D RY P ROCESSING

1.5.1 D RY P ROCESSING

Dry fractionation of cereals to produce fractions enriched in fibers and β-glucans may provide an economic advantage over methods utilizing solvents to produce β-glucan-

rich fractions from cereals. Various dry-milling and sieving processes have been applied to whole cereal grains for production of β-glucan-rich fractions with improved functionality and dietary fiber characteristics. The applications of enrichment of cereal grains in β-glucans in a laboratory and large scale are summarized in Table 1.1 and Table 1.2, respectively. Oat materials are more difficult to fractionate than barley due to high levels of fat; fractionation of oats is successful after removal of fat. 56,134

The concentration of β-glucans in the oat subaleurone layer, as revealed by micro- scopic examination, 105,106 has led to milling procedures that produce fractions with concentrated β-glucans, such as oat bran. The thickened cell walls at the aleu- rone–endosperm junction, and resistance of this region to milling attribution, result in

a coarser particle that forms part of the β-glucan-rich bran, although other physical properties of the seed, such as cell size, might influence milling characteristics. 78

12 TABLE 1.1

Laboratory-Scale Enrichment of Cereal Grains in β-Glucans by Dry Processing

β-Glucan

Weight (%) of

Content (%)

Process

Coarse Enriched

β-Glucan Content

Enrichment (number of cultivars)

Starting Material of Starting

Fractions

(%) of Coarse

Material

Milling Particle Fractionation

(yield)

Enriched Fractions

Factor a References

1.4–1.5 Wood et al. 5 Hulless oats (2)

Dehulled oats (2)

Falling number mill

Sieving (45 μm)

7.5 1.7 Dehulled oats (9)

1.3–1.6 Wood et al. 78 Hulless oats (2)

Falling number mill

Sieving (45 μm)

1.4–1.5 Oat bran (1)

1.7–23.0 (28.5) b 26.4–27.2 (22.6) b 2.6–2.9 (2.4) b Knuckles et Rolled oat (1)

9.6 Abrasive Udy mill

Sieving (45 μm) and

5.2–18.3 (36.9) b 21.2–23.6 (12.5) b 3.7–4.9 (2.7) b al. 56 Dehulled barley (2)

4.7 resieving after

2.1–20.7 (26.8-27.6) b 17.1–22.5 (14.2–14.9) b 3.3–4.3 (2.8) b Hulless barley (2)

regrinding (45, 147,

2.2–30.1 (29.1–48.9) b 16.0–21.3 (11.4–19.5) b 2.4–3.0 (1.7–2.7) b Hulless barley (1)

75 μm)

4.6 Cyclotec sample mill

Sieving (125 μm)

8.5 1.8 Cavallero et

al. 31 Hulled barley (1)

(0.5 mm)

35.0 8.9 2.0 Knuckles and Chiu 59 Functional F Dehulled barley (1)

4.5 Hammer mill

Air classification

13.2 14.7 2.5 Wu et al. 58 Hulless barley (1)

5.8 Grinder and pin mill

Sieving (500-43 μm)

27.3 14.6 1.8 Defatted hulless waxy

8.0 and

31.0 31.3 1.6 barley (1)

19.6 air classification

ood Carboh Hulled feed barley (2)

1.1–1.2 Bhatty 64 Hulled malt barley (2)

Udy cyclone mill (1.00 Short flow

1.2 Hulless barley (2)

Allis–Chalmers roller

Hulless waxy barley (5) 7.6–11.3

1.3–1.7 ydrates Hulless normal barley (4)

Cereal Rye

1.2 Roller mill

Short

Bran

11.0 2.3 1.9 Harkonen et

19.0 2.9 2.4 al. 108 a Ratio of β-glucan concentration in enriched fraction to β-glucan concentration in starting material.

flow

Shorts

-Glucans: Structures, Physical Properties, and Physiological Functions β

b Range of values for all coarse (remained on the screens) enriched fractions obtained after regrinding and resieving through 325-, 100-, and 200-mesh screens; values in paretheses are for the coarse enriched fraction obtained after the first grinding and sieving through a 325-mesh screen.

