With the realization of numerous researches, considerable knowledge has been accumulated about volatile compounds that contribute to the texture and aroma of yogurt. Most of the aroma compounds in yogurt are produced from the lipolysis of milk fat and the microbiological transformation of lactose and citrate. Over 100 volatiles, including carbonyl compounds, alcohols, acids, esters, hydrocarbons, aromatic compounds, sulfur-containing compounds, and heterocyclic compounds, are found in yogurt at low concentrations. In addition to lactic acid, acetaldehyde, diacetyl, acetoin, acetone, and 2-butanone contribute the most to the typical aroma of yogurt. However, extended storage of yogurt leads to the development of undesirable flavors, mainly attributed to the production of undesired aldehydes and fatty acids during lipid oxidation. Regarding the description of the action of volatile compounds on yogurts, we cite the study entitled "Volatile Flavor Compounds in Yogurt: A Review," authored by Hefa Meyer from the State Key Laboratory of Organic Geochemistry, Illinois University.

An ancient indulgence

Yogurt is a cultured dairy product that is widely consumed as a healthy and nourishing food, as well as for its sensory properties. Chemically, yogurt is a complex gel system that incorporates proteins, polysaccharides, and lipids. It is produced from milk through fermentation, one of the oldest methods practiced by humans to transform milk into products with an extended shelf life (Tamime and Robinson, 1999).

Today, yogurt is commonly made by fermenting cow's milk using a symbiotic culture of the bacteria Lactobacillus bulgaricus and Streptococcus thermophilus, especially under controlled temperature and environmental conditions in industrial production (Marshall, 1993). Fermentation of the milk sugar (lactose) produces lactic acid, which denatures milk proteins to give yogurt its texture (by forming the coagulated gel) and its characteristic flavor.

The naturalness and pleasant taste of yogurt make it an attractive food ingredient. The main quality characteristics of yogurt include texture, taste, and aroma (Sodini et al., 2004). Yogurt is usually characterized as a smooth, viscous product with a characteristic acidified taste and a green apple flavor (Bodyfelt et al., 1988). Some yogurts have a thick consistency resembling cream or milk pudding, while others are soft and essentially drinkable.

Aroma Determines Acceptability

Aroma is one of the most important properties of food products and is a significant factor in determining their acceptability and preference. The sensory properties of dairy products largely depend on the relative balance of aroma compounds derived from the fats, proteins, or carbohydrates in milk.

The distinct flavor of yogurt is attributed to lactic acid and a complex mixture of aroma compounds, including volatile substances already present in milk, as well as specific compounds produced during milk fermentation (Imhof et al., 1994).

So far, over 90 different volatile substances have been identified in yogurt, including carbohydrates, alcohols, aldehydes, ketones, acids, esters, lactones, sulfur-containing compounds, pyrazines, and furan derivatives (Marshall, 1982). During storage, the volatile constituents of yogurt can undergo changes depending on the culture, blend formulation, and storage conditions (Brauss et al., 1999).

Like many other dairy products, yogurt is prone to deterioration, especially at room temperature, over a few days. Various microbial, enzymatic, or chemical reactions occurring in yogurt during storage can modify its physical, chemical, and microbiological structure, leading to spoilage.

Hence, the generation of volatile secondary products results in undesirable aromas, making the product unsatisfactory for consumers' tastes. The evolution of volatile compounds often determines the storage duration and shelf life of yogurt.

The basic aroma of dairy products primarily arises from the native volatile constituents in cow's milk, influenced by pasteurization, fermentation, processing, and storage. Many organic volatile compounds found in yogurt are not produced by starter bacteria but originate from milk (Imhof et al., 1994).

Differences in the active aromatic compounds of yogurt, compared to milk, are most likely generated by the metabolism of acid-producing bacteria. These aroma compounds generated by cultures result from microbial, enzymatic, or chemical transformations of lactose, lipids, citric acid, and milk proteins/amino acids (Boelrijk et al., 2003; McGorrin, 2001; Tamime and Robinson, 1999). The aroma compounds present in yogurt can be divided into four main categories:

(i) non-volatile acids (e.g., lactic, pyruvic, oxalic, and succinic);

(ii) volatile acids (e.g., acetic, propionic, and butyric);

(iii) carbonyl compounds (e.g., acetaldehyde, acetone, acetoin, and diacetyl); and

(iv) various compounds (e.g., certain amino acids and/or constituents formed by the thermal degradation of proteins, fats, and lactose) (Tamime and Robinson, 1999).

