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Physio-chemical changes in tubers during storage
Sprouting of Tubers
- Sprouting is unwanted germination of tubers. It starts
at temperatures more than 50 deg F in storage.
- If more light falls on tubers at 40 deg F, it
also starts growing in tubers. It reduces
- Storage life
- Level of vitamin C
- Quality of cooking, processing and germination
- It also changes chemical composition.
Greening of Tubers
- Greening in potatoes is being induced when they are
exposed to light continuously for sometime.
- The colour becomes green and taste bitter. The growing
occurs:
- Glycoalboloid
- Solenine
- Chaconine
It deteriorates the quality of cooking and
changes chemical composition of Tubers also.
Sweetening of Tubers
- When potatoes are stored at temperature below 35 deg
F, the starch starts converting into sugar by which
tubers become sweet.
- The accumulation of sugar changes the peeled tubers
to brown and chips become black. If respiration rate
is low, sweetening will be high. This changes flavor
and quality of potatoes.
Rotting of Tubers
- The rotting of tubers is caused by micro-organisms
entered in peeled, cut, bruised and contaminated tubers
in presence of favorable conditions available for
their survival. It results
- Odour becomes foul
- Eyes become black
- When they are pressed, leak water
- Black heart is caused due to abnormal rate of respiration
in poor ventilation and fungus causing blight grows
in tubers which are stored at low temperature for 10-15
days. Any bruise on tubers may cause infection by leak
fungus.
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Factors Governing Safe Storage of Potatoes
Potato Conditions
Tuber Variety
- The autumn crop can resist a wide range of temperature
and withstand low humidity.
- While crop is not capable of tolerating higher temperatures.
- Kufri sheetman can tolerate frost and low temperatures,
while Kufri Sandhuri higher temperatures.
Tuber Maturity
- Mature tubers can be stored for long period under safe
storage conditions while immature tubers are easily affected
by fluctuations in environmental conditions and infected
by micro-organisms.
Tuber Health
The potatoes free from mechanical injuries like
cuts, bruises and scraping as well as from biological injuries
like
- Disease Infection
- Insect Attack
- Rotten Condition
- Sprouting
Can be stored for longer periods as compared
to unhealthy tubers
Handling Conditions
Careful Handling
- During harvesting, grading, plucking and stacking, the
potatoes should be handled carefully so that there may
be no damage to outer peel through cutting, bruising and
cracking etc.
Suitable Farm Practices
- Before harvesting the potato crop should be undergone
efficient and suitable farm practices like earthing, irrigation,
fertilization and pesticide application so that the tubers
may achieve proper health and strengths before harvesting.
Sanitary Conditions In Storage
- The storage walls, ceiling, filling boxes and equipment
should be free from insects and disease organisms. The
air circulation and water distribution should be free
from contamination.
Environmental Conditions
Temperature
- The best storage temperature for potatoes are reported
to be 40-45 deg F.
- The lower and higher temperature cause some changes
in cooking, processing and germination qualities, increase
storage losses and also reduce storage life of potatoes.
At lower temperature below 40 deg F
- The tubers start converting starch into sugars and tubers
become sweet.
- The peeled tubers turn brown and colour of chips become
black
- Seed potatoes become less affective for germination
as their dormancy breaking capacity decreases rapidly
- There is rapid loss of vitamin C
- It causes infection of blight, blank heart mahogany
diseases
- The wound healing and skin hardening capacity decrease
rapidly.
At high temperature above 60 deg F
- The respiration rate increases while reduces storage
life.
- There is moisture loss and cause shrinkage, writhing
and sponginess in tubers.
- Cause sprouting which reduces storage life and quality
of germination.
Humidity
- The best relative humidity for storage of potatoes is
80-85 per cent at which there occurs minimum shrinkage
and best physiological processes can be maintained.
- If relative humidity is lowered below 70%, the loss
of moisture by transportation becomes faster than the
loss of solids by respiration, while results in decrease
in specific gravity and tubers become light and spongy.
- The specific gravity is the index of eating and cooking
quality of tubers.
- The temperature and ventilation are the controlling
agencies of humidity.
- The control of humidity in storage poses problems of
condensation on walls and ceiling.
