of the dependence of the unit operation on a physical principle, or
a small group of associated
principles, quantitative relationships in the form of mathematical equations
can be built to describe them. The equations can be used to follow
is happening in the process, and to control and modify the process if
Important unit operations in the food industry are fluid flow, heat transfer,
drying, evaporation, contact equilibrium processes (which include distillation,
extraction, gas absorption, crystallization, and membrane processes),
mechanical separations (which include filtration, centrifugation, sedimentation
and sieving), size reduction and mixing.
These unit operations, and in particular the basic principles on which
they depend, are the subject of this book, rather than the equipment used
or the materials being processed.
Two very important laws which all unit operations obey are the laws of
conservation of mass and energy.
Conservation of Mass and Energy
law of conservation of mass states that mass can neither be created
nor destroyed. Thus in
a processing plant, the total mass of material entering the plant must
equal the total mass of material leaving the plant, less any accumulation
left in the plant. If there is no accumulation, then the simple rule
that "what goes in must come out". Similarly all material entering
a unit operation must in due course leave.
For example, if milk is being fed into a centrifuge to separate it into
skim milk and cream, under the law of conservation of mass the total number
of kilograms of material (milk) entering the centrifuge per minute must
equal the total number of kilograms of material (skim milk and cream)
that leave the centrifuge per minute.
Similarly, the law of conservation of mass applies to each component in
the entering materials. For example, considering the butter fat in the
milk entering the centrifuge, the weight of butter fat entering the centrifuge
per minute must be equal to the weight of butter fat leaving the centrifuge
per minute. A similar relationship will hold for the other components,
proteins, milk sugars and so on.
The law of conservation of energy states that energy can neither be created
nor destroyed. The total energy in the materials entering the processing
plant, plus the energy added in the plant, must equal the total energy
leaving the plant.
This is a more complex concept than the conservation of mass, as energy
can take various forms such as kinetic energy, potential energy, heat
energy, chemical energy, electrical energy and so on.
During processing, some of these forms of energy can be converted from
one to another. Mechanical energy in a fluid can be converted through
friction into heat energy. Chemical energy in food is converted by the
human body into mechanical energy.
Note that it is the sum total of all these forms of energy that
For example, consider the pasteurizing process for milk, in which milk
is pumped through a heat exchanger and is first heated and then cooled.
The energy can be considered either over the whole plant or only as it
affects the milk. For total plant energy, the balance must include: the
conversion in the pump of electrical energy to kinetic and heat energy,
the kinetic and potential energies of the milk entering and leaving the
plant and the various kinds of energy in the heating and cooling sections,as
well as the exiting heat, kinetic and potential energies.
To the food technologist, the energies affecting the product are the most
important. In the case of the pasteurizer, the energy affecting the product
is the heat energy in the milk. Heat energy is added to the milk by the
pump and by the hot water passing through the heat exchanger. Cooling
water then removes part of the heat energy and some of the heat energy
is also lost to the surroundings.
The heat energy leaving in the milk must equal the heat energy in the
milk entering the pasteurizer plus or minus any heat added or taken away
in the plant.
Heat energy leaving
in milk = initial heat energy
heat energy added by pump
heat energy added in heating section
heat energy taken out in cooling section
heat energy lost to surroundings.
law of conservation of energy can also apply to part of a process.
For example, considering
the heating section of the heat exchanger in the pasteurizer, the heat
lost by the hot water must be equal to the sum of the heat gained by
milk and the heat lost from the heat exchanger to its surroundings.
From these laws of conservation of mass and energy, a balance sheet for
materials and for energy can be drawn up at all times for a unit operation.
These are called material balances and energy balances.
Overall View of an Engineering Process
a material balance and an energy balance, a food engineering process
can be viewed
overall or as a series of units. Each unit is a unit operation. The unit
operation can be represented by a box as shown in Fig. 1.1.
Fig. 1.1 Unit operation
the box go the raw materials and energy, out of the box come the desired
wastes and energy. The equipment within the box will enable the required
changes to be made with as little waste of materials and energy as
In other words, the desired products are required to be maximized and
the undesired by-products and wastes minimized. Control over the process
exercised by regulating the flow of energy, or of materials, or of both.
Introduction > DIMENSIONS
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