Bread Aroma and Flavour Creation Factors

Abstract

Bread is typically defined as being light and airy, with palatable and mild taste, and is very common in the diet of people, regardless of gender, person’s age, or socioeconomic standing. It is a typical food that supplies the majority of daily energy intake and consists mainly of carbohydrates. When it comes to food, the aroma of freshly baked bread is regarded as one of the most preferred ones. Numerous studies have been conducted on aroma of bread, and over 540 volatile components have been found. Alkaloids, aldehydes, esters, ketones, acids, pyrazines, and pyrrolines are the most significant groups in terms of quantity; furans, hydrocarbons, and lactones are also mentioned. Nevertheless, the final bread aroma is mostly determined by a very small number of these chemicals. Numerous factors, including the flour and its extraction milling rate, additional ingredients in the bread recipe, the method, regimes, and time frame for fermentation process, affect the final aroma of the bread. Since the final bread aroma is created during baking, baking conditions are equally essential to its creation. In this sense, the text states the factors that affect aroma of bread and describes their function.

1. Introduction

A wide range of products with varying forms, dimensions, textures, crusts, colours, softness, eating characteristics, and aromas are referred to as bread. Since the Neolithic era, certain rudimentary varieties of unleavened bread have been discovered, and numerous artefacts indicate its importance. Even more, the spiritual and psychological importance of bread has remained today in many cultures around the world. Bread’s favoured and distinctive flavour and aroma, together with certain nutritional benefits, are major contributors to its widespread acceptance as a basic food.

The term bread, in a more restrictive meaning according to National Regulation concerning a baking product quality, is defined as a product made from flour of various grains, water, or other allowed liquid, baker’s yeast, or other fermentation components, additives, and other ingredients. In addition, bread is a baked product that weighs more than 250 g and is made by mixing, fermenting, and baking [1].

Because fermentation process introduces CO₂ and ethanol, leavened bread has a lot more air and is lighter and more palatable. It is easy to chew and hence better for digestion. In much of Europe, bread is the main source of carbohydrates. Different kinds of bread have varying contents, ranging from whole-grain rye bread with whole kernels and sourdough to white wheat bread with sifted flour, that has both the germ and the outer layers of the grain removed. This makes an important difference when it comes to health perspective [2].

Consuming whole grains has been linked to several health advantages, including a lower risk of type 2 diabetes, insulin resistance, coronary heart disease, unwanted weight gain, and colorectal cancer. The official definition of whole grains, as adopted by the Whole Grains Council in May 2004 [3], is as follows: “Whole grains or foods made from them contain all the essential parts and naturally-occurring nutrients of the entire grain seed in their original proportions. If the grain has been processed (e.g., cracked, crushed, rolled, extruded, and/or cooked), the food product should deliver the same rich balance of nutrients that are found in the original grain seed.”

The importance of bread in people’s diet is evidenced by the data that the average bread and bakery consumption volume per person is around 57 kg from 2017 to 2021 in Europe, and it has been rather stable over the past 10 years [4]. The usage of white bread variants has significantly decreased in Western countries during the past several decades. Due to their high salt intake and poor dietary fibre consumption, which are associated with a number of health problems, these countries have boosted their use of wholemeal bread, which has greater dietary fibre content and is generally a healthier choice [5].

It is clear how current studies and trends have affected the relative popularity of bakery goods, or the amount of a particular kind of bread consumed. It is evident, also, that while selecting a bakery product, considerations other than health advantages are equally crucial, such as flavour and aroma [6]. In relation to eating behaviours, it should be noted that one of the most preferred aromas is that of freshly baked bread.

Numerous volatile substances, such as alcohols, aldehydes, esters, ethers, ketones, acids, hydrocarbons, pyrazines, pyrrolines, furans, lactones, or sulphur compounds, have been linked to wheat bread aroma. These volatile substances could find in the crust, the crumb, or from both. The volatile components in the crumb are created by enzymatic processes that occur during dough kneading and, primarily, during yeast and lactic acid bacteria (LAB) fermentation of dough carbohydrates, while the aromatic components generated during thermal processing, i.e. baking, are the most representative in the crust [7]. A number of investigations on bread aroma and flavour have contributed to the reaffirmation of the contribution of traditional production methods regarding the complex aroma and taste profile of the product [8].

2. What is aroma in general

Food sensory characteristics are extremely important because they are frequently the determining factor in food selection. The basic sensory characteristics of food are smell (odour, fragrance), taste, appearance, texture, and sound. Thereby, smell and taste are chemical senses while appearance, touch, and sound are physical sense. The chemical compounds that are responsible for the taste sensations are water soluble and mostly non-volatile. Those compounds are detected by test buds located in oral cavity, majority in the tongue. Taste is determined by five taste modalities (sweet, sour, salty, bitter, and umami), and recognition of them seems to be easy. However, it is difficult since different chemical molecules (acetic acid in vinegar and ascorbic acid in strawberries) produce the same taste sensation but have distinct sensory characteristics. Furthermore, the taste of a particular ingredient does not exist in isolation; rather, it interacts with other molecules that are released when food is consumed.

Unlike taste, odour is a very complex sensation due to huge number of stimuli that can generate it. There are more than 10.000 compounds identified in food that may contribute to smell perception [9]. The smell is a sensory impression caused by the stimulation of the olfactory receptors. Smell can be perceived with sniff when the smell compound is inhaled directly into the nose by the respiratory airflow (orthonasal). However, smell can also be perceived by olfactory receptors from the mouth as one eats food, creating an aroma. The term “aroma” refers to smell experienced through the buccal cavity (retronasal) and perceived at the olfactory receptors (Figure 1).

Figure 1.

Aroma compounds are received via the nasal and buccal cavity (Source: the author’s private archive).

The term aroma does not have the same meaning in different languages. In the French language, the aroma is a pleasant odour; on English, it is odour that comes to the brain via retronasal pathways. Colloquially, in the Bosnian language, aroma implies the impression generated in the mouth as a result of smell and taste. However, according to ISO 5492, aroma is a sensory attribute perceptible by the olfactory organ via the back of the nose when tasting [10].

