frying process
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The frying process
Processi della Tecnologia Alimentare - Prof. Davide Barbanti
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Processi della Tecnologia Alimentare - Prof. Davide Barbanti
Frying is a unit operation which is mainly used to alter the eating quality of afood. 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: foods that retain a moist interior (for example doughnuts, fish and
poultry products which may also be breaded or battered), have a relatively
short shelf life, owing to moisture and oil migration during storage.
These foods are important in catering applications and are produced on a
commercial scale for distribution to retail stores, preserved by chilling and/or
gas packing. Foods that are more thoroughly dried by frying, for example
potato chips, maize and other potato snackfoods, have a shelf life of up to 12
months at ambient temperature. The quality is maintained by adequate barrier
properties of packaging materials and correct storage conditions.
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Processi della Tecnologia Alimentare - Prof. Davide Barbanti
Raw materials : meat, seafood
products, vegetables, dough-
based foods, re-formed foods.
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Processi della Tecnologia Alimentare - Prof. Davide Barbanti
The thickness of the layer of oil varies as a result of irregularities in the surface of the
food. This, together with the action of bubbles of steam which lift the food off the hotsurface, causes temperature variations as frying proceeds and produces the characteristic
irregular browning of shallow fried foods. Shallow frying has a high surface heat transfer
coefficient (200–450Wm-2K-1), although not uniformly across the entire surface of the
food.
Shallow (or contact) frying
This method is suited to foods
with large surface-area-to-volume
ratio (bacon slices, eggs, burgers
and other types of pattie). Heat is
transferred to the food mostly by
conduction from the hot surface
of the pan through a thin layer of
oil.
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Processi della Tecnologia Alimentare - Prof. Davide Barbanti
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Processi della Tecnologia Alimentare - Prof. Davide Barbanti
Deep-fat frying
Heat transfer is a combination of convection
within the hot oil and conduction to the interior of
the food. All surfaces of the food receive a similarheat 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 absorband entrain a greater volume of oil. Heat transfer
coefficients are 250–300Wm-2K-1 before
evaporation of moisture from the surface begins,
but subsequently increase to 800–1000Wm-2K-1
owing to the violent turbulence caused by steam
escaping from the food.
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Processi della Tecnologia Alimentare - Prof. Davide Barbanti
The rate of heat penetration into the food is controlled by
the thermal conductivity of the food. The surface crust hasa 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. Moisture moves from the surface of the food
through a boundary film of oil, the thickness of which
controls the rate of heat and mass transfer.
When food is placed in hot oil, the surface
temperature rises rapidly and water is vaporised assteam. The surface then begins to dry out in
similar way to that described during baking and
roasting. 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 hotoil, and the internal temperature rises more slowly
towards 100ºC. The rate of heat transfer is
controlled by the temperature difference between
the oil and the food and by the surface heat transfe
coefficient.
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Processi della Tecnologia Alimentare - Prof. Davide Barbanti
Oil absorption may depend on:
- surface/volume of foods;
- oil temperature- crust composition
- moisture content of food
- thermal hystory of oil
- ......
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Processi della Tecnologia Alimentare - Prof. Davide Barbanti
Frozen French Fries process flow sheet
Raw potatoes Sorting and grading Washing (water)
Steam peelingCuttingStick thickness and
lenght sorting
Steam or water
blanching
Partial drying Deep-fat frying
Oil draining and
coolingFreezing
Frozen storage
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Processi della Tecnologia Alimentare - Prof. Davide Barbanti
Steam peeling
Stick thickness and
lenght sorting
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Processi della Tecnologia Alimentare - Prof. Davide Barbanti
Steam blancher
Water blancher
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Processi della Tecnologia Alimentare - Prof. Davide Barbanti
Deep-fat frying