Storage of fruits and vegetables

        Proper marketing of perishable commodities such as fruits and vegetables often requires some storage to balance day-to-day fluctuations between harvest and sale for long-term storage. Storage improves commodities quality, and usefulness and also controls a market glut. The principal goal of storage is to control the rate of transpiration, respiration, disease and insect infestation. Storage life can be prolonged by harvesting at proper maturity, control of post-harvest diseases, regulation of the atmosphere, chemical treatments, irradiation, refrigeration and controlled and modified atmosphere.

The main goals of storage are:

 Slow the biological activity without chilling injury

 Slow the growth of micro-organisms.

 Reduce transpiration loss.

The factors which need to be taken into account before embarking on crop storage are:

 Knowledge of the appropriate storage conditions

 Cultivar or a variety of crops suitable for storage

 Availability of appropriate storage facilities

 Availability of suitable management.

Fruits and vegetables are living organisms. Their condition and marketable life will deteriorate during storage through—

1) Loss of moisture

2) Loss of stored energy—carbohydrates

3) Loss of other foods

4) Physical losses through pest and disease attack

5) Loss in quality from physiological disorders, Fibreness (asparagus) Greening (potatoes) Rooting (due to increased humidity) Shoot growth and elongation (Asparagus, Carrot, Beet) Seed germination Fruit growth Sprouting (potatoes, onion, ginger, garlic) Toughening (due to high-temperature beans and sweet corn)

Factors affecting storage:

a) Temperature

b) Relative humidity

c) Air velocity

d) Atmosphere composition

e) Light

f) Storage operations

Methods of storage: Mainly there are two methods of storage i.e. traditional methods and advanced methods.

a) Traditional methods (Low-cost storage structures) not requiring refrigeration include: in situ, sand, coir, pits, clamps, windbreaks, cellars, barns, evaporative cooling, and night ventilation:

b) In situ. This method of storing fruits and vegetables involves delaying the harvest until the crop is required. It can be used in some cases with root crops, such as cassava, but means that the land on which the crop was grown will remain occupied and a new crop cannot be planted. In colder climates, the crop may be exposed to freezing and chilling injury. In some commodities development of undesirable fibre and starch occurs. There are chances of occurring damage due to insect pests and diseases.

c) Sand or coir: This storage technique is used in countries like India to store potatoes for longer periods of time, which involves covering the commodity underground with sand.

d) Pits or trenches: These are dug 1.0-1.5m deep at the edges of the field where the crop has been grown. Usually, pits are placed at the highest point in the field, especially in regions of high rainfall. The pit or trench is lined with straw or other organic material and filled with the crop being stored, then covered with a layer of organic material followed by a layer of soil. Holes are created with straw at the top to allow for air ventilation, as lack of ventilation may cause problems with the rotting of the crop. This method is suitable for storing ginger. This method is not suitable for fruits and leafy vegetables demanding high humidity because it cannot maintain high humidity. The stored commodity can not be examined frequently for rotting etc.

e) Clamps. This has been a traditional method for storing potatoes, cassava etc. In some parts of the world, such as Great Britain. A common design uses an area of land at the side of the field. The width of the clamp is about 1 to 2.5 m. The dimensions are marked out and the potatoes are piled on the ground in an elongated conical heap. Sometimes straw is laid on the soil before the potatoes. The central height of the heap depends on its angle of repose, which is about one-third the width of the clump. At the top, the straw is bent over the ridge so that rain will tend to run off the structure. The straw thickness should be from 15-25 cm when compressed. After two weeks, the clamp is covered with soil to a depth of 15-20 cm, but this may vary depending on the climate. Produce may desiccate because of low relative humidity. Large heaps may result in more incidence of rotting.

f) Windbreaks are constructed by driving wooden stakes into the ground in two parallel rows about 1 m apart. A wooden platform is built between the stakes about 30 cm from the ground, often made from wooden boxes. Chicken wire is affixed between the stakes and across both ends of the windbreak. This method is used in Britain to store onions.

g) Cellars. These underground or partly underground rooms are often beneath a house. This location has good insulation, providing cooling in warm ambient conditions and protection from excessively low temperatures in cold climates. Cellars have traditionally been used on a domestic scale in Britain to store apples, cabbages, onions, and potatoes during winter. Produce may desiccate due to low relative humidity.

h) Barns. A barn is a farm building for sheltering, processing, and storing agricultural products, animals, and implements. Although there is no precise scale or measure for the type or size of the building, the term barn is usually reserved for the largest or most important structure on any particular farm. Smaller or minor agricultural buildings are often labelled as sheds or outbuildings and are normally used to house smaller implements or activities.

i) Evaporative cooling. When water evaporates from the liquid phase into the vapour phase energy is required. This principle can be used to cool stores by first passing the air introduced into the storage room through a pad of water. The degree of cooling depends on the original humidity of the air and the efficiency of the evaporating surface. If the ambient air has low humidity and is humidified to around 100% RH, then a large reduction in temperature will be achieved. This can provide cool moist conditions during storage.

