Frying Tonight!

Introduction

Frying foods in hot oil has been a common cooking practice for all those looking for crispy textured and flavoursome foods. The rapid and simultaneous heating and mass transfer of oil, food, and air during deep-fat frying produces the desirable and unique qualities so special to fried foods.

The process of frying is extremely versatile and flexible (Raoult-Wack et al., 2000). It ranges from cooking at a domestic level with a frying pan or saucepan of hot oil to massive industrial fryers that continuously process fried food at tonnes per hour.

A commercial chip fryer might generate product rates of 7,000 lb/hr with a feed rate of 26,000 lb/hr which demands high oil usage and energy.

What does frying do to a food from a basic sensory point of view?

-The Frying Process

Frying is a process of very rapid heat transfer which is why it is so effective. It is also extremely complex. It involves simultaneous heat and mass transfer between the frying medium and food. It causes rapid chemical changes which alter and  improve the sensory appeal of the food. That rapid heat flux is transferred from the oil to the food surface using convection and from the surface of the food into its core by conduction (Farinu & Baik, 2005).

Mass transfer, especially of water involves egress of moisture from the food through capillary pressure and molecular diffusion (Moyano & Berna, 2007). At the same time there is movement of oil into the empty spaces of the food when the water has been driven out.

From a sensory perspective, frying generates a crispiness which can be seen best in potato crisps and fries, differing degrees of golden colour to browning and even burning. Frying also gives a food a particular fried flavour found in no other process. Air expansion cooking and even air frying have yet to match the pearl that is deep-fried flavour.

Advantages and Disadvantages

There are few other heating technologies that offer the same advantages: the efficient heat transfer ensures the food has a crispy texture and fine taste. In terms of food safety, it destroys pathogens making it safer than other conventional heat processes, it is quick and convenient and cooks the food effectively.

There are some disadvantages too. One aspect to contend with is the degradation of the cooking medium – the oils themselves. A great deal of research has been devoted to understanding the mechanisms of degradation.

Frying is also dangerous if not monitored carefully as the oils are usually flammable. The evaporating water is also known to carry volatile organic compounds which pose a safety hazard themselves through ingestion or breathing. Entrained oil droplets contribute to pollution.

From a nutritional perspective, frying also implies obesity and heart disease because of the fat residues which remain within the food.

However, advanced food frying technologies are being developed to minimise the impact of retained fat or removing altogether. In fact, whilst fat will contribute to a certain characteristic mouthfeel it can also ruin crispiness when it is retained and so its removal is an imperative from both the health and sensory perspective.

Types of Oil For Frying

The choice of frying oil is a critical decision in snack production, as it influences the sensory attributes and quality of the final product.

Generally, vegetable oils are preferred but each individually shows varying degrees of stability during the frying process. The main Vegetable Oils are:

• • Soybean Oil: Soybean oil is widely used due to its neutral flavor, high smoke point, and affordability.
  • Sunflower Oil: Sunflower oil is known for its high smoke point and light, neutral taste.
  • Canola Oil: Canola oil is low in saturated fat and offers a mild flavor, making it suitable for various snack products.

Corn oil is chosen for its mild flavor and high smoke point, making it suitable for frying snacks. Rapeseed oil is ideal for stir-fries etc. but olive oil is not so even though its use lends authenticity in Mediterranean cooking for example. Palm oil is another commonly used frying oil due to its stability at high temperatures. It is often used in snack production, especially in regions where it is readily available. Sesame, avocado and other culinary oils are not used because of their propensity for degradation. Cottonseed oil has a high smoke point, which makes it a viable option for snack frying. However, it may have a slightly stronger flavor. Some manufacturers use blended oils, which combine the desirable qualities of different oils. Blends can offer a balance of flavor and performance.   

The better frying oils are generally unsaturated even though economically they have a lower shelf-life with continued use. However, given oil is frequently removed with the food the issue is not as constraining on costs.

Frying oil degrades with heating over time. The rate of degradation increases with heating temperature. A number of reactions occur such as oxidation, polymerisation and hydrolysis. Oxidation produces rancid off flavours and other off tastes which are imparted to the food being fried. The polyunsaturated oils are more vulnerable to oxidation that more saturated oils. However, saturated oils are regarded as more unhealthy because they contribute to raising blood cholesterol levels which is associated with heart disease.

To improve thermal stability, polyunsaturated oils are partially hydrogenated which unfortunately leads to the production of trans-fatty acids which are unhealthy too. Free fatty acids (FFAs) have a bearing on the quality of the frying oil. The FFAs increase the thermal oxidation of oils and their unsaturation, rather than their chain length, which leads to significant losses in frying power. The oxidation rate of oil increases as the content of any unsaturated fatty acids in the frying oil rises. One fatty acid, linolenic acid and its content is essential in frying as it influences oil stability and ultimately the flavour quality of the fried food (Liu and White, 1992; Xu et al., 1999).

