Low temp presentation

Food preservation by use
of low temperatures
CREDIT SEMINAR
Name of student : Anwar Hussain
Regd. No. : J-12-D-167-A
Division : Post Harvest Technology

Food Preservation
The aim of commercial food preservation is to
prevent undesirable changes in the
wholesomeness, nutritive value, or sensory
quality of food by economical methods which
control growth of microorganisms, reduce
chemical, physical and physiological changes
of an undesirable nature and avoid
contamination.

Techniques of preservation
The various techniques of preservation of food are
enlisted as:
Asepsis (Absence of infection)
Preservation by high temperature
Preservation by low temperature
Preservation by chemicals
Preservation by drying
Preservation by filtration
Preservation by fermentation
Preservation by salt and sugar
Preservation by acids
Preservation by irradiation
Srivastava and Kumar, (2008)

Preservation by low temperature
The use of low temperatures to preserve foods is
based upon the fact that the activities of food-
borne microorganisms and enzyme reactions
can be slowed down and/or stopped at
temperatures above freezing and generally
stopped at subfreezing temperatures.
Low temperature can be produced by:
1.Refrigeration or chilling
2.Freezing

Preservation of
food by
CHILLING

Chilling
 Preservation of foods at temperatures above
freezing and below 15ºC is known as
refrigeration or chilling. (Sareen, 2006)
 It is used to reduce the rate of biochemical and
microbiological changes and hence to extend
shelf life of fresh and processed foods.
 It causes minimal changes to sensory
characteristics and nutritional properties of
foods.

Chilling retards:
 Growth of microorganisms.
 Postharvest and post slaughter metabolic activities
of plant and animal tissues respectively.
 Deteriorative chemical reactions, including enzyme-
catalyzed oxidative browning or oxidation of lipids
and chemical changes associated with color
degradation, autolysis of fish and loss of nutritive
value of foods.
 Moisture loss.

Categories of chilled food
According to storage temperature range:
 -1ºC to 1ºC (fresh fish, meats, sausages,
smoked meat and ground meat)
 0ºC to 5ºC (pasteurized milk, canned meat,
cream, yoghurt, salad, sandwiches, baked
foods, soups and sauces)
 0ºC to 8ºC (Soft fruits and fruit juices, cooked
rice, hard cheese and butter)
Fellows, (2000)

Chilling equipments
Mechanical refrigerators
Cryogenic systems

Mechanical refrigerators
 Mechanical refrigerators have four basic
elements: an evaporator, a compressor, a
condenser and an expansion valve.
 A refrigerant circulates between the four
elements of the refrigerator, changing state
from liquid to gas and back to liquid.

Cryogenic systems
 Cryogenic chillers use solid carbon dioxide,
liquid carbon dioxide and liquid nitrogen.
 Solid carbon dioxide removes latent heat of
sublimation and liquid cryogens remove latent
heat of vaporisation.

Effect on foods
 The most significant effect of chilling on the
sensory characteristics of processed foods is
hardening due to solidification of fats and oils.
 Other effects include enzymatic browning,
lipolysis, colour and flavour deterioration and
retrogradation of starch to cause staling of
bread, protein denaturation, vitamin
degradation.

Loss of Vitamin C in selected vegetables during
cold storage
Storage conditions
Produce Days ºC ºF Losses (%)
Asparagus 1 1.7 35 5
7 0 32 50
Broccoli 1 7.8 46 20
4 7.8 46 35
Green beans 1 7.8 46 10
4 7.8 46 20
Spinach 2 0 32 5
3 1.1 34 7
Porter and Hotchkiss (2007)

Desirable consequences of chilling
temperature
 Growth of mesophilic and thermophilic microbes
is greatly retarded at chilling temperature.
Psychrotrophic microorganism, of course, grow
well in the range of 0ºC to 15ºC but is much
slower in this range.
 Rate of respiration and ripening usually declines
as the temperature is reduced below 4ºC in case
of climacteric fruits.

