Structured emulsion gel systems for delivery of bioactive compounds : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Manawatū, New Zealand
The structure of solid/semi-solid foods greatly impacts on how the food is broken down and digested in the human body, which affects its sensory perception, and the bioaccessibility of nutrients. In this project, heat-set whey protein emulsion gel was used as a model system for solid/semi-solid foods for the delivery of capsaicinoids (CAP); the capsaicinoids were dissolved in the emulsion droplets. The aim was to investigate the effect of emulsion gel structure on the breakdown properties and sensory perception of the gel in human mouth and to understand how gel structure affects its digestion behaviour as well as the release of capsaicinoids during in vitro gastrointestinal digestion. Small and large deformation properties as well as the microstructure of the emulsion gel were evaluated. Eighteen human subjects were used to investigate in vivo oral processing behaviour and sensory perception. The Human Gastric Simulator (HGS) was used for in vitro dynamic gastric digestion and pH-stat for simulated intestinal digestion. Human intestinal epithelial cells Caco-2 were used to evaluate the irritation effect of gastric digesta by the quantification of human interleukin-8 (IL-8) production using enzyme-linked immunosorbent assay (Elisa).
Based on the rheological properties, the gels were classified into three groups: semi-solid gel (whey proteins as emulsifier, 10 mM NaCl with d4,3 of ~ 0.2 µm); soft and elastic gels (whey proteins as emulsifier, 10 mM NaCl with d4,3 of ~ 4, 1 and 0.5 µm); hard and brittle gels (whey proteins as emulsifier, 100 mM NaCl with d4,3 of ~ 4, 1, 0.5 and 0.2 µm). Results from in vivo study indicated that the degree of gel fragmentation during mastication was positively correlated with gel hardness (represented by Young’s modulus). A higher degree of fragmentation led to a greater surface exposure during mastication and, therefore, a greater release of capsaicinoid molecules, resulting in greater mouth burn perception. Results from in vitro gastrointestinal digestion of CAP-loaded soft gel and CAP-loaded hard gel showed that the hard gel was disintegrated and hydrolysed slower than the soft gel during gastric digestion. The rate and extent of lipid digestion during intestinal digestion were affected by several factors, such as fat content, gel structure, gel particle size and initial oil droplet size of the gastric digesta. Generally, the soft gel had higher degree of lipid digestion, mainly because of its soft gel structure and lower fat content. The bioaccessibility of CAP was found to be positively correlated with the extent of lipid digestion.
The effect of active (whey proteins as emulsifier) versus inactive (Tween 80 as emulsifier) filler particles of CAP-loaded emulsion gels was also investigated. CAP-loaded Tween-80-coated oil droplets were not bound within the whey protein matrix (i.e. emulsion gels containing inactive filler particles) and appeared to be flocculated and heterogeneously distributed in the gel matrix; this led to drastically decreased gel strength. On the other hand, the whey-protein-coated oil droplets had strong interactions with surrounding whey protein matrix contributing to gel strength, and the oil droplets were relatively evenly distributed in gel matrix in CAP-loaded whey protein emulsion gels (i.e. emulsion gels containing active filler particles). During in vivo oral processing, CAP-loaded Tween 80 emulsion gels were readily broken down into small fragments in the mouth at small deformations with less chewing and released large amounts of oil droplets from the gel matrix. In general, the mouth burn perception was positively correlated with degree of gel fragmentation. The large amounts of oil droplets released from the gel matrix during mastication and the inhomogeneous distribution of the oil droplets of the CAP-loaded Tween 80 emulsion gels also contributed to their greater mouth burn perception. During in vitro gastric digestion, the gel with inactive filler particles was disintegrated and emptied out faster than gel with active filler particles, due to its significantly smaller masticated particle size entering the stomach. Large amounts of oil droplets were released during gastric digestion from the gel with inactive filler particles while gel with active filler particles had minor release of oil droplets at the end of digestion. During intestinal digestion, the presence of Tween 80 in gel with inactive filler particles has slowed down the rate and extent of lipolysis, because Tween 80 had certain resistance against replacement by bile salts from the interface. Moreover, the Tween 80 molecules, once displaced by bile salts from the interface, would also participate in the formation of mixed micelles and help solubilize the released CAP molecules, therefore, leading to improved bioaccessibility of CAP.
An in vitro method was developed to quantify the gastric irritation of CAP-loaded food formulations during gastric digestion. Results suggest that Caco-2 cells had immune responses to CAP-loaded samples by secreting significant amounts of IL-8, confirming that CAP molecules are inflammatory to Caco-2 cells. The emulsion gel structure was modified using different emulsifiers: whey proteins versus Tween 80. The gastric digesta from CAP-loaded Tween 80 emulsion gel was able to stimulate more IL-8 production than CAP-loaded whey protein emulsion gel. Tween 80 was found to be a proinflammatory factor to Caco-2 cells and could stimulate IL-8 secretion.
Overall, this research provided new information on the use of solid/semi-solid systems for delivery of capsaicinoids and how food structure affects disintegration and digestion behaviour and eventually the release of capsacinoids. The outcomes have potential for designing functional foods containing capsaicinoids, with increased incorporation of capsaicinoids in the foods / pharmaceuticals, reduced irritation in the mouth and stomach and increased bioaccessibility in the intestine.