why is kidney bean skin tough
up vote 20 down vote accepted. Soften. Other things that typically are added with salt will tend to toughen the beans, but it isn't the fault of the salt. For decades, chefs have circulated the oral tradition that adding salt hardens beans, but it's a myth.n short: salt replaces calcium and magnesium in the beans' skins that make them tougher. The result is that when beans are soaked in salt water the skin softens at the same rate as the bean interior and the beans do not blow out.