Aroma compounds must be volatile in order to reach the olfactory bulb. They also have a relatively low boiling point and limited solubility in water [11]. They are mainly hydrophobic, small molecules with low molecular weight, usually lower than 300 Dalton [12]. Aroma compounds are mostly organic compounds, but also inorganic compounds could be important contributors to the general aroma of some foods, such as hydrogen sulphide (H₂S).

There are several sources of aroma compounds in foods. They are natural constituents of non-process food or fresh food. It is the so-called primary aroma that originates from raw material as a result of enzymatic and microbial processes. During different technological processes of food production, a so-called secondary aroma is forming. The most important types of food processes responsible for the synthesis aroma compounds in food are fermentation [13] and non-enzymatic processes resulting from thermal treatments [14]. Some foods require more than one technological process, and they are the source of a great number of various aroma components. Bread making is one of them since dough fermentation and baking are two major sources of aroma compound formation in the bread [15]. In this case, the aroma of bread is generated by primary (raw material), secondary (fermentation), and tertiary (baking) aroma compounds. Spaho et al. [16] stated that aroma compounds of spirits originate from even four sources: primary – raw material, secondary – fermentation, tertiary – distillation, and maturation – quaternary.

Aroma compounds belong to a wide group of chemicals: alcohols, aldehydes, ketones, esters, acids, terpenes, lactones, and phenols. Quantitatively, they are present in food in very low concentrations, but qualitatively, aroma compounds are dominant in whole list of components present in some foods. The number of aroma compounds is extremely high, especially in foods that undergo Maillard reactions. As previously stated, the concentration of volatile aroma compounds is very low, often ranging from 10 to 15 ppm [14], but it can sometimes be extremely low measured in μg/kg [17].

However, the intensity of odour is not conditioned by the concentrations of aroma compound. Some compounds can be present in extremely low concentrations, but their contribution to overall aroma sensation could be remarkable and vice versa. In fact, the influence of the aroma components on the sensory quality of food is closely related to their sensory threshold. Threshold is the lowest concentration of aroma components that is perceivable by the human sense of smell. The compounds with very low odour threshold are usually aroma-active compounds. This suggests that they actively add to the aromatic profile of foods. Among hundreds aroma compounds present in some foods, only small part of them is crucial for the final aroma character of those foods. Aroma compound contribution is assessed in terms of aroma odour activity values (OAVs). It is the ratio of the detected compound concentration (C) in the sample to its orthonasal detection threshold (ODT). Orthonasal thresholds are mostly taken value from the literature and books.

Components with OAV higher than 1 are aroma-active compounds. Among 54 identified aroma compounds of white bread, 30 of them have OAVs ≥1 and were identified as key aroma profile compounds [18]. The importance of threshold value in OAV computation is best demonstrated by the example of 2-acetyl-1-pyrroline content in bread. Among the aroma compounds found in bread, this component showed the highest OAV due to its extremely low threshold while being present in the lowest concentration [19]. Therefore, 2-acetyl-1-pyrroline should be considered as the primary odorant of the wheat bread crust [20].

All the above-mentioned facts make the research of aroma components (analytically and sensory) very demanding and challenging. At the same time, we must remember that it is quite difficult for a person to isolate retronasally merely the smell without aroma’s interaction with taste, tactile, and trigeminal sensations. All these sensations combine to generate a flavour experience. According to ISO 5492, flavour is the complex combination of olfactory, gustatory and trigeminal sensations perceived during tasting [10]. It is regarded as one of the most essential factors influencing consumer acceptability of food.

A very broad multimodal aspect of taste perception includes the food matrix with all its characteristics [21]; the eating process (mastication and saliva); the release of volatile and non-volatile compounds [22]; the physiology of person (sensory system activation) and central cognitive processing of information (psychology of person).

In this extraordinarily complex multidisciplinary phenomenon of flavour perception, all conceivable interactions within and across modalities are occurring. These interactions of all stimuli are nearly infinite in flavour perception, where aroma is most essential [17].

3. Elements that contribute to aroma and flavour of bread

Bakery products in general have a distinct and pleasant aroma and flavour, which play a major role in influencing customer preferences and popularity of a certain bakery product, and they are significantly reliant on individual sensibilities and preferences.

There have been reports of over 540 volatile substances in bread. Still, the favourable aroma attributes of bread are mostly due to a relatively small amount of the volatile components present [15].

The aroma and flavour characteristics of bread are influenced by a variety of elements, including the recipe, particularly the flour and other non-wheat ingredients added, the method of fermentation, the addition of enzymes and improvers, and baking. Additionally, storage could have significant effects on the aromatic profile [20]. In general, the ingredients and processing techniques used for fermented products and bread determine how their flavour and aroma develop (Figure 2).

Figure 2.

Elements responsible for bread aroma creation.

3.1 Ingredients as contributors of bread aroma and flavour

3.1.1 Wheat flour

The most important ingredient in breadmaking is undoubtedly wheat flour which is used in the greatest quantity in the breadmaking process. The quality of the flour is crucial for achieving the appropriate quality of the final bakery product. Therefore, flour quality analyses are very detailed and complex in order to produce a product of uniform quality and excellent physical and chemical characteristics. It does, however, have very little influence on the aroma and flavour of the final product and provides very little to the group of volatile compounds and their precursors [23]. The high milling extraction rate of wheat flour contributes to a certain extent to enriching bread with specific flavour and aroma components since the outer layers parts and germ of the grain are present to a greater extent than in white flour. This is particularly pronounced in baking products made using wholemeal flour [24]. Therefore, a key factor in final bread aroma may be whether wholemeal wheat flour or white wheat flour is used [25].

