1.Introduction Food science is a discipline concerned with all
technical aspects of food, beginning with harvesting or
slaughtering, and ending with its cooking and consumption. It is
considered one of the agricultural sciences, and is usually
considered distinct from the field of nutrition. Food science is a
discipline concerned with all technical aspects of food, beginning
with harvesting or slaughtering, and ending with its cooking and
consumption. It is considered one of the agricultural sciences, and
is usually considered distinct from the field of
nutrition.foodharvestingslaughteringcookingagricultural
sciencesnutritionfoodharvestingslaughteringcookingagricultural
sciencesnutrition
Slide 3
Some of the sub disciplines of food science include: Food
safety - the causes, prevention and communication dealing with
foodborne illness Food safety - the causes, prevention and
communication dealing with foodborne illness Food safetyfoodborne
illness Food safetyfoodborne illness Food microbiology - the
positive and negative interactions between micro-organisms and
foods Food microbiology - the positive and negative interactions
between micro-organisms and foods Food microbiology Food
microbiology Food preservation - the causes and prevention of
quality degradation Food preservation - the causes and prevention
of quality degradation Food preservation Food preservation Food
engineering - the industrial processes used to manufacture food
Food engineering - the industrial processes used to manufacture
food Food engineering Food engineering Product development - the
invention of new food products Product development - the invention
of new food products Product development Product development
Sensory analysis - the study of how food is perceived by the
consumer's senses Sensory analysis - the study of how food is
perceived by the consumer's senses Sensory analysis Sensory
analysis
Slide 4
Food chemistry - the molecular composition of food and the
involvement of these molecules in chemical reactions Food chemistry
- the molecular composition of food and the involvement of these
molecules in chemical reactions Food chemistry Food chemistry Food
packaging - the study of how food is packaged to preserve the food
after it has been processed. Food packaging - the study of how food
is packaged to preserve the food after it has been processed. Food
packaging Food packaging Molecular gastronomy - the scientific
investigation of processes in cooking, social & artistic
gastronomical phenomena Molecular gastronomy - the scientific
investigation of processes in cooking, social & artistic
gastronomical phenomena Molecular gastronomygastronomical Molecular
gastronomygastronomical Food technology - the technological aspects
Food technology - the technological aspects Food technology Food
technology Food physics - the physical aspects of foods (such as
viscosity, creaminess, texture...) Food physics - the physical
aspects of foods (such as viscosity, creaminess, texture...) Food
physics Food physics
Slide 5
aims of The food industries ( food processing) today 1. To
extend the shelf life to allow time for distribution, sales and
home storage. 2. To increase variety in the diet by providing a
range of attractive flavours, colours, aromas and textures in food
(collectively known as eating quality, sensory characteristics or
organoleptic quality);. 3. To provide the nutrients required for
health (termed nutritional quality of a food). 4. To generate
income for the manufacturing company.
Slide 6
Food processing divided into: Unit operations combination of
procedures to achieve the intended changes to the raw materials.
Unit operations are grouped together to form a process. The
combination and sequence of operations determines the nature of the
final product.
Slide 7
Consumer demand for foods fewer synthetic additives, fewer
changes during processing. healthy or natural image
Slide 8
This demands effect on food processing industry to launch food
products: Free from synthetic additives low-fat, sugar-free
low-salt Supplemented with vitamins, minerals and probiotic
cultures Organic products
Slide 9
Motivation and changes of food industry improved quality
assurance and quality control reduces production costs Reduce
wastage increases production efficiency, automation
Slide 10
Heat has important influences on food processing in a number of
respects: it is the most convenient way of extending the shelf life
of foods by destroying enzymic and microbiological activity, or by
removing water to inhibit deterioration; it changes the nutritional
and sensory qualities of foods; and generation of heat is a major
processing cost.
Slide 11
Unit operations that take place at ambient temperature and
involve minimum heating of foods operations that heat foods to
extend the shelf life or to alter the eating quality; operations
that remove heat from foods to extend the shelf life with minimal
changes in nutritional qualities and sensory characteristics; the
final part, is concerned with operations that are integral to a
food process but aresupplementary to the main method of
processing.
