Digestibility of legume starches as influenced by their physical and structural properties

Digestibility of common legumes in India (black gram, chickpea, mung bean, lentil, field pea and pigeon pea) was studied and related to their physical (thermal and pasting) and structural (amylose content and crystallinity) properties. All legume starches exhibited a characteristic C-type diffraction pattern with relative crystallinity ranging between 27.2% and 33.5%. Pigeon pea starch showed the highest values for molecular weight (M_w) of amylopectin (389 × 10⁶ g/mol) and amylose (3.64 × 10⁶ g/mol) whereas, field pea starch showed the lowest values for M_w of amylopectin (239 × 10⁶ g/mol). The enthalpy for melting (ΔH) was the highest for field pea starch whereas, mung bean showed the lowest. Peak viscosity (PV) ranged from 3942 mPa s (chickpea) to 6107 mPa s (mung bean). Slowly digestible starch (SDS) content followed the order: mung bean > chickpea > field pea > lentil > black gram > pigeon pea, whereas, the resistant starch (RS) content followed the following order: pigeon pea > lentil > black gram > field pea > chickpea > mung bean. The hydrolysis indices (HI) of the legume starches ranged from 8.2 to 20.0, and the estimated glycemic indices (GI) based on the HI were between 44.2% and 50.7%. Several significant correlations were observed among different starch properties as revealed both by Pearson correlation (PC) and principal component analysis (PCA). Together, the first two PCs represent 86.6% of total variability. Digestibility of starch was negatively correlated with starch granule diameter (r = -.791, p < .05) and M_w of amylopectin and amylose (r = -.845 and -.837, respectively, p < .05). A negative correlation between relative crystallinity and amylose content was observed (r = -.775, p < .5). M_w of amylopectin was positively correlated to relative crystallinity (r = .914, p < .01) and negatively correlated to amylose content (r = -.874, p < .05).