W ET P ROCESSING

1.5.2 W ET P ROCESSING

Methods such as wet milling–sieving also aim to partially remove starch from cereal grain. From 20 to 55% of the barley β-glucan was found to be soluble in water at room temperature, 70,75 whereas only 5% was soluble in ice-cold water. 135 Similarly, little of the cell wall β-glucan is soluble in ethanol or ethanol/water mixtures or aqueous solutions of certain salts. Therefore, for wet milling–sieving, cold water or the latter solvents are used.

18 Functional Food Carbohydrates

Removal of starch from the cell wall material is accomplished by homogenization of barley flour with water saturated with a salt (sodium sulfate) and sieving the

slurry; 135 the remaining material on the screen yielded 44% of the starting flour and contained 10.4% β-glucan. From dehulled oats, containing 5.6% β-glucan, Wood et al., 134 by pin milling, dry sieving, reflux in ethanol, sieving in ethanol (screen, 150

μm), and drying, prepared in a large scale, a fraction with a 16.6% β-glucan level and

a 21% yield of the starting material. Vasanthan and Temelli 142 suggested the recovery of β-glucans with wet sieving from ground/milled barley or oat whole grain, or any β-glucan-rich fraction of the grain obtained by dry processes such as milling (i.e., pin, hammer, attrition) and air classification or sieving. This material is mixed with

a mixture of ethanol/water (40 to 50% ethanol) at room temperature for 10 to 30 min, and optionally, protease and amylase were used at this step for the removal of starch and proteins. The slurry was separated by a screen (40 to 70 μm), and the retentate portion was washed with the ethanol/water mixture and air dried. This method pro- vided β-glucan concentrates from barley and oat having a β-glucan concentration of

40 to 70%, depending on whether the protease and amylase were employed. β-Glucan-enriched products from oats and barley were prepared using wet grinding in cold water (8 to 12˚C), which may contain an organic solvent (ethanol),

about 20% by weight of the water, and following sieving and drying. The β-glucan concentration of the obtained fiber was 18 to 31% and the β-glucan yield, calculated

from the β-glucan of the initial materials, was between 75 and 90%. 143,144 In pilot and small industrial scale, oat bran concentrates containing 14.7 to 15.5% β-glucans were produced by wet milling in neutral or acidified cold-water (<14˚C) suspensions and shifting. Moreover, two other concentrates have been prepared by wet milling and sieving in ethanol–water suspensions; for wet milling, 70% (v/v) ethanol at 20˚C and 90% (v/v) ethanol at 75˚C were used, yielding oat bran concentrates with 16.3 and 18.9% β-glucans, respectively. 136

In general, cereal β-glucan concentrates and isolates resulting from a number of investigations at pilot scale have been obtained by water, acidified water, and

aqueous alkali (i.e., NaOH or Na 2 CO 3 ) extraction from whole ground cereal grains or high β-glucan fractions produced by the various aforementioned dry and wet processing of cereal grains. The resultant slurry is then processed by techniques such as filtration, centrifugation, and alcohol precipitation to separate the β-glucan from the slurry (Table1.3). In combination with extraction, treatments with amylases and proteases can be used for removal of starch and proteins. 112,119,120,145,146 Use of α-amylase may yield substantial amounts of glucose, which can provide sweetness to food formulations and promote the formation of colored and bitter products on heating in the presence of amino acids (Maillard reaction products), consuming simultaneously lysine, an essential amino acid. Therefore, the use of β-amylase instead of α-amylase has been suggested in combination with an ultrafiltration step (Table 1.3) for the purification of the product from low molecular weight constitu- ents. 146 Beer et al. 112 examined different recovery methods of β-glucans from oat bran extracts and demonstrated that for production of large amounts of good-quality oat gum rich in β-glucans, an alcoholic precipitation would be the process of choice, but ultrafiltration and dialysis are quite useful alternatives. However, these conven- tional processes have a number of technical problems, which limit commercial uses

Cereal

TABLE 1.3

Large-Scale Production of Cereal β-Glucan Concentrates or Isolates by Solvent Extraction -Glucans: Structures, Physical Properties, and Physiological Functions β

Extraction

β-Glucan

Content of Final Material

Starting Preprocessing for

Conditions/Purification

Enrichment in β-Glucans

Procedures

Recovery Methods

Products References

Cavallero et al., 31 Knuckles et al. 28 Oat flour

Barley flour Dry milling and sieving

Aqueous (4–5˚C ×

18 h)

Centrifugation and freeze drying

Aqueous (90–100˚C) with

Centrifugation and freeze drying

Inglett 145

α-amylase digestion

Oat flour Dry milling–sieving and

Oste Trantafyllou 146 heat treatment

Aqueous (55˚C) with β-amylase

Centrifugation or filtration, ultrafiltration, and

digestion

pasteurization–concentration or drying (spray or freeze drying)

