Interactions between skimmed bovine milk and tea infusions under an acidulous environment: a thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Physicochemical and microstructural properties of acidified dairy gels produced with tea infusions (ADG), as well as the secondary structure of protein components, were investigated to explore the practicality of using green tea, oolong tea and black tea extracts as an additive in milk products. Reconstituted skim milk was supplemented with different levels of green, oolong or black tea infusion (15%, 30%, 45% 60%) and acidified by glucono-δ-lactone (GDL) at 35 ± 1 °C until a pH value of 4.55 was reached. The physicochemical and textural properties were measured by texture analyser, and the antioxidant capacity of the gels was determined through four assays - Total phenolic content (TPC) by Folin-Ciocalteu assay, free radical scavenging ability by DPPH assay along with ABTS assay and FRAP assay. The cellular antioxidant activity and anti-proliferative activity were conducted on HepG2 cell lines. The microstructure of the ADGs was observed using different microscopic techniques (SEM (scanning electron microscope) and CLSM (confocal laser scanning microscopy)). The secondary structure of proteins in ADGs were assessed by FT-IR, Circular dichroism and X-ray diffraction. Results showed that green, oolong and black tea infusions significantly improved the antioxidant capacity of the gel system. However, lower phase stability was observed in the ADG with a high portion of tea infusion (60%), and this appeared to be related to the water holding capacity (WHC) and microstructure. The microstructure of the ADGs observed is related to the phase stability and textural attributes. The addition of tea extracts had no significant impact on the secondary structure of proteins, the addition of oolong tea and black tea extracts significantly changed the particle size of the ADG which could be related to the changes of textural properties. The results suggest that tea polyphenols improved antioxidant capacity in all samples and interactions between tea polyphenols and dairy components improved the texture of ADG only at low portions of tea infusion which could due to the changes of Coulombian barrier and microstructure as well as the formation of large networks of protein/polyphenols/Ca2+.