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

SECONDARY PROCESSING 117
malt. These levels are 16 to 18% higher than reported in comparable barley and
barley malt, respectively where the starch lipids are not included. Shifts in chemical
structure occur in surface and internal lipids associated with starch granules
during malting, which suggests that such lipid changes may be responsible for
oxidation and the formation of off-flavors in some malts.
SECONDARY PROCESSING
Extrusion
Extrusion is a technology used for conversion of basic dense grain formulations
into light, puffed, and crisp products that are cooked (RTE) or partially cooked.
It is a process using high-temperature short-term cooking applicable to producing
RTE breakfast cereal and snack foods and for direct expansion and forming of
solid shapes. The basic components and operation of extruders are screw(s) rotating
in a barrel propelling food material forward through a die or restrictive orifice,
generating heat and pressure in the process. The action of these components on
the substance being extruded is influenced directly by the operating pressure in
the system. Miller (1990) provided a geometric and physical explanation of the
principles of die flow. Raw ingredients are first moved into a cooking chamber,
where they are compressed and shredded at elevated temperatures to undergo
a melt transition and form a viscous fluid. The extruder develops the fluid by
shearing the biopolymers, particularly starch. Shear is a term referring to the
resistance of molecules as they move around a mixture, due to the presence of
intermolecular forces. Starch has a high water-binding capacity during cooking,
which causes viscosity development. The die pressure and increased temperature
cause reorganization of the food molecular structure. The fluid is then forced
through dies to form a variety of shapes with low density (Guy 1991).
Extruders may consist of single or twin screws with spirally arranged flights
for conveying material through a barrel or tube with a capacity for developing
pressure. Heat is produced by steam injection as well as the mechanical energy
developed by the pressure applied. Single-screw extruders were first used to produce
RTE breakfast cereals in the 1960s. Problems were related to movement
through the barrel and excessive shear, affecting color and appearance of finished
products as well as overgelatinization. Twin-screw extruders have greater
efficiency and produce increased solubility of the material being extruded, which
allows for greater control (Kent and Evers 1994). Fast (2001) described innovations
in twin-screw extruders and the advantages in control of the various steps
in the extrusion process, including advantages in die design and cutting mechanisms.
Extrusion is a rapidly expanding technology for processing cereals into
RTE breakfast cereals, snack foods, and similar products. New developments
include improved designs in all phases of the equipment that allow for multiple
use and greater control of shear, compression, and pressure at the die. The promotion
of whole grains in modern dietary recommendations and subsequent health
claim labeling for whole grains (FDA 1999) has resulted in new extruded products