Effect of temperature on rates of respiration
in fruits and vegetables
Btu per ton per 24 hr at:
Commodity 0ºC (32ºF) 4.5ºC (40ºF) 15.5ºC (60ºF)
Apples 300-1500 600-2700 2300-7900
Peas (green) 8200-8400 13200-16000 39300-44500
Strawberries 2700-3800 3600-6800 15600-20300
Sareen (2006)

Undesirable consequences of chilling temperature
Cold shortening:
Animal muscle which is exposed immediately after
slaughter can undergo a detrimental occurrence
known as cold shortening if it is promptly cooled
to a temperature range of 0ºC to 5ºC
Chilling injury:
A substantial number of fruits and vegetables
especially those of tropical or subtropical origin,
develop a physiological disorders when exposed
to temperature but above freezing temperature

Superchilling
 Superchilling is one of the method that can be
used to maintain food products at a low
temperature.
 Generally, superchilling is positioned between
freezing and refrigeration (conventional
chilling), where the surrounding temperature is
set below the initial freezing point.
 It is a process by which the temperature of a
food product is lowered to -1 to -4 °C, by
means of slurry ice or in superchilled
chambers without ice.

Definitions of superchilling
Superchilling is that where temperature of food
is maintained below 0°C but ice crystals are
not generated. (Ando et al. 2004)
Superchilling is defined as a technology where
food is stored just below the initial freezing
temperature. (Beaufort et al. 2009)

Shelf-life aspects in relation to superchilling
technology
 Superchilling of fish: with the reduction of
temperature from -1°C to -3°C, the shelf-life
increased from 21 to 35 days (Carlson, 1969)
 Superchilled salmon: Shelf-life doubled at
storage temperature of -1.4°C to -3.6°C compared
to ice chilled storage with respect to microbial
and chemical analysis. (Duun and Rustad, 2008)
 Superchilled pork roasts: Shelf-life of at least
16 weeks compared to 2 weeks for the chilled
products (Duun et al. 2008)

Quality aspects in relation to superchilling
technology
 Storage of cold-smoked salmon at -2 °C for 14 days
did not have any serious consequences on the
quality compared to controls (absence of
superchilling) (Beaufort et al. 2009)
 During superchilled storage of kuruma prawn, the
brightness of tail colour could be retained compared
to traditional refrigeration (Ando et al. 2004)
 Drip loss was found to be lower in superchilled
samples than in chilled samples both in cod and
salmon fillets as well as in pork roasts (Duun and
Rustad, 2007)

Preservation of
food by
FREEZING

Definitions
Freezing is the unit operation in which the temperature of a
food is reduced below its freezing point and a proportion of
the water undergoes a change in state to form ice crystals.
Fellows, (2000)
Freezing process is a combination of the beneficial effects of
low temperature at which micro-organisms cannot grow,
chemical changes are reduced and cellular metabolic reactions
are delayed.
Delgado and Sun, (2000)

Theory of freezing
 Sensible heat is first removed to lower the temperature
of food to the freezing point.
 Heat produced by respiration in case of fresh foods
(fruits and vegetables) is also removed.
 Then latent heat of crystallisation is removed to form ice
crystals.
 The latent heat of crystallisation of other components of
food (e.g., fats) is also removed.

A schematic illustration of general freezing process

Calculation of freezing time
tF = ρfLf / TF - Ta (Pa / h + Ra2
/ k)
Plank’s equation

Effect of freezing air velocity
on freezing time
Effect of freezing air
temperature
on freezing time
Jafari et al. (2008)

Freezing techniques
Plate Freezing: Product is pressed
between hollow metal plates, either
horizontally or vertically with a refrigerant
circulating inside plates.
immersion Freezing: Food can be frozen
rapidly by direct immersion in liquid such as
brine, syrup, glycerol, etc. at low temperature
(-18 ºC).
CaBinet Freezing: Cold air is circulated in
a cabinet where product is placed on a tray.

air Blast Freezing: Refers to vigorous
circulation of cold air in order to freeze the
product. Air temperature is approx.-18 to -34 ºC.
Fluidized-Bed Freezing: Air is forced
upward through perforated belt to partially lift or
suspend particles.
tunnel Freezing: Product on trays are
placed in racks or trolleys and frozen with cold
air circulation inside the tunnel.

