Refrigeration
Module- 15
Lec- 15
Dr. Shishir Sinha
Dept. of Chemical Engineering
IIT Roorkee
Chilling equipment
•
Mechanical Refrigerators
- Evaporate and compress the refrigerant in a continuous cycle
•
Cryogenic Systems
- Use solid and liquid CO , N directly in contact with the food
2
2
Alternative Classification Based upon the rate of movement of ice front
•
Slow Freezers 0.2 cm/hr
- Still air and cold stores
•
Quick Freezers 0.5-3 cm/hr
- Air blast and plate freezers
•
Rapid Freezers 5-10 cm/hr
- Fluidized bed freezers
•
Ultra rapid Freezers 10-100 cm/hr
- Cryogenic freezers
Slow Freezers:
Still Air and Cold Stores
•
Food is frozen in air circulating with natural convection. Air temp -20 to -30 C. Freezing
Rate 3-72 hrs. (Low heat transfer coefficient).
•
Fans are used in cold stores to increase heat transfer coefficient. Cold stores are used to
store foods, meat, to harden ice cream frozen by other methods
Quick Frreezer
Blast typ
pe, Plate
Blast Typ
pe
•
Air
A is circulatted at -20 to -40 C at 1.5
5-1.6 m/s
-
Batch
B
type
Air flow
w is parallel or perpendiccular to the food
f
Batch Frreezer
Blast Typ
pe
Source: Unit
U operatio
ons for food the food ind
dustries by: W.A. Gould
Rapid Frreezer: Fluiidized Bed
•
Food is contaained on a peerforated tray
y or conveyeer belt.
•
Air
A between -25
- to -35 C is passed at high velocitty (2-6 m/s)
•
Each
E
food com
mes in contaact with air individually
i
pid:
Ultra rap
Direct Co
ontact Liquid Nitrogen Tunnel
T
Freezzer
Cryogen
nic
What is chilling?
•
A unit operatiion in which
h temperaturee of a food iis reduced beetween -1 annd 8 C.
Effect
E
of temp
perature
•
Biochemical
B
used by micro-organismss and naturallly occurringg enzymes
changes cau
in
ncrease with temperaturee.
Therefore,
T
by
y reducing teemperature we
w reduce thee biochemiccal changes.
Why we chill foods?
Enhances Shelf life
•
To reduce the rate of biochemical activity (respiratory rate of foods)
•
To reduce the rate of microbial activity
•
To preserve sensory and nutritional value of foods
•
Consumers consider chilled foods as convenient, easy to prepare, high quality, healthy
natural and fresh
First order Kinetics
A+B=C+D
[A]=Concentration of A
d  A
dt
  k  A
Where k is reaction constant, given by Arrhenius equation:
k  Ae
Ea
RT
A is a constant, E is activation energy, R is universal gas constant
a
Categories of refrigerated foods based upon storage temperature
•
-1 to +1 C (Fresh meats, sausages and ground meats, smoked meats and breaded fish)
•
0 to +5 C (Pasteurized canned meat, milk, cream, yoghurt, prepared salads, sandwiches,
baked goods, fresh pasta, fresh soups and sauces, pizzas, pastries and unbaked dough
•
0 to +8 C (fully cooked meats, fish pies, cooked or uncooked cured meats, butter,
margarine, hard cheese, cooked rice, fruit juices and soft fruits
Effect of respiration rate
•
Refrigeration system should be designed so that respiration heat of foods is removed.
•
Therefore, it is important to know
– Refrigeration Heat of foods
– Whether the food is climacteric or non-climacteric
Climacteric Foods
•
Climacteric foods show a burst of ethylene production when they ripen
•
Their rate of respiration increases near the point of optimum ripeness
•
These foods ripen after harvesting
•
They soften and become sweeter during storage
Examples: apple, apricot, avocado, banana, mango, peach, pear, plum, tomato
Non-Climacteric Foods
•
Non climacteric foods do not show a burst in ethylene production when they ripen
•
They do not show increase in respiration rate
•
These foods ripen before harvest
•
These foods do not undergo many changes during storage
•
Examples: cherry, cucumber, fig, grape, grapefruit, lemon, pineapple and strawberry
Heat of respiration (Watts/tonne)
0C
10 C
15.5 C
Apples
10-12
41-61
58-87
Bananas
73-82
65-116
-
Oranges
9-12
35-40
68
Carrots
46
93
-
Potatoes
-
20-30
-
Chilling Injury
•
Chilling injury causes undesirable physiological changes (external or internal browning,
skin blemishes, failure to ripen)
•
Fruits and vegetables undergoing chilling injury should be stored above a certain
temperature
•
Chilling injury results from imbalance of metabolic activity
•
There is an overproduction of some toxic metabolites
Examples of foods undergoing chilling injury
•
Apples (<2-3 C)
•
Bananas (< 12-13 C)
•
Lemons (< 14 C)
•
Mangoes (<10-13 C)
•
Melons, pineapples, tomatoes
(< 7-10 C)
Effect on Meats
•
In slaughtered animals oxygen supply stops and aerobic respiration declines.
•
During anaerobic respiration glycogen is converted into lactic acid and pH of the meat
decreases. Rigor mortis starts.
•
Cooling during anaerobic respiration produces desirable texture and color and reduces
bacterial contamination.
•
Do not cool meat before rigor mortis has occurred otherwise cold shortening may occur.
Cold Shortening
•
Extremely tough meat (meat is inedible).
•
Darker meat color.
Heat Shortening of Meats
Heat shortening occurs when chilling is not sufficiently fast.
•
Meat becomes pale and tough
References

http://www.refrigerationbasics.com/

http://www.tulstar.com/products/chemicals/refrigerants/?gclid=CLbqv5Lpo7wCFWFS4g
odzFgANA

http://en.wikipedia.org/wiki/Heat_pump_and_refrigeration_cycle

http://en.wikipedia.org/
http://www.worldscientific.com/worldscinet/ijacrwiki/Ton_of_refrigeration

http://en.wikipedia.org/wiki/Refrigeration

http://www.worldscientific.com/worldscinet/ijacr