Production of flavor constituents

A major pathway for the production of flavor constituents in yogurt is through lipolysis, or oxidation of fatty acids from milk fat. Considerable amounts of short-chain fatty acids, which strongly contribute to flavor formation in crop products, are produced from saturated fatty acids.

Unsaturated fats are oxidized in the presence of free radicals to form hydroperoxides, which rapidly decompose to form hexanal or unsaturated aldehydes (McGorrin, 2001). 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-ketoacids.

Another major pathway is represented by the microbiological transformations of lactose (and the produced lactate and citrate) by acid-producing bacteria (lactococci and lactobacilli), which produce aromatic compounds such as lactic acid, acetaldehyde, diacetyl, acetoin and ethanol . The alcohols in the yogurt can later combine with the free acids to form esters such as ethyl acetate and butyl acetate.

Analysis of Aroma Compounds

Despite the utmost importance of aroma as an indicator of product quality and compliance, the chemical analysis of aroma compounds in dairy products is complicated due to the heterogeneous nature of matrices.

The relatively high levels of lipids, proteins, and carbohydrates in dairy products make it difficult to separate active aromatic chemical substances based on general properties such as polarity or volatility.

Because the most volatile organic compounds are present only in small concentrations (ranging from μg/kg to mg/kg) in yogurt (Carrillo-Carriona et al., 2007), isolating volatiles from the matrix is complex. Unfortunately, the extraction and concentration of volatile components from dairy products before their analysis represent a major analytical challenge. The main issues encountered during this process are:

(i) the tendency of compounds to degrade or form residues in the presence of heat and/or oxygen;

(ii) the potential formation of secondary volatiles through enzymatic reactions; and

(iii) the incomplete recovery of polar/semi-volatile aromatic constituents.

Carbonyl Compounds

Carbonyl compounds comprise the main aromatic substances in yogurt, among which acetaldehyde is the compound that contributes the most to the typical aroma of yogurt (Pourahmad and Assadi, 2005). Pure acetaldehyde possesses a pungent, irritating smell, but at diluted concentrations, it gives a pleasant fruity aroma. Acetaldehyde imparts yogurt with its characteristic green apple or nutty aroma, and it is readily produced by Lactobacillus bulgaricus and Streptococcus thermophilus (Bodyfelt et al., 1988).

Acetaldehyde is an essential aroma compound in yogurt; yogurt with good flavor is produced when appropriate levels (23–40 mg/kg and at least 8–10 mg/kg) of acetaldehyde are present (Gaafar, 1992). Diacetyl is an important aromatic compound that gives butter its aroma and can enhance the aroma quality of yogurt at high concentrations.

Diacetyl seems to contribute to the delicate appearance, aroma, and full-bodied flavor of yogurt and is especially crucial for products containing low concentrations of acetaldehyde (Groux, 1973). Diacetyl is a diketone derived from the fermentation of citrate present in milk and milk blends (Vedamuthu, 2006). Small amounts of diacetyl, ranging from traces to 0.90 mg/kg or more, contribute to the pleasant and delicate aroma of yogurt (Rasic and Kurmann, 1978).

Typical concentrations of diacetyl in yogurt have varied between 0.2 mg/kg and 3 mg/kg (Georgala et al., 1995). There are different opinions regarding the role of diacetyl in yogurt aroma formation. Some researchers consider it a dominant component of the aroma only when acetaldehyde is at a low level (Rysstad and Abrahamsen, 1987), while others attribute to it the primary role in aroma formation (Imhof et al., 1994).

Acetoin and acetone

Acetoin is a common flavoring substance in many cultured dairy products. Acetoin has a light, slightly sweet, butter-like creaminess that is similar to that of diacetyl. The flavor of acetoin is considerably weaker than that of diacetyl and tends to reduce the hardness of diacetyl.

Acetoin is readily converted from diacetyl by the enzyme diacetyl reductase (Collins, 1972). Typical concentrations of acetoin in yogurt ranged from 1.2 to 28.2 mg/kg (Pourahmad and Assadi, 2005). Diacetyl combined with acetoin gives a mild, pleasant, buttery taste and are essential to the rich perception of yogurt.

Acetone and 2-butanone are contained in milk and are described as two volatile compounds of minor importance in the flavor contribution in dairy products (Carcoba et al. 2000).

Acetone has a sweet, fruity flavor and is known to influence the flavor and aroma qualities of yogurt. Small amounts of acetone usually come from milk, but certain amounts are produced by yogurt bacteria (Georgala et al., 1995). The typical acetone content of yogurt varied from 0.3 to 4.0 mg/kg (Pourahmad and Assadi, 2005), while 0.8–2.7 mg/kg acetone was found in cow's milk ( Hilde, 1979).

