Direct quantification of protein partitioning in oil-in-water emulsion by front-face fluorescence: Avoiding the need for centrifugation

Abstract

The quantification of proteins adsorbed at the oil-in-water interface is often difficult since it requires separation of fat globules from the aqueous phase that may damage the fat globule size and/or modify the interfacial composition. Front-face fluorescence spectroscopy was used to characterize the protein partitioning between the aqueous and oil phases of emulsions without separating these two phases. Different emulsions based on skim milk powder (SMP), two mono- and di-glyceride (MDG) mixtures (saturated and partially unsaturated), and three fats (hydrogenated and refined coconut oils and refined palm oil) were studied. The impact of an ageing period (24 h at 4 °C) was also investigated to typify the first step of ice cream processing. The emulsions were characterized for protein partitioning, immediately following emulsification and after ageing, using the Bradford spectrophotometric method, applied to the aqueous phase recovered after emulsion centrifugation. In parallel, the emulsions were characterized by their tryptophan emission fluorescence spectra. The area of the peaks at 333 nm, of the fourth-derivative fluorescence spectra corresponding to the amount of proteins present in the aqueous phase of emulsions, was well correlated with the Bradford measurements ( r 2 = 0.91). This amount was also calculated from the fluorescence calibration curve obtained with SMP in solution. In conclusion, front-face fluorescence spectroscopy appeared to be a powerful and simple technique allowing the quantification of different populations of protein in an emulsified system, i.e., in the aqueous phase and loaded at the fat globule interface.