Development and characteristics of green tea kombucha : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Albany, New Zealand

Abstract

Fermentation by microorganisms plays an important role in the development of manufactured food due to its ability to extend the shelf-life of the products as well as improving the nutritional value and sensory properties. Demand and interest in the consumption of fermented products, such as yoghurt, cheese, buttermilk and meat has continued to increase in the past few decades, due to their beneficial health effects which include prevention of chronic disease and enhancement of the immune system. However, because of the health challenges of these products such as lactose intolerance, high cholesterol and fat content, as well as protein allergies, consumer interest in the consumption of non-dairy and plant-based fermented products such as water kefir and kombucha is growing. Kombucha is a traditional refreshing home-made beverage with a slightly acidic, sweet and alcoholic taste, which is thought to have originated in Germany, China and Russia, but is now consumed worldwide. Kombucha is usually produced by the fermentation of tea and sugar with a symbiotic consortium of acetic acid bacteria and yeasts, commonly known as the kombucha starter culture. The physico-chemical characteristics, microbial profiles and sensory properties of kombucha are significantly affected by fermentation conditions including sugar concentration, fermentation time and temperature. The majority of previous studies have focused on the comparison of different substrates for kombucha preparation and their effects on the final composition of kombucha. Therefore, there is scanty information in the development and characteristics of kombucha under different production conditions. The present study investigated the effects of sugar concentration, fermentation time and temperature on the fermentation of green tea kombucha aimed at optimising the process to develop a consistent high quality beverage. The development and characteristics of green tea kombucha were investigated in four- integrated experimental phases. Phase 1 enumerated the microflora in the kombucha starter culture (acetic acid bacteria, lactic acid bacteria and yeasts). The effect of fermentation time (7, 10 and 14 days) on the development of kombucha was studied in Phase 2 in order to select the optimum fermentation time. In Phase 3, the effects of two sugar concentrations (7% and 10%) and two fermentation temperatures (22℃, 24℃) on the physico-chemical, microbial and sensory characteristics of green tea kombucha were studied, with the aim of selecting the optimum sugar concentration and fermentation temperature for the development of green tea kombucha. The antibacterial activity of the final optimised green tea kombucha were investigated and the stability of the beverage was monitored during storage (4℃) for 4 weeks in Phase 4. Various analyses of green tea kombucha samples were conducted during fermentation and storage in order to investigate the physico-chemical, microbial and sensory characteristics of the beverage: sugars, organic acids, ethanol, antioxidant, titratable acidity (TA) and viable cell counts (VCC) of kombucha microorganisms were analysed, pH, total soluble solids (TTS) and colour were also measured. Acetic acid bacteria (6.08±0.06 log cfu/ml) and yeasts (7.13±0.07 log cfu/ml) were present in the kombucha starter culture used in this study, while no lactic acid bacteria were found. Results from Phase 2 showed that fermentation time contributed to the physico-chemical, microbial and sensory properties of green tea kombucha. In Phase 2, TA increased steadily from Day 7 (0.36±0.02 - 0.42±0.04) to Day 14 (0.88±0.04 - 1.01±0.06) (p<0.05), while pH, total soluble solids, VCC and overall consumer acceptability decreased (p<0.05). In Phase 3, kombucha samples containing 7% or 10% sugar and fermented at 24℃ for 7 days were characterised by higher levels (p<0.05) of organic acids with lower pH, TSS and VCC than kombucha fermented at 22℃. No significant differences (p<0.05) in colour, VCC and levels fermentation. Based on the physico-chemical, microbial and sensory characteristics of green tea kombucha beverage in Phases 2 and 3, the optimum fermentation conditions were kombucha containing 7% sugar and fermented at 22℃ for 7 days. The results of the disc diffusion studies showed that the final optimised green tea kombucha had antibacterial activities against Escherichia coli 111, Listeria monocytogenes 15E03-1, Salmonella typhimurium ESR3479, Staphylococcus aureus MU-A57 and Pseudomonas aeruginosa MU-A26. High quantities of antioxidants (gallic acid = 5.7±0.04 µg/ml, EGC = 130.89±6.86 µg/ml, EGCG = 152.26±39.70 µg/ml and ECG = 41.11±16.23 µg/ml) were also present in this beverage. These observations suggested that consumption of green tea kombucha may exert beneficial health effects. During storage (4℃) for 4 weeks, the colour of the optimised green tea kombucha was stable and the consumer acceptability of green tea kombucha beverage remained high. Green tea kombucha containing 7% sugar and fermented at 22℃ for 7 days was well-liked by consumer panellists (n=60) and this beverage contained 0.35±0.03% (w/v) gluconic acid, 0.31±0.00% (w/v) acetic acid and high levels of certain antioxidants which may confer beneficial effects on human health.