Growth of Nutraceutical Auxenochlorella protothecoides and Yarrowia lipolytica Heterotrophs on Rendered Hydrolysates and Determination of Accessible Carbon Substrates

Abstract Rendered hydrolysates are a large and growing fraction of heat-sterilized aqueous phases produced by meat rendering. Processing of hydrolysates involves energy-intensive evaporation and wastewater treatment, creating an expensive waste stream. This work investigates the growth of nutraceutical Auxenochlorella protothecoides (microalgae) and Yarrowia lipolytica (yeast) heterotrophs on undiluted hydrolysates. Two hydrolysates were produced from thermal hydrolysis at 121 °C, one from 7% and another from 25% solid loading, representing wet and dry rendering techniques, respectively. Robust heterotrophic growth was observed on hydrolysates with Yarrowia lipolytica and Auxenochlorella protothecoides reaching 1.3–2.2 grams per liter (g/L) after 96 h. Microbial protein content was around 40%, but hydrolysates slightly reduced amino acids relative to control media. Robust growth suggested an abundance of unknown organic molecules in hydrolysates, which could be metabolized. Direct aqueous injection gas chromatography followed by mass spectrometry showed high concentrations of glycerol (up to 10 g/L) in the hydrolysates, which was utilized by algae and yeast. The high abundance of glycerol indicated fat hydrolysis at temperatures less than half those normally required (250 °C versus 121 °C). Follow-up experiments indicated a catalytic mechanism involving inorganic metals. This work expands our chemical knowledge of rendering processes and makes important progress toward integrating biotechnology into the rendering industry.