**2.2 Effect of different lactic acid bacteria on yogurt flavor**

The starter cultures, processing conditions, sources of milk, and some other ingredients all have an impact on the flavor of yogurt [15]. However, within

these parameters, the development of the flavor components in yogurt is mostly influenced by the starter cultures used. The culture used for yogurt is primarily composed of *Streptococcus thermophilus* and *Lactobacillus delbrueckii* subsp. bulgaricus. As a result, some nations only allow the use of the word "yogurt" for products prepared using starters that include bacteria from both of these strains [16]. The symbiotic connection between these two bacteria is referred to in mixed cultures as proto-cooperation and makes them mutually advantageous during fermentation even though each grows well in milk on their own [16]. Due to their associative development and mutual stimulation, the number of flavor constituents in the mixed cultures is significantly higher than that in either of the two individual cultures. It was discovered that the largest percentage of flavor compounds, including acetaldehyde, diacetyl, acetoin, acetone, ethanol, and 2-butanone (**Figure 3**), were produced when mixed cultures were utilized during lactic acid fermentation [7]. Additionally, the levels of methylated sulfides and dimethyl trisulfide were extremely low in *Lactobacillus bulgaricus* and *S. thermophilus* monocultures, suggesting that perhaps the mixed culture's higher levels were the result of interspecies interaction. The proto-cooperation of the mixed cultures is also significantly influenced by proteolytic activity. Compared to pure cultures or cultures with proteolytic *S. thermophilus*, this combination will generate higher aroma volatiles and nonvolatile metabolites. Thus, the combination of both microorganisms influences the synthesis of volatile and nonvolatile compounds essential to flavor development. The Lactobacillus strains are the most widely used commercial probiotics in yogurt. In addition to imparting yogurt's flavor and enhancing organoleptic qualities, these strains also provide health benefits. Yogurt flavor development is influenced by a variety of environmental factors, such as the composition of the culture medium, competition for nutrients, and interactions between microorganisms.

### **2.3 Metabolic engineering application for flavor enhancement**

A significant approach for genetically modifying strains in order to increase the production of flavor compounds such as acetaldehyde, diacetyl, and esters is metabolic engineering. The changing of one or more genes or enzymes is a popular method for producing many different flavor molecules [11]. For example, formation of acetaldehyde by yogurt bacteria occurs via a variety of pathways, with threonine aldolase likely being the primary enzyme in this process. Serine hydroxymethyltransferase (SHMT), which is produced by the glyA gene in *S. thermophilus*, also has threonine aldolase activity. According to Chaves et al. [17], overexpression of the glyA gene in *S. thermophilus* increases acetaldehyde synthesis by 80–90%, whereas inhibition of the gene completely abolishes acetaldehyde formation. The main goal of these numerous metabolic engineering solutions for LAB has been to efficiently produce diacetyl for its significance in yogurt flavor. The als or ilvBN genes, ldh gene, and aldB gene that are coded for the enzymes α-acetolactate synthase, lactate dehydrogenase, and α-acetolactate decarboxylase, respectively, are the main enzymes involved in the synthesis of diacetyl.

## **2.4 Correlation between lactic acid bacteria and flavor compounds in yogurt**

In several studies, certain volatile compounds found in yogurt have been linked to LAB species, demonstrating that LAB significantly affect the flavor of many

*Volatile Aromatic Flavor Compounds in Yogurt: A Review DOI: http://dx.doi.org/10.5772/intechopen.109034*

fermented foods, including yogurt. Yogurt and other dairy products are often fermented from milk from various plants and animal sources. The characteristic LAB species found in these fermented dairy products come from the genera Lactobacillus and are naturally prevalent in a variety of environments; however, they are mostly used for fermentation purposes [18–20]. Lactobacillus was found to be the most prevalent species in most samples of fermented yak milk. Microbial analysis, as well as the flavor profile of the product, revealed that these bacteria were significantly correlated with flavor compounds such as ethanol, benzaldehyde, ethyl acetate, 2, 3-pentanedione, and benzaldehyde [21]. Through correlational analysis using bidirectional orthogonal partial least square, it was determined that bacteria contribute more to flavor production than fungus. The majority of studies that compared the relationship between the LAB community and volatile chemicals found a strong correlation between LAB and the development of flavor in yogurt and other fermented foods. In yogurt, the main species, *Lactobacillus*, is predominant and helps create esters, aldehydes, acids, ketones, and alcohols.
