Pres3. Processing by Application of Heat

CHAPTER TWO
PROCESSING BY APPLICATION
OF HEAT
A. Heat processing using steam or water
B. Heat processing using hot air
C. Heat processing using hot oil

Introduction
• Heat treatment remains one of the most important methods
used in food processing, not only because of the desirable
effects on eating quality but also because of the preservative
effect on foods by the destruction of enzymes, micro-
organisms, insects and parasites.
• The other main advantages of heat processing are:
relatively simple control of processing conditions
capability to produce shelf-stable foods that do not
require refrigeration
destruction of anti-nutritional factors (e.g. trypsin
inhibitor in some legumes)
improvement in the availability of some nutrients (e.g.
improved digestibility of proteins, gelatinization of
starches and release of bound niacin).

Introduction…
• Another important effect of heating is the selective
removal of volatile components from a food. E.g.
• In evaporation and dehydration, the removal of water
inhibits microbial growth and enzyme activity and thus
achieves preservation.
• In distillation either alcohol is selectively removed to
produce concentrated spirits, or flavour components are
recovered and added back to foods to improve their sensory
characteristics.
• However, heat also alters or destroys components of foods
that are responsible for their individual flavour, colour, taste
or texture and as a result they are perceived to have a lower
quality and lower value.

A. Heat processing using steam or water
1. Blanching
• Blanching serves a variety of functions, one of the main
ones being to destroy enzymic activity in vegetables and
some fruits, prior to further processing.
• As such, it is not intended as a sole method of
preservation but as a pre-treatment which is normally
carried out between the preparation of the raw material and
later operations (particularly heat sterilisation, dehydration
and freezing).
• Blanching is also combined with peeling and/or cleaning
of food, to achieve savings in energy consumption, space
and equipment costs.

1. Blanching…
• A few processed vegetables, for example onions and green
peppers, do not require blanching to prevent enzyme
activity during storage, but the majority suffer considerable
loss in quality if blanching is omitted or if they are under-
blanched.
• To achieve adequate enzyme inactivation, food is heated
rapidly to a pre-set temperature, held for a pre-set time and
then cooled rapidly to near ambient temperatures.
• The factors which influence blanching time are:
type of fruit or vegetable
size of the pieces of food
blanching temperature
method of heating.

1. Blanching…
• The two most widespread commercial methods of blanching
involve passing food through an atmosphere of saturated
steam (steam blancher) or a bath of hot water (hot water
blancher).
Effect of blanching on foods
• Blanching brightens the colour of some foods by removing
air and dust on the surface and thus altering the wavelength
of reflected light. The time and temperature of blanching
also influence the change in food pigments.
• Blanching soften the texture of vegetables to facilitate
filling into containers prior to canning.

Effect of blanching on foods…
• However, when used for freezing or drying, the time–
temperature conditions needed to achieve enzyme
inactivation cause an excessive loss of texture in some types
of food (e.g. certain varieties of potato) and in large pieces
of food.
• Under-blanching can lead to the development of off-
flavours during storage of dried or frozen foods.
• Some minerals, water-soluble vitamins and other water-
soluble components are lost during blanching.
• Losses of vitamins are mostly due to leaching, thermal
destruction and, to a lesser extent, oxidation.

2. Pasteurization
• Pasteurization is a relatively mild heat treatment, in which
food is heated to below 100ºC.
• In low acid foods (pH>4.5, for example milk) it is used to
minimise possible health hazards from pathogenic micro-
organisms and to extend the shelf life of foods for several
days.
• In acidic foods (pH<4.5, for example bottled fruit) it is
used to extend the shelf life for several months by
destruction of spoilage micro-organisms (yeasts or moulds)
and/or enzyme inactivation.
• In both types of food, minimal changes are caused to the
sensory characteristics or nutritive value.

Effect of pasteurization on foods
• Pasteurization is a relatively mild heat treatment and even
when combined with other unit operations (for e.g.
irradiation and chilling) there are only minor changes to the
nutritional and sensory characteristics of most foods.
• The difference between the whiteness of raw milk and that
of pasteurised milk is due to homogenisation, and
pasteurisation alone has no measurable effect.
• Other pigments in plant and animal products are also mostly
unaffected by pasteurisation.
• Loss of volatiles from raw milk removes a hay-like aroma
and produces a blander product.

