In deep-frying, the first batch is never the best. Food writer Russ Parsons explains in How to Read a French Fry that fry oil has five stages: break-in (too fresh to fry well), fresh, optimum, degrading (on the way to spoiling), and runaway (dark, smelly, and prone to smoking). Food fried in optimum oil is golden and crisp. Break-in and fresh oil yield paler, less crisp food. Degrading and runaway oil produce dark, greasy food with rancid odors. Could we create optimum oil by mixing used and new?
We fried shrimp, fish, and French fries in fresh oil and in a mixture of fresh oil and oil that had been used once to make French fries (and then strained through a coffee filter to remove any solids).
Food fried in the mix of new and used oil was crisper and more uniformly golden than food fried in fresh oil.
Oil that is too fresh can’t penetrate the barrier of moisture that surrounds food as it fries. Over time, as the oil continues to be exposed to heat, it breaks down, producing slippery, soaplike compounds that can penetrate the water barrier. This increased contact between oil and food promotes browning and crispness.
Save a cup or two of used oil to mix with fresh the next time you fry (we found that a ratio of 1 cup of used oil to 5 cups of fresh oil worked best). Just make sure that you don’t save oil used to fry fish—the smell will permeate the new oil.