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. 2018 Oct 5;13(10):e0204831. doi: 10.1371/journal.pone.0204831

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Fig 1 — (A) Trend in VFA metabolites over the course of bioreactor operation. AA, acetate; PA, propionate; BA, butyrate; IVA, isovalerate; VA, valerate; CA, caproic acid. (B) Ratio of propionate to acetate as an indicator of bioreactor process performance. Ratios above 1.4 are considered an indication of imminent process failure. The ratio of PA to AA was observed to increase substantially after the impeller speed was doubled (200 rpm to 400 rpm; dotted vertical lines) shortly after 50 days of bioreactor operation. A return of the impeller to 200 rpm at 99 days of operation resulted in the drop of the ratio.