- The condensed moisture dripping on the tubers may cause
rotting and other excessive losses.
- When moisture is added to the storage atmosphere, all
water droplets should completely vaporize into air before
coming in contact with tubers.
Ventilation
- Ventilation affects the quality of potatoes in storage
because of its relation to temperature and humidity of
the storage atmosphere.
- Ventilation usually may imply the introduction of outside
air or merely the circulation of air within the storage.
- The introduction of outside air is frequently a quick
means to change temperature and humidity in the storage
with motor operated thermostatically controlled ventilation.
- Ventilation usually tends to lower the relative humidity
surrounding the tubers unless the incoming air or recirculated
air reaches to saturation condition.
- Storage operations should be aware of the influence
of ventilation on the humidity in air surrounding the
tubers as well as in providing a more uniformity desirable
temperature.
- Hence Ventilation can have a considerable effect on
the quality of potatoes.
- The optimum air velocity for storage Ventilation is
about 250-300 ft / min
Light
- Tuber exposed to light either natural or artificial
gradually develops chlorophyll and Solanine formation
in their tissues and become green in colour.
- The green tuber have bitter taste and slowly develop
undesirable quality due to green colour.
- If the tubers are exposed to light a continuously long
period, photosynthesis occurs frequently which brings
some physio-chemical changes in the tubers. and hence
affects the chemical composition of tubers.
- The transportation losses of moisture also occur rapidly
in presence of light.
- Photeolytic enzymes become active during light and cause
early ripening of tubers.
- Further activity of these enzymes may cause rotting
of tubers.
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Five basic requirements for safe storage of potatoes
- To provide satisfactory environmental conditions
- To control disease causing organisms, enzymes and
insects
- To control sprouting in tubers during storage
- To maintain potato firmness to prevent black spot
- Gupta, R.R. 1978. Evaluation of storage characteristics
of potatoes with evaporative cooling system.
- Master of technology PAU Ludhiana thesis.
- In Punjab, Seed tubers are stored in bags in cold stores
at 4 deg-5 deg C and 90-95% relative humidity.
- The seed material should be kept in cold store by
15 March as the delay affects the productivity of
seed tubers.
- High temperature leads to increased rate of respiration
and water loss in tubers which affect their metabolic
balance during storage.
- Compared to ordinary store, the weight losses and
rotting of tubers are negligible in cold stores.
- Tubers moth, which is a serious problem before the
advent of cold stores, has almost disappeared.
- The only disadvantage of cold storage is that the
leaf-roll virus is preserved with the seed tubers
at low temperature.
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Storage Design
- Scientifically the most suitable shape is a sphere but
the most practical shape is cube. The economical shape
depends on the balance in cost between two opposing factors,
viz., (a) narrower span reduces cost and (b) the shortest
length of walls is provided by square shaped building.
- Increase in height of building, as much as possible,
reduces cost per unit material stored
- Exterior surfaces should be protected against wind and
rain. They must be water proof but not impermeable to
the passage of water vapour from inside. Oil tempered
hard wood is resistant to moisture and could be used as
cladding material for both external and internal applications.
- Solar radiation falling on internal surface may raise
its temperature as much as 15.6 deg C above that of the
atmosphere. Part of the heat is dissipated but part is
transmitted through walls and roof. Heat gains through
walls at 20 deg N latitude are lower on the south than
on the east and west. South-East and South- West orientation
occupies intermediate position. Heat gains through roofs
are the lowest in the case of plaster ceiling with roof
space alone, provided with tiles on board and felt.
- Good insulation is important not only for refrigerated
types of stores but also with the others where ventilation
with the outside air is the only means of controlling
store temperature. Extruded expanded polystyrene appears
to be most suitable insulation material as it can be used
without a vapour barrier which is essential with other
materials such as insulation boards, glass fiber that
absorbs moisture affecting their insulating property.
Strawbale is considered expensive as it has the shorter
life as compared with commercial insulants.
- High relative humidity in stores reduces evaporation
losses from potatoes but may lead to condensation on inside
faces of walls and roofs. This can be avoided by a surface
covering of straw to absorb condensation unless better
insulation keeps up air temperature in the store roof
space above or store air is recirculated whenever necessary.
