**1. Introduction**

Yogurt is defined as the product of fermented milk by *L. bulgaricus* and *Streptococcus thermophilus*. Yogurt was first discovered in the Middle East and has been a part of the human diet for thousands of years. However, it was not until the twentieth century that scientists started to provide scientific evidence for the health benefits associated with yogurt consumption. In 1905, a Bulgarian scientist, Stamen Grigorov, was the first to report on *Bacillus bulgaricus* (now *L. bulgaricus*), a lactic acid bacterial strain found in Bulgarian yogurt. Then, in 1908, the Russian scientist Elie Metchnikoff theorized that one's health could be improved, and senility delayed by colonizing the gut with the host-friendly bacteria found in yogurt. The popularity of yogurt is attributed to its various health benefits as well as the flavors and sensory characteristics [1]. Yogurt is described as having a smooth, viscous gel-like texture

with Sharpe acid associated with green apple aroma. These characteristics play a significant role in the consumer acceptability to yogurt products (**Table 1**). The traditional yogurt flavor is a combination of aroma and taste that are typically produced during lactic acid fermentation by the yogurt starter cultures. During fermentation, the yogurt starter culture coverts lactose and other nutrients in milk to several chemicals that lead to the production of various flavor and aromatic compounds. More than 100 different volatile compounds have been reported and produced by various yogurt cultures. Several advanced techniques and instrumentations have been applied to determine the volatile compounds in yogurt products. **Table 2** lists some of the common analytical instrument used for flavor analysis with advantages and limitations of each instrument.

The selection of suitable strain combinations in yogurt starter culture is important for achieving the best technological performance and desirable sensory characteristics. However, the use of genetically modified lactic acid bacteria with encoded and targeted flavoring [7] is not an acceptable solution primarily due to the lack of consumer acceptance of this technology, and restricted regulations for the use of such bacterial strains in food products; this is especially true for the European market [7]. In this regard,


#### **Table 1.**

*Some identified volatile compounds in yogurt with their description of odors.*


#### **Table 2.**

*Analytical techniques for the determination of volatile compounds in fermented dairy products.*

knowledge of the aromatic-metabolic profile of the starter cultures used and the influence of the profile on the sensory characteristics of the fermented products is essential, both for the individual consumer and for the food industry. It should be noted that an important consideration in the selection of starter cultures for the production of yogurt and other dairy products is the ability of the starter culture strains to produce metabolites that shape the sensory qualities of the product. Thus, in the selection of strains included in the composition of starter cultures used for the production of dairy products, the metabolic profile of each strain is examined, particularly since some metabolic products involved in the formation of the aroma have antimicrobial activity.

In this chapter, we present a comprehensive review of the general aromatic components that are present in yogurt products. First, we introduce the role of lactic acid bacteria with regard to yogurt flavor. Next, we present the aromatic compounds and group them based on the chemical structure into carbonyl compounds, organic acids, alcohols, and esters as major compounds. We then go on to discuss advanced instrument techniques for yogurt analysis. It is these techniques that could help us to reach a more advanced level of understanding of the impact of specific yogurt strains for obtaining the desired sensory qualities of yogurt products and other fermented lactic acid products.
