Transcript FOOD ENGINEERING APPLICATIONS FST 318 BY SANNI, L.O. (Prof)/KAJIHAUSA, O. E Department of Food Science & Technology, University of Agriculture, PMB 2240, Abeokuta, Nigeria.

FOOD ENGINEERING APPLICATIONS
FST 318
BY
SANNI, L.O. (Prof)/KAJIHAUSA, O. E
Department of Food Science & Technology,
University of Agriculture, PMB 2240, Abeokuta, Nigeria.
Course Objectives
• To expose the students the relevance of thermo
physical properties to food processing.
• To teach them the importance of thermal
processing and its application in the food
industry.
• To teach the students the theory of food
dehydration and train them on the use of
different drying equipments.
• To expose them to the application of fluid flow to
food processing.
Grading
• Continuous Assessment Test – CAT - 20%
• Examination
- 70%
• Attendance
- 10%
• Total
- 100%
COURSE OUTLINE
• Lecture 1- Thermophysical Properties
• Lecture 2- Thermal Processing
• Lecture 3- Types of Thermal Treatments
• Lecture 4- Reaction Kinetics During Thermal Processing
• Lecture 5- Food Dehydration Theory and Applications
• Lecture 6- Application of Fluid Flow Theory
LECTURE ONE
Thermophysical Properties
Thermophysical properties can be simply defined
as material properties that vary with temperature
without altering the material’s chemical identity.
These
properties
will
include
thermal
conductivity and diffusivity, heat capacity,
thermal expansion and thermal relative
properties as well as viscosity and mass and
thermal diffusion coefficients, speed of sound,
surface and interfacial tension in fluids
Relevance to food processing
• They have important application in unit
operations which involve conduction of heat
through food to remove water e.g. drying,
frying, freeze- drying.
• They are important in the design of
manufacturing equipments.
LECTURE TWO
THERMAL PROCESSING
Thermal processing involves heating food, either in
a sealed container or by passing it through a heat
exchanger, followed by packaging.
Reasons for Heating Foods:
• to
inactivate
pathogenic
or
spoilage
microorganisms.
• to inactivate enzymes.
• to
induce physical changes and chemical
reactions, such as starch gelatinization protein
denaturation or browning.
Safety and Quality Issues
• The two most important issues connected
with thermal processing are food safety and
food quality.
• The two most important issues connected
with thermal processing are food safety and
food quality.
• Quality issues revolve around minimizing
chemical reactions and loss of nutrients and
ensuring that sensory characteristics
(appearance, colour, flavour and texture) are
acceptable to the consumer.
• There may also be conflicts between safety
and quality issues. For example, microbial
inactivation and food safety is increased by
more severe heating conditions, but product
quality in general deteriorates. To summarise,
it is important to understand reaction kinetics
and how they relate to:
 microbial inactivation
 chemical damage
 enzyme inactivation
 physical changes
Product Range
The products of thermal processing include
those which can be filled into containers and
subsequently sealed and heat-treated and
those which can be processed by passing
them through a continuous heat exchanger.
LECTURE THREE
Different types of Thermal Treatments
There are three different categories of thermal
treatments that have been developed to
obtain optimum quality products.
In-container processing
Aseptic processing
HIST (High Temperature short time)
processing.
1. In-container (In-can) processing – This is the
most conventional concept for thermal
processing. It involves placing a product in a
hermetic container and then thermally
processing the container and product. The
thermal process may be:
(a) Batch using a resort to provide the heating
holding and cooling phases of the thermal
investment.
(b) Continuous, with products rolling on
conveyors into a tunnel with three sections
(heating, holding, cooling).
In in-can processing, retorting (heat
processing) can be achieved through the
following ways:
 By saturated steam – latent heat is
transferred to food when saturated steam
condenses on the outside of the container.
 By hot water: Food are processed in glass
containers or flexible pouches bender hot
water with an over pressure of air.
 By flame – at atmospheric pressure using
direct flame heating of spinning cans.
2. Aseptic Processing
In aseptic processing, the product is
packed only after processing. It must
therefore be transported through equipment
where it will be heated, hold at the required
temperature for the required time, and then
cooled, and it must then be packed in an
aseptic environment, into sterilized packages.
The advantage stems from the resistance to
heat transfer that food products themselves
exhibit.
3. HTST (High Temperature Short Time) process
Whether the food is processed in-container or
aseptically, a HTST (High Temperature Short Time)
process would result or significant quality gains
and also minimizes energy costs and maximizes
productivity.
If we assume a constant temperature for
treatment, we could use any temperature high
enough to kill microbes.
The higher the temperature, the shorter the time
required. Therefore, if we use a higher
temperature we can achieve a microbial safety
target in shorter time – the higher the better.
