This jargon term of the food-processing industry is not yet, as far as I know, in any dictionary. But it is one that has been turning up more frequently in the past few years as a result of increasing concern over side-effects of the technologies that produce some of the processed foods we eat. Manufacturers of products such as cakes and biscuits need fats in solid form, but unsaturated fats usually occur as liquids, so makers have commonly converted them to solids by hydrogenation. The problem is that some of the fat is converted to a type called trans fat, which humans can’t easily digest. As a result, firms are instead starting to turn to interesterification, in which acids or enzymes modify the fats to make them solid. (The name comes about because the component fatty acids in the oils are combined with other organic groups and are so technically esters; these are shifted about within the oil molecules during the reaction.) The process has been known for at least the last two decades, but is slowly becoming commercially viable. Some critics claim it is open to similar objections to the older hydrogenation method. A variant spelling, interestification, sounds as though it might be something that enhances your interest in a subject. It’s certainly a process being carefully watched by nutritionists.