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

WHOLE-GRAIN PROCESSING 103
axis through a hole in the center of the stones. The stones may be as large as 1.2
m in diameter and as thick as 0.3 m. The stone disks may have several grooves
extending from the center hole to the outer edges. The grooves facilitate movement
of the ground grain toward the edge of the stone. Grain is fed through the
center hole as the upper stone is rotated in close contact with lower stone. As the
grain is mashed or ground it moves outward through the grooves in the lower
stone to the periphery, where it is collected. Advanced stone mills generally have
three sets of stone disks. The first cracks the hull, which is removed by a fan
mechanism. A second set of disks produces coarse-meal flour, and a third set
grinds the meal to fine flour, which may be sieved to remove large particles.
Mostly stone mills were replaced by roller mills in the mid-nineteenth century,
except for those mills that produce stone milled barley, wheat, and other grains
for specialty markets (Kent and Evers 1994; Owens 2001). The Barony Mill at
Birsay (www.birsay.org.uk/baronymill.htm) near Kirkwall in the Orkney Islands
of Scotland is still in operation. It is a water-powered stone mill that specializes in
producing whole-meal barley flour from Bere barley, an ancient cultivar grown in
Scotland for at least 2000 years (Theobald et al. 2006). Stone milling is an effective
method of producing whole-grain products when complete separation milling
or concentration of endosperm for flour is not required for the final product.
In modern processing, hammer mills and pin mills are used more commonly to
grind whole-grain or pearled barley for direct use or prior to further processing.
A hammer mill is a machine designed to shred material, including grain, into fine
particles. Hammer mills include a delivery system to introduce grain into the path
of rotating hammers. These mills consist of a steel drum or housing containing
a rotor mounted on a horizontal shaft on which pivoting free-swinging hammers
are suspended from rods running parallel to the shaft and through the rotor disks.
The rotor is spun at high speed inside the drum while grain is metered into the
grinding area. The grain is contacted there by the hammers, reducing the particle
size and forcing the particles through a perforated screen into a discharge opening.
Optimal design and placement of the hammers provide maximum contact with
the grain. The ground grain is removed either by gravity or is air-assisted using
a blower system through a discharge vent. Small screen openings and high-speed
rotation of the hammers increase the effectiveness of particle size reduction. The
design and placement of hammers are determined by operating parameters such
as rotor speed, motor horsepower, and open area in the screen. Rotor speed and
the amount of open area in the screen are the primary determinants of grinding
efficiency and finished particle size. The screen must be designed to maintain its
integrity and provide the greatest amount of open area. Screen openings (holes)
that are aligned in a 60 ◦ staggered pattern optimize open area while maintaining
screen strength. Hammer mills are capable of producing a wide range of particle
sizes, which will generally be spherical, with a surface that appears polished,
although particle size will be less uniform than with some other methods of
grinding (Wikipedia 2007).
Pin milling is a means of grinding, sizing, deagglomerating, and homogenizing
various materials, including cereal grains. Pin milling is accomplished with