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Deep frying - what you should know

What is deep frying?  
Deep frying involves plunging food into hot oil at a temperature of 160-190°C, in effect boiling in oil to produce a deep-fried quality. The food remains moist on the inside, and thanks to the good heat transfer properties of the hot oil, the cooking process is very fast.

What happens?  
When food is placed in the hot oil, rapid surface heating takes place. This leads to a quick change in the form of the surface proteins, which in turn allows only a limited flow of water from the product being deep fried.

Since the surface is quickly sealed and liquid flow from the inside is limited by these changes, moisture inside the food stays largely in place. Successful deep frying produces food which is crispy on the outside and cooked right through on the inside.

How is quality achieved?  
For good quality you need to use:

  • A fat/oil specially intended for deep frying
  • The right temperature and timing for the particular food
  • Good equipment, soundly maintained 
If the above conditions are met, the oil for deep frying will be preserved for as long as possible. Poor quality oil for deep frying naturally results in a poor end product.


Ideal temperature for:
French fries - 180°C
Fish - 160-170°C
Croquettes - 170-180°C
Doughnuts, etc. - 160°C

Four factors that affect the ageing of deep frying fats/oils:
Oxygen, light, heat and catalysts.

Oxygen in the air makes deep frying oils turn rancid

Ultraviolet light helps the oxygen in the air to break down the oil. You can mitigate this effect by putting a lid over the fat during a pause in the cooking. You should also cover the oil when the deep fryer is turned off.

The rate of oxidation increases as the temperature rises. If the temperature rises by 10°C, the break down speed is doubled.

For example: A temperature increase from 180°C to 190°C means that the oil will be broken down at twice the rate.

Materials which, although not consumed themselves, break down the oil more quickly.

For example

  • Copper and brass
  • Salt (never salt food over the hot oil)
  • Food remains (filter the oil at least once per day to remove any food remains)
  • Poor cleaning
  • Old and new oils should not be mixed together (since the old might already be oxidised, the new oil will be broken down very quickly).  

Be aware also that water escaping from the frying food has a negative effect on the oil (known as hydrolysis), in which the water breaks down the oil into free fatty acids. These produce a rancid smell and taste. They also lower the smoking and combustion temperature of the oil.

When should I change my deep fat frying oil?

  • if it smokes at a low temperature
  • if it smells and/or tastes bad
  • if it has darkened appreciably
  • if it forms a froth
  • if it has thickened up


A good rule of thumb is:
A fresh oil for deep fat frying behaves like sparkling mineral water when opened. It bubbles up, then the froth subsides. Old oil behaves more like beer: the froth is fine and takes a long time to disperse.

What fat/oil should I use for deep frying?  
There are three types of deep frying fat/oil

  • Semi-solid. In this case, PREP Supreme is a good choice.
  • Liquid, ideal for "normal" deep frying, such as potatoes. PREP Ultra is recommended.
  • Solid, suitable for all kinds of deep frying, especially when many of the foodstuffs to be fried are "fat-destroyers", such as breaded products, doughnuts and other confectionery. Used widely by bakers and confectioners. We recommend our Fritex 35.  


Good practice: summary

  • Deep fry at the correct temperature
  • Don't fry too much at a time: it causes the temperature to sink and the product to become limp or dry.
  • If you have fried fish in the oil, you can expect any French fries fried in the same oil to acquire a fishy taste. Ideally you should use different oils for different products.
  • Never salt food over the oil
  • Clean the fryer every day
  • Put on the lid and lower the temperature when using infrequently
  • Filter the oil at the end of the day an place in the refrigerator.

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