Effect of pH, Calcium, and Heat Treatment on Curd Tension of Casein Fraction Fortified Skim Milk

Abstract

The effect of α- or β-casein fortification (12 to 28% above control at 3.3% total protein) on skim milk curd strength was studied. After addition of 1N hydrochloric acid to pH 6.0, α-casein fortified skim milk produced a weaker curd than β-casein fortification. With added calcium chloride (.01M), both fractions formed a firmer curd with α-casein slightly stronger than β-casein.Alpha and β-casein fortified skim milk cultured with Streptococcus lactis had similar curd tension within a pH range of 5.20 to 5.85. Calcium addition (.01M) increased α-casein fortified skim milk curd tension but had a depressing effect on β-casein as compared to their respective curd strengths without added calcium. However, as pH approached 5.20, β-casein curd strength increased faster per pH unit than that of the α-casein fortified samples.Heat treatments within an effective heat range of .05 to .42 (holding time 10min) allowed α-casein fortified skim milk to form a stronger curd than that fortified with β-casein. When compared to unfortified skim milk, the greatest difference in curd tension between the α- and β-casein fortifications was at effective heat .42. Alpha casein curd tension was reduced nearly three times as fast as β-casein when skim milk fortified samples were heated at .42 in the presence of .005M calcium chloride addition.