freezingoffruitsandvegetables

1. Freezing of Fruits and Vegetables
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2.  WHAT IS FREEZING?
• It is a unit operation.
• Temparature of the food is reduced below its
freezing point generally to -18 °C or below.
• Change in state(Liquid state to Solid state).
• Allows preservation of taste, texture, and nutritional
value in foods.
• Provide a signifant extended shelf life.
• Retards growth of microorganisms.
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3. GRAPH SHOWING FREEZING PROCESS
FOR FOODS
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4. • Freezing point is defined as the temperature at
which the first ice crystal appears and the liquid at
that temperature is in equilibrium with the solid.
• Freezing time is defined as time required to lower
product temperature from its initial temperature to a
given temperature at its thermal center.
• Freezing rate is defined as the ratio of difference
between initial and final temperature of product to
freezing time.
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5. Principles of Freezing
Does not sterilize food.
Extreme cold (0 oF or -18 oC) colder):
◦ Stops growth of microorganisms and
◦ Slows chemical changes, such as enzymatic
reactions.
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6. Advantages of Freezing Foods
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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.
◦ But this is personal preference.
Foods can be frozen in less time than they can
be dried or canned.

7. Disadvantages of Freezing Foods
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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.

8. Chemical changes
• Enzymes in vegetables
 Are destroyed by heat, called blanching, before packaging and freezing.
• Enzymes in fruit
 Usually controlled by ascorbic acid (vit- C) or some other additives.
 Fruits are usually not blanched.
• Rancidity
 Another natural change that causes off-flavors, particularly in fatty foods.
 Best control is to keep as much air out of the package as possible.
Texture Changes
• Expansion of food
Water freezes and expands foods.
Vegetables with very high water content do not freeze well. Ex. Celery,
lettuce and tomatoes
Vegetables with lower water content become more compact. Ex. Spinach
and broccoli
• Ice crystals
How Freezing Affects Food
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9. Freezing Fruits
Frozen in many forms
◦ Whole, sliced, crushed, juiced.
Best quality
◦ Optimum maturity and freshness.
◦ Immature or over ripe both produce lower quality
when frozen.
Wash and work with small amounts at a time to
preserve best quality.
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10. Raw material
Water
Removal of low quality
produce
Sorting - grading
Packaging
Sugar addition
Washing
Freezing
Frozen storage -18˚C
Sale
Flow diagram of freezing process for fruit-based product.
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11. Sweetened Packs For Fruit
Syrup Pack
◦ Better texture.
◦ Not needed for safety.
◦ Fruits should be covered with syrup.
Place crumpled water-resistant paper in top of container.
Sugar Pack
◦ Sliced soft fruits (strawberries, peaches, etc.) make
their own syrup when mixed with the right
proportion of sugar.
◦ Layer fruit and sugar in bowl or pan.
◦ Allow mixture to stand 15 minutes to make juice
or “syrup” before packaging.
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12. Unsweetened Packs For Fruit
Dry Pack
◦ Good for small whole fruit such as
berries that don’t need sugar.
◦ Simply pack into containers and
freeze.
Dry Tray Pack
◦ Fruit pieces may be frozen individually,
in single layer, on a tray first.
◦ Freeze until firm then package in rigid
container or bag.
◦ Will pour out of container easily when
frozen.
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13. Freezing Vegetables
Select young, tender, high-quality vegetables.
Sort for size and ripeness.
Wash and drain before removing skins or shells.
Wash small lots at a time, lifting out of water.
DO NOT SOAK.
Work in small quantities, preparing as directed.
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14. Raw material
Water
Removal of low quality
produce
Sorting - grading
Hot water & Steam
Washing
Peeling-Trimming-Cutting-Sizing
Blanching(85-90˚C)
Deep in cold water (1-5 ˚C)
Packaging
Freezing
Frozen storage -18˚C
Sale
Flow diagram of freezing process of vegetable-based produ1c4t.

