MECHANISM OF HARD-TO-COOK DEFECT IN COWPEAS - VERIFICATION VIA MICROSTRUCTURE EXAMINATION

It has been hypothesized that part of the hard-to-cook (HTC) defect in cowpeas is due to decreases in solubility and thermal stability of in tracellular proteins during storage since coagulated proteins would li mit water to starch and prevent full swelling during cooking. To verif y this hypothesis, effects of soaking and heating temperature on textu re, water absorption, protein water extractability and microstructure of control and HTC cowpea seeds (aged or treated) were studied. Scanni ng electron microscopy showed no structural difference between dry con trol and dry aged seeds. However, after soaking, in contrast to the co ntrol, aged seeds exhibited a coarse protein matrix with tightly embed ded starch granules and resistance to fracture. Treated seeds also sho wed tight embedment of starch granules. This indicates that storage pr oteins had coagulated or aggregated during aging or treatments, as evi denced also by very low extractability. Unlike 60-degrees-C, heating a t 85-degrees-C dramatically decreased protein extractability and led s tarch granules to swell partially in control seeds and little in HTC s eeds. When cooked (100-degrees-C), HTC seeds showed lack of cell separ ation and restricted starch swelling, all of which were in sharp contr ast with the control. Results not only verified the aforementioned hyp othesis but also reaffirmed the role of cell middle lamella, implying involvements of multiple mechanisms in bean hardening.