THERMAL-STABILITY OF SKIM MILK WITH PROTEIN-CONTENT STANDARDIZED BY THE ADDITION OF ULTRAFILTRATION PERMEATES

The protein content of skim milk was adjusted to values in the range s imilar to 3.4-2.4% (w/w) by the addition of various quantities of perm eate obtained by ultrafiltration of skim milk (SMP), sweet whey (SWP) or acid whey (AWP). The pH of the standardized samples was adjusted to values in the range 6.4-7.1 and heat stability determined at 140 degr ees C. Skim milk containing increasing amounts of SMP or SWP exhibited increased heat stability at all pH values, especially from pH 6.8-6.9 . The stabilizing action of these permeates was likely related to thei r low calcium content. When AWP was added to skim milk, drastic decrea ses in heat stability occurred, especially at the unadjusted pH of pro tein-standardized samples even though the pH drop due to the permeate addition was minor. Decreased heat stability was attributed to an incr eased level of soluble calcium which appeared to cause a faster rate o f pH decline on heating and possibly shifted the isoelectric point of the caseins to more alkaline pH values.