drying (partial), freeze drying, freeze concentration, and pasteurisation class lecture

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The heat used to dry foods or concentrate liquids by boiling removes water and therefore preserves the food by a reduction in water activity (A low moisture content is only an indication of food stability and not a guarantee. It is the availability of moisture for microbial growth that is more important and the term 'Water Activity' is used to describe this) The heat also causes a loss of sensory characteristics and nutritional qualities. In freeze drying and freeze concentration a similar preservative effect is achieved by reduction in water activity without heating the food, and as a result nutritional

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Page 1: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

The heat used to dry foods or concentrate liquids by boiling

removes water and therefore preserves the food by a reduction in

water activity (A low moisture content is only an indication of food

stability and not a guarantee. It is the availability of moisture for

microbial growth that is more important and the term 'Water Activity'

is used to describe this) The heat also causes a loss of sensory characteristics and

nutritional qualities. In freeze drying and freeze concentration a similar preservative

effect is achieved by reduction in water activity without heating the

food, and as a result nutritional qualities and sensory characteristics

are better retained. Both operations (freeze drying and freeze concentration) are slower than conventional dehydration and evaporation methods.

Page 2: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Energy costs for refrigeration are high and, in freeze drying, the

production of a high vacuum is an additional expense. This, together

with a relatively high capital investment, results in high production

costs for freeze-dried and freeze concentrated foods. Freeze drying is the more important operation commercially and is

used to dry expensive foods which have delicate aromas or textures

(for example coffee, mushrooms, herbs and spices, fruit juices, meat,

sea foods, vegetables and complete meals for military rations or

expeditions) for which consumers are willing to pay higher prices for

superior quality.Freeze concentration is not widely used in food processing but has

found some applications such as pre-concentrating coffee extract

prior to freeze drying, increasing the alcohol content of wine and

preparation of fruit juices, vinegar and pickle liquors.

Page 3: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Differences between conventional drying and freeze drying

Page 4: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Effect of freeze drying on foods

Freeze-dried foods have a very high retention of sensory

characteristics and nutritional qualities and a shelf life of longer than 12

months when correctly packaged. Volatile aroma compounds are not entrained in the water vapour

produced by sublimation and are trapped in the food matrix. As a result,

aroma retention of 80–100% is possible.The texture of freeze-dried foods is well maintained; there is little

shrinkage and no case hardening. The open porous structure (as shown in the figure) allows rapid and

full rehydration, but it is fragile and requires protection from mechanical

damage. There are only minor changes to proteins, starches or other

carbohydrates. However, the open porous structure of the food may

allow oxygen to enter and cause oxidative deterioration of lipids. Food is

therefore packaged in an inert gas.

Page 5: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Changes in thiamin and ascorbic acid content during freeze drying

are moderate and there are negligible losses of other vitamins.

However, losses of nutrients due to preparation procedures, especially

blanching of vegetables, may substantially affect the final nutritional

quality of a freeze-dried food.

Fig. Porous structure of freeze-dried food

Page 6: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

FREEZE DRYING • Moisture is removed from the solid state (ice)

directly to the vapor state by sublimation. Drying actually occurs in two steps, primary and secondary drying stages. It is in the primary stage that water is removed by sublimation, whereas vaporization of unfrozen liquid water molecules occurs in the secondary stage of drying.

Page 7: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

• Foods are dried in two stages: first by sublimation to approximately 15% moisture content and then by evaporative drying (desorption) of unfrozen water to 2% moisture content. Desorption is achieved by raising the temperature in the drier to near ambient temperature whilst retaining the low pressure.

Page 8: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Steps in Freeze Drying

• Freezing (-40°C) • Primary Drying.

• Secondary Drying.

• Heat and Mass Transfer in Freeze Drying

Page 9: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture
Page 10: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Primary Drying

Sublimation of ice is accomplished by controlling the vacuum

level in the freeze dryer and through careful heat input. A high

vacuum is desired to enhance sublimation rate. Dry ice sublimes at atmospheric pressure and room temperature. Frozen water will sublime if the temperature is 0 C or below and

the frozen water is placed in a vacuum chamber at a pressure of 4.7

mm or less. If the vacuum is maintained sufficiently high, usually within a

range of about 0.1 – 2 mm Hg, and enough heat (38 C) is supplied

to the material for the water to sublimate, it provides the driving

force for rapid sublimation. Within the vacuum chamber, heat is applied to the frozen food to

speed sublimation.

Page 11: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Introduction of heat is to supply energy to a plate on which the food

is sitting (conduction heating), while also providing a radiation source

above the product.

Page 12: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Sublimation Front

• The ice recedes into the food product as drying occurs. This boundary between frozen and dried product is called the sublimation front. Heat must be transferred into the product to this front to promote sublimation, and water vapor must then be removed by mass transfer through the dried product

Page 13: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture
Page 14: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Secondary Drying

• The secondary drying phase aims to remove unfrozen water

molecules, since the ice was removed in the primary drying

phase.

• In this phase, the temperature is raised higher than in the

primary drying phase, to break any physico-chemical

interactions that have formed between the water molecules and

the frozen material.

• Usually the pressure is also lowered in this stage to encourage

desorption (typically in the range of microbars, or fractions of a

pascal).

• At the end of the operation, the final residual water content in

the product is extremely low, around 1% to 4%.

Page 15: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Collapse Behavior*

• Rapid heat addition causes the temperature of the product to exceed its collapse temperature. product becomes sufficiently flowable that it "collapses!' During collapse, the pockets where ice crystals have sublimed disappear as the food slowly flows into these regions. This causes product to have higher density and reduces its ability to be rehydrate.

Page 16: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Collapse Behavior*Collapse is a phenomenon that occurs when the solid matrix of the

food stuff can no longer support its own weight, leading to drastic loss of

structure, decrease in volume, and reduction of porosity.

Freeze dried products are susceptible to collapse during storage

because of their high porosity.

Collapse in freeze dried matrices results in loss of desirable appearance

and in poor rehydration capacity.

If collapse in freeze drying, it causes sealing of capillaries, which in

turn makes dehydration difficult and gives uneven moisture distribution

throughout the product.

The collapse temperature may be defined as the threshold temperature

reached during elevation of freeze drying temperature at which the

normal freeze drying patterns ceases and collapse of the structure occurs.

Page 17: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture
Page 18: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Freeze concentration

Freeze concentration of liquid foods involves the fractional

crystallisation of water to ice and subsequent removal of the ice. This is achieved by freezing, followed by mechanical separation

techniques or washing columns. In particular, the low temperatures used in the process cause a

high retention of volatile aroma compounds. The process has high refrigeration costs, high capital costs for

equipment required to handle the frozen solids, high operating costs

and low production rates, compared with concentration by boiling. The degree of concentration achieved is higher than in membrane

processes, but lower than concentration by boiling. As a result of

these limitations, freeze concentration is only used for high-value

juices or extracts.

Page 19: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Equipment

The basic components of a freeze concentration unit are shown in

the Fig. These are:

• a direct freezing system (for example solid carbon dioxide) or

indirect equipment to freeze the liquid food

• a mixing vessel to allow the ice crystals to grow

• a separator to remove the crystals from the concentrated solution.Separation is achieved by centrifugation, filtration, filter pressing

or wash columns. Wash columns operate by feeding the ice-concentrate slurry into

the bottom of a vertical enclosed cylinder. The majority of the concentrate drains through the crystals and is

removed.

Page 20: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Fig. Freeze concentration plant

Page 21: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

The ice crystals are melted by a heater at the top of the column and

some of the melt water drains through the bed of ice crystals to

remove entrained concentrate.

Concentration takes place in either single-stage or, more commonly,

multi-stage equipment. Multi-stage concentrators have lower energy

consumption and higher production rates. Improvements in

techniques for generating large ice crystals and more efficient

washing have increased the maximum obtainable concentration to

45% solids.

Page 22: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Pasteurization and Blanching

The processes that utilize relatively mild thermal treatments to

achieve the desired results are pasteurization and blanching. Both

processes apply thermal treatment to food products in an effort to

improve the stability of the product during storage.

Although the magnitude of the thermal processes is similar,

application of the processes involves two distinctly different types of

food products. Pasteurization is most often associated with liquid

foods, while blanching is most often associated with solid foods.

The magnitude of thermal treatment used for both processes is not

sufficient to establish storage stability at room temperature. The

criteria utilized in establishing these modest thermal treatments are

rather specific and are different for different food commodities.

Page 23: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Purpose of Pasteurization Processing

Pasteurization is a mild thermal process applied to a liquid food

to increase the product shelf life during refrigeration and to destroy

vegetative pathogens (brucellosis and tuberculosis), Salmonella and

Listeria. In fruit juice, to inactivate enzymes.

Purpose of Blanching Processing

In fruits and vegetables to inactivate enzymes. To removes air from intercellular spaces of a fruit or vegetable Before canning raw fruits and vegetables and the more severe

thermal processes associated with commercial sterilization.

Page 24: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Description of Processing systems

Batch-type operation pasteurization Continuous high temperature, short-time (HTST) pasteurization

system Rotary hot water system blanching system Steam blanchers

Batch-type operation pasteurization

The vessel containing the product has a jacket where a heating

medium is introduced. The liquid product in the vessel is mixed to ensure uniform

temperature rise until the desired temperature is reached. The same jacket is utilized during cooling of the product by

introducing a cold medium and cooling the product contact surface.

Page 25: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture
Page 26: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Batch pasteurization systems can be relatively inexpensive and will

vary in capacity from small vessels to relatively large vessels. The

primary disadvantage of these systems is the inefficiency associated

with batch processing.

Continuous HTST Pasteurization System

Page 27: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

HTST Pasteurization System

Page 28: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

In one form of the HTST method, the milk is retained at a

temperature of 71.7°C (161°F) for 15 seconds. This is sufficient to kill the disease-causing bacteria, including the

tuberculosis bacillus, and most of the bacteria which cause souring. The milk is the cooled rapidly to below 10°C (50°F). If the milk were allowed to cool slowly conditions would

encourage the growth again of the milk-souring bacteria and so rapid

souring would take place. The cold incoming milk is first heated by the hot outgoing

pasteurized milk as the latter passes from the holding tube through

the regenerator. The pasteurized milk loses heat to the cold milk and thus is itself

partly cooled. The warmed incoming milk is then heated by hot

water in the heater to 71.7°C (161°F).

Page 29: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Its temperature is carefully maintained so that it enters the holding

tube at 71.7°C. The milk takes only 15 seconds to pass through the

holding tube its temperature remaining at 71.7°C during this time. It

then passes to the regenerator where it heats more incoming cold

milk (itself being slightly cooled) and is piped to the cooler in which

it is cooled by brine or chilled water to 4.4°–7.2°C (40°–45°F). A safety device, the flow diversion valve, automatically opens if

the temperature of the milk at the end of the holding period is below

71.7°C (161°F) and the milk is returned to the heating section. When

the required temperature is regained the valve closes and the

pasteurized milk passes to the regenerator. Freshly pasteurized milk is immediately filled into containers,

which have been thoroughly sterilized before hand to prevent the

milk from being reinfected with bacteria.

Page 30: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

system six essential components

• Three of the six components are heat exchangers (regeneration, heating, cooling).

• Timing pump• Flow diversion valve

• Holding tube

Page 31: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Heat exchangers• The regeneration, heating, and cooling sections of the

pasteurization system are heat exchangers. Most often, plate

heat exchangers are used. The plate heat exchanger is divided

into three sections, with the middle section serving as the

regeneration component, while the sections of the plate heat

exchanger on either side are used for heating and cooling.

• Plate heat exchangers are ideal for pasteurization systems with

these configurations. The heat exchange component of the

system is very compact. Plate heat exchangers provide very

efficient heat transfer from one low-viscosity liquid to another.

Page 32: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Ultra-high-temperature (UHT) pasteurization• When temp. exceeding the boiling point of water is used for pasteurization, It

is ultra-high-temperature (UHT) pasteurization, The systems requires

pressure control in regions where the product is elevated to temperatures

above the boiling point of water. In these systems, the pressure control would

be maintained throughout the time that the product is in the holding tube.

The timing pump

• The timing pump is a critical component of the pasteurization

system. This pump must be positive displacement and must be set

at a flow rate to ensure an established mass flow rate of product

through the system as long as the system is operating in forward

flow.

Page 33: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

The flow diversion valve• The FDV is controlled by a temp.-sensing device located at the exit of

the heating section. If temp. is below the desired temp., the valve

diverts flow to the entrance point. As soon as the established temp. is

reached, the flow diversion valve changes and the product moves

forward through the holding tube. This control device ensures safety

of product. The holding tube

• A holding tube has a known-diameter pipe designed to provide an

established residence time for product at the pasteurization temp.,

the critical time/temp. relationship needed for pasteurization is

achieved by the residence time requirement in the holding tube.

the length of holding tube ensures the necessary residence time

of product.

Page 34: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture
Page 35: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture
Page 36: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

The plate heat exchanger normally consists of corrugated plates

assembled into a frame. The hot fluid flows in one direction in

alternating chambers while the cold fluid flows in true counter-

current flow in the other alternating chambers.

Channels are formed between the plates and the corner ports are

arranged so that the two media flow through alternate channels.

The heat is transferred through the plate between the channels,

and complete counter-current flow is created for highest possible

efficiency. The corrugation of the plates provides the passage

between the plates, supports each plate against the adjacent one

and enhances the turbulence, resulting in efficient heat transfer.

Page 37: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Pasteurisation

Pasteurisation is a relatively mild heat treatment, in which food is

heated to below 100ºC. In low acid foods (pH > 4.5, for example milk) it is used to

minimize possible health hazards from pathogenic micro-organisms

and to extend the shelf life of foods for several days. In acidic foods (pH < 4.5, for example bottled fruit) it is used to

extend the shelf life for several months by destruction of spoilage

micro-organisms (yeasts or moulds) and/or enzyme inactivation. In both types of food, minimal changes are caused to the sensory

characteristics or nutritive value.

Page 38: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Pasteurisation of unpackaged liquids

The large scale pasteurisation of low viscosity liquids (for

example milk, milk products, fruit juices, liquid egg, beers and

wines) usually employs plate heat exchangers. Some products (for example fruit juices, wines) also require de-

aeration to prevent oxidative changes during storage. They are

sprayed into a vacuum chamber and dissolved air is removed by a

vacuum pump, prior to pasteurisation. The plate heat exchanger consists of a series of thin vertical

stainless steel plates, held tightly together in a metal frame. The plates form parallel channels, and liquid food and heating

medium (hot water or steam) are pumped through alternate

channels, usually in a counter-current flow pattern (Fig. 1).

Page 39: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Figure 1. Counter-current flow through plate heat exchanger: (a) one pass with four channels per medium; (b) two passes with two channels per pass and per medium.

Page 40: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Figure 2. Pasteurising using a plate heat exchanger

Page 41: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Each plate is fitted with a synthetic rubber gasket to produce a

watertight seal and to prevent mixing of the product and the heating

and cooling media.The plates are corrugated to induce turbulence in the liquids and

this, together with the high velocity induced by pumping, reduces the

thickness of boundary films to give high heat transfer coefficients. In operation (Fig. 2), food is pumped from a balance tank to a

‘regeneration’ section, where it is pre-heated by food that has already

been pasteurised. It is then heated to pasteurising temperature in a

heating section and held for the time required to achieve

pasteurisation in a holding tube. If the pasteurising temperature is not reached, a flow diversion

valve automatically returns the food to the balance tank to be

repasteurised.

Page 42: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

The pasteurised product is then cooled in the regeneration section

(and simultaneously preheats incoming food) and then further cooled

by cold water and, if necessary, chilled water in a cooling section. Pasteurised food is immediately filled into cartons or bottles and

sealed to prevent recontamination. Significant levels of spoilage and risks from pathogens can arise

from post-pasteurisation contamination, particularly when foods (for

example milk) are not re-heated before consumption, and great care

with cleaning and hygiene is therefore necessary. Products should then be stored at refrigerated temperature until

consumption.

Page 43: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

The canning of fruits and vegetables is a growing, competitive

industry. The industry is made up of establishments primarily

engaged in canning fruits, vegetables, fruit and vegetable juices;

processing ketchup and other tomato sauces; and producing natural

and imitation preserves, jams, and jellies.

Process Description

The primary objective of food processing is the preservation of

perishable foods in a stable form that can be stored and shipped to

distant markets during all months of the year. Processing also can change foods into new or more usable forms

and make foods more convenient to prepare.

Canned Fruits And Vegetables

Page 44: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

The goal of the canning process is to destroy any microorganisms

in the food and prevent recontamination by microorganisms. Heat is the most common agent used to destroy microorganisms. Removal of oxygen can be used in conjunction with other methods

to prevent the growth of oxygen – requiring microorganisms. In the conventional canning of fruits and vegetables, there are

basic process steps that are similar for both types of products. However, there is a great diversity among all plants and even

those plants processing the same commodity. The differences include the inclusion of certain operations for

some fruits or vegetables, the sequence of the process steps used in

the operations, and the cooking or blanching steps.

Page 45: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Figure 1. Generic process diagram for fruit canning.

Page 46: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Figure 2. Generic process diagram for vegetable canning.

Page 47: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Figure 3. Generic process diagram for juice canning.

Page 48: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Production of fruit or vegetable juices occurs by a different

sequence of operations and there is a wide diversity among these

plants. Typical canned products include beans (cut and whole), beets,

carrots, corn, peas, spinach, tomatoes, apples, peaches, pineapple,

pears, apricots, and cranberries. Typical juices are orange, pineapple,

grapefruit, tomato, and cranberry.Generic process flow diagrams for the canning of fruits, vegetables,

and fruit juices are shown in Figures 1, 2, and 3. The steps outlined in these figures are intended to the basic

processes in production. A typical commercial canning operation may employ the following

general processes: washing, sorting/grading, preparation, container

filling, exhausting, container sealing, heat sterilization, cooling,

labeling/casing, and storage for shipment.

Page 49: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

In these diagrams, no attempt has been made to be product specific

and include all process steps that would be used for all products. Figures 1 and 2 show optional operations, as dotted line steps, that

are often used but are not used for all products. One of the major differences in the sequence of operations between

fruit and vegetable canning is the blanching operation. Most of the fruits are not blanched prior to can filling whereas

many of the vegetables undergo this step. Canned vegetables generally require more severe processing than

do fruits because the vegetables have much lower acidity and contain

more heat-resistant soil organisms. Many vegetables also require more cooking than fruits to develop

their most desirable flavor and texture.

Page 50: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

The methods used in the cooking step vary widely among facilities.

With many fruits, preliminary treatment steps (e. g., peeling, coring,

halving, pitting) occur prior to any heating or cooking step but with

vegetables, these treatment steps often occur after the vegetable has been

blanched.

For both fruits and vegetables, peeling is done either by a mechanical

peeler, steam peeling, or lye peeling. The choice depends upon the type

of fruit or vegetable or the choice of the company.

Some citrus fruit processors produce dry citrus peel, citrus molasses

and D-limonene from the peels and pulp residue collected from the

canning and juice operations.

The peels and residue are collected and ground in a hammer mill, lime

is added to neutralize the acids, and the product pressed to remove

excess moisture.

Page 51: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

The liquid from the press is screened to remove large particles,

which are recycled back to the press, and the liquid is concentrated to

molasses in an evaporator. The pressed peel is sent to a direct-fired

hot-air drier. After passing through a condenser to remove the D-

limonene, the exhaust gases from the drier are used as the heat source

for the molasses evaporator. Equipment for conventional canning has been converting from

batch to continuous units. In continuous retorts, the cans are fed through an air lock, then

rotated through the pressurized heating chamber, and subsequently

cooled through a second section of the retort in a separate cold-water

cooler.

Page 52: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Commercial methods for sterilization of canned foods with a pH of

4.5 or lower include use of static retorts, which are similar to large

pressure cookers. A newer unit is the agitating retort, which mechanically moves

the can and the food, providing quicker heat penetration. In the

aseptic (Aseptic technique refers to a procedure that is performed

under sterile conditions ) packaging process, the problem with slow

heat penetration in the in-container process are avoided by sterilizing

and cooling the food separate from the container. Presterilized containers are then filled with the sterilized and

cooled product and are sealed in a sterile atmosphere. To provide a closer insight into the actual processes that occur

during a canning operation, a description of the canning of whole

tomatoes is presented in the following paragraphs.

