Effect of relative humidity on oxidation of flaxseed oil in spray dried whey protein emulsions

Flaxseed oil was emulsified in whey protein isolate (WPI) and spray-dried. Powder characteristics and oxidative stability of oil at relative humidities (RH) from RH ∼0% to RH 91% at 37 °C were analyzed. Oil droplets retained their forms in drying and reconstitution, but the original droplet size of the emulsion was not restored when the powder was dispersed in water. The particles seemed to be covered by a protein-rich surface layer as analyzed by electron spectroscopy for chemical analysis (ESCA). Oxidation of flaxseed oil dispersed in the WPI matrix was retarded from that of bulk oil but followed the same pattern as bulk oil with respect to humidity. A high rate of oxidation was found for both low and high humidity conditions. The lowest rate of oxidation as followed by peroxide values was found at RH 75%, a condition that is likely to diverge significantly from the monolayer moisture value. A weak baseline transition observed for the WPI matrix in a differential scanning calorimetry (DSC) thermogram suggested a glassy state of the matrix at all storage conditions. This was not consistent with the observed caking of the powder at RH 91%. Scanning electron microscopy (SEM) images revealed a considerable structural change in the WPI matrix in these conditions, which was suggested to be linked with a higher rate of oxygen transport. Possible mechanisms for oxygen transport in the whey protein matrix under variable RHs are discussed.