ARABINOXYLANS AS TECHNOLOGICALLY FUNCTIONAL FOOD INGREDIENTS

7.7 ARABINOXYLANS AS TECHNOLOGICALLY FUNCTIONAL FOOD INGREDIENTS

The role of arabinoxylans in the bread-making processes and their effects on the final bread product have been studied extensively over the last few decades. The

results reported by earlier studies were sometimes contradictory, primarily due to utilization of polymers with varying degrees of purity and composition, different levels of supplementation, and various baking procedures utilized by different work- ers. Clear distinction has to be made between water-soluble arabinoxylans and those that are not readily extractable with water. When added to wheat flour, water-soluble arabinoxylans compete for water with other flour constituents. As a consequence, dough consistency is increased. Michniewicz et al. 115 and Wang et al. 116 reported also that water-soluble arabinoxylans negatively affect gluten yield and increase its resistance against extension. These effects, however, can be corrected by adding more water or adding xylanase before dough mixing. Biliaderis et al. 117 used two different preparations of arabinoxylans to demonstrate the effect of molecular size of these polymers on baking absorption and showed that the high molecular weight arabinoxylans increased the farinograph water absorption to a greater extent than their low molecular weight counterparts. Water-soluble arabinoxylans are believed to increase the viscosity of the dough aqueous phase, and therefore to have a positive effect on the dough structure and its stability, especially during the early baking processes, when a relatively high pressure is generated inside the gas cells. The increased stability of the film surrounding the gas cells is useful in prolonging the oven rise and preventing coalescence. These phenomena lead to a higher loaf volume and improve crumb structure. McCleary 118 convincingly demonstrated the beneficial role of arabinoxylans on the loaf volume and appearance when wheat flours, treated with an excessive amount of purified endoxylanase, were shown to produce breads with low loaf volume and soggy texture. The overall effect of arabinoxylans on the bread-making process is, however, dependent on the concentration of these polymers in the dough system. A higher than optimum amount of arabinoxylans may cause viscosity buildup and hinder their beneficial effects. 117,119 The optimum concentration of arabinoxylans may depend on their molecular structure (size) and the baking

Functional Food Carbohydrates

characteristics of the wheat flours. 117 Despite a number of investigations, the mech- anism of interactions between arabinoxylans and gluten has not been fully deter-

mined. Wang et al. 116,120 observed that water-soluble pentosans lower the extensibility of dough, and that this effect is related to the ferulic acid content of arabinoxylans. The authors proposed that covalent cross-linking between protein and arabinoxylans might be partly responsible for changes in the rheological properties of gluten. However, two separate studies by Hilhorst et al. 121 and Labat et al. 122 reported no evidence for covalent complexes between arabinoxylans and protein in mixed, oxi- dized, or overmixed doughs. Labat et al. 122 proposed that free ferulic acid esters are likely to react with proteins during mixing and, therefore, these free ferulic acid residues, rather than feruloylated arabinoxylans, might be involved in gluten break- down during dough overmixing. Santos and coworkers 123 reported that the presence of water-soluble pentosans reinforced the gluten network and decreased the irrevers- ible changes occurring during heating the gluten.

In contrast to water-soluble arabinoxylans, their insoluble counterparts, which are not readily extractable from the cell walls with water, destabilize the dough structure and have a negative effect on the loaf volume and other bread character- istics. 124,125 They also absorb a large amount of water, thus depleting the pool available for proper gluten development and film formation. Water-insoluble arabi- noxylans that are present in dough as discrete cell wall fragments can form physical barriers for the gluten network during dough development. The resulting gluten has lower extensibility and a lower rate of aggregation, and therefore a different network structure. 125 During fermentation, water-insoluble arabinoxylans decrease the film stability. Their presence in dough results in lower loaf volume and coarser and firmer crumbs. However, Courtin and Delcour 124 showed that the negative effects of water- insoluble arabinoxylans in bread could be reversed by using endoxylanases with specificity toward the water-insoluble arabinoxylans. These enzymes catalyze the conversion of detrimental water-insoluble arabinoxylans to the high molecular weight water-soluble counterparts. The addition of optimal doses of endoxylanases during the bread-making processes has been found to positively affect the dough handling and bread properties. The addition of xylanases has been reported to improve dough consistency, fermentation stability, oven rise, loaf volume, and crumb structure and softness. 124–128 An excessive amount of endoxylanases or addition of endoxylanases with specificity toward water-soluble arabinoxylans may lead to too extensive degradation of these polymers and to negative effects on bread loaves. The use of xylanase in combination with peroxidase prevents extensive degradation of arabinoxylans by cross-linking them into larger aggregates. 121

Another functional property of arabinoxylans may be associated with their role in bread staling. Bread staling is a complex phenomenon involving loss of aroma, deterioration of crust characteristics, and increase in crumb firmness. Biliaderis et al. 117 measured bread staling by monitoring the crumb firmness of breads fortified with water-soluble arabinoxylans. It was shown that over a 7-day storage period, the arabinoxylan-fortified breads exhibited lower crumb firmness than the controls. The observed effects were attributed to a higher moisture content of breads substituted with arabinoxylans and to the plasticizing effects of water. Contrary to the observed decrease in crumb firmness, starch retrogradation, as

Arabinoxylans

measured by calorimetry and x-ray diffraction, increased in the presence of ara- binoxylans. The enhanced kinetics of chain ordering were attributed to a higher water content in the breads and a correspondingly greater mobility of the starch molecules. As established by Zeleznak and Hoseney, 129 starch retrogradation increases with increasing moisture content between 20 and 45% moisture in the system. Gudmudson et al. 130 showed that the rate of amylopectin crystallization in starch gels containing arabinoxylans is either increased or decreased, depending on the final water content of starch; retrogradation was minimal in starch gels with

a moisture content below 20% and increased significantly in gels with a moisture content between 20 and 30%.

The recent revival of interest in incorporating arabinoxylans into food products is due to the beneficial effects of these polymers on human health. However, as

pointed out above, the addition of arabinoxylans to food products may have some negative effects on their quality, depending on the amount and properties of arabi- noxylans. The food industry is therefore faced with a challenge to create foods that are both healthy and appealing in terms of taste, aroma, and appearance. Several attempts to enrich various food products with arabinoxylans have been documented.

Lu et al. 32 incorporated between 7 and 14% of arabinoxylan-rich fiber into bread and reported that the product’s palatability was equal to that of 50% whole wheat bread. Utilization of barley pearling by-products enriched in dietary fiber for making pasta has been reported by Marconi et al. 131 Substitution of 50% of durum wheat semolina with pearling by-products resulted in a darker pasta with good cooking characteristics as regards stickiness, firmness, and cooking losses. The high dietary content, including arabinoxylans, has made this product a healthy alternative for health-conscious consumers. Izydorczyk et al. 132 incorporated barley fiber-rich frac- tions enriched in arabinoxylans and β-glucans into noodles and baked products. Appropriate baking procedures, protein content of wheat flour, and addition of specific hydrolytic enzymes in the bread formula resulted in satisfactory crumb texture and loaf volume of bread baked with partial replacement (15%) of wheat flour with the barley fiber-rich fractions. 133 Izydorczyk et al. 132 showed that yellow alkaline and white salted noodles enriched in barley fiber-rich fractions offer con- sumers convenience due to shorter cooking time, acceptable cooking quality, and increased and diversified nutritional value.