Page 53: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture
Page 54: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

This description provides more detail for each of the operations

than is presented in the generic process flow diagrams in Figures 1, 2,

and 3.

Preparation –

The principal preparation steps are washing and sorting. Mechanically harvested tomatoes are usually thoroughly washed

by high-pressure sprays or by strong-flowing streams of water

while being passed along a moving belt or on agitating or revolving

screens. The raw produce may need to be sorted for size and maturity.

Sorting for size is accomplished by passing the raw tomatoes through

a series of moving screens with different mesh sizes or over

differently spaced rollers.

Page 55: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Separation into groups according to degree of ripeness or perfection of

shape is done by hand; trimming is also done by hand.

Peeling And Coring –

Formerly, tomatoes were initially scalded followed by hand peeling,

but steam peeling and lye peeling have also become widely used.With steam peeling, the tomatoes are treated with steam to loosen the

skin, which is then removed by mechanical means. In lye peeling, the fruit is immersed in a hot lye bath or sprayed with a

boiling solution of 10 to 20 percent lye. The excess lye is then drained and any lye that adheres to the tomatoes

is removed with the peel by thorough washing. Coring is done by a water-powered device with a small turbine

wheel. A special blade mounted on the turbine wheel spins and removes

the tomato cores.

Page 56: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Filling –

After peeling and coring, the tomatoes are conveyed by automatic

runways, through washers, to the point of filling. Before being filled, the can or glass containers are cleaned by hot

water, steam, or air blast. Most filling is done by machine. The containers are filled with the solid product and then usually topped

with a light puree of tomato juice. Acidification of canned whole tomatoes with 0.1 to 0.2 percent citric

acid has been suggested as a means of increasing acidity to a safer and

more desirable level.Because of the increased sourness of the acidified product, the addition

of 2 to 3 percent sucrose is used to balance the taste. The addition of salt is important for palatability (the quality of a food

that makes it acceptable or agreeable to one's personal taste )

Page 57: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Exhausting –

The objective of exhausting containers is to remove air so that the

pressure inside the container following heat treatment and cooling will be

less than atmospheric. The reduced internal pressure (vacuum) helps to keep the can ends drawn in, reduces strain on the containers during processing, minimizes the level of oxygen remaining in the headspace. helps to extend the shelf life of food products and prevents bulging of the container at high altitudes. Vacuum in the can may be obtained by the use of heat or by mechanical

means. The tomatoes may be preheated before filling and sealed hot. For

products that cannot be preheated before filling, it may be necessary to pass

the filled containers through a steam chamber or tunnel prior to the sealing

machine to expel gases from the food and raise the temperature.

Page 58: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture
Page 59: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Vacuum also may be produced mechanically by sealing containers

in a chamber under a high vacuum.

Sealing –

In sealing lids on metal cans, a double seam is created by

interlocking the curl of the lid and flange of the can. Many closing machines are equipped to create vacuum in the

headspace either mechanically or by steam-flow before lids are

sealed.

Heat Sterilization –

During processing, microorganisms that can cause spoilage are

destroyed by heat. The temperature and processing time vary with

the nature of the product and the size of the container.

Page 60: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Acidic products, such as tomatoes, are readily preserved at 100°C

(212°F). The containers holding these products are processed in

atmospheric steam or hot-water cookers. The rotary continuous cookers, which operate at 100°C (212°F),

have largely replaced retorts and open-still cookers for processing

canned tomatoes. Some plants use hydrostatic cookers and others use continuous-

pressure cookers.

Cooling –

After heat sterilization, containers are quickly cooled to prevent

overcooking. Containers may be quick cooled by adding water to the

cooker under air pressure or by conveying the containers from the

cooker to a rotary cooler equipped with a cold-water spray.

Page 61: Drying (Partial), Freeze Drying, Freeze Concentration, And Pasteurisation Class Lecture

Labeling And Casing –

After the heat sterilization, cooling, and drying operations, the

containers are ready for labeling. Labeling machines apply glue and labels in one high-speed

operation. The labeled cans or jars are the packed into shipping

cartons.