Literature Review: Network of flavor compounds formation and influence factors in yogurt

Journal of Dairy Science

Available online 28 June 2024

Die Li ¹,Yutong Cui ¹,Xinying Wu ¹,Jiyong Li ²,Fuhai Min ²,Tianrui Zhao ¹,Jianming Zhang ³,Jiliang Zhang ¹¹Faculty of Food Science and Engineering, Kunming University of Science and Technology, Kunming 650550, China²Shangri-la Kangmei Dairy Products CO.\, Ltd., Diqing Prefecture 674400, China³Zhejiang Academy of Agricultural Sciences, Hangzhou, Zhejiang, 310016, China

Received 5 March 2024, Accepted 2 June 2024, Available online 28 June 2024.

Under a Creative Commons license

ABSTRACT

Yogurt is popular as a natural and healthy food, but its flavor greatly affects acceptability by consumers. Flavor compounds of yogurt is generally produced by the metabolism of lactose, protein and fat, and the resulting flavors include carbonyls, acids, esters and alcohols, etc. Each flavor compounds could individually provide the corresponding flavor, or it can be combined with other compounds to form a new flavor. The flavor network was formed among the metabolites of milk components, and acetaldehyde, as the central compounds, played a role in connecting the whole network. The flavor compounds can be affected by many factors, such as the use of different raw milks, ways of homogenization, sterilization, fermentation, post ripening, storage condition and packaging materials, etc., which can affect the overall flavor of yogurt. This paper provides an overview of the volatile flavor compounds in yogurt, the pathways of production of the main flavor compounds during yogurt fermentation, and the factors that influence the flavor of yogurt including type of raw milk, processing, and storage. It also tries to provide theoretical guidance for the product of yogurt in ideal flavor, but further research is needed to provide a more comprehensive description of the flavor system of yogurt.

INTRODUCTION

With the end of the COVID-19 pandemic, people pay more attention to their health, thus food with natural desirable flavor and better nutrition are getting a lot of attention. For these reasons, fermented dairy products have undoubtedly become one of the preferred products in people's daily lives. Among those dairy products, yogurt is favored for its high nutritional value, desirable flavor and less salt intake compared with that of others.

With the increasing demand for natural flavor in yogurt, the research focuses on flavor formation became one of the hotspots (Bai et al., 2020, Tian et al., 2020b, Li et al., 2022, Peng et al., 2022). The flavor of yogurt is affected by many factors such as milk metabolism, milk composition, treatment of raw milk and milk sources, etc.

In this paper, the classification of volatile flavor compounds of yogurt, the general way of flavor compounds formation, flavor compounds interaction, and the factors affecting the flavor formation were systematically summarized.

YOGURT CONSUMPTION AND BENEFIT FOR HEALTH

The consumption of yogurt could provide many benefits to health. Yogurt is a good source of protein, calcium, iodine and vitamin B12, lower the risk of obesity and cardiometabolic risk in child and adult. Meanwhile the ideal calcium-phosphorus ratio of yogurt is good for bone strengthen. Furthermore, yogurt could enhance the proliferation and colonization of probiotics (L.acidophilus and B.bifidus) for strengthen immunity, weight and fat loss regulation, and beneficial for digestive and reduction in the symptoms caused by lactose maldigestion(Moore et al., 2018, Hasegawa and Bolling, 2023).

The outbreak of the epidemic prompted consumers to pay more and more attention to their own immune health, we began to learn how to enhance immunity. In addition to adequate sleep and regular exercise, nutritious, healthy food is essential. Among them, yogurt is favored for its high-quality protein, vitamins and calcium as well as its good flavor. Other studies have shown the potential role of yogurt consumption in preventing diabetes, its benefits to gastrointestinal health, as well as its ability to boost immunity and prevent obesity. The yogurt market is growing rapidly due to health claims amidst the epidemic, and the global yogurt market is expected to exceed $237 billion by 2028(Shahbandeh, 2023a). In the US, both the annual yogurt consumption category and per capita consumption have increased since the pandemic outbreak in 2019. In 2022 US yogurt consumption is about 4.65 billion pounds, per capita consumption of about 13.9 pounds(Shahbandeh, 2023b), the consumption of yogurt and its benefits is shown in Figure 1.

