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.