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

136 Cereal grains for the food and beverage industries
which are then lifted from the grain mass and discharged by gravity when
the indent cannot hold the grain any longer (Wimberly, 1983; Bond, 2004).
Milling
Three methods, namely wet, dry and semi-dry milling, have been used to
grind polished rice kernels or brokens into flour (Chiang and Yeh, 2002).
Wet grinding is a traditional process used to prepare rice flour and includes
five successive steps: soaking, adding excess water during grinding, filtering,
drying and sieving. This process comprises the use of several machines and
much manpower. Additionally, this process is known to have significant
flour loss and high energy consumption (Yeh, 2004). Dry grinding does
not use water (no waste water generation) and is characterized by its
limited consumption of energy. Herein, the rice kernel is ground with dry
grinding machinery such as a hammer mill, pin mill, roller mill or disc mill,
etc. Nevertheless, many food items made of dry ground flour (for instance,
noodles) have inadequate rheological properties for many consumer uses
(Yeh, 2004).
In the semi-dry grinding methods, the properties of flour are intermediate
to those of both dry and wet ground flour from the perspectives
of particle size, viscosity, damaged starch, etc. (Ngamnikom and
Songsermpong, 2011). The semi-dry grinding process is characterized by
three consecutive steps: soaking, drying to remove excess water (15-17 %
wet basis) and grinding with dry grinding machinery (Yeh, 2004). Nevertheless,
the semi-dry method has some disadvantages, such as the longer time
required to adjust the rice kernel moisture content, the high energy consumption
needed for the drying step, the excessive consumption of water
and the generation of waste water. Generally, wet-milled flour is superior
to dry-milled flour for making traditional baked products (Bean et ai, 1983)
and Thai rice-based snack food (Jomduang and Mohamed, 1994). The superior
quality of wet-milled flour is mainly due to its low proportion of
damaged starch and finest relative particle size (Chen et al., 1999).
Ngamnikom and Songsermpong (2011) investigated the performance of
a new grinding process, which includes freezing rice with liquid nitrogen
prior to dry grinding. In terms of damaged starch content, average particle
size, particle size distribution, microscopic structures and energy consumption,
they found that the freeze grinding resulted in (i) reduced particle size
and damaged starch content due to the extremely low temperature of the
sample prior to grinding, and (ii) a smaller flour particle size than from dry
grinding resulting in a greater yield after sieving. Moreover, freeze grinding
produced a higher flour yield after sieving in comparison with dry grinding
using an identical grinder. Finally, the energy consumption of freeze grinding
was similar to dry grinding and much lower than that detected for the
wet grinding process. Consequently, the authors conclude that the freeze
grinding process was a viable alternative to the traditional wet grinding