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Blanching
• Blanching is a unit operation prior to freezing,
canning, or drying in which fruits or
vegetables are heated for the purpose of
inactivating natural/endogenous enzymes;
modifying texture; preserving color, flavor,
and nutritional value; and removing trapped
air.
Blanching (contd.)
Blanching serves a variety of functions!
--one of the main ones being to destroy enzymatic
activity in vegetables and some fruits, prior to
further processing by heat.
As such, 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
(particularly heat sterilization, dehydration and
freezing.
Blanching is also combined with peeling and/or
cleaning of food, to achieve savings in energy
consumption, space and equipment costs.
Without blanching, the fresh cauliflower on the left can
darken and become unappealing
Fresh After Browning reaction
BLANCHING OF FRUITS AND
VEGETABLES
OBJECTIVES:
1. Inactivate enzymes
A. Metabolic
B. Maintain color
C. Texture
D. Flavor
E. Nutritive value
BLANCHING OF FRUITS AND
VEGETABLES
OBJECTIVES (cont.):
2. Lower microbial load (about 90%)
3. Aids in packaging – wilts vegetables and
removes respiratory gases
4. Removes dirt, leaves, etc.
5. Aids in peeling
Blanching
 A few processed vegetables, for example onions
and green peppers, do not require blanching to
prevent enzyme activity during storage, but the
majority suffer considerable loss in quality if
blanching is omitted or if they are under-
blanched.
 To achieve adequate enzyme inactivation, food is
heated rapidly to a pre-set temperature, held for
a pre-set time and then cooled rapidly to near
ambient temperatures.
The factors which influence blanching time are:
• Type of fruit or vegetable
• Size of the pieces of food
• Blanching temperature
• Method of heating.
The factors which influence blanching time are:
• type of fruit or vegetable
• size of the pieces of food
• blanching temperature
• method of heating.
Theory
 The maximum processing temperature in freezing
and dehydration is insufficient to inactivate
enzymes. If the food is not blanched, undesirable
changes in sensory characteristics and nutritional
properties take place during storage.
 In canning, the time taken to reach sterilizing
temperatures, particularly in large cans, may be
sufficient to allow enzyme activity to take place.
It is therefore necessary to blanch foods prior to
these preservation operations.
 Under-blanching may cause more damage to
food than the absence of blanching does,
because heat, which is sufficient to disrupt
tissues and release enzymes, but not inactivate
them, causes accelerated damage by mixing the
enzymes and substrates. In addition, only some
enzymes may be destroyed which causes
increased activity of others and accelerated
deterioration.
 Enzymes which cause a loss of eating and
nutritional qualities in vegetables and fruits
include lipoxygenase, polyphenoloxidase,
polygalacturonase and chlorophyllase. Two heat-
resistant enzymes which are found in most
vegetables are catalase and peroxidase.
`
 Although they do not cause deterioration during
storage, they are used as marker enzymes to
determine the success of blanching.
 Peroxidase is the more heat resistant of the two,
so the absence of residual peroxidase activity
would indicate that other less heat-resistant
enzymes are also destroyed.
The factors that control the rate of heating at
the centre of the product can be summarized
as:
 the temperature of the heating medium
 the convective heat transfer coefficient
 the size and shape of the pieces of food
 the thermal conductivity of the food.
 Blanching reduces the numbers of contaminating
micro-organisms on the surface of foods and
hence assists in subsequent preservation
operations. This is particularly important in heat
sterilization, as the time and temperature of
processing are designed to achieve a specified
reduction in cell numbers.
 Blanching also softens vegetable tissues to
facilitate filling into containers and removes air
from intercellular spaces which increases the
density of food and assists in the formation of a
head-space vacuum in cans
Equipment
 The two most widespread commercial methods of
blanching involve passing food through an
atmosphere of saturated steam or a bath of hot
water. Both types of equipment are relatively
simple and inexpensive. Microwave blanching is
not yet used commercially on a large scale.
Steam blanchers
 In general this is the preferred method for foods
with a large area of cut surfaces as leaching
losses are much smaller than those found using
hot-water blanchers.
