Potato

Harvesting And Storage

Harvesting and Yield

Lifting of Potato tubers

• Harvesting can be done when plants turn to yellow mixed brown colour and starts drying.
• Rainfed crop comes to harvest during August-September and irrigated crop during February-April months and this also depends on varietal characters.
• While digging land for harvesting tubers, care should be taken not to injure tubers.
• Irrigation should not be given one week prior to harvest.
• After harvesting, tubers should be airs dried for 10-15 days for the purpose of curing.

Yield

• Rainfed 7500 to 10000 kg.
• Irrigated 15000 to 20000 kg.

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Harvest care and grading of potato tubers

Drying and curing

• During harvesting tubers undergo a considerable skinning, bruising and other damages leading to rottage, if not handled properly and these damages are much greater when tubers are immature at harvest.
• The potato on the hills is normally harvested in wet weather favouring greater susceptibility to bruising and rot-causing organisms like Phthium, Phytophthora and Erwinia spp.
• Where as the crop on the plains suffers from heat injury due to direct exposure to sunlight and temperature above 25° C for long periods favouring bacterial soft-rot (Erwinia and Bacillus spp.) and charcoal-rot (Macrophomina phaseolina).
• Potato curing can be very properly done by heaping the tubers under shade in a ventilated place at around 15° C - 20° C.
• The heaps can be made about 1 - 1.5 m high and 3 - 3.5 m wide for 10-15 days in Indo-Gangetic plains.
• In the hills the potato is harvested under wet weather where the tubers should be spread in thin layers for proper drying of skin before heaping.

Sorting

• Sorting leads to removal of diseased, damaged or rough tubers where as grading leads to classification of healthy tubers into sizes of accepted standards.
• These operations are essential in order to fetch good price in the market and to the uniform emergence of the crop.

Grading of tubers

In order to assure uniform quality of superior grade tubers and to get remunerative returns, it is essential to grade them according to their respective sizes.

• The potatoes of the plains may be graded as follows:
• Seed size tubers,
• Large size tubers, and Chats
• The last two grades can be sold as ware potatoes and the seed size tubers, depending upon their health standards, should be separated out and kept for seed.

The potato tubers obtained from hills are classified or graded as under:

• Special size with a diameter of 8 cm and above.
• Phool size with a diameter of 3 to 5 cm, and
• Ration size with a diameter of 2.5 cm and below
• While grading the tubers, it must be kept in mind that damaged, diseased and tubers not true to the types should be separated out and discarded.

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Quality factors in potato

• Potato is no more a vegetable but it has become a staple food crop of India.
• Therefore, potato tubers have to be evaluated for seed quality and cooking quality separately. These factors have been described below:

A) Seed qualities in potato

• No aspect of potato growing is more important than the selection of best quality of seed as the yield depends more on quality of seed to be planted than any other single factor.
• The following points should be taken into account while evaluating the seed quality of tubers:
  1. True to the type.
  2. Adaptability of the variety in the region for which it has to be purchased.
  3. Abnormalities in the tubers.
  4. Desirable size.
  5. Sprouted conditions of the tubers.
  6. Proper grading.
  7. Freedom from insect/pest and diseases, etc.

B) Cooking quality in potato

• This is most desirable quality in potato for which the consumer pays money.
• The factors responsible for a better cooking quality are mentioned below:

Dry matter (DM)

• This increases as specific gravity increases from 1.07 to 1.09 with a concomitant increase in carbohydrate.
• As the size of the tuber increases say from 20 to 80 mm per cent dry matter also increases from 15 to 24.
• Mealiness or disintegration of boiled tubers is associated with dry matter content. Mealiness is given as:

Mealiness = Albuminoid matter / Starch X 100

Mealiness = Nitrogenous matter / Starch X 100

The range of values that describe the quality are given as: The mealiness may be interpreted as follows

The factors that influence DM are

• variety
• day length
• temperature
• light intensity
• water
• N supply

Reducing sugars

• Most of the processing requires lower content of reducing sugars such as glucose and fructose (0.2 to 0.4%).
• On frying, the potatoes darken due to the reaction between reducing sugars and amino acid (non-enzymic browning) which is known as Mallard reaction.
• The reducing sugar content depends on variety, growing conditions and storage temperature.
• Immature tubers will have more reducing sugars than mature ones.