15. Types of Pack for Vegetables
Dry Pack
◦ Pack after the vegetables are blanched, cooled,
and drained.
◦ Pack quickly, pushing air out of package as you
work towards top.
Tray Pack
◦ After draining, spread pieces in a single layer
on a shallow pan.
◦ Freeze firm.
◦ After first hour, check often.
◦ Package quickly, pushing air out as you work.
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16. Fruits
• Most frozen fruits maintain high quality for 8 to 12
months
• Unsweetened fruits lose quality faster than fruits packed
in sugar or sugar syrups
Vegetables
• Most vegetables will maintain high quality for 12 to 18
months at 0°F or lower
• Use your home-frozen vegetables before the next year’s
crop is ready for freezing
Recommended Storage Times
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17. Thawing Foods For Serving
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18. Fruits
Best if served with ice crystals present.
Thaw:
o In refrigerator 6 to 8 hours per pound of fruit in syrup.
o At room temperature 1 to 2 hours per pound.
o At room temperature in cool water 1/2 to 1 hour per
pound.
o In microwave oven - follow manufacturer’s
instructions.
Dry sugar packs thaw faster than syrup packs.
Unsweetened packs thaw the slowest.
When used in recipes, allow for added sugar and more
juice.
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19. Vegetables
Most vegetables can be cooked without
thawing.
Leafy greens cook more evenly if partially
thawed.
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20. Packaging Materials
Moisture-vapor resistant
Durable and leak-proof
Not become brittle and crack at low temperatures.
Resistant to oil, grease, or water
Protects foods from absorption of off-flavors or odors
Easy to seal and mark
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21. Types of Packaging Materials
Rigid Containers
o Plastic freezer containers.
o Wide-mouth canning/freezing jars.
Good for liquids or soft, juicy, or liquid-
packed foods.
May be reusable.
Hold their shape and can be stored upright.
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22. Types of Packaging Materials
Non-Rigid Containers
o Bags
o Wrappings – plastic (such as polyethylene),
heavy-duty aluminum foil, laminated paper
 Good for firm, non-juicy foods.
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23. Labels
Name of product
Added ingredients
Form of food - halves, whole,
ground, etc.
Packaging date
Number of servings or
amount
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24. Consider
• Size
• Shape
• Efficiency
• Defrosting features
• Available floor area
• Amount of freezer space needed
Other features to consider
• Self defrosting or manual defrost.
• Receptacle clips prevent accidental disconnecting.
• Door locks.
• Drains for defrosting.
Selection Of Freezer
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25. Types of Freezers
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26. Refrigerator - Freezer
Combination
Air Blast Freezer Belt Freezer
Plate Freezer Immersion Freezer
Liquid Nitrogen Freezer 26

27. Location and Placement of Freezer
Place in convenient, cool, dry, well-ventilated
area.
Do not place by stove, range, water heater or
in the sun.
Do not push flush against wall. Leave space
for air circulation and cleaning.
Be sure freezer is level.
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28. Freezer Emergencies
If know power will be off, set freezer controls on -10oF
to -20oF immediately.
Do NOT open the door.
Foods stay frozen longer if freezer is full, well insulated
and in cool area.
◦ Full freezer - keeps 2 to 4 days.
◦ Half-full freezer - 24 hours.
Place dry ice on boards or heavy cardboard on top of
food.
Do not touch dry ice to skin.
Do not open freezer.
Room should be ventilated.
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29. Best Advice for Freezing
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Freeze foods quickly.
◦ Set freezer temperature at -10o F at least 24
hours ahead of freezing large quantities of
fresh food.
◦ Spread packages out around the freezer, until
frozen, then stack.
Hold at 0oF for best quality.

