Revisiting the mechanisms of oil uptake during deep-frying
Résumé
Highlights • Oil penetrates in parfried frozen products during the first minute of deep-frying, and during cooling, once the product is removed from the oil bath. • The mechanisms of oil penetration during frying involves a Carnot cycle mediated by steam between the crust and the frozen core. • All regions connected by fissures are accessible to oil penetration during deep-frying regardless of their water content. • The deep penetration of oil during cooling and immersion stages are under enthalpic control and requires steam condensation. • Surface oil penetration is conversely under entropic control and is possible only when air (non-condensable phase) can be displaced by oil. Abstract Deep-frying is one of the most used and versatile technique for cooking foods. During immersion stage, it has been well accepted that the internal vaporization created an overpressure across the crust higher than the oil capillary pressure. As a result, oil could penetrate inside the food product only during the very last stages of frying or during cooling. This study shows a * Correspondence: olivier.vitrac@agroparistech.fr more complex picture in parfried frozen French fries with oil being capable of penetrating deep inside the product during the first minute of immersion. The new mechanism invokes a Carnot cycle between hygroscopic and frozen regions mediated by steam along cracks and fissures. Oil labeling and microscopic analysis show that both enthalpic and entropic forces drive oil transport and that oil can replace or be replaced by another phase: new oil, water, air. This work suggests new strategies based on thermodynamics to minimize oil pickup in fried products.
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