477 Czech J. Food Sci. Vol. 32, 2014, No. 5: 477–484 Optimisation of the Antioxidant Activity of Kombucha Fermented Milk Products Radomir MALBAŠA, Jasmina VITAS, Eva LONČAR, Jovana GRAHOVAC and Sp asenija MILANOVIĆ Faculty of Technology, University of Novi Sad, Novi Sad, Serbia Abstract Malbaša R., Vitas J., Lončar E., Grahovac J., Milanović S. (2014): Optimisation of the antioxidant activity of kombucha fermented milk products. Czech J. Food Sci., 32: 477–484. The antioxidant activity of fermented milk products obtained by using kombucha starter produced by fermentation on sweetened wild thyme extract was investigated. The starter was added to milk containing 0.8, 1.6, and 2.8% milk fat, at fermentation temperature of 37, 40, and 43°C. The fermentation process was terminated when the pH reached 4.5. Antioxidant activities to DPPH and hydroxyl radicals, the contents of MUFAs, PUFAs, vitamin C, and sensory mark, were monitored using the response surface methodology (RSM) and the method of desired function. Kombucha fermented milk products containing wild thyme (WT) showed opposite antioxidant response to DPPH and hydroxyl radicals in terms of milk fat. Optimum processing conditions for WT products in terms of antioxidant activity were: milk fat 2.78% and process temperature 37°C. In order to obtain WT products with a high sensory mark, these condi- tions are completely different (milk fat 1.10% and process temperature 43°C). Keywords: tea fungus; fermentation; antioxidants; wild thyme; RSM Recent studies have shown the possibility of obtain- ing fermented milk products by using kombucha as a starter culture (Malbaša et al. 2009; Iličić et al. 2012; Pejić et al. 2012; Vitas et al. 2013). The symbiotic association of acetic acid bacteria and yeasts is known as kombucha ( Dufresne & Farnworth 2000; Teoh et al. 2004; Nguyen et al. 2010). Kombucha beverage is the most commonly prepared by the biotransformation of sweetened black or green tea, but other substrates, such as herbal teas, can be also used (Dufresne & Farnworth 2000; Malbaša et al. 2009). Kombucha is rich in compounds that are known to be strong antioxidants, such as vitamins C and B 2 , as well as polyphenols, primarily catechins (Dufresne & Farnworth 2000; Jayabalan et al. 2008; Malbaša et al. 2011a). It is very well known that herbal extracts may pro- vide a variety of health benefits. The extract of wild thyme possesses antioxidant and antihypertensive activities (Mihailovic-Stanojevic et al. 2013) and other biological activities (Rehman et al. 2009). Some of the naturally occurring antioxidant con- stituents of the kombucha fermented milk products with winter savory and stinging nettle are vitamin C (mainly from kombucha) and polyunsaturated fatty acids (PUFAs) (primarily from milk fat) (Vitas et al. 2013). The antioxidant properties of PUFAs, as well as of monounsaturated fatty acids (MUFAs) are described in the literature (Berry 1997; Ambrozova et al. 2010). The aim of this article is the optimisation of the antioxidant activity of fermented milk products obtained with kombucha starter prepared on wild thyme substrate considering the fact that no kind of antioxidative analysis of this type of kombucha products has been performed up to our knowledge. Also, the influence of the content of MUFAs on the antioxidant activity of the kombucha fermented milk products has been estimated for the first time besides the influence of PUFAs and vitamin C con- tents on the antioxidant activity to hydroxyl and DPPH radicals. Supported by the Ministry of Education, Science and Technological Development of Republic of Serbia, Project No. III-46009.