Post on 28-Nov-2014
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.
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.
Differences between conventional drying and freeze drying
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.
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
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.
• 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.
Steps in Freeze Drying
• Freezing (-40°C) • Primary Drying.
• Secondary Drying.
• Heat and Mass Transfer in Freeze Drying
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.
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.
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
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%.
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.
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.
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.
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.
Fig. Freeze concentration plant
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.
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.
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.
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.
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
HTST Pasteurization System
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).
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.
system six essential components
• Three of the six components are heat exchangers (regeneration, heating, cooling).
• Timing pump• Flow diversion valve
• Holding tube
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.
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.
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.
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.
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.
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).
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.
Figure 2. Pasteurising using a plate heat exchanger
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.
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.
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
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.
Figure 1. Generic process diagram for fruit canning.
Figure 2. Generic process diagram for vegetable canning.
Figure 3. Generic process diagram for juice canning.
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.
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.
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.
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.
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.
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.
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.
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 )
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.
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.
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.
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.