* Priyanka, Dr. K Narsaiah, Dr. Bhupinder Ghodki, Chitranayak Sinha & Prashant Minz
Frying is the most convenient, easy and effective method of food preparation employed all over the world for various traditional cousines. It is a process of cooking food in vegetable/animal fat at a high temperature ranging between 150 to 190°C. The typical flavor, color and texture attributed to foods by the frying oil (heat transfer medium) during frying is what makes fried food desirable among customers. The unit operation involved in imparting the required texture and color is heat and mass transfer between oil, air and food. Frying can be classified into three categories based on the quantity of frying medium used, which includes sautéing (little fat in the pan), shallow frying (fat comes partly up the sides of the food) or deep fat frying (food completely dipped in fat).
The principle behind frying is dehydration of food. As the surface starts to dehydrate, the crust formed inhibits further oil absorption while heat transfer to the interior via conduction is continued. Heat transfer causes structural and chemical changes in the components of food such as gelatinization of starch, denaturation of proteins and softening of food fibers, thus imparting typical flavor, color and texture to fried food. Many other physical and chemical reactions occurring also contribute to positive changes in the fried foods owing to physical and chemical changes in the various components of food. These reactions include oxidation, hydrolysis, polymerization etc.
Adverse effects of deep fat fried foods
Despite the taste, rapidity of making and ease of preparation, fried foods are not considered healthy for the consequences on bodily functions. Chronic non-communicable diseases (NCDs) are some of the problems associated with fried foods. These include heart diseases, stroke, hypertension, diabetes, mellitus and obesity. There is no clear explanation on the correlation between consumption of fried foods on health. Gadiraju et al., 2015 reviewed the effect of consumption of fried food on the cardiovascular health and found a positive association of chronic diseases with frequent consumption of fried foods. There are many factors that influence the nutritional quality of fried foods, like the time for frying, contact surface area, type of food in terms of moisture content, and last but most important is the frying oil. The oil absorption of the originally healthier foods with low fat content converts them to high energy after frying in oil (Sanchez-Muniz, 2006). The oil absorption differ depending on the type of fried food from 10% to 40% for French fries and potato chips, respectively.
The chemical reactions involved in altering the properties of fried foods during frying by producing volatile and non-volatile compounds include hydrolysis, oxidation and polymerization. While the volatile compounds evaporate with steam in the atmosphere, non-volatile compounds make fried foods unhealthy. The process of oxidation during frying of food is associated with the risk of hypertension due to increase in amount of trans-fatty acids in food. The amount of unsaturated fatty acids is found to decrease in oils during deep frying along with other changes including changes in foaming characteristics, changes in chemical composition (amount of free fatty acids, polar materials, polymeric compounds) and changes in physical properties(color, viscosity, density, specific heat) (Choe and Min, 2007). The products of oxidation of frying oils such as acrylamide (a highly reactive polar organic substance), have toxic health effects including ataxia, memory impairment, decrease of serum level, mitochondrial dysfunction etc. as reviewed by many researchers. Repeated heating of oil ignites several chemical reactions leading to degradation products, makes it compulsory to change the oil for safer products which then adds to the cost of the product.
Due to the negative health effects deep frying, focus is shifted on alternatives frying methods providing the same properties in terms of taste and flavour, enhanced nutritional quality and ease of use. Air frying is emerging as the suitable alternative for producing fried foods with lesser absorption of fat on the food surface. This technology uses hot air as the heating medium instead of hot oil, as is used in conventional frying. Hot air is circulated using a fan near the heating element. In conventional frying, submerging food in oil at high temperatures of between 140 to 165°C causes Maillard reaction to occur which induces the required color in the fried foods. In air frying, browning is achieved by covering the food by a layer of oil vapors in the form of oil mist and circulating hot air at 200°C to start the browning reaction (Harold, 2004). It gives the appearance of crust on the surface of food which is obtained by deep frying traditionally.
Fig: Diagrammatic explanation of working of an air fryer
After being first launched in Europe, air frying has become popular frying technology all over Europe for the short time of cooking, pleasant taste and absorption of around 80% less fat as compared to conventional frying (Zaghi, A.N. et al., 2019). Lower lipid degradation and lesser oxidation reaction compounds in the air fried foods along with lesser fat absorption, makes it a healthier substitute for conventional deep fat frying. Environmental advantage is added with reduced oil consumption and effluents emitted (Shaker, M. A., 2014). Energy savings is another added advantage when air frying is analyzed against deep fat frying, as air frying is reported to show 70% more energy saving compared to conventional frying techniques (Giovanelli, G. et al., 2017). There are some constraints observed in dis technology as well such as enhanced water losses due to thinner crust formation. The losses are avoided in deep fat frying due to rapid hard crust formation. For the similar thickness of crust to avoid mass losses, air frying requires longer frying time (double than that of conventional frying). Air frying presented lower lipid and phenolic compounds degradation.
Despite the few limitations air frying is considered a healthy alternative since there is significant reduction in oil uptake by food, for the health conscious generation. Also it would be an economical option for fast food businesses since it cuts the vegetable oil expenses. In terms of the overall sensory attributes, air fried foods were preferred in terms of food properties after frying including textural properties (crispness, hardness, oiliness) and color. Till now, this technology has not been studied in detail and should be explored for evaluation of its mechanism of action on properties of food and followed effects on human health, so that it can replace deep fat frying on commercial scale.
Sanchez-Muniz. (2006). Oils and fats: changes due to culinary and industrial processes. International journal for vitamin and nutrition research, 76(4), 230-237.
Choe, E., & Min, D. B. (2007). Chemistry of deep‐fat frying oils. Journal of food science, 72(5), R77-R86.
Zaghi, A. N., Barbalho, S. M., Guiguer, E. L., & Otoboni, A. M. (2019). Frying process: From conventional to air frying technology. Food Reviews International, 35(8), 763-777.
Giovanelli, G., Torri, L., Sinelli, N., & Buratti, S. (2017).Comparative Study of Physico-Chemical andSensory Characteristics of French Fries Prepared from Frozen Potatoes Using DifferentCooking Systems. European Food Research Technology, 243(9), 1619–1631.
Shaker, M. A. (2014). Air Frying a New Technique for Produce of Healthy Fried Potato Strips. Journal of Food and Nutrition Sciences, 2(4), 200–206.
Priyanka, recently joined as Scientist at ICAR-National Dairy Research Institute, Karnal in Dairy Engineering has 7 years of research experience in food technology with specialization in dairy. Her area of expertise includes novel food processing techniques like ohmic heating, machine design and nano-technology.