Chalier et al. [26] analysed the volatile components of organic wheat flour and fine and coarse brans using the HS-SPME method. Regarding the quantity, the total amount of volatile components was about 8 mg/kg (dry basis) in coarse and fine bran, while the flour had only about 2 mg/kg (dry basis). The main constituents of volatile compounds are lipids; therefore, compared to coarse or fine brans, the flour with the lowest fat content had a lower concentration of volatile components. The predominant chemicals in the flour were 2-heptanone, limonene, 1-hexanol, and nonanal. Limonene was undoubtedly the main component in the brans, although 1-hexanol and 2-heptanone were also found. Regarding the compounds found in each fraction, most of them had a green, vegetal aroma that was deemed to be very pleasant, apart from acetic acid, 1-pentanol, and 1-hexanol (in high concentration), which are thought to be pungent. The sensory quality associated with citrus aroma can be enhanced by the presence of limonene [26].

According to Pico et al. [20], hexanal is the main component in wheat flour, followed by heptanal, pentanal, or 1-hexanol.

Compared to wholemeal wheat bread, the crust of white bread has higher amounts of compounds produced by Maillard’s reaction, such as 2-acetyl-1-pyrroline, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, 2-phenylethanol, 2-acetyl-2-thiazoline, and 2,4-dihydroxy-2,5-dimethyl-3(2H)-furanone, but lower amounts of lipid oxidation off-flavour compounds, such as 2-(E)-nonenal and 2,4-(E,E)-decadienal. As a result, wholemeal wheat bread has a reduced crust aroma strength and expressiveness. This may be because, while wholemeal bread bakes, ferulic acid is released and inhibits the production of 2-acetyl-1 pyrroline, a crucial aroma component of the bread crust, due to the interaction of methylglyox, which is the main precursor of 2-acetyl-1-pyrroline, and ferulic acid [25].

In general, the kind of wheat flour used, that is the milling extraction rate, will have an initial effect on the bread aroma. Bread samples prepared from red and white hard winter wheat, for example, showed relatively slight variations in volatile components, unlike the bread samples made from white and wholemeal flour, which differed greatly in volatile component content [27].

There are two ways that wheat flour affects the ultimate flavour and aroma of bread. While some unaltered volatile components directly affect the bread’s aroma, such as aldehydes, ketones, and alcohols, other flour components, including amino acids, fatty acids, and phenol acids, operate as aroma and flavour precursors creating the final aroma during processing [28].

However, strange flavour and unpleasant aroma develop along with oxidative rancidity if the wheat grain becomes contaminated with mould because of improper harvesting or storage. Excessive heating during the drying process might also result in the unusual aroma of wheat grains. Besides, wheat is sensitive to external smells; thus, during storage or transportation, smell of engine oil, petrol, or other chemicals may be absorbed and cause the unusual aroma of wheat grains. Naturally, processing should not be done on wheat that has altered its smell or flavour [24, 29]. Regardless of the grain cleaning and washing in the mill or baking techniques used, the foreign flavour and odour would spread through the final product if such wheat were processed. These defects of wheat grain and flour are easily identified while analysing wheat in the mill or evaluating the flour’s quality in bakeries, and such wheat are not processed any further [24].

The addition of non-wheat cereals into the recipes of the baking products could also influence their aroma and flavour. Because of its high phytate and tannin content, buckwheat may be less digestible for proteins and cause products to taste bitter and with altered aroma characteristics [30], particularly when using Tartary buckwheat in baked goods [31, 32]. On the other hand, the pleasant nutty aroma and flavour of barley flour can enhance baked products [32, 33].

The components and precursors of rye’s aroma contribute to the distinctive and powerful flavour and aroma of the products, primarily of the bread [34]. Rye flour is essential to making rye bread and is a crucial component in the fermentation process of sourdough. It contributes significantly to the providing of reducing sugars, free amino acids, free fatty acids, and free phenolic acids [35].

The addition of a liquid malt extract had a significant impact on the wheat bread’s aroma. Malt is dried and powdered barley that has germinated. Clean sound barley is soaked first to hydrate the grain and start the germination process in order to produce malt. Germination is carried out under appropriate and controlled conditions for approximately 5 days. During drying of germinated barley, enzymes are partially inactivated when high temperatures are used, and the result is a dark, rich malt with a distinct “malt” flavour. Lower temperatures provide malt with comparatively little flavour influence but significant enzyme activity [36].

Bread aroma, which had dark liquid malt extract added, was more pronounced in the crumb than in the crust. Sotolon (4-hydroxy-2,3-dimethyl-2H-furan-5-one), with spice-like flavour (curry, walnut, and caramel), was the main reason of the altered aroma in the crust. On the other hand, a notable rise in sotolon concentrations and the molecules maltol (3-hydroxy-2-methyl-4-pyrone) and 4-hydroxy-2,5-dimethylfuran-3(2H)-one which has a caramel-like aroma were responsible for the crumb feature. Whereas 4-hydroxy-2,5-dimethylfuran-3(2H)-one is generated from precursors derived from malt extract, the rise in sotolon and maltol (3-hydroxy-2-methyl-4-pyrone) was explained by direct transfer from the liquid malt extract to the bread [37].

Fruit fibres and other ingredients with distinct aroma profiles can likewise impact the flavour and aroma of bread [38].

It is important to note that each stage of the breadmaking process has a significant impact on bread aroma, and there is no doubt that bread aroma is very different and more complex than flour aroma. As a result, the flour lacks some substances found in bread, such as acetoin, ethyl octanoate, and 3-methyl-1-butanol [20].

3.1.2 Water

Water is essential to bread making and serves as a solvent and plasticizer for many components. A gluten network cannot form without water, and yeast cannot release enough gas to leaven the dough, as well. Gluten, starch, damaged starch, and non-starch polysaccharides are ingredients of flour which have a hygroscopic nature. When they are sufficiently hydrated, they perform their functionality [36]. The kind and quantity of dissolved minerals in water can have a significant impact on the fermentation process and dough rheology. Hard water makes the gluten tougher, which slows down the fermentation. On the other hand, soft water causes the dough to become too soft and sticky, which makes it unsuitable for dough machine processing [24, 29]. Water generally has an indirect effect on the development of flavour and aroma through its influence on dough rheology and activities that occur throughout the fermentation process.

