels to flow freely through the process.
The shredding rolls are from 15.2 cm to 20.3 cm in diameter and as wide as
60 cm or more. They are much smaller than flaking rolls. On one roll of the
pair is a series of about 20 shallow corrugations running around the periphery.
In cross section, these corrugations may be square, rectangular, or a
combination of these shapes. The other roll of the pair is smooth. Soft and
cooked rice is drawn between these rolls as they rotate and issues a continuous
strand of dough.