The general formula of any edible oil or fat is shown by the attached file. As you know, an oil or a fat is also described as triester of glycerol or triglyceride. The 3 groups in the formula (R,R',R") are long chains which may be saturated (largely in a fat) or unsaturated (largely in an oil) . If chemical or enzymatic modification is to be carried out, it will involve partial or total changes on (R,R',R") but not from the other side (i.e. the -CH2-CH-CH2-). Any change on the groups will make the fat inedible. Imagine that Br2 is added to a single C=C in R' , then the product will simply become a poison.