Slide 12
Food processing Food processing is the set of methods and
techniques used to transform raw ingredients into food or to
transform food into other forms for consumption by humans or
animals either in the home or by the food processing industry. Food
processing typically takes clean, harvested crops or slaughtered
and butchered animal products and uses these to produce attractive,
marketable and often long-life food products. Similar process are
used to produce animal feed. Food processing is the set of methods
and techniques used to transform raw ingredients into food or to
transform food into other forms for consumption by humans or
animals either in the home or by the food processing industry. Food
processing typically takes clean, harvested crops or slaughtered
and butchered animal products and uses these to produce attractive,
marketable and often long-life food products. Similar process are
used to produce animal feed.ingredientsfoodconsumptionhumansfood
processing industryharvestedslaughteredbutchered marketableanimal
feedingredientsfoodconsumptionhumansfood processing
industryharvestedslaughteredbutchered marketableanimal feed
Slide 13
Food processing methods Removal of unwanted outer layers, such
as potato peeling or the skinning of peaches. Removal of unwanted
outer layers, such as potato peeling or the skinning of peaches.
potatopeaches potatopeaches Chopping or slicing e.g. diced carrots.
Chopping or slicing e.g. diced carrots.carrots Mincing and
macerating Mincing and macerating Mincing Liquefaction, such as to
produce fruit juice Liquefaction, such as to produce fruit
juicefruit juicefruit juice Fermentation e.g. in beer breweries
Fermentation e.g. in beer breweries Fermentationbeer breweries
Fermentationbeer breweries Emulsification Emulsification
Emulsification Cooking, such as boiling, broiling, frying, steaming
or grilling Cooking, such as boiling, broiling, frying, steaming or
grilling Cookingboilingbroilingfrying steaminggrilling
Cookingboilingbroilingfrying steaminggrilling
Slide 14
Deep frying Deep frying Deep frying Deep frying Baking Baking
Baking Mixing Mixing Mixing Addition of gas such as air entrainment
for bread or gasification of soft drinks Addition of gas such as
air entrainment for bread or gasification of soft drinks
breadgasificationsoft drinks breadgasificationsoft drinks Proofing
Proofing Proofing Spray drying Spray drying Spray drying Spray
drying Pasteurization Pasteurization Pasteurization Packaging
Packaging Packaging
Slide 15
Chapter 3.Dehydration Dehydration (or 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 (or
in the case of freeze drying by sublimation). 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,
but the processing temperature is usually insufficient to cause
their inactivation.
Slide 16
Drying causes deterioration of both the eating quality and the
nutritional value of the food. Examples of commercially important
dried foods are coffee, milk, raisins, and other fruits, pasta,
flours (including bakery mixes), beans, nuts, breakfast cereals,
tea and spices.
Slide 17
There are a large number of factors that control the rate at
which foods dry, which can be grouped into the following categories
those related to the processing conditions those related to the
nature of the food those related to the drier design.
Slide 18
2.1.Drying using heated air There are three inter-related
factors that control the capacity of air to remove moisture from a
food: 1. the amount of water vapor already carried by the air 2.
the air temperature 3. the amount of air that passes over the food.
The amount of water vapour in air is expressed as either absolute
humidity2 or relative humidity3 (RH) (in per cent). Psychrometry is
the study of inter-related properties of airwater vapour
systems.
Slide 19
Slide 20
Heat from drying air is absorbed by food and provides the
latent heat needed to evaporate water from the surface. The
temperature of the air, measured by a thermometer bulb, is termed
the dry-bulb temperature. If the thermometer bulb is surrounded by
a wet cloth, heat is removed by evaporation of water from the cloth
and the temperature falls. This lower temperature is called the
wet-bulb temperature. The difference between the two temperatures
is used to find the relative humidity of air on the psychrometric
chart Adiabatic cooling lines are the parallel straight lines
sloping across the chart, which show how absolute humidity
decreases as the air temperature increases.
Slide 21
Mechanism of drying The third factor that controls the rate of
drying, in addition to air temperature and humidity, is the air
velocity. When hot air is blown over a wet food, water vapor
diffuses through a boundary film of air surrounding the food and is
carried away by the moving air A water vapour pressure gradient is
established from the moist interior of the food to the dry air.
This gradient provides the driving force for water removal from the
food.
Slide 22
The boundary film acts as a barrier to both heat transfer and
water vapor removal during drying. The thickness of the film is
determined primarily by the air velocity; if the velocity is low,
the boundary film is thicker and this reduces both the heat
transfer coefficient and the rate of removal of water vapor. Water
vapor leaves the surface of the food and increases the humidity of
the surrounding air, to cause a reduction in the water vapour
pressure gradient and hence the rate of drying. Therefore the
faster the air, the thinner the boundary film and hence the faster
the rate of drying.
Slide 23
Slide 24
(a) and (b) Drying curves. The temperature and humidity of the
drying air are constant and all heat is supplied to the food
surface by convection.
