Effects of pH, Calcium and Unheated Casein Micelle on Physical Properties of High-Temperature Heated Milk Rennet Curd

Abstract

Skim milk, separated from bulk cow's milk gotten from a dairy district in Miyazaki Prefecture, was heated at 85°C for 5min, immediately followed by heating at 135°C in a moment on an experimental scale. The effects of pH, Ca and intact casein micelle on the physical properties (hardness, breaking energy and elastic modulus) of the rennet curd prepared from this high-temperature heated milk were investigated using a rheometer.
The results obtained were summarized as follows:
(1) The physical properties of rennet curd of high-temperature heated and pH-adjusted (6.7-6.0) milk could not be improved up to those of unheated milk. (2) Using Ca-added (80mg/100ml) and pH-adjusted (6.7-6.0) milk, it could be seen that the physical properties of high-temperature heated milk were improved with pH lowering, whereas even the highest values of the physical properties obtained for pH 6.0-adjusted milk rennet curd could not come up to those of unheated one. (3) When unheated casein micelle (360mgN/100ml) of 0.35 or above as unheated casein micelle ratio were added to high-temperature heated milk at 6.6 or 6.2, the physical properties of milk rennet curd were greater than those of unheated one and the rennet curd of pH 6.2-adjusted milk was somewhat better than those of pH 6.6-adjusted one. (4) Rennet curd of high-temperature heated milk at pH 6.6 or 6.2 was remarkably improved by the combination of Ca addition (80mg/100ml) and unheated casein micelle fortification; pH 6.6 or 6.2-adjusted milk rennet curd was attained nearly same as unheated one by fortifing to 0.2 or 0.15 respectively, as unheated casein micelle ratio. (5) From those results it was confimed that an adequate combination of both Ca addition and native casein micelle fortification as well as sutitable acidification was essential for the improvement of high-temperature heated milk rennet curd.