effects on food
By- ketaki b.patil
• Frequently in the food industry a raw material or a
potential foodstuff contains more water than is
required in the final product.
• When the food stuff is liquid, the easiest method of
removing the water, in general, is to apply heat to
• Evaporation is thus a process which is often used by
the food technologist.
• Evaporation is a physical separation process, which
removes a volatile component from a liquid solution
or mixture by vaporization, obtaining a concentrated
product of the nonvolatile components.
• For liquid foods, evaporation removes most of water
resulting in concentrated product which may be used
as such or processed further, e.g., by drying.
• Evaporation is used extensively in concentrating fruit
and vegetables juices, milk, coffee extracts and in
refining sugar and salt.
• Reduction of the water content reduces weight and
volume of the product, cutting storage and
transportation costs, and improving the storage
stability of the product.
• Evaporation is established as the major process of
concentrating liquid foods, although some new
methods offer special advantages, such as freeze
concentration and reverse osmosis.
Evaporation and Vaporization
Process by which liquid changes to its
vapors at a temperature below its
Process by which liquid changes to its
vapors at its Boiling Point
Takes place at all temperatures Takes place at only fixed temperature
i.e. its boiling point temperature
Temperature may changes during
Temperature during vaporization does
A slow and silent process A fast and violent process
Evaporation takes place only at
surface of liquids
Takes place over the entire mass of
• While the design criteria for evaporators
of the industry, two questions always exist:
1)Is this equipment best suited for the duty?
2) Is the equipment arranged for the most
efficient and economical use?
Energy Saving Evaporation
• Single effect evaporators
• Multiple effect evaporators
• Vapor Recompression Evaporators
The single effect evaporator
• The typical evaporator is made up of three functional
• The heat exchanger
• The evaporating section – where liquid boils and
• Separator – in which vapor leaves the liquid and
passes off to the condenser or the other equipment.
• In many evaporators all these three sections are
contained in a single vertical cylinder.
• In the centre of cylinder there is steam heating
section, with pipes passing through it in which the
evaporating liquors rise.
• At the top of the cylinder the baffles which allow the
vapours to escape but check liquid droplets which
may accompany the vapors from the liquid surface
this type of evaporator which may be called
• In the heat exchanger section, called a
calandria in this type of evaporator, steam
condenses in the jacket and the liquid being
evaporated boils on the inside of the tubes
and in the space above the upper tube plate.
• The circulation of liquids greatly affects
evaporation rates but circulation rates and
pattern are very difficult to predict in any
• As evaporations proceeds the remaining
liquors become more concentrated and
because of this the boiling temperature rise.
• The rise in temperature of boiling reduces the
available temperature drop, assuming no change in
the heat source. And so the total rate of heat transfer
will drop accordingly.
• Also the viscosity of liquid will increase, often quite
substantially, this affects circulations and heat
transfer coefficients leading again to lower rates of
Multiple effect evaporator
• Multi-Effect Evaporation is an important procedure adopted
to economize the consumption of energy.
• Multi effect evaporation system is based on the
repeated use of the water vapors from one
evaporation unit (effect) to heat the next effect, which
operates at a lower pressure.
• Thus, 1kg of steam can evaporate more water,
depending on the number of effects and the operating
Steam Economies (SE) of
Evaporator system SE, kg water / kg steam
Single effect 0.90- 0.98
Double effect 1.70- 2
Triple effect 2.40- 2.80
Six effect 4.6- 4.9
Thermo compressor, 3 effect 4-8
Mechanical vapor recompression 10-30
• There are two feed operations - backward feed and
forward feed operations.
• In the backward operation, the raw feed enters the
last (coldest) effect and the discharge from this effect
becomes a feed for the next to last effect.
• This technique of evaporations is advantageous, in
case the feed is cold, as much less liquid must be
heated to the higher temperature existing in the early
effects. The procedure is also used if the product is
viscous and high temperatures are required to keep
the viscosity low enough to produce good heat
forward feed operation
• In the case of a forward feed operation, the raw feed
is introduced in the first effect and is passed from
effect to effect parallel to steam flow. The product is
withdrawn from the last effect. This procedure is
highly advantageous if the feed is hot. The method is
also used if the concentrated product may be
damaged or may deposit scale at high temperature.