14 TABLE 1.2

Large-Scale Enrichment of Cereal Grains in β-Glucans by Dry Processing

β-Glucan

Weight (%) of

Content (%)

Process

Coarse Enriched β-Glucan Content (%)

Starting Material (number of

Enrichment cultivars)

of Starting

Particle

Fractions

of Coarse Enriched

Factor a References

Dehulled barley (1)

Knuckles and Chiu 59 Hulled barley (1)

Pin mill

Sieving (45 μm)

4.5 Pin mill

Sieving (45 μm)

Hulless waxy barley (1)

Pin mill

Sieving (45 μm)

Defatted oat flakes (1)

38 11.2 2.5 Wood et al. 134 Dehulled oat (1)

5.5 Pin mill

Air classification

5.6 Pin mill

Air classification and

sieving (300 μm)

Dehulled oat (1)

5.6 Pin mill

Sieving (355 μm)

Hulless normal barley (1)

10.4 13.1 2.2 Vasanthan and Hulless waxy barley (1)

5.9 Pin mill

Air classification

7.2 7.6 23.8 3.3 Bhatty 60 Dehulled high-amylose barley (1)

~1.5 Andersson et al. 67 Functional F Hulled high-amylose barley (1)

Hulled normal barley (1)

4.6 Impact mill

Air classification

~1.4 Hulled waxy barley (1)

~1.8 Hulless normal barley (1)

~2.1 Hulless high-amylose barley (1)

~1.7 ood Carboh Hulless waxy barley (1)

~2.0 Hulless waxy barley (1)

~1.4 Dehulled oat (1)

Westerlund et al., 130 Wikstrom et al. 132 Rolled oat (1)

Roller mill

Short flow

ydrates

Cereal Hulless barley (1)

28 7.7 1.4 Bhatty 120 Hulless normal barley (1)

5.6 Buhler roller mill Short flow

16.3 8.1 2.4 Izydorczyk et al. 71 Hulless high-amylose barley (1)

3.4 Pearling to 10% Short flow

6.1 and Buhler

28.9 13.4 2.2 β -Glucans: Structures, Physical Properties, and Physiological Functions

Hulless waxy barley (1)

5.7 roller mill

Rye

1.5 Dehuller and

Short flow

3.3 2.2 Glitso and Bach

Knudsen 89 Wheat

roller mill

2.0 1.7 4.3 Dexter and Wood 138 Wheat

0.4 Debranning commercial system

2.6 5.2 Cui et al. 119 a Ratio of β-glucan concentration in enriched fraction to β-glucan concentration in starting material.

0.5 Debranning commercial system

16 Functional Food Carbohydrates

According to a definition adopted by the American Association of Cereal Chemists (AACC), oat bran is characterized as the milled fraction that does not exceed 50% of the initial oat groats or rolled oats, has a total β-glucan content of at least 5.5% (dry

weight basis), a total dietary fiber of at least 16.0%, and at least one third of the total dietary fiber as soluble fiber. 139

Fractionation of several oat cultivars into coarse (bran) and fine fractions by a simple dry-milling and sieving procedure gave a 7.4% mean β-glucan content of all

the brans, with an average enrichment factor of 1.5 from an average bran yield of 53.3%, close to the maximum suggested in the AC definition. The value of a

particular cultivar as a source of β-glucan in the bran is clearly not solely dependent on the concentration of β-glucan in the groat; it seems that both cell wall thickness and the degree to which this varies throughout the endosperm differ among culti-

vars. 78 It is, however, possible to increase the β-glucan concentration of the brans by alternative milling procedures designed to improve the fractionation of coarse from fine particles, although this is associated with a decrease in bran yield. Large differences in yields of the rich β-glucan fractions have been demonstrated among the various types of mills used for the dry-milling procedure. Grinding of dehulled barley with the ball mill, roller mill, pin mill, abrasive stone disk mill, and abrasive Udy mill resulted in 21, 65, 51, 70, and 30%, respectively, of the dehulled barley weight remaining on a screen with 45-

A pilot-scale fractionation of several barley cultivars by pin milling and screening resulted in a coarse fraction containing 16 to 18% β-glucans (Table 1.2) and 40 to 45% total dietary fiber. 59 Application of repeating grinding and sieving techniques yielded fractions with high levels of β-glucans up to 28%. 56,135,140 Furthermore, pearling has been found to increase β-glucan content in coarse fractions, and therefore is used for enrichment of grain fractions in β-glucans alone or in combination with other processes, such as repeated milling/grinding and sieving. 30,56,71,135,140

