The effect of green tea (Camellia Sinensis) on lactic acid fermentation of milk : effects on physical, nutritional and functional properties of yogurt / Shabboo Amirdivani

Amirdivani, Shabboo (2014) The effect of green tea (Camellia Sinensis) on lactic acid fermentation of milk : effects on physical, nutritional and functional properties of yogurt / Shabboo Amirdivani. PhD thesis, University of Malaya.

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Abstract

Yogurt is a healthy food and it has become good candidate for fortification with many healthful ingredients including fruits, herbs, prebiotics, probiotics and omega-3 fatty acids. Whatever the fortification strategies, it is essential that the finished product retain a desirable taste, becomes more nutritious and has long shelf life. The present study investigated the benefits green tea would bring to increase yogurt properties by virtue of the high levels of flavonoids which can protect cells and tissues from oxidative damage by scavenging oxygen-free radicals. The presence of green tea would however affect yogurt bacteria fermentation of milk and subsequently the typical fermentation products responsible for the physicochemical properties (e.g organic acids), nutritional (e.g. digested protein and carbohydrate) and functional (e.g. bioactive peptides, organic acids, viable lactic acid bacteria) values of yogurt. On the other hand changing acidic environment and microbial metabolic activities are expected to exert profound effects on the breakdown of green tea phytochemical contents and subsequently any biological activities associated with these compounds. Milk was incubated (41⁰C) with starter culture (SC) in the presence of two types of green tea (MGT and JGT) until pH of yogurt reached 4.5. The resulting green-tea-yogurts were evaluated with respect to changes in antioxidant activity, phenolic compounds, the growth of lactic acid bacteria (LAB), acidification, exopolysaccharide (EPS) content, organic acids and rheological characteristics of yogurt during fermentation and storage at 4⁰C. The total phenolic content (TPC) was higher in MGT than those from JGT leaves infusion. Major phenolic compounds (gallocatechin, epicatechin, epigallocatechin gallate, quercetin-rhamnosylgalactoside, and kaempferol-3-O-rutinoside) in MGT leaves infusion were higher in those JGT leaves infusion. Diphenyl picrylhydrazyl (DPPH, %) and ferric reducing antioxidant power (FRAP, mmol /L) methods were used to evaluate the antioxidant capacity of yogurts. Inhibition of DPPH oxidation was highest for MGTY followed by JGTY and PY whereas FRAP value showed highest (p>0.05) ferric reduction power for MGTY followed by JGTY and PY. Viable S.thermophillus and Lactobacillus spp. counts in yogurt were highest in MGTY followed by JGTY and PY. Proteolysis in green tea yogurts increased with increasing fermentation time resulting in highest OPA values for MGTY followed by PY at pH 4.5. Viable cells count (VCC) of S. thermophilus spp. increased by day 7 of storage with highest number present in MGTY followed by JGTY and PY. Lactobacilus spp. VCC decreased gradually for all yogurts during the 28 days of storage. Maximum DPPH inhibition by yogurts was shown on day 7 of storage by MGTY followed by JGTY and PY. The addition of green tea increased the FRAP values in MGTY which was 3 fold higher than JGTY and PY. Four major compounds were detected in green tea yogurts (quercetin-rhamnosyl, gallocatechin, kaempferol-3-rutinoside and epicatechin) with higher amounts present in MGTY than in JGTY but these tend to diminish by 28 days of refrigerated storage. The presence of green tea resulted in higher score recorded for overall appearance, colour, aroma and flavor in MGTY and JGTY compared with PY. However green tea yogurts showed more fluid like characteristics with distinct pseudo plastic properties and lesser ability to resist deformation upon applied shear. The presence of green tea during fermentation of milk increased the radical scavenging activities of yogurt and it stimulated the growth of both S. thermophillus and L. bulgaricus resulting in increased proteolysis of milk protein, acidification, enzymes activity and organoleptic properties. The catechin-related compounds, despite being not stable during refrigerated storage and had negative effects on rheology, have promising biological effects in increasing the nutritional and functional properties of yogurt.

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