S TRATEGIES TO E XTRACT A RABINOXYLANS FROM A GRICULTURAL B Y - PRODUCTS

7.3.2 S TRATEGIES TO E XTRACT A RABINOXYLANS FROM A GRICULTURAL B Y - PRODUCTS

A considerable effort has recently been directed toward extraction and purification of arabinoxylans from agricultural by-products. Brewer’s (spent) grain, wheat or rye

bran, sugar beet pulp, corncobs, and banana peels constitute abundant low-value by- products of the food industry. These materials are potential sources of arabinoxylans as they are rich in noncellulosic polysaccharides, with arabinoxylans being a primary

component. Bataillon et al. 29 combined delignification (with 37% sodium chlorite) and alkali extraction (with 43% sodium hydroxide) to obtain arabinoxylan prepara- tions from de-starched wheat bran. The extracted arabinoxylans were purified by a microfiltration and dried by an atomization system (Figure 7.2); the yield and purity

of this preparation were 13 and 75%, respectively. 29 In cereal grains, the extractability of arabinoxylans from the outer layers of the kernel is lower than that from the starchy endosperm, and physical and chemical treatments are often needed to ease

their solubilization. However, Bergmans et al. 30 observed that autoclaving, alkaline peroxide treatment, or chlorite delignification of wheat bran was not effective in

Functional Food Carbohydrates

WHEAT FLOW

+ WATER (250 L)

(50 Kg)

Stirring 30 min at room temperature

Horizontal decanter vertical centrifuge

RESIDUE SUPERNATANT

Alcalase treatment Heat treatment 2 h/55°C/pH 7.5

10 min/100°C

Centrifugation Centrifugation

Residue + water Supernatant

Centrifugation Amyloglucosidase treatment

Termamyl treatment 2 h/59°C/pH 6.5

Cross-flow UF

Centrifugation Freeze-drying

Residue + water WATER-SOLUBLE

PENTOSANS Centrifugation

Freeze-drying

WATER-INSOLUBLE PENTOSANS

FIGURE 7.1 Flow scheme for extraction procedure and purification of water-extractable and water-insoluble arabinoxylans from wheat flour. (Adapted from Faurot, A.-L. et al., Lebensm. Wiss. Technol ., 28, 436, 1995. With permission.)

Arabinoxylans

De - starched wheat bran

Sodium chlorite

Delignification, 2 h/70°C/pH 3

Sulfuric acid

Washing, sieving

Delignified bran

Sodium hydroxide

Arabinoxylan extraction, 6 h/40°C

Decantation

Solid residue

Liquid residue

Washing decantation

Liquid residue Sulfuric acid

Precipitation pH 4.8

Centrifugation

Supernatant

Tangential microfiltration

Retentate

Atomization

Purified arabinoxylans

FIGURE 7.2 Flow scheme for extraction of arabinoxylans from wheat bran. (Adapted from Bataillon, M. et al., Ind. Crop Products, 8, 37, 1998. With permission.)

increasing the extract yield in the subsequent saturated barium hydroxide extraction of glucuronoarabinoxylans. Consequently, researchers have turned to various hydro- lyzing enzymes, such as β-glucanases, arabinofuranosidases, endoxylanases, and ferulic acid esterases to facilitate the liberation of arabinoxylans from the cell wall

materials. Figueroa-Espinoza et al. 31 combined various physical treatments (extru- sion and high shear) with the use of different endoxylanases (from Aspergillus niger, Talaromyces emersonii , and Bacillus subtilis) to investigate the release of high

Functional Food Carbohydrates

molecular weight arabinoxylans from rye bran. The best results were obtained when the rye bran was extruded at high temperature (~140˚C) and extracted in the presence

of endoxylanase from Bacillus subtilis. It was pointed out, however, that the amount of hydrolytic enzymes needs to be controlled to avoid hydrolysis of arabinoxylans and a consequent reduction of their molecular weight and ability to form gels. Further improvement in solubilization of arabinoxylans was achieved when β-glucanase was added to the extraction buffer. The high-shear treatments, achieved by pumping of rye bran slurry through a reactor used for production of microemulsions, did not improve the extractability of arabinoxylans. The use of hydrolytic enzymes to assist the extraction of arabinoxylans from brewer’s grain and wheat bran has also been

explored by Faulds et al. 4 An enzyme preparation from the thermophilic fungus Humicola insolens , containing endoxylanases and feruloyl esterases, proved efficient in releasing ferulic and diferulic acid residues and solubilizing arabinoxylans present in these by-products.

Lu and coworkers 32 utilized a by-product generated during processing of wheat flour into starch and gluten to produce arabinoxylan-rich fiber. The arabinoxylan-

enriched residue left after flour processing was simply collected on a sieve (75 μm), washed with water, and spray dried to a powder. Another product, containing 60%

of arabinoxylan-enriched dietary fiber, was obtained from wastewater, remaining after starch extraction, after a series of enzymatic and fermentative treatments fol-

lowed by cross-flow ultrafiltration and spray drying. 33