Abstract:
The importance of common beans (Phaseolus vulgaris) in addressing food insecurity cannot
be underestimated. However, their utilization is hampered by development of the hard-tocook
(HTC) defect i.e. the inability of cotyledons to soften sufficiently within a reasonable
time during cooking, presence of flatulence causing oligosaccharides, antinutrients and low
digestibility of macronutrients. The objective of this study was to determine the effect of
storage conditions (time, temperature and relative humidity) on pectic polysaccharides of
selected common bean varieties during the evolution of the hard cook problem. First, alcohol
insoluble residue (AIR) was extracted from the bean flour. The AIR was fractionated into
water, chelator and Na2CO3 soluble pectin fractions and a hemicellulose fraction. The
galacturonic acid content, neutral sugars, degree of methylesterfication (DM), degree of
acetylation (DAc) and molar mass distribution for pectin fractions were determined. In
addition, filterable residual protein in various fractions was estimated. Results on the acidic
and neutral sugars revealed that common beans contained highly branched, arabinose-rich
pectic polysaccharides. Storage of common beans for more than 4 months at high relative
humidity (83%) and high temperature (45°C) resulting in HTC development showed a
decrease in pectin extractability in water paralleled by an increase in the alkaline soluble
fraction. Other pectin characteristics such as DM and DAc showed minor variations upon
storage of beans. The hydrolysis of both starch and proteins before AIR extraction decreased
with increasing storage time, temperature and relative humidity. The increase in residual
starch and protein might be linked to the protein-starch hypothesis where predominance of
protein denaturation leads to restricted starch gelatinization. The results reveal that, the
contribution of pectic polysaccharides to development of HTC defect during the storage of
Canadian wonder and Red haricot common beans at elevated temperature and relative
humidity is due to reduced pectin solubility. However, the influence of starch and proteins
seems evident.
