Evaluation of waste products in the synthesis of surfactants by yeasts

The highest yields of biosurfactants were obtained by: (i) Pseudozyma antarctica (107.2 g L^?1) cultivated in a medium containing post-refining waste; (ii) Pseudozyma aphidis (77.7 g L^?1); and (iii) Starmerella bombicola (93.8 g L^?1) both cultivated in a medium with soapstock; (iv)Pichia jadinii (67.3 g L^?1) cultivated in a medium supplemented with waste frying oil. It was found that the biosurfactant synthesis yield increased in all strains when the cell surface hydrophobicity reached 70–80 %, enabling the microbial cells to make good contact with hydrophobic substrates. The lowest surface tension of the post-cultivation medium was from 32.0 mN m^?1 to 37.8 mN m^?1. However, this parameter (which was also determined by a drop collapse assay) was of limited use in monitoring biosurfactant synthesis in this study. The crude glycerol was not a good substrate for biosurfactant synthesis although, in the case of P. aphidis, 67.4 g L^?1 of biosurfactants were obtained after cultivation in the medium supplemented with glycerol fraction (GF2). In a low-cost medium containing soapstock and whey permeate or molasses, about 90 g L^?1 of mannosylerythritol lipids were synthesised by P. aphidis and approximately 40 g L^?1 by P. antarctica.