Types of evaporator
• Batch pan
• Forced circulation
• Natural circulation
• Wiped film
• Rising film tubular
• Plate equivalents of tubular evaporators
• Falling film tubular
• Rising/falling film tubular
1)Long Residence –Time evaporator
a)Jacketed vessel evaporator
c)Short tube evaporator
2) Short Residence-Time Evaporator
a) Long tube vertical evaporators
c) Agitated film evaporators
d)Centrifugal film evaporator
Falling Film Evaporators Rising Film Evaporator
Thin Film Evaporators Forced Circulation
Falling film evaporation.
• The liquid feed is pumped through a preheater to the
top of the evaporator tube, and it is distributed as a
liquid film on the tube walls.
• The main body of the evaporator consists of a vertical
tube, surrounded by a steam jacket.
• Evaporation takes place as the liquid film lows down
the tube, and the generated vapors flow concurrently
with the liquid into a vapor-liquid separator, located
at the bottom of the tube.
• pressure drop (up to 100 psig), measured with a
pressure gauge at the entrance of the preheater.
• Vacuum evaporation is the process of causing the
pressure in a liquid-filled container to be reduced
below the vapor pressure of the liquid, causing the
liquid to evaporate at a lower temperature than
• Although the process can be applied to any type of
liquid at any vapor pressure.
• It is generally used to describe the boiling of water by
lowering the container's internal pressure below
standard atmospheric pressure and causing the water
to boil at room temperature .
• Evaporation is considered to be the most
economical and most widely used method of
concentration. However, it is not suited for
food products with very delicate flavors.
Commercial processes for the concentration of
such products by membrane separation
techniques are not yet available.
• As compared to the conventional evaporation
processes, concentration by freezing is
potentially a superior and economic process
for aroma-rich liquid foods.
• In the past, the process, however, was seldom used because of
the investment cost and the considerable loss of concentrate in
the withdrawn ice, and hence, the quality.
• Recent technological developments have minimized these two
drawbacks associated with the earlier freeze concentration
• In the coming decade, freeze concentration is seen as a
potentially attractive method for the concentration of aroma-
rich liquid foods, including fruit juices, coffee, tea, and
selected alcoholic beverages.
• In this article, several aspects of the theoretical considerations
behind freeze concentration of fruit juices, the development of
new and cheaper designs, and commercially available freeze
concentration processes are reviewed.
• Evaporation results in a loss of fresh juice flavors, color
degradation and “cooked” taste due to the thermal effects.
• Methods using less heat for thermal damage reduction must be
investigated to establish parameters for future commercial
• The promising alternative is Reverse Osmosis (RO),but it
cannot achieve concentrations greater than 30ºBrix.
• RO has advantages over traditional evaporation techniques in
removing water. Because less heat is used, thermal damage to
products is generally eliminated.
• Vapour recompression is an alternative method to
multiple effect to save energy and that can be
achieved with two different technologies :
1)Thermal Vapour Recompression (TVR) and
2) Mechanical Vapour Recompression (MVR).
Thermal Vapour Recompression (TVR)
• The main features of TVR are as follow :
• Usable when high pressure steam is available
• Single or multiple effect
• Energy and cooling water savings
• No moving parts in steam jet ejector
• Operating close to design conditions
• Suitable for low boiling elevation and non-incrusting
Mechanical Vapour Recompression (MVR).
• The main features of MVR are as follow :
• Most energy efficient evaporation technology
• Flexible and simple to use
• Generally single effect
• Almost no steam and cooling water consumption
(boiler and cooling tower cost reduction)
• Suitable for moderate boiling elevation products
• Capital cost vs. operating cost to be studied
Application of evaporators
• Chemical Industry –
Dyes, Pickle Liquor, Plating
bath, Salt, Glycerin, Sodium
Nitrate, Sodium Sulphate,
• Food Industry -
Milk, Fruit Juice, Pulps,
Tomato Puree, Herbal
Extracts, Coffee, Tea,
Protein, Gelatin, Coconut
• Corn Industry -
Whey, Gelatin, Malt
Fructose, Dextrose, Corn
Steep Liquor, Sorbitol,
• Waste Water Treatment
Blow Down, RO reject,
Floor washings, Process
effluents, Black Liquor,
Fish Sick Water.
