Storage of fruits and vegetables
Storage improves the quality, usability of goods and also controls the market glut. The main goal of storage is to control the rate of transpiration, respiration, disease, and insect infestation. Harvesting at proper maturity, post-harvest disease control, atmospheric regulation, chemical treatment, irradiation, refrigeration, and controlled and modified environments can extend storage life.
The main goals of storage are:
- Slowing down biological activity without chilling injury.
- Slowing down the growth of micro-organisms.
- Minimizing transpiration loss.
The factors that should be kept in mind before starting crop storage are:
- Knowledge of appropriate storage conditions.
- Variety of crops suitable for storage.
- Availability of suitable storage facilities.
- Availability of appropriate management.
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Causes of poor condition and marketable life of fruits and vegetables during storage –
1) lack of moisture
2) Loss of stored energy (carbohydrates)
3) Loss of other foods
4) Physical damage caused by insect and disease attack
5) loss of quality from physical disorders,
Rooting (due to increased humidity)
Sprouting (potatoes, onion, ginger, garlic)
Toughening (due to high-temperature beans and sweet corn)
Factors affecting storage:
b) Relative humidity
c) Air velocity
d) Atmosphere composition
f) Storage operations
Methods of storage: There are mainly two methods of storage, i.e. traditional method, and the advanced method.
Traditional methods (Low-cost storage structures):
Traditional methods not requiring refrigeration include: in situ, sand, coir, pits, clamps, windbreaks, cellars, barns, evaporative cooling, and night ventilation:
I. In situ:
In this method of storing fruits and vegetables, the crop is not harvested until it is needed. It can be used with most root crops, such as cassava. Due to the occupation of the land on which the crop was grown, a new crop cannot be planted. In cold climates, the crop may suffer chilling injury. Some commodities develop unwanted fibers and starches. There is also the possibility of damage by pests and diseases.
II. Sand or coir:
This storage technique is used in countries like India for the long-term storage of potatoes, in which the product is covered with sand or coir.
III. Pits or trenches:
These are pits or trenches dug 1.0-1.5 m deep on an elevated or high edge of the same field (especially in areas with high rainfall) where the crop has been grown. Straw or other organic material is laid in the pit or trench and the crop is stored, then covered with a layer of organic material followed by a layer of soil. Holes are made along with the pipe at the top for air movement, as lack of ventilation can lead to crop rot. This method is suitable for the storage of ginger.
This method is not suitable for high humidity demanding fruits and leafy vegetables as high humidity cannot be maintained in it. Stored items cannot be repeatedly checked for decay etc.
This has been a traditional way of storage of potatoes, cassava, etc. in some parts of the world such as Great Britain. A typical design uses an area of land on the side of a farm. The width of the clamp is kept at about 1 to 2.5 m. A long conical heap of potatoes is made by marking the length and width. Sometimes straw is laid on the soil under the potatoes. The central height of the pile depends on the angle of its repose, which is placed approximately one-third of the width of the clamps. At the top, the straw is folded from side to side so that rainwater drains out of the structure. The thickness of the straw when pressed should be from 15-25 cm. After two weeks, the clump is covered with a layer of soil 15-20 cm thick, but this may vary depending on the climate. Yields may dry out due to low relative humidity. Larger heaps can result in greater chances of rot.
Windbreaks are created by driving wooden poles into the ground in two parallel rows spaced about 1 meter apart. At a height of about 30 cm from the ground, a wooden platform is made between the poles, often made of wooden planks. Chicken wire is affixed between the poles and at both ends of the windbreak. This method is used to store onions in Britain.
These underground or partially underground rooms are often at the bottom of a house. The location has good insulation, providing cooling in hot ambient conditions and protection from extremely low temperatures in cold climates. Usually, the temperature inside is not much lower than the outside temperature and rarely below 15 °C. This temperature is not low enough to protect microorganisms and plant enzymes from spoilage, although decomposition is quite slow. Cellars are traditionally used domestically in Britain to store apples, cabbage, onions, and potatoes during the winter. The yield may dry out due to the low relative humidity in it.
A barn is a farm building for shelter, processing, and storage of agricultural products, animals, and equipment. While there is no exact 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 a particular farm. Small agricultural buildings are often referred to as sheds or outbuildings and are typically used for small equipment or for activities.
VIII. Evaporative cooling:
Energy is required when water evaporates from the liquid state to the gaseous state. This principle can be used to cool the store by blowing air into the storage room through a water pad. The amount of cooling depends on the original humidity of the air and the efficiency of the evaporating surface. If the ambient air is humidified to about 100% RH, a large reduction in temperature will be achieved. It can provide cool moist conditions during storage.
