Determination of Location of Zooglea Growth
Comments: The first two pH tests were done with lithmus paper. It was very difficult to get a good reading since there is not enough of a color difference below 7 pH. The last pH test was done with an electronic pH meter which was calibrated with a 4.0 pH buffer. The measurements were repeated once.
The absence of oxygen in container A was verified by lowering a lit match after removal of the airlock. It was extinguished in an instant.
What surprised the most was the fact that the tea in container A tasted almost flat with little acidity even though it tested at pH 3.65. Sample B had the typical sour taste of regular KT. The pH difference between the two samples, on the other hand, was only 0.35. Although the number of H+ ions accounting for acidity increases exponentially compared to the decrease in pH value, the striking difference in sourness was unexpected. (Example: A solution with a pH of 4 has 10 times more acidity than one with a pH of 5. A solution with a pH of 3 is 100 times more acid than one with a pH of 5.) According to the literature, human taste sensation of sourness not only depends on H+ concentration but is also influenced by the shape of the molecules in question. Vinegar tastes considerably more sour than lactic acid with the same pH.
It has been suggested that one could cover the brewing container with a plate if a reduction in sour taste was desired. This would cause more lactic and less acetic acid to be produced. My experiment seemed to confirm this as far as flavor was concerned.
Since the acetic acid bacteria are known to be aerobic, sample A without the oxygen consequently produced no vinegar and no zoogloea. The observed acidity must have been completely due to other acids present. Lactic acid, for example, is produced as a by-product of fermentation under anaerobic conditions.
As Reiss has shown in his research, most of the acidity in K.T. consists of acids other than acetic acid . He found that K.T. grown under normal conditions after 10 days had only about 0.1 g/liter acetic acid, but 15 times as much gluconic acid and lactic acid. Another interesting result was that acetic acid leveled off after 10 days while the other two continued to accumulate. This would explain why I measured a pH for sample A that was lower than B by only a small number. Eliminating acetic acid in sample A had only a small effect on measured acidity.
Sample C showed the most acidity but the zoogloea was fragmented and not in a continuous layer like
in sample B. This probably was caused by the agitation of the surface because of the bubble stone which probably prevented acetic acid bacteria from settling on the surface where the most oxygen was
available. It is not clear at this time why the acidity of this sample was higher than sample B.
Last updated 3/24/2000