Slide 25
Follow: factors affecting on drying The composition and
structure of the food has an influence on the mechanism of moisture
removal. For example, the orientation of fibres in vegetables (e.g.
celery) and protein strands in meat allow more rapid moisture
movement along their length than across the structure. The amount
of food placed into a drier in relation to its capacity (in a given
drier, faster drying is achieved with smaller quantities of
food).
Slide 26
2.2 Drying using heated surfaces Slurries of food are deposited
on a heated steel drum. Heat is conducted from the hot surface,
through the food, and moisture is evaporated from the exposed
surface. The main resistance to heat transfer is the thermal
conductivity of the food Additional resistance arises if the partly
dried food lifts off the hot surface, forming a barrier layer of
air between the food and the drum. Knowledge of the rheological
properties of the food is therefore necessary to determine the
thickness of the layer and the way in which it is applied to the
heated surface
Slide 27
2.3 Types of Driers 2.3.1 Hot-air driers Bin driers Bin driers
are large, cylindrical or rectangular containers fitted with a mesh
base. Hot air passes up through a bed of food at relatively low
velocities Cabinet driers (tray driers) These consist of an
insulated cabinet fitted with shallow mesh or perforated trays,
each of which contains a thin (26 cm deep) layer of food. Hot air
is blown at 0.5 5ms1 through a system of ducts and baffles to
promote uniform air distribution over and/or through each
tray.
Slide 28
Tunnel driers Layers of food are dried on trays, which are
stacked on trucks programmed to move semi continuously through an
insulated tunnel, having one or more types of air flow Typically a
20m tunnel contains 1215 trucks with a total capacity of 5000 kg of
food.
Slide 29
Conveyor driers (belt driers) Continuous conveyor driers are up
to 20m long and 3m wide. Food is dried on a mesh belt in beds 515
cm deep. The air flow is initially directed upwards through the bed
of food and then downwards in later stages to prevent dried food
from blowing out of the bed.
Slide 30
(a) Conveyor drier and (b) three-stage conveyor drier.
Slide 31
Fluidized-bed driers The main features of a fluidised-bed drier
are a distributor to evenly distribute the air at a uniform
velocity around the bed of material; a plenum chamber below the
distributor to produce an homogenous region of air and prevent
localised high velocities; and a disengagement or freeboard region
above the bed to allow disentrainment of particles thrown up by the
air. Air from the fluidised bed is usually fed into cyclones to
separate out fine particles, which are then added back to the
product or agglomerated. Above the distributor, mesh trays contain
a bed of particulate foods up to 15 cdeep. Hot air is blown through
the bed, causing the food to become suspended and vigorously
agitated (fluidised), exposing the maximum surface area of food for
drying
Slide 32
Slide 33
2.4 Drying Effect on foods All products undergo changes during
drying and storage that reduce their quality compared to the fresh
material and the aim of improved drying technologies is to minimize
these changes while maximizing process efficiency. The main changes
to dried foods are to the texture and loss of flavor or aroma, but
changes in color and nutritional value are also significant in some
foods.
Slide 34
2.4.1 Texture Changes to the texture of solid foods are an
important cause of quality deterioration, The loss of texture in
these products is caused by gelatinization of starch,
crystallization of cellulose, and localized variations in the
moisture content during drying, which set up internal stresses.
These rupture, crack, compress and permanently distort the
relatively rigid cells, to give the food a shrunken shrivelled
appearance. On rehydration the product absorbs water more slowly
and does not regain the firm texture of the fresh material. There
are substantial variations in the degree of shrinkage and
rehydration with different foods
Slide 35
In general, rapid drying and high temperatures cause greater
changes to the texture of foods than do moderate rates of drying
and lower temperatures. As water is removed during drying, solutes
move from the interior of the food to the surface. The mechanism
and rate of movement are specific for each solute and depend on the
type of food and the drying conditions used Evaporation of water
causes concentration of solutes at the surface. High air
temperatures (particularly with fruits, fish and meats), cause
complex chemical and physical changes to solutes at the surface,
and the formation of a hard impermeable skin. This is termed case
hardening and it reduces the rate of drying to produce a food with
a dry surface and a moist interior It is minimised by controlling
the drying conditions to prevent excessively high moisture
gradients between the interior and the surface of the food.
Slide 36
2.4.2 Flavor and aroma 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 extent of volatile loss depends on the
temperature and moisture content of the food and on the vapour
pressure of the volatiles and their solubility in water vapour.
