## 3 Results and Discussion
### Volatile Compounds
#### 3.1.3 Volatile Acids  
Both non-volatile and volatile carboxylic acids are important flavor compounds in dairy products, as they enhance product sensory properties [35]. Even though lactic acid is not a volatile carboxylic acid, it is included in the present discussion due to its crucial importance in yogurt flavor. Lactic acid is the prime source of flavor in fermented dairy products, responsible for the refreshing sour/acidic flavor of yogurt [36]. During fermentation, a good portion (20-40%) of lactose is transformed into lactic acid. Acidity is a key factor in yogurt flavor, producing a pH of around 4.4-4.2. [PERSON] et al. [37] showed that there are important flavor differences between traditional acidic and mild, less acidic yogurts, which are mainly due to the differences in acidity and not due to different concentrations of the three flavor impact compounds (acetaldehyde, diacetyl, and 2,3-pentanedione). These authors considered lactic acid as the most important component of yogurt flavor. Carboxylic acids usually derive from lipolysis, proteolysis or lactose metabolism [15; 38].  
Regarding the volatile acid content of the yogurt samples, it was observed that the different samples contained different carboxylic acids. More specifically, in dessert yogurts made from cow milk without the addition of Bifidobacterium BB-12, hexanoic acid (contributing with pungent, rancid, flowery notes) was determined in small amounts in the case of the mild and classic cultures, while in the case of the acidic culture, no volatile acids were identified. When Bifidobacterium BB-12 was added, acetic acid was determined in particularly high concentrations (120 to 151 mg kg\({}^{-1}\)), except in the case of the classic culture. In a previous study, it was reported that the main difference in volatilesbetween cow milk either unfermented or fermented with Bifidibacterium BB-12, was the high concentration of acetic acid produced in the latter [9]. Despite the weak ability of bifidobacteria to ferment lactose, the acetic acid content in fermented milk confirmed their heterofermetative nature.  
Acetic acid imparts a vinegar flavor to yogurt; therefore, when found at high concentrations, it can render the dessert yogurt unacceptable by consumers. Therefore, these samples are expected to show inferior organoleptic properties compared to yogurt desserts from cow milk, to which no Bifidobacterium BB-12 are added. This was validated, as can be seen from the organoleptic evaluation results in Section 3.2. [PERSON] and [PERSON] [39] reported an acetic acid concentration ranging from 0.5 to 18.8 mg kg\({}^{-1}\) in commercial yogurts.  
In the case of dessert yogurts from goat milk without the addition of _Lactobacillus acidophilus_ LA-5, no volatile acids were identified, while in dessert yogurts prepared with the addition of _Lactobacillus acidophilus_ LA-5, only butanoic acid was identified. Butanoic acid imparts a cheesy flavor, and therefore the corresponding samples are expected to have improved organoleptic properties [38]. This was validated, as can be seen from the organoleptic evaluation results in Section 3.2.  
The thermophilic Streptococcus and Lactobacillus cultures are known to produce acetic, butyric, and caproic (hexanoic) acids [31].