Studies on the hard-to-cook defect of beans (Phaseolus vulgaris L.)
del Valle, Jose Manuel
Ph.D., University of Guelph (Canada), 1992, 147 pages.
1992
บทคัดย่อ
Beans undergo an undesirable textural defect, referred to as 'hard-to-cook', during postharvest storage at adverse conditions, which is characterized by limited softening after prolonged cooking, and is responsible for nutritional and economic losses of tremendous practical importance in Third World countries.
A popular source of support for the hypothesis that the hard-to-cook defect is due to impaired cell separation during cooking due to formation of ionic crosslinkages between middle lamellar pectates has come from the softening effect of chelating and ion exchange agents. The present work, however, suggested that salt-induced improvements in thermal denaturation of proteins makes a major contribution to softening of hard-to-cook beans.
Cooked hardness of beans kept at elevated temperature and high relative humidity increased with storage time, even when cooking was preceded by soaking in a concentrated solution of a strong chelating agent, and the leaching rate of pectins during cooking of hard-to-cook beans in distilled water was slower than the softening rate, suggesting that softness is not determined by cell attachment through crosslinked pectates alone. Covalent cell wall strengthening by phenolics (lignification) may also contribute to bean toughness. The 'irreversible' component of the hard-to-cook defect, that was not eliminated by soaking in a concentrated EDTA solution, was smaller in white than black beans, and in dehulled than nontreated samples, suggesting that seed coat tannins may participate in this lignification-like reaction. The 'reversible' component of the hard-to-cook defect, that was eliminated by EDTA, approached equilibrium conditions faster than the irreversible component, supporting the contention that ionic
interactions contribute to the hard-to-cook defect early.
The rate and extent of hardening were strongly dependent on storage conditions. The hard-to-cook defect appeared to be due to a multiple-step reaction(s), possibly enzymatic, controlled by reactant mobility. Data also suggested temperature-dependent activation energies for the hardening reaction, indicating the possibility of a switch in reaction control. The irreversible hardening rate was more dependent on temperature than that of reversible toughening, helping to explain temperature-dependent activation energies.
The hard-to-cook defect appears to be a physiological response of beans to environmental stresses. These stresses are not limited to storage at elevated temperature and high relative humidity, but also included soil composition and weather factors during plant development, and soaking at low temperature.