Bulk drugs, Glycerin,
Sweet water, Yeast
Products which are treated to
evaporation• Apple juice
• Pear juice
• Amino acids
• Fruit purees
• Beef broths
• Pharmaceutical products
• Chicken broth
• Mango juice
• Beet juice
• Grape juice
• Pineapple juice
• Lime juice
• Skim milk
• Liquid egg
• Cheese whey
• Low alcohol beer
• Physical properties :-
– The physical properties of the liquid, which are of
direct importance to evaporation, are the
– Viscosity (or rheological constants)
– Thermal conductivity
– Specific heat
– Surface tension
– Boiling point elevation
• Surface tension of water decreases significantly
when organic components are present in the aqueous
• The surface tension of liquid food material is lower
due to the surface active component present.
• Boiling Point Elevation (BPE)is caused by
solute/water interaction and it is undesirable in
evaporation, since it requires a higher temperature of
heating medium to affect the same driving force
• It is particularly high in concentrated aqueous
solutions of salts and alkalis (e.g., sodium hydroxide)
Fauling in Evaporators
• Fouling is the formation of deposits on the heat
transfer surface, which reduces heat transfer and
evaporation rates and may damage the quality of the
• Fauling is costly to industrial operations because of
the higher capital cost (oversized plants), the energy
losses, the higher maintenance, and the loss of
production during shutdown for cleaning.
• Fauling includes scaling, which is the precipitation
of inorganic salts on the heating surface.
Food quality consideration
• Evaporation should minimize the undesirable changes
in quality and nutritive value of heat sensitive food
materials. Such changes are losses of organoleptic
quality( flavor, aroma, and color), losses of vitamins,
and production of undesirable compounds( browning,
• Losses of aroma components during evaporation of
fruit juices and aqueous extracts can be restored by
aroma recovery in the distillate or add-back of fresh
• Efficient operation of evaporation system requires
process control, which is achieved mainly by simple
or advanced control system.
• The control system should respond to :-
• Changes in feed composition
• Change the heat supply
• Testing of evaporators :-
– The testing and evaluation performance of
evaporators can be performed using a procedure
developed by the American Institute Of
– Standardized measurements are made, after the
evaporator has reached steady-state operation.
– It includes feed, product, steam rates, temperature,
evaporator body, and condenser pressure, cooling
water temperature, boiling point elevation etc
• Preventing excessive vacuum
• Excessive vacuum can produce too much vapor flow
for processing by the emissions control system.
Control of the vacuum can significantly decrease
• Air drawback into the evaporator after the vacuum
pump is shut down can result in unsafe conditions
when flammable solvents are distilled. A flammable
mixture can be created in the equipment that is a
potential source for an explosion.
• Hygienic (sanitary) guidelines and codes for food
processing equipment are of particular importance to
the design, operation, and cleaning of food
• The most important hygienic problems is the fouling
of evaporation surfaces, caused by deposition of food
colloids and other food components, which reduces
heat transfer and evaporation rate(capacity).
• Fouling is an empirical function of the operation
time, and periodic cleaning is necessary.
• The operation cycle may be long (a week)
for sugar evaporators, and shorter for
dairy units (a day), due to the precipitation
of milk heat sensitive proteins.
• CIP cleaning systems can be used for all
parts of food contact equipment.
• Evaporator - Wikipedia, the free encyclopedia
• Calandria Evaporator Swenson Technology, Inc_
• Principles of Food Processing - Dennis R_ Heldman, Richard
W Hartel - Google Books
• Applications of Evaporators - Comp Engineering and Exports,
• Food, Dairy and Beverage Evaporator Applications
• Handbook of food processing equipment by George
• Unit operations in food processing by R.L.EARLE