METHODOLOGY OF YOGURT FOR RELATED PRACTITIONERS

Dairy products including cheese, butter, yogurt, ice cream and new products etc. Sensory evaluation of those products is necessary especially for the new products before and after launched into the market. For dairy products, conventional descriptive analysis has been well applied with presents validity and robustness, however, expensive, time consuming limit the professionalism of its application. Recently, new sensory methodologies with good results easily the evaluation process for many dairy related practitioners.

The new sensory methodologies mainly including about 10 ways to evaluate the milk products, and check all that apply (CATA), rate all that apply (RATA), just about right (JAR), preferred attribute elicitation (PAE), projective mapping (PM)/napping and sorting), belongs to global differences and similarities, has been used for yogurt evaluation.

CATA could discrimination the difference based on sweet and milk aroma, kinds and concentration of ingredients, and flavor. RATA, PAE and PM/snapping evaluation based on the kinds of sweetener, but RATA also evaluated the similarity of sweetener with sucrose, PM/snapping optimize the mixture of sweetener. JAR evaluates the application of excipients in yogurt through consumer acceptance. Sorting is help to discrimination of the products based on protein ratio and fat type(Pereira et al., 2021, Soutelino et al., 2022, Ribeiro et al., 2024). All those new methods provide fast and flexible way for yogurt sensory evaluation.

MAJOR FLAVOR COMPOUNDS IN YOGURT

Research has been showed that about 117 kinds of flavor compounds are present in yogurt, including carbonyl compounds, acids, alcohols, esters and other aromatic, etc (Liu et al., 2022). Most compounds could provide typical flavor, however, it is often the small number of those compounds play a decisive role. Compounds such as acetaldehyde, 2,3-butanedione, acetoin, 2-butanone, 2,3-pentanedione and acetic acid contribute most to the typical flavor of yogurt (Božanić et al., 2003, Routray and Mishra, 2011, Settachaimongkon et al., 2014). Other compounds provide yogurt with extra pleasant flavor, such as fruity, creamy- and comforting-flavor. Most of those compounds have auxiliary effects on flavor, they not only provide flavor individually, but also replace individual flavor and/or provide new flavor when multiple flavor compounds are exerting their function together. The flavor formation associated with acid, carbonyl compounds, alcohol, ester and etc, which are introduced respectively, and Table 1 is present the flavor or the function of compounds.

Table 1. - Flavor compounds and flavor contribution

-(c) represent compounds do not directly provide flavor, they can be converted into flavor compounds.

-(e) represent compounds in low concentration in yogurt, but could increase flavor as enhancer

Acids: Research showed that more than 10 kinds of organic acids have been proved to have a great impact on yogurt flavor, which divided into non-volatile acids and volatile acids (Akbaridoust et al., 2015). Most volatile acids are composed by C₂-C₁₀ saturated fatty acids, such as acetic acid, capric acid and octanoic acid, and they provide vinegar, pungent and rancid, wax and animal flavor, respectively (Ai et al., 2015, Wang et al., 2020). Non-volatile acids such as actic acid, pyruvic acid, oxalic acid and succinic acid, etc. (Cheng, 2010). Although these non-volatile acids do not directly provide flavor to yogurt, they can be converted into flavor compounds through a variety of pathways. Those pathways are introduced in the following.

Acid could contribute desirable and/or undesirable flavors to the yogurt according to different acceptance. Acids like butyric acid, hexanoic acid and hexadecanoic acid could provide desirable flavor like fruity-, creamy- and sweat- flavor, respectively. Lactic acid imparts an acidic and refreshing flavor (Božanić et al., 2003). However, octanoic acid and N-decanoic acid provide waxy-, unpleasant- and soapy- flavor, respectively. Some acid like acetic acid provides vinegar flavor, whose acceptance depends on the concentration.

