###### Abstract  
Lactic acid bacteria (LAB) are industrially important bacteria that are widely used in the fermented food industry, especially in the manufacture of yogurt. The fermentation characteristics of LAB are an important factor affecting the physicochemical properties of yogurts. Here, different ratios of _L. delbrueckii_ subsp. _bulgaricus_ IMAU20312 and _S. thermophilus_ IMAU80809 were compared with a commercial starter JD (control) for their effects on viable cell counts, pH values, titratable acidity (TA), viscosity and water holding capacity (WHC) of milk during fermentation. Sensory evaluation and flavour profiles were also determined at the end of fermentation. All samples had a viable cell count above 5.59 \(\times\) 10\({}^{7}\) CFU/mL at the end of fermentation, and a significant increase in TA and decrease in pH were observed. Viscosity, WHC and the sensory evaluation results of one treatment ratio (A3) were closer to the commercial starter control than the others. A total of 63 volatile flavour compounds and 10 odour-active (OAVs) compounds were detected in all treatment ratios and the control according to the results from solid-phase micro-extraction-gas chromatography-mass spectrometry (SPME-GC-MS). Principal components analysis (PCA) also indicated that the flavour characteristics of the A3 treatment ratio were closer to the control. These results help us understand how the fermentation characteristics of yogurts are affected by the ratio of _L. delbrueckii_ subsp. _bulgaricus_ to _S. thermophilus_ in starter cultures; this is useful for the development of value-added fermented dairy products. starter culture; fermentation characteristics; volatile flavour compounds; SPME-GC-MS 20232821232  
Lactococcus lactis_ or _Lactobacillus plantarum_. These species are commonly known as 'yogurt starter cultures' [6]. Strains of _L. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ have been isolated from a variety of habitats, particularly traditionally fermented food [7]. These strains are defined as a ubiquitous family of microbes that can ferment glucose into lactic acid as the major catabolic end product in a specific dairy environment. Starter cultures are one of the most important factors in the delivery of the technological and organoleptic characteristics of fermented dairy food [8]. The ratio of _L. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ in yogurt starter cultures has attracted wide attention. Generally, the ratio of _L. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ in yogurt starter cultures is 1:1 or 1:2. The different ratios of starter cultures have a significant effect on fermentation time, pH, viscosity and other characteristics of the product. [PERSON] et al. concluded that a long fermentation time reduced the efficiency of yogurt production and increased production costs and the rate of contamination by harmful microorganisms [9]. [PERSON] et al. found that different ratios of _L. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ had different viable cell counts [10]. Under the same fermentation conditions, the selection of starter cultures with good fermentation characteristics can improve the texture of yogurt. _L. delbrueckii_ subsp. _bulgaricus_ and _S. thermophilus_ used in a mixed culture have a symbiotic relationship in milk when used as a yogurt starter culture via the exchange of metabolites that are necessary for the growth of each bacterium [11; 12]. For example, _S. thermophilus_ uses the peptides and free amino acids produced by _L. delbrueckii_ subsp. _bulgaricus_, while _L. delbrueckii_ subsp. _bulgaricus_ uses the pyruvate acid, formic acid, folic acid and long-chain fatty acids produced by _S. thermophilus_[13]. Mixed fermentation with the two microorganisms gives the yogurt good textural properties. In addition, the production of metabolites such as amino acids and short peptides contributes to the formation of flavour substances in the yogurt [14]. Our enjoyment of yogurt is governed by our perception of aroma, taste and texture.