A RABINOXYLAN -E NRICHED F RACTIONS O BTAINED BY P HYSICAL G RAIN F RACTIONATION

7.3.3 A RABINOXYLAN -E NRICHED F RACTIONS O BTAINED BY P HYSICAL G RAIN F RACTIONATION

It is prudent to predict that with the growing demand for functional and designer foods, physical fractionation of grain to obtain fractions enriched in specific dietary

fiber components will become more desirable than isolation of a single constituent. This approach will ensure that the preparations are enriched in the component of

interest but are not depleted of other dietary fiber ingredients and phytochemicals (polyphenols, phytosterols, vitamins, etc.) naturally present in grain tissues. The

location of arabinoxylans in cereal grains and their interactions with other grain constituents influence commercial processing, such as milling and isolation pro-

cedures aiming at obtaining arabinoxylan-enriched fractions. Various approaches to milling whole rye grain have recently been investigated to obtain fractions with improved functionality and dietary fiber characteristics. 25,34 As a consequence of variable concentration of arabinoxylan in various grain tissues, milling leads to fractions differing in arabinoxylan content. In the study by Glitsø and Knudsen, 34 the whole rye grain was passed twice through a dehuller and the outer layers were ground in a fine grinder yielding the pericarp/testa-enriched fractions. Part of the residual rye material was passed through three corrugated roller mills and sieved to obtain particles smaller than 1200 μm, thus producing the fraction enriched in aleurone cells. Another part of the rye material, remaining after dehulling, was passed through one smooth roller mill retrieving particles smaller than 1200 μm and through another smooth roller mill retrieving particles smaller than 250 μm,

thus producing the endosperm-enriched fraction. 34 The fractions varied in the

Arabinoxylans

TABLE 7.2 Composition, Extractability of Arabinoxylans (% Total), and Arabinose-to- Xylose Ratio of Whole Rye and Enriched Rye-Milling Fractions

Aleurone Endosperm Whole Rye

Pericarp/Testa

Fraction

Fraction Fraction

Total dietary fiber, % 15.1 73.3 28.3 6.5 Cellulose, %

1.3 13.6 2.0 0.4 β-Glucans, %

1.5 0.46 3.3 0.75 Lignin, %

1.5 11.0 3.9 0.2 Total AX, %

9.0 39.5 17.1 4.2 Water-extractable AX, %

41 14 27 70 Ara/Xyl (WE)

0.64 1.17 0.57 0.71 Ara/Xyl (WUE) b 0.63 1.02 0.35 0.83

Ferulic acid, mg/100 g

193 17 Note : AX = arabinoxylans; WE = water-extractable arabinoxylans; WUE = water-unextractable arabi-

noxylans. Source : Adapted from Glitsø, L.V. and Knudsen, K.E.B., J. Cereal Sci., 29, 89, 1999.

content and composition of dietary fiber; a wide range of arabinose-to-xylose (Ara/xyl) ratios and the proportions of soluble-to-insoluble arabinoxylans implied

that variations in arabinoxylans’ structure were obtained (Table 7.2). Fractionation of barley grain via pearling and roller milling was also shown to afford fractions

enriched in specific components of dietary fiber (Figure 7.3). 35 The roller milling of whole or pearled barley involved several break and sizing passages using

corrugated rolls, and reduction passages with smooth frosted or corrugated rolls. It was shown that addition of impact milling between grinding passages enabled better separation of the bran and endosperm cell walls and resulted in fiber-rich fractions containing between 15 and 25% of β-glucans and 8 and 12% of arabi- noxylans, depending on the barley genotype and certain preprocessing practices,

such as pearling and tempering of grain. 35 Most recently, Harris et al. 36 described

a large-scale procedure for the production of aleurone-rich and pericarp-rich frac- tions from hard Australian wheat. The pericarp fraction was obtained after wheat grain, conditioned to 16% moisture, was abraded using a debranning machine to remove 2.5% of bran by weight. The debranned wheat was put onto the first break of the pilot mill. The bran and pollard produced by this milling were mixed to form the aleurone-rich fraction, which included the seed coat with its cuticle. Both fractions were enriched in arabinoxylans, but the concentration of ester-linked ferulic acid and β-glucans was greater in the aleurone than in the pericarp-rich

fraction. Consistent with the results of Glitsø and Knudsen, 34 it was found that the arabinoxylans in the walls of the pericarp-rich fraction were more highly substituted (Ara/Xyl = 0.93) than those in the walls of the aleurone-rich fraction (Ara/Xyl = 0.59). 36

Functional Food Carbohydrates

20 Pearling by-products 18 16 14 12

20 Fiber-rich fractions 18 16 14 12

amylose barley FIGURE 7.3 Content of arabinoxylans and β-glucans in barley pearling by-products (obtained

barley

barley

by abrading barley grain to 10%), flour, and fiber-rich fractions (obtained via roller milling of whole barley grain). (Adapted from Izydorczyk, M.S. et al., Cereal Chem., 80, 637, 2003.)