**3.4 Esters**

Esters are another volatile compound found in fermented yogurt and have a similar role to alcohol and acid in flavor development. A total of three to six ester compounds were detected in fermented milk [34, 35] depending on the starter culture. This included ethyl ester, butyl ester, and ethenyl ester. Ethyl ester, which is significant among other esters in flavor development, is derived from the enzymatic or chemical esterification of acids with ethanol. Ethyl ester adds unique fruity and floral aroma and flavor while minimizing the sharpness and bitterness imported by fatty acids and amines [1, 33]. Esters contained in yogurt are in a low amount and are able to withstand extended storage periods.

#### **3.5 Sulfur compounds**

Volatile sulfur compounds are significant contributors to the off-flavors of yogurt. As a class, sulfur compounds are typically present in foods at extremely low concentrations and have low sensory detection thresholds. Some of these compounds provide background sensory nuances to the flavor, while others provide unique flavor characterizing identities to the products. Sulfur compounds including methanethiol, dimethyl disulfide, and hydrogen sulfide can be detected in fermented milk products, and their presence contributes to the strong, unacceptable aroma [33–35].

The sensory properties of yogurt depend largely on the relative balance of chemical compounds derived from carbohydrates, protein, or fat in the milk base. The flavor components of yogurt include the volatile compounds naturally present in the milk and specific compounds produced from milk fermentation. It has been reported that more than 100 different volatile compounds have been identified in yogurt, including carbonyl compounds, alcohols, acids, esters, and sulfur-containing compounds.

The major compounds responsible for imparting the desirable flavor in yogurt are acetaldehyde diacetyl, acetoin, 2-butanone, and lactic acid. Moreover, the optimum flavor of yogurt results when proper levels of these compounds are produced. The desirable concentrations of acetaldehyde in yogurt ranged from 23 to 40 mg/kg, while lesser concentrations resulted in weak flavor and higher acetaldehyde led to an off-flavor. Additionally, as with many other dairy products, yogurt is prone to deterioration, especially under improper storage conditions. The generation of volatile byproducts leads to off-flavors, which make the product unsatisfactory for consumers.
