Michigan State University Extension
Preserving Food Safely - 01600605
In general, it is not a good practice to add oil or margarine to fruits or vegetables that will be processed. Oil or margarine will not go into solution but will form an unstable emulsion with tiny oil droplets scattered throughout the product. Heat does not penetrate the oil droplets very well, therefore microorganisms or spores in the oil droplet may survive processing. Even in an acid product (such as tomato sauce), the micro-environment immediately surrounding an oil droplet may not be the same pH as the rest of the product, thus permitting the growth of microorganisms that could not otherwise grow in an acid medium. It is best to process the product without added oil; if desired, oil can be added just before serving. Another alternative is to freeze the product.