FREEZING OF

FRUITS AND

VEGETABLES

In the 19th century the principles involving mechanical methods of producing intense cold were understood.

In 1823, Michael Faraday demonstrated that a gas could be changed to a liquid by the application of pressure.

By 1890 cold storage by mechanical refrigeration was established. Meat and fish could be stored successfully

In the 1920s, Clarence Birdseye started quick freezing foods, mainly fish, but also fruits, vegetables, meats and poultry.

He advocated the use of good quality raw materials, and careful handling, good packaging, quick freezing, and keeping the products at low temperatures during storage.

HISTORYHISTORY

INDIAN SCENARIO

The total production in the world is around 370 MT. India ranks first in the world with an annual output of 32 MT,

accounts for about 8% of the world’s fruit production. The fruit production in India has recorded a growth rate of 3.9%,

whereas the fruit processing sector has grown at about 20% per annum.

The growth rates have been extensively higher for frozen fruits & vegetables (121%)

There exist over 4000 fruit processing units in India It is estimated that around 20% of the production of processed

fruits is meant for exports India is the second largest producer of vegetables in the world

(ranks next to China) and accounts for about 15% of the world’s production of vegetables. The current production level is over 71 million MT

The chief function of freezing is to preserve food while maintaining its high quality.

This is accomplished by reducing the product temperature, thereby slowing the quality deterioration processes:

– The oxidation of fat– The growth of microorganisms– Enzymatic reactions– The loss of surface moistures (dehydration)

INTRODUCTIONINTRODUCTION

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- A method of food preservation whereby: The heat is removed (heat of fusion)

The temperature of the food is reduced below its freezing point (T<Tf)

A portion of water in food undergoes a change in state to form ice crystals (Aw lowered)

– Preservation achieved by: Low temperature Reduced water activity due to ice formation High concentration of solutes in unfrozen water Blanching of some foods

WHAT IS FREEZING ?WHAT IS FREEZING ?

NEED FOR THE DEVELOPMENT OF FROZEN FOODS

Now-a-days customer are more aware about the food safety

issues Customers are more interested in fresh foods Improvement in life style: customer can spend more money Frozen foods are chemical preservative free: therefore liked by

customers To deliver safe nutritive. wholesome and fresh processed food

to the customer Frozen foods have more become an essential item in retail food

industries

– Preservation:

Pathogen growth is halted below -4oC

Spoilage microorganisms don’t grow below -10oC.

Chemical reaction rates are significantly reduced.

–Processing aid:

Freezing changes the texture and viscosity for further processing, e.g. slicing meat products

– Product definition:

Freezing defines some food products, e.g. ice cream and frozen deserts.

WHY FREEZING WHY FREEZING ??

– To prevent growth of microorganisms by

Killing some bacteria (little effect) Reducing water activity Mechanical formation of ice crystals Osmotic changes in cell fluids Tying up some free water

– To lower temperature enough to slow down chemical reactions

Every 10°C decrease in temperature halves the reaction rate

CONT…..CONT…..

FACTORS AFFECTING QUALITY

1. ENZYMES 2. AIR 3. MICROORGANISMS 4. ICE CRYSTALS 5. FREEZER TEMPERATURE 6. EVAPORATION OF MOISTURE

– Water is the basis for freezing foods.

– In addition to water, foods contain a lot of soluble materials which slow down the movement of water molecules, and hence the freezing occurs at lower temperature.

– 1g of soluble materials will decrease the freezing point by 1oC.

PRINCIPLE OF FREEZINGPRINCIPLE OF FREEZING

– During freezing, heat is conducted from the interior of a food to the surface and is removed by the freezing medium.

– Sensible heat is first removed to lower the temperature of a food to the freezing point.

– In addition to the sensible heat, most foods have high specific heat and latent heat due to a large proportion of water.

– Therefore, a substantial amount of energy is needed to remove latent heat, form ice crystals, and hence to freeze foods .

CONT…..CONT…..