μm openings. 56

Combinations of dry-milling processes or sieving with air classification were also used to improve the β-glucan level in fractions of oat and barley grains in a

laboratory and large scale (Table 1.1 and Table 1.2), achieving fractions containing ~7 to 31% total β-glucans, with enrichment factors from 1.4 to 3.3 and yield in the range of ~8 to 39%. 58–60,67,134

The yields of β-glucan fractions obtained from abrasion milling and subsequent sifting are relatively low, and a prolonged sieving time is required. Roller milling has the capacity for large-scale processing, produces numerous products of highly variable composition, and is used in many cases for production of rich β-glucan fractions in large-scale trials. Quantities of 400 to 500 Kg dehulled oat grains with or without prior steam flaking were processed in a pilot plant roller mill of the type used in commercial wheat milling, yielding three discrete fractions: bran, outer starchy endosperm, and inner starchy endosperm. 130,132 The inner endosperm com- prised about 50%, and each of the other fractions about 25%; the content of β- glucans was generally higher in the bran fraction (7.9 to 8.4%) than in the starchy endosperm (2.0 to 2.3%) (Table 1.2).

Bhatty 64 dry milled, in an experimental roller mill, 15 diverse cultivars and geno- types of barley varied in β-glucans from 4.2 to 11.3%. The resulting bran fractions contained β-glucan from 4.9 to 15.4%, with an average yield value and β-glucan

Cereal β -Glucans: Structures, Physical Properties, and Physiological Functions 17

enrichment in bran of 30.3% and 1.4-fold, respectively (Table 1.1). Due to poor separation of bran and shorts in barley roller milling, these fractions were combined to obtain a composite bran sample in approximately 30% yield. 64,120 Although for barley bran there is as yet no definition like that of oat bran, Bhatty 141 claims that hulless barley bran of about 30% extraction is a true bran containing the seed coat, germ, aleurone, and subaleurone layers. On the average of bran fractions of 15 different cultivars and genotypes of barley at ~30% extraction, the β-glucan enrichment is about 37%, yielding bran that may contain 7 to 14% soluble fibers — far higher than in oat bran.

Unlike wheat and oats, barley does not have a long tradition of being fractionated by roller milling, and the fractions derived by this process have not yet been standardized in terms of quality, composition, or even terminology. Izydorczyk and coworkers 71,72 roller-milled hulless barley cultivars of variable amylose content and generated mill streams with variable composition. β-Glucans from endosperm cell walls are highly concentrated in the shorts from the reduction system, designated as the fiber-rich fraction (FRF). Generally, for high β-glucan cultivars the FRF yields are greater than 20% (whole barley basis), with β-glucan contents above 15%, having obvious potential as a functional food ingredient (Table 1.2).

Recently, various dry-milling approaches for rye grain have also been investi- gated to obtain fractions with higher contents of dietary fiber components and to

add them to rye breads. Harkonen et al. 108 fractionated a rye cultivar by a laboratory- scale roller mill and obtained bran and short fractions enriched in β-glucans with contents of 2.3 and 2.9%, respectively (Table 1.1). Glitso and Bach Knudsen 89 separated rye by dry milling into three fractions enriched in different rye grain tissue (pericarp/testa, aleurone, and endosperm). The pericarp/testa-enriched fraction, obtained after dehulling of the whole kernels, had a low concentration of β-glucans (0.46%). The dehulled grain was roller milled and, depending on the subsequent sieving procedure, two different fractions were obtained; the highest amount of β- glucans was found in the aleurone-rich fraction (3.3%) (Table 1.2), whereas the β- glucan content was lower in the starchy endosperm (0.75%).

Studies on wheat β-glucans are limited because of their low contents in the grain. However, a newly developed wheat preprocessing technology produces debranned by-products, enriched in β-glucans (up to 1.7%) (Table 1.2), using a commercial system of friction–abrasion technology. 138 Cui and coworkers developed some interest in wheat β-glucan due to this preprocessing technology, which enriches the β-glucan content from 0.5% in whole wheat to 2.6 to 3.0% in one of the bran fractions 107,119 (Table 1.2).