Barley (Hordeum vulgare L.) is the most commonly used cereal in beer production, and its endosperm represents the main tissue of the grain, which is mainly composed of starch granules (62.7% of the total grain weight) embedded in a protein matrix [25,26]. Other cereals, such as wheat, rye, oat, triticale, sorghum, maize, etc., can also be used as raw material for beer production [27], as long as we are aware that this procedure may affect the sensory properties of the final beer. It is true that the usage of other cereals may confer beer with new properties and organoleptic features. However, at the same time, the incorporation of new cereals that do not contain the necessary enzymes may involve certain technological issues related to mash lautering, beer filtration, extract recovery, or production forecasting and scaling [28]. As an example, let us mention wheat beer, which is especially consumed in Bavaria and Austria and is characterized by clove-like, banana-like, vanilla, and fresh fruit scents [29,30]. Sorghum beers are described as one of the most subtle beers with regard to their sensory properties and mild taste [31]. Another study where different proportions of sorghum were used concluded that these beers have a lower acetaldehyde and ester content, and a greater proportion of higher alcohols [32]. The same authors described the beers that contained between 30 and 40% oat as better beverages in terms of aroma and taste purity than 100% malted barley beers, which contain lower amounts of lactones and higher alcohols, and a greater proportion of esters [33]. Other studies have concluded that different unmalted cereal adjuncts can replace malted barley at rates of up to 40–60% to produce beers with a sensory profile comparable to that of 100% malted barley beers [21,34].

Different barley genotypes lead to different chemical compositions, i.e., different enzymes and metabolites and, therefore, unequal results are expected from similar malting procedures [35,36]. Beers with higher fruity, floral, and grassy flavors are produced from Golden Promise barley—a classic British spring barley variety with a light malty flavor and beautiful mouthfeel—whereas other beers that are rich in toffee and toasted flavors, while exhibiting lower harshness or astringency, are obtained from Full Pint barley [37]. In this sense, CDC Copeland barley, a two-rowed malting barley, produces neutral flavors and pale colored beers. It has also been reported that, depending on the “terroir” where barley is grown, beers with different flavors can also be obtained [38].

The quality of the barley grain used as raw material is, therefore, another very important factor and it largely depends on agronomic practices, as well as on genetic and environmental variables [39]. Moreover, grain is required to present the adequate plumpness and kernel weight, with a high germination potential (≥95%) and the appropriate protein content. In this regard, barley’s protein content should be below 11%, otherwise the proteins that are soluble in wort may confer off-flavors to the final product. Moreover, high protein concentrations traditionally correlate with low carbohydrate levels and lower extract yields. The quality of barley may also be altered by microbial infections, being fungi the most commonly found microorganisms. These can infect barley while in the fields, especially during its wet growing season, while it may infect barley or malt while in storage under moist conditions. Primary gushing is associated with the use of defective malted barley, when harvested under wet conditions, while secondary gushing may be caused by solid particles, which may arise from various sources (dust polluted containers, faulty filtration, haze particles developing in aged beer, etc.), or adsorbed gas residues acting as nuclei for bubble formation. [40].

Malt contamination with Aspergillus fumigatus has been proven to be responsible for a noticeable rancid taste of beer [45]. Fungi growth on malt also has a negative effect on beer foam quality, because of the β-glucanases and xylanases produced that decrease the viscosity of the wort [16]. These and other negative aspects of the fungi that may grow on barley represent a hazard for the organoleptic properties of beer that may lead to consumers’ rejection [46]. Barley is also the vehicle for a variety of contaminating microorganisms other than fungi or molds and that may negatively affect the germination of barley prior to malting. Clostridium and Bacillus bacteria, which are generally associated with the production of butyric acid and sulfides, are amongst these other polluting organisms [47]. An excessive moisture level after completing the kilning or malting roasting process should be avoided if certain barley pollutants are to be prevented. In other words, proper storage and preservation procedures for the barley and the malted barley are crucial factors [16].

Barley is subjected to malting in order to solubilize the proteins and to break down the starch into fermentable sugars. Specific malting procedures also provide beer with characteristic colors and flavors [48]. During the malting process, the grains undergo chemical and structural changes that result in the generation of a number of precursors that will determine the organoleptic properties of the final beer, including its color, aroma, and flavor [49].

