Cold storage is the one widely practiced method for bulk handling of the perishables between production and marketing process. It is one of the methods of preserving perishable commodities in fresh and wholesome state for a longer period by controlling temperature and humidity within the storage system. Maintaining adequately low temperature is critical, as otherwise it will cause chilling injury to the produce. Also, relative humidity of the storeroom should be kept as high as 85-90 per cent for most of the perishables.
Most fruits and vegetables have a very limited life after harvest if held at ambient harvesting temperatures. Post-harvest cooling rapidly removes field heat, allowing longer storage periods. Proper post-harvest cooling can:
- Reduce respiratory activity and degradation by enzymes;
- Reduce internal water loss and wilting;
- Slow or inhibit the growth of decay-producing microorganisms;
- Reduce the production of the natural ripening agent, ethylene.
In addition to helping maintain quality, post-harvest cooling also provides marketing flexibility by allowing the grower to sell produce at the most appropriate time. Having cooling and storage facilities makes it unnecessary to market the produce immediately after harvest. This can be an advantage to growers who supply restaurants and grocery stores or to small growers who want to assemble truckload lots for shipment. Post-harvest cooling is essential to delivering produce of the highest possible quality to the consumer. Cold storage can be combined with storage in an environment with addition of carbon dioxide, sulfur dioxide (in case of grapes) nitrogen, etc. according to the nature of product to be preserved. The cold storage of dried/dehydrated vegetables can be successfully carried out for a storage time of more than one year, at 0°-10°C with a relative humidity of 80-95 per cent.
The cold storage of perishables has advanced noticeably in recent years, leading to better maintenance of organoleptic qualities, reduced spoilage, and longer shelf lives. These advances have resulted from joint action by physiologists to determine the requirements of fruit and vegetables, and by refrigerating specialists to design and run refrigerating machines accordingly.
Care should be taken to store only those things, which do not show incompatibility of storage, when storing multi-produce in the same room. For example, apple can be stored with grapes, oranges, peaches, and plums and not with banana. However, with potato and cabbage slight danger of cross actions can occur. Contrary to this, grape is incompatible to all other vegetables except cabbage. To resolve the incompatibility during cold storage, foodstuffs are grouped into three temperature ranges.
Based on their thermal incompatibility the produce is classified into:
- Most perishable products, not sensitive to cold (0-4°C)
e.g. Apple, grape, carrot and onion
- Vegetable produce moderately sensitive to cold (4-8°C)
e.g. Mango, orange, potato and tomato (ripened)
- Vegetable produce sensitive to cold (>8°C)
e.g. Pineapple, banana, pumpkin and bhendi
Based on the purpose, the present day cold stores are classified into following groups:
- Bulk cold stores: Generally, for storage of a single commodity which mostly operates on a seasonal basis e.g.: stores for potatoes, chilies, apples etc.
- Multi-purpose cold stores: It is designed for storage of variety of commodities, which operate practically throughout the year.
- Small cold stores: It is designed with pre-cooling facilities. For fresh fruits and vegetables, mainly for export-oriented items like grapes etc.
- Frozen food stores: It is designed for with (or) without processing and freezing facilities for fish, meat, poultry, dairy products and processed fruits and vegetables.
- Mini units or walk in cold stores: It is located at distribution center etc.
- Controlled atmosphere (CA) stores: It is mainly designed for certain fruits and vegetables.
General Arrangements and Consideration
If produce is to be stored, it is important to begin with a high quality product. The produce must not contain damaged or diseased units, and containers must be well ventilated and strong enough to withstand stacking. In general, proper storage practices include temperature control, relative humidity control, air circulation and maintenance of space between containers for adequate ventilation and avoiding incompatible product mixes. Commodities stored together should be capable of tolerating the same temperature, relative humidity and level of ethylene in the storage environment. High ethylene producers (such as ripe bananas and apples) can stimulate physiological changes in ethylene sensitive commodities (such as lettuce, cucumbers, carrots, potatoes, sweet potatoes) leading to often undesirable color, flavor and texture changes.
