A study published in the Journal of Food Science reveals what happens to food and its microstructure when it is fried. A University of Illinois food scientist set out to examine food’s uptake of oil during frying and how that oil gets distributed throughout the food. During deep frying, as food is immersed in hot oil, water in that food quickly evaporates and steam pressure builds. This pressure affects the microstructure, including the porosity—the number and size of pores in the food as well as the twistiness of the pathways between those pores (tortuosity). This determines how and how much oil gets taken up into the food.
To understand the distribution of oil better, the researchers conducted a study using X-ray micro-computed tomography (micro-CT) to gain 3D images of the microstructure of fried potato disks after they had been fried for various lengths of time. For the study, russet potatoes cut into disks that were 45-mm in diameter and 1.65 mm thick were fried at 190°C for 20, 40, 60, or 80 sec, freeze dried, and scanned.
The researchers collected about 986 2D images of the potato samples and then combined them to produce 3D images. Using the 3D images, they were able to gain more information about the pores and pore networks in the material. The researchers observed that as frying time increased, pore size increased, allowing for greater uptake of oil. They also saw a correlation between oil content and how the network of pathways between the pores changed throughout the frying time. These pathways act like channels for water and vapor flow and oil penetration in the food.
The researchers observed oil distributed across the full thickness of the potato disks. In thicker materials with lots of moisture (like chicken nuggets and French fries), they have observed the oil to remain near the surface as continuous evaporation helps to resist oil penetration.
“It is not easy to make a product that has no oil and still provides taste, flavor, and texture that consumers enjoy,” said Pawan Takhar, study author and University of Illinois food scientist. “People like that fried flavor and the texture of crispiness outside and softness inside. At the same time you want to reduce the oil content to make the food healthier. With this network study we wanted to see how those networks are formed, because networks are also related to texture.” It’s a combination of the oil content and air pockets in the pore structure that provide the desired crispy texture.