The malting process usually involves steeping, germination, and kilning. During the steeping process, cold water (10–15 °C) and oxygen are supplied into hygienic and calibrated kernels in order to maintain moisture levels at approximately 38–45% and promote the germination of the grains. At this stage, the grains’ endosperm cell walls and its proteins are broken through the action of certain enzymes, such as protease, amylase, or β-glucanase [50,51]. Different aspect of this process can be improved in several ways, as follows: using standardized seeds to achieve a uniform germination; using plump kernels to achieve maximum malt extract yields; and low protein content to attain higher extract levels and to enhance beer stability [52]. The quality of the malting process can be evaluated through ‘fine-grind’, which allows measurement of the soluble malt material, including fermentable sugars [53]. Other quality parameters used to evaluate the quality of the malting are kernel size fraction, kernel weight, protein contents, β-glucan, α-amylase activity, viscosity, and soluble nitrogen ratio [54].

Germination is ended by drying the grains (moisture content down to 3–4%) through a gradual increment of the temperature from 50 to roughly 85 °C or more (kilning). The kilning process has a crucial impact on beer color and flavor [51], mainly as a result of Maillard reaction, which produces maltoxazine, maltol, isomaltol, and ethyl maltol, among other substances responsible for the caramel, bread, or cotton candy-like flavors in beer [56]. Therefore, through the control of the temperature, Maillard reaction can be adjusted to determine color formation and obtain different types of malt (base, caramel, special, amber, chocolate, or black) [26], which will result in variations of the compounds responsible for wort flavor and for the different organoleptic profiles of the final beers [36,57]. It should also be noted that the melanoidins generated through Maillard reactions may promote the growth of certain undesirable microorganisms. In fact, melanoidins have been used as antimicrobial agents against different pathogenic bacteria strains [58]. Apart from melanoidins, certain malt alkaloids, mainly hordatines, have also been proven to have an influence on beer flavor by increasing its astringency [59,60].

Mashing is an enzymatic process that produces sugars from malt to obtain wort, which is in turn fermented to produce beer. During this stage, the amylases, β-glucanases, and proteases degrade carbohydrates, β-glucans, and proteins, respectively. Their activities are affected by the temperature, pH, and composition of the solution, as well as by the processing time [71]. The action of these enzymes results in a final beer that contains a small amount of residual fermentable sugars (maltose being the most abundant one), a variable amount of dextrins, such as maltodextrin, and a small amount of peptides, which have an influence on the sensory properties and the palate fullness of the final beer.

Water is one of the most important ingredients during this mashing stage, because it represents most of the beer’s composition. The chemical composition of water, as well as the presence of pathogenic and/or non-pathogenic microorganisms, also has a considerable influence on the final result, so that it may even spoil beer to the point of rendering it unsuitable for human consumption [72].

The mineral composition of wort and beer, where the principal cations, such as calcium, magnesium, sodium, and potassium, as well as anions, such as sulfate, nitrate, phosphate, chlorides, and silicate, may also determine beer quality. The minor ions are iron, copper, zinc, and manganese [73]. Ions are necessary for the correct course of the fermentation process and for the growth of beneficial microorganisms [74], but also contribute directly to the flavor of beers as non-volatile taste-active compounds [75]. The mineral composition of the wort also depends on the nature of the raw materials [76]. Therefore, this factor must be taken into consideration when using cereals other tan malt.

As an example, Briggs et al. [26] established that the presence of calcium ions in the water used to make beer had a relevant influence on the mashing process and affected final beer flavor. According to Montanari et al. [73], calcium has the capacity to extract fine bittering principles from the hops and to reduce wort color, while sodium contributes to the perceived flavor of the beer by enhancing its sweetness. Other authors [77] have observed that “hard water” (with a high concentrations of salts; pH 8.47 ± 0.08) seemed to be a better extractor of the total carbohydrate content and B vitamins (riboflavin and niacin) than soft water (with a low concentrations of dissolved salts; pH 7.68 ± 0.23), whereas organic acid and iso-α-acid concentrations were not influenced by water pH values. It is a fact that the composition of wort has a great influence on the molecules that result from the fermentation process and, consequently, on the organoleptic profile of final beers. Therefore, as an example, wort sugar content levels and free amino nitrogen and lipids, as well as aeration [78] or temperature [79], are parameters that condition the subsequent production of aromatic esters by the microbiota [80,81]. Sucrose, fructose, glucose, maltose, maltotriose, and some dextrins, with maltose and maltotriose as the most abundant ones, are the main sugars that can be found in wort. Their concentrations depend on the characteristics of the barley and on the malting process [20].

