Barley for Food and Health: Science, Technology, and Products

124 BARLEY PROCESSING: METHODS AND PRODUCT COMPOSITION
classification, but significant variations occurred in the nutrient concentrations
due to starch and hull type. The reader is referred to Fedec (2003) for a more
detailed description of the principles of operation and uses of air classification
in the food industry, including sources for equipment and plant system designs.
Sieving
Sieving is an alternative method to air classification for producing protein- and
fiber-rich fractions from pin- or hammer-milled barley (Knuckles et al. 1992;
Yoon et al. 1995) and can be combined successfully with air classification
(Knuckles and Chiu 1995; Sundberg et al. 1995). The term sieving as used
in the grain milling and processing industries refers to the separation of ground
material into various particle size classifications, utilizing size-designed sieves or
screens (Posner and Hibbs 1997). Sieving is also a component of the roller-milling
process used to produce high-quality bread flour, but also may be applied to
whole-meal flour prepared with other milling machinery, such as hammer mills or
pin mills. Separation of ground barley by sieving is more effective with dehulled
barley (blocked), pearled or hulless barley, than with hulled barley.
Separation of flour-meal particles in a sieve is accomplished by moving the
particles across a sieve screen, resulting in the smaller particles passing through
the screen and the larger particles being removed to a collection point by a
conveyor. Posner and Hibbs (1997) described the six principles that govern the
results obtained through sieving as direction of movement of the sieve (gyrating,
reciprocating, a combination of the two, or rotating), rate of movement of the
flour relative to sieve surface area, the size of the apertures of the sieve, the
amount of sieve surface area, the amount of flour on the sieve surface, and the
granulation and shape of the flour particles.
In a commercial wheat flour milling operation, which may be adapted to
barley, a sifter section is a box containing a stack of sieves arranged to separate
material flowing over and through the sieves into different particle size fractions
or streams as determined by the principles outlined above. The sieve arrangement
is known as the flow of the sifter, and arranging of the sifter sections is called
flowing the sections. Sifter sections can be flowed to remove coarse material
first and the fine material last at the bottom of the sifter stack or arranged in
reverse with fine material removed first and coarse material removed last. The
sieve arrangement is referred to as sieve stacking (Posner and Hibbs 1997).
Air classification and sieving are effective methods to separate materials such
as whole-meal barley or barley flour into fractions with different levels of protein,
starch, and dietary fiber for application in a variety of food products. A
combination of air classification and sieving is often more effective in isolating
particles of different sizes and composition than is either procedure alone. Future
food and industrial processors may require ingredients for standard as well as
specialty products containing a wide variety of physical, chemical, and functional
properties, depending on end use. Thus it is not appropriate to describe a particular
product or fraction of the milling processes to be superior to another, only that