Pre-drying thermal treatment affects commercial dried milk powder protein functionality.

The properties of nonfat dried milk (NDM) powder, a multifunctional ingredient, are intimately linked to how long it is heated before it is dried. Commercial NDM is classified, based on the extent of its heat pretreatment, as either a high-heat (HH), medium-heat (MH) or low-heat (LH) ingredient.

The effect of heat-treating fluid milk proteins has been extensively studied. However, how a predrying heat treatment affects the functionality of the NDM protein is poorly understood. The aim of research at Pennsylvania State University was to determine the effect of a pre-drying thermal treatment on NDM protein functionality. The results suggest that the pre-drying treatment greatly affects the characteristics and functionality of NDM proteins.

The total and free sulfhydryl content of LH-NDM and HH-NDM powders was determined using a commercial assay. Researchers isolated casein and whey protein fractions from commercial LH-NDM and HH-NDM using ammonium sulfate precipitation. They then subjected them to physicochemical analysis to determine lysine concentration, surface hydrophobicity, nominal molecular weight and the extent of denaturation.

Finally, the functionality of LH-NDM and HH-NDM was compared by the oscillatory rheology of dglucono-lactose (GDL) induced gels. NDM total sulfhydryl content (micromoles per g) did not change significantly with different heat pre-treatments: 2.6 [+ or -] 0.9 for LH and 2.5 [+ or -] 0.11 for HH pre-treatments. The exposed soluble sulfhydryl (SH) content, however, showed significant differences with high values for HHNDM powders: 0.91 [+ or -] 0.03 for LH and 1.3[+ or -]0.04 for HH pre-treatments.

A physiochemical analysis of the casein and whey protein fractions showed an increase in the extent of surface hydrophobicity and denaturation, and a decrease in lysine concentration when using a higher heat treatment. Rheological analysis demonstrated that the gels from HH-NDM powders had a higher G' value (109.4 Pa) than the LH-NDM powders (23.1 Pa). The HH-NDM powders also showed a greater stability against the effects of aging.

Further information. Robert Roberts, Department of Food Science, Pennsylvania State University, 212 Borland Laboratory, University Park, PA 16802; phone: [phone]; fax: [phone]; email: [e-mail].