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The synthesis of acetic acid can occur through direct oxidation of ethanol, though this process is very slow in anaerobic conditions [ref]. Some bacteria, noteably Acetomonas and Acetobacter can catalyze the conversion of ethanol to acetic acid, greatly speeding up the process. The former can continue the oxidation process to fully convert ethanol to carbon dioxide and water, but the latter species is incapable of progressing beyond acetic acid. Both of these bacteria require oxygen for the reaction to take place, so the production of acetic acid occurs early in the fermentation process, when the wort is still well-oxygenated. After about 6 weeks, the production of acetic acid trails off and the concentration will remain the same until the end of fermentation <ref name=Oevelen77 >D. Van Oevelen, M. Spaepen, P. Timmermans and H. Verachtert, [http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1977.tb03825.x/abstract|MICROBIOLOGICAL ASPECTS OF SPONTANEOUS WORT FERMENTATION IN THE PRODUCTION OF LAMBIC AND GUEUZE], 1977</ref>. As acetic acid bacteria are ever-present in the wort, exposure to excessive oxygen at any time during fermentation will lead to the formation of large amounts of acetic acid, sometimes up to 4 g/l <ref name=Oevelen77 >D. Van Oevelen, M. Spaepen, P. Timmermans and H. Verachtert, [1http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1977.tb03825.x/abstract|MICROBIOLOGICAL ASPECTS OF SPONTANEOUS WORT FERMENTATION IN THE PRODUCTION OF LAMBIC AND GUEUZE], 1977</ref>, at which point the lambic is referred to as "hard", though it is still useful for blending <ref name=Guinard >Jean-Xavier Guinard , [[Books#Classic_Beer_Styles:_Lambic|Classic Beer Styles: Lambic]], 1990</ref>.