# The Influence of Fermentation Vessels on Yeast Microbiota and Main Parameters of Sauerkraut
## Results and Discussion
### The Influence of the Vessel Used for Fermentation on the Yeast Microbiota of Sauerkraut
In the following days, increases in the number of bacteria were observed regardless of the vessel used for fermentation. In the jars, the maximum level of LAB was detected after 7 days of fermentation (9.43 log cfu/g), which was a result similar to that in other studies [21; 30]. In the following days, the number slowly decreased, but still reached a value of 8.23 log cfu/g at the end of fermentation, higher than the highest number of LAB detected in stoneware vessels. In stoneware vessels, the maximum amount of LAB occurred on the fourth day of fermentation (7.55 log cfu/g), then decreased in subsequent measurements to increase again to the value reached after four days. The presence of oxygen during vegetable fermentation may inhibit the growth of some lactic acid bacteria, especially strictly anaerobic ones. It can also modify the metabolism of others, causing an increase in the amounts of oxidized components (such as acetic acid, aldehydes, and others) [31]. The pH value of sauerkraut (Table 2) gradually decreased during the entire fermentation, from the level of 6.04 to below 4. The greatest drop occurred between the second and fourth days in jars, and the third and seventh days in stoneware vessels. In both cases, a similar level was finally achieved, safe for fermented vegetables. The pH of fermented vegetables should be at or below 4 to limit the development of putrefractive bacteria [32]. This level was achieved significantly faster (by about 3 days) in cabbage fermented in jars, which suggests that the process itself may be shorter there. A number of factors are responsible for obtaining the appropriate level of acidity during vegetables' fermentation, including both acid-producing bacteria and the yeast responsible for their metabolism. Among the organic acids, lactic acid and acetic acid are considered the most important in terms of influencing the pH of the sauerkraut [33]. In both vessels, the concentration of lactic acid reached its maximum level on day 10, after which the level of this compound decreased (Table 2). Statistically significant differences appeared between the samples from the seventh day of fermentation, when the concentration of lactic acid in the sauerkraut in the jars was almost twice as high as in the stoneware vessels. After the tenth day of fermentation, a decrease in the level of lactic acid was observed, with a decrease of 0.41 g/kg (2.5%) in the jars and 1.53 g/kg (13.1%) in the stoneware vessels. The conditions in the jars made it possible to maintain anaerobic conditions to a greater extent, and thus a better course of lactic fermentation, allowing for obtaining a high-quality product. At the end of fermentation, the lactic acid content should be between 1.5 and 2.0% [34]. Typically, the fermentation of sauerkraut is considered complete when it has reached a pH of approximately 3.8 or 1.5% lactic acid [35]. Such parameters were obtainedonly in sauerkraut in glass jars on the tenth day of fermentation. The detected changes could be related to the microbiota present during fermentation, which created or degraded lactic acid. Moreover, the anaerobic conditions in the jar also modify the composition of the yeast microbiota, which eliminates cultures that can use lactic acid as a carbon source (deacidify), such as _Rhodotorula_ yeast [36]. Acetic acid in fermented cabbages in both vessels was detected from the first day of fermentation, and several times more of this compound was found during fermentation in stoneware vessels than in jars. Acetic acid is formed by acetic acid bacteria and heterorefmentative LAB, especially in aerobic conditions, as well as by some yeast cultures. They produce acetic acid mainly from sugars, but some strains can also convert lactic acid [37]. The concentration of acetic acid reached a maximum on the seventh day and then decreased in the following days.