LECTURE FOUR
REACTION KINECTICS
• When heat inactivation studies are carried out at
constant temperature, it is often observed that
microbial inactivation follows first order reaction
kinetics i.e. the rate of inactivation is directly
proportional to the population.
• The heat resistance of an organism is
characterized by its decimal reduction time (D),
which is defined as the time required in reducing
the population by 90% or by one order of
magnitude or one log cycle, i.e. from 104 to 103,
at a constant temperature, T.
• Temperature Dependence
Food scientists use a parameter known as the
z value, to describe temperature dependence.
This is based on the observation that, over a
limited temperature range, there is a linear
relationship between the log of the decimal
reduction time and the temperature .
This is used to define the z value of
inactivation of that particular microorganism
as follows: the z value is the temperature
change which results in a tenfold change in
the decimal reduction time.
LECTURE FIVE
FOOD DEHYDRATION
Food drying also called dehydration is the
process of removing water from a product in
order to reduce considerably the reactions
which lead to the product’s deterioration.
Removing water from the food product
inhibits the growth of microorganisms
(enzymes) and bacteria. The water is
eliminated by evaporation into the
surrounding air.
Drying properly requires mastering three fundamental
properties:
(i) The added thermal energy which heats the
products and set water migrating towards its surface
and turning into water vapour.
(ii) The capacity of the surrounding air to absorb the
water vapour given off by the product. This capacity
depends on the percentage of moisture already
present in the air before it enters the dryer and on the
air temperature.
(iii) The velocity of the air going over the products
surface, which must be high (up to a certain limit)
especially at the beginning of the drying process, in
order to take the moisture away rapidly.
Heat and Mass transfer
Whatever method of drying employed, food
dehydration involves getting heat into the product and
getting moisture out. These two processes – heat
transfer and mass (water) transfer out are not always
favoured by the operating conditions. The following
considerations are important in this regard.

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
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
Surface area
Temperature
Air velocity
Humidity
Atmospheric pressure and vacuum
Evaporation and temperature
Time and temperature
The Three Phases in a Drying Process
• This first phase (in which drying velocity increases) is short,
to non-existent, and corresponds to the rise in temperature
of the product until it reaches an equilibrium when the
product receives as much as heat from the air as it needs to
give to the water to vapourise.
• Constant-rate period
This is known as the constant-rate period and
continues until a certain critical moisture content is reached
.
• Falling-rate period
When the moisture content of the food falls below the
critical moisture content, the rate of drying slowly
decreases until it approaches zero at the equilibrium
moisture content (that is the food comes into equilibrium
with the drying air). This is known as the fallingrate period.
Drying methods and Equipment.
Some of the more common drying methods include
drum drying, spray drying, vacuum shelf drying,
vacuum belt drying, atmospheric belt drying, freezedrying, fluidized-bed drying, rotary drying, cabinet
drying, land drying, tunnel drying and others. Some of
these methods are particularly suited to liquids foods
and cannot handle solid food process, others one
suitable for solid foods or mixtures containing food
pieces. One useful division of drier types separates
them into air converting driers, drum or roller driers,
and vacuum driers, drum or roller driers, and vacuum
driers.
LECTURE SIX
APPLICATION OF FLUID FLOW THEORY
• Basic Fluid Properties.
The transport of a fluid (especially liquid) food
in a transport system is directly related to
liquid properties, primary viscosity and
density. These properties will influence the
power required for liquid transport as well as
the flow characteristics within the pipeline
LIQUID TRANSPORT SYSTEM
A typical liquid transport system will consist of
four basic components. The liquids products
will be contained in some vessels is the
conductor pipeline for liquid flow. Unless flow
can be achieved by gravity, the third primary
component is the pump, where mechanical
energy is used to enhance product transport.
The fourth components of the system is the
valve used to control or direct flow. The
vessels used in these types of system may be
of any size and configuration.
Pipelines for Processing Plants
The pipelines used for liquids foods
components have numerous unique features.
Probably the most evident feature is the use
of stainless steel for construction. This metal
provides smoothness, clean ability and
corrosion prevention. The corrosion resistance
of stainless steel is attributed to “passivity”
the formation of a surface film on the metal
surface when exposed to air.
Types of Pumps
Within the exception of situations where
gravity can be used to move liquid product or
product components, some type of
mechanical energy must be introduced to
overcome the forces opposing transport. The
two most popular types of pumps in the food
industry are centrifugal and positive
displacement. There are variations of each
type, but the concept of operation for each
type is the same.
1 centrifugal pumps
 volute type
 turbine type
2 positive displacement pumps
 reciprocating- I piston pump
II diaphragm
 rotary pumps- I lobe pump
II gear
III screw
IV peristaltic