Effect of phytosterol enrichment on the crystallisation, physiochemical, and interfacial behaviour of bulk and emulsified milk fat triacylglycerol matrices

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Zychowski, Lisa
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University College Cork
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Phytosterols possess the ability to significantly lower levels of low-density lipoprotein (LDL) cholesterol in the blood, but their bioaccessibility is dependent upon the solubility of the phytosterol in the consumable food or pharmaceutical product. Phytosterols are one of the most commonly used groups of bioactive compounds in the functional food industry. However, very little research has examined how phytosterols crystallise within food systems or how the physiochemical properties of the food system change upon phytosterol addition. The studies in this thesis investigated phytosterol addition in bulk and emulsified milk fat matrices, as dairy products are a common matrix for phytosterol enrichment. The main objectives of the thesis were to: (i) characterise the collective crystalline behaviour of both milk fat and phytosterols; (ii) quantify how phytosterol enrichment influences the physiochemical properties of the food system; (iii) investigate how an oil-in-water (o/w) interface influences phytosterol and milk fat crystalline behaviour; and (iv) develop and assess the means by which phytosterol solubility could be improved in milk fat matrices. In both the emulsion and bulk milk systems employed, the level of phytosterol-enrichment in milk fat was either 0 (the control), 3, or 6%. In phytosterol-enriched emulsions, whey protein (1%) was employed as the emulsifier in emulsions with 10% oil and 89% water. During cooling, phytosterols addition altered the nucleation temperature of emulsions, but no such effect was identified in bulk milk fat. During the crystallisation process, emulsified milk fat triacylglycerols (TAG) packing expanded upon phytosterol enrichment, which was observed as an increase in the triple-chain length (3L). In bulk milk fat, both doublechain length (2L) and 3L TAG packing was seen during cooling; the 3L spacing also increased with phytosterol enrichment, but no differences were seen in the 2L packing. These results suggest that phytosterols can insert themselves into the milk fat TAG network during cooling; however, the overall polymorphic form did not change. After storage, the milk fat TAG network developed into more structured polymorphic forms, and phytosterols were no longer found within the TAG packing. Phytosterols were also found to be able to decrease the average size of droplets in an emulsion and participate in a synergistic interaction with whey protein at the emulsion interface. In addition, phytosterol enrichment was found to have no negative effect on the creaming behaviour of the emulsions. Phytosterol crystallisation was altered by the addition of low molecular weight surfactants, lecithin and monoacylglycerol (MAG), and by changing the average droplet size from 1.0 to 0.2 µm. Lecithin and MAG addition significantly decreased phytosterol crystallisation in the bulk form, but changes in phytosterol crystallisation behaviour in the emulsified form were mainly driven by the decrease in droplet size. The lecithin-containing emulsions with the smaller droplets, however, showed the greatest potential for improved phytosterol solubility; in addition, they possessed better emulsion stability, as compared to the control and MAG-enriched emulsions. As the crystallisation properties of an emulsion are greatly affected by the o/w interface, two purified milk fat TAG lipids without surfactants or emulsifiers were studied in a grazing incidence synchrotron system. Differences between the lipid droplets were distinguishable, but further work is needed on the droplet deposition. In conclusion, the studies conducted in this thesis provide important insight on the behaviour of phytosterols in a model TAG-based system and can be utilised by the functional food or pharmaceutical industries to improve the bioaccessibility of phytosterols and possibility other lipophilic bioactives.
β-sitosterol , Emulsion , Phytosterols , Milk fat , Functional foods , X-ray diffraction
Zychowski, L. M. 2018. Effect of phytosterol enrichment on the crystallisation, physiochemical, and interfacial behaviour of bulk and emulsified milk fat triacylglycerol matrices. PhD Thesis, University College Cork.