Acid-tolerant yeasts often inhabit extremely acidic environments: mine drain-ages, hot springs, and even fermented foods. Some of them also possess the ability to neutralize acidic media. However, the examples of these yeasts that are already known were isolated from acidic environments. In this study, the isolation of acid-tolerant yeasts from natural neutral aquatic environments and the identification of yeasts able to neutralize an acidic medium (acid-neutralizing yeast) in Japan were carried out. Various kinds of acid-tolerant and acid-neutralizing yeasts were obtained. In a neutralizing test using an acidic casamino acid solution adjusted to a pH of 4.0 with sulfuric acid, the obtained acid-neutralizing yeasts elevated the pH to approximately 7.0, and their neutralizing abilities were similar to those of previously reported yeasts that had been isolated from acidic environments. These results showed that acid-tolerant yeasts and acid-neutralizing yeasts exist widely in neutral environments, and little difference was found in the neutralizing abilities of yeasts obtained from neutral environments in comparison to those obtained from acidic environments.
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