Volatiles which have a high relative volatility and diffusivity are
lost at an early stage in drying. Foods that have a high economic
value due to their characteristic flavours (for example herbs and
spices) are dried at low temperatures
Slide 37
Flavour changes, due to oxidative or hydrolytic enzymes are
prevented in fruits by the use of sulphur dioxide, ascorbic acid or
citric acid, by pasteurisation of milk or fruit juices and by
blanching of vegetables. Other methods which are used to retain
flavours in dried foods include: recovery of volatiles and their
return to the product during drying
Slide 38
mixing recovered volatiles with flavour fixing compounds, which
are then granulated and added back to the dried product (for
example dried meat powders) addition of enzymes, or activation of
naturally occurring enzymes, to produce flavours from flavour
precursors in the food (for example onion and garlic are dried
under conditions that protect the enzymes that release
characteristic flavours).
Slide 39
2.4.3 Colour 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. In fruits
and vegetables, chemical changes to carotenoid and chlorophyll
pigments are caused by heat and oxidation during drying and
residual polyphenoloxidase enzyme activity causes browning during
storage.
Slide 40
This is prevented by blanching or treatment of fruits with
ascorbic acid or sulphur dioxide. For moderately sulphured fruits
and vegetables the rate of darkening during storage is inversely
proportional to the residual sulphur dioxide content. However,
sulphur dioxide bleaches anthocyanins, and residual sulphur dioxide
is also linked to health concerns. Its use in dried products is now
restricted in many countries.
Slide 41
The rate of Maillard browning in stored milk and fruit products
depends on the water activity of the food and the temperature of
storage. The rate of darkening increases markedly at high drying
temperatures, when the moisture content of the product exceeds 45%,
and at storage temperatures above 38C
Slide 42
2.4.4 Nutritional value Large differences in reported data on
the nutritional value of dried foods are due to wide variations in
the preparation procedures, the drying temperature and time, and
the storage conditions. In fruits and vegetables, losses during
preparation usually exceed those caused by the drying operation For
example Escher and Neukom (1970) showed that losses of vitamin C
during preparation of apple flakes were 8% during slicing, 62% from
blanching, 10% from pureeing and 5% from drum drying
Slide 43
Vitamins have different solubilities in water and as drying
proceeds, some (for example riboflavin) become supersaturated and
precipitate from solution, so losses are small. Others, for example
ascorbic acid, are soluble until the moisture content of the food
falls to very low levels and these react with solutes at higher
rates as drying proceeds. Vitamin C is also sensitive to heat and
oxidation and short drying times, low temperatures, low moisture
and oxygen levels during storage are therefore necessary to avoid
large losses.
Slide 44
Vitamin losses in selected dried foods
Slide 45
2.4.5 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 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
Slide 46
Chapter 4. Blanching Blanching serves a variety of functions,
one of the main ones being to destroy enzymatic activity in
vegetables and some fruits, prior to further processing by heat. 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
Slide 47
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.
Slide 48
The factors which influence blanching time are: type of fruit
or vegetable size of the pieces of food blanching temperature
method of heating.
Slide 49
3.1 Theory The maximum processing temperature in freezing and
dehydration is insufficient to inactivate enzymes. If the food is
not blanched, undesirable changes in sensory characteristics and
nutritional properties take place during storage. In canning, the
time taken to reach sterilizing temperatures, particularly in large
cans, may be sufficient to allow enzyme activity to take place. It
is therefore necessary to blanch foods prior to these preservation
operations.
Slide 50
Under-blanching may cause more damage to food than the absence
of blanching does, because heat, which is sufficient to disrupt
tissues and release enzymes, but not inactivate them, causes
accelerated damage by mixing the enzymes and substrates. In
addition, only some enzymes may be destroyed which causes increased
activity of others and accelerated deterioration. Enzymes which
cause a loss of eating and nutritional qualities in vegetables and
fruits include lipoxygenase, polyphenoloxidase, polygalacturonase
and chlorophyllase. Two heat-resistant enzymes which are found in
most vegetables are catalase and peroxidase.
Slide 51
` Although they do not cause deterioration during storage, they
are used as marker enzymes to determine the success of blanching.
Peroxidase is the more heat resistant of the two, so the absence of
residual peroxidase activity would indicate that other less heat-
resistant enzymes are also destroyed.
Slide 52
The factors that control the rate of heating at the centre of
the product can be summarized as: the temperature of the heating
medium the convective heat transfer coefficient the size and shape
of the pieces of food the thermal conductivity of the food.
Slide 53
Blanching reduces the numbers of contaminating micro- organisms
on the surface of foods and hence assists in subsequent
preservation operations. This is particularly important in heat
sterilization, as the time and temperature of processing are
designed to achieve a specified reduction in cell numbers.