FREEZING CURVEFREEZING CURVE

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– The thermal conductivity of the food

– The area of food available for heat transfer

– The distance that the heat must travel through the food (size of the pieces)

–The temperature difference between the food and the freezing medium

–The insulating effect of the boundary film of air surrounding the food

-Packaging is an additional barrier to heat flow

FACTORS INFLUENCING FREEZING RATES

Tf = λp/(Өf - Өa)* [L/6(1/h+x/k1)+L2/24/k2 ]

Where,

Tf = freezing time, sec

L = length of cube (m) ,

H = surface heat transfer coefficient (Wm-2K-1)

Өa = temperature of the freezing medium

Өf = temperature of the food

Λ = latent heat of crystallization (J Kg-1)

P = density of the food (K gm-3)

X = thickness of the packaging, (m)

K1 = thermal conductivity of the packaging (Wm-1K-1)

k2 = thermal conductivity of the frozen, (Wm-1K-1)

FREEZING TIME CALCULATTION

FoodFood Water contentWater content

(%)(%)Freezing PointFreezing Point

((ooC)C)

VegetablesVegetables 78-9278-92 -08 to -2.8-08 to -2.8

FruitsFruits 87-9587-95 -0.9 to -2.7-0.9 to -2.7

Food processing technology by: P.J. Fellows

WATER CONTENT AND FREEZING POINT OF SELECTED FOODS

Freezing point of food : the temperature at witch a minute crystals of ice exists in equilibrium with the surrounding water

There are two type of nucleation:

Homogeneous nucleation (the chance orientation and combination of water molecules)

Heterogeneous nucleation ( the formation of nucleus around suspended partials or at a cell wall)

it occur in foods and take place during super cooling

ICE CRYSTAL FORMATIONICE CRYSTAL FORMATION

High rates of heat transfer produce large number of nuclei and as water molecule migrate to existing nuclei in preference to form new nuclei

Fast freezing therefore produces a large number of small ice crystals

Large differences in crystal size are found with similar freezing rate due to different type of food and even in similar foods which have received different pre-freezing treatment

CONT…..CONT…..

The volume of ice is 9% greater than the volume of water. Expansion of foods after freezing would be expected and depends

on: Moisture content (higher moisture, greater changes in volume) Cell arrangement (plant materials have intercellular air spaces, no

large changes in their overall size) Concentration of solutes (higher concentration less expansion) Freezer temperature Crystallized components (ice, fats, and solute) contract when they

are cooled and this reduces the volume of the food

CHANGES IN VOLUME

Fruits (whole, pureed or juice

concentrates): Strawberries Oranges Raspberries

– Vegetables: Peas Green been Spinach Sprouts and potatoes

MAJOR GROUPS OF COMMERCIALLY FROZEN FOODSMAJOR GROUPS OF COMMERCIALLY FROZEN FOODS

StrawberriesStrawberries Peas Broccoli

STEPS IN FREEZING FRUITS

1. Do not use underripe or overripe fruit. 2. Wash and sort fruit according to size. 3. Pare and remove pits, seeds, and blemishes. 4. Use an anti-darkening agent on fruits that turn

brown. Ascorbic acid preparations or mixtures of sugar and citric acid are the most effective

5. Use dry sugar, syrup, or unsweetened pack. The flavor, color, and texture of most fruits are retained best

when some sweetening is used. Dry Sugar—Sprinkle sugar over fruit (see specific fruit for

amount). Gently stir until pieces are coated with sugar and the juice is drawn out. Pack in containers, allowing recommended headspace for syrup-packed fruit.

Syrup—A 30 to 40% syrup is used for most fruits (see Syrup Proportions Chart on page 4). Dissolve sugar in water. Heavier syrups may be needed for very sour fruits. Allow ½ to 2⁄3 cup syrup for each pint. Add fruit and cover with additional syrup, leaving ½-inch headspace for wide-top pints, 1 inch for wide-top quarts, ¾ inch for narrow top pints and 1 ½ inch for narrow top quarts.

Unsweetened—Fruit also may be packed in its own juice, other fruit juices, or water to which an anti-darkening agent is added (1 teaspoon ascorbic acid per quart of water). Use an anti-darkening agent if the fruit is light colored or if you choose to use a nonnutritive sweetener. Unsweetened fruits lose quality faster than those sweetened with sugar or syrup.

6. Spread small, whole fruits like whole berries, in a single layer on a tray and freeze until nearly solid. Transfer to freezer bag or containers.

7. Store at 0⁰F or lower

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Freezing Fruits

Frozen in many forms – Whole, sliced, crushed, juiced.

Best quality – Optimum maturity and freshness. Immature or overripe both produce lower quality when

frozen. Wash and work with small amounts at a time to preserve best

quality.

Sweetened Packs for Fruit

Sugar Syrup Pack Better texture. Not needed for safety. Fruits should be covered with syrup.

Place crumpled water-resistant paper in top of container. 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.

