Curdling of market-returned UHT fresh milk by direct acidification and an addition of calcium chloride and rennet

摘要

An investigation into the possibility of curdling the market-returned UHT fresh milk (hereafter called UHT milk) by direct acidification together with the use of calcium chloride (CaCl_2) and rennet was performed. Results showed that the UHT milk samples which were collected from the market during the period of l-s month, before the expiry date were still suitable for protein coagulation. The coagula prepared by acidifying the UHT milk with lactic acid (together with CaCl_2 and rennet) at pH 5.0 had the highest protein content (N X 6.38) and their percentage curd yield (weight of curd on dry basis / weight of milk) and moisture content were significantly (p <= 0.05) lower than those samples prepared at higher pH values, but the values for these two parameters were not significantly (p > 0.05) different from those of the samples treated at pH 4.7. It was apparent that type of acids influenced characteristics of the coagula. Moisture contents of the samples prepared by acetic and lactic acid were significantly (p <= 0.05) the lowest but protein contents of these two samples were significantly the highest. It was also found that the amounts of CaCl_2 and rennet significantly (p < 0.05) affected the percentage curd yield and the moisture content ofthe coagula. Therefore, acetic acid (10 percent w/v) together with an addition of 200 ppm CaCl_2 and 50 ppm rennet were appropriate for curdling of the market-returned UHT milk.