The characteristic aroma of 2-butanone is similar to that of acetone. Concentrations of 2-butanone in yogurt ranged from 0.1 to 7 mg/kg (Kaminarides et al., 2007), while its content in milk was up to 0.2 mg/kg (Imhof and Bosset, 1994b ; Ott et al., 1999). Gallardo-Escamilla et al. (2005) reported that 2-butanone is significant for causing yogurt odor and contributes to fruity aroma.

Although each of these carbonyl compounds constitutes a recognizable flavor, yogurt flavor is determined by a mixture of important volatile compounds. For example, a ratio of acetaldehyde to diacetyl of 1:1 would give a preferred ratio typical of yogurt flavor (Zouari and Desmazeaud, 1991), while too much acetaldehyde, compared to diacetyl, would result in a "green" flavor .

Also, the ratio of acetaldehyde to acetone plays a significant role in yogurt flavor development (Panagiotidis and Tzia, 2001) and a ratio of 2.8:1 results in the desired fullness flavor (Bottazzi and Vescovo, 1969).

Acids

Lactic acid and other acids also play specific roles in the aromatic properties of yogurt. Although lactic acid exhibits low volatility, it is included in this discussion due to its importance in aroma formation.

Lactic acid is a primary source of aroma and functionality for many fermented food products and is responsible for the refreshing tart aroma of yogurt (Panagiotidis and Tzia, 2001).

During the yogurt production process, approximately 20-40% of lactose present in milk is transformed into lactic acid, and the lactic acid content in yogurt is around 0.9%. Acidity is an important factor in the perception of yogurt aroma, and the pH of yogurt is typically close to 4.4.

Milder yogurts, which are preferred by some consumers, are evaluated as less aromatic. Ott et al. (2000) showed that there are significant aroma differences between traditional acidic yogurts and milder, less acidic ones, but this is primarily due to differences in acidity rather than different concentrations of the three aroma impact compounds (acetaldehyde, diacetyl, and 2,3-pentanedione). Panagiotidis and Tzia (2001) found that lactic acid and acetaldehyde contribute significantly to the typical aroma of yogurt.

Some researchers have considered lactic acid as the most important component of yogurt aroma (Ott et al., 2000). Besides lactic acid, other fermentation acids are also produced in yogurt, both through lipolytic (C4-C20) and bacterial processes (C2-C4).

Acetic acid is an important compound produced by lactic starter cultures (Alonso and Fraga, 2001). Concentrations ranging from 0.5 to 18.8 mg/kg have been reported in yogurt (Alonso and Fraga, 2001). High levels of acetic acid impart a "vinegar" taste that may not be accepted by consumers (Tamime and Robinson, 1999). Acids with longer chains (e.g., octanoic acid) can contribute to the characteristic "soapy" aroma (Guler, 2007).

Alcohols and Esters

Alcohol is another group of volatile compounds found in yogurt, with ethanol being the main alcohol present in yogurt, which is a common end product in the breakdown of glucose and amino acid catabolism (Guler, 2007). Ethanol content in the range of 0.2 to 9.9 mg/kg has been found in yogurt made from cow's milk (Pourahmad and Assadi, 2005), while its content in yogurt made from goat and sheep milk is generally lower (Georgala et al., 1995).

Despite ethanol being a major volatile compound in yogurt, its contribution to the overall aroma is not clear. It has been reported that ethanol probably provides a complementary aroma (Vedamuthu, 2006), while others believe that ethanol produced during acid fermentation has no practical importance in yogurt aroma at low levels (Rasic and Kurmann, 1978).

Esters, which correspond to the acids and alcohols present, are also found in yogurt. Ethyl esters, resulting from the enzymatic or chemical esterification of acids with ethanol (Molimard and Spinnler, 1996), possess a sweet, pleasant taste, and fruity notes, contributing to the aroma of cultured dairy products such as cheese (Mariaca et al., 2001).

Esters are usually found at low concentrations in yogurt that has undergone prolonged storage. Traces (0.03 mg/kg) of ethyl acetate have been found in fresh yogurt (Imhof et al., 1995), while Guler (2007) observed that ethyl acetate only started appearing in salty yogurt after 30 days of storage, and its concentration increased slowly with prolonged storage.

Most esters have a fruity and floral aroma and contribute to the aroma by minimizing the clarity and bitterness conferred by fatty acids and amines (Guler, 2007; Molimard and Spinnler, 1996).