Flavor Changes in Whole Milk Powder During Storage

The kinetics of chemical changes are usually described as being zero order, first order, and so forth. A common and generally accepted description of fat oxidation in foods is that the reaction has an initial linear phase which gradually changes to an exponential phase. In the present investigation a generalized mathematical model was developed for the overall characterization of the formation of volatile fat oxidation products. The model is continuous, and describes the rate of the oxidation during both the linear phase (zero order) and the exponential phase (first order). The break point at which the oxidation changes from zero to first order was determined by a stepwise procedure. The zero order and first order models are special cases of the generalized model. Zero order, first order, and generalized mixed models were used to describe the formation of straight chain aldehydes, methyl ketones, Strecker aldehydes, alkyl furans, sulfur compounds, and hydrocarbons in spray dried, whole milk powder during storage. The milk powder was prepared without and with additions of either a mixture of the antioxidants BHA and BHT or an antioxidative mixture of Maillard reaction products. The milk powder was stored in either air or nitrogen for 84 weeks. The formation of volatile compounds was analyzed using gas chromatographic headspace methods. A static equilibrium sampling method was used for the most volatile compounds and a dynamic sampling method for the less volatile compounds.