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THEORY OF DEEP FRYING
Deep-frying is a cooking process, with which water containing foodstuff is immersed into edible oils or
fats at temperatures between 140 – 180 °C. In the first phase, within a few seconds, a thin crust forms,
whose structure crucially affects the deep-frying process and the quality of the food with regards to fat
absorption and crispness.
Fats and oils have a high heat capacity, thereby enabling heat transfer at temperatures far above that of
the boiling point of water. Due to the evaporation in the boundary zone between food and oil, the water
bound in the food is gradually transported from the inside to the boundary layer into the surrounding oil
(mass transfer). The speed of transfer depends more or less on the structure of the outer crust. As soon
as the transfer of water ends, the temperature inside the food starts to rise above 100 °C. At this point the
typical deep-frying aromas and flavours as well as the gold-yellow colour begin to develop. With the rise of
temperature in the boundary layer to more than 120°C, the formation of acryl amide begins, in particular
in the presence of reducing sugars and asparagine like in grain- or potato products.
The moisture released from the food acts as a protective shield, preventing direct contact of oxygen to
the fat surface. Consequently, frying fats that are constantly used for the preparation of meals deteriorate
more slowly as if being heated up frequently without food.
Deep-frying process – Product safety and quality
During deep-frying not only desired changes occur, like crusting of the outer area, cooking processes
within the food and the formation of typical flavour components, deep-frying also leads to unwanted
changes, which finally results in the deterioration of the frying oil/fat. These changes affect both the quality
of the deep-frying medium and the quality of the fried food.
Changes in the deep-frying medium
Depending upon temperature and the duration of the deep-frying process, the heating of fats and oils
will change the composition of the medium and eventually lead to the degradation of the fat. In order to
produce optimal sensory results the oil must have experienced some initial thermal decomposition. On
the other hand fat degradation is not reversible, the processing must aim at obtaining and maintaining
optimal conditions for the production of tasty food for as long a time as possible. This is not a simple
task, as complex physical procedures and chemical reactions take place during deep frying, which are
influenced by temperature and also by the interaction between the oil/fat, the food item and atmospheric
oxygen.
Decomposition:
Food fats and oils are (from a chemical point of view) mixtures of triacylglycerols (non-polar components),
which are composed of fatty acids and glycerol. Such triacylglycerols are affected by oxygen and heat,
whereby, due to oxidation and polymerization more polar compounds like free short-chain fatty acids,
mono- and diglycerides, aldehydes, ketones, polymers, cyclic and aromatic compounds are formed.
Some of these compounds are responsible for the pleasant flavour of the deep-fried products. At the
same time however, oxidation products such as short-chain fatty acids lead to a decrease of the smoke
point, so that the fat starts smoking at clearly lower temperatures than the fresh fat. In addition it develops
a gritty taste. Polymeric components lead to the formation of foam and increase the viscosity (i.e. the
deep-frying medium becomes more viscous). Especially when palm kernel oil and coconut fat are used,
water from the fried food splits off fatty acids (hydrolysis), forming an eye-irritating smoke due to the formation
of toxic acrolein and free fatty acids. With other fats and oils the hydrolytic splitting rises only to a
very small extent.
Changes in the fried foods
Desired Changes
At the beginning of the process the agglutination of starch and the denaturing of proteins occur. These
changes in the fried food improve the digestibility and taste of the food. The colourization (browning) of
the food and the accumulation of flavour components are to be attributed to the Maillard reaction. The
reduction of the moisture at the surface leads to the development of the crust.
Undesired Changes
Depending upon the surface/mass ratio of the fried food, the amount of fat uptake in the deep-fryer will
be affected, with potato chips or crisps ranging from 30-40%, with donuts 15-20% and with fried potatoes
(french fries, chips) 6-12%.
Since food products absorb the fat, the selection of the frying oil/fat is of nutritional-physiologically relevance.
In addition, the correct processing methods need to be taken into consideration in order to steer
the oil uptake of the fried foods.
The greater part of oil uptake (80%) takes place after removing the fried foods from the fryer. Studies
show that on the surface fat is absorbed through the cooling effect. During deep-frying the water vapours
are released from the inside of the fried foods by small channels. When the fried food is taken out of the
frying medium, air cools down into the channels and develops a vacuum, which absorbs the fat on the
surface and sucks it into the channels. Therefore in order to reduce the fat absorption of the fried foods,
it is important that the fried foods are allowed to drip off under warm conditions.
Acrylamide
The heating of foods (rich in starch and poor in water content) by baking, roasting, grilling, frying and
deep-frying may cause the formation of acrylamide in the presence of protein components like asparagine
and reducing sugars like glucose.
The formation of acrylamide can clearly be minimised through a short deep-frying process with temperatures
not exceeding 165 – 175°C and by the selection of food with a suitable volume/surface ratio. A
large volume and a small surface reduce the acrylamide content of the product further.The rule of thumb
is that food should not be heated longer than necessary, especially not potato products.
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