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Food Structure

Abstract

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 intracellular proteins during storage since coagulated proteins would limit water to starch and prevent full swelling during cooking. To verify this hypothesis, effects of soaking and heating temperature on texture, water absorption, protein water extractability and microstructure of control and HTC cowpea seeds (aged or treated) were studied. Scanning electron microscopy showed no structural difference between dry control and dry aged seeds. However, after soaking, in contrast to the control, aged seeds exhibited a coarse protein matrix with tightly embedded starch granules and resistance to fracture. Treated seeds also showed ti ght embedment of starch granules. This indicates that storage proteins had coagulated or aggregated during aging or treatments , as evidenced also by very low extractability. Unlike 60°C, heating at 85°C dramatically decreased protein extractability and led starch granules to swell partially in control seeds and little in HTC seeds. When cooked (IOO ' C), HTC seeds showed lack of cell separation and restricted starch swelling, all of which were in sharp contrast with the control. Results not only verified the aforementioned hypothesis but also reaffirmed the role of cell middle lamella, implying involvements of multiple mechanisms in bean hardening.

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