Abstract
Although Aspergillus niger, strain 72-4, has been studied previously because of the high yields of citric acid it gives when fermenting sugars, it also produces oxalic acid under the proper conditions. The present work is a study of the conditions under which oxalic acid is formed, the substrates that are converted to it, and the mechanism of its formation from these substrates. Oxalic acid is never found in normal growth fermentations with strain 72-4 until after autolysis has begun. However, in replacement fermentations in which preformed, washed mycelium is resuspended in nitrogen-free medium, oxalic acid is often formed together with citric acid. It was originally suggested (1) that oxalic acid is made by hydrolytic splitting of oxalacetate. Later work (2, 3), although consistent with this theory, gave data more in line with the results of the present work, which indicate that oxalacetate is oxidatively split to 2 molecules of oxalic acid. Direct oxidation of acetate to oxalate was ruled out by the work of Bomstein and Johnson (3) and also by the present work. The present study also shows that, at neutral pH, a great variety of sugars and related compounds can be fermented to oxalic acid by fluoride and fluoroacetate-insensitive pathways. Compounds part of or related to the Krebs cycle still appear to form oxalic acid by oxidative splitting of oxalacetate.
Highlights
Effect of pH and Bu$er on Oxalic Acid Formation from Glucose- in normal growth fermentations at low pH oxalic acid is not formed, it frequently is formed with citric acid in replacement fermentations
If one increases the pH of the medium by addition of buffer, oxalic acid formation is favored over citric acid production and completely replaces it at a pH above 5, as can be seen from Table I
The results above leave little doubt that oxalic acid is formed as the result of raised internal pH in mycelium grown at low pH for citric acid production
Summary
Effect of pH and Bu$er on Oxalic Acid Formation from Glucose- in normal growth fermentations at low pH oxalic acid is not formed, it frequently is formed with citric acid in replacement fermentations. This effect was found to bc reproducible and a function of the citric acid concentration These data suggest that oxalic acid formation is due to a higher than normal pII inside t,he cell. The yields from citrate, succinate, and tartrate may have been high because of utilizat)ion of the buffer itself; for this reason phosphate was used routinely as a buffer to study the oxalic acid fermentation at neutral pH, except when acidic substrates n-hich could serve as their own buffers were employed. (Roth inhibitors prevent germination of the mold and fluoride inhibits the formation of citric acid from glucose at low pH completely.) Sugars, sugar alcohols, and related acids are possibly converted to oxalate through glycolaldehyde, glycolate, and glyoxylate (see under “Discussion”).
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