Homendra Kumar Sahu, Pramod Kumar Omre, Priya Gond,
Department of Post-Harvest Process and Food Engineering,
Govind Ballabh Pant University of Agriculture and Technology, Panatnagar Utttarakhand
Rakesh Kumar Gupta, Agricultural and Food Engineering, Indian Institute of technology, Kharagpur,

1. Introduction
Air frying is a cooking method that utilizes hot air circulation to cook food, creating a crispy exterior similar to deep-frying but with significantly less oil content yet retaining a similar texture and appearance to deep-fat frying. Unlike deep frying, air frying has emerged as an alternative and healthier frying technique which intends to produce a variety of fried food. In air-frying, higher temperatures over 140 °C are applied for shorter periods of time, and hot air is constantly circulated throughout the system to preserve nutrients and phenolic compounds. It also inhibits oil deterioration and preserves nutrients and vitamins, which makes fried foods more effective. A hot-air frying process, designed to replace oil with air, has been found to offset the disadvantages of the common frying method.

Although air frying generally produces low-fat products, they exhibit a variety of sensory qualities, including taste, appearance, hardness, crispiness, oiliness, color, and aroma. It also contributes to environmental benefits, such as reducing the amount of oil consumed and generating fewer effluents discarded after frying. A 70 % energy savings were achieved with air frying equipment compared to others. Compared to deep-fat frying, this method uses less oil, reducing problems with oil uptake and degradation and may reduce the formation of acrylamide. However, it takes twice as long to process the product that is why this is a relative disadvantage because the current lifestyle requires agility and rapid meal preparation.

2. Mechanism of air frying
In hot air fryer, high-temperature airflow (140–200 °C) is supplied to surround the food with a tremendous and homogeneous temperature gradient that simultaneously induces a fast mass transfer in a transient state between a hot air medium, water, and oil inside the foods, gradually dehydrating the food and simultaneously inducing the creation of superficial crust similar to that obtained by deep-fat frying. Sometimes, in hot air frying, a small amount of oil is added to improve the organoleptic and textural properties, as in potatoes and other carbohydrate-based products. In other cases, only fat naturally contained in foods is used, as in meat, chicken, and sea foods. Therefore, calorie-reduced foodstuffs containing less fat than those processed by deep-fat frying can be obtained. Some reports indicate that the final content of oil can be reduced to 90 %. To illustrate hot air frying and the heat and mass transfer phenomena involved, Fig. 1a shows a simplified diagram of a home hot air fryer, where the fan is placed on the top of equipment and is responsible for circulating the hot air heated by electrical resistance around the sample in a small chamber. This design allows the circulation of hot air at a high flow rate in a manner that mimics the movement and flow of heat currents produced in a pot of boiling oil, such as in a deep-fat fryer, producing browning and an external crust in the sample as it cooks the inside of the food (Fig. 1b).

Figure: 1. (a) Simplified diagram of a hot air fryer, (b) transport phenomena in the transient state during hot air frying (T: temperature, T∞: air frying temperature, Ts: surface temperature, Ti: internal temperature, q: heat flux, NW: water molecular flux, and NF: fat molecular flux), and (c) comparative images of the processes of raw, deep-fat and hot air frying of sturgeon steaks.

In addition, the airflow inside the hot air fryer is forced on the food, similar to a convection oven. The heat and mass transport phenomena that occur during hot air frying are illustrated in Fig. 1b, which shows a sample cross-section at the initial time. This sample has a homogeneous distribution profile of water and fat, which is immediately subject to a large temperature gradient. Water and fat molecular fluxes increase drastically as frying proceeds, controlled by high heat transfer. At the beginning of the process, the giant water flux can induce a slight decrease in the temperature gradient, but it can be recovered when the heat source remains constant. Then, this phenomenon gradually induces the formation of a crust on the sample surface. Towards the end of the process, the water and fat fluxes decrease until they reach a quasi-equilibrium state or adequate moisture and sensorial conditions for their consumption. Additionally, when a small quantity of oil is sprayed on the sample, it must be considered in the whole mass balance (the addition of oil to the food is optional, but it is added so that the food obtains characteristics similar to those achieved during traditional frying). However, heat and mass transfer are faster in deep-fat frying than in hot air frying. Even though the frying temperature is the same in both cases, longer frying times are needed in hot air frying than in deep-fat frying. In contrast, Fig. 1c shows that foodstuffs produced by deep-fat frying exhibit more browning and thicker crusts than those produced by hot air frying.

