3.1. pH and Microbial Populations
The initial pH of the wort was 5.9 and it was reduced to 4.6 - 4.7 after the first fermentation (
Table 1). To evaluate the pH, a control sample was used, in which no probiotic strain was inoculated, but only the yeast US-05. At the end of brewery, this control formulation reached a pH of 4.2, while the formulations with probiotic strains had a pH of 3.9 (
Table 1).
In general, beer is an inhospitable environment to bacteria, due to its compounds and characteristics, such as the presence of iso-α-acids, ethanol, low pH, high CO
2, and low O2 [
5]. Using ANOVA and comparing counts obtained by pour plating and by PMA-qPCR (
Table 1), the PMA-qPCR revealed much higher populations (p<0.05), indicating that, even with a satisfactory count, the LABs were under stress, which is understandable, due to the high quantity of hops in the formulations. This difference between pour-plate and PMA-qPCR counts is not surprising, as beer is a stressful vehicle for bacteria.
The daily amount of probiotic ingested is still a matter of debate among scientific sources. According to the government of Canada [
25], a food product must contain a minimum level of 1.0 x 10
9 CFU/mL of one or more probiotic microorganisms.
In our study, probiotic cultures were stable throughout the production process and storage, showing a population above 10 log CFU per daily portion of 350 mL in almost all steps. These formulations not only achieved the requirements to be considered beverage with probiotic potential, but also show stability in the cultures used. There were no differences in populations between the F19 and L431 strains by PMA-qPCR. Therefore, both strains have the same growth advantage and are good candidates for fermenting hop beers.
Similarly, Alcine Chan et al [
10] reported stability in yeast and LAB probiotic populations. The authors concluded that co-culture of the probiotic strain Lacticaseibacillus paracasei L26 with S. cerevisiae S-04 in unhopped wort showed excellent growth and stability and the yeast performance was not affected by the probiotic strain. Likewise, the growth of S. cerevisiae strain US-05 (US-05) in our study was not affected. In addition, like the study performed by Dysvik et al [
26], our data showed that the LAB strains did not interference in the yeast growth.
3.2. A-Acids Stress and Hop Resistance Genes Expression
The presence of iso-α-acids was reported as inhibitory for bacterial growth [
5]. The mechanism of action of these acids is simple. As they are weak acids, they have an affinity for the cytoplasm of bacterial cells and undissociated can cross the cell membranes of bacteria. Once in their cytoplasm, they promote the release of protons and, consequently, the drop in intracellular pH, affecting the metabolism of the entire cell [
5].
Some strains are resistant to the antimicrobial actions of the hops [
27]. This ability is due the action of the specific genes here analyzed. In the gene expression analysis, Recombination Factor A (recA) did not show any expression in both formulations. The expression of the other genes was well evidenced in the two beer samples (
Figure 2). However, with some differences among them: the expression of Hop-Resistant B (horB) was observed to be absent in F19, while the expression of Hop-Resistant A (horA) was absent and, of Beer-Spoilage Related A (bsrA) was significantly higher in 431 (p<0.05).
It is known that hops provide protection to beer due to the concentration of iso-α-acids, which have a bacteriostatic effect [
5,
27]. Despite this imposed barrier, beer spoilage is not uncommon. These genes, which confer hops or α-acids resistance, for our study are of great interest, even as genetic molecular markers, since the strains here studied bring health benefits [
16,
17,
28,
29]. Among these genes, horA and horC are considered the most important ones.
The horA gene encodes an ABC transporter capable of expelling hop bitter acids from cells [
30]. The horC gene encodes a proton motive force (PMF)-dependent multidrug effluence pump [
31], and bsrA as multidrug ABC transporters [
32], that is, they are involved in the transport of compounds or substances. On the other hand, horB is proposed as the transcriptional regulator of horC [
24], but evidence to confirm this function is lacking. Besides, the influx of hop bitter acids into the cell is diminished by the contribution of horA and horC [
5].
Despite several evidence and studies that point out their functions, the importance of these genes is still discussed and questioned, and still not well understood [
33]. For example, in Lactobacillus brevi NBC strain UCCLB556, horA was found. However, this strain was described as unable to grow with hops and the strain UCCLBBS124, which, on the contrary, was reported as growing in the presence of hops, had plasmids with horA and horC, and not hitA [
34]. Gene horA was not found in any of the isolates in Lactobacillus rossiae strains isolated from beer, while horC was found in four isolates and hitA in two [
35].