Unsweetened Packs for Fruit

Dry Pack Good for small whole fruits such as berries that don’t need sugar. Simply pack into containers and

freeze. Or may be frozen individually, in

single layer, on a tray first.

Dry Tray Pack for Fruit

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.

Can remove only the amount needed at one time.

Fruit pieces retain shapes. Fruit pieces do not “clump” as when

packed directly into containers or with sugar syrup.

STEPS IN FREEZING VEGETABLES

1. Choose young, tender vegetables at their peak of flavor andtexture.

2. 2. Work in small quantities.3. Blanch in boiling water or steam 4.Cool immediately in cold water (60OF or less) for the same time as for blanching5. Two basic packing methods for freezing vegetables: DRY PACK—Pack tightly in freezer bags or rigid containers. Squeeze as much air as possible from bags before sealing. Leave ½- to ¾ inch headspace for expansion in containers. TRAY PACK—Place well dried vegetables in single layer on trays or plates6. Label with product and date. Freeze at once at 0OF or lower.7. Vegetables may be placed in a single layer on a tray and frozen until nearly solid8. Store at 0OF or lower.

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Preventing Flavor and Color Changes in Vegetables

Blanching

Primary method to destroy enzymes for vegetables. Will also soften hard veggies to make packaging easier. Will also remove some microorganisms. Under-blanching can be harmful; it will stimulate

enzymes and not destroy them. Check required blanching times for each food.

FREEZING EQUIPMENTS AND APPLICATION

FREEZING EQUIPMENTS AND APPLICATION

TYPES OF FREEZING

Quiescently –slow and large ice crystals form

Blast freezing –high volume of air aids in heat transfer

IQF (individually quick frozen) very low temperatures (sometimes as low as –198°C using liquid nitrogen)

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The rate of freezing required

The size, shape and packaging requirements of the food

Batch or continuous operation

The scale of production

Range of products to be processed

SELECTION OF FREEZING EQUIPMENTSSELECTION OF FREEZING EQUIPMENTS

FREEZING METHODS

1. Freezing in air Still-air sharp freezer Blast freezer Fluidized-bed freezer

Blast freezer

-Temp -300C and -400C

-Air velocity 1.5-6 m/s

-Throughput 200-1500 kg/hr

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2. Indirect contact freezing

–Single plate–Double plate–Pressure plate–Slush freezer (In all cases, food or food packages are in

direct contact with a surface that is cooled by a refrigerant)

3. Immersion freezing

–Heat exchange fluid–Compressed gas–Refrigerant spray Very fast freezing using liquid nitrogen IQF = Individually Quick Frozen

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INDIVIDUAL QUICK FREEZING

• Quick freezing is the only process whereby all the properties of most foodstuffs can be preserved.

• Process is ultra-rapid and practised at very low temperatures (-30°C to - 40°C) designed to halt the activities of the microorganisms

• It is possible to preserve and store raw fruit and vegetables in the same farm-fresh condition for more than a year, with the color, flavor and texture of produce remaining as good as fresh from the farm.

• In IQF, each piece is frozen individually using technique of fluidization resulting in freezing of fruit and vegetables only in 10 to 12 minutes which otherwise takes at least 3 to 4 hours or even more in the blast freezer.

• This results into better texture and there is no lump/ block formation and the product is free flowing. One does not have to thaw or defrost the whole packet to take out only a portion, and the rest will remain frozen till required again.

TRAY IQF FLUIDIZED TUNNEL FREEZER

The Perforated Tray Tunnel System utilizes a combination of horizontal and vertical pulsating air, continuously turning the product in the fluidization form.

The Tray tunnel are sectioned with different air flow to allow more air in the precool section for fast crust freezing, then it flows towards freeze section for more gentle and less air buoyant action to remove product latent heat.

The products are then fed onto the tray by shaker or infeed conveyor, and then poured onto the built up fluidized product about two to six inches deep depending on the product size.

The product line continuously meets an upward high velocity air stream, where the product is levitated in the mid air stream until it is crust frozen.

Then it flows and settles down the tunnel in fluidized state As the product is backed up to adjusted depth it

overflows the discharge gate. The gate height is adjusted according to the various product sizes and shape. Usually higher depth for a larger sized product and a lower depth for smaller sized product.

Pre assembled and tested at our plant; then it is disassembled in sections for shipping, and quickly reassembled at your site.

