When scientific studies exposed the inherent dangers to public health in trans fats, many processed food manufacturers scrambled to find a suitable replacement. They needed to find a form of fat which would still provide the extended shelf life of partially-hydrogenated oils, but did not contain trans fatty acids. One solution arrived in the form of interesterified fat, a fully hydrogenated product with many of the same characteristics as trans fat, but closer to saturated fat chemically. Interesterified fat is produced through a process called interesterification, which rearranges the molecular structure of fatty plant oils.
It might be helpful to look at the term as "inter-ester-ified fat," since the interesterification process generally affects compounds known as esters in the oils used. The chemistry which produces interesterified fat is a bit complicated, but essentially, a natural vegetable oil such as rapeseed is combined with stearic acid and various alkylinic catalysts. The fat components of the oil are chemically altered to become more saturated, much like artificially turning margarine (a partially hydrogenated fat product) into beef lard, a saturated fat. Interesterified fat provides the best of both worlds for manufacturers of processed foods-- an increased shelf life and a better mouth feel.
The problem with interesterified fat for consumers is that in many ways the cure is worse than the disease. The reduction or elimination of harmful trans fat should have led to a safer fat product for general consumption. Instead, interesterified fat may be even worse than the partially-hydrogenated products it replaced. Recent studies on the effects of interesterified fat on humans revealed that interesterified fat has roughly the same negative effects on HDL/LDL cholesterol as trans fat. Interesterified fat is believed to lower levels of the good HDL cholesterol while raising the levels of bad LDL cholesterol.
In addition, interesterified fat also showed signs of raising glucose levels as much as 20%, which could lead to the development of pre-diabetic conditions or diabetes itself. The scientists who conducted these studies on interesterified fat suggest that a diet containing the previously maligned saturated fat in moderation would be safer than a diet high in interesterified fat. Although interesterified fat is not the same as partially-hydrogenated oils, the unnatural rearrangement of the lipid molecules does create many of the same health concerns for consumers. Chemically speaking, interesterified fat is the saturated fat alternative the developers of trans fat were hoping to create but couldn't. Time will tell if interesterified fat will face the same fate as the trans fat it replaces.