There were no statistical differences in the populations comparing each group in the two methodologies, the traditional plate-count method and PMA-qPCR (
Table 1), at the end of the production process (day 28). However, the plate-count method applied during the storage period evaluated, showed a lower population for the F19 strain (p < 0.05), which may be evidence of stress. Thus, the difference in the modulation of the expression of these genes may not only be associated with the survival of the strains in the wort with hops, but the different genes expressed may have an influence on the bacteria balance. Further studies are necessary to better understand the role of these genes.
Despite this, as mentioned previously, the expression of bsrA is higher in L431, and horA is present only in F19. The horA gene encodes an ABC transporter [
30] and bsrA a multidrug ABC transporter [
32]. ATP-Binding Cassette (ABC) transporters are responsible for expelling toxins from cells, and when found in bacteria they can confer antimicrobial resistance [
36]. Interestingly, the α-acids found in hops confer antimicrobial properties [
5]. Thus, both horA and bsrA seem to perform the same function, since they translate the same class of proteins. Both proteins play an important role in expelling alpha acids from the bacterial cell. The expression of bsrA is practically double in L431 compared to the same expression in F19, and horA expressed in F19. About this, we can think in two hypotheses (
Figure 3).
It seems that F19 can not sufficiently express bsrA and, therefore, it expresses horA, which has a protein with the same function, to compensate for this bsrA insufficiency (
Figure 3). The stimulus for the expression of horA and bsrA happened simultaneously in F19 and, thus, shows that a higher expression of bsrA, as in the case of 431, is more efficient in eliminating hop bitter acids. This can be corroborated by the fact that the F19 population determined by the agar-plating method is lower than that of 431 (
Table 1) on day 2, and the PMA-qPCR counting method is the same between the two strains (
Table 1) in the same period, which can evidence that the F19 is more stressed.
3.3. A new Hop Sour Beer Style?
According to the Beer Judge Certification Program (BJCP) (
https://www.bjcp.org/), sour beers are characterized by having a freshness and lactic acid flavor, low pH, and the vast majority do not have much hop content. Most of them have an IBU up to 10 or 12. However, Flanders Red Ale and Oud Bruin are the only ones that can reach an IBU of 25. So far, there is no sour registered that has an IBU above 25 (
Table 2). In this way, a beer with an IBU equal to 29.65 can be considered a very hop beer, although it does not have the same content as most IPAs do, which are considered the most hop beers. Even among the IPAs, there are those for which an IBU of 25 is acceptable, as in the case of Brut IPA and Hazy IPA. However, it has a value that is within the acceptable range for a German Pilsen and might be regarded as a beer with a pronounced hop flavor (
Table 2) (BJCP).
As far as we know, this is the first study and the first evidence that bacteria can survive in beers with high quantity of hops. It is known that the survival of bacteria together with hops depends on their ability to expel α-acids from their cytoplasm, which are involved in membrane transport mechanisms and pumps [
5]. Thus, perhaps due to their ability to survive in the gastro-intensive tract, some strains of probiotic bacteria may be the best candidates to survive in beers with more hops.
3.4. Final Considerations
In the production of a sour beer, therefore, little or no hops are used in its production. Some specific styles, as demonstrated earlier, may contain a larger amount of these ingredients. This is mainly due to the difficulty that most microorganisms have in growing together with hops. An alternative would be the production of sour beers with hops added late, that is, during the fermentation process. In this way, the release of α-acids during boiling is avoided [
37]. However, this methodology demands an increased maturation time and, therefore, a higher cost in each bottle that is taken to the consumer.
The use of hop-resistant strains, in this case the probiotic strains Lacticaseibacillus paracasei subsp. paracasei F19 and L431, not only can provide functional properties to the beer, but could also accelerate the brewery of sour beers with more hops, and possibly making these beers cheaper to the consumer. These strains are strong candidates to be used in the production of other sour beers, as well as in the development of other beer styles in this category. In addition, the data presented here, as far we know, is the first evidence of bacterial survival in formulations with an IBU greater than 29, and the beer formulations here presented do not have parameters for any of the beer styles found in the BJCP database. As a consequence, they are possibly new styles of sour beer.
It is a consensus that moderate consumption of beer brings health benefits [
38]. Beer contains components associated with health benefits, such as melatonin [
39], hop xanthohumol (XN) [
9] and iso-α-acid [
40], vitamin B, minerals, and flavonoids [
41]. Hops also have medical properties. Several studies have demonstrated the beneficial effects of molecules present in hops on weight gain, lipid metabolism, glucose homeostasis, insulin sensitivity, and inflammation, acting on different targets [
42]. They have also been shown to improve gastrointestinal mucosal integrity and reduce systemic endotoxemia in high-fat diet mouse models [
42]. Allied to these benefits that a beer with a high quantity of hops can provide, a beer with probiotics can provide even more benefits in the case of moderate or low beer consumption.