Frying - Theory and Equipment

Food Technology and engineering - Frying theory and Equipment
professional , FSSAI Standards

1. FRYING – THEORY AND EQUIPMENT DN VAISNAVI 17BBT057

2. CONTENTS : Introduction Theory Types of frying: deep fat , shallow , dry , stir , air fryer Equipment oil selection Effect on food Advantages and disadvantages

3. Introduction : Frying is a unit operation which is mainly used to alter the eating quality of a food. A secondary consideration is the preservative effect that results from thermal destruction of micro-organisms and enzymes, and a reduction in water activity at the surface of the food (or throughout the food, if it is fried in thin slices). The shelf life of fried foods is mostly determined by the moisture content after frying, food that retain moisture content have a relatively short life period . Foods that are more thoroughly dried by frying , for example potato crisps , maize and other potato snack foods, have a shelf life of up to 12 months at ambient temperature. The quality is maintained by adequate barrier properties of packaging of materials and correct storage conditions .

4. FRYING THEORY : When food is placed in hot oil , the surface temperature rises rapidly and water is vaporised as steam. The plane of evaporation moves inside the food, and a crust is formed. The surface temperature of the food then rises to that of the hot oil, and the internal temperature rises more slowly towards 100 degree celisius . The rate of heat transfer is controlled by the temperature difference between the oil and the food and by the surface heat transfer coefficient. The rate of heat penetration into the food is controlled by the thermal conductivity of the food The surface crust has a porous structure, consisting of different-sized capillaries. During frying, both water and water vapour are removed from the larger capillaries first, and replaced by hot oil. Foods that retain moisture interior are fried until the thermal centre has received sufficient heat to destroy contaminating microbes , specifically meat products.

5. THEORY CONTI….. The time taken for food to be completely fried depends on : the type of food the temperature of oil the method of frying ( shallow or deep-fat frying) the thickness of the food the required change in eating quality . The temperature used for frying is determined mostly by type of food product and economic consideration , At high temperatures (180–200oC), processing times are reduced and production rates are therefore increased. High temperature causes deterioration of oil and formation of free fatty acids , which also alters the viscosity , flavour and colour of the oil and promote foaming . This increases the frequency with which the oil must be changed and hence increases the costs

6. TYPES OF FRYING : SHALLOW FRYING (CONTACT FRYING): Involves the cooking of food in layer of hot fat that comes half way up the food. Heat is transferred to the food mostly by conduction from the hot surface of the pan through a thin layer of oil. The thickness of the layer of oil varies as a result of irregularities in the surface of the food , together with the action of bubbles of steam which lift the food off the hot surface, causes temperature variations as frying proceeds and produces the characteristic irregular browning of shallow fried foods. Shallow frying has the highest surface heat transfer coefficient 200 – 450 W .  

7. Types of frying : 2 . DRY FRYING : Some foods Eg :sesame seeds for purpose of fine grinding, bacon , sausages etc can be fried without the addition of fat , as they contain sufficient fat to prevent sticking to the pan . Non stick pans are most suitable for this type of cooking

8. TYPES CONTI….. 3.STIR FRYING: All ingredients should be finely chopped into pieces so that it cooks quickly. A teaspoon of oil is dropped in the pan and the food is put in and stirred continuously to ensure even cooking. Few nutrients are lost and the food remains crisp and well flavoured. It is quick and healthy method of cooking . Stir frying of vegetables is an example.

9. Types conti ….. 4 . AIR FRYER : An air fryer is a kitchen appliance that cooks by circulating hot air around the food using the convection mechanism . A mechanical fan circulates the hot air around the food at high speed, cooking the food and producing a crispy layer . The food itself sits in a perforated basket which increases its contact with the hot moving air.

10. TYPES CONTI…. 5 . DEEP FAT FRYING : Heat transfer is a combination of convection within the hot oil and conduction through the interior of the food. All the surface of food receive similar heat treatment to produce a uniform colour and appearance. Deep-fat frying is suitable for foods of all shapes, but irregularly shaped food or pieces with a greater surface : mass ratio tend to absorb and entrain a greater volume of oil when it is removed from the frier . Heat transfer coefficients are 250–300 W  

11. Equipment :

12. EQUIPMENT : In batch operation the food is suspended in a batch of hot oil and retained for the required degree of frying . Continuous deep-fat friers consist of a stainless steel mesh conveyor which is submerged in a thermostatically controlled oil tank. Depending upon the products there are different types of conveyer belts.

13. EQUIPMENT : The size of pieces, conveyor speed and oil temperature control the frying time. An inclined conveyor then removes the food and allows excess oil to drain back into the tank . The equipment operates automatically at production rates of up to 15 t of fried product per hour. Oil is continuously recirculated through external heaters and filters to remove particles of food, and fresh oil is added automatically to maintain the desired level in the tank. These features extend the useful life of the oil by removing food particles that would otherwise burn and affect the flavour and colour of the product . The viscosity of the oil is important for optimum heat transfer and minimum entrainment in the food. viscosity is achieved when the oil is heated until the free fatty acid content reaches 0.4%. Methyl silicone may be added to prevent foaming .

14. Oil selection depends on : Which should be suitable to maintain : Product flavour Product texture Product appearance Shelf life of product Nutritional requirement Any selection of frying oil must be : Low in saturated fat Low in linolenic acid High oxidative and flavour stability

15. Effect on foods:

16. Effect of heat on oil :

17. Effect of heat on oil :

18. Effect of heat on fried foods : Main purpose – to develop characteristic colour , flavours and aroma in the crust of fried foods. The main factors that control the changes are : The type of oil used for frying The age and thermal history of oil The interfacial tension between oil and the product The temperature and time of frying The size , moisture content and surface characteristics of the food Post frying treatments

19. Conti……… Each of these factors, together with the pre - treatment such as blanching accounts for 45% oil content in the product. Excess fat consumption leads to various heath issues such as obesity , coronary heart disease and cancer – use of vacuum frying plays a major role here ( air fryer – les consumption of oil but costly). The texture of fried food is produced due to changes to proteins , fats , polymeric carbohydrates which is similar to those produced by baking Changes of protein quality results in mailliard reaction with amino acid in the crust . Fat content of the food increases owing to oil absorption and entrainment . Difficult to determine nutritional significance , as it varies to number of factors.

20. Reference : https://www.slideshare.net/ranasalva/vacuum-frying-technology https://www.slideshare.net/bavaneethan/baking-and-roasting-and-frying https://www.slideshare.net/mahmudulmithun/frying P. fellows book