3. Heat sterilization
• Heat sterilisation is the unit operation in which foods are
heated at a sufficiently high temperature and for a
sufficiently long time to destroy microbial and enzyme
activity.
• As a result, sterilised foods have a shelf life in excess of six
months at ambient temperatures.
• The severe heat treatment during the older process of in-
container sterilisation (canning) may produce substantial
changes in nutritional and sensory qualities of foods.
• Developments in processing technology therefore aim to
reduce the damage to nutrients and sensory components, by
either reducing the time of processing in containers or
processing foods before packaging (aseptic processing).

3. Heat sterilization…
• The length of time required to sterilise a food is influenced by:
the heat resistance of micro-organisms or enzymes likely to be
present in the food
the heating conditions
the pH of the food
the size of the container
the physical state of the food.
• In order to determine the process time for a given food, it is
necessary to have information about both the heat resistance of
micro-organisms, particularly heat resistant spores, or enzymes
that are likely to be present and the rate of heat penetration into
the food.
• Sterilizing retorts may be batch or continuous in operation.
Batch retorts may be vertical or horizontal; the latter are easier to
load and unload and have facilities for agitating containers, but
require more floor space.

Ultra high-temperature (UHT)/aseptic processes
• Higher processing temperatures for a shorter time are possible if
the product is sterilized before it is filled into pre-sterilised
containers in a sterile atmosphere. This forms the basis of UHT
processing (also termed aseptic processing).
• It is used to sterilise a wide range of liquid foods, including
milk, fruit juices and concentrates, cream, yoghurt, wine, salad
dressing, egg and ice cream mix.
• It can also be used to process foods which contain small discrete
particles, for example cottage cheese, baby foods, tomato
products, fruit and vegetables, soups and rice desserts.
• The main limitations of UHT processing are the cost and
complexity of the plant, which arises from the necessity to
sterilise packaging materials, associated pipework and tanks, the
maintenance of sterile air and surfaces in filling machines, and
the higher skill levels required by operators and maintenance
staff.

Effect of sterilization on foods
• The time–temperature combinations used in canning have
a substantial effect on most naturally occurring pigments in
foods. For example, in meats the red oxymyoglobin
pigment is converted to brown metmyoglobin.
• In milk, the development of a cooked flavour is due to
denaturation of whey proteins to form hydrogen sulphide
and the formation of lactones and methyl ketones from
lipids.
• In canned meats, changes in texture are caused by
coagulation and a loss of water holding capacity of proteins,
which produces shrinkage and stiffening of muscle tissues.
• Proteins are coagulated and, in canned meats, losses of
amino acids are 10–20%.

4. Evaporation and distillation
• In common with other unit operations that are intended to
separate components of foods, evaporation and distillation
aim to separate specific components to increase the value of
the food.
• In both types of operation, separation is achieved by
exploiting differences in the vapour pressure (volatility) of
the components and using heat to remove one or more from
the bulk of the food.
4.1. Evaporation: Evaporation, or concentration by
boiling, is the partial removal of water from liquid foods
by boiling off water vapour. It increases the solids content
of a food and hence preserves it by a reduction in water
activity.

4.1. Evaporation…
• Evaporation is used to pre-concentrate foods (e.g. fruit
juice, milk and coffee) prior to drying, freezing or
sterilization and hence to reduce their weight and volume.
This saves energy in subsequent operations and reduces
storage, transport and distribution costs.
• There is also greater convenience for the consumer (e.g.
fruit drinks for dilution, concentrated soups, tomato or
garlic pastes, sugar) or for the manufacturer (e.g. liquid
pectin, fruit concentrates for use in ice cream or baked
goods).

Effect of evaporation on foods
• Aroma compounds that are more volatile than water are
thus lost during evaporation.
• Evaporation darkens the colour of foods, partly because of
the increase in concentration of solids, but also because the
reduction in water activity promotes chemical changes, (e.g.
Maillard browning).
• Changes to food quality that result from the relatively
severe heat treatment are minimized by the design and
operation of the equipment.
• Evaporation is more expensive in energy consumption than
other methods of concentration (membrane concentration)
and freeze concentration but a higher degree of
concentration can be achieved

