A QUEOUS E XTRACTION

7.3.1 A QUEOUS E XTRACTION

The most common approach to isolating arabinoxylans from various plant materials involves aqueous or alkali extraction of these polymers either from whole grains or from specific plant tissues. Once isolated from the cell wall matrix, arabinoxylans are water soluble; however, in the intact wall, these polymers are cross-linked with other wall constituents to form a structural fabric that is not soluble in an aqueous

environment. 19 Some of the cross-links involved are noncovalent and, while individ- ually weak, they may confer strength and insolubility if present in large numbers (e.g., hydrogen bonds). Arabinoxylan chains can also be covalently cross-linked to each other or to other cell wall constituents. As a consequence of these cross-links,

a certain portion of arabinoxylans cannot be easily extracted from the plant materials with water and requires harsher treatments with alkali solutions to liberate them from the networks of covalent and noncovalent bonds, as well as physical entangle- ments. Various methods and procedures for obtaining highly purified water-soluble arabinoxylans from common cereals for analytical purposes have been published. 20–23 Extractions are usually conducted in water or in buffers. After extraction, purification procedures usually involve inactivation of endogenous enzymes in aqueous extracts and the use of hydrolytic enzymes to eliminate contaminating proteins and starch

from the preparations. Crowe and Rasper 21 and Izydorczyk et al. 23 obtained nearly protein-free pentosan extracts from wheat flour after adsorption of contaminating

proteins in the water extracts on various clays.

Arabinoxylans

Recently, Faurot et al. 24 developed a large-scale procedure for isolation of water- soluble and insoluble pentosans from wheat flour (Figure 7.1). The procedure was

based on mixing flour and water (50 kg/250 l), separating the soluble arabinoxylans from the insoluble residue by centrifugation, and heat treating the supernatant. The

insoluble residue was then treated with Alcalase and Termamyl to solubilize the initially water-unextractable polymers. The yields of water-soluble and -insoluble arabinoxylans ranged from 100 to 200 g and from 250 to 350 g, respectively. Both preparations, however, were contaminated with either soluble proteins or starch, but the enrichment factor ranged from 98 to 68 for water-soluble and 27 to 19 for water- insoluble arabinoxylans compared to their initial content in wheat flour. A pilot-

scale isolation of water-extractable arabinoxylans from rye 25 involved a heat treat- ment of the ground whole meal (130˚C, 90 min) to inactivate the endogenous

enzymes, followed by stirring of the rye whole meal (10 kg) and deionized water (100 l) at room temperature (90 min) in a pilot-scale brewing vessel. After decanting of the supernatant, the water-extractable arabinoxylans were purified by treatments with a heat-stable α-amylase, protein coagulation, and partial concentration by heat evaporation. Subsequently, arabinoxylans were precipitated by addition of ethanol. The precipitated material contained about 54% arabinoxylans, 22% proteins, 4.4% arabinogalactan peptide, and 4.7% β-glucans. An additional purification step, based on a treatment of the dissolved material with montmorillonite clay, increased the arabinoxylan content to about 93%, while leaving almost no protein in the extract. 25

Gruppen et al. 26 reported a large-scale isolation of highly purified arabinoxylan- enriched cell wall material from wheat endosperm based on dough kneading in combination with wet sieving. Mares and Stone 27 and Selvedran and Dupont 28 developed a method for preparation of cell wall material enriched in arabinoxylans based on wet sieving of wheat flour in aqueous ethanol to remove starch granules, followed by sonication or removal of starch and intracellular proteins by organic solvents to improve the purity of the preparations.