Wort is also moderately rich in amino acids, peptides, and proteins [72]. Some amino acids are required for the healthy growing of yeast. Such amino acids, together with certain small peptides, constitute what is known as Free Amino Nitrogen (FAN). Total FAN is important for the fermentation (via yeast nutrition) and the stability of flavor. A high FAN content may affect beer flavor stability because of the production of vicinal diketones (VDK) such as diacetyl and 2,3-pentanedione, through the differential utilization of amino acids (valine and isoleucine, respectively) by yeast, which may provide beer with a butter- or butterscotch-like flavor or toffee-like flavor, respectively [82].

The releasing of free amino nitrogen and reducing sugars during the mashing stage contributes to a minor set of flavor precursors that can develop during the Maillard reaction, principally during wort boiling [83]. They are transformed during the mashing stage and through the metabolism of the yeasts during the fermentation stage into other new substances that contribute to the organoleptic profile of beer [17,84].

During mashing, a key cascade reaction is also initiated, where the products from lipid oxidation generate hydroperoxides that form active volatile compounds [83].

On the other hand, during this stage, unmalted adjuncts such as rice, wheat, corn, honey, or fruit can be added as an alternative cost-efficient source of extract that enables the production of innovative products that increase the content of bioactive compounds and generate unique flavors and bitterness and improve mouthfeel [5,85,86]. It has been observed that when rice is used in the brewing process, it provides neutral, clean, and dry sensory characteristics, whereas adding corn results in a fuller mouthfeel [87].

More sour, grainy, and sweet corn aroma beers were obtained when 60% torrefied maize was added to the wort [34]. The addition of unmalted barley at up to 50% resulted in beers with a preference rating that was comparable, with regard to odor and taste, to that of all-malt beers [88]. In contrast, when the added unmalted barley reached 90%, it resulted in more astringent beers, while 100% unmalted barley produced final beers with less body and mouthfeel [89].

Certain extracts from medicinal plants can also be added to the wort in order to produce beers with unique sensory characteristics and an increased concentration of various bioactive compounds, such as phenols [90].

Resins and essential oils can be found in the lupulin glands of female hop flowers, which, even when used in small amounts, contribute to bitterness and aroma (sensorially characterized by descriptors such as ‘fruity’, ‘floral’, ‘spicy’, ‘herbal’, or ‘woody’) [91,92]. In fact, hops are the main ingredient responsible for the bitterness of beer because of their polyphenols and α-acids contributions [93,94,95]. Hops contain a complex mixture of volatile compounds (essential oils), among which linalool, geraniol, and 4-methyl-4-sulfanylpentan-2-one are of particular importance [96].

Hop varieties can be classified as aroma hops, dual-purpose hops (aromatic and bitter), and bittering hops (very bitter) [97]. Saaz and the rest of the “noble hops”—Hallertauer Mittelfrüh, Tettnang, and Spalt—belong to the first category and are traditionally used for pilsners and lagers produced in the Bavaria and Bohemia regions. Another Saaz aroma hop, Styrian Goldings, is often preferred for Belgian-style ales. Bitter (high α-acid) or dual-purpose hops such as Citra, Centennial, Cascade, or Amarillo, among others, are typically used for American IPAs [98].

During wort boiling, the humulones (α-acids) that are found in the soft resins of hops are isomerized into isohumulones, which are the main components responsible for the bitterness of beer [99]. It has also been recently observed that the oxidized forms of humulones, humulinones that are present in dry-hopped and hop-forward beers, can also contribute to beer bitterness [100].

During wort boiling, the majority of the volatiles derived from hops are lost through evaporation. Thus, by the late addition of multiple dosages, we can obtain beers with hop aroma but without any extra hop bitterness. So, for a less bitter beer, hops can be added toward the end of the wort boiling stage, or to the whirlpool (late hopping) or to green and bright beer (dry-hopping) [101]. The flavor descriptors that are most often detected in late hopped beers are spicy, noble, herbal, woody, and, to a lesser extent, estery or fruity. Dry-hopping consists of the cold extraction of volatile and non-volatile hop compounds. This technique is widely employed by brewers to increase the aroma and stability of beer flavor [102]. The descriptors that are most frequently found in dry-hopped beers are floral, citrus, or pine [103,104]. Unlike in boiling hopping, dry-hopping does not allow for the thermal isomerization of the α-acids into iso-α-acids, which makes beer more prone to microbial instability [105]. Recent investigations on the microbial contamination hazards associated with dry-hopping techniques have detected spore-forming bacteria such as Bacillus spp., as well as Enterobacteriaceae, yeast, and fungi [106].