4.2. Distillation
• Although common in the chemical industry, distillation in food
processing is mostly confined to the production of alcoholic
spirits and separation of volatile flavour and aroma compounds
(e.g, production of essential oils by steam distillation).
• When a food that contains components having different
degrees of volatility is heated, those that have a higher vapour
pressure (more volatile components) are separated first. These
are termed the ‘distillate’ and components that have a lower
volatility are termed bottoms’ or residues.
• Equipment such as; batch distillation, continous distillation
column, spinning cone column are used for distillation.
• Processes: Feed liquor flows continuously through the column
and as it is heated, volatiles are produced and separated at the
top of the column as distillate and the residue is separated at the

5. Extrusion
• Extrusion is a process which combines several unit
operations including mixing, cooking, kneading, shearing,
shaping and forming.
• Extruders are classified according to the method of
operation (cold extruders or extruder-cookers) and the
method of construction (single- or twin-screw extruders).
• Cold extrusion, in which the temperature of the food
remains at ambient is used to mix and shape foods such as
pasta and meat products.
• Low pressure extrusion, at temperatures below 100ºC, is
used to produce, for example, liquorice, fish pastes, surimi
and pet foods.

5. Extrusion…
• Extrusion cooking is a high-temperature short-time (HTST)
process which reduces microbial contamination and
inactivates enzymes.
• However, the main method of preservation of both hot-
and cold-extruded foods is by the low water activity of the
product (0.1–0.4), and for semi-moist products in particular,
by the packaging materials that are used.
• Extrusion has gained in popularity for the following reasons:
Versatility, Reduced costs, High production rates and
automated production, Product quality, No process effluents.
• Extrusion can be seen as an example of a size enlargement
process, in which granular or powdered foods are re-formed into
larger pieces.

Effect of extrusion on foods
• The HTST conditions in extrusion cooking have only minor
effects on the natural colour and flavour of foods.
• Added flavors are mixed with ingredients before cold extrusion,
but this is largely unsuccessful in extrusion cooking as the
flavours are volatilised when the food emerges from the die.
• Vitamin losses in extruded foods vary according to the type of
food, the moisture content, the temperature of processing and the
holding time. Generally, losses are minimal in cold extrusion.
• High temperatures and the presence of sugars cause Maillard
browning and a reduction in protein quality.
• Lower temperatures and low concentrations of sugars result
in an increase in protein digestibility, owing to
rearrangement of the protein structure.

B. Heat processing using hot air
1. Dehydration
• Dehydration (drying) is defined as ‘the application of
heat under controlled conditions to remove the majority
of the water normally present in a food by evaporation’
(in the case of freeze drying (by sublimation).
• This definition excludes other unit operations which
remove water from foods (e.g. mechanical separations and
membrane concentration, evaporation and baking as these
normally remove much less water than dehydration).
• The main purpose of dehydration is to extend the shelf life
of foods by a reduction in water activity. This inhibits
microbial growth and enzyme activity,

1. Dehydration…
• The reduction in weight and bulk of food reduces transport
and storage costs. For some types of food, dehydration
provides a convenient product for the consumer or more
easily handled ingredients for food processors.
• Examples of commercially important dried foods are coffee,
milk, raisins, sultanas and other fruits, pasta, flours
(including bakery mixes), beans, pulses, nuts, breakfast
cereals, tea and spices.
• Examples of important dried ingredients that are used by
manufacturers include egg powder, flavourings and
colourings, lactose, sucrose or fructose powder, enzymes
and yeasts.

Effect of dehydration on foods
• Changes to the texture of solid foods are an important cause of
quality deterioration. The loss of texture in these products is
caused by gelatinisation of starch, crystallisation of cellulose,
and localised variations in the moisture content during drying,
which set up internal stresses.
• Heat not only vaporises water during drying but also causes loss
of volatile components from the food and as a result most dried
foods have less flavour than the original material.
• The open porous structure of dried food allows access of
oxygen, which is a second important cause of aroma loss due to
oxidation of volatile components and lipids during storage.
• There are a number of causes of colour loss or change in dried
foods; drying changes the surface characteristics of a food and
hence alters its reflectivity and colour.

Rehydration
• Water that is removed from a food during dehydration cannot be
replaced in the same way when the food is rehydrated (that is,
rehydration is not the reverse of Dehydration or drying);
• loss of cellular osmotic pressure, changes in cell membrane
permeability, solute migration, crystallisation of polysaccharides
and coagulation of cellular proteins all contribute to texture
changes and volatile losses and are each irreversible.
• Heat reduces the degree of hydration of starch and the elasticity
of cell walls, and coagulates proteins to reduce their water-
holding capacity.
• Therefore, The rate and extent of rehydration may be used as
an indicator of food quality; those foods that are dried under
optimum conditions suffer less damage and rehydrate more
rapidly and completely than poorly dried foods.