30. INDEX
Freezing fruits
• Production and harvesting
• Pre-process handling and operations
• Effect of ingredients
• Packaging
Freezing vegetables
• Crop cultivar, production, and maturity
• Harvesting
• Pre-process handling
• Blanching
• Packaging

31. A general flow chart
of frozen fruits and
vegetables

32. INTRODUCTION
• The quality demanded in frozen fruit products is mostly based on the
intended use of the product. If the fruit is to be eaten without any further
processing after thawing, texture characteristics are more important when
compared to use as a raw material in other industries.
• Fruits to be frozen are harvested in a fully ripe state and are soft in texture.
• Fruits have delicate flavors that are easily damaged or changed by heat
• attractive color is important for frozen fruits.
• Chemical treatments or additives are often used to inactivate the
deteriorative enzymes in fruits. Therefore, proper processing is essential
for all steps involved, from harvesting to packaging and distribution.

33. Fruit freezing
guide

34. Production and harvesting
• characteristics of raw materials are of primary importance in determining
the quality of the frozen product
 genetic makeup
 climate of the growing area
 type of fertilization
 maturity of harvest
The ability to withstand rough handling, resistance to virus diseases,
molds, uniformity in ripening, and yield are some of the important
characteristics of fruits in terms of economical aspects considered in
production.

35. Production and harvesting
• harvesting of fruits at an optimum level for commercial use is difficult.
Simple tests like pressure tests are applied to determine when a fruit has
reached optimum maturity for harvest.
• Color is also one of the characteristics used in determining maturity since
increased maturation causes a darker color in fruits.
• A combination of color and pressure tests is a better way to assess
maturity level for harvesting

36. Pre-process handling and operations
• Peeling and slicing
• fruits should be prepared prior to the freezing process in terms of
peeling, slicing or cutting.
• Peeling is done by scalding the fruit in hot water, steam or hot lye
solutions
• The effect of peeling on the quality of frozen products has been
studied for several fruits, including kiwi , banana, mango
• The rate of freezing can be increased by decreasing the size of
products frozen, especially for large fruits.
• An increase in the freezing rate results in smaller ice crystals, which
decreases cellular damage in fruit tissue.
• Banana, tomato, mango, and kiwi are some examples of large fruits
commonly cut into smaller cubes or slices prior to freezing

37. Pre-process handling and operations
• Blanching
• The objective of blanching is to inactivate the enzymes causing detrimental
changes in colour, odour, flavour, and nutritive value, but heat treatment
causes loss of such characteristics in fruits
• Therefore, only a few types of fruits are blanched for inactivation of
enzymes prior to freezing.
• The loss of water-soluble minerals and vitamins during blanching should
also be minimized by keeping blanching time and temperature at an
optimum combination

38. OBJECTIVES OF BLANCHING
• Removing the peel,
• Modifying texture,
• Brightens color
• Helps slow vitamin losses
• Inactivating enzymes (which otherwise cause loss of flavor, color and
texture)

40. Pre-process handling and operations
• Effect of ingredients
• Addition of sugars
• Sugars when dissolved in solutions act by withdrawing water from
cells by osmosis, resulting in very concentrated solutions inside the
cells.
• The high concentration of solutes depresses the freezing point and
therefore reduces the freezing within the cells, which inhibits
excessive structural damage
• Sugar syrups in the range of 30-60 percent sugar content are
commonly used to cover the fruit completely, acting as a barrier to
oxygen transmission and browning.

41. Packaging
• Fruits exposed to oxygen are susceptible to oxidative degradation,
resulting in
 browning
 reduced storage life of products
• methods currently employed for packaging frozen fruits.
1. Replacement of oxygen with sugar solution or inert gas,
2. consuming the oxygen by glucose-oxidase
3. use of vacuum and oxygen-impermeable films

42. Packaging
• most commonly used packaging materials based on penetration
properties and thickness
 Plastic bags
 plastic pots
 paper bags
 cans
• There are several types of fruit packs suitable for freezing:syrup pack
 syrup pack
 sugar pack
 unsweetened pack
 Tray pack
 sugar replacement pack

