Ancient artifacts from Babylonia, 5,000 years old, suggest that “frying” was used as a cooking method in those times. In addition, artifacts from ancient Egypt suggest “deep frying” was popular 4,000 years ago. Considering how long frying has been part of our history, it’s surprising how little people in the cooking industry (and the public at large) actually know about how frying works, and how to extend the life of cooking oil in their fryer.
While cooking in oil is nothing new per se, it’s only in the last hundred or so years that major changes have been made. Traditionally, cooking was done in animal fat, tallow or “lard”. Near the turn of the twentieth century, people recognized that animal fat had a short shelf life and was becoming expensive. In 1903, hydrogenation was introduced as a way to improve vegetable oils. By hydrogenating the vegetable oil, the cooking temperature, melt point and shelf life were increased. However, the hydrogenation process created trans fats, which over the last several decades has been shown to pose a health risk. Today’s oil selections include “High Oleic Acid” oils (derived from canola, sunflower, or soybean), which offer many benefits, including:
- A healthier choice
- Easier to clean
- A pleasing flavor profile
- A more desired appearance and texture
Breaking Down & Extending Cooking Oil Life
To better understand methods to extend cooking oil life, let’s take a look at the chemical and physical changes that occur during the cooking process.
- First, heat is applied to the oil to bring it to the proper frying temperature. Whether the oil is being used for cooking or not, it’s also certain that the oil will be exposed to air and light, and heat, air, and light are three of the many components that break down oil.
- After reaching the desired temperature, a food product (protein, starch or vegetable) is added to the oil where almost immediately a “crust” begins to form on the food product via the excellent heat conduction properties of cooking oil.
- As the heat penetrates the product, the water inside the product converts to steam and expands approximately 1,600 times in volume, creating a pressurized “steam envelope” around the product, pushing the oil away from the product.
To the operator, it appears the product is “boiling in the oil,” however, the temperature inside the product never exceeds 212⁰F at normal atmospheric pressure.
The steam released from the product reacts with the oil in a process called hydrolysis, further breaking down the oil. If the product is over cooked, the steam envelope will collapse (no more water) and the oil will directly contact the food, overcooking the surface, and penetrating deeply into the product, leaving it both dry and greasy to the palate. Any “cracklings” that fall off the food during this process, fall to the bottom of the fryer, and further degrade the oil as those bits overcook. Also, if the fryer temperature is set to low, the oil begins to penetrate the product before the steam envelope forms, resulting in a greasy texture.
During oil’s cooking life, there are several stages from fresh (new) to fully oxidized:
- When new oil is nearly clear to white, it has a higher viscosity (or “thickness”) and surface tension so it resists penetrating the food. Typically, the final product is not browned enough and lacks the fried flavor profile.
- During “breaking in”, the viscosity and surface temperature reduce and the oil starts to become a brown color, imparting slight coloring of the food and some flavor profile.
- At the “optimum” stage, the product gets a good browning and optimal flavor profile from oil penetration (or absorption).
- The final stages to full oxidation, include degradation and runaway; the viscosity and surface tension are very low (thin versus thick) and too much oil is absorbed in the food, food color is generally too brown, and the flavor and odor are undesirable.
Optimum Cooking Stage of Oil Life
So, from the cooking process, we know that heat, water, light, air, and debris all contribute to reducing oil life. Knowing that, operators should take the following steps to stay in the optimum cooking stage of oil life:
- Turn the temperature down to a “ready” state (below 300⁰F) when the fryer won’t be used for long periods.
- Shake excess water (or ice) and debris off the product away from the fryer before frying.
- Keep the cover over the frying kettle when not in use to minimize exposure to air, light, water and debris.
- Filter frequently to remove debris (or cracklings). “Active” filtration with a powdered reagent (or impregnated pad) is far more effective. The reagent neutralizes undesirable, free fatty acids and other polar compounds that oxidize oil. With or without active filtration, filtering improves the color of the oil.
- Use a filtering method that minimizes exposure to air.
- Oil exposed to soft metals (bronze, brass, sodium, etc.) or detergents will breakdown extremely rapidly. It’s, therefore, important to insure the fryer, filtering mechanism, and accessories do not use brass or bronze.
- Avoid exposure to sodium (or salt) by seasoning fried products away from the fryer, never over the oil.
- If soap must be used to clean the frying kettle, ensure the fryer is properly neutralized, rinsed, and dried before refilling with oil.
While extending the life of cooking oil saves money and time, more importantly, it also makes the finished product look and taste better.