Blanching also softens vegetable tissues to facilitate filling into
containers and removes air from intercellular spaces which
increases the density of food and assists in the formation of a
head-space vacuum in cans
Slide 54
3.2 Equipment The two most widespread commercial methods of
blanching involve passing food through an atmosphere of saturated
steam or a bath of hot water. Both types of equipment are
relatively simple and inexpensive. Microwave blanching is not yet
used commercially on a large scale.
Slide 55
3.2.1 Steam blanchers` In general this is the preferred method
for foods with a large area of cut surfaces as leaching losses are
much smaller than those found using hot-water blanchers. At its
simplest a steam blancher consists of a mesh conveyor belt that
carries food through a steam atmosphere in a tunnel. The residence
time of the food is controlled by the speed of the conveyor and the
length of the tunnel. Typically a tunnel is 15m long and 11.5m
wide.
Slide 56
In conventional steam blanching, there is often poor uniformity
of heating in the multiple layers of food. The timetemperature
combination required to ensure enzyme inactivation at the centre of
the bed results in overheating of food at the edges and a
consequent loss of texture and other sensory characteristics.
Slide 57
Advantages and limitations of conventional steam and hot- water
blanchers
Slide 58
3.2.2 Hot-water blanchers There are a number of different
designs of blancher, each of which holds the food in hot water at
70100C for a specified time and then removes it to a dewatering-
cooling section. In the widely used reel blancher, food enters a
slowly rotating cylindrical mesh drum which is partly submerged in
hot water. The food is moved through the drum by internal flights.
The speed of rotation and length control the heating time. Pipe
blanchers consist of a continuous insulated metal pipe fitted with
feed and discharge ports. Hot water is recirculated through the
pipe and food is metered in. The residence time of food in the
blancher is determined by the length of the pipe and the velocity
of the water
Slide 59
Blanchers: (a) IQB steam blancher (after Timbers et al.
(1984)); (b) blancher cooler (from Hallstrom et al. (1988)) and (c)
counter- current blancher (after Wendt et al. (1983)).
Slide 60
IQB: IQB: Individual Quick Blanching
Slide 61
3.3 Blanching Effect on foods The heat received by a food
during blanching inevitably causes some changes to sensory and
nutritional qualities. However, the heat treatment is less severe
than for example in heat sterilization, and the resulting changes
in food quality are less pronounced. In general, the
timetemperature combination used for blanching is a compromise
which ensures adequate enzyme inactivation but prevents excessive
softening and loss of flavor in the food
Slide 62
3.3.1 Nutrients 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. The extent of vitamin loss depends on a
number of factors including: the maturity of the food and variety
methods used in preparation of the food, particularly the extent of
cutting, slicing or dicing
Slide 63
the surface-area-to-volume ratio of the pieces of food method
of blanching time and temperature of blanching (lower vitamin
losses at higher temperatures for shorter times) the method of
cooling the ratio of water to food (in both water blanching and
cooling).
Slide 64
Effect of blanching on cell tissues: S, starch gelatinised; CM,
cytoplasmic membranes altered; CW, cell walls little altered; P,
pectins modified; N, nucleus and cytoplasmic proteins denatured; C,
chloroplasts and chromoplasts distorted.
Slide 65
Effect of blanching method on ascorbic acid losses in selected
vegetables Differences in both steam versus water blanching and air
versus water cooling are significant at the 5% level Adapted from
Cumming et al.
Slide 66
Losses of ascorbic acid are used as an indicator of food
quality, and therefore the severity of blanching
Slide 67
3.3.2 3.3.2 Colour and flavour 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
according to their D value. Sodium carbonate (0.125% w/w) or
calcium oxide are often added to blancher water to protect
chlorophyll and to retain the colour of green vegetables, although
the increase in pH may increase losses of ascorbic acid.
Slide 68
Enzymatic browning of cut apples and potatoes is prevented by
holding the food in dilute (2% w/w) brine prior to blanching. When
correctly blanched, most foods have no significant changes to
flavor or aroma, but under-blanching can lead to the development of
off- flavors during storage of dried or frozen foods.
Slide 69
3.3.3 3.3.3 Texture One of the purposes of blanching is to
soften the texture of vegetables to facilitate filling into
containers prior to canning. 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 (for example certain varieties of potato) and in large pieces
of food. Calcium chloride (12%) is therefore added to blancher
water to form insoluble calcium pectate complexes and thus to
maintain firmness in the tissues
Slide 70
Chapter 5.Pasteurisation Pasteurization is a relatively mild
heat treatment, in which food is heated to below 100C. In low acid
foods (pH>4.5, for example milk) it is used to minimize possible
health hazards from pathogenic micro- organisms and to extend the
shelf life of foods for several days. In acidic foods (pH