Discolouration after peeling and black spot

• This is caused by the formation of melanin from tyrosine.
• It is enzymic browning and catalyzed by the enzyme phenolase.
• The extent of discolouration depends on the tyrosine content and phenolase activity.
• It is accepted that potassium decreases tyrosine content in the tuber, while nitrogen increases it.
• The black spot appears as blue-grey patches of various sizes under the skin.
• In wounded cells the colourless phenolic amino acid-tyrosine is converted into black coloured melanin.
• The incidence of black spot depends on the cell turgidity and susceptibility of tubers to damage, storage conditions and roughness in handling.

Discolouration after cooking

• Grey discolouration after cooking is non-enzymic reaction.
• This is caused by a grey pigment consisting of ferrous iron and chlorogenic acid.
• A colourless complex of ferrous iron and chlorogenic acid is formed first, which on exposure to air is oxidized to ferric complex.
• This discolouration is influenced by pH and the ratio of citric acid to chlorogenic acid.
• An increase in pH (upto 8) stimulates discolouration.
• Discolouration is normally greater (due to high pH) at heel and of tuber than at the rose end.
• Higher ratio of citric acid to chlorogenic acid would lower the discolouration after cooking.
• Citric acid binds the iron in the tuber, hence it is not available to react with chlorogenic acid.
• Higher contents of citric acid lowers the pH, hence allows less colouration.
• It is widely accepted that K increases citric acid while chlorides decrease it.
• Nitrogen increases the chlorogenic acid but potassium decreases it.
• Potatoes from peat soils are prone more to discolouration than those from silt or loam soil.
• Varieties differed in respect of blackening and the varieties which had more chlorogenic acid blackened most, whilst those low in chlorogenic acid blackened the least.

Glycoalkaloids

• These are potentially toxic compounds formed in the plants of Solanaceae.
• These exist in all parts of the potato plants but the highest concentration is in flowers (3,000 to 5,000 mg/kg) and sprouts (1,000 to 5,000 mg/kg).
• A major glycoalkaloid is soladine glycosides (SG), which is composed of alpha-solanine and a-chaconine.
• At low concentration, SG enhances flavour but higher concentration causes bitter taste. Consumption of SG causes gastroenteritis and the safe limit is 150 mg/kg fresh tuber weight.
• In general glycoalkaloids offer resistance to Colorado potato beetle and potato leaf hopper. Peeling tubers decreases SG content.
• The conditions, which favour SG content of tubers, are immature tubers, small tubers, exposure to sunlight immediately after the harvest, short storage in light, etc.

Consistency

• The potato may be mealy or sometimes consistent also because the starch does not integrate due to high temperature and more starch. Thus the mealiness and consistency go together.

Colour

• There are two main discolorations, which appear in cooked potatoes
• The browning of the chipped, fried and crisped potatoes brought about by the process of caramilization of sugars.
• The blackening of potato pieces is developed due to the oxidation of tyrosine and O. Dehydric enzymes.
• In immature tubers more free tyrosine is present which gets blackened on wilting or skin drying of tubers.
• Sometime blackening is due to ferrous compounds present in the tubers.

Flavour

• The flavour is found to be related with oil content, mineral salts and ratio between contents of potassium and nitrogen.
• The burnt taste, which occurs in chipped, fried and crisped potatoes, is due to presence of sugars.
• The potatoes are graded into three classes viz., mealy, soggy and waxy.

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Storage

Potatoes are perishable in nature and have to be kept in proper stores to avoid rotting of tubers.

• In plains the seed tubers may be kept in cold stores between February-March to September-October.
• At places where cold stores are not available, the potatoes may be stored in country stores.
• A thatched house be build in a cool place, preferably under the trees.
• The potatoes may be stored in it on wooden or bamboo racks.
• Potatoes may be kept in layers 20-30 cm high and covered with a layer of sand.
• The ventilators of the store should be closed during the day time and opened late in the evening.
• The potatoes may be periodically examined to remove the rotting tubers and to break the large sprous.
• Potato tubers in the hills are subjected to chilling due to low temperature during
• December to February resulting into 4.6 to 9.4 per cent loss in tuber weight and poor emergence where as in he plains, seed tubers are stored in bags in cold stores at 4°- 5°C and 90-95 per cent humidity.
• The installed storage capacity in India is around 4.5 m.
• Tonnes which is not enough to store even one third of potato produce.

The CPRI has developed store run on passive evaporative cooling which is more efficient and economic at high temperatures and lower humidities.

Caution

• Do not grow potato crop after raising tomato, chilli, brinjal and tobacco.
• Do not use potato leaves to cover harvested tubers so as to avoid infestation of tuber moth.
• Use sulphate of potash (SOP) as potassic fertilizer so as to prolong shelf life.

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