## 3 Discussion  
Milk can be transformed into other food products, such as yogurt, using fermentation, which requires particular attributes of lactic acid bacteria [34]. The most effective strains are adapted to environmental conditions, produce acid rapidly from the lactose in milk and large amounts of volatile flavour compounds [2], such as diacetyl and acetaldehyde. Traditionally fermented dairy products rarely achieve fermentation as a result of a single bacterial strain and rely on spontaneous growth of bacteria from the environment. Usually, yogurt starter cultures are comprised of _L. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ in a 1:1 or 1:2 ratio, but with the increasing demand for commercial starter cultures in the fermentation industry, the impact of using different ratios of _L. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ on dairy flavour substances has been of increasing interest to the industry. However, little information is available on the effect of different combinations of _L. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ on the flavour of yogurt produced by conventional milk fermentation. Today, starter cultures are carefully selected to produce dairy products with particular attributes. With the rapid development of the dairy industry, some tailor-made mixed cultures of known species of lactic acid bacteria have been developed as starter cultures. Mixed cultures promote biosynthesis of metabolites, which are important fermentation characteristics in a starter culture. These include mixtures with different proportions of _L. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ with good fermentation characteristics that produce specific flavours. The main quality characteristics of yogurt include texture, taste, aroma and flavour [35]. In this study, the effect of five different combinations of _L. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ on the fermentation characteristics of milk and on the volatile flavour compounds produced was evaluated. In addition, it should be noted that this study focused mainly on determining the differences in fermentation characteristics and flavour profiles between the different combinations and the commercial yogurt starter culture used as a control. In this study, we evaluated the effect of different ratios of _L. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ as starter cultures on pH, TA, viscosity, WHC and viable cell counts during the fermentation of yogurt. The fermentation characteristics of lactic acid bacteria are important factors in screening starter culture strains. One of the most important indicators for screening starter cultures is fermentation time. We observed different fermentation times in each sample compared with the control (Table 1). [PERSON] et al. suggested that yogurt tasted better when the titratable acidity of the fermented milk was controlled between 70 and 110 \({}^{\circ}\)T [36]. During fermentation, the TA increased significantly, and pH decreased in all samples and the control. At the end of fermentation, the TA was above 70 \({}^{\circ}\)T in all samples (Figure 1). As an indicator of viability, viable cell counts are considered a key parameter in the development of fermented dairy products. It is generally accepted that the viable cell counts in yogurt should be maintained at 10\({}^{7}\) CFU/mL or