**2. The role of lactic acid bacteria in yogurt flavor**

Yogurt is one of the most popular fermented dairy products worldwide nowadays. Moreover, consumption of yogurt has been increasing globally as a result of its pleasing sensory qualities, including texture, color, and flavor. Being one of the key food preservation methods, fermentation has significantly increased the nutritional value, shelf life, and sensory qualities of foods. This process involves a variety of microorganisms that break down the biochemical components of the food's basic materials (carbohydrates, proteins, and lipids), improving catabolism (digestion), taste, and enhancing the pharmacological and nutritional benefits of the food [8]. Most of the flavor compounds found in yogurt are a result of the activity of microbes in starter cultures, lactic acid bacteria (LAB). Microbes found in this starter culture carry out three key biochemical tasks during fermentation, which include the breaking down of milkfat into free fatty acids (lipolysis), caseins into peptides and free amino acids (proteolysis), and carbohydrates into lactic acid or other metabolites (glycolysis) [7]. Flavor is very important in food; consumers consider flavor to be one of the most significant aspects of food since it affects how well a particular product is liked and its overall acceptability.

## **2.1 Metabolic pathways of flavor compounds formation in yogurt**

During fermentation, lactic acid bacteria processes create flavor precursors that are then transformed into flavor compounds. Enzymes hydrolyze several dietary components, including carbohydrates, proteins, and lipids. Carbohydrate metabolism (glycolysis), amino acid metabolism (proteolysis), and fatty acid metabolism (lipolysis) are the three main metabolic processes of LAB that lead to the formation of volatile compounds [9].

## *2.1.1 Flavor compounds from LAB carbohydrate metabolism*

Lactic acid bacteria use the sugar lactose that is present in milk as their primary source of energy and carbon [10]. In fact, the distinctive acidic flavor of yogurt can be attributed to the conversion of lactose to lactic acid by LAB. The two distinct

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

carbohydrate fermentation pathways in LAB—homo-fermentation and heterofermentation result in various metabolic end products, depending on the LAB species, substrate, and environmental factors. Homofermentative LAB, which include *Pediococcus*, *Lactococcus*, *Streptococcus*, etc., use the Embden-Meyerhof-Parnas (EMP) pathway to produce lactic acid as the main by-product. However, heterofermentative LAB such as *Leuconostoc*, *Oenococcus*, *Lactobacillus*, etc., use the phosphoketolase pathway (PKP), which also produces other end products, such as ethanol, carbon dioxide, acetic acid [9]. Homofermentative metabolism could also switch to a mixed-acid metabolism with a variety of molecules under specific situations such as carbon limitation, carbon excess of slowly metabolized sugars, aerobic conditions. The metabolic by-products of this metabolism would include multiple flavor compounds such as acetaldehyde, ethanol, and diacetyl. Acetaldehyde, for example, dominates the flavor of yogurt in its normal form and helps in its distinctive flavor. Pyruvate is a crucial metabolic precursor that is usually catalyzed by aldehyde dehydrogenase or α-carboxylase to produce acetaldehyde. The characteristic flavor of yogurt is produced in fermented dairy products by a variety of C4 molecules such as diacetyl, acetoin, and 2, 3-butanediol [7]. These molecules may be produced by the citrate or glycolysis metabolism of certain LAB (**Figures 1**–**3**). Diacetyl is the predominant significant flavor compound among these C4 chemicals, and both *S. thermophilus* and *L. bulgaricus* are capable of producing it. Acetoin, which is diacetyl's reduced form, is important for decreasing the sharpness of diacetyl and also adds to the pleasant, creamy flavor of yogurt.

### *2.1.2 Flavor compounds from amino acid metabolism by LAB*

In order for yogurt to have a pleasant taste and aroma (flavor), proteolysis is a crucial biochemical step. Proteolytic abilities in certain LAB allow them to undergo hydrolysis of proteins, which leads to the production amino acids and peptides [9]. Proteolysis and the breakdown of an amino acid (amino acid degradation) make up the first two phases of this process. The enzyme cell-envelope proteinases (CEPs) help to break down the protein into oligopeptides, causing casein to begin to undergo proteolysis by LAB. The second phase then begins and involves the transport of di-, tri-, and oligopeptides into the cell. Peptidases further hydrolyze casein-derived peptides to amino acids after these casein-derived peptides have been absorbed by LAB cells. In a single bacterial genome, peptidases can be encoded in several copies. Free amino acids generated by the breakdown of proteins (proteolysis) may be transformed into a variety of flavoring substances, including those ammonia, amines, aldehydes, phenols, indole, and alcohols, and these compounds all have imparted the flavor of the yogurt. The primary sources of flavor substances obtained from milk protein are mostly branched-chain amino acids such as Val, Leu, Ile, aromatic amino acids such as Phe, Tyr, Trp, and sulfuric amino acids such as Cys, Met [11]. Transamination of amino acids to their respective α-keto acids is the first stage of amino acid breakdown. The α-keto acids then go through several enzymatic processes, such as reduction to produce flavorless α-hydroxy acids and decarboxylation to produce aldehydes that can subsequently be reduced to an alcohol, or oxidative decarboxylation to produce acyl-CoA, and finally, carboxylic acids [7]. After that, esterases or acyltransferases catalyze the formation of esters or thioesters in processes involving alcohols and carboxylic acids [12]. As a member of a different class of lyases, threonine aldolase may convert threonine straight into acetaldehyde.

**Figure 1.** *Pathways of citrate metabolism by lactobacillus [1, 7].*

**Figure 2.** *Pathways of lactic acid production by lactobacillus [1, 7].*

#### **Figure 3.** *Pathways of alcohol production by glucose metabolism [1, 6, 7].*