Barley and Oat —

Cahill et al. 147 flours

Mild alkaline (pH = 8, 50˚C × 1 h) Centrifugation, spray drying, and agglomeration

and acidification (pH = 4)

Oat flakes Hexane defatting, pin Mild alkaline (20% Na 2 (CO 3 ), pH = Centrifugation, concentration, precipitation with 2- 78% (18.6 kg) a Wood et al. 134 milling–air classification

10, 45˚C ×

30 min) and

propanol (50%), centrifugation, repeated blending

(2000 kg), a and refluxing

precipitation of proteins at pH = 4.5 with ethanol or propanol (100%), and

with 70–75% ethanol

centrifugation and air desolventization Same as the above process with an additional

Wood et al. 5

purification step of the (NH 4 ) 2 SO 4 (20% w/v) precipitation

Oat bran —

Beer et al. 112 concentrates

Mild alkaline (20% Na 2 (CO 3 ), pH = Centrifugation, ultrafiltration, and freeze drying

10, 40˚C ×

30 min) and

precipitation of proteins at pH = 4.5 Same as the above process with an

additional step of pancreatin digestion

Hulless barley Roller mill

Alkaline (0.25 N NaOH, 25˚C) with Precipitation with ethanol and freeze drying

50% (0.5 kg) a Bhatty 120

(500 kg) a (5 kg) a Termamyl digestion

TABLE 1.3 (continued) Large-Scale Production of Cereal β-Glucan Concentrates or Isolates by Solvent Extraction

Extraction

β-Glucan

Content of Final Material

Starting Preprocessing for

Conditions/Purification

Enrichment in β-Glucans

Procedures

Recovery Methods

Products References

Wheat flour

Cui et al. 119 commercial system

Branning with debranning Alkaline (0.25 N NaOH, 25˚C × 1 Precipitation with ethanol and freeze drying

h) with thermostable -amylase digestion

75% (145 g) a Wang et al. 148 Barley flour

Oat flour (7 kg) a Aqueous (<50˚C) and heating b Centrifugation, precipitation, and dehydration with

87% (3.5 kg) a (100 kg) a Waxy barley

2-propanol (50%), screening and grinding

Goering and Eslick 149 meal

Aqueous (40–60˚C)

Centrifugation, heating (90–95˚C × 5 min),

centrifugation, ultrafiltration, and drum or spray drying

Cereal grain or Wet screening

Potter et al. 150 bran or spent

Alkaline (55–65˚C × 1–2 h)

Centrifugation,

Concentration, drying milling

acidification,

brewer’s grain

heating, cooling Evaporation, c skimming,

centrifugation b filtration, drying milling Same as the above process

Functional F

with additional repeated ultrafiltration

Pearled barley Roller and hammer milling Aqueous (50˚C) with cellulase,

Morgan 135 and screening

Gelation (0˚C ×

24 h), washing, centrifugation, and

xylanase, and amyloglucosidase

spray drying

ood Carboh

digestion, centrifugation,

Gelation (–18˚C ×

48 h and 25˚C ×

72 h),

70–90% d Morgan 151

concentration (falling film

decanting, and freeze drying

evaporator)

a In parentheses is the amount of starting material or enriched fraction in β-glucan or final concentrate/isolate.

ydrates β-Glucanase inactivation or protein coagulation. c For production of a skin (solid film) enriched in β-glucans.

d Increasing with repeated freezing–thawing cycles.

Cereal β -Glucans: Structures, Physical Properties, and Physiological Functions 21

by the cost of the isolated material, particularly for food applications. The most common technical problems come about from the viscosity rise in the slurry during extraction of β-glucans, causing clotting of the filter upon filtration and inefficient separation of flour components during centrifugation, as well as from the use of large amounts of organic solvents for precipitation and the high cost for their recycling (distillation).

Van Lengerich et al. 152 suggested digestion with exogenous enzymes (cellulases, hemicellulases, xylanases, and pentosanases) of the aqueous slurry containing the β-glucan grain material to reduce viscosity and optimize separation of insolubles from the extract solution. For the recovery of β-glucan from cereal water extracts without using the precipitation method with organic solvents, Morgan 135,151 suggested gel formation processes (Table 1.3); gelation was induced by various procedures such as resting, shearing, cooling, or freezing the solution for a period. However, the molecular weight of the isolate obtained by this protocol was about 50,000 Da, because no deactivation of endogenous enzymes was carried out before extraction. 135 Later, Morgan 151 proposed a process for controlling the average molecular weight of β-glucan extracted from cereals by managing the extraction time; decreasing the extraction time from 1 h to 30 min resulted in a β-glucan isolate with a molecular weight of more than 100,000 Da.