3.1.3 Salt

The amount of salt in bread varies depending on regional preferences. Taste of salt modifies the flavour of bread by giving it its salty taste and by enhancing perception of other possible flavours and aromas during chewing [23]. Besides improving and strengthening the flavour, salt used in breadmaking process contributes to the texture and stability of final products [28]. It also controls the yeast activity, enhances the gluten network, and helps the dough retain gas. In addition, salt can also cover off-flavoured bitterness. One of the most continuous issues confronting food manufacturers is lowering the salt level of processed foods, particularly bread [39].

The flavour and aroma features of baking products are also contributed by other common ingredients such as fat, sugar, milk, and whey.

3.2 Creation of aroma during breadmaking process

Aroma components can be produced during kneading, dough fermentation by yeast and/or LAB, enzyme activity, and thermal reactions that occur during baking, primarily through caramelization and Maillard reactions [15].

3.2.1 Kneading

Kneading is the first step in the breadmaking process. During the kneading procedure, all components are homogenized. The process of mixing flour and water produce dough and makes possible interactions between distributed and hydrated gluten-forming proteins, which results in the development of gluten and achievement of the appropriate rheological characteristics of the dough. Along with the solid and water phases, air is incorporated into the dough during the kneading process, creating the gas phase.

The simplest method for dough making is “straight dough process” (SDP). This process entails adding all the ingredients at once and kneading until the dough achieves the desired consistency. The two-stage process includes the production of preferments. This method has several different variants, such as polish (preferment made from flour, water and commercial yeast) or levain method (naturally leavened preferment by wild yeast and LAB). Furthermore, rye flour and water in preferment, is usually referred to when the term sourdough is used [29]. In a word, sourdough presents mixture of flour and water that is fermented by yeasts and LABs, which control the leavening, acidity, flavour and aroma [8]. Sourdough can be dried, but it must contain live microflora: 10⁶ to 10⁷ yeast/g and 10⁸ bacteria/g, and they should be in active form to provide the dough leavening [40]. There has been extensive research done on the processes, ingredients, starters, and characteristics of sourdough bread [41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53].

According to Wirtz [29], the method of kneading plays a significant role in the creation of aroma through acetic and other acids. Thus, compared to the two-stage method, substantially less acetic acid is produced in the SDP. In addition to acting as a transporter for some aromatic components, acetic acid can also intensify the aroma characteristics of baking products [30]. In any case, it may be argued that a longer dough kneading time contributes to the creation of more aromatic components.

On the other side, too long kneading period requires more energy, which raises the dough temperature significantly and can decrease the concentration of volatile aroma components like methylpropane, 2-methylbutanal, and 3-methylbutanal [54].

3.2.2 Fermentation

The period required for the dough structure to expand as a result of the leavening process is known as fermentation. It may be the final fermentation just before baking, or it may be an intermediate step in a process, often called a dough resting, that is followed by other dough manipulations. The final product’s physical characteristics (texture, porosity, volume, bread form, etc.), aroma and flavour all depend significantly on the fermentation process.

The microorganisms that carry out the fermentation of sugars are yeasts and LABs. In order to make dough, yeast is introduced together with flour and water. The purpose of adding LABs to sourdough is to acidify the dough and develop new compounds, including lactic acid, which adds new and pleasant organoleptic notes and enhances bread flavour and aroma [20].

3.2.2.1 Aroma development during alcoholic fermentation

The primary process that produces volatile components is the fermentation of dough sugars. The flour might naturally contain sugars that are incorporated into the dough. However, the amount of sugar in sound wheat flour (1–1.8%) is insufficient for fermentation, that is, for yeast activity. A certain amount of sugar is added according to recipes, usually sucrose (mostly 0.5–3.0% on a flour basis). Owing to the activity of amylolytic enzymes, the majority of the sugar used by yeasts during fermentation originates from starch (Figure 3) [7].

Figure 3.

Enzymatic production of sugar from starch during fermentation.

Saccharomyces cerevisiae (yeast) as the microbiologic leavening agent is widely employed in bread products to ferment glucose, fructose, sucrose, and maltose. That is a species that has diverged significantly from its contemporary wild relatives throughout time, with a focus on the quick synthesis of ethanol and carbon dioxide from carbohydrates. Yeasts in doughs are living organisms that consume nutrients, particularly sugar from carbohydrates, for growth and energy [55].

Carbon dioxide and ethanol are developed in yeast dough as leavening gases. The dough expands due to the carbon dioxide created during fermentation and the early stage of baking, whereas alcohol only leavens during baking. During fermentation, the carbon dioxide that the yeast produces mostly takes the form of tiny, finely split bubbles that accumulate inside the gas. Gluten is stretched at the surface of a developing bubble, which largely depends on the gluten quality, that is, on the quality and suitability of the flour. This process influences the bread crumb appearance and porosity. During yeast fermentation, precursors of flavour and aroma are also created (Table 1) [24].

