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.