 At its simplest a steam blancher consists of a
mesh conveyor belt that carries food through a
steam atmosphere in a tunnel. The residence
time of the food is controlled by the speed of the
conveyor and the length of the tunnel. Typically a
tunnel is 15m long and 1–1.5m wide.
 In conventional steam blanching, there is often
poor uniformity of heating in the multiple layers
of food. The time–temperature combination
required to ensure enzyme inactivation at the
centre of the bed results in overheating of food
at the edges and a consequent loss of texture
and other sensory characteristics.
Advantages and limitations of conventional
steam and hot-water blanchers
Hot-water blanchers
 There are a number of different designs of
blancher, each of which holds the food in hot
water at 70–100ºC for a specified time and then
removes it to a dewatering-cooling section.
 In the widely used reel blancher, food enters a
slowly rotating cylindrical mesh drum which is
partly submerged in hot water.
 The food is moved through the drum by internal
flights. The speed of rotation and length control
the heating time.
Blanchers:
(a) IQB
steam
blancher
(after
Timbers et
al. (1984));
(b)
blancher–
cooler
(from
Hallstrom
et al.
(1988)) and
(c) counter-
current
blancher
(after
Wendt et al.
(1983)).
Blanching Effect on foods
 The heat received by a food during blanching
inevitably causes some changes to sensory and
nutritional qualities.
 However, the heat treatment is less severe than
for example in heat sterilization, and the
resulting changes in food quality are less
pronounced.
 In general, the time–temperature combination
used for blanching is a compromise which
ensures adequate enzyme inactivation but
prevents excessive softening and loss of flavor in
the food
#Nutrients
 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).
Effect of blanching on cell tissues:
S=starch gelatinised; CM=cytoplasmic membranes
altered; CW=cell walls little altered; P=pectins
modified; N=nucleus and cytoplasmic proteins
denatured; C=chloroplasts and chromoplasts distorted.
Effect of blanching method on ascorbic acid
losses in selected vegetables
Losses of ascorbic acid are used as an index of food
quality, and therefore the severity inidcator of blanching
Colour and flavour
 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.
 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.
 Enzymatic 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 flavor or aroma, but under-
blanching can lead to the development of off-
flavors during storage of dried or frozen foods.
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

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Blanching of Food

  • 1. Blanching • Blanching is a unit operation prior to freezing, canning, or drying in which fruits or vegetables are heated for the purpose of inactivating natural/endogenous enzymes; modifying texture; preserving color, flavor, and nutritional value; and removing trapped air.
  • 2. Blanching (contd.) Blanching serves a variety of functions! --one of the main ones being to destroy enzymatic activity in vegetables and some fruits, prior to further processing by heat. As such, 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 (particularly heat sterilization, dehydration and freezing. Blanching is also combined with peeling and/or cleaning of food, to achieve savings in energy consumption, space and equipment costs.
  • 3. Without blanching, the fresh cauliflower on the left can darken and become unappealing Fresh After Browning reaction
  • 4. BLANCHING OF FRUITS AND VEGETABLES OBJECTIVES: 1. Inactivate enzymes A. Metabolic B. Maintain color C. Texture D. Flavor E. Nutritive value
  • 5. BLANCHING OF FRUITS AND VEGETABLES OBJECTIVES (cont.): 2. Lower microbial load (about 90%) 3. Aids in packaging – wilts vegetables and removes respiratory gases 4. Removes dirt, leaves, etc. 5. Aids in peeling
  • 6. Blanching  A few processed vegetables, for example onions and green peppers, do not require blanching to prevent enzyme activity during storage, but the majority suffer considerable loss in quality if blanching is omitted or if they are under- blanched.  To achieve adequate enzyme inactivation, food is heated rapidly to a pre-set temperature, held for a pre-set time and then cooled rapidly to near ambient temperatures.
  • 7. The factors which influence blanching time are: • Type of fruit or vegetable • Size of the pieces of food • Blanching temperature • Method of heating.
  • 8. The factors which influence blanching time are: • type of fruit or vegetable • size of the pieces of food • blanching temperature • method of heating.
  • 9. Theory  The maximum processing temperature in freezing and dehydration is insufficient to inactivate enzymes. If the food is not blanched, undesirable changes in sensory characteristics and nutritional properties take place during storage.  In canning, the time taken to reach sterilizing temperatures, particularly in large cans, may be sufficient to allow enzyme activity to take place. It is therefore necessary to blanch foods prior to these preservation operations.
  • 10.  Under-blanching may cause more damage to food than the absence of blanching does, because heat, which is sufficient to disrupt tissues and release enzymes, but not inactivate them, causes accelerated damage by mixing the enzymes and substrates. In addition, only some enzymes may be destroyed which causes increased activity of others and accelerated deterioration.  Enzymes which cause a loss of eating and nutritional qualities in vegetables and fruits include lipoxygenase, polyphenoloxidase, polygalacturonase and chlorophyllase. Two heat- resistant enzymes which are found in most vegetables are catalase and peroxidase.
  • 11. `  Although they do not cause deterioration during storage, they are used as marker enzymes to determine the success of blanching.  Peroxidase is the more heat resistant of the two, so the absence of residual peroxidase activity would indicate that other less heat-resistant enzymes are also destroyed.
  • 12. The factors that control the rate of heating at the centre of the product can be summarized as:  the temperature of the heating medium  the convective heat transfer coefficient  the size and shape of the pieces of food  the thermal conductivity of the food.
  • 13.  Blanching reduces the numbers of contaminating micro-organisms on the surface of foods and hence assists in subsequent preservation operations. This is particularly important in heat sterilization, as the time and temperature of processing are designed to achieve a specified reduction in cell numbers.  Blanching also softens vegetable tissues to facilitate filling into containers and removes air from intercellular spaces which increases the density of food and assists in the formation of a head-space vacuum in cans
  • 14. Equipment  The two most widespread commercial methods of blanching involve passing food through an atmosphere of saturated steam or a bath of hot water. Both types of equipment are relatively simple and inexpensive. Microwave blanching is not yet used commercially on a large scale.
  • 15. Steam blanchers  In general this is the preferred method for foods with a large area of cut surfaces as leaching losses are much smaller than those found using hot-water blanchers.  At its simplest a steam blancher consists of a mesh conveyor belt that carries food through a steam atmosphere in a tunnel. The residence time of the food is controlled by the speed of the conveyor and the length of the tunnel. Typically a tunnel is 15m long and 1–1.5m wide.
  • 16.  In conventional steam blanching, there is often poor uniformity of heating in the multiple layers of food. The time–temperature combination required to ensure enzyme inactivation at the centre of the bed results in overheating of food at the edges and a consequent loss of texture and other sensory characteristics.
  • 17. Advantages and limitations of conventional steam and hot-water blanchers
  • 18. Hot-water blanchers  There are a number of different designs of blancher, each of which holds the food in hot water at 70–100ºC for a specified time and then removes it to a dewatering-cooling section.  In the widely used reel blancher, food enters a slowly rotating cylindrical mesh drum which is partly submerged in hot water.  The food is moved through the drum by internal flights. The speed of rotation and length control the heating time.
  • 19. Blanchers: (a) IQB steam blancher (after Timbers et al. (1984)); (b) blancher– cooler (from Hallstrom et al. (1988)) and (c) counter- current blancher (after Wendt et al. (1983)).
  • 20. Blanching Effect on foods  The heat received by a food during blanching inevitably causes some changes to sensory and nutritional qualities.  However, the heat treatment is less severe than for example in heat sterilization, and the resulting changes in food quality are less pronounced.  In general, the time–temperature combination used for blanching is a compromise which ensures adequate enzyme inactivation but prevents excessive softening and loss of flavor in the food
  • 21. #Nutrients  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
  • 22. 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).
  • 23. Effect of blanching on cell tissues: S=starch gelatinised; CM=cytoplasmic membranes altered; CW=cell walls little altered; P=pectins modified; N=nucleus and cytoplasmic proteins denatured; C=chloroplasts and chromoplasts distorted.
  • 24. Effect of blanching method on ascorbic acid losses in selected vegetables Losses of ascorbic acid are used as an index of food quality, and therefore the severity inidcator of blanching
  • 25. Colour and flavour  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.  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.
  • 26.  Enzymatic 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 flavor or aroma, but under- blanching can lead to the development of off- flavors during storage of dried or frozen foods.
  • 27. 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