**5. Aromatic components produced by** *Lactobacillus delbrueckii* **subsp.**  *bulgaricus*

Recent work has focused on the isolation and characterization of *L. bulgaricus* with a particular interest in the metabolite profile of these strains. For example, eight strains were examined for the metabolic profiles and found at least 47 different aromatic compounds that were recently identified (**Table 4**) [5]. These aromatic compounds were divided into six main groups based on the chemical structure of each, as follows: 1. organic acid group 2. alcohol group, 3. aldehyde group, 4. ketone group, 5. ester group, and 6. aromatic group. As expected, the primary aromatic component, acetaldehyde, was produced by all of the bacterial strains. Importantly, acetaldehyde is recognized as a major flavor component in yogurt and provides the traditional strong, fruity aroma, sometimes described as green-apple-like that is characteristic of yogurt products. All of the tested strains produced pentanoic acid along with octanoic acids and acetone in a wide range of concentrations. We also observed that using different combinations of yogurt cultures led to the formulation of a wide range of unknown aromatic compounds and higher levels of acetaldehyde. Results showed that the interaction between strains generated a favorable yogurt volatile profile



#### **Table 4.**

*Aromatic components produced by symbiotic starter cultures of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus.*

resulting in the highest numbers of compounds identified as key-aroma compounds that are desirable for an acceptable organoleptic yogurt quality. Moreover, a different starter culture preparation could also lead to the production of different metabolites. Preparation steps would include bulk growth media, media additives, protein sources, and amino acid composition (especially glycine). The use of direct-to-vat culture or a lyophilized starter culture could also impact the aromatic profile of yogurt products. Our work also demonstrated that there are synergistic effects among the aromatic compounds that contributed to the overall yogurt flavor.