ADVANTAGES OF IQF TECHNIQUE Ice crystals formed are much smaller—

less damage to the cell structure or texture of the food

Very slow freezing period—less diffusion of salts and the separation of water in the form of ice

Product is quickly cooled below the temperature for growth of micoorganisms

The inherent speed and greater output

ADVANTAGES OF IQF FOODS

Close to natural freshness Better taste, flavor, aroma, color and appearance Higher nutritive value Require less time in cooking Greater convinience in handling and preparation More hygienic & chances of adulteration are

minimum Cent per cent edible portion of food available in

package— no syrup, no brine, no gravy Great variety of seasonal fruits and vegetables all the

year

LIMITATIONS OF IQF FOODS

Higher investments for freezing equipments Thawing problems Need for cold chain throughout the country Products are more prone to dehydration

because of large exposed surface area Small fluctuations in storage temperature

can result in thawing and refreezing of the product into a large lump

The main effect of freezing on food quality is damaged caused to cells by ice crystal growth.

The extent of damage depends on the

-size of the crystal

-The rate of heat transfer

-Differences in the verity and quality of raw materials

-The degree of control over pre-treatment

EFFECT OF FREEZING

a. slow freezing b. Fast freezing

Effect of freezing on plant tissue

CHANGES TO FROZEN FOODS DURING STORAGE

Degradation of pigments: -Chloroplast and chromoplasts are broken down and chlorophyll

is slowly degraded to brown pheophytin even in blanched vegetables.

In fruits, changes in pH due to precipitation of salts in concentrated solutions change in the colour of anthocyanin

Loss of vitamins: water soluble vitamins are lost at sub freezing temperature Losses of other vitamins are mainly due to drip losses, particularly in meat and fish

Residual enzyme activity : In vegetables which are inadequately blanched or in fruits, the most important loss of quality due to polyphenoloxidase activity which causes browning

and lipoxygenases activity which produces off-flavour and off-odours from lipids and causes degradation of carotene.

proteolytic and lipolytic activity in meats may alter the texture and flavour over long storage periods

Oxidation reaction: This reaction takes places slowly at -180C and causes off-odour and off-flavours

–Physically changes to ice crystals in their, shape size or orientation are collectively known as Recrystallisation

1.Isomass Recrystallisation: This is a change in surface shape or internal structure, usually resulting in a lower surface area to volume ratio

2.Accretive Recrystallisation: Two adjacent ice crystals join together to form a larger crystal and cause an overall reduction in the number of crystals in the food

3.Migratory Recrystallisation: this is an increase in the average size and a reduction in the average number of crystals, caused by the growth of larger crystals at the expanse of smaller crystals.

RECRYSTALLISATION

Migratory Recrystallisation is most important in most of foods and is largely caused by fluctuations in the storage temperature

Therefore a gradual reduction in the number of small crystals and an increase in the size of larger crystals, resulting in loss of quality similar to slow freezing

PACKAGING

keep food from drying out and to preserve nutritive value, flavor, texture and color.

A good packaging material should have the following characteristics:

• Moisture/vapor-proof or at least moisture resistant.

• Made of food grade material, i.e. designed to be used for food products.

• Durable and leakproof.

• Doesn’t become brittle and crack at low temperatures.

• Resistant to oil, grease or water.

• Protect foods from off flavors and odors.

• Easy to fill and seal.

• Easy to mark and store.

Types of Packaging Materials Rigid Containers

Plastic freezer containers. 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.

Non-Rigid Containers Bags Wrappings – plastic (such as polyethylene), heavy-duty

aluminum foil, laminated paper Good for firm, non-juicy foods.

Wholesomeness and freshness of the product is maintained

Loss of nutritive value very less as compared to the other preservation process

Least changes in sensory attributes of the products Shelf life is more. No need of chemical preservatives Food product can be

preserved in its natural form Least chemical, physical and microbial changes during

the storage

ADVANTAGES OF FREEZING

Comparatively costly technology

Lack of conceptual understanding

Processing cost is more than other preservation techniques

During storage, always low temperature (-180C) is required which becomes ultimately costly and also difficult to maintain in Indian conditions due to shortage of power

Difficult in the transportation of frozen foods

LIMITATIONS

Preservation and consumption of frozen fruits and their products is presently very negligible.

This technology needs to be adopted in our country to protect foods from spoilage and maintain the quality.

But this industry has expanded rapidly in industrially advanced countries like Australia and united states. This has been largely because of the publics acceptance of frozen foods as a high quality menu item.

Education of operative in cold storage and in the transport system is probably the best way to achieve these improvements. And the frozen foods can be made available in the off season also.

CONCLUSIONCONCLUSION

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