INFLUENCE OF ICE-CREAM MIX COMPONENTS ON THE THERMAL-STABILITY OF BOVINE-MILK GAMMA-GLUTAMYL-TRANSPEPTIDASE AND LISTERIA-INNOCUA

The influence of various components of ice cream mixes (milkfat, milk- solids nonfat, sucrose, stabilizer) on the thermal stability of bovine milk gamma-glutamyl transpeptidase (TP) and Listeria innocua (LN) was determined. A factorial Screening Design experiment revealed that D v alues for TP at 71, 74, and 77 degrees C were significantly (P less th an or equal to 0.05) increased in ice-cream mixes over the values foun d in whole milk, with an average 8.9-fold increase for D-71. In contra st, D values for LN at 60, 63, and 66 degrees C were not significantly influenced by mix components, although an average 2.4-fold increase w as found for D-60 indicating that mix components exert some protective effect toward LN. Likewise, the TP z-value was not affected by the co mponents of the mix. The Screening Design was augmented into a Central Composite Design and a response surface model for TP was derived for log(10)D(71). A total of 90% of the variation in the data was explaine d by the model which incorporated linear, quadratic and interactive ef fects of the four mix components. Copyright (C) 1996 Elsevier Science Limited.