The general features of a cold store operational program (products, chilling and chilled storage and freezing) include total capacity, number and size of rooms, refrigeration system, storage and handling equipment and access facilities. The relative positioning of the different parts will condition the refrigeration system chosen. The site of the cold chambers should be decided once the sizes are known, but as a general rule they should be in the shade of direct sunlight. The land area must be large enough for the store, its annexes and areas for traffic, parking and possible future enlargement. A land area about six to ten times the area of the covered surface will suffice.
There is a general trend to construct single-storey cold stores, in spite of the relatively high surface: volume ratio influencing heat losses. The single storey has many advantages: lighter construction; span and pillar height can be increased; building on lower resistance soils is possible; internal mechanical transport is easier. Mechanical handling with forklift trucks allows the building of stores of great height, reducing the costs of construction for a given total volume.
The greater the height of the chambers the better, limited only by the mechanical means of stacking and by the mechanical resistance either of the packaging material or of the unpackaged merchandise. The length and width of the chambers are determined by the total amount of merchandise to be handled, how it is handled (rails, forklift trucks), the number of chambers and the dimensions of basic handling elements. A design that opts for fewer, larger chambers represents in the first place an economy in construction costs as many divisional walls and doors are eliminated. Refrigeration and control equipment is simplified and reduced, affecting investment and running costs. Large chambers allow easier control of temperature and relative humidity and also better use of storage space. Only in very particular situations should the cold store be designed with more than five or six cold chambers. Store capacity is the total amount of produce to be stored. If the total volume of the chambers is filled, the quantity of produce by unit of volume will express storage density.
Several parameters must be defined within a cold store. The total volume is the space comprised within the floor, roof and walls of the building. The gross volume is the total volume in which produce can be stored that is excluding other spaces not for storage. The net volume represents the space where produce is stacked, excluding those spaces occupied by pillars, coolers, ducts, air circulation and traffic passages inside the chambers that are included in the gross volume. Storage density referred to as net volume is expressed in kg/m3, but is the most commonly referred to as gross volume. About 3.4 m3 of volume is required per ton of potato to be preserved while for onions this value is about 5.7 m3/t. Thus, one can calculate the total volume of storage space as soon as the amount of storage product is known. An index of how reasonably and economically the cold store has been designed is the gross volume divided by the total volume. It must be in the range of 0.50 to 0.80. Similarly, gross volume is about 50 per cent greater than net volume, and gross area (same concept as volume) is about 25 per cent greater than net area. The extent of occupation is the ratio between the actual quantity of produce in storage at a given moment and that which can be stored. Equally the extent of utilisation is the average of the extent of occupation during a given period — usually a year, but it can also be per month.
The earlier cold storages were cubical in shape in order to minimise the surface area for a given volume, i.e., a = b = H =V1/3
Where a, b, H and V are width, breadth, height and volume of storage space.
Temperature management during storage can be aided by constructing square rather than rectangular buildings. Rectangular buildings have more wall area per square meter of storage space, so more heat is conducted across the walls, making them more expensive to cool. Temperature management can also be aided by shading buildings, painting storehouses white or silver to help reflect the sun’s rays, or by using sprinkler systems on the roof of a building for evaporative cooling. Facilities located at higher altitudes can be effective, since air temperature decreases as altitude increases. Increased altitude therefore, can make evaporative cooling, night cooling and radiant cooling more feasible.
The air composition in the storage environment can be manipulated by increasing or decreasing the rate of ventilation (introduction of fresh air) or by using gas absorbers such as potassium permanganate or activated charcoal. Large-scale controlled or modified atmosphere storage requires complex technology and management skills; however, some simple methods are available for handling small volumes of produce.
