Modeling heat and mass transfer in immersion frying. I, model development

Deep-fat frying is a rapid and low cost process widely used to prepare tasty food. During this cooking process, oil is used both as the heating medium and as an ingredient producing calorific products. Nutrition has become a major health issue, especially in developed countries where increasing obesity is a problem, particularly among children. Many food research projects involving snack food industries therefore attempt to understand oil uptake during the frying process in order to control and reduce the fat content of fried products without deteriorating their desirable organoleptic characteristics. The main objectives of this paper are to review the literature on the frying process and more precisely the mechanisms and parameters involved in the oil uptake phenomenon. Both products and processes will be considered and their influence via experimental results will be discussed.

Review of mechanisms, conditions, and factors involved in the oil uptake phenomenon during the deep-fat frying process

Water loss and oil uptake as a function of frying time

Oil uptake in fresh, blanched and, blanched and dried potato slices was studied during frying. Potato slices blanched in hot water (85 °C, 3.5 min) and potato slices blanched (85 °C, 3.5 min) and then dried until to a moisture content of ∼60 g/100 g (wet basis) were deep fried in sunflower oil at 120, 150 and 180 °C. A control treatment consisted of unblanched potato slices without the pre-drying treatment (fresh samples). It was studied applying two empirical kinetic models in order to fit the oil uptake during frying: (i) a first order model; (ii) a proposed model, with a linear time behavior for short times, while time independent for long times. Oil uptake was high even for short frying times at the different temperatures tested suggesting that oil wetting is an important mechanism of oil uptake during frying. For control slices, oil uptake increased approximately by 32% as the frying temperature decreased from 180 to 120 °C at moisture contents ⩽1 g water/g dry solid. No apparent effect of frying temperature in oil uptake was observed at moisture contents ⩽0.5 g water/g dry solid in fried slices previously blanched and dried. The two kinetic models studied fitted properly the values of oil uptake during frying, with similar correlation coefficient r2.

Kinetics of oil uptake during frying of potato slices : Effect of pre-treatments

During the last decade much attention has been given to investigating oil absorption during frying. Several studies have suggested the key role of the dehydrated crust, which is thought to control the main mechanisms in oil absorption, that is drainage and capillary suction during post-frying cooling. To contribute to a better understanding of these mechanisms, we have used infrared microspectroscopy at the Synchrotron Radiation Source at Daresbury (UK) to monitor the oil distribution in fried potato cylinders. We report what we believe to be the first quantitative data on oil distribution within a fried product. Results show that the oil distribution within the crust reflects the anisotropic nature of the porous region in accordance with the proposed mechanism of oil absorption. Pendant ces dernieres decenies, une grande attention a ete donnee a l'etude de l'absorption d'huile pendant la friture. Pour une meilleure comprehension des mecanismes d'absorption de l'huile, cet article rapporte une etude sur l'utilisation de la microspectrometrie infrarouge a haute resolution spatiale pour suivre la penetration de l'huile dans des cylindres de pommes de terre pendant la friture. L'hypothese proposee est que le croute deshydratee joue un role tres important dans l'absorption de l'huile, de drainage et de succion capillaire pendant le refroidissement apres friture.

Oil Distribution in Fried Potatoes Monitored by Infrared Microspectroscopy

Using a diffusion model for water transport and a zero order kinetics model for oil absorption, it was possible to correlate both drying and oil absorption for French fries. The zero order rate constant includes terms for texture and for moisture content of the potato strip

Correlation of moisture and oil concentration in french fries

A model for predicting the production capacity of a single drum dryer for potato flakes was developed. The model uses process parameters for drum speed, steam pressure and number of spreader rolls. It also includes the texture of the raw potato, degree of cooking, starch content of the raw potato and the bulk density of the dried raw potato. The model accounted for variation observed due to cultivar and production year

J. C. Craig

Papers

Correlation of moisture and oil concentration in french fries

Effect of potato composition on drum dryer capacity.