2. Preservation of food
• Preservation is the “action taken to retain nutrients
and quality of perishable foods until final use”
• All living organisms are carrying on respiration,
the biochemical oxidation of organic nutrients.
• Principal reaction of combustion of carbohydrates
C6H12O6 + 6O2 6CO2+ 6H2O + heat
• Preservation techniques cannot improve
deterioration but only can slow down deterioration
to an acceptable rate
3. Preservation of food (Cont‘d...)
The respiration is often modeled to the entire
mass (Agri-products + all living organisms) by
combustion of carbohydrates
The reaction uses oxygen and assumes an
aerobic state in the agricultural mass
The inter-granular atmosphere (about 45% of
grain mass volume) supplies oxygen
continuously unless the mass is sealed
4. Preservation of food (Cont‘d...)
Measurements of CO2 evolved is often used to
gauge progression of the reaction
The living organism can include
Viable grain kernels
Fungi
Bacteria
Insects
Mites
Rodents and birds etc
5. Preservation of food (Cont‘d...)
Reasons of deterioration
At the time of harvest or sometimes before, these
living organisms begin to compete for grain
nutrients
As they succeed, the grain deteriorates
Preservation slows their reaction
Effectiveness of preservation
It cannot improve upon the agricultural products or
grains as it was at the time of harvest
It cannot even stop the deterioration completely
Preservation slows the deterioration to an
acceptable rate
7. Refrigeration
Refrigeration is a process of moving
heat from one location to another in
controlled conditions. The work of heat
transport is traditionally driven by
mechanical work, but can also be driven
by heat, magnetism, electricity, laser, or
other means.
8. Refrigeration- An effective
preservation method
Refrigeration works because the respiration and
metabolic rate of living organisms retard when their
temperature is decreased below an optimum range
The rate of reaction is approximately halved for
every 10°C decrease below the optimum range
Most common micro organisms in perishable foods
are mesophiles
9. Refrigeration- An effective
preservation method (cont’d…)
More quickly the product is cooled, the longer
it will remain marketable
Every hour lost before cooling to storage
temperature result in a loss of one day or more
of shelf life
10. Refrigerator and cold storage
plants
• A machine for producing cold
• Refrigerators and cold storage
plants are used for the
manufacturing of ice and for
cooling of perishable foods in the
storage chambers
11. Principle of refrigeration
Any reversible heat engine acts as a
refrigerator when the cycle is run in reverse
direction by means of external power
The reversible engine becomes a heat pump
which pump heat from a cold body and delivers
it to a hot body
14. Laws of thermodynamics
First law of thermodynamics
Heat and work are mutually convertable
Second Law of thermodynamics
Heat flows from a hotbody to a cold body
unaided but it is impossible for the heat to
flow from a cold body to hot body without
the aid of external work
15. Units of Heat
Calorie (Cal)
It is the amount of heat energy required to raise
the temperatre of one gram of water through 1°C
1 kCal = 1000 Calories (1 kg through 1°C)
1 Cal = 4.185 J
British Thermal Unit (BTU)
It is the abount of heat energy required to raise the
temperature of 1 pound (lb) of water through 1°F
1 BTU = 252 Cal = 1054.6 J
16. Units of Heat (Cont‘d...)
Centigrade heat unit (CHU)
It is the amount of heat energy required to raise
the temeperature of one pound of water through
1°C
Joule
It is the amount of heat energy required to raise
the temperature of 1/4.187 gram of water through
1°C
1 kJoule(kJ) = 1000 Joules
17. Work and Power
Work
Product of force into displacement; Work =
force×displacement (F×d)
Joule
When a force of one Newton is acted upon a
body and produces a displacement of one
meter. Then it is called one joule work
1 joule = 1N.m; and 1 kiloJoule (kJ) = 1 KN.m
Other units of work are erg and foot pound
kWh= 3600 kJ
18. Work and Power (Cont‘d...)
Power
It is the unit of power. Power =Work/time
Watts (W)
The rate of workdone @1Joule/sec
kiloWatts (kW) is the rate of workdone @
1kJ/sec
Other units are horse power. 1 horsepower =
33000 ft-lb/min or 550 ft-lb/sec
19. Sensible Heat
Sensible heat (Qs) amount of heat energy
required to raise the temperature of a substance
by increasing its internal energy
Mathematically
Q = m×Cp ×ΔT
Where
Qs= Heat supplied kJoule
M = mass of a substance(kg)
Cp = specific heat (kJ/kg.K)
ΔT = change in temperature, K
20. Sensible heat (Cont‘d...)
When an object is heated, its temperature rises
as heat is added. The increase in heat is called
sensible heat. Similarly, when heat is removed
from an object and its temperature falls, the
heat removed is also called sensible heat. Heat
that causes a change in temperature in an object
is called sensible heat.
21. Latent heat
Latent heat (Ql) amount of
heat energy used in
changing the phase of a
substance at constant
presure
Mathematically
Ql = m*hfg
Where
Ql = Latent heat (kJoule/kg)
m = mass of a substance (kg)
Hfg = specific latent heat of
freezing (kJ/kg)
22. Latent heat (Cont‘d...)
Latent heat: the heat required to convert a
solid into a liquid or vapour, or a liquid into a
vapour, without change of temperature.
The enthalpy of fusion also known as Latent
heat of fusion is the change
in enthalpy resulting from heating a given
quantity of a substance to change its state from
a solid to a liquid. The temperature at which
this occurs is the melting point.
24. Specific Heat
The specific heat is the amount of heat per
unit mass required to raise the temperature by
one degree Celsius.
The specific heat of wet agricultural materials
is the sum of sp. Heat of bone dry material and
its moisture contents. If “cd” and “cw” are the
specific heats of bone dry material and water,
“m” is the moisture content of the material in
% on wet basis, then the specific heat is
expressed as;
C=
𝑚
100
𝑐𝑤 +
100−𝑚
100
𝑐𝑑 K Cal/kg 0C
25. Units of Refrigeration
Practical unit of refrigeration is expressed
in terms of tonne of refrigeration (TR)
The amount of refrigeration effect produced
by the uniform melting of one tonne (1000
kg) of ice from and at 0°C in 24 hours
The latent heat of ice is 335 kJ/kg
1 TR = 1000 x 335 kJ / 24 hours
= 1000 x 335 /( 24 x 60)
= 232.6 kJoule/min
In actual practice, one tonne of refigeration
is taken as equivalent to 210 kJ/min or 3.5
kW (as 1kJ/sec= 1kW)
26. Units of Refrigeration (Cont‘d...)
A ton of refrigeration is a unit of power to
describe the heat-extraction capacity
of refrigeration and air conditioning equipment.
The roots for refrigeration are in the ice making
industry, and the ice manufacturers wanted an
easy way of understanding the size of a
refrigeration system in terms of the production
of ice. If 288,000 Btu are required to make one
ton of ice, divide this by 24 hours to get 12,000
Btu/h required to make one ton of ice in one
day. This is the requirement for the phase
change from liquid to solid — to convert water
at 0°C (+32°F) into ice at 0°C (+32°F). As a
practical matter, additional refrigeration is
required to take water at room temperature and
turn it into ice.
27. Co-efficient of performance
The ratio of heat extracted from the refrigerator to
the work done on the refrigerator
COP = Q / W
where
Q = Heat abstracted from the refrigerator
W = Amount of work on the refrigerator