43. Sweetened Packs For Fruit
Syrup Pack
◦ Better texture.
◦ Not needed for safety.
◦ Fruits should be covered with syrup.
Place crumpled water-resistant paper in top of container.
Sugar Pack
◦ Sliced soft fruits (strawberries, peaches, etc.) make
their own syrup when mixed with the right
proportion of sugar.
◦ Layer fruit and sugar in bowl or pan.
◦ Allow mixture to stand 15 minutes to make juice
or “syrup” before packaging.
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44. Unsweetened Packs For Fruit
Dry Pack
◦ Good for small whole fruit such as
berries that don’t need sugar.
◦ Simply pack into containers and
freeze.
Dry Tray Pack
◦ Fruit pieces may be frozen individually,
in single layer, on a tray first.
◦ Freeze until firm then package in rigid
container or bag.
◦ Will pour out of container easily when
frozen.
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45. Freezing vegetables
• Freezing is often considered the simplest and most natural way of
preservation for vegetables
• Frozen vegetables and potatoes form a significant proportion of the market
in terms of frozen food consumption
• The quality of frozen vegetables depends on the quality of fresh products,
since freezing does not improve product quality.
• Pre-process handling, from the time vegetables are picked until ready to
eat, is one of the major concerns in quality retention.

46. Crop cultivar, production, and maturity
• The choice of the right cultivar and maturity before crop is harvested are
the two most important factors affecting raw material quality.
• Characteristics used as selection criteria
 Suitability for mechanical harvesting
 Uniform maturity
 Exceptional flavour
 uniform colour
 desirable texture
 Resistance to diseases
 High yield

47. Harvesting
• At optimum maturity, physiological changes in several vegetables take
place very rapidly. Thus, the determination of optimum harvesting time is
critical
• Some vegetables such as green peas and sweet corn only have a short
period during which they are of prime quality.
• If harvesting is delayed beyond this point, quality deteriorates and the crop
may quickly become unacceptable

48. Table. Vegetable
freezing guide

49. Pre-process handling
• Vegetables at peak flavor and texture are used for freezing.
• Postharvest delays in handling vegetables are known to produce
deterioration in flavor, texture, color, and nutrients
• Cooling vegetables by cold water, air blasting, or ice will often reduce the
rate of post-harvest losses sufficiently, providing extra hours of high
quality retention for transporting raw material to considerable distances
from the field to the processing plant

50. Blanching
• Blanching is the exposure of the vegetables to boiling water or steam for a
brief period of time to inactivate enzymes.
• Blanching also causes wilting or softening of vegetables, making them
easier to pack.
• It destroys some bacteria and helps remove any surface dirt
• Vegetables can be blanched in hot water, steam, and in the microwave.
• Blanching in hot water at 70 to 105 °C has been associated with the
destruction of enzyme activity.

51. THAWING
• Thawing is performed more slowly than freezing. This is mainly inherently
to the process for the following reasons
(1) the thawed outer layer of food has a lower thermal conductivity
than the frozen part
(2) the ambient temperature cannot be high, because of high
temperatures promoting biochemical processes, as enzymatic activity,
microbial decay or protein denaturation having adverse effects on
product quality.
Enzymes and microbes are often not (fully) inactivated, and proceed with
their activity upon rewarming

52. THAWING
Thawing is often left to the consumers, who either leave it in the open air in
the kitchen, or in the refrigerator or immersed in tap wate
Thawing can be achieved via
• pressure-shift freezing
frozen food are subject to high pressures of about 200 MPa at − 15 ∘C, by which the
freezing point shifts. Thawing can happen at subzero temperatures, and subsequently
rewarmed to room temperature
• Tempering
i.e., raised in temperature from − 18 to − 5 ∘C to allow for easy slicing
• electromagnetic means
• microwave,
• radio-frequency (RF) waves
• ohmic heating

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