Components	Yeast dough at the start of fermentation; (μg/kg)	Yeast dough after fermentation; (μg/kg)	Found in bread	Odour
1-Propanol	2.44	7.02	Crumb	Fruity, alcoholic, plastic, pungent
Isobutanol	205.44	343.43	Crumb, crust	Glue, alcoholic, wine-like, malty
butanol	1.03	2.16	Crumb	Fruity, solvent
1-Penten 3-ol	1.48	1.6	Crumb	Green type
3-Methyl-1-butanol	790.49	1447.26	Crumb, crust	Balsamic, alcoholic, malty
1-Pentanol	10.49	13.12	Crumb	Balsamic, fruity, fusel-like sweet
2-Heptanol	1.68	1.70	Crumb	Green
1-Hexanol	61.12	74.13	Crumb, crust	Green grass, flowery, woody, mild, sweet
1-Propanol 3-ethoxi	0.00	1.68	Crumb, crust	Fruit
3-Hexen 1-ol	0.84	0.83	Crumb	Grassy-green
2-Butoxy ethanol	1.93	1.78
1-Octen 3-ol	3.22	3.87	Crumb, crust	Mushroom-like
1-Heptanol	7.87	9.81	Crumb	Green
2-Ethyil 1-hexanol	1.32	1.26
1-Octanol	3.10	4.11	Crumb, crust	Earthy, mouldy vegetable
2-Octenol	0.79	0.80
1-Nonanol	2.96	3.61	Crumb, crust	Citrus
3-Nonen-ol	5.64	11.72	Crumb	Waxy
3-Decen 1-ol	7.04	18.90	Crumb
Benzyl alcohol	13.76	9.55	Crumb	Pleasant aromatic
2-Phenyl ethyl alcohol	90.26	366.15	Crumb, crust	Rose-honey-like, wilted rose
1-Dodecanol	1.20	1.08	Crumb	Waxy type
Alcohols – total	1214.70	2325.57
Hexanal	2.78	1.58	Crumb, crust	Green, grassy, tallow
Heptanal	1.84	1.62	Crumb, crust	Green, fatty
Nonanal	1.26	1.31	Crumb, crust	Citrus, soapy
2-Octenal	0.00	0.00	Crumb, crust	Fatty, nutty, roasted
2-Nonenal	0.00	0.00
2,4-Decadienal	6.11	2.86	Bread	Deep fat fried, waxy
Aldehydes – total	11.99	7.37
3-Methyl butyl acetate	0.00	1.04	Crumb, crust	Banana-like
Ethyl hexanoate	0.00	1.98	Crumb, crust (traces)	Apple peel, fruity
Ethyl heptanoate	0.00	0.56	Bread	Grapes
2-Phenyl ethyl acetate	2.22	1.89	Crumb	Rose
Esters – total	3.56	16.34
3-Hydroxy 2-butanone	14.15	121.93	Crumb	Butterscotch, butter, yoghurt, cream
Hidroxyacetone	1.02	1.32	Crumb	Pungent, sweet caramellicethereal
3,5-Octadiene 2-one	0.47	0.00
γ-Valerolactone	2.29	2.96
γ-Butyrolactone	6.53	7.77
γ -Caprolactone	2.57	2.86
γ -Nonalactone	4.58	8.98
γ -Decalactone	1.84	1.46	Crust	Sweet, soapy
Ketones and lactones – total	33.43	147.28
Methionol	1.83	3.20	Crumb	Potato

Table 1.

Volatile aromatic components in yeast dough (μg/kg) at the beginning and the end of fermentation and presence in bread [7] (Approved by author and editor).

However, not only the flour quality is crucial in the breadmaking process. The choice of yeast strain is very significant in terms of the aroma and physical features of the bread and of dough fermentation time [56, 57, 58].

Towards the finalization of fermentation there is a discernible rise in the aroma-producing substance in the dough, particularly alcohol and esters (Table 1). It is those components that influence the aromatic character of bread crumb [15]. Alcohols, aldehydes, 3-hydroxy-2-butanone (acetoin), and esters are the primary elements of aroma and are mainly created by yeast metabolism [57].

The most significant components of bread aroma are aldehydes with low odour threshold values, such as (E)-2-nonenal, nonanal, hexanal, heptanal, and octanal. They are frequently referred to as “off-aroma compounds” because of their scents of green, tallow, mushrooms, and citrus [20].

According to Burch et al. [56], increasing the yeast concentration increased the formation of most aroma compounds (2-methyl-1-propanol, 2-phenylethanol, phenylacetaldehyde, 2,3-butanedione, ethyl acetate, ethyl 3-methylbutanoate, ethyl hexanoate, ethyl octanoate, and phenylethyl acetate) which are generated from the fermentative activity of yeast. The most sensory significant of these chemicals are phenylacetaldehyde and 2,3-butanedione, while most of the lipid oxidation products were unaffected by yeast concentration [56].

The concentrations of the lipid oxidation products 1-heptanol, hexanal, heptanal, octanal, decanal, and 2-pentylfuran were found to increase when the fermentation temperature was raised from 5 to 35°C. Hexanal and heptanal were the most aroma active of these components and are associated with off-flavours. On the low fermentation temperature to 5°C, the creation of ethyl acetate, ethyl hexanoate, and ethyl octanoate in bread increases. They are distinguished by a fruity and pleasant aroma [56].

The fermentative activity of yeast, the oxidation of flour lipids and Maillard reactions, which mostly originate in the crust, are the principal factors in creating aroma components found in wheat bread [57]. Under anaerobic conditions, yeasts transform more than 95% of glucose into CO₂ and ethanol. Although most of the ethanol is lost during baking, some remains and serves as a direct flavouring agent [7, 59] and is able to take part in secondary fermentation processes such as pyruvic acid glycolysis [20].

Amino acids from flour participate in the production of aromatic substances through the reaction described by Erlich. According to the Ehrlich pathway, the flour amino acids are broken down inside the yeast cell to produce the aldehydes and their corresponding aliphatic and aromatic alcohols known as fusel alcohols. These higher alcohols get their name from the German term fusel, which means “bad liquor.” Low quantities of these molecules and their esters are crucial to the aroma and flavour of fermented foods, whereas excessive concentrations of fusel alcohols create off-flavours [60].

Furthermore, aldehyde dehydrogenase can oxidize aldehydes produced via the Ehrlich pathway to their corresponding acids, such as 3-methylbutanoic acid and 2-methylpropanoic acid. Aldehydes from Maillard reactions, including 3-methylbutanal, can also be formed during baking [60].

In general, during fermentation, yeast cells gradually incorporate the amino acids from wheat flour, and the initial transamination occurs, producing α-keto acid, which is converted to fusel aldehyde, which can subsequently be reduced to fusel alcohol or oxidized to fusel acid in ester form. At some point, created substances are released from the yeast cell [20].

