STORAGE CONDITIONS AND STRUCTURES FOR FRUITS AND VEGETABLES ON FARM
Once a crop is harvested, it is almost impossible to improve its quality. Proper storage conditions- temperature and humidity-are needed to lengthen storage life and maintain quality once the crop has been cooled to the optimum storage temperature. Fresh fruits and vegetables need low temperatures (32 to 55°F) and high relative humidities (80 to 95 percent) to lower respiration and to slow metabolic and transpiration rates.
Relative Humidity: Transpiration rates (water loss from produce) are determined by the moisture content of the air, which is usually expressed as relative humidity. At high relative humidity, produce maintains salable weight, appearance, nutritional quality and flavor, while wilting, softening and juiciness are reduced. Low relative humidity increase transpiration rates. A hygrometer or a sling psychrometer, not the appearance of the produce, should be used to monitor humidity. Control can be achieved by a variety of methods:
- Operating a humidifier in the storage area.
- Regulating air movement and ventilation in relation to storage room load.
- Maintaining refrigeration coil temperature within 2°F of the storage room air temperature.
- Wetting the storage room floor.
Temperature: Respiration and metabolic rates are directly related to room temperatures within a given range. The higher the rate of respiration, the faster the produce deteriorates. Lower temperatures slow respiration rates and the ripening and senescence processes, which prolongs the storage life of fruits and vegetables. Low temperatures also slow the growth of pathogenic fungi which cause spoilage of fruits and vegetables in storage.
Ethylene: Ethylene (C2 H4) ia an air pollutant gas wherever fruit ripening takes place. Mature but unripe fruits are placed in well ventilated rooms and exposed to ethylene with acetylene. Carbon dioxide and temperature over 30o C inhibit ethylene action.
Desirable effects: Ripening, color development, degreasing, shuck loosing, sprout induction.
Undesirable effects:Accelerates ripening, accelerates yellowing,induces leaf loss, bitter taste in carrots, induces sprouting in potatoes.
Measures to control effects of Ethylene:
- Eliminate sources of ethylene
- Ventilation one air charge per hour
- Inhibiting ethylene effects by CAS-low oxygen or high carbon dioxide
- Chemical removal-activated charcoal, potassium permanganate, UV lamps,etc.
Freezing Injury: Freezing will occur in all commodities below 32°F. Produce that is likely to be injured by one freezing is classified as "most susceptible." The "moderately susceptible" produce will recover from one or two freezings. Produce which is "least susceptible" can survive several freezings without injury. Injury from freezing temperatures can appear in plant tissues as loss of rigidity, softening and water soaking. Injury can be reduced if the produce is allowed to warm up slowly to optimum storage temperatures, and if it is not handled during the thawing period.
Chilling Injury: Fruits and vegetables that require warmer storage temperatures (40 to 55°F) can be damaged if they are subjected to nearfreezing temperatures (32°F). Cooler temperatures interfere with normal metabolic processes. Injury symptoms are varied and often do not develop until the produce has been returned to warmer temperatures for several days.
Objectives of on-farm storage of fruits and vegetables:
- These are perishables
- Face marketing challenges
- Distance from major production to consumption
Product quality for longer life
- High economic returns
SOME STORAGE STRUCTURES:
- Storage of fruits and vegetables at home
- Cold storage
- Controlled atmosphere storage
- Plastic shallow storage
- Perforated plastic shallow
Storing Vegetables and Fruits at Home: Many vegetables and fruits can be stored in pits, cellars or basements without refrigeration during cool fall and cold winter months.
Outdoor Storage: Produce that requires cool-to-cold moist surroundings can be stored outdoors. A well-drained location is essential to prevent excessive accumulation of water. Insulating materials commonly used are straw, hay, dry leaves, corn stalks, or wood shavings, and some soil.
In-Garden Storage: It is possible to leave some root crops, such as carrots, turnips, and parsnips in the garden where they grew, for part or all of the winter. After the ground begins to freeze in the late fall, cover the root crops with a foot or more of mulch straw, hay, or dry leaves. Do not place mulch on warm soil because doing so will cause vegetables to decay rapidly. Wait until the ground is cold.
Mounds, Pits: Mounds or pits are a very economical way to store cabbage and root crops, such as carrots, beets, celeriac, kohlrabi, rutabagas, turnips, and winter radishes.
Buried Containers: A 20-gallon trash can, buried in the ground, makes a convenient and economical storage for many kinds of vegetables. The container must be free of substances that might give off-flavor to the produce. Never use drums or containers that might have held pesticides or other chemicals.
Indoor Storage Area: There are many areas in dwellings that naturally provide, or can be adapted to provide a variety of temperature and moisture conditions for storage. These include the attic, unheated rooms, the basement, or cellar. They can provide cool, moderately moist conditions and can be used to store some types of apples or a variety of root crops. A warm storage area, such as an attic, can be a good environment in the fall for drying herbs, beans, walnuts, or hickory nuts.