IX. Zero energy cool chamber (ZECC):
It is a low-cost storage structure suitable for the short-duration storage of fruits and vegetables. No power source i.e. electricity, diesel, petrol, etc. is required for cooling, hence, these are named zero energy cooling chambers. Zero energy cool chambers are based on evaporative cooling systems. Evaporation occurs when air that is not already saturated with water moves over any wet surface. Thus an evaporative cooler consists of a wet porous bed through which the air is drawn, cooled, and humidified by the evaporation of water. In summer, when the outside temperature is 44°C, the maximum temperature inside the room never exceeds 28°C, with a relative humidity of 90%.
X. Night ventilation:
In hot weather, the difference between day and night temperatures can be used to keep the store cool. The storage room should be well insulated when the crop is stored inside. A fan is installed in the storeroom, which is turned on at night when the outside temperature is lower than the inside temperature. When the temperature is equal, the fan turns off. The fan is controlled by a differential thermostat, which continuously compares the outside air temperature to the internal storage temperature. This method is used to store onions in bulk.
Advanced (high cost) methods of storage:
I. Low-temperature storage (Refrigerated or cold storage):
Food storage at low temperatures retards microbial growth and enzyme reactions. The lower the temperature, the greater the retardation. Planned low temperature can be
a) Refrigeration or chilling temperature (O° C to 5°C)
b) Freezing temperature(Cold storage) (-18°C to -40°C)
a) Refrigeration or chilling temperatures (0° to 5 °C):
Refrigerated storage or low-temperature storage is the most common method of storing both fruits and vegetables worldwide. Refrigeration is the process of removing heat from an enclosed space or room or from a substance or object. The primary purpose of refrigeration is to reduce the temperature of an enclosed space or substance or object and then to maintain that low temperature.
b) Cold storage:
The growth of microorganisms and enzyme activity is minimal at temperatures below the freezing point of water (-18°C to -40°C). Many micro-organisms can survive this treatment and become active which can spoil the food if the food is subsequently kept at a high temperature. Therefore frozen foods should always be stored at a temperature below -5°C. Enzymes in some vegetables may continue to function after early freezing and therefore must be given a heat (above 80°C) treatment known as blanching before freezing to prevent the development of an off flavour in vegetables. There are two methods of freezing:
- Quick Freezing: If the temperature is reduced rapidly, it is called quick freezing and food kept at these temperatures can be preserved for many months. Foods can be frozen quickly in approximately 90 minutes or less by (1) placing them in contact with coils through which the refrigerant flows (2) Blast freezing in which cold air is passed throughout the food, (3) by immersion in liquid nitrogen. Quick-frozen foods retain their identity and freshness after thawing (brought to room temperature) because very small crystals form when foods are frozen by these methods.
- Slow Freezing: In this method, the food is frozen at a slower rate as compared to the quick freezing, due to which large crystals are formed in the food cells during freezing which injures their membranes. After thawing the food loses its identity and freshness.
II. Modified Environment Storage (MAS):
In this system, the product is kept in modified atmospheric conditions by package, overwrap, box lining, or pellet cover. In MAS the composition of the storage environment is not closely controlled i.e. the temperature of the product is controlled but the CO2 and O2 levels are not changed. Several types of whole and minimally processed fruits and vegetables have been successfully stored in modified atmosphere storage.
III. Controlled Atmosphere Storage (CAS):
The first requirement of CAS is a sufficiently gas-tight envelope around the product and the second requirement is some means of maintaining the concentration of CO2 and O2 at the desired level. This method, along with refrigeration, significantly extended the storage life of the fruits. The best environment for storing mango in CAS is 5% CO2 and 5% O2 at 13OC. CAS improves the appearance of pineapple fruit by reducing the growth of superficial mould. Keeping green banana fruits in an environment of 1-10% O2, 5-10% CO2, or a combination of low O2 and high CO2 While banana ripening can be delayed for weeks or months, in general, the response to citrus fruits has been disappointing in CAS.
IV. Hypobaric (sub-atmosphere) storage:
The product is placed in a vacuum-tight (1.3 to 13 kPa) and refrigerated container and the pressure is lowered to the desired level by a vacuum pump. The process of ripening and aging is greatly slowed down by the removal of the ethylene given by the product and by decreased respiration. This is an expensive method.