Heat Load Calculations
The optimal storage temperature must be continuously maintained to obtain the full benefit of cold storage. To make sure the storage room can be kept at the desired temperature, calculation of the required refrigeration capacity should be done using the most severe conditions expected during operation. These conditions include the mean maximum outside temperature, the maximum amount of produce cooled each day, and the maximum temperature of the produce to be cooled. The total amount of heat that the refrigeration system must remove from the cooling room is called the heat load. If the refrigeration system can be thought of as a heat pump, the refrigerated room can be thought of as a boat leaking in several places with an occasional wave splashing over the side. The leaks and splashes of heat entering a cooling room come from several sources:
- Transmission Load: Heat entering through the insulated walls, ceiling, and floor. This heat gain is directly proportional to the Temperature Difference (T.D.) between the two sides of the wall. The type and thickness of insulation used in the wall construction, the outside area of the wall and the T.D. between the two sides of the wall are the three factors that establish the wall load.
- Air Change Load: When the door to a refrigerated room is opened, warm outside air will enter the room. This air must be cooled to the refrigerated room temperature, resulting in an appreciable source of heat gain. This load is sometimes called the infiltration load. The probable number of air changes per day and the heat that must be removed from each cubic foot of the infiltrated air.
Field Heat: Whenever a product having a higher temperature is placed in a refrigerator or freezer room, the product will lose its heat until it reaches the storage temperature. This heat load consists of separate components:
Specific Heat: The amount of heat that must be removed from one pound of product to reduce the temperature of this pound by 1ºF, is called its specific heat. It has two values: one applies when the product is above freezing; the second is applicable after the product has reached its freezing point.
Latent Heat: The amount of heat that must be removed from one pound of product to freeze this pound is called the latent heat of fusion.
Estimating specific and latent heats:
Sp. Ht. above freezing = 0.20 + (0.008 X % water)
Sp. Ht. below freezing = 0.20 + (0.008 X % water)
Latent Heat = 143.3 X % water
Pull down Time: When a product load is to be calculated at other than a 24 hour pull down, a correction factor must be multiplied to the product load. The hourly heat load serves as the guide in selecting equipment. It is found by dividing the final BTU/24 hour load by the desired condensing unit run time.
- Heat of Respiration: Heat generated by the produce as a natural by-product of its respiration. Fresh fruits and vegetables are alive. Even in refrigerated storage they generate heat which is called the heat of respiration. They continually undergo a change in which energy is released in the form of heat, which varies with the type and temperature of the product.
- Service Load: Heat from lights, equipment, people, and warm, moist air entering through cracks or through the door when opened.
Fundamentals for Designing a Cold Storage Project
The design of cold storage facilities is usually directed to provide for the storage of perishable commodities at selected temperature with consideration being given to a proper balance between initial, operating, maintenance, and depreciation costs. As per the directions of the MIDH ,the projects shall be recommended as per the following component wise cost.
The basic procedures for construction of the cold store units should have the following requirements:
a) Process Layout
The most important requirement for any food project using insulated envelopes is to determine the process layout of the operation which is to be housed by the envelope. In the case of a meat plant, this can be a carcass dressing line or a boning room, or for a cold store, the pallet layout and mode of operation must be established. It is simply no good building an envelope and then attempting to place the processing machinery inside it.
b) Planning Drawings & Application
It is only after concluding the process layout that a planning application can be made when the dimensions of the envelope and supporting buildings can be frozen.
c) Design Drawings & Specifications
Once planning approval has been obtained then the preparation of design drawings and specifications can proceed. For a competitive design and construct tender, it is essential to prepare some 15 – 20 detailed drawings covering, at the minimum, the process layout, elevations and sections, the refrigeration system layout, mechanical and electrical systems reticulation and the lighting layout.
In addition to make up package at least six separate detailed specifications are required covering the project’s requirements on:
- Contractual requirements
- Building specification
- Refrigeration specification
- Insulation panel supply and erection
- Electrical requirements
- Mechanical services.
