Effects of carbon monoxide supply on gas fermentations in serum bottles investigated by online monitoring of gas transfer rates

Abstract

Serum bottles are widespread in the early stage of anaerobic gas fermentation processes. These cultivation devices are particularly cost-effective and easy to use. However, serum bottles used for gas fermentation processes face the challenge of low substrate availability. Additionally, serum bottles are commonly operated as “black-boxes” without online monitoring technologies. In this study, an in-house built device for online monitoring the absolute pressure in serum bottles is presented. Furthermore, biomass was online monitored by scattered light measurements. Therefore, Clostridium ljungdahlii, grown on carbon monoxide, was used as model acetogen. The effects of batch gassing, re-gassing, and continuous ventilation on gas transfer characteristics were analyzed. In addition, the impact of the shaking frequency on gas transfer rates was investigated. The absolute pressure measurement provides crucial information on gas transfer rates and microbial activity. Scattered light signals were successfully correlated to the offline optical density. The availability of the gaseous carbon source is limited by the gas transfer and influences product formation. In batch gassed serum bottles, exponential growth of the microorganisms is observed, until the cultivations transient in a gas transfer limitation. The finite available carbon monoxide led to starvation and morphological changes, which were indicated by scattered light measurements. These starvation-based morphological changes could be temporarily prevented by re-gassing of the serum bottles and fully prevented by continuous gassing.