E35 Behavior of oil during frying

Topic

During deep-frying, the food submerged in hot oil undergoes three coupled phenomena: the oil transfers heat to the food via convection—a process described by another model in this set; the water vapor generated inside the food creates—for the duration of the frying process—an overpressure and an outward flow of vapor that hinder the oil from penetrating the interior, a phenomenon described in greater detail by a separate model in this set; and, upon removing the food from the hot oil and allowing it to cool, that same water vapor condenses inside, lowering internal pressure and facilitating the oil's entry into the pores of the food's crust. Prolonged submersion allows heat transfer, water loss, and the formation and modification of the porous structure to continue progressing within the food.

Oil penetration through the porous crust can be described using Darcy's law. The crust's permeability—determined by characteristics such as pore size, connectivity, and tortuosity—influences the ease with which oil can pass through it: a structure with lower permeability impedes oil flow, whereas a network of large, well-connected pores facilitates penetration. Oil temperature also influences the rate of water evaporation and crust development; consequently, an appropriate frying temperature can reduce final oil absorption by promoting rapid surface dehydration and maintaining a strong outward vapor flow throughout the frying process.

Capillary pressure within the crust's pores depends on pore size, interfacial tension, and, crucially, the wettability of the pore surfaces by the oil. When the oil effectively wets these surfaces, capillary forces promote penetration; When wettability is lower, capillary penetration decreases and may even require an additional pressure difference. Furthermore, during frying, the pressure and outward flow of water vapor counteract the oil's entry. Consequently, both reduced wettability by the oil and high internal vapor pressure can limit penetration while the food remains submerged.

However, a significant portion of oil absorption occurs during cooling. When the food is removed from the hot oil, the water vapor within its pores begins to condense, causing the internal pressure to drop. This pressure differential, combined with capillary forces, draws the oil adhering to the food's surface into the pores. Thus, a substantial fraction of the oil ultimately retained may be absorbed after the food is removed from the frying bath, rather than while it remains fully submerged.

Oil temperature and viscosity also influence these processes. As temperature rises, oil viscosity decreases, allowing the oil to move more easily through the porous structure; simultaneously, a sufficiently high temperature intensifies water evaporation, accelerates crust formation, and maintains an outward vapor flow that resists oil penetration during frying. Conversely, an excessively low temperature prolongs the time required for dehydration and crust development, potentially leading to higher final oil absorption. The resulting fat content therefore depends on the interplay between heat transfer, water loss, crust structure and permeability, capillary forces, internal pressure, and oil viscosity—both during frying and the subsequent cooling phase.

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