Aldehydes, alcohols, and acids are the volatile substances produced by the Ehrlich pathway in the yeast cell during dough fermentation from wheat flour amino acid. The following processes can ocur: 3-methylbutanal, 3-methyl-1-butanol, and 3-methylbutanoate are produced from leucine; 2-methylpropanal, 2-methylpropanol, and 2-methylpropanoate are produced from valine; 2-methylbutanal, 2-methyl-1-butanol, and 2-methylbutanoate are produced from isoleucine; 2-phenylethanal, 2-phenylethanol, and 2-phenylethanoate are produced from phenylalanine; and 3-(methylthio) propanal, 3-(methylthio)- 1-propanol, and 3-(methylthio)-propanoate are produced from methionine [60].

Lipid oxidation by lipoxygenase activity in oxygen presence may also result in creating the crumb’s aldehydes, ketones, alcohols, and esters. This occurs when linoleic and linolenic acid transforms into unstable hydroperoxides, which degrade the most in hexanal and hexenal during baking. Pentanol is also frequently formed in this breaking phase. Other aldehydes, such as acetaldehyde, are produced with further breakings. Oxidation of aldehydes onwards produces acids and esters [20]. The aroma profile of wheat bread is significantly influenced by the degradation of unsaturated fatty acids [57].

A pleasant, sweet and fruity scent is generally associated with the most aroma-active esters in bread, which include ethyl 3-methylbutanoate, ethyl hexanoate, ethyl octanoate, ethyl acetate, and ethyl nonanoate. The interaction between acetyl coenzyme A derivatives of fatty acids (C6–C10) and alcohols (mostly ethanol) in the yeast cell, which is catalysed by acetyltransferases, is a particular source of some esters found in the crumb. Additionally, lactones are found in wheat bread crumb and are produced by the yeast’s activity. It is well known that lactones contribute to the nice and fruity aroma of the bread [20, 57].

3.2.2.2 Aroma development during sourdough fermentation

One of the oldest food fermentations is sourdough fermentation, which is also one of the opportunities presented by modern commercial advancements for the use of novel microorganisms and novel fermentation processes in bread production [52], with the growing use of indigenous wheat cultivars to maintain genetic variety and potential value for farmers and breeding initiatives [61].

The primary metabolic processes involving sourdough microbiota are linked to three main processes: bread dough fermentation by LABs, bread aromatic component synthesis by LABs and yeasts, and bread dough leavening by yeasts and heterofermentative LABs.

Sourdough microbiota and their metabolic activity, raw ingredients, and technological parameters of fermentation and proofing are the main factors contributing to the formation of volatile organic compounds in sourdough and sourdough bread [23, 48, 62].

Sourdough is a characteristic example of the association of LABs and yeast. Its functionality is reflected in the appropriate balance between lactic acid and alcoholic fermentation. The greatest obstacles in studying the complex biological system of sourdough are represented by numerous interactions between the conditions that prevail in the sourdough, the microorganisms present, and the technological treatment. Sourdough processes are influenced by both internal and external variables. Internal variables include flour quality (extraction milling rate, nutrients, minerals, development activators, enzymatic activity, etc.) and microflora composition (type, number, optimal operating factors, fermentation, and enzymatic activity). The consistency of the dough (water content), temperature, duration of fermentation, oxygen concentration, addition of yeast, additives, etc., are examples of external influences. In dough with a softer consistency, that is, dough with more additional water, LABs produce more lactic acid than acetic acid. This is significant because the flavour of lactic acid is moderate acidic and slow acting, but acetic acid has a strong acidic taste that is instantly perceptible [8].

In terms of temperature, increasing the temperature from 25 to 30°C improves yeast fermentation. At 25°C, yeasts create ethyl acetate, acetic acid, and lactic acid; at 30°C, they make ethanol, 1-propanol, 2-methyl-1-propanol, and 3-methyl-1 butanol. When the temperature is elevated to 35°C, the aromatic volatile profile does not alter [7].

In baking rye bread, the sourdough method is frequently employed, and the starch gelatinization and enzyme activity are significantly impacted by the acidic conditions produced. Rye dough baking qualities and properties of the bread are influenced by both α-amylase and enzymes that break down cell walls [35].

Sourdough is a multicomponent ecosystem where, depending on the conditions of the process, yeast and LAB interact with one another and with the dough ingredients. To increase the microflora fermentation, additional ingredients like sugar or enzymes can be added along with flour and water. A unique, desirable, volatile profile of the sourdough is determined by the ratio of these components as well as the regimes of the breadmaking process, and the amount of sourdough has a significant impact on the bread aroma and flavour [44]. Sourdoughs include approximately 108–109 CFU (colony-forming units) per gram of LAB and 106–107 CFU/gram of yeast. The ratio of LAB to yeast is 100:1. LABs, unlike yeast, are responsible for the majority of acidity. Lactic acid is presented in negligible quantities in bread fermented by yeast, but it is present in sourdough bread at a concentration of 5000 ppm. The concentration of acetic acid in yeast-fermented bread is 55 ppm; however, it is substantially greater in sourdough bread at 1093 ppm [40].

Homofermentative LABs can convert more than 85% of sugars into lactic acid, whereas heterofermentative LABs create lactic acid as well as acetic acid, ethanol, and CO₂. Both yeast and LAB, can create aroma precursors, particularly free amino acids, and their concentrations grow significantly during fermentation [63]. The fermentation process of sourdough can start in several ways: spontaneously, leaving a mixture of water and flour in suitable conditions, using defined microbiological starters and using the remaining dough from the previous fermentation [8]. Knowing the differences between spontaneous and inoculated sourdough is very important in selecting fermentation conditions.

There are three distinct kinds of sourdoughs, which differ in their production procedures [46, 47]:

1. Type I is a traditional kind of sourdough produced with part of previous fermentation. This type is stable and contains both LABs and yeast. LABs contribute to dough acidity, whereas yeasts with LABs affect the flavour and aroma, amount of CO₂, and texture, as well as retard bread stalling.

