## 3 Results and Discussion
### Volatile Compounds
#### 3.1.2 Ketones  
Ketones are a major class of volatile compounds identified in yogurt. They are derived both from raw milk (and as such, dessert yogurts from cow milk are expected to differ from those from goat milk in their ketone content) and from the processing parameters of yogurt production, due to the \(\beta\)-oxidation metabolic pathway of unsaturated fatty acids [15; 26]. Dessert yogurts made from cow milk, prepared with the mild culture, without the addition of Bifidobacterium BB-12, had had a ketones content of around three times lower than the corresponding dessert yogurts prepared with the classic and acidic cultures (52.52 \(\pm\) 12.98 mg kg\({}^{-1}\) for the mild, 160.90 \(\pm\) 52.47 mg kg\({}^{-1}\) for the classic and 147.60 \(\pm\) 50.54 mg kg\({}^{-1}\) for the acidic culture), probably due to the specific action of the mild culture. The same difference in the content of ketones was observed between the dessert yogurts from cow milk with different cultures, to which Bifidobacterium BB-12 were added (68.39 \(\pm\) 39.45 mg kg\({}^{-1}\) for the mild, 143.46 \(\pm\) 23.83 mg kg\({}^{-1}\) for the classic and 202.52 \(\pm\) 5.98 mg kg\({}^{-1}\) for the acidic culture). However, for a given culture, no significant differences were observed in the ketone content between yogurts with or without Bifidobacterium bacterium BB-12, which could be associated with the specific culture used and not with the addition of Bifidobacterium bacterium BB-12. In the case of dessert yogurts from goat milk without _Lactobacillus acidophilus_ LA-5, the same rough tendency was observed in the content of ketones. That is, dessert yogurts with the mild culture had a significantly lower content of ketones than those prepared with the classic and acidic culture (128.72 \(\pm\) 69.16 mg kg\({}^{-1}\) for the mild, 184.22 \(\pm\) 26.41 mg kg\({}^{-1}\) for the classic and 187.93 \(\pm\) 77.79 mg kg\({}^{-1}\) for the acidic culture). However, the case of dessert yogurts from goat milk to which _Lactobacillus_ LA-5 was added was quite different. In this case, the dessert yogurts with the mild culture had ca. a 2.6 times higher content of ketones, compared to the corresponding dessert yogurts to which no _Lactobacillus acidophilus_ LA-5 were added (338.34 \(\pm\) 128.24 mg kg\({}^{-1}\) vs. 128.72 \(\pm\) 69.16 mg kg\({}^{-1}\)). This value was ca. 4 times higher than that of ketones in dessert yogurts with _Lactobacillus acidophilus_ LA-5, which were prepared using the classic and acidic cultures (338.34 \(\pm\) 128.24 mg kg\({}^{-1}\) vs. 87.85 \(\pm\) 18.30 and 74.04 \(\pm\) 10.97 mg kg\({}^{-1}\)). These two dessert yogurts showed ca. 50% reduction in the content of their ketones, compared to desserts prepared without _Lactobacillus acidophilus_ LA-5 (87.85 \(\pm\) 18.30 vs. 184.22 \(\pm\) 26.41 mg kg\({}^{-1}\) for the classic and 74.04 \(\pm\) 10.97 vs. 187.93 \(\pm\) 77.79 mg kg\({}^{-1}\) for the acidic culture). This finding is consistent with results previously reported in the literature, as it is reported that the addition of bacterial strains that grow easily in milk, such as _Lactobacillus acidophilus_, affects the formation of ketones [27]. Therefore, in the case of dessert yogurts derived from cow milk, the addition of _Bifidobacterium_ BB-12 does not seem to significantly affect their ketone content and as such, their odor. In contrast, in dessert yogurts from goat milk, the addition of _Lactobacillus acidophilus_ LA-5 significantly increased the ketone content when the mild culture was used while decreasing the ketone content when the other two cultures were used. Among the determined ketones, diacetyl (2,3-butanedione) is an important aroma compound, contributing to yogurt aroma with buttery notes. It is formed through the fermentation of citrate present in milk. Typical concentrations of diacetyl in yogurt range from 0.2-3 mg kg\({}^{-1}\)[28]. In the present study, substantially higher concentrations of diacetyl were recorded.