- Ventilation is necessary to dissipate the metabolic
heat produced by potatoes through respiration. In an unventilated
store, temperature may increase by 2 deg C / m storage
depth.
The amount of ventilation and fan capacity can
be calculated on the basis of 4 factors
- Average outside temperature and its range.
- Heat gains from potato respiration
- Solar radiation and thermal transmittance of the building.
- Heat gains from equipment.
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Cooling Systems and Equipment
- Duncan (1955) designed an evaporative cooling system
using spray water as cooling agent.
- A pump recirculates water at a rate greatly in excess
of the evaporation.
The essential requirements for better operation
of this system are as under
- Uniform distribution of air across the spray chamber.
- Suitable air velocity of about 300-700 fpm in washer
chamber
- An adequate amount of spray water broken into fine droplets
- Good spray distribution across the air stream
- Sufficient length of travel through the spray and wetted
surfaces
- Elimination of free moisture from the outgoing air.
- Wilson (1971) designed a forced evaporative humidifier.
He concluded that the rate of evaporation of water is
limited by the following factors:
- Water surface area exposed to air
- Rate of air flow in the chamber
- Mixing motion of air with water
- Temperature of water in circulation.
- Fergin (1974) studied various combinations of heat recovery
and evaporative cooling.
- In a simple evaporative cooling system, Outside air
is cooled by water spray to the desired supply air conditions.
- The second part consists of spray nozzles or cooling
pads. Entering that side, air is cooled by water spray.
- The cooled air then absorbs heat from a second outside
air stream via heat recovery unit.
- This second air stream is then further cooled by a second
water spray and then introduced into the control chamber.
- Subramanium (1970) studied about of thermal insulation
for cold stores.
- He used different insulation materials of different
thickness for comparative performance.
- He recommended paddy husk for walls and rock wool or
thermocol for ceiling and other parts of the storage structure
for better performance.
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Uses of Potatoes in Indian Kitchens
- Potato is used in Indian kitchens in one way or the
other and no meal is considered perfect without potatoes.
- It is mixed in large number of dishes and hoped that
these dishes whether exclusively potatoes or not, will
form either a part of the basic diet or shall be used
for special occasions, etc.
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Item
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Ingredients |
Remarks |
| Potato-Cheese
crispies |
Medium
sized
cooked potato,
Butter, Mint,
Coriander
chutney,
Craft cheese |
Grate
boiled potatoes
Beast in the butter when cold.
Chopped craft cheese and a little mint coriander
chutney
Shape the mixture into small balls, toss them lightly
and fresh breadcrumb and deep-fry them in ghee oil
until golden brown. |
| Potato
Cheese Splits |
Medium
Sized
PotatoesCottage
or Craft
CheeseTomato
sauce or Mango
relishButter |
Bake
the washed potatoes until they feel soft when pinched
or forked. Split each potato into halves, place
a cheese slice, little chutney and ½ teaspoonful
(2-3 g) butter or as per taste. |
| Banana
Split |
Banana
per
personCrushed
potato
Wafers
(crisps)Whipped
cream to
decorate the
bananasApple
/ Apricot or
any Jam |
Peel
bananas. Brush with a thick coating of the jam.
Coat with potato crisps. Pipe cream on each banana
in a simple line. Serve immediately. |
| Potato
Straws |
Large
PotatoesSaltIce
WaterOil or
Ghee for frying |
Peel
and cut potatoes into very thin long strips. Immerse
them in a solution of ice water and salt for about
½ hour. Drain and dry them on a piece of cloth.
Heat oil and deep-fry the potato straws until crisp
and golden brown. Serve them sprinkled in the salt
and pepper as per taste |
| Potato
Crisp Praline |
Potato
WafersSugar |
Heat
sugar in a small thick pan, unless melted continue
to heat until goes golden brown on very low heat.
