Structure and Viscoelastic Properties of Starches Separated from Different Legumes

Abstract

Abstract A comparison between the morphological, structural, thermal and viscoelastic properties of starches separated from pigeon pea, chickpea, field pea, kidney bean and blackgram was made. The shape of the starch granules in the different legumes varied from oval to elliptical or spherical. X‐ray diffraction of the legume starches indicated a typical C‐pattern (mixture of A‐ and B‐type). Granules of blackgram and pigeon pea starch had a higher degree of crystallinity than those of field pea and kidney bean starches. Apparent amylose content of field pea, kidney bean, chickpea, blackgram and pigeon pea starch was 37.9%, 36.0%, 34.4‐35.5%, 32.9‐35.6% and 31.8%, respectively. Distribution of isoamylase‐branched materials among the starches revealed that the proportions of long and short side chains of amylopectin ranged between 13.6‐18.5% and 41.7‐46.5%, respectively. Field pea and kidney bean starch had the highest apparent amylose content and the lowest amount of long side chains of amylopectin, respectively. Blackgram and pigeon pea starch possessed higher proportions of both long and short side chains of amylopectin than field pea and chickpea starches. The onset, peak and conclusion temperatures of gelatinization ( T o T p and T c , respectively) were determined by differential scanning calorimetry. T o and T c ranged from 59.3 to 77.3°C, 66.8 to 79.6°C, 55.4 to 67.6°C and 68.3 to 69.3°C, respectively, for chickpea, blackgram, field pea and kidney bean starch. The enthalpy of gelatinization (Δ H gel ) of field pea, kidney bean, chickpea, blackgram and pigeon pea starches was 3.6, 3.0, 2.6‐4.2, 1.6‐1.7 and 2.6 J/g, respectively. Pastes of blackgram and pigeon pea starches showed lower storage and loss shear moduli G′ than field pea, kidney bean and chickpea starches. The changes in moduli during 10 h at 10°C revealed retrogradation in the order of: field pea> kidney bean> chickpea> blackgram> pigeon pea starch. In blackgram and pigeon pea starches, the lower proportion of amylose plus intermediate fraction and higher proportion of short and long side chains of amylopectin are considered responsible for the higher crystallinity, gelatinization temperature and enthalpy of gelatinization.