Aldehyde: unlike the acids, the number of aldehyde found in yogurt is relatively less, and they barely provide undesirable flavor. Research found that 4 main aldehydes provide pleasant flavor to yogurt. Acetaldehyde provides green apple-, nutty- and yogurt- flavor (Chen et al., 2017b), heptanal and nonanal provide flavor like green-, sweet- and grass-like (Moio et al., 1996), hexanal provide a fruity, fresh flavor, as well as, a fatty flavor (Mounchili et al., 2005). Acetaldehyde as the core substance of metabolic conversion pathway, it plays an important role in the formation of yogurt flavor, it will introduce later.

Ketone: like the aldehyde, ketone belongs to carbonyl compounds. Different ketones have completely opposite contribution to yogurt flavor. Most of ketones provide desirable flavor to yogurt. Three ketones could provide fruity flavor to yogurt, 2-undecanone and 2-tridecanone provide one flavor like pineapple- and peach- flavor, respectively. However, acetone provide apple- and pear- flavor at the same time. Four ketones provide creamy-, sweet- and buttery- flavor including 2-butanone, 2,3-butanedione, 2,3-butadione and 3-hydroxy-2-butanone. Unlike other ketones, 2-heptanone and 2-nonanone could provide yogurt with undesirable flavor like mustard and spicy. Bicyclo[2,2,1]heptanone make the yogurt with camphoreous like flavor.

Esters: esters are not only important flavor contributor to yogurt, but also mask the undesirable flavor including astringent and bitter (Cheng, 2010, Liu et al., 2022). Most esters are often described as having floral- and fruity- aromas that provide desirable flavors to yogurt. For example, Ethyl acetate, ethyl hexanoate and ethyl caprylate could provide yogurt with fruity flavor like pineapple and apple. γ-lactones and δ-lactones could provide yogurt with different fruity flavor like coconut and peach flavor, respectively. However, δ-decalactone could provide both peach and coconut flavor to yogurt. Butyl acrylate and ethyl butanoate provide tropical fruity flavor, like banana. Besides those fruity flavor provider esters, δ-Octalactone provide multiple desirable flavors including floral, fruity, creamy and buttery. δ-Nonalactone, ethyl caprate and methyl benzoate provide milky, wine-like and vanilla-like flavor, respectively.

Alcohols are present in low concentrations in yogurt made from goat milk and sheep milk, and generally do not contribute to flavor, but yak yogurt is an exception (Liu et al., 2022). Certain concentration of alcohol provides yak yogurt with mellow flavor, however, the high level of ethanol provides wine flavor, a controversial flavor. 3-methyl-1-butanol gives a fresh alcoholic-, fruity- and grainy- flavor (Reyes-Diaz et al., 2020, Capitain et al., 2022). 1-octen-3-ol provides yogurt flavor with mushroom-like. In addition to providing flavor directly, alcohol can also esterify with acids during the storage process to form new flavor compounds and play an auxiliary role in aroma production.

THE ROLE IN THE PRODUCTION OF FLAVOR COMPOUNDS BY LAB

During the fermentation, the lactic acid bacteria (LAB) metabolism in milk could produce many flavor compounds, especially for acetaldehyde and diacetyl. LAB could participate in homofermentation, and heterofermentation according to the species, substrate and environmental conditions. Lactose under the glycolysis to produce the puryvate, which could also produce by oxalacetate. Puryvate converted into the acetyl-CoA by pyruvate decarboxylation complex, which consists of 3 Associated subunits including pyruvate dehydrogenase, acetyl-CoA synthetase and acetyl-CoA carboxylase. acetyl-CoA converted into the main flavor compounds acetaldehyde by acetyl-CoA reductase. Acetaldehyde could also produce in protein degrades, and it will introduce in the following.

The concentration of acetaldehyde in yogurt ranges from 2.0 to 41 mg/kg depending different strains and process during the fermentation, and it could provide yogurt with good flavor only the concentration greater than 8.0 mg/kg. The concentration of diacetyl in yogurt ranges from 0.2 to 3 mg/kg, which is relatively lower than that of acetaldehyde, and it provides distinctive aroma when the concentration reached 1 mg/kg(Chen et al., 2017a).

