Addressing spatiotemporal signal variations in pair correlation function analysis.

Fluorescence correlation spectroscopy (FCS) is a cornerstone technique in optical microscopy to measure, for example, the concentration and diffusivity of fluorescent emitters and biomolecules in solution. The application of FCS to complex biological systems, however, is fraught with inherent intricacies that impair the interpretation of correlation patterns. Critical among these intricacies are temporal variations beyond diffusion in the quantity, intensity, and spatial distribution of fluorescent emitters. These variations introduce distortions into correlated intensity data, thus compromising the accuracy and reproducibility of the analysis. This issue is accentuated in imaging-based approaches such as pair correlation function (pCF) analysis due to their broader regions of interest compared with point-detector-based approaches. Despite ongoing developments in FCS, attention to systems characterized by a spatiotemporal-dependent probability distribution function (ST-PDF) has been lacking. To address this knowledge gap, we developed a new analytical framework for ST-PDF systems that introduces a dual-timescale model function within the conventional pCF analysis. Our approach selectively differentiates the signals associated with rapid processes, such as particle diffusion, from signals stemming from spatiotemporal variations in the distribution of fluorescent emitters occurring at extended delay timescales. To corroborate our approach, we conducted proof-of-concept experiments on an ST-PDF system, wherein the, initially, uniform distribution of fluorescent microspheres within a microfluidic channel changes into a localized accumulation of microspheres over time. Our framework is offering a comprehensive solution for investigating various phenomena such as biomolecular binding, sedimentation, and particle accumulation.

A mechanistic kinetic model for lipid oxidation in Tween 20-stabilized O/W emulsions.

Models predicting lipid oxidation in oil-in-water (O/W) emulsions are a requirement for developing effective antioxidant solutions. Existing models do, however, not include explicit equations that account for composition and structural features of O/W emulsions. To bridge this gap, a mechanistic kinetic model for lipid oxidation in emulsions is presented, describing the emulsion as a one-dimensional three phase (headspace, water, and oil) system. Variation in oil droplet sizes, overall surface area of oil/water interface, oxidation of emulsifiers, and the presence of catalytic transition metals were accounted for. For adequate predictions, the overall surface area of oil/water interface needs to be determined from the droplet size distribution obtained by dynamic and static light scattering (DLS, SLS). The kinetic model predicted well the formation of oxidation products in both mono- and polydisperse emulsions, with and without presence of catalytic transition metals.

Influence of emulsifier on lipid oxidation in spray-dried microencapsulated O/W emulsions.

Lipid oxidation limits the shelf-life of dried microencapsulated oils (DMOs), such as infant formula. However, it is poorly understood how lipid oxidation is affected by different types of emulsifiers. To improve our understanding, we prepared DMOs with different emulsifiers (whey protein isolate (WPI), pea protein isolate (PPI), and non-proteinaceous CITREM) and studied lipid oxidation in both the free and encapsulated fat. Only a small difference in oxidation rate was observed between these fat fractions for all formulations. We ascribed this to a non-discrete distribution of the fractions and the subsequent low fractionation selectivity as shown by Raman microscopy. The DMO with PPI showed hardly any oxidation during a 7-week incubation at 40 °C, whereas the DMOs with WPI and CITREM both reached significantly higher contents of oxidation products (lipid hydroperoxides, aldehydes, and epoxides). The enhanced stability of DMO-PPI could not be ascribed to the presence of phytic acid. In conclusion, we demonstrate the potential of using PPI to produce oxidatively stable DMOs.

Unravelling the effect of droplet size on lipid oxidation in O/W emulsions by using microfluidics.

Lipid oxidation in emulsions is hypothesised to increase with decreasing droplet size, as this increases the specific oil-water interfacial area, where lipid oxidation is expected to be initiated. In literature, however, contradictory results have been reported, which can be caused by confounding factors such as the oil droplet polydispersity and the distribution of components between the available phases. In this work, monodisperse surfactant-stabilised emulsions with highly controlled droplet sizes of 4.7, 9.1, and 26 µm were produced by microfluidic emulsification. We show that lipid oxidation increases with decreasing droplet size, which we ascribe to the increased contact area between lipids and continuous phase prooxidants. Besides, a significant amount of oxygen was consumed by oxidation of the surfactant itself (Tween 20), an effect that also increased with decreasing droplet size. These insights substantiate the importance of controlling droplet size for improving the oxidative stability of emulsions.