Novel methods of freezing
 HigH Pressure Freezing: Use of high pressure
promotes uniform and rapid ice nucleation thus
produces smaller crystals.
 deHydroFreezing: Food is dehydrated to a
desirable moisture and then frozen. It has the
advantage of less damage to plant texture.
 aPPliCation oF iCe nuCleation and
antiFreeze Proteins: ice nucleation protein
directly acts as a nucleus thus causes rapid
nucleation and ultimately rapid freezing while as
antifreeze proteins can retard recrystallisation in
frozen storage.
Bing L. et al, (2001)

0
50
100
150
Raw NT SO SU MA
Content(mg/100g)
Effect of osmotic pre-treatmentson Vit. C and Chlorophyll content
offrozen kiwifruit slices after 6 months of storage
Vitamin C
Chlorophyll
Torreggianni et al. (2000)

Changes during freezing
Loss in texture is the main problem that occur
during freezing. Water in inter cellular spaces of
fruits and vegetables freezes and ice-crystals are
formed which cause adjacent cell walls to rupture.
Freezing causes negligible change to pigments,
flavours or nutritionally important components,
although these may be lost in preparation
procedures.
Freezing causes death of 10% to 60% of the
microbe population and the %age gradually
increases during frozen storage.

PHysiCal asPeCts:
Freezer Burn: It is caused by sublimation
of ice from the surface of food into the air
inside the freezer. The surface of freezer
burned food appears as dry, grainy and
brownish spots and the tissues become dry
and tough.
reCrystallisation: This is largely caused
by fluctuations in the storage temperature.
When heat is allowed to enter the freezer, this
causes crystals to melt partially. When
temperature falls again, large sized crystals are
formed resulting in loss of quality.
Changes in frozen storage

CHemiCal asPeCts:
Loss of natural pigments like
chlorophyll and flavour in vegetables.
The major problem associated with
fruits is the oxidative rancidity and
loss of vitamin C due to the action of
enzymes.
Oxidative rancidity also cause off-
flavours.

Comparative organoleptic quality of apple slices
preserved by different methods
S.No
.
Quality
Attributes
Frozen slices
(% Score)
Canned slices
(% Score)
Dried slices
(% Score)
1. Freshness 71 14 5
2. Flavour 56 30 7
3. Appearance 62 28 3
4. Overall
acceptability
66 24 5
Pruthi (2006)

Changes in levels of chemical components in frozen tomato
cubes depending on the time and temperature of storage
isiewska et al,(1999)

IMPACT OF FREEZING RATE ON A PlANT
CEll

Advantages of quick freezing over
slow freezing
1. Ice-crystals formed are much smaller,
therefore cause much less damage to cell
structure or texture of foods.
2. There is more rapid prevention of microbial
growth.
3. Brief exposure of food components to
adverse constituents of freezing.
4. No microbial adaptation to low temperatures.
5. There is slowing of enzyme action.
6. Drip loss is less.

0
1
2
3
4
5
6
7
8
Colour Texture Flavour Wholeness
Evaluationnumber
Sensory evaluationresults of strawberry frozenby different freezing
methods after three months of storage
Quick freezing
method
Slow freezing
method
Source: Sahari et al. (2003)

0
1
2
3
4
5
6
7
8
9
Colour Texture Flavour Wholeness
Evaluationnumber
Sensory evaluation results of strawberry frozen
at different temperatures (˚C) after three
months of storage
-12
-18
-24
Source: Sahari et al. (2003)

Advantages of Freezing
 Many foods can be frozen.
 Good natural color, flavor and nutritive value can be
retained.
 Texture usually better than for other methods of food
preservation.
 Foods can be frozen in less time than they can be dried
or canned.
 Simple procedures.
 Adds convenience to food preparation.

Disadvantages
of Freezing
 Texture of some foods is undesirable because
of changes due to the freezing process.
 Initial investment and cost of maintaining a
freezer is high.
 Storage space is limited by how much the
freezer will hold.

Conclusion
Preservation by low temperature techniques is
superior to other methods of long term
preservation because it is more effective in
retaining flavour, colour and nutritive value of
food and moderately effective for preservation
of texture. It is comparatively a less time
consuming preservation method.

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