2. Baking and roasting
• Baking and roasting are essentially the same unit operation:
they both use heated air to alter the eating quality of foods.
• The terminology differs in common usage; baking is usually
applied to flour-based foods or fruits, and roasting to meats, nuts
and vegetables. In this context the term baking is used to include
both operations.
• A secondary purpose of baking is preservation by destruction of
micro-organisms and reduction of the water activity at the
surface of the food. However, the shelf life of most baked foods
is short unless it is extended by refrigeration or packaging.
• Baking involves simultaneous heat and mass transfer; heat is
transferred into the food from hot surfaces and air in the oven
and moisture is transferred from the food to air that surrounds it
and then removed from the oven

Effect of baking on foods
• A characteristic of many baked foods is the formation of a
dry crust which contains the moist bulk of the food.
• In cereal foods, changes to the granular structure of starch,
gelatinisation and dehydration produce the characteristic
texture of the crust.
• The aromas produced by baking are an important sensory
characteristic of baked goods. The severe heating conditions
in the surface layers of food cause Maillard browning
reactions between sugars and amino acids.
• The main nutritional changes during baking occur at the
surface of foods, and the ratio of surface area to volume is
therefore an important factor in determining the effect on
overall nutritional loss.

C. Heat processing using hot oils
1. Frying
• Frying is a unit operation which is mainly used to alter the
eating quality of a food.
• 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 (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.
• The quality is maintained by adequate barrier properties of
packaging materials and correct storage conditions.

1. Frying…
• The time taken for food to be completely fried depends
on:
the type of food
the temperature of the oil
the method of frying (shallow or deep-fat frying)
the thickness of the food
the required change in eating quality.
• There are two main methods of commercial frying which
are distinguished by the method of heat transfer involved:
these are shallow frying and deep-fat frying.
Effect of frying on foods
• The effect of frying on foods therefore involves both the
effect on the oil, which in turn influences the quality of the

Effect of frying on foods…
• Prolonged heating of oils at the high temperatures used in
frying, in the presence of moisture and oxygen released
from foods, causes oxidation of the oil to form a range of
volatile carbonyls, hydroxy acids, keto acids and epoxy
acids.
• Oxidation of fat-soluble vitamins in the oil results in a loss
of nutritional value.
• The texture of fried foods is produced by changes to
proteins, fats and polymeric carbohydrates which are similar
to those produced by baking.
• Frying operations that are intended to dry the food and to
extend the shelf life cause substantially higher losses of
nutrients, particularly fat-soluble vitamins.

MODEL QUESTION
From chapter two
1. What does it mean by Ambient temperature processing and
its use?
2. What is cleaning? Compare and contrast between wet and
dry cleaning?
3. What is sorting? Explain the four main physical properties
used to sort foods?
4. What is contaminants and list classification of
contaminants? (explain by giving examples)
5. Clearly identify between Grading and Sorting operation?

6. What size reduction? And explain it’s benefit in food
processing?
7. What is homogenization and emulsification?
8. What is mixing and forming and their use in food
processing?
9. Explain unit operations used in separation and
concentration of food components one by one? Also
describe their effect on food?
10. List the main advantage of fermentation and enzyme
technology in food processing?

MODEL QUESTIONS
From Chapter Three
1. What are the main advantage of heat processing and also
explain its drawback (negative side of heat processing)?
2. What are the pro (positive side) and cons (negative side)of
blanching and list factors that affect blanching time?
3. Explain the use of pasteurization in low and high acid food
and also describe the effect of pasteurization?
4. What is heat sterilization? What are factors affecting the
length of time to sterilize food?
5. What is UHT and explain its use and limitation?

6. Clearly distinguish between evaporation and distillation
(their definition, use and their effect in food processing) ?
7. Explain extrusion? Its classifications? Why extrusion
gained popularity and also why it is considered as size
enlargement operation?
8. Define dehydration, explain its main purpose and its effect
on food processing. Explain about rehydration too?
9. What are the purpose of baking and explain the effect of
baking on food? How baking simultaneously involve the
heat and mass transfer operation?
10. What is frying? List factors affecting time needed to
completely fry food? Also explain the effect of frying on
food?

Pres3. Processing by Application of Heat

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Pres3. Processing by Application of Heat