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

GENETICS AND NUTRIENT COMPOSITION 75
Cereal grains have long been noted as good sources for certain vitamins,
especially some of the B-complex vitamins. The barley caryopsis contains all
of the vitamins and choline with the exception of vitamins A, D, K, B 12, and
C. Reported analytical values for many of the vitamins in barley often disagree
widely, the differences being either real or due to analytical techniques (Briggs
1978). Recently, there has been a surge in investigative activity concerning vitamin
E in cereals, due to reported effects of some of the vitamin E components
on human health. Vitamin E is a complex of eight isomers, four tocopherols and
four tocotrienols, collectively called tocols. Of the major cereals, barley contains
the highest amount of total tocols, according to Kerckhoffs et al. (2002). The
tocols are composed of a chromanol ring with an attached phytyl C16 side chain.
Tocotrienols (T3) differ from tocopherols (T) in that their phytyl side chain contains
three double bonds at carbons 3, 7, and 11. The four isomers in T and
T3 differ in the number and positions of methyl groups on the chromanol ring;
α-(5,7,8-trimethyl), β-(5,8-dimethyl), γ-(7,8-dimethyl), and δ-(8-methyl) (Hunt
and Groff 1990). For many years the biological activity of vitamin E has been
ascribed primarily to α-tocopherol; however, McLaughlin and Weibrauch (1979)
calculated a vitamin E equivalent value utilizing a sum of α-tocopherol and
percentages of α-tocotrienol and the other isomers.
The tocols are associated with lipid components in aleurone, endosperm, and
embryo tissue, and concentrations are positively correlated with oil content.
α-Tocopherol and α-tocotrienol concentrations in barley oil were reported to be
24 and 17 times greater, respectively, that those in corn oil (Wang et al. 1993).
Peterson (1994) showed that hull, endosperm, and embryo of Morex barley contained
5.4, 0.6, and 93.7%, respectively, of the total tocopherol and 25.9, 45.7,
and 28.5%, respectively, of the total tocotrienols. Thus, the majority of tocopherols
are found in the embryo, whereas tocotrienols are more evenly dispersed
throughout the kernel.
Cavallero et al. (2004) confirmed earlier reports of Peterson and Qureshi
(1993) and Wang et al. (1993) for the total amount and relative abundance of T
and T3 isomers in barley. Cavallero et al. (2004) indicated that both genotype
and environment influenced total tocol content. A recent investigation indicated
a wide range of tocol concentration in 13 different barley cultivars produced in
three growing seasons under two cropping systems (Ehrenbergerovά et al. 2006).
Differences in tocol concentrations were associated not only with cultivar but
also with climate (year) and soil fertilization practice. In this study, total tocols
(T + T3) in the kernel ranged from 46.7 to 67.6 m/kg, with an average of 53.8
mg/kg. Tocotrienols comprised 77% of the total tocols, with 23% coming from
tocopherols. Of the eight isomers, α-T3 was the major compound, comprising
73% of the total tocols in these barleys. These authors reported higher levels of
all tocols in hulless and waxy barley types. Wabet, a waxy hulled cultivar used
in a reciprocal crossing program, resulted in a higher content of tocols and the
isomers of α-T and α-T3 in its progeny than that in the other waxy varieties,
confirming the findings of Vaculová et al. (2001), cited by Ehrenbergerovά et al.
(2006). In another recent study, Moreau et al. (2007) evaluated the tocol content