2. Type II sourdough is created with specific strains (starter) to initiate fermentation and has been utilized in industry.

3. Type III comes in dried form.

Commercial sourdough cultures (type II) include (i) pure starter culture in powder form, or lyophilized culture that may contain only one species, or composed of several species of microorganisms and (ii) active sourdough. The use of pure cultures has not been particularly successful in the production of sourdough, as is common in the production of dairy products, wine or meat products [64]. To ensure good acidification and aromatization, commercial sourdough starters typically contain combinations of different LAB groups: heterofermentative and homofermentative. The heterofermentative LAB produces a combination of lactic and acetic acids while the homofermentative LAB is rapidly acidifying and produce mostly lactic acid [40]. It should be noted that differing fermentation conditions have a similar impact on the volatile content of sourdough made with various mixed starters.

The sourdough fermentation improves the rheological, sensory, and shelf-life attributes of baking products, and they also have a lower glycaemic index, improved mineral bioavailability due to the reduction of the phytic acid, and lower gluten content. In addition, the content of free phenolic content and free antioxidant activity increases [40, 47, 65].

These products take longer to produce since the creation of substantial amounts of volatile compounds during fermentation requires a multi-step process that takes around 12–24 hours, whereas fermentation using baker’s yeast alone takes only a few hours [63]. It is important to point out that the concentration of bioactive compounds from flour fermented by yeast and LAB might remain constant or even increase [66].

The type of bread determines the choice of sourdough, that is, starter, which is crucial in achieving the desired aroma and taste of the bread. Bread with a large proportion of rye flour requires a high degree of acidity, and a larger amount of sourdough is added to achieve a strong sour aroma and taste. Wheat bread with sourdough provides a slightly sour, aromatic taste. Breads with an increased proportion of grains and seeds require a sufficient degree of acidity, which will not suppress the taste and aroma of grains and seeds. The most significant characteristic of the crust aroma of sourdough wheat bread is attributed to 2-acetylpyrroline [6, 7].

Certain aromatic compounds have only been generated from LAB and yeast mixed cultures (Figure 4). Additionally, it has been demonstrated that yeast produces more of a number of aroma components – most notably propionic acid – when LAB is present [55].

Figure 4.

Some aromatic components produced by yeasts and LAB in sourdough. This figure is created thanks to the author [7].

Some of the aromatic components generated by yeasts are shown in Figure 4, along with how they contribute to the predominant sour flavour found in sourdoughs.

Even though the aromatic component composition of bread crumbs made with and without sourdough is similar, it appears that adding sourdough increases the concentration of some components, like ethanol, 2-methyl-1-butanol, 3-methyl-1-butanol, n-propanol, 2-phenylethanol, benzyl alcohol, 2-methylpropanoic acid, 3-methylbutanoic acid, or acetic acid. It should be pointed out that bread aroma and flavour are influenced by the type of LAB and yeasts employed, particularly in sourdough [20].

The combination of different LABs contributes to the creation of a harmonious relationship between aroma components. When both LAB and yeasts are present in the dough, a greater variety of alcohols are formed than when yeast is the only present in the dough [7]. Bread made using sourdoughs contains considerably fewer alcohols, esters, and diacetyl than sourdoughs, as the majority evaporate during baking. On the contrary, the concentration of 3-methylbutanal increased considerably when baking. Propanone, 3-methyl butane, benzyl alcohol, and 2-phenylethanol express the strongest and bread-like flavour and aroma profile in crumb of rye bread produced by sourdough [63].

The primary sensory characteristics that demonstrated the distinctiveness of sourdough breads are their mild or strong acidic taste and aroma, attractiveness, distinguished colour of the crumb and crust, crispness of the crust, freshness, fruitiness, high porosity, and sourness. In general, sourdough imparts a distinct and superior flavour and aroma, particularly due to the release of amino acids during fermentation, which either directly influence the volume of the flavour and aroma or function as precursors of volatile components [48]. When compared to analogous bread that is not made with sourdough and is instead produced by SDP, these additional volatile compounds provide sourdough bread increased ranking and a wider range of aroma and flavour components [67].

3.2.3 Baking

Baking is the final stage of bread production in which the dough is turned into bread, and the crumb and crust are developed, while the aroma and flavour are completely defined.

The gas produced during dough fermentation remains in the crumb, resulting in bread porosity. Furthermore, aroma-creating reactions occur in both the crust and the crumb, as do browning reactions in the crust. In fact, the majority of aroma components are generated during baking, while ethanol and other volatile components leave the dough/bread.

Because the temperature of the surface regions of the bread during baking reaches 180°C, while the temperature of the middle of the bread never exceeds 100°C, the crust contains more aromatic components than the crumb. However, some components of the bread crust aroma slowly migrate to bread crumb [55].

Two thermochemical reactions are responsible for the creation of bread aroma – caramelization and the Maillard reaction, and they take place simultaneously. The primary distinction between these two processes is that caramelization is the pyrolysis of sugars, whereas the Maillard reaction is the interaction of sugars and amino acids (Figure 5). The above processes are also determined by the ingredients and preceding steps in bread making, which define the precursors of aroma and flavour creation.

Figure 5.

Factors that contribute to bread aroma formation during baking.

Caramelization is the complex process of converting various types of sugar that remain in the dough after fermentation into dark-coloured polymer complexes in the amount of 2 to 3%. Caramelization begins with the melting of sugar on the dough’s surface at 130–140°C and progresses to a rise in temperature, resulting in the synthesis of coloured multiple oligosaccharides with varying flavours ranging from sweet to bitter.

The type of sugar and temperature are the most important factors that influence the level of caramelization. Sugars require different temperatures to caramelize. Fructose caramelizes the fastest of all sugars at 110°C, glucose at 160°C, and maltose at 180°C. The pH, or acidity, also determines the level of caramelization. Caramelization occurs more slowly when the pH is close to neutral. When the pH is relatively low in an acidic medium such as sourdough, caramelization occurs more rapidly [7].

