Because partially hydrogenated oils and shortenings (with trans fat) have a long fry life and shelf life, they became popular over the last 50 years as the "heavy-duty" fry products of choice. Now that we know how unhealthy industrially produced trans fat is, chefs are returning to traditional heavy-duty oils and using some newer heavy-duty alternatives.
These highly stable oils are slow to break down through multiple fryings and can withstand deep frying for extended periods. The traditional “heavy-duty” fry products and newer heavy-duty alternatives should function in the fryer the same as partially hydrogenated vegetable oils/shortenings, and some seem to last even longer. As an example, if the current partially hydrogenated vegetable oil/shortening lasts 10 days in the fryer in your kitchen, then a similar performance can be expected from one of these highly stable oils.
Heavy-duty oils typically cost more per gallon than light/medium-duty oils. However, because they last longer in the fryer, you may find that they are cost-neutral.
There are several choices for heavy-duty and extended deep frying:
Naturally stable plant oils. These include cottonseed, peanut, corn, rice bran and palm. Corn and peanut are traditional favorites for heavy-duty frying. Palm oil, a tropical import, is stable but very high in saturated fat. It should be used sparingly.
Modified composition oils. "Low-linolenic," "mid-oleic," and "high-oleic" are terms used to describe newer oils with a fatty acid composition that’s very stable and good for extended deep frying. These oils come from plant sources (mainly soy, canola and sunflower) that have been bred for this purpose. Under the right conditions (see Deep-Frying Tips) these oils can last a week or longer.
Light/medium-duty and heavy-duty oil blends. You can increase the stability of low-cost medium-duty oils by blending them with small amounts of naturally stable plant oil or modified composition oil. You can also buy premade blends (see our 0 grams of trans fat fry oils and shortenings list). A blend made this way can be used for extended deep frying, but it won’t last as long as a 100 percent naturally stable oil or modified composition oil. Typical blends contain 75–90 percent soy or canola oil with TBHQ mixed with 10–25 percent peanut, cottonseed, rice bran or a modified composition oil.