The challenge with preparing “twigim” -- the Korean street food similar to Japanese tempura -- at home lies in managing the leftover oil. Disposing of it after a single use can feel wasteful. Therefore, the question arises: how many times can oil for frying be reused? The answer is contingent upon several factors including temperature, frying method, fried ingredients, and whether a deep fryer is employed. Under optimal conditions, it may be viable to reuse the oil three to four times or even seven to eight times. A study conducted by Seoul National University in 2010 revealed that home cooks could safely reuse frying oil up to three times, with each reuse being spaced three days apart. Remarkably, even after the oil had been used for frying three times and stored for 10 days, its acid value and peroxide levels remained below the standard, and sensory evaluations indicated no abnormalities. Interestingly, whether the frying oil was filtered or stored unfiltered had no discernible impact on its suitability for reuse.

Reusing oil after frying once isn't necessarily bad. In fact, it can enhance the flavor of subsequent frying sessions. This occurs because the flavor compounds from the ingredients dissolve into the oil during the initial frying process. When ingredients are heated in oil, sugars and amino acids undergo the so-called “Maillard reaction,” producing compounds that contribute to the characteristic flavor and browning of fried foods. Therefore, the oil retains some Maillard starter materials that help initiate the reactions after its initial use, facilitating the browning process in subsequent frying sessions.

The difference is noticeable when grilling meat. If you retain the oil in the pan used to cook the meat, and then add a bit more oil before cooking again at high heat, the outer layer will brown more rapidly. This explains why steaks cooked a second time in the same pan tend to have a richer flavor. Similarly, chefs often blend fresh frying oil with reused oil for the same reason—to leverage the enhanced flavor and browning properties achieved through prior use.

While one might assume that frying at home entails using cleaner oil compared to restaurant frying, the reality is quite the opposite. Reused frying oil at home is more susceptible to rancidity than oil used in restaurants. This is because dedicated deep fryers commonly employed in restaurants are engineered to heat the oil from the top rather than the bottom. Consequently, the bottom layer of oil remains at a relatively low temperature. Even if the top of the fryer reaches temperatures of 170 to 180 degrees Celsius, the cooler zone beneath never exceeds 100 degrees Celsius.

Even if small pieces fall into the oil, they tend to sink to the bottom rather than burn. In contrast, when using a pot or pan at home, it's common for crumbs to burn on the bottom and oxidize the oil.

Frying is one of the most dangerous cooking methods at home. In the event the frying oil ignites, there's a risk of fire. Additionally, inhaling the fumes generated during frying isn't conducive to good health. Hence, whether you're toasting or frying with oil, it's advisable to activate the hood and ensure adequate ventilation by opening windows.

You can safely reuse oil unless it becomes rancid and emits a foul odor. However, consuming excessive amounts of leftover oil can lead to an intake of surplus calories. In several respects, purchasing twigim from outside establishments may be preferable to making it at home.

Jeong Jae-hoon is a food writer and pharmacist. He covers a variety of subjects, including trends in food, wellness and medications. This column was originally published in Korean in Joongang Ilbo on Feb. 15, 2024. – Ed.