## 3 Results and Discussion
### Volatile Compounds
#### 3.1.1 Aldehydes  
Aldehydes can be formed through either the degradation of milk fat (unsaturated fatty acids are oxidized to hydroperoxides, which in turn yield aldehydes through the action of hydroperoxidase) or from amino acid catabolism [6; 21]. However, aldehydes can also be oxidized to produce carboxylic acids or be reduced to yield alcohols via enzymatic reactions. Therefore, the final amount of aldehydes in yogurts is dependent on the activity of the enzymes present in microorganisms [21]. With regard to the aldehydes identified in cow dessert yogurts, without the addition of _Bifidobacterium_ BB-12, only acetaldehyde was determined (54.87 \(\pm\) 12.15 mg kg\({}^{-1}\)), in the sample prepared using the classic culture. In contrast, when _Bifidobacterium_ BB-12 was added, acetaldehyde, pentanal, hexanal, and nonanal were determined in dessert yogurts (total aldehyde content: 30.36 \(\pm\) 23.66 mg kg\({}^{-1}\) for the classic and 12.86 \(\pm\) 0.47 mg kg\({}^{-1}\) for the acidic culture). The addition of _Bifidobacterium_ BB-12 led to changes in the pH and composition of milk during fermentation (unlike dessert yogurts prepared without the addition of probiotic bacteria). The initial pH of cow milk was 6.70 and for goat milk it was 6.65. On day 21, The pH of cow dessert yogurt without _Bifidobacterium BB-12_ was: 4.36 for the mild culture, 4.28 for the classic culture, 4.23 for the acidic culture and with _Bifidobacterium BB-12_: 4.31 for the mild culture, 4.22 for the classic culture and 4.16 for the acidic culture. These pH changes caused the activation of enzymes in _Bifidobacterium_ BB-12, resulting in their combination with oxidation in the production of other aldehydes, in addition to acetaldehyde [8; 9]. Acetaldehyde is one of the most important compounds in fermented dairy products, as it contributes a light, fresh, green, and strong odor [6; 10]. Generally, it has been suggested that acetaldehyde may result from the degradation of various compounds such as fatty acids, glucose, catechol, glyceraldehydes, and amino acids such as threonine and glycine [11]. However, the most important pathway of acetaldehyde formation is reported to be the breakdown of threonine into acetaldehyde and glycine, which is catalyzed by the enzyme threonine aldolase [12]. This enzyme is present in both the _Streptococcus thermophilus_ and _Lactobacillus bulgaricus_ microorganisms, but the production of acetaldehyde by the two microorganisms is not identical. Bacterial cultures that allow acetaldehyde to form without particularly acidifying the yogurt are more desirable [13]. Acetaldehyde was mainly identified in larger amounts in the cow milk; the dessert yogurt prepared using the classic culture was found to be consistent with previous studies in which the classic culture produces a more desirable aroma in the dessert yogurt due to acetaldehyde [14]. However, the above-mentioned trend was not observed in the dessert yogurts from goat milk. Both with the addition or in the absence of _Lactobacillus acidophilus_ LA-5, no aldehydes other than acetaldehyde were determined (7.12 \(\pm\) 4.08 mg kg\({}^{-1}\) for the mild and 66.09 \(\pm\) 18.12 mg kg\({}^{-1}\) for the classic culture in the absence of _Lactobacillus acidophilus_ LA-5 and 54.50 \(\pm\) 1.00 mg kg\({}^{-1}\) for the mild, 11.13 \(\pm\) 0.04 mg kg\({}^{-1}\) for the classic and 17.06 \(\pm\) 4.35 mg kg\({}^{-1}\) for the acidic culture in the presence of _Lactobacillus acidophilus_ LA-5). This may be due to the fact that different probiotic bacterial strains were used to prepare dessert yogurts from cow and goat milk. Similarly, differences in the amount of acetaldehyde in goat dessert yogurts with the addition of _Lactobacillus acidophilus_ LA-5 compared to those prepared without _Lactobacillus acidophilus_ LA-5 may be due to the specific enzymatic activity of _Lactobacillus acidophilus_ LA-5 bacteria [22; 23].