The location chosen for the cooling facility should reflect its primary function. If the plan is to conduct retail sales of fresh produce from the facility, it should be located with easy access to public roads. If, however, the primary function of the cooling facility is to cool and assemble wholesale lots, ease of public access is less important. In this case, the best location may be adjacent to the packing or grading room. In addition to housing grading and packing equipment, the space could be used to store empty containers and other equipment and supplies when it is not needed for cooling. Regardless of how it is used, the facility will need access to electrical power and water. For larger cooling rooms requiring more than about 10 tons of refrigeration in a single unit, access to three-phase power will be necessary. The location of existing utility lines should be carefully considered, as connection costs can be prohibitive in some rural areas. One can consult local power company for details. In addition, it is a good idea to anticipate any future growth when locating and designing any facility. A refrigerated store, with one (or) more thermally insulated places, and refrigerating machines can be planned with the aim of assuring certain services. The details about:
- Nature of the products
- Frequency of loading and unloading
- Calendar for harvest and dispatch
- Field heat of the produce
- Daily tonnage of produce to be handled
- Daily tonnage of ice to be manufactured
- Nature and dimension of packages
Availability of skilled and unskilled labor from the local area is the major factor to be considered for the successful operation.
When the hourly BTU load has been determined, equipment can now be selected. Some of the factors affecting equipment selection are:
- Equipment Balance
- Temperature Difference (TD)
- Capacity Control or Product Safety
- Type of Operation or Air Flow
The condensing unit is generally selected first to have capacity greater than the calculated cooling or freezing load. The unit cooler(s) must be selected to balance the capacity of the condensing unit. The capacity of the condensing unit should be selected at a suction temperature (after correction for suction line pressure drop) which will balance with the unit cooler(s) at a desirable TD between the refrigerant in the unit cooler and the air in the refrigerated storage room. The condensing unit capacity must also be selected at a condensing temperature corresponding to the condensing medium (ambient air or water) temperature available at the job location.
(For Storage Rooms Above 32ºF (0ºC.)
The nature of the product determines the desirable relative humidity for the storage room. The desirable relative humidity, in turn, dictates the approximate design TD between the air in storage room and the refrigerant in the unit cooler. For the general purpose, cooler involving meats, vegetables, and dairy products, it is common procedure to balance the low side to the condensing unit at a 10ºF to 12ºF. TD. It has been learned by experience that if this is done, one may expect to maintain in a cooler 80 per cent to 85 per cent relative humidity, which is a good range for general storage. TDs can be approximated by dividing the unit cooler capacity at a 1º TD into the condensing unit capacity at the desired saturated suction temperature (SST). In low temperature rooms, the amount of dehydration of unwrapped products is proportional to the TD. Since the prevention of excess dehydration is important and since low temperature condensing unit capacities drop off sharply as the suction temperature reduced, it is considered good practice to use a maximum TD of 10ºF.
Product Safety or Capacity Control
In large boxes, it is recommended that the load be divided among multiple units. A load that requires more than a 10 HP unit should be split to provide the customer with some refrigeration level in the event of mechanical failure. In addition, as refrigeration is selected for the 1 per cent worst occurrence of the year, multiple units provide for some capacity control. In low load situations, some units can be turned off and the box maintained adequately with a fraction of the horsepower necessary for the summer operation. Multiple units on staged start up also cut the demand charges assessed by the utility company which cut your customer’s electric bill.
Type of Operation or Air Flow
Two important considerations in the selection and location of the unit cooler are uniform air distribution and air velocities which are compatible with the particular application. The direction of the air and air throw should be such that there is movement of air where there is a heat gain; this applies to the room walls and ceiling as well as the product. The unit cooler(s) should be arranged to direct its discharge air at any doors or openings, if it all possible. Avoid placing the unit cooler in a position close to a door where it may induce additional infiltration into the room; this can cause fan icing and a condition known as hoar-frost. Also, avoid placing a unit in the air stream of another unit, because defrosting difficulties can result.
For general storage coolers and holding freezers, there are not criteria for air velocities within the room. The total supply of air is such that approximately 40 to 80 air changes occur each hour.
*includes all unit coolers and auxiliary fans