There is also evidence that the amylolytic enzymes present in hops can biochemically modify dry-hopping beer, which may lead to the degradation of long-chain, unfermentable dextrins into fermentable sugars [107]. This increase in fermentable sugars can, in the presence of yeast, give rise to a slow secondary fermentation, which is referred to as “hop creep” [108]. “Hop creep” represents a problem for brewers, because it modifies the specific density, flavor profile, and alcohol content of beers. Bruner et al. [109] revealed that hop creep resulted in 1.06% (v/v) alcohol increments in dry-hopped lager beers and 0.88% (v/v) in ale ones, over 30 day periods.

Beer aroma can also be modified by adding pure aroma hop extract [110]. Moreover, the addition of hop extracts to unhoped beer has been demonstrated to improve mouthfeel and fullness while increasing the bitter perception of beer [111]. Hop extracts are also commonly added for extra bitterness and to obtain a greater content of aromatic compounds from the different stages of the brewing process [112].

Beer is an unstable product whose composition can change during storage and bottling [36] through different types of reaction.

During the maturation phase, some off-flavor compounds from previous stages may reduce their concentrations and facilitate the production of a more balanced product. The bitterness provided by the hops and by some polyphenols such as gallic acid, flavonoids, and tannins, is also dependent on the specific conditions under which this phase takes place. Generally, during maturation, bitterness decreases and sweetness increases. Nevertheless, the extent to which this phenomenon occurs depends on a number of factors, including the type of beer [93]. In the case of lager beers, certain aromatic changes may take place during storage, together with a linear decrease in bitterness, because of the degradation of isohumulones and/or humulinones, and an increment of sweet aroma, toffee flavor, cardboard taint, and ribes off-flavor [143,144].

Certain compounds such as the furfural extracted from wood, and the esters generated by the esterification reactions that take place between alcohols—mainly ethanol and acids—during beer aging in wood change their concentrations, which increases beer bitterness as greater amounts of tannins are extracted from the wood [93]. Another aspect that should be considered during this particular maturation is that different microorganisms can contribute with different compounds to beer, but their presence will depend on the state and type of wood used for the aging [145]. For example, lambic beer matures in wooden casks, and yeasts such as Brettanomyces bruxellensis, Brettanomyces anomalous, and Pichia membranifaciens; acetic acid bacteria; and the LAB Pediococcus damnosus and Lactobacillus brevis, among others, play an important role in the process because they contribute to the typical Brett flavor of lambic beer, characterized by spicy and medicinal notes, and also fruity and floral ones. Thus, the ester-synthesizing activity of Brettanomyces contributes to the production of various ethyl esters, such as ethyl caproate or ethyl caprylate, that contribute to floral notes, at concentrations significantly higher than those found in other beers. In addition, the Brettanomyces yeast species that contain a superoxide dismutase enzyme with vinyl phenol reductase activity can form 4-ethylphenol and 4-ethylguaiacol, which are responsible for spicy and medicinal notes. Brettanomyces can also produce isovaleric acid from leucine, and this acid is responsible for sweaty and cheesy flavors, and may also produce mousy off-flavors that are associated with 2-ethyltetrahydropyridine and 2-acetyltetrahydropyridine. The presence of acetic acid and lactic acid bacteria also contributes to the high concentrations of ethyl acetate and ethyl lactate. In addition, acetoin, which is produced by AAB species through the utilization of lactate, may contribute to undesirable buttery notes [146,147].

It is also known that, after bottling, beer flavor is affected by certain chemical reactions that lead to instability, being an indicator of the increment of sensory-active aldehydes, which are generated in the sequence of radical reactions initiated by reactive oxygen species [149,150]. These aldehydes are also produced during mashing and wort boiling, but they decrease during the fermentation stage. It has been demonstrated that hop polyphenols slow down the sensory deterioration of pale lager beer as they suppress the formation of sensory-active aldehydes.

A traditional method to achieve beer carbonation consists of bottle re-fermentation, which is initiated by adding yeast and fermentable carbohydrates. As a result of yeast multiplication, carbonation increases and the concentration of flavor-active compounds is also affected, so that beer aroma and taste are also modified [11]. New flavors are produced as a result of the yeast activity, which incorporates higher alcohols, esters, aldehydes, vicinal diketones, and sulfur compounds that have an influence on beer aroma [17]. So, there are certain yeast strains that produce phenolic flavors resembling clove, smoked meat, or medicinal odors, among others [153]. Furthermore, the increment of carbon dioxide concentrations enhances beer effervescence. An additional effect of bottling is the prevention of oxidative damage, as yeast consumes oxygen.

Another factor to take into account is the presence of contaminants from previous stages that may reach the beer storage phase.