Changes in protein nutritional quality in fresh and recombined ultra high temperature treated milk during storage.

The objective of this study was to determine the changes in nutritional quality of protein in stored UHT milk. Commercial fresh and recombined ultra high temperature (FUHT and RUHT respectively) treated milk samples were used. Pasteurized milk (P-milk) samples were used for comparison. Milk samples were freeze-dried for the use in diet formulation. Pasteurized milk samples were freeze-dried within 24 h of production. FUHT and RUHT milk samples were freeze-dried after production (zero time), 3 and 6 months of storage at 37 degrees C. Three nitrogen balance experiments were done at zero time, after 3 and 6 months of storage. The diet used in all experiments were similar in composition except for protein source. In each experiment egg white (low protein), casein (control), P-milk, and either 0, 3 or 6 months old FUHT and RUHT milk samples were used as the source of protein in the diets. In each experiment 30 (six for each diet) male Sprague-Dawley rats were used. Apparent digestibility (AD), true digestibility (TD), biological value (BV), and net protein utilization (NPU) were calculated for protein in casein and all milk samples. The nutritional quality of protein in FUHT milk before storage was similar to that of protein in pasteurized milk, but it decreased during storage for 3 months and then remained at the same quality up to 6 months of storage. BV and NPU of protein in FUHT milk after storage for 3 and 6 months were lower than that of protein in pasteurized milk by 4% (not significant) and 5% respectively. On the other hand, the nutritional quality of protein in RUHT milk before storage was lower than that of protein in pasteurized milk and remained at the same level after storage for 3 months: however, it decreased significantly after storage for 6 months. AD and TD of protein in RUHT milk stored for 6 months were lower than that of protein in pasteurized milk by 5% and BV and NPU were lower than that of protein in pasteurized milk by 9 and 12% respectively. The above reduction in nutritional quality of protein in UHT milk may be of significant value in feeding young children who might be dependent on this type of milk in their diet.

Measurement of Microbial Protease Activity Using a pH-Stat Titration.

The activity of purified and crude microbial proteases was measured by titration at pH 9 using an automatic pH-stat instrument. The ability of the pH-stat titration method and the trinitrobenzenesulfonic acid (TNBS) method to detect protease activity was compared. The pH-stat titration produced higher measurements of activity than the TNBS method when purified protease from Bacillus amyloliquefaciens was tested. Protease activity determination using the two methods resulted in a linear correlation of R2 = 0.985 and the same repeatability (C.V.=2.6%). The pH-stat titration method was more sensitive than the TNBS method in measuring activity of the purified protease, and it indicated greater proteolytic activity than the TNBS method when culture filtrates from five Pseudomonas spp. which produced proteolytic enzyme with greater activity at pH 9 than pH 7.5 were tested. When culture filtrates from four Pseudomonas spp. with similar but low proteolytic activity at pH 9 and 7.5 were tested, the two methods produced similar results. For proteases with optimum activity at or above pH 9, the pH-stat titration method was simplier, faster, and more sensitive than the TNBS method in determining activity.