Traditional yogurt fermentation by the action of S. thermophilus and Lb. delbrueckii ssp. Bulgaricus (dominant starters), due to the 2 strains have genes that produce the enzymes for process mentioned above. Other strain, like Lb. rhamnosus GG, has been used for yogurt fermentation in many countries due to its contribution to produce non-volatile compounds. Yogurt fermented by Lb. casei individually lacks of the typical volatile compounds compared with that of dominant starter, they could be defined as auxiliary starter. With the advent of molecular biology, the glyA gene of LAB was proved directly associated with the acetaldehyde yield. In the case of glyA gene overexpression and glyA gene complete inactivation, the yield of acetaldehyde is 1.8–1.9 times and 0 times, respectively. Another main flavor compounds, diacetyl was associated with α-acetolactate synthase, lactate dehydrogenase and α-acetolactate decarboxylase encoded by als or ilvBN, ldh and aldB genes, respectively. Unlike the regulation of glyA gene, the yield of diacetyl was not affected by overexpression of the als- or ilvBN-gene or the inactivation of the ldh- or aldB-gene(Chaves et al., 2002, Chen et al., 2017a).

THE MAIN FLAVOR FORMATION NETWORK IN YOGURT

Flavor formation is a complex process, mainly consists of 2 aspects. On the one hand, the metabolism of the 3 major nutrients produces flavor compounds, and on the other hand, the flavor compounds transformation into new flavor compounds. The 3 major nutrients including saccharometabolism, protein (mainly casein)- and lipid- catabolism (Jia et al., 2021). They could not only form flavor compounds individually, but also interconversion to other flavor compounds. These compounds form a larger network of flavor formation through their interaction, the whole network is shown in Figure 2.

Flavor Compounds Formation by Saccharometabolism

Saccharometabolism would mainly generate 3 kinds of flavor compounds including acids, ketones and aldehydes. One of the most important compounds is pyruvate, which is link the whole saccharometabolism pathway. Pyruvate can be produced in the tricarboxylic acid cycle by citrate lyase to oxaloacetate, which directly generated α-acetolactic acid. Oxaloacetate and α-acetolactic acid can form pyruvic under the action of decarboxylation. Pyruvate as the flavor precursor could converted into 3 important flavor contributors, including acetoin, diacetyl and acetaldehyde. Pyruvate could directly or indirectly convert into acetoin, and the indirectly way could transform into another important flavor compounds like diacetyl. First, pyruvic converted into α-acetolactic acid by the action of α-acetolactic acid synthase. Then α-acetolactic acid converted into diacetyl under the action of chemical oxidative decarboxylation, which transformed either into acetoin by diacetyl reductase, or directly into acetoin under the decarboxylation by α-acetolactic acid decarboxylase (Comasio et al., 2019).

Flavor Formation Affected by Protein Catabolism

The milk proteins, especially casein, decompose slowly and thus form free amino acids. Amino acids could provide flavor either individually, or as precursors for flavor compounds formation, particularly branched chain-, aromatic- and sulfuric- amino acids (Martin-Dejardin et al., 2011). Different amino acids provide different flavor, such as arginine is associated with bitterness, while proline, serine and asparagine are associated with sweetness (Niro et al., 2017). The biosynthetic pathway of flavor compounds from amino acids relies on transaminases and the chemical pathway is through Strecker degradation (Afzal et al., 2017). Several amino acids, such as leucine, methionine, and threonine could convert into flavor compounds mainly involves amino acid exchange and deamination. Threonine and glycine produce acetaldehyde in yogurt under the action of threonine aldolase (Ott et al., 2000). The exchange via aminotransferases, forming the corresponding α-keto acids, followed by the formation of aldehydes, esters, and alcohols. 3-Methylbutyraldehyde is formed by the leucine catabolism under the biosynthetic pathway and the chemical pathways, which provides nutty- or chocolate- flavor (Smit et al., 2004). ‘Goaty' aroma always associated with 3-methylbutan-1-ol, 2-phenylethanol and 4-methylphenol, which are produced by the metabolism of amino acid decomposition (Zabaleta et al., 2017).

