Food structure modification in the gastrointestinal environment and its impact on the delivery of lipophilic bioactive compounds : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology, Riddet Institute, Massey University, Palmerston North, New Zealand

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Massey University
Thesis will be uploaded when the journal embargo on Chapter 5 expires in July 2024.
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Lipophilic bioactive compounds such as curcumin, polyphenols, etc. are often encapsulated as lipid-based delivery systems before adding into various foods The food matrix and its structural reorganizations under the influence of gastric and intestinal conditions affect their release, and uptake and utilization by the human body. Understanding the digestion behaviour of different food matrices is mandatory to modulate the release kinetics of bioactive ingredients. This thesis focuses on understanding how microstructural rearrangement of the dairy and starch-based foods during gastric digestion influences the bio-accessibility of curcumin. Initially a curcumin nanoemulsion (CNE) was optimized with high encapsulation efficiency (~ 94%) along with acceptable shelf stability. Further, these CNEs were incorporated into milk, milk gels or corn starch gels. Using the human gastric simulator (HGS), these food systems were digested before being subjected to intestinal digestion in a static in vitro intestinal model. The results showed that milks reconstituted from low-heat, medium-heat, and high-heat skim milk powders exhibited significantly distinct crud structures and disintegration behaviours in the stomach due to the varying degrees of casein/whey protein interactions that occurred during milk powder manufacture. The reconstituted milk made using high-heat milk powder formed soft curd under dynamic gastric conditions, resulting in a faster outflow of proteins and entrapped curcumin nanoemulsion droplets. Thus, the changes in the gastric digesta profiles influenced both the rate of lipid hydrolysis and the bioaccessibility of curcumin during intestinal digestion. Milk gels were formed using rennet enzyme or acid with similar rheological and compositional profiles. The gastric emptying was significantly impacted by the way these gels disintegrated during dynamic gastric digestion. The curd particles from the acid- gel were digested much faster than that from the rennet gel. The composition of the digesta was affected by these changes to the curd structures and stomach emptying rates during the gastric phase, which altered how the oil droplets were released from the stomach. This in turn affected the related lipophilic curcumin's bioaccessibility during the intestinal phase. Furthermore, the digestion behaviour of corn starch gels made from waxy, native, and high amylose corn starches with added CNE were investigated. The physicochemical properties of the gels were drastically altered by the addition of curcumin nanoemulsion. Because of the waxy gel's adhesive character, most of the oil droplets were held inside the gel fragments throughout the dynamic gastric phase. This resulted in the delayed breakdown and emptying of gels from the stomach. This variation in the compositional and structural characteristics of the gastric digesta was further connected to the varying rates of starch hydrolysis, the release of free fatty acids, and the associated proportion of bioaccessible curcumin. These findings in this thesis highlight how the release of health-promoting bioactive compounds from food matrix can be manipulated by understanding the complex dynamic processing behaviour of the food materials within the gastrointestinal tract. This can further help in designing novel functional foods for various populations.