**3. Volatile and aromatic compounds in yogurt**

Flavor is one of the most important properties of food products and is an important factor determining consumer acceptability. With regard to dairy products, their sensory properties largely depend on the relative balance of flavor compounds derived from fat, protein, or carbohydrates in the milk. For example, the distinctive flavor of yogurt is contributed by lactic acid and a complex mixture of flavor compounds that include the volatiles already present in the milk and specific compounds produced during lactic fermentation [22]. More than 100 different volatiles have been identified in yogurt, including carbohydrates, alcohols, aldehydes, ketones, acids, esters, lactones, sulfur-containing compounds, pyrazines, and furan derivatives [3].

Characterization of the volatile compounds allows for examination of the mechanism of formation of the aromatic profile of the product. Knowledge of the primary flavor compounds and their origin will thus support the production of dairy products of consistent quality that will be more readily accepted by consumers. For example, routine analysis of the primary aroma compounds can be used for quality monitoring during yogurt production. In addition, the profile of volatile compounds in yogurt can be used as a parameter to provide consumers with a better quality and safer food [3].

One major pathway for the production of flavor compounds in yogurt is through lipolysis or oxidation of the fatty acids in milk fat. Unsaturated fatty acids are oxidized in the presence of free radicals to form hydroperoxides, which rapidly decompose to form hexanal or unsaturated aldehydes. Unsaturated fatty acids also lead to the formation of 4- or 5-hydroxyacids, which readily cyclize to γ- or δ-lactones and odd-carbon methyl ketones by decarboxylation of β-keto acids. Another major pathway would be the microbiological transformation of lactose (and produced lactate) and citrate by acid-producing bacteria into acetaldehyde, diacetyl, acetoin, and ethanol. The alcohols in the yogurt can then combine with the free acids to form esters such as ethyl acetate and butyl acetate. In addition, biogenic amines and nitrogencontaining compounds can be transformed from proteins and amino acids, and sulfur compounds can be derived from organosulfur compounds [23].

However, not all volatile components found in foods are important for the foods' organoleptic properties. For example, in most studies, despite the long list of volatile compounds found in yogurt (**Table 1**), only a few had relatively high concentrations. Only acetaldehyde, ethanol, acetone, diacetyl, and 2-butanone exert a strong influence on the desired aroma and are also present in amounts detectable by common laboratory techniques. The main volatile compounds commonly reported to be responsible for imparting the desired aroma to yogurt are the carbonyl compounds acetaldehyde, diacetyl, acetone, acetoin, and 2-butanone. Although present in small amounts in yogurt, these compounds are important organoleptic factors.

The primary volatile components involved in the formation of the aroma of typical Bulgarian yogurt are acetaldehyde, acetone, 2-butanone, diacetyl, ethyl acetate, and ethanol. Kaminarides et al. [24] found that acetic acid, acetaldehyde, acetone, diacetyl, 2-butanone, acetoin, and 3-methyl-2-butanone were the primary volatile aroma compounds in yogurt made from sheep's milk. The primary aroma components in Swiss yogurt as determined by GC-sniff technique are acetaldehyde, diacetyl, 2, 3-pentanedione, methional, 2-methyltetrahydrothiophen-3-one, 2-neonal, 3-methylbutyric acid, guaiacol, benzothiazole, and two unidentified compounds [3]. The aromatic compounds in Swiss yogurt were investigated and found that few major compounds that had high-impact yogurt flavor, these compounds are acetaldehyde, dimethylsulfide, Diacetyl, 2, 3-pentanedione, L-limonene, and undecanal. However, other major constituents (fat, protein, and carbohydrates) in yogurt could play a major factor in the release of volatiles compounds. The aromatic components produced by the starter culture can be grouped into separate classes as carbonyl compounds, organic acids, alcohols, and esters, depending on their respective chemical structure.

#### **3.1 Carbonyl compounds**

The quality of yogurt is heavily reliant on the relative balance of volatile compounds including carbonyl substances derived from fat, protein, and carbohydrate in the milk base during the fermentation process. Carbonyl compounds are the primary aromatic substances in fermented yogurt where more than 38 of these compounds have been detected [3]. They are composed of aldehydes and ketones. The type and level of compounds derived during fermentation depend on the starter culture, variety of milk, and the conditions of the fermentation process. The metabolism of citric acid and amino acids by lactic acid bacteria—*Lactobacillus acidophilus*—and *Streptococcus thermophiles*, both of which are commonly used in the yogurt industry, produces the flavor compounds characteristic of yogurt products. **Table 3** shows the most common carbonyls compounds in yogurt and typical concentrations in yogurt products [3].

