CHARACTERIZATION OF THE ARABINOGALACTAN FROM TEPARY BEAN (PHASEOLUS ACUTIFOLIUS VAR. LATIFOLIUS).

Abstract

An investigation was made of physicochemical and functional properties of a polysaccharide, arabinogalactan (TAG), found in the tepary bean, Phaseolus acutifolius var. latifolius, an arid adapted legume with substantial protein and starch content. TAG has specific functional and sensory properties which could be of value in foods and industrial products. The polysaccharide was isolated by a sequence of steps depending on initial extraction by trichloroacetic acid. Isolated TAG was obtained from bean flour in 3.7% yield and contained 3.6% protein; fractional precipitation provided further purification and gave fractionated TAG in 1.8% yield with a protein content of 0.9%. The purified product was composed primarily of two monosaccharide monomers, arabinose and galactose, in the ratio 2.07:1.00. Fractionated TAG was separated further by gel filtration and showed the presence of two arabinogalactans in the ratio of 9:4 with similar properties and approximate molecular weights of 500,000 and 30,000. The influence of concentration, temperature, pH and presence of sugars and electrolytes on the viscosity of fractionated TAG was studied. The specific viscosity at 1% w/v (17.84), was much greater than that of larch arabinogalactan (0.08) or soluble starch (0.39). Optical rotation measurements of temperature dependent conformational changes gave evidence for the maintenance and stabilization of the native conformation of TAG by guar gum in a mixed gel. No synergism was observed. TAG in 4% concentration produced oil in water emulsions with excellent stability over at least five months. Loaf and crumb characteristics of wheat bread were influenced by TAG at a supplementation level of 0.2%. Sensory evaluation indicated excellent color, flavor, aroma, mouthfeel, texture and overall acceptability. Substantial advantages were also afforded by TAG supplementation of rice flour bread which is formulated specifically for gluten-allergic people. Significant improvements were found in loaf and crumb quality and in sensory properties. The inclusion of TAG in a variety of other foods could be expected to enhance rheological properties as well as improve thickening, emulsification, emulsion stabilization, gas-holding and dough-raising.