Traditional and Emerging Feedstocks for Food and Industrial Bioproduct Manufacturing - Food and Industrial Bioproducts and Bioprocessing - page 9

practices and environmental conditions (Griffey et al ., 2009 ; Aman et al ., 1985 ; Oscarsson
et al ., 1996). Barley hull comprises about 13% of the kernel and consists mostly of cellulose,
hemicelluloses (xylans), lignin and a small amount of protein (Andersson et al ., 1999 ).
Hulls adhere to the caryopsis of the hulled barley while they are not attached or loosely
attached to the grain surface of hulless barley. According to Bhatty (1999), hulless barley
would ideally have less than 5% adhering hulls. The thickness of the hulls varies. Thick
hulls adhere to the caryopsis less firmly than thin hulls. Presence or absence of hulls
significantly affects grain composition. Hulless barley generally has lower ash and dietary
fiber but higher starch, protein and oil content due to the absence of the hull.
Carbohydrates comprise about 80% of the barley grain. Starch is the major grain
component and barley may contain up to 65% starch (Song and Jane, 2000). Starch and
protein contents of hulless barley can be as high as those of field corn. A study on the
chemical composition of 92 Swedish barley varieties showed that two-rowed barley varieties
had slightly higher starch content while six-rowed barleys had higher protein and fiber
(Aman et al ., 1985). Waxy barley varieties contain 5-8% less starch than that of non-waxy/
regular barley varieties (Bhatty, 1999). Starch type and properties have a significant effect
on barley end use. Amylopectin comprises 72-78% of the total starch in barley (Bhatty,
1999). Waxy barley varieties contain very high levels of amylopectin. The existence of
barley cultivars with 100% amylopectin has been reported (Bhatty, 1997). Zero or waxy,
normal and high amylose barley contain 0-5%, 20-30% and up to 45% amylose (based on
grain weight), respectively (Baik and Ullrich, 2008). Starch granule size in hulless barley
ranges from 2 to 30 μm (Bhatty, 1999; You and Izydorczyk, 2002). Among four types of
hulless barley (normal, high, waxy and zero amylose), normal amylose type has the greatest
amount of large granules (74.7%). Waxy, zero and high amylose starches consist of 66.4,
43.9 and 19.4% large granules, respectively (You and Izydorczyk, 2002).
The major non-starch carbohydrates in barley comprise (1,3)(1,4)- β -D-glucans and

arabinoxylans. -Glucans, which are mainly present in the endosperm cell walls (Oscarsson

et al ., 1996), consist of high molecular weight linear chains of

β

β -glucosyl residues

polymerized through β -(1-3) and β -(1-4) linkages (Newman and Newman, 2008). The high

β -glucan content of barley (2.5-11.3%) is notable (Izydorczyk and Dexter, 2008). High

amylose and waxy barley have been reported to contain higher β -glucan than normal

amylose type (You and Izydorczyk, 2002). -Glucan is partially soluble in aqueous solutions

due to the molecular, structural and solubility differences of polysaccharides present in its
chemical structure (Newman and Newman 2008). The water soluble part of

β

-glucan
produces high viscosity starch slurries that can cause problems during industrial processing.
High viscosity mash increases pumping costs and complicates production. Low

β

-glucan
content in the grain leads to low viscosity and little need of expensive enzymes to break it
down for efficient processing and fermentation. Cellulose (1,4-

β

-D-glucan), fructans,
arabinoxylans, glucommannan, galactomannan, arabinogalactan and a number of simple
sugars and oligosaccharides are also present in barley grain in relatively small quantities.
Although phenolic compounds are minor constituents in barley, they play an important
role in the nutritional quality of the grain. Phenolic compounds bind proteins, carbohydrates
and minerals, thereby affecting the nutritional and functional value of the bound constituents.
The major nutritional concern is the ability of condensed and hydrolysable tannins to bind
strongly to large proteins, specifically to proteins high in proline, thereby reducing protein
digestibility. Significant differences were found in flavanol and total phenol content among
diverse barley genotypes (Griffiths and Welch, 1982). The variations in phenolic content
were not correlated with grain size, malting quality and oil and protein contents. Barley grain

β