Preliminary Study on the Role of Ionic Calcium in Gelation and Proteolysis of UHT Milk

A study was conducted to determine the influence of ionic calcium on gelation and proteolysis in milk. Raw milk was heated to 142 °C for 2s followed by cooling to 5 °C to make UHT milk. Two sequestering agents, Trisodium citrate (TSC) and Sodium hexametaphosphate (SHMP) were added at 0.04 - 0.08% (w/v) to the processed UHT milk to reduce the ionic calcium levels. Proteolysis and gelation were induced by addition of trypsin (248 BAEE units), 0.03% (v/v) chymosin and 106 cfu/mL Pseudomonas fluorescens NCIMB 702085 (Ps. fl. 416) to UHT milk. Samples were stored at 25 °C for 2½ weeks to monitor gelation and Ca2+ and some were incubated at 37 °C for 2h to monitor proteolysis. SHMP reduced more Ca2+ than TSC. Ca2+ reduction was accompanied by an increase in pH, most evident with TSC at 25 °C. Gelation was not observed in samples inoculated with Ps. fl. 416 (with sequestering agents) even after 9 days of storage, suggesting the importance of calcium in gelation. Chymosin treated samples gelled on day 0, whereas other samples gelled after 4 days. Trypsin increased Ca2+ to levels higher than originally present in control UHT skim milk. Although in the current study, proteolysis was higher in samples inoculated with Pseudomonas fluorescens 416, no clear relationship was established between proteolysis and gelation in UHT milk. This observation implies that proteolytic activity is not influenced by Ca2+. However, longer time study using sodium azide to prevent bacterial contamination would be required to confirm these findings.