Effect of NaCl on textural changes and protein and lipid degradation during the ripening stage of sufu, a Chinese fermented soybean food
Bei-Zhong, Han | Jai-Huai, Wang | Rombouts, Frans M. | Nout, M.J.Robert
Sufu is made by solid state fungal fermentation (using Actinomucor elegans) of tofu, followed by salting and maturation in dressing mixtures containing salt, alcohol and various other ingredients. NaCl in dressing mixtures strongly affected the changes in textural properties and the hydrolysis of protein and lipid of sufu. Higher salt contents (14% w/w) resulted in increased hardness (+100%) and elasticity (+18%) and reduced adhesiveness (-30%). Hardness and elasticity could be used to judge the extent of sufu ripening. SDS-PAGE showed the disappearance of all protein subunits at 80 and 110 g kg(-1) salt content; however, some protein subunits were still detectable at 140 g kg(-1) salt content after 60 days of ripening. Higher ratios of free amino nitrogen to total nitrogen (FAN/TN = 0.4-0.45) and free amino acids to crude protein (FAA/CP = 0.24-0.26) were observed in sufu with lower (80 g kg(-1)) salt content. FAN/TN and FAA/CP in white sufu (obtained with dressing mixtures containing only salt and alcohol) were higher than those in red sufu (obtained with dressing mixtures containing angkak or kojic red rice) owing to different dressing mixture compositions. Increases in free fatty acids (FFA) were also observed during ripening. FFA levels in sufu with lower salt content increased rapidly during the first 30-40 days and then increased slowly, probably resulting from the formation of fatty acid esters. Lowering the salt content (80 g kg(-1)) can shorten the ripening time to 40 days, which is of benefit to manufacturers. However, sufu will spoil, ie undergo souring, during the ripening stage at salt contents of 50 g kg(-1) or lower.
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