I’m going to use the manual for my Bella 1.5L Deep Fryer as an example. There are two different kinds of reasons: the superficial reason, and the underlying reason.
The superficial reason is a difference in behavior between lard (and other solid or semi-solid fats) and oils (and other liquid fats). Here is the full text of the warning in my deep fryer’s manual:
- Always use oil with low water content like sunflower oil, grapeseed oil, vegetable oil or corn oil. Never use hard fats, olive oil or oil with a high water content.
WARNING! Never, under any circumstances, add water or any other liquid to the oil.
- Never mix different oils together to fry foods.
- Never use butter or margarine to fry foods.
Butter, according to a very old Iowa State University survey, contains at least 8.6% water, with most containing between 13% and 16% water. The USDA’s Food Data Central says about 16% for salted butter (i.e., 16 grams out of every 100 grams). Unsalted butter is higher, at 17%. (Judging from other sources even besides the old survey, that number varies considerably.)
Oils, such as sunflower oil, typically contain practically no water, on the order of less than a percent. The USDA’s Food Data Central doesn’t report any amount for water in safflower oil, sunflower oil, peanut oil, or canola oil.
The more water, the greater the chance of sputtering, throwing hot fat out of the fryer.
Mind you, they also report no water in lard or olive oil. The wording in that manual may be unfortunate. Judging from the hard fats warning being bracketed by warnings about water, it may reflect an incorrect belief on the writers’ part that hard fats and olive oils contain more water than sunflower, etc., oil. Or it may be differentiating three types of oil: those that are hard, those that are olive, and those that have a high water content.
If its presence in the line that starts with “Always use oil with low water content” is unrelated to that opening and similar closing, then the warning to “never use hard fats” contains no reason. However, there are some anecdotal sources that attribute this to solid fats heating less evenly until they melt, suggesting the potential for the heating element to have trouble if there isn’t anything to heat around it. Given the speed at which lard melts, this sounds almost urban-legendish to me, but you may wish to pre-melt your lard if you’re going to use it in a deep fryer.
Beef tallow is, in this respect, similar to lard, a solid fat with practically no water.
While the manual doesn’t mention it, another concern is likely the smoke point. Oils have a much higher smoke point than animal fats, with lard coming in at 370°F/185°C, butter at 350°F/175°C, and most common cooking oils well above that. Sunflower oil starts at 440°F/225°C, peanut oil is at 450°F/230°C, and safflower oil has a smoke point of 510°F/265°C. Since these fryers tend to have maximums above 370°F/185°C (mine goes to 375°), recommending only oils vastly reduces the chance of reaching a fat’s smoke point.
This concern is also touched on in Julia Kiene’s 1954 Electric Fryer-Cooker Recipes.
WHICH FATS ARE BEST
If you prefer oils for frying, there are excellent ones, such as Wesson, Mazola, and peanut oil. Each section of the United States and each foreign country offers its favorite oils. Olive oil is fine but much too expensive, of course.
Perhaps you prefer a hydrogenated vegetable fat such as Spry, Crisco, or Snowdrift. Each has been used with success.
Leaf or fine-quality lard is a possibility. The National Livestock and Meat Board recommends that the temperature for lard for deep-fat frying should not go above 350° F. There are also combinations of lard and vegetable oils as in Swift’ning.
Note that when she writes “deep-fat frying” she is talking about all frying oils. She refers to all of them as “fats” (for example, see the facing page where she describes how to “clarify fat” and “how many times fat may be used”, and of course the title to the section I quoted, “Which fats are best” where she includes various vegetable oils under that heading.
However, all of those concerns could be handled by recommending people use care when frying, as Kiene does, perhaps even including a chart of smoke points. Very likely the real concern is over lawsuits. Here’s another warning in my deep fryer’s manual:
- Do not immerse detachable power cord in any liquid. If the cord falls in water or other liquid, DISCARD IMMEDIATELY and replace it with a new cord.
There is no ambiguity to that statement, no “if water gets inside the plug holes” or “if hot fat melts the plastic around the wire”. If any part of the cord gets into water or any other liquid, throw it out. This is almost certainly there purely for legal reasons. Likely the same is true for their warning about kinds of oils. That’s why I quoted an older cookbook above; I specifically went looking for an older book which, if my theory were correct, would not contain such inflexible warnings. This does not prove the theory true, of course, but it also does not falsify it.
Anecdotally, I use lard almost exclusively in this deep fryer. It fries wonderfully and tastes great.