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ABSTRACT Hydrophobic feedstocks such as waste cooking oil have recently been considered for microbial biotransformation due to their abundance, low cost, and unique advantage for lipid‐derived fermentation products. Most fermentations with hydrophobic substrates are conducted at the tube or flask scale (less than 1 L total volume) or with the hydrophobic substrate comprising a small fraction of the media. Low substrate concentrations require additional feeding. Alternatively, high concentrations do not require significant dilution of the oil feedstock, which reduce volumetric requirements for larger scale fermentations. However, high‐oil‐density fermentations complicate efficient mixing and mass transfer challenges which are exacerbated at larger scales. To address this, computational fluid dynamics (CFD) models were explored to simulate three‐phase (hydrophobic, hydrophilic, and gaseous) bench (3 L) and pilot scale (4000 L) bioreactors, highlighting challenges and potential considerations. Bioreactor fermentations ofYarrowia lipolyticastrain L36DGA1 with substrate loadings of 5%, 10%, 20%, 30%, 40%, and 50% (v/v) waste cooking oil were also conducted, representing one of the highest concentrations in the reported literature. This work supports future research into and implementation of high‐oil‐density fermentations at the bench and pilot bioreactor scale.