3. Quality characteristics of fried food

  • Moisture content: If the quantity of moisture is insufficient, the food will become excessively dry, and in excess, the food will become overly soft, leading to potential rejection by the consumer. The high moisture content of hot air-fried products can affect their shelf life, deteriorating their sensory attributes, modifying their physicochemical properties, and accelerating spoilage by microorganisms.
  • Fat content: During frying, water vapour migrates from the food to the hot oil, and the hot oil penetrates the food simultaneously. Hot air frying technology offers more advantages than traditional frying, such as reduced oil absorption, few calories, and healthy food. This phenomenon is beneficial because a high-fat content in the final product can risk becoming rancid, impacting its shelf life and consumer sensory quality. Factors favouring fat oxidation include light, oxygen, the presence of metals, and water activity.
  • Colour: During the frying process, such as starch gelatinization, Maillard reactions, protein denaturation, dough browning, and breading, affect the development of the crust and the colour of the fried food. According to various studies in air frying, a colour change is generated, reducing the lightness with time and the temperature of the process; thus, the food obtains its characteristic colour.
  • Acrylamide content: Acrylamide is a component generated in fried foods as an intermediate of Maillard reactions among reducing sugars, the amino acid asparagine, and the high temperatures applied. It is affected by many factors, including biological cultivars, fertilization, climatic conditions, soil conditions, storage practices, frying time and frying temperature. Therefore, hot air frying has been proven to generate a lower acrylamide concentration.
  • Texture analysis: The texture of fried foods is mainly characterized by developing a superficial crust, which is a parameter particularly appreciated by consumers. It has been found that hardness values increase with increasing frying temperature and time. It was found that air-fried samosa has a slightly harder texture, higher cohesion, and higher elasticity than traditionally fried samosa.
  • Microstructure: It was found that increased frying time increases water evaporation, favouring porosity and void space formation, and a highly porous structure usually results in accelerated rehydration and reconstitution and improved solubility.

4. Advantages of air frying
Hot air frying has emerged as a modern, health-conscious alternative to traditional deep frying methods. By using rapidly circulating hot air instead of submerging food in oil, it reduces oil consumption by nearly 80–90%, making food lighter and less greasy. The technology ensures uniform heating throughout the product, leading to even cooking and enhanced texture. The resulting crust is thinner, more homogeneous, and visually appealing without irregularities. Additionally, hot air frying significantly lowers the formation of acrylamide, a harmful compound associated with high-temperature frying. Overall, it represents a sustainable, energy-efficient, and healthier approach to food preparation in today’s kitchens.

5. Limitation of air frying
Although hot air frying offers several health benefits, it also has certain limitations compared to traditional deep-fat frying. The process generally takes a longer time to cook food, which can affect convenience and energy efficiency. Products prepared by hot air frying often have reduced crust color, texture, and moisture content, leading to less appealing sensory characteristics. Additionally, mass loss is higher than in deep-fat frying, resulting in slightly drier products. The technology is still developing, and there are limited commercial products specifically designed for hot air frying, restricting its widespread application in food industries.

6. Future Possibilities for Research by Students
Air frying technology offers vast opportunities for future research, particularly for students interested in food processing, nutrition, and thermal engineering. Studies can focus on optimizing air frying parameters such as temperature, time, and airflow rate to enhance product quality and reduce energy consumption. Further research may explore the impact of air frying on nutrient retention, texture, and flavor for various food materials. Comparative analyses of different coating materials, oil types, and pre-treatments could help improve sensory and shelf-life characteristics. Advanced modeling of heat and mass transfer can also aid in designing more efficient air fryers. Additionally, exploring hybrid systems that combine air frying with microwave or infrared heating may open new pathways for faster and more sustainable processing. Overall, student-led innovation in this area can significantly contribute to healthier and environmentally friendly food technologies.

7. References

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Castro-López, R., Mba, O. I., Gómez-Salazar, J. A., Cerón-García, A., Ngadi, M. O., & Sosa-Morales, M. E. (2023). Evaluation of chicken nuggets during air frying and deep-fat frying at different temperatures. International Journal of Gastronomy and Food Science, 31, 100631.

Ghaitaranpour, A., Koocheki, A., Mohebbi, M., & Ngadi, M. O. (2018). Effect of deep fat and hot air frying on doughnuts physical properties and kinetic of crust formation. Journal of Cereal Science, 83, 25-31.

Giovanelli, G., Torri, L., Sinelli, N., & Buratti, S. (2017). Comparative study of physico-chemical and sensory characteristics of French fries prepared from frozen potatoes using different cooking systems. European Food Research and Technology, 243, 1619-1631.

Gouyo, T., Goujot, D., Bohuon, P., & Courtois, F. (2021). Multi-compartment model for heat and mass transfer during the frying of frozen pre-fried French fries. Journal of Food Engineering, 305, 110587.

Heredia, A., Castelló, M. L., Argüelles, A., & Andrés, A. (2014). Evolution of mechanical and optical properties of French fries obtained by hot air-frying. LWT-Food Science and Technology, 57(2), 755-760.

Liu, L., Huang, P., Xie, W., Wang, J., Li, Y., Wang, H., & Zhao, Y. (2022). Effect of air fryer frying temperature on the quality attributes of sturgeon steak and comparison of its performance with traditional deep fat frying. Food Science & Nutrition, 10(2), 342-353.

Nandasiri, R., Semenko, B., Wijekoon, C., & Suh, M. (2023). Air-frying is a better thermal processing choice for improving antioxidant properties of brassica vegetables. Antioxidants, 12(2), 490.

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.