Cold Storage of Agricultural Products
Introduction
After produce has been pre-cooled it needs to be transported and put into a cold storage facility where, in order to maximize shelf-life and quality, it should be stored at its optimum storage temperature which varies according to different items making the storage of several items in one facility problematic if multiple storage chambers are not available. Storage and shelf life depends on intrinsic characteristic of items stored as well as the storage conditions.
In general, the optimum storage temperature is as low as possible but still above the level of chilling sensitivity of the respective crop. Another important factor affecting shelf-life of fresh produce is the level of relative humidity, impacting water loss, decay, physiological disorders, and the uniformity of ripening. Relative humidity levels should be high and saturated for most types of produce[1]. Finally, ethylene also impacts shelf life: whilst some items are ethylene producers after harvest, other crops are sensitive to ethylene exposure[2]. Subsequntly, if produce cannot be stored separately, their requirements regarding temperature, relative humidity and ethylene production/exposure need to be considered.
According to the Postharvest Centre of California[3], for the sake of short-term storage or transport, three groups of produce can be kept together:
1. most leafy vegetables, cole crops, fruits and berries from temperate climates
- 0°-2°C, 90%-98% relative humidity
2. citrus fruits, most tropical fruits, many fruit-type vegetables
- 7°-10°C, 85%-95% relative humidity
3. root-type vegetables, winter squashes, most tropical fruits and melons
- 13°-18°C, 85%-95% relative humidity
More detailed information on optimum storage conditions for specific items can be found here.
In the Indian context, the Indian National Board for Horticulture has divided produce into 7 categories according to optimum storage temperature and relative humidity. The categories can be found here.
Cold storage technologies for small-scale production
Cooling and cold storage are very energy intensive and expensive undertakings that require reliable electricity supply. This precondition is often not given and capital and running costs for farmers in developing countries are generally too high. Hence, unbroken cold chains and the use of appropriate cold storage facilities are generally not accessible for small-scale farmers[4] [5].
Below, several low technology options are being presented that can operate in electricity scarce regions and are feasible for small-scale farming operations:
passive/evaporative cooler: works in naturally dry areas without the input of energy and can provide an environment of 10-25°C. Passive methods include nocturnal cooling, architectural measures, such as the creation of shade, fountains, etc.[4][6].
Fuel: no energy input required
Absorption refrigerator: heat-driven technology than can cool below 10°C
Fuel: solar, kerosene[4]
refrigerators: electricity-driven and dependent on reliable and continuous supply of electricity
Fuel: grid, diesel, renewable sources such as hydro or solar, batteries, etc.[4]
There are multiple factors to consider in the technology and site selection of a cold storage facility:
location: the location chosen should be easily accessible and close to market hubs and/or produce collection points[7]
electricity: the access to and the supply of electricity to the cold storage facility should be given. Electricity supply has an impact on the size and equipment feasible of the cold storage[7]
Drainage and waste disposal: good drainage options and waste management equipment needs to be readily available[8]
Water: supply of water is important for matter of sanitation. Also the removal of wastewater generated has to be given[7]
Economic viability: the value generated though cold storage needs to justify investment and running costs[7]
In general, choosing an appropriate cold storage technology depends on the temperatures and level of reliability required, resources available – including skilled labor, capital, electricity and water supply[6]
Cold Storage Management
Temperature control can have huge benefits for the improvement of shelf-life and product; however, benefits will only realize if the cold storage is managed properly with regards to temperature, relative humidity levels, air circulation, adequate space between storage bins, trays and containers, the mixing of compatible produce, as well as the management of product in and outflow which should follow the ‘First In, First Out’ principle[7].
The following points should therefore be considered:
Temperature management
According to information of the FAO[7], cold storage facilities should be cooled to the storage temperature required at least three days before produce is being entered.
Chilling injury
Some produce, especially of tropical origin, is sensitive to chilling, which means that it will incur physiological damages if stored at a certain time period below a certain temperature but above their freezing points. In general, the longer the time period that produce is exposed to temperature below their level of chilling sensitivity and the lower the temperature, the faster damages will occur. It should also be noted that effects can be of a cumulative nature, i.e. the time periods of storage below the level of chilling sensitivity add up even if produce is stored at optimal conditions in between. Several factors, such as the level of maturity and level of ripeness at the point of harvest can affect chilling sensitivity[7].
Common visual symptoms:[7]
Surface lesions (pitting, large sunken areas and discoloration)
Water soaking (disruption of cell structure and accompanying release of substrate favors the growth of microorganisms)
Internal discoloration of the pulp
Failure to ripen in the expected pattern
Accelerated rate of senescence
Increased susceptibility to decay
Compositional changes
Freezing injury
Below 0°C all type of produce freezes due to dissolved soluble solids which are present in cell saps. Often, damages incurred through freezing only become visible once the produce is returned to temperatures above 0°C[7].
Symptoms[7]
Waterlogging
Glassy areas in the flesh
Storage mix
Different produce has different storage demands with regards to factors, such as optimum temperature, level of relative humidity, levels of ethylene production and sensitivity, and chilling sensitