Hard-to-cook defect in black beans: hardening rates, water imbibition and multiple mechanism hypothesis
Jod M. Aguilera & R. Rivera
Department of Chemical Engineering, Catholic University, PO Box 6177, Santiago, Chile
The existence of a slower initial hardening period during storage of dry black beans, independent of cooking time and heatpretreatment, was corroborated. Water vapor production during storage for 1 year at 25 and 35°C did not change appreciably the moisture in dry beans stored at 10 and 12% moisture content. Further support to the hypothesis that water in soaked hard beans is held differently from that in soft beans was based on differences in the rate of drying and microstructural evidence. A dual contribution to thehardening mechanism, one reversible and another irreversible, was postulated based on soaking experiments with monovalent salts and chelating agents. Cell separation was not changed by salt treatment although the cell wall morphology was modified.
Keywords: beans, hardening,
mechanism, salts, hard-to-cook.
INTRODUCTION Hardening of legumes, also known as the hard-toisthe failure of imcook (HTC) phenomenon, properly stored seeds to soften enough to be eaten after cooking for a reasonable time. It is a major constraint to the consumption of Phasedus and Vigna cultivars in Latin America and Africa, where high ambient temperatures and relative humidities exist all year round (Aguilera & Stanley, 1985; McWatters, 1987). The visible result of HTC at themicrostructural level appears to be the failure of the middle lamella of the cotyledon cells to soften or dissolve and the cells to separate (Jones & Boulter, 1983). The biochemical mechanisms leading to this gradual deterioration are still not well understood (Stanley & Aguilera, 1985). There has been a renewed and consistent effort recently to understand the biochemical nature of the hardening phenomenon inlegume seeds. An impediment to this work is that determining biochemical changes in situ is extremely complicated so most experimental results are based on assays
Food Research International 0963-9969/92/$05.00 0 1992 Canadian Institute of Food Science and Technology
of extracts from the seed. Recent research suggests that multiple mechanisms are responsible for hardening (Hincks &Stanley, 1986). Hardening can be partially alleviated by soaking beans in a solution of chelating agents and monovalent cations (Rockland & Jones, 1974) but a residual hardness persists that may be due to a lignification-type reaction and formation of a threedimensional phenolic polymer (Varriano-Marston & Jackson, 1981; Hincks & Stanley, 1987). A complementary approach to this biochemical work is tostudy the response of the hardening rate curves to controlled changes in storage temperature and water activity of the bean (Aguilera & Ballivian, 1987). It is now well documented that these two environmental variables are key factors in the rate and extent of hardening. A mathematical model has been developed allowing for accurate prediction of bean hardening during storage (Aguilera & Hohlberg,1989). This work addresses several issues of the bean hardening phenomenon: the existence of a pseudoinduction period in the hardening rate; changes in moisture content of packaged beans; the location of water inside soaked seeds; the influence of salts on texture; and the multiple mechanism question.
J. M. Aguilera, J. Rivera
To test whether an inductionperiod existed, sun dried black beans (Phaseolus vulgaris var. Orfeo) harvested in March 1988 were placed in storage 40 days after harvest. Beans (approx 200 g) with a moisture content of 13-14% were placed in aluminium foil/polyethylene laminated bags and kept at 35-37°C. Beans retrieved from storage were either soaked in distilled water or not soaked. Cooking was performed after discarding the...