Fries are highly valued for their colour, texture as a crunch, and flavour. Studies on optimising the crispiness of chips and fries without too much oil uptake have been extensively examined (Pedreshi and Moyano, 2005). Optical sorters can remove those too lightly coloured for further frying or those too dark allowing them to be rechecked for inclusion.

Recent development have included developing non-digestible oils. A good example is the sugar polyester called  olestra™. It suffers however from causing gastric distress in some and has not been exploited as fully as possible. One other issue is its high melting point which makes it difficult to melt and thus transport. It also has a peculiar crystallisation property.

Conventional Frying

Most industrial processes rely on a standard technique of bathing the food in very hot oil. Foods such as snacks, chips or fries, fish, etc. are fried at atmospheric pressure usually in  batch containers or if larger systems are employed, continuously using conveyoring. The food is moved through hot oil continuously – you can often see doughnuts and pastries being fried this way.  The process is so controlled that time, temperature and positioning of the food in the oil is strictly monitored to achieve consistency and quality with minimum cost. The bigger the food, the longer the processing required because of the larger distances needed for heat to reach the critical central point within the food. If a food floats such as a doughnut then at some point, it needs to be flipped over part way through frying to obtain a similar heating to the initial product.

Fryer design affects oil deterioration too. Rapid heat transfer prevents the formation of hot spots and oil scorching. Polymerised fats deposited in the fryer causing gum formation, foaming, darkening of oil colour and ultimately loss of frying potential. A small surface-to-volume ratio in the fryer is ideal for achieving the minimum content of oil with the air (Negeshi et al., 2003).

Oil is heated directly using burning fires, electricity and gas. Frying occurs at the relatively high temperatures of 150 to 190 °C.  Rapid heating is achieved using conventional heat exchangers and even high pressure steam.  Some heat exchangers rely on preheated oil or water to maximise energy recovery. Oil needs to be filtered regularly to remove any charred fragments, burnt food particles and even air because they all promote degradation. Entrained oil droplets are also removed and this too helps in oil recovery to reduce costs further.

 

Good Practice In Frying

 – Surface treatment and draining of fried food.

Mass transfer between food and oil is most affected by the surface of the food. The types of materials moving across the boundary include primarily fats and water, polar compounds and cracking particles. Crackling which is heard when fresh food for example enters the fryer causes oil degradation. Any mass transfer from oil to food always involves absorption of oil (Quaglia & Bucarelli, 2001).

To reduce crackling, breadcrumbs and batter must be shaken or blown loose. Air flow to the blower required for coating a food must be optimised. This helps reduce oil degradation and maintains a relatively high level of oil.

The cooked food must be drained immediately after frying to prevent as much overall oil absorption as possible

– The Correct Cleaning Of The Fryer

All traces of polymerised oil which is linked to edible oil degradation must be removed in the cleaning process. The power of a hot caustic wash with detergent is without par. It needs to be performed weekly when fryers are used heavily.

The fryer needs to be well rinsed and drained to remove traces of detergent before oil is added. One of the main causes of edible oil degradation is the presence of alkaline cleaning chemicals. Good operators will also check for the presence of detergents.

More developed frying processes are coming onstream as we now discuss.

Monitoring Oil Quality

The flavour of the finished product is paramount. This depends on the quality of the frying oil. To ensure peak performance the oil quality needs to checked at all stages of the operation especially where degradation is likely to occur. The methods employed include:

• peroxide value (PV) a measure of the degree of oxidation during oil storage and handling. This value should be monitored regularly before oil is heated and following use.
• Active oxygen method: a measure of the suitability of oil when processing breaded products in particular. Good for checking crispness
• Total polar materials (TPM): a method for monitoring the extent of oil degradation during processing. This is a measure to be taken daily and especially before the start of frying. It is an easier measure to obtain than one for oxidized fatty acids. Quoted as a percentage
• Free fatty acids (FFAs) – another measure for degree of degradation during processing because of hydrolysis. Another daily measure.
• Polymeric materials (%). Used to determine oil degradation especially due to continuous exposure to heat and improper cleaning. Another measure for pre-frying checks.

Pressure frying

Meats such as chicken is cooked this way in various foodservice situations.  Pressure frying is currently a batch process. It relies on using a pressured vessel where released steam increases the pressure so raising the boiling point of water and thus the temperature at which the food is cooked is lowered to 175 °C-184 °C. This shortens the cooking time significantly and improves moisture retention compared to atmospheric frying.  It is thought that pressure frying would reduce the oil’s usable shelf-life but as the atmosphere is largely steam, the air is displaced and so the chance for oxidation is minimised.

