How to partially hydrolyze triglycerides?
Over the last decades, partial hydrolysis of triglycerides has been examined in various contexts. 

Note that this isn't my field of work and that there might hundreds of better and/or more instructive recipes, but a cursory search yielded the following freely available references:

THE DIGESTION IN VITRO OF TRIGLYCERIDES BY PANCREATIC LIPASE, 
F. H. Mattson, and L. W. Beck,
J. Biol. Chem., 1955, 214, 115-125 (PDF)

A stereospecific analysis of triglycerides, 
H. Brockerhoff, Journal of Lipid Research, 1965, 6, 10-15
(PDF).

The products from lipase-catalysed hydrolysis of bovine milkfat kill
Helicobacter pylori in vitro, 
Cynthia Q. Sun, Charmian J. O’Connor, Alastair K. H. MacGibbon, and Anthony M. Roberton, FEMS Immunology &amp; Medical Microbiology, 2007, 49, 235-242 
(DOI)

It seems that the digestion of triglycerides by lipases in buffered solutions is a viable approach.