Realizing Power-to-Food: Optimizing microbial hydrogen fermentation for maximum protein yields

The non-pathogenic bacteria Cupriavidus necator represents a novel and sustainable food protein source reaching protein contents above 75% in its dry matter. The chemolithoautotroph proliferates on H₂ (electron donor), O₂ (electron acceptor), and CO₂ (carbon source), whereby these feedstocks are ideally generated/harvested using renewable energy – the "Power-to-Food" concept. In this study, variations in the decisive cultivation parameters for C. necator were tested to determine optimum parameters for rapid growth, high biomass concentration, and maximized protein yield.

A top-stirred tank reactor designed for safe hydrogen fermentation was inoculated with precultures of C. necator (DSM 428) and operated in batch cultivation. Parameters including the volumetric flow rate of supplied gases, ratio of CO₂ in the gas mix, and nitrogen supply were changed. The growth rate analyzed inline by optical density measurements, biomass formation determined as change in dry weight over time, and protein yield tested with the Dumas method were recorded as a function of the operated cultivation parameters.

As expected, a clear correlation between increased flow of the gases and higher growth rates was seen. Lowering the CO₂ concentration in the gas mixture led to more rapid growth and overall higher biomass formation, but lower protein yields. Still, the nitrogen availability proved to be the most important factor in protein synthesis. Here, a nitrogen feeding protocol with high feeding frequency in the early exponential growth phase (20-40h) resulted in a very high protein content of 83% in cell dry weight, while periodical feeding at a constant frequency revealed significantly lower protein contents around 30%.

These findings point the way to the establishment of microbial hydrogen fermentation for single-cell protein production, which surpasses protein yields from the cultivation of other microorganisms (algae, yeast, etc.) and reveals the potential towards the production of sustainable food.