Provided are methods for preparing chemically modified lipids. The lipids are obtained from heterotrophically cultured microalgae and are subjected to an epoxidation reaction. The microalgae include those from the genus Parachlorella, Prototheca, Chlorella, or strains having at least 85% nucleotide sequence identity in 23S rRNA sequences to a Parachlorella, Prototheca, or Chlorella strain that are cultured in a bioreactor substantially in the absence of light.