Several carbonyl compounds including diacetyl, acetoin, and butanediol are derived from citrate metabolism while several amino acids are converted into the intermediate metabolite pyruvate and finally acetaldehyde or directly into acetaldehyde.

In citric acid metabolism (**Figure 1**), citrate is converted into acetate and oxaloacetate with the presence of citric acid lyase catalyze. Next, oxaloacetate is decarboxylated and produces pyruvate and carbon dioxide. Subsequently, pyruvate is metabolized by lactic acid bacteria to produce different end products, including diacetyl, acetoin, and butanediol [25].

The crucial role of carbonyl compounds in yogurt can be identified when considering the sensory attributes of yogurt. Although each of these carbonyls is responsible for its characteristic flavor or aroma, the ultimate sensory properties of yogurt are decided by a relative balanced mixture of all flavored substances as well as their dominant properties.

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


#### **Table 3.**

*List of common carbonyl compounds found in yogurt.*

Acetaldehyde is an essential aroma and flavor compound found in fermented yogurt and provides the essential unique green apple or nutty flavor in fermentation by *L. bulgaricus* and Streptococcus thermophiles. However, a proper concentration level of acetaldehyde is required in order to obtain the most desired sensory quality. For example, although acetaldehyde gives a pleasant fruity aroma at diluted concentrations, high levels can result in a pungent irritating odor [3].

Diacetyl, which produces a characteristic buttery flavor in yogurt, is derived by fermentation of the citrate present in milk. It is equally as important as acetaldehyde with regard to the sensory quality of yogurt. The preferred typical yogurt flavor would thus be obtained by a 1:1 mixture of acetaldehyde and diacetyl. However, when the acetaldehyde level in yogurt is low, diacetyl contributes to producing a delicate, full flavor and aroma in the product. At higher concentrations, diacetyl can act as a flavor and quality enhancer as well [3].

Another flavored substance commonly available in yogurt is acetoin, which gives a mild creamy, slightly sweet, butter-like flavor. While acetoin is converted from diacetyl by the diacetyl reductase enzyme [5, 7], its flavor properties are also similar to those of diacetyl. A proper combination of both substances thus results in a typically mild, pleasant, buttery yogurt taste. Moreover, acetoin tends to reduce the harshness of diacetyl.

Acetone and 2-butanone reportedly have similar flavor characteristics with regard to minor but important flavor compounds found in yogurt. Both compounds make a positive contribution to sweet, fruity aroma and flavor qualities. Typical concentrations of acetoin in yogurt range from 1.2 to 28.2 mg/kg [26, 27]. Diacetyl in combination with acetoin is responsible for the soft, pleasant, fatty taste of yogurt that is crucial to yogurt's widespread appeal.

Acetone and 2-butanone are two volatile compounds with a minor contribution to aroma in dairy products [1, 24, 27, 28]. For example, acetone has a sweet fruity aroma and is known to affect the flavor and taste of yogurt. Small amounts of acetone typically originate from milk, but certain amounts are produced by bacteria in yogurt and the concentration of acetone in yogurt ranges from 0.3 to 4.0 mg/kg [27]. The taste characteristic of 2-butanone is similar to that of acetone and the concentrations in yogurt range from 0.1 to 7.0 mg/kg [24, 27]. Gallardo-Escamilla et al. [28] reported that 2-butanone is important for the aroma development of yogurt and contributes to its fruity flavor.

However, many of the carbonyl compounds also play a role in the loss of yogurt taste stability by developing off flavor during storage. For example, reactions from carbonyl compounds can generate off-flavor chemicals. Lipid oxidation in milk results in an undesirable stale, oxidized flavor. Moreover, some malodorous compounds such as 2, 4, 5-trimethyloxazole can be generated from diacetyl and acetaldehyde in

the presence of ammonia [29]. Due to their off-character and low aroma and taste thresholds, these compounds can lead to serious taste and aroma defects.

