2. STERILIZATION
Heat sterilization 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, sterilized foods have a shelf life in excess of
six months at ambient temperatures.
3. Enzyme and microbial resistance to heat
Thermal calculations involve the need for knowledge of
the concentration of microorganisms to be destroyed
The acceptable concentration of microorganisms that can
remain behind
Thermal resistance of the targeted microorganisms
Temperature time relationship required for destruction of
the targeted organisms
4. Factors affecting sterilization time
The time and temperature required for the sterilization
of foods are influenced by several factors;
the heat resistance of micro-organisms or enzymes
likely to be present in the food
the heating conditions
the type of microorganisms found on the food
the size of the container
the acidity or pH of the food
and the method of heating
5. Enzyme and microbial resistance to heat
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.
The preservative effect of heat processing is due to the
denaturation of proteins which destroys enzyme
activity and enzyme-controlled metabolism in micro-
organisms
6. Enzyme and microbial resistance to heat
The rate of destruction is a first order reaction; - when
food is heated to a temperature that is high enough to
destroy contaminating micro-organisms, the same
percentage die in a given time interval regardless of
the numbers present initially
the rate of microbial death remains proportional to the
number of viable microorganisms present at any given time.
This is known as the logarithmic order of death
It illustrates how the number of viable microorganisms
decreases exponentially as they are exposed to a lethal
agent.
It is described by a death rate curve
7. Enzyme and microbial resistance to heat
The time required for the heat treatment is determined
by the D-value of the most heat resistant enzyme or
micro-organism which may be present in the product.
The D-value also known as the decimal reduction time
is the time required to reduce the targeted
microorganisms by a factor of 10 / 90% of the initial
load, or one log10 unit.
Can be used to calculate sterilization time
D-values differ for different microbial species
A higher D-value indicates greater heat resistance
8.
9. Enzyme and microbial resistance to heat
D= t / {log (a) – log (b)}
Where a and b represent the survivor counts
following heating for t min
10. Enzyme and microbial resistance to heat
Factors on which it depends:
Type of microbes: smaller microbes will die faster in
comparison to larger microbes
The shape of bacteria: bacteria with capsule and
cocci structure are harder to kill.
Temperature: More temperature will lead to more
effectiveness.
11. Enzyme and microbial resistance to heat
Z-value is the no-of degrees the temperature has to be increased to
achieve a tenfold (i.e. 1 log 10) reduction in the D-value.
the change in temperature required to achieve a tenfold (one log) change
in the D-value.
For example, if the Z-value of a population is 10 degrees, then
increasing the sterilization temperature 10 degrees will result in a log
reduction of the D-value.
The smaller the Z value, means greater sensitive are the
microorganisms towards change in temperature
For e.g.: bacteria with spores have a Z value ranging from 10°C to
15°C and bacteria without spores has a z value of 4°C to 6°C, now we
can notice that z value of bacteria without spores is lower as
compared to bacteria with spores because they are far easier to kill
and they are less resistant to heat
The value of Z for C. botulinum is 10˚C
12.
13. Enzyme and microbial resistance to heat
Z = T(2) – T(1) / {logD(1) – logD(2)}
Where D1 and D2 are D values at T1 and T2
respectively.
14. F-value
This is the time that is required to kill a specific population of
bacteria at a specific temperature / constant temperature
defined as an equivalent heating of 1 min at a reference
temperature, which is usually 121.1ºC
For e.g., if we want to kill one million bacteria of the same type it
requires one minute, at a temperature of 121°C to attain
commercial sterility. That one minute is the F-value for this
bacteria.
It is denoted by Fo.
F = D(logn1- logn2 )
where n1 = initial number of micro-organisms and n2=final number
of micro-organisms.
A reference F value (F0) is used to describe processes that operate
at 121ºC which are based on a micro-organism with a z value of
10ºC.
Typical F0 values are 3–6 min for vegetables in brine, 4–5 min
for cream soups and 12–15 min for meat in gravy
15. Heat penetration rate
The rate of heat penetration is measured by measuring
the temperature at the centre of a container (the point
of slowest heating) ]food during processing
The rate will depend on the following factors;
Type of product
Size of container
Agitation of the container
Temperature of the retort
Shape of the container
Type of glass
16. Types of sterilizable containers
There are four types;
a) Metal cans
b) Glass jars
c) Flexible pouches
d) Rigid trays
18. EVAPORATION
Evaporation, or concentration by boiling, is the partial
removal of water from liquid foods by boiling off water
vapour.
Simultaneous heat and mass transfer process occur resulting
in the separation of vapour from a solution.
Evaporation occurs where molecules obtain enough energy to
escape as vapour from a solution
It increases the solids content of a food and hence preserves
it by a reduction in water activity
commonly used to remove water from dilute liquid foods to
obtain a product with a desired solid concentration.
19. Evaporation is used to pre-concentrate foods (for
example fruit juice, milk and coffee) prior to drying,
freezing or sterilization
It reduces weight and volume.
This saves energy in subsequent operations and reduces
storage, transport and distribution costs.
There is also greater convenience for the consumer (for
example fruit drinks for dilution, concentrated soups,
tomato or garlic pastes, sugar) or for the manufacturer
(for example liquid pectin, fruit concentrates for use in
ice cream or baked goods).
Changes to food quality that result from the relatively
severe heat treatment are minimized by the design and
operation of the equipment.
20. During evaporation, sensible heat is transferred from
steam to the food, to raise the temperature to its
boiling point.
Latent heat of vaporization is then supplied by the
steam to form bubbles of vapor, which leave the surface
of the boiling liquid.
The rate of evaporation is determined by both the rate
of heat transfer into the food and the rate of mass
transfer of vapor from the food
21.
22. Factors affecting the rate of evaporation
Rate at which heat can be transferred to the liquid
Quantity of heat required to evaporate each kg of
water
Maximum allowable temperature of the liquid
Pressure at which the evaporation takes place
Changes that may occur in the food stuff during the
coarse of the evaporation process
23. HEAT AND MASS BALANCE
Heat and mass balances are used to calculate the
degree of concentration, energy use and processing
times in an evaporator.
The mass balance states that ‘the mass of feed
entering the evaporator equals the mass of product
and vapour removed from the evaporator’.
For the water component, this is given by:
24. HEAT AND MASS BALANCE
For solutes, the mass of solids entering the
evaporator equals the mass of solids leaving the
evaporator:
Total mass balance :
25. Factors influencing the rate of heat transfer
1. Temperature difference between the steam and
boiling liquid.
2. Deposits on heat transfer surfaces - fouling
3. Boundary firms -
26. EVAPORATOR COMPOSITION
A heat source: It is generally comprised of a heat
exchanger for the evaporation of water.
An evaporation vessel: It drives out water from the
product as vapor.
A vapor separation vessel: It separates vapor and
product.
A vacuum system: It draws water vapor out of the
separation vessel. This vacuum also reduces pressure
in the evaporation vessel, which reduces the boiling
point.
