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The overall goal of this study was to characterize the distribution of a model bioactive encapsulant in the lipid domain of SLNs and NLCs and its relationship with loading efficiency and reactivity of the model encapsulant with oxidative stress agents. Distribution of a model bioactive (beta-carotene) was compared to that of a fluorescent dye (Nile red) in SLNs, 10% NLC, 30% NLC, 50% NLC, 70% NLC (the number represents the percentage of liquid lipid within the total lipid amount) and emulsions. Fluorescence imaging shows that the distribution of Nile red in the lipid domain of colloidal carriers was similar to that of beta-carotene in all formulations. Based on the combination of imaging observations and loading efficiency measurements, the results demonstrate that beta-carotene was excluded from the lipid domain in both SLNs and NLCs. The extent of exclusion decreased, while uniformity in the distribution of encapsulant in the lipid domain of colloidal carrier increased with an increase in percentage of liquid lipid content of NLCs. Oxidative stability of the encapsulated beta-carotene in SLN and NLCs (at least until 30% liquid lipid composition) was significantly lower compared to that in emulsion. Only for the NLCs with 50 and 70% liquid lipid content, oxidative stability of the encapsulated compound was significantly higher than that in emulsions. Overall, the results demonstrate that differences in loading efficiency and oxidative stability of beta-carotene in SLNs and NLCs may be explained by the differences in the distribution of beta-carotene.

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Efficient delivery of bioactives remains a critical challenge due to their limited bioavailability and solubility. While many encapsulation systems are designed to modulate the digestion and release of bioactives within the human gastrointestinal tract, there is limited understanding of how engineered structures influence the delivery of bioactives. The objective of this study was to develop a real-time quantitative method to measure structural changes in emulsion interface during simulated intestinal digestion and to correlate these changes with the release of free fatty acids (FFAs). Fluorescence resonant energy transfer (FRET) was used for rapid in-situ measurement of the structural changes in emulsion interface during simulated intestinal digestion. By using FRET, changes in the intermolecular spacing between the two different fluorescent probes labeled emulsifier were characterized. Changes in FRET measurements were compared with the release of FFAs. The results showed that bile salts and pancreatic lipase interacted immediately with the emulsion droplets and disrupted the emulsion interface as evidenced by reduction in FRET efficacy compared to the control. Similarly, a significant amount of FFAs was released during digestion. Moreover, addition of a second layer of polymers at emulsion interface decreased the extent of interface disruption by bile salts and pancreatic lipase and impacted the amount or rate of FFA release during digestion. These results were consistent with the lower donor/acceptor ratio of the labeled probes from the FRET result. Overall, this study provides a novel approach to analyze the dynamics of emulsion interface during digestion and their relationship with the release of FFAs.

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