#### **3.2 Organic acids**

The most perceptible chemical compound in yogurt in flavor detection is carbonyl compound followed by organic acids. Degradation of polysaccharides by lactic acid bacteria during fermentation produces monosaccharaides and acids. Organic acids contribute significantly to the sensory properties in fermented yogurt, especially with regard to acidity. For example, changes in acid concentration lead to the development of a characteristic flavor and aroma along with desirable consistency. Lactic acid is the major organic acid found in fermented yogurt, and it has both positive and negative impacts on taste (**Figure 2**). Approximately 20–40% of lactose present in milk base is metabolized into lactic acid, which increases the acid concentration up to 0.9%.

Lactic acid bacteria utilize lactose and then glucose as the carbon source to produce pyruvate through glycolysis. Lactic acid is produced by lactate dehydrogenase. Taste and mouthfeel of the final product can vary with the concentration of lactic acid regardless of the flavor compound contained [30]. Moreover, formation of acid directly involves the texture development. To obtain desirable consistency, it should reach the optimum pH level. Typical pH level in yogurt is 4.4 [3].

Acetic acid, folic acid, and longer-chain organic acid are generated during yogurt fermentation in addition to lactic acid for example, acetic acid amounts range from 0.5 to 18.8 mg/kg in typical products. However, high levels of acetic acid impart an unacceptable vinegar-like taste [31]. Folic acid is mainly derived by *Streptococcus thermophiles* by amino acid utilization. Accumulation of folic acid stimulates the growth of other lactic acid bacteria including *Lactobacillus acidophilus* in the fermentation medium. Longer-chain acids such as octanoic acid develop a characteristic soap-like flavor [3].

Thus, in order to obtain a yogurt with desirable properties, acid production should be controlled. Extended acidification during fermentation or in storage results in the development of off-flavors. Syneresis, the most common issue associated with the sensory quality of yogurt, is a qualitative defect in the yogurt structure that tends to lower consumer acceptability by weakening the appearance, texture, and consistency of the product. Syneresis develops as a result of post acidification, which causes some leakages of whey proteins.

Post acidification depends on the type of strain, microbial ratio in the yogurt starter culture, the storage temperature, and the storage time. Post acidification manipulation can be done changing the microbial ratio, and it increases the shelf life of the yogurt. Volatile acids are also important from a nutritional and therapeutic point of view in addition to their influence on organoleptic properties of the products.

In addition to lactic acid, other acids are produced during the fermentation of yogurt, both by lipolytic processes and by bacterial fermentation. For instance, acetic acid is an important compound produced by lactic acid starter cultures [26]. Acetic acid has been reported at a concentration of 0.5–18.8 mg/kg in yogurt [26]. High levels of acetic acid impart a vinegar-like taste that may not be accepted by consumers [32]. Longer-chain acids (e.g., octanoic acid) may contribute to the characteristic soap-like aroma [33].

#### **3.3 Alcohols**

In addition to carbonyl compound and acid, another volatile compound generated during yogurt fermentation is alcohol. However, the contribution of alcohol

compounds in flavor development is comparatively less. A total of eight to nine alcohol compounds associated with fermented yogurt have been detected [34, 35]. The type and concentration of compound primarily depend on the starter culture used (**Figure 3**).

Ethanol is considered to be the principal alcohol derived in lactic acid fermentation. It is produced by breakdown of glucose and catabolism of amino acids. In the ethanol production pathway, glucose breaks down into lactic acid, ethanol, and CO2 with the presence of ATP. As acetaldehyde degradation occurs during alcohol production, the amount of acetaldehyde in the medium is reduced.

As ethanol has an effective olfactory and trigeminal stimulus, it can act as a flavor enhancer to some extent. In typical yogurt made by cow milk fermentation, ethanol content ranged from 0.2 to 9.9 mg/kg [3] while yogurt made by other milk contained a lower amount. In addition to ethanol production, 1-hexanol and 1-heptanol production was also detected during fermentation. However, production of high levels of alcohol, particularly ethanol, was measured during storage while a reduction in acetaldehyde was reported. The level of ethanol ranged between 8.13 and 10.99% throughout the storage period [34, 35].