Effect of Flavor Formation by Fat Catabolism

Lipolysis or oxidation of triacylglycerides generates free fatty acids, which has 2 pathways to form flavor compounds mainly including ketones and esters. One pathway is free fatty acids by β-oxidation convert into β-ketoacids, which further transformed into ketones. Under the action of decarboxylation, β-ketoacids converted into methyl ketones, which is an important flavoring compounds and provides different flavors depending on the number of carbons (Cao et al., 2014). For example, 2-Heptanone offers banana flavors in kurut, while 2-Nonanone offers rose like flavors (Wang et al., 2020, Wang et al., 2021a). Free fatty acids also could undergo fatty acid hydroxylation during β-oxidation to generate γ-hydroxy acids and δ-hydroxy acids, which are subsequently converted to γ-lactones and δ-lactones by removing one molecule of water under the action of cyclase. γ-lactones and δ-lactones provide coconut- and peach- flavor, respectively. Another way is that free fatty acids with ethanol converted by acetaldehyde to produce esters by esterification. For example, short chain free fatty acids react with methyl mercaptan to form methyl thioesters (Cheng, 2010).

The Linkage Function of Acetaldehyde in Metabolic Network

Acetaldehyde plays an important role in linking the whole metabolic network of flavor compounds. There are 2 main pathways to generate acetaldehyde including glycometabolic pathway and protein pathway (Ott et al., 2000). For former one, one way is that lactose produces glucose under the action of β-galactosidase, which convert into pyruvate through glycolysis, then pyruvate forms acetyl-CoA under the action of pyruvate dehydrogenase or pyruvate formate lyase, and acetyl-CoA forms acetaldehyde through aldehyde dehydrogenase. Another is that pyruvate could be directly converted into acetaldehyde under the action of pyruvate decarboxylase or pyruvate oxidase. For latter one, amino acids generated from proteins undergo a deamination reaction to form α-keto acids, which are further decarboxylated to form acetaldehyde. Acetaldehyde transformed into ethanol under the glyoxylate reductase, and ethanol and free fatty acids could generate the esters. Hence, acetaldehyde links the glycometabolic, protein catabolism and fat catabolism together.

Extra Effects of Compounds on Flavor

In fact, these flavor compounds cannot only affect the yogurt flavor individually, but also have the combination function of flavor complementation, flavor enhancement or convert to other compounds contribute new the flavor. Study demonstrated that the lack of acetaldehyde can probably be substituted by a relatively high concentration of diacetyl (Chen et al., 2019). Acetoin has a similar flavor to 2,3-butanedione, however, the flavor of acetoin is considerably weaker than that of 2,3-butanedione, which provide harshness flavor alone. When they combined with each other, the resulting mixture could provide the mild, pleasant and buttery flavor, and acetoin tends to reduce the harshness of 2,3-butanedione (Cheng, 2010). Diacetyl couple with acetoin or ethyl provide yogurt with creamy and buttery flavor (Chen et al., 2017b). Diacetyl, acetaldehyde, and acetoin (3-hydroxy-2-butanone) have been known to increase desirable flavor to yogurt (Tian et al., 2020a, Zheng et al., 2021). There are several ketone compounds found in the yogurt in low quantities, and always used as flavor enhancers. Those compounds including 4-pentanal, 2-methylbutanal, 2-butanone, 3-methyl-2-butanone, 2-hexanone, 2-heptanone and 2-nonanone, etc. flavor compounds such as 2-methyl-butanol and 3-methyl-butanol increase the alcohol flavor (Toso et al., 2002). Diacetyl contributes the delicate, full flavor of yogurt, if acetaldehyde content is low it will compensate the flavor (Božanić et al., 2003).