j) Zero energy cool chamber (ZECC): It is a low-cost storage structure suitable for short-duration storage of fruits and vegetables There is no need for any power source i.e. electricity, diesel, petrol etc. for cooling, thus, the name zero energy cool chamber. The zero-energy cool chamber is based on the evaporative cooling system. Evaporation occurs when air that is not already saturated with water is blown across any wet surface. Thus an evaporative cooler consists of a wet porous bed through which air is drawn, cooled and humidified by the evaporation of water. In summer, when the outside temperature is 44 degree C, the maximum temperature inside the chamber never goes beyond more than 28OC, the relative humidity being 90% .

k) Night ventilation. In hot climates, the variation between day and night temperatures can be used to keep stores cool. The storage room should be well insulated when the crop is placed inside. A fan is built into the store room, which is switched on when the outside temperature at night becomes lower than the temperature within. The fan switches off when the temperatures equalize. The fan is controlled by a differential thermostat, which constantly compares the outside air temperature with the internal storage temperature. This method is used to store bulk onions.

l) Controlled atmospheres are made of gastight chambers with insulated walls, ceiling, and floor. They are increasingly common for fruit storage at a larger scale. Depending on the species and variety, various blends of O2, CO2, and N2 are required. Low-content O2 atmospheres (0.8 to 1.5%), called ULO (Ultra-Low Oxygen) atmospheres, are used for fruits with long storage lives (e.g., apples).

II. Advanced (high-cost) methods of storage:

1. Low-temperature storage (Refrigerated or cold storage): Microbial growth and enzyme reactions are retarded in food storage at low temperatures. The lower the temperature, the greater the retardation. Low temperatures employed can be

a) Cellar storage temperature (about 15 degree C)

b) Refrigeration or chilling temperature (0-5 degree C)

c) Freezing temperature (Cold storage) (-18 to -40 degree C)

(a) Cellar storage temperature (about 15 degree C): The temperature in cellar (underground rooms) where food is stored in many villages are usually not much below that of the outside air and is seldom lower than 15 degree C. The temperature is not low enough to

prevent the action of many spoilage organisms and of plant enzymes. Decomposition is however slowed down considerably. Root crops, potatoes, onions, apples and similar foods can be stored for a limited during the winter months.

(b) Refrigeration or chilling temperatures (0-5 degree C): Refrigerated storage or low-temperature storage is the most common method of storage throughout the world both for fruits and vegetables. Refrigeration is the process of removing heat from an enclosed space or room or a substance or commodity. The primary purpose of refrigeration is to lower the temperature of the enclosed space or substance or commodity and then maintain that lower temperature.

(c) Cold storage: At temperatures below the freezing point of water (-18 to -40 degree C) growth of microorganisms and enzyme activity are reduced to the minimum. Most perishable foods can be preserved for several months if the temperature is brought down quickly (called quick freezing) and food is held at these temperatures. Foods can be quickly frozen in about 90 minutes or less by placing them in contact with the coil through which the refrigerant flows (2) through blast freezing in which cold air is blown across the food, (3) by dipping in liquid nitrogen. Quick-frozen foods maintain their identity and freshness when they are thawed (brought to room temperature) because very small crystals are formed when foods are frozen by these methods. Many microorganisms can survive this treatment and may become active and spoil the food if the foods are held at higher temperatures. Frozen foods should always, therefore be held at temperatures below -5OC. Enzymes in certain vegetables can continue to act even after being quickly frozen and so vegetables have to be given heat treatment called blanching(above 80 degree C) before they are frozen to prevent the development of off flavours.

(2) Controlled / Modified atmosphere storage: In this system, the product is held under atmosphere conditions modified by package, overwrap, box liner or pellet cover. The first requirement of CAS is sufficiently gas-tight envelops around the product and the second requirement is some means of maintaining the concentration of CO2 and O2 at the desired level. This method in combination with refrigeration markedly enhanced the storage life of fruits. The fruit that has derived the most benefit is the apple. Among the tropical fruits, the best atmosphere for storage of mangoes is 5% CO2 and 5% O2 at 13 degree C. CAS improved the appearance of pine apple fruit by reducing the superficial mould growth. The optimum O2 level was 2%. Levels of oxygen below that were ineffective in extending storage life. Benefits could be obtained with papaya when the fruits are stored in 5% CO2 and 1 % O2 for 3 weeks at 13OC. Initiation of ripening in banana can be delayed for weeks or months by holding the green banana fruits in an atmosphere of 1-10% O2, 5-10% CO2 or low O2 and high CO2 combination, In general, the response of citrus fruits to CAS has been disappointing. In MAS the composition of the storage atmosphere is not closely controlled.

(3) Hypobaric (Sub atmosphere) storage: The commodity is placed in a vacuum-tight and refrigerated container and evacuated by a vacuum pump to the desired low pressure. The process of ripening and senescence are greatly

retarded by decreasing respiration and evacuation of ethylene given out by the poduce. This is an expensive method.

(4) Irradiation: The application of irradiation for suppressing sprouting and hence the extension of shelf life has been allowed in India. Sprouting onion can be checked by gamma irradiation at a dose of 0.06 - 0.1 kGY. In potato gamma irradiation at -0.1 kGY can inhibit sprouting completely. The irradiated potatoes could be stored successfully for 6 months at 15OC with 10% loss. Irradiation in banana, guava, mango and papaya improves shelf life due to delay in rate of ripening and senescence.