Maillard reaction, that is, the formation of melanoidin, begins at a temperature of about 100°C and is accelerated by increasing the temperature. It involves very complex interactions between reducing sugars (glucose, fructose, and maltose) and amino acids (lysine), proteins, or peptides, resulting in the formation of coloured polymers aldehyde-amine and heterocyclic components with nitrogen [29].

Most furans, pyrazines, pyrroles, pyrrolines, oxazoles and sulphuric compounds (thiophenes) are formed during Maillard reactions at high baking temperatures [68].

Strecker degradation is an important stage in the Maillard process, in which amino acids combine with dehydro-reductones to form aldehydes that have a comparable structure as the initial amino acid. As a result, alanine produces acetaldehyde, glycine produces formaldehyde, serine produces glyoxal, and threonine produces 2-hydroxypropanal which greatly contributes to the bread aroma [20]. Martínez-Anaya [69] reported aldehydes produced by Strecker degradation during Maillard reactions or the Ehrlich pathway during fermentation, such as 2-methylpropanal (malty) from valine, 3-methylbutanal (malty, roasty cucumber-like) from leucine, 2-methylbutanal (almond, malty) from isoleucine, phenylacetaldehyde (honey-like, sweet) from phenylalanine, and methional (boiled-potato, malty, waxy) from methionine.

The intensity of the Maillard reaction and the amount of carbonyl compounds are affected by the type of sugar. On the other hand, the type of amino acid determines the aroma characteristic and the kind of carbonyl compounds formed. Thus, lysine, leucine, and isoleucine are responsible for pleasant aroma, whereas methionine causes disagreeable aromas. Among sugars, xylose is more reactive than glucose and maltose [19]. Sourdough in bread making stimulates the generation of Maillard’s volatile compounds in a low pH medium due to organic acids formed by LAB. The low pH is conducive to the initiation of Maillard reactions [68]. The major bread aroma component, 2-acetyl-1-pyrroline, is produced during Maillard reactions. It has a pleasant roasty odour and the highest value of OAV in the crust [7, 20].

Other important components for bread crust pleasant aroma include 3-methylbutyric acid, 2-methylbutanal, 3-methylbutanal, and methional, which can also be created by the Ehrlich pathway; 2,3-butanedione, which may be originated from fermentation as well; and 4-hydroxy-2,5-dimethyl-3(2H)-furanone and 2-methyl-propanal. In general, they are attractive and intensive aroma components with high OAVs. Bread aroma includes Maillard products such as furfural, benzaldehyde, and 3-hydroxy-2-butanone.

Furthermore, during baking, the thermal disintegration of sugars and amino acids occurs at approximately 220°C, as does the creation of some volatile aroma components. Glucose causes the release of components including furan, acetic acid, furfural, 3-furfural, 5-methyl-2-furfural, or 1,2-propanedione, while amino acid alanine promotes the release of acetaldehyde and 2-methyl-5-ethylpyridine, among others [20].

4. Conclusion

The aroma of bread, which contributes a lot to the sensory impression, is influenced by the presence of volatile aromatic components produced during manufacture. Two elementary processes of aroma development may be recognized under the influence of different temperatures: the formation of the aroma of the bread crumb when the temperature does not exceed 100°C, and the aroma of the crust when the temperature reaches 180°C or more.

Alcohols, aldehydes, ketones, acids, and esters are formed in the greatest quantities during fermentation due to the action of yeasts and LAB, lipid oxidation, and enzymatic reactions. They have an impact on the bread crumb qualities, with 3-methyl-1-butanol representing the most important aroma component.

Higher baking temperatures result in aromatic crust components such as furans, pyrazines, pyrroles, pyrrolines, pyridines, oxazoles, thiophenes, and sulphur compounds. Under the effect of thermal treatment at 180°C or more, the Maillard reaction, caramelization, and thermal degradation occur on the dough/bread’s surface. The primary component of bread crust aroma is 2-acetyl-1-pyrroline.

A number of investigations have been conducted in search of ways to improve the aroma and flavour of bakery products. However, nutritional advances present an equally important challenge to research.

Food innovation in general is being impacted by the increased interest in functional foods and their effects on human health. Nutrition expertise has been used to promote consumer health which reflects the functional food idea in general. As a consequence, the baking industry faces two separate issues. Products are meant to have both health-promoting characteristics and pleasant, attractive aroma and flavours. In this sense, both manufacturers and scientists have to focus on the enrichment of bakery products consumed every day.

It is sourdough bread that meets a wide range of requirements, including superior nutritional benefits, enhanced biocomponent content, extended shelf life, and, most importantly, excellent aroma and flavour properties. Improving the aroma of bread with sourdough is the subject of many different studies by scientists around the world, and it can be carried out by improvements and inventions through several activities: (i) by choosing appropriate microflora for the fermentation conducting and selecting new strains of yeasts and LABs with beneficial production properties in relation to bread manufacturing; (ii) by investigating the effects of various extracts, aromatic agents, and other unconventional compounds; (iii) by testing novel and inventive methods of combining, baking, freezing, storing, and packing.

Even though it is a traditional method of bread production, using improved sourdough fermentation is the first one among innovative baking processes, as it offers undeniable advantages over conventional SDP bread in sense of sensory, rheology, shelf life, and advanced nutritional properties. Sourdough in baking is an area where modern commercial and societal changes permit many new techniques for the utilization of innovative microorganisms, their interactions and unique fermentation processes.

When combined, new methods, microorganisms, and substrates provide an abundance of creative possibilities for innovation, and the search for innovative sourdough bread continues.

However, any innovations aiming at improving bakery products in terms of nutrition and sensory appeal should be adjusted to customer expectations and demands, which differ based on the geographical, socio-cultural, and economic factors of a specific region or country.