New Oil, Old Oil

Apple fritter lover Emma wants to know whether it’s OK to combine fresh oil with older oil when you’re frying. She says she keeps seeing recipes that specifically instruct her never to do that. I’ve seen those as well, Emma, and all I can think is that none of these folks have done very much frying, for not only can you combine old fry oil with new, you absolutely, positively should.

Which of course raises the question: why? The short answer is because oil and water don’t mix. Drop a wet food like doughnut batter into a pan of hot oil and the food and the medium will repel each other. That’s good to a large extent, since that action — combined with the outrush of steam from the food — is what’s responsible for keeping food free of soaked-in oil.

Over time that mechanism breaks down, however. Heat and oxygen exposure take their toll on fat molecules, breaking them into smaller pieces. Some of these pieces are chemical soaps. What do soaps do? Why, they allow fat and water to mix of course. So as the proportion of soaps in the fry oil increases, oil starts sneaking past the steam and water barrier, soaking into the food and creating a limp and greasy end product.

The thing is, on the one hand you don’t want fry oil that’s too old and soapy. On the other you don’t want your oil to be too totally fresh either. No soap whatsoever means the oil will stay so far away from the food you’ll hardly get any surface drying or browning, which is the whole point of frying. You’ll also sometimes get a funny, almost synthetic, aftertaste.

Avoid the trap of pathologically fresh fry oil by cooling, storing and re-using it. A single two or three-quart batch should be good for half a dozen uses, provided you’re not frying ten pounds of fritters at a time. Just top it off to whatever level is appropriate and carry on with confidence, knowing that you can tell too-old oil by its dark color and its fishy smell (not actually caused by fish but by smelly chemical compounds called ketones, a by-product of oil breakdown). When it finally comes time to throw it out, save a couple of tablespoons to infuse the next batch with soaps.

Oh and, you know never to deep fry in cast iron, yes? Iron speeds oil breakdown by about 100 times over stainless steel. A sure way to ensure your oil will only last you for one use.

15 thoughts on “New Oil, Old Oil”

    1. Nope, as long as the cast iron is covered by the coating you’re good!


      – Joe

  1. Does the type of oil — canola, corn, sunflower — affect the result? Sunflower seed oil smells dreadful after one use. I can’t imagine using even a spoonful in the next batch.

    1. Hey LML!

      The type of oil does indeed matter, and some simply break down faster than others. If the oil is a lost cause when you’re done then it’s best to dispose of it…but keep a couple of spoonfuls behind to add to the next batch!


      – Joe

  2. Hi Joe,
    This is off topic for this question, but something that you might be able to help me with.
    A few months ago I won a “Le Crueset” cast iron saute pan, with a wooden handle. I know that you are supposed to season the pan before use, but I’ve only been able to find methods that involve putting the entire pan into the oven. I’m reluctant to do that with the wooden handle. Can you suggest a method? I’ve tried just heating oil in the pan, but that doesn’t do the trick.

    1. Great question. I’ll answer that on the blog if you don’t mind!


      – Joe

  3. As a chef who works a fast serve seafood restaurant and cooks a large amount of fish and chips, and is frustrated as anything every Monday when we have fresh oil in the fryer, all I can do is agree fervently.

    We always keep a little bit of the old oil, filter it well and add it to the new batch just so things don’t look too anemic on Monday until we dirty up the new oil appropriately. If that doesn’t happen(the old oil was burned for whatever reason, or there just wasn’t time to filter it), I’ll add a handful or two of flour to the oil just to help get the breakdown started before service.

    1. Great stuff, Pete! And it’s always nice to have a little validation from a pro fryer. Thanks very much,

      – Joe

  4. Hi Joe,

    What about a well- seasoned cast iron pan? Does that have the same effect for deep frying as an enamel-coated one?


    1. Hello Tanyeem!

      Once the pan is seasoned it’s good to go. Deep frying in it will only add to the seasoning layer you already have going. The more you use it, the more resistant to soap it becomes and the better it performs.

      Thanks for the question, Tanyeem!

      – Joe

  5. Love your website! What is the best oil for frying pastry and sweets, such as cannoli, zeppole, etc? The Babbo cookbooks all use olive oil, which I find odd, but I otherwise like the books.


Leave a Reply

Your email address will not be published. Required fields are marked *