Microbial contamination control mechanism in lipid production using distillery wastewater and oleaginous yeast - Antimicrobial compounds in wastewater as a double-edged sword.

Microbial contamination and the high expense of sterilization are the key factors limiting the application of resource recovery in processes such as producing lipids (can be converted to biodiesel via transesterification) from wastewater. This study was conducted to study the succession of contaminating and indigenous microorganisms, analyze the mechanism and propose a control strategy for undesirable microorganisms in the non-sterile lipid production process using distillery wastewater and oleaginous yeast. In the early stage, indigenous microorganisms (Pichia, Saccharomyces, Acetobacter and Gluconobacter) were the main competitors. Based on antimicrobial experiment and analyses of liquid chromatography-tandem mass spectrometry (LC-MS/MS) and gas chromatography mass spectrometry (GC-MS), the antimicrobial compounds (such as lactic acid 10,011-17,498 mg/L, succinic acid 210-325 mg/L and furfural 0.63-1.23 mg/L) combined with the low pH (3.2-3.8) in distillery wastewater played the primary role in the prevention of contaminating bacteria in this stage rather than the potential antimicrobial compounds from oleaginous yeast. Cinnamic acid (56-143 mg/L) was the main inhibitor against oleaginous yeast among the major antimicrobial compounds in wastewater. Its inhibition decreased when pH increased from 3.2 to 5.5. In the later stage, as the pH increased to over 7 during the culture, heterotrophic bacteria (Chryseobacterium and Sphingobacterium) with a relatively low tolerance for acidic conditions became the dominant undesirable microorganisms. Utilizing the antimicrobial activity of distillery wastewater combined with a high inoculum size and proper pH control could be effective for achieving dominant oleaginous yeast growth and improving lipid production in non-sterile conditions.