Basement: A well-ventilated basement with central heating is generally dry and has a temperature range of 50˚F to 60˚F. It may be used for ripening tomatoes and for short-term storage of pumpkins, winter squash, potatoes, sweet potatoes, and onions.
Basement Storage Room: For long-term storage over winter you will need to partition off a room and insulate it to control temperature and moisture. Build a storage room on the north or east side of the basement, if practicable, and make sure there are no heating ducts or pipes running through it. You will need at least one window for cooling and ventilating the room.
Managing a storage area:
Sanitation: At least once a year, remove all containers from the storage area and clean and air them in the sun. The room itself should also be thoroughly aired, cleaned and washed down with a disinfectant, such as diluted chlorine bleach (1 cup bleach mixed with 9 cups water), to kill off any molds or bacteria that could lie dormant and ruin future crops.
Handling Vegetables and Fruits: Vegetables and fruits that are to be stored should be handled carefully to prevent damage. Protruding wire staples in baskets and hampers are particularly damaging. Lightweight tub buckets and plastic-coated stave baskets (egg baskets) are good containers for harvesting. If the soil is sandy, rinse the containers frequently to reduce skin breaks. Standard apple boxes and lug boxes used for shipping tomatoes, grapes, and nectarines are good storage containers.
Cold Storage of Fruit and Vegetables: Cold storage reduces the rate of biochemical changes in fresh foods (known as 'respiration' and 'senescence') and also slows down the growth of contaminating micro-organisms. The reason for storing fruits and vegetables in a cold store is therefore to extend their life beyond the harvest season. Some of the factors that control the shelf life of fresh crops in cold storage include:
* The type of food and variety.
* The temperature during harvest.
* The composition of the storage atmosphere.
Preparation of crops for cold storage: Most crops are likely to contain contaminants, to have parts that are inedible, or to have variable shape or size. To ensure that foods have a uniformly high quality for sale in the fresh market sector, it is necessary to clean, sort and grade the crop before cooling and cold storage.
Harvesting and cleaning: Cut or bruised produce is susceptible to post-harvest infection and has a much shorter storage life and poor appearance after storage. Crops should be harvested carefully using a sharp stainless steel blade and should not be placed on the ground where they could pick up dirt.
Construction and operation of a cold store:
Most cold stores are now constructed from prefabricated panels that have:
* structural steel or concrete to give them strength
* insulation (fibrous material such as rock wool or cellular plastics such as polyisocyanurate)
* a vapour barrier to prevent movement of water vapour, and
* an outer and inner facing material that is bonded to the core.
Packing systems: A packing system is required in the store so that produce can be loaded and unloaded easily and safely. This system can involve stacked crates or a more complicated system using racks and trays.
Temperature at storage: Cold stores are cooled by circulation of cold air produced by refrigeration units. All cold stores should lower the temperature of crops as quickly as possible through the 'warm zone' (50 -> 10°C) where maximum growth of micro-organisms occurs
Humidity of storage: There is always some moisture loss from fruits and vegetables during cold storage but excessive moisture loss is a problem. It is prevented by keeping the humidity of air in the store above 85%.
Controlled Atmosphere Storage: Controlled atmosphere (CA) storage involves altering and maintaining an atmospheric composition that is different from air composition (about 78% N2, 21% O2, and 0.03% CO2); generally, O2 below 8% and CO2 above 1% are used.
Some Beneficial Effects of CA (optimum composition for the commodity):
- i. Retardation of senescence (including ripening) and associated biochemical and physiological changes, ie., slowing down rates of respiration, ethylene production, softening, and compositional changes.
- ii. Reduction of sensitivity to ethylene action at O2 levels < 8% and/or CO2 levels > 1%.
- iii. Alleviation of certain physiological disorders such as chilling injury of avocado and some storage disorders, including scald, of apples.
Detrimental Effects of CA (above or below optimum composition for the commodity):
- i. Initiation and/or aggravation of certain physiological disorders such as internal browning in apples and pears, brown stain of lettuce, and chilling injury of some commodities.
- ii. Irregular ripening of fruits, such as banana, mango, pear, and tomato, can result from exposure to O2 levels below 2% and/or CO2 levels above 5% for > 1 mo.
Commercial Application of CA Storage: Several refinements in CA storage have been made in recent years to improve quality maintenance; these include creating nitrogen by separation from compressed air using molecular sieve beds or membrane systems, low O2 (1.0 to 1.5%) storage, low ethylene (< 1 μL L-1) CA storage; rapid CA (rapid establishment of optimal levels of O2 and CO2, and programmed (or sequential) CA storage, eg., storage in 1% O2 for 2 to 6 weeks followed by storage in 2 to 3% O2 for the remainder of the storage period.
Editted and formatted by: Shweta Singh
Submitted by ncshahi on Wed, 03/06/2009 - 13:40