Frying oils interact with foods such as those coated or containing salt. This hydrolyses the oil to mono- and diglycerides and then to less useful free fatty acids and glycerol. Glycerol in turn will breakdown into acrolien which is a relatively unpleasant smelling compound and a component of tear gas. Hot oils also form carbon deposits on surfaces which are difficult to clean.

The more oil that is removed the product, the fresher on average the oil left in the fryer. Fresh oil must be added to replenish the level though. When steps are taken to reduce oil pickup, the average age of the oil increases and there is a greater risk of rancidity and degradation. In the extreme, the oil that is too aged must be removed which creates waste and adds to production downtime.

Waste oil issues are being resolved by converting the waste oil into a biodiesel or other fuel. Waste oil is relatively cheap and plentiful compared to palm oil or other waste oils. Some restaurants are even paid for disposing of it to processors which contradicts earlier practices where processors had to pay for disposal. The current issue now is minimising the variability in such waste materials.

Vacuum Frying

Vacuum frying is the converse of high pressure frying and is more popular in Asia than in other continents. Here, the adverse characteristics of frying generally can be reduced because of the use of lower temperatures than at atmospheric or high pressure. Lowering the pressure lowers the temperature of water boiling and thus using a lower oil temperature. For fruits and chips, most atmospheric frying occurs at 180 C but under vacuum the oil temperature is reduced to 160 °C which significantly helps economically.

In atmospheric frying, the oil content of a fried food is between 30 to 40%. In vacuum frying, the oil content is reduced to between 20 to 25%. Another benefit is the reduction in formation of acrylamide which is a breakdown product  of nitrogen rich proteins in particular and a potent carcinogen and neurological poison (Granda et al., 2006).

The shelf-life of the oil is also increased because the temperature of processing drops. The oil does not breakdown as rapidly. Likewise, the oxygen exposure is also reduced in a vacuum system. Polyunsaturated frying oils are more popular in vacuum frying because they do not degrade so readily.

Like pressurised systems, vacuum frying must be conducted in batches. It has not yet found favour with industrial manufacturers because of the costs.

Reducing Retained Oil On Products

Various innovators have sought to overcome the various issues of frying. Charlie Baggs Culinary Innovations (Chicago, Ill. USA) (www.charliebaggsinc.com) has developed SpinFresh™ technology for frying breaded and battered chicken. The company’s researchers found that fried chicken had less retained oil whilst keeping their flavour, colour and texture which was similar or better than chicken fried in a conventional restaurant fryer.

SpinFry Inc., Woodbridge, Ill. USA (www.spinfresh.com) has developed a number of frying systems using centrifugal forces to spin out the oil from the food’s surface. The food is batch fried in a basket containing the frying oil which is then removed and spun above the bath. Surface oil is removed and returned to the heating bath whilst the food continues to cook in the convective heat above the hot bath. SpinFry has leased its technology to Middleby Worldwide, Elgib, Ill. (www.middleby.com). They manufacture equipment for foodservice providers such as pizza ovens. Midddleby is also marketing a vented model under the Pitco™ brand and a countertop model by Perfect Fry Co.

1st version: 23/01/2015. Revised with addition of new material on monitoring etc. 16/10/2020.

References

 Liu, H.-R., White, P.J. (1992) High-temperature stability of soybean oils with altered fatty acid compositions. J. Am. Oil Chem. Soc.  69 pp. 533–7.

Negishi, H., Nishida, M., Endo, Y., Fujimoto, K. (2003) Effect of a modified deep-fat fryer on chemical and physical characteristics of frying oil. J. Am. Oil Chem. Soc. 80  pp.163–6

Quaglia, G.B., Bucarelli, F.M. (2001) In: Frying: Improving Quality. Edt.J.B. Rossell. Woodhead Publ. Food Sci. Technol (CRC) ISBN 1 85573 556 3

Pedreshi, F., Moyano, P. (2005) Oil uptake and texture development in fried potato slices. J. Food Eng. 70 pp. 557–63.

Raoult-Wack, A.L., Vitrac, O., Trystram, G., Lucas, T. (2000) Water-mediated phenomena in some food processes. In:  8th International Symposium on the Properties of Water, ISOPOW 16–21 September 2000, Israel, pp. 16–22.

Xu, X.-Q,, Tran, V.H., Palmer, M., White, K., Salisbury, P. (1999) Chemical and physical analyses and sensory evaluation of six deep-frying oils. J. Am. Oil Chem. Soc.  76  pp. 1091–9.