Blanching:
Blanching
is a unit operation prior to freezing, canning, or drying in which fruits and
vegetables are heated for the purpose of inactivating enzymes, modifying
texture, preserving color, flavor and nutritional value and removing trapped air.
It
is a mild heat treatment but is not a method of preservation. It is a
pretreatment usually performed between preparation and subsequent processing.
It
is not intended as a sole method of preservation but as a pre-treatment which
is normally carried out between the preparation of the raw material and later
operations
How to done
blanching?
Blanching consists
of heating the food rapidly to a predetermined temperature, holding for a
specified time and then cooled rapidly to near ambient temperature or passing
immediately to the next processing stages.
Purpose of
blanching:
- The
major purpose of blanching is frequently inactivate enzymes which
otherwise would lead to quality reduction in the processed product.
Eg. With frozen products,
deterioration could take place, during any delay of processing, during freezing
or during subsequent thawing.
- Blanching
causes the removal of gasses from
plant tissues specially intercellular gas
Eg. Especially useful prior to
canning where help to achieve or create vacuum in container preventing
expansion of air during processing.
- To
bring about shrinkage in order to pack sufficiently high filling weights.
- It
act as a final cleaning and decontamination process.
Eg. It remove pesticide residue,
toxic constituent s, from the food material by leaching process during
blanching.
- To
prevent an increase in pressure developing In the can s during processing.
- To
prevent loss of color and flavor.
- It
also reduces microbiological content or vegetative cell content during
processing.
Eg. Which is useful for frozen or dry food where the chance
of multiplying microorganism have during thawing or dehydration.
Processing
condition:
- Under
blanching:
- Marker
enzymes
Factors that are necessary to considered
during blanching process:
Following factor
are must be considered for selecting time and temperature during blanching.
Fruits
and vegetables properties
-Thermal conductivity
-Degree of maturity]
Overall
blanching effect required for the processed product.
-achieving specified center
temperature
-inactivation peroxidase
-treating the nutrient content
Size
and shape of the food pieces
Method
of heating and temperature of the blanching medium.
N.B: time and
temperature combination may vary very widely for different food and different
process and must be determined specially any situation holding time 1-15 minutes at 70-1000C are normal.
Blanching
equipment:
There are two type
of equipment are used in blanching
- Steam
blanching.
- Hot
water blanching.
Steam blanching:
Steam blanching
consist of a mesh conveyor belt that carries food through a steam atom tunnel
,the residence time of the food is controlled by the space of the conveyor and
length of the tunnel .steam blancher are two type:
- Individual
quick blancher(IQB)
- Batch
fluidized-bed blancher
Individual quick blancher
(IQB):
Fig: IQB steam blancher
The equipment for IQB steam blancher (Fig. 10.1(a))
consists of a bucket elevator which carries the food to a heating section. The
elevator is located in a close fitting tunnel to reduce steam losses. A single
layer of food is heated on a conveyor belt and then held on a holding elevator
before cooling. The cooling section employs a fog spray to saturate the cold
air with moisture. This reduces evaporative losses from the food and reduces
the amount of effluent produced. Typically the equipment processes up to 4500
kg h of À 1food. The complete inactivation of peroxidase is achieved with a
minimum loss in quality, indicated by the retention of 76–85% of ascorbic acid.
It is three stage
blanching process in which the vegetable pieces are heated rapidly in the thin
layers by steam followed by holding in a deep bed where temperature equilibration take places, after which the
material is cooled down. the method has the advantages of short residence time
improved yield and reduced wastewater.
Batch
fluidized-bed blancher:
Batch fluidized-bed
blanchers operate using a mixture of air and steam, moving at approximately
4.5 m/s, which
fluids and heats the product simultaneously. The design of the blanching
chamber promotes continuous and uniform circulation of the food until it is
adequately blanched. Although these blanchers have not yet been widely used at
a commercial scale, they are reported to overcome many of the problems
associated with both steam and hot-water methods (Gilbertet al., 1980). The
advantages include:
•
faster,
more uniform heating
•
good
mixing of the product
•
a
substantial reduction in the volume of effluent
•
Shorter
processing times and hence smaller losses of vitamins and other soluble heat
sensitive components of food.
Hot water
blancher type:
- Reel hot water blancher:
- Pipe hot water blancher
Reel blancher:
Food enters a
slowly rotating cylinder mesh drum which is partially submerged in hot water.
The food is moved through the drum by internal flight. The speed of rotation
and length control the heating time.
Pipe blancher:
Pipe blancher
consists of a continuous insulated metal pipe fitted with feed and discharge
ports. Hot water is recirculates through the pipe and food is metered in. The
residence time of food in the blancher is determined by the length of the pipe
and the velocity of the water. The blanchers have the advantage of a large
capacity while occupying a small floor space. In some applications they may be
used to transport food simultaneously through a factory.
v
Advantage and
limitations of conventional steam and hot water blancher:
Effect
on food during blanching:
- Nutrient
content:
Some minerals, water-soluble
vitamins and other water-soluble components are lost during blanching. Losses
of vitamins are mostly due to leaching, thermal destruction and, to a lesser
extent, oxidation. The extent of vitamin loss depends on a number of factors including:
·
the
maturity of the food and variety
·
methods
used in preparation of the food, particularly the extent of cutting, slicing or
dicing.
·
the
surface-area-to-volume ratio of the pieces of food
·
method
of blanching
·
time
and temperature of blanching (lower vitamin losses at higher temperatures for shorter
times)
·
the
method of cooling
·
The
ratio of water to food (in both water blanching and cooling).
Losses of ascorbic
acid are used as an indicator of food quality, and therefore the severity of
blanching (Table 10.2).
- Color
and flavor:
Blanching brightens the colour of
some foods by removing air and dust on the surface and thus altering the
wavelength of reflected light. The time and temperature of blanching also
influence the change in food pigments according to their D value (Chapter 1).Sodium
carbonate (0.125% w/w) or calcium oxide are often added to blancher water to protect
chlorophyll and to retain the colour of green vegetables, although the increase
in pH may increase losses of ascorbic acid. Enzymic browning of cut apples and
potatoes is prevented by holding the food in dilute (2% w/w) brine prior to
blanching. When correctly blanched, most foods have no significant changes to
flavour or aroma, but under-blanching can lead to the development of off-flavours
during storage of dried or frozen foods (Chapters 15 and 21). Changes in colour
and flavour are described in more detail by Selman (1987).
- Texture:
One
of the purposes of blanching is to soften the texture of vegetables to
facilitate filling into containers prior to canning.
However, when used for freezing or drying, the time–temperature conditions
needed to achieve enzyme inactivation cause an excessive loss of texture in
some types of food (for example certain varieties of potato) and in large
pieces of food. Calcium chloride (1–2%) is therefore added to blancher water to
form insoluble calcium pectate complexes and thus to maintain firmness in the
tissues.