Crush the wafers with Belan and quickly stir into
the sugar and pour at once in a thin layer over
a greased baking sheet or Thali. Leave to set. Crush
it when dry. Store in airtight jar until required. |
| Potato-Rice
Hamburger |
Boiled
PotatoSalt
Cooked
salted
RicePepper |
Combine
potato with salt & pepper
Form a potato partly covered with mashed cooked
rice to form flat balls
Role in fine bread crumbs and fry. |
| Potato
Balls |
Boiled
potatoesFresh
bread slicesSalt
Pepper
Coriander,
Chilli,Cumin,
Anardana etc. |
Mesh
potatoes
Soften the bread (after removing their all 4 crusts)
in water and press them within palms to drain off
excess water
Mix this bread with potatoes and the herbal powder
Make into patties of heart, leaf or any other shape
Deep fry in extra hot medium. |
| Potato
Soup |
Potatoes
Carrot
Piement
Fresh
BeansWater
Onion
Chopped
ButterCorn
flourSour
Cream Or
CurdChopped
Parsley Or
CorianderSalt
And Pepper |
Peel
and wash potato and other vegetables
Cut potatoes and carrot into small pieces
Boil in ½ liter water until soft
Strain and put them into a pan
Add other vegetables, 400 ml water and boil once
In another frying pan, Saute onions in butter until
brown
Add the flour and saute again
Mix cooked ingredients in a pan, Pepper to taste,
mix curd, sprinkle with garam Masala and chopped
parsley and serve hot. |
| Potato
or Crunchy Omelet |
Egg
per
person Course
chopped potato
Wafers or
boiled
grated
potatoesPepper
& SaltGhee |
Beat
well the eggs
Mix with pepper and salt
Heat the oil in a 8-10" pan and cover the base with
thin layer of crushed wafers or grated potatoes
Pour over the egg mixture, stir very lightly cook
until golden brown and serve. |
| Baked
Potatoes |
Large
potato
per person
ButterSauce,
Chutney,
Salt, Pepper |
Wash
and dry the potatoes
Wrap in aluminum foil to be baked with hot coal
or burnt cows dung or hot oven at 350 deg F.
Pierce the fork while cooking to let the steam out
Peel and serve with salt, butter, and sauce
Use small potatoes for quicker cooking. |
| Potato
Salad |
Medium
sized
potatoesOnion
choppedGarlic
cloves
chopped
Green
chillies chopped
MayonnaiseOil
& vinegar
salad
dressingTo
lightly cover
the salad |
Peel
, slice and boil potatoes until just cooked
Do not overcook otherwise they will break
Drain and mix potatoes with remaining ingredients
except mayonnaise and chillies
Add mayonnaise just before serving
Garnish it with shaved carrot, coriander leaves
or cabbage or chopped tomato.
For oil and vinegar dressing: mix 2 tablespoonful
salad oil and 1 tablespoonful vinegar, 1 spoonful
salt + 2 spoonfuls freshly ground pepper and shake
well. |
| Potato
Pan Cakes |
Potato
Egg
Onion
gratedSalt
Tomato
Sauce or
Mango
Chutney |
Wash,
peel and finely grate potatoes in salted water
Slieve grated potatoes
Mix grated potatoes with onion in a bowl
Add beaten eggs and mix well.
Make into thick cake and fry until brown on both
sides.
Serve hot with chutney |
| Potato
charchari |
Medium
sized
potatoes,
Small
dry
red chillies
halved, Turmeric powder, Water,
Coriander
leaves
chopped,
Poppy
seeds
Salt
Chilli
powder
Lime juice |
Peel
and cut potatoes after washing potatoes.
Fry the poppy paste in same in the same oil after
frying potatoes
Add potatoes, salt, turmeric, chilli powder and
fry 1-2 minutes.
Add water, cover and simmer till potatoes are tender
and water absorbed
Add lime juice and serve hot. |
Crispy Tips to Instantly Add the Potato Flavour
- Replace French fries with the wafers. Crumble the
wafers on scrambled eggs or panion bhurji or salads
for better taste.
- Enhance flavour of thick soups with a topping of broken
potato crisps.
- Finally crushed potatoes are a better substitute for
bread crumbs for the pakoras / bondas, etc.
- Add crunchiness of potato by layering broken potato
wafers between butter and / or vegetables in the bread
sandwiches.
- Plain cakes taste and look better if a mixture of
chopped wafers and sugar is put on the top of plain
cake before baking.
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