FACTORS AFFECTING THE FLAVOR OF DAIRY PRODUCTS

The formation of flavor compounds is not only affected by the metabolism, but also by the character of raw materials, and milk collection season, treatment process and storage conditions, etc. Every process from the collection of raw milk to the marketing of yogurt could affect the flavor significantly.

Effect of Fat Composition on Flavor

The composition and concentration of raw fresh milk fatty acids produced from ruminants such as cow, goat, cotton sheep or yak has distinctive different. Fat concentration was a key factor that influence the yogurt like flavor, higher fat content in yogurt protected the degradation of flavor compounds like diacetyl and acetaldehyde by lipase compared with that of lower fat resulting yogurt (Rychlik et al., 2006). A study found that milk fatty acids with different carbon numbers could affect the flavor (Mitani et al., 2016). Sheep and goat milk have a waxy and animal flavor with more medium and short chain saturated fatty acids (C6:0, C8:0, C10:0, and C12:0) compared with that of other ruminants (Jia et al., 2021). Soapy flavor was positively correlated with total fatty acids, such as C8:0, C10:1 and C14:0. Long-chain fatty acids can be hydrolyzed into short-chain fatty acids to affect the flavor, such as butanoic acid (C4:0), hexanoic (C6:0) and octanoic acids (C8:0). Yak milk has a distinctive milky flavor associated with its high unsaturated fatty acids content, such as eicosapentaenoic acid (C20:5) and docosahexaenoic acid (C22:6) providing sweet and tangy flavor.

Effect of Compounds Concentration on Flavor

In addition to the composition, the concentration of flavor compounds also play an important role in affecting the yogurt flavor (Kaminarides et al., 2007). Some flavor compounds in a relative low concentration provide more obvious effect. A study found that yogurt made with cow's and goat's milk had initial diacetyl concentrations of 14.20 and 15.59 mg kg^–1, respectively, and during storage, diacetyl concentrations were 16.60 and 16.80 mg kg^–1, respectively. As a result, the former provides more creamy flavor than the latter. (Božanić et al., 2003). Acetaldehyde, diacetyl and ethyl provide green apple, creamy, buttery flavor, and their odor thresholds are 0.11, 0.003 and 8 mg/L, respectively (Chen et al., 2017b). However, along with the concentration increases, those flavor decreases simultaneously. The relatively high concentration of acetaldehyde found in ovine yogurt (29.49–42.94 mg/kg) than that found in bovine yogurt (1.8–16.8 mg/kg), resulting the former one has a less acceptable flavor compared with that of latter one. Study also demonstrated that relative high concentration of fat could affect certain volatile flavor compounds, including 4-pentanal and hexanal. However, the fat concentration not affected the flavor compounds like 3-methyl-2-butanone, 3-methyl-1-butanol and 3-methyl-2-pentanol at all.

Effect of Different Forage and Season on Flavor

The composition of milk constitution decided by the feeding forage, which indirectly effect the flavor compounds in both concentration and variety. Studies have been demonstrated that the milk from pasture fed cows contained higher levels of aldehydes than that of whole diet fed cows (Carpino et al., 2004), compounds including octanal and nonanal to be present only in grass silage milk, but pasture milk (Boltar et al., 2014). Silage usually contains ethanol, especially corn silage (Kilcawley et al., 2018), the co-feeding of pasture and silage to cows may produce milk with the best flavor because carboxylic acids from plants and ethanol from silage form more desirable esters (Kalač, 2011). Besides that, the milk contained high amounts of esters fed with pasture and concentrate with high ryegrass hay, corn meal, corn silage and grains, etc. (Carpino et al., 2004). Besides the forage, the collecting season also effect the flavor compounds. Study demonstrated that cows fed with grass silage had higher concentration levels of 2-butanone and ethyl acetate in milk produced in winter than that in summer (Boltar et al., 2014). The milk collected in winter has a higher ethanol content than that in other seasons (Gonzalez-Martin et al., 2016).

THE EFFECT OF PROCESSING ON THE FLAVOR

After the collection of raw milk, which will undergo the process including homogenization, sterilization, fermentation and post ripening for yogurt production. Each process might affect the flavor in yogurt. These processes can lead to the formation of new flavor compounds in the milk, and also effect flavor of the resulting yogurt, the result is shown in Figure 3.

Effect of Fat Globule Treatment Ways on Flavor

The size of fat globules directly affects the quality of yogurt, especially in whey extraction. There are 2 ways to guarantee the milk fat globules in the ideal size including homogenization or ultrasound. Both ways affect the flavor compounds in treated milk and the flavor in the resulting yogurt through different ways.

In general, parts of yogurt flavor compounds come from the fat. It has been found that as the pressure of homogenization increases, consequently, the fat globule size and the off flavors such as cooked and cardboard decreases (Whitson et al., 2010, Park and Drake, 2017). Research also found that homogenization easily generates important flavor compounds including acetaldehyde, methyl ketone and others (Vélez et al., 2017). Ultrasonic is another way to ensure the milk fat globule to the ideal size. A study showed that ultrasonic time could affect the generation of flavor compounds in milk. Several flavor compounds generated in relative short ultrasonic time, and their concentration slightly changed when extended the ultrasonic time. Those flavor compounds including 1,3-butadiene, 1-buten-3-yne, 1-hexene, and hexanal (Riener et al., 2009, Sfakianakis and Tzia, 2017).

Both ways have the same effect on the fat globule size, however, they had different effect on flavor generation. Study have been demonstrated that the concentration of 2,3-butadione and 3-hydroxy-2-butanone was insignificantly different compared between ultrasound and homogenization. However, higher concentration of the long-chain volatile compounds including aldehydes, ketones, carboxylic acids and dimethyl sulphide were found in resulting yogurt treated by ultrasound (Sfakianakis and Tzia, 2017).

The Effect of Sterilization on Flavor Compounds Formation

A necessary process before yogurt fermentation is sterilization, which changes flavor compounds in milk and resulting yogurt (Ruiz Pérez-Cacho et al., 2019). Sterilization has 2 effects on flavor compounds, one is to alter the concentration of the original flavor compounds (Li et al., 2013), another is to generates new flavor compounds (Jo et al., 2018, Tong et al., 2019).

For former one, the content of natural volatile compounds such as aldehydes, methyl ketones, alkanes, olefins and alkenones in milk increases with the intensity of heat treatment (Li et al., 2013). For latter one, desirable and undesirable flavor compounds could generation. The lipids oxidation produces flavor compounds including esters, methyl ketones and free fatty acids during the sterilization. For example, the concentration of butanedione, 2-heptanone and 2-pentyl ketone was positively correlated with the heating temperature (Vázquez-Landaverde et al., 2005, Tong et al., 2019). The Maillard reaction between the lactose and amino groups produces flavor compounds such as nitrogenous compounds, maltol and diacetyl groups (Jo et al., 2018). However, the breakdown of sulfur-containing proteins to form sulfides produces a cooked flavor, and oxidation of unsaturated fatty acids to form aromatically active aldehydes and ketones produces a cardboard-like flavor (Vázquez-Landaverde et al., 2006, Amador-Espejo et al., 2014, Amador-Espejo et al., 2017).

The Effect of Different Sterilization Ways on Flavor Compounds Formation

The sterilization methods of milk including pasteurization, ultra-high temperature (UHT), high intensity ultrasound (US) treatment, high pressure processing (HPP) techniques and ohmic heating. Each methods have different impact on the raw milk and the subsequent flavor compounds in yogurt.

The pasteurization destroys the lipoprotein lipase present in raw milk, and the resulting yogurt presents excessive rancidity and goat flavors, However, the concentration of off flavor compounds like caprylic and capric acid was reduced in milk and yogurt (Sulejmani and Hayaloglu, 2018). The flavor of ultra-pasteurized milk has a distinct sulfur, egg, boiled and cabbage flavor compared with that of UHT milk, associated with the release of sulfhydryl compounds from whey proteins, especially β-lactoglobulin (Vázquez-Landaverde et al., 2006, Lee et al., 2017). UHT treatment generates higher concentration of flavor compounds including 2,3-butanediol, hexanal, 2-octanone and 2-heptanone (Munir et al., 2022). In addition, it prevents the occurrence of off flavor compounds including methyl ketone and biogenic amines (Voigt et al., 2010, Calzada et al., 2013, Calzada et al., 2014a, b). US treatment could cause lipid photo-oxidation to generates more desirable flavor compounds like acetaldehyde, acetone and hexanal, etc. However, it could also generates undesirable volatile compounds, such as glutaraldehyde, heptanal carbonyl, and maximum amounts of heptanal, 3-octanone, amyl isobutyrate, dodecanoic acid and octanoic acid (Munir et al., 2022). In addition, the resulting yogurt has an undesirable flavor including “rubbery,” burnt, pungent and fatty (Riener et al., 2009, Chouliara et al., 2010, Sfakianakis and Tzia, 2017). Although many undesirable volatile compounds generated from US, but the resulting yogurt has more desirable flavor than that of pressure homogenized. Ohmic heating increases the degree of milk protein hydrolysis, the resulting yogurt with positive flavor including vanilla flavor, sweetness and creaminess (Rocha et al., 2020). It also produces off flavor compounds like furfural, 5-hydroxymethylfurfural, furanone and caramel (Ferreira et al., 2019, Rocha et al., 2020). It has been reported that the production of acetaldehyde 88 is increased by strengthening and heat treatment of milk, which may be due to the subsequent increase of free amino acids.

OTHER WAYS TO AFFECT THE FLAVOR IN YOGURT

In addition to the above influencing factors on the flavor of yogurt, there are 2 other important influencing factors, including storage conditions and packaging conditions.

Effect of Storage Conditions on Flavor Compounds

The concentration of key flavor compounds in yogurt changed during the ripens within days, which mainly decided by storage ways. For example, the concentration of diacetyl decreases during ambient storage (Tahmas-Kahyaoğlu et al., 2022), however, its concentration tends to increase slightly under refrigerated conditions(Božanić et al., 2003, St-Gelais et al., 2009). Concentration of lactic acid, acetic acid and butyric acid tended to increase after ambient storage, which participated in esterification with ethanol to increase the concentration of ethyl butyrate (Chen et al., 2019). The concentration of acetaldehyde, diacetyl and acetone increased during the refrigerated storage, however, their concentrations showed opposite trend when storage during ambient condition. Acetoin, another important flavor compounds, its concentration decreased during refrigerated storage, but unlike the 3 compounds mentioned above, its concentration was not affected by the storage time under ambient condition. (Tahmas-Kahyaoğlu et al., 2022). Concentration of lactate and orotate was increased during the storage under ambient condition, while the concentration of pyruvate and citrate was not affected (Miyaji et al., 2021). During the storage, the concentration of off flavor tyrosine significantly increased, which concentration increased from 0.17 to 0.20 mg/g, the bitterness of yogurt increased.

Effect of Packaging on Flavor

Packaging used for yogurt during the storage may produce flavor compounds such as hexanal, heptanal and octanal in cardboard . It has been shown that odors from packaging materials can migrate into dairy products, and 3 flavor compounds, nonanal (fatty, citrusy, grassy), 2-ethyl-1-hexanol, and D-limonene (woody), were detected in both off-flavored yogurt and paper-based packaging materials (Zhang et al., 2022). Therefore, the packaging of dairy products is also an important factor affecting the flavor.

CONCLUSIONS

In recent years, the research on yogurt focuses on the identification of flavor compounds and the determination of key aroma compounds, but the research on yogurt flavor is not systematic enough. In addition, processing and storage of raw milk also affect the flavor. With the development of technology, there are more new heat treatment technology applied to the processing of yogurt, it is possible to form unique flavor compounds. But the application of the new technology requires more research. To understand the way of flavor formation, the factors affecting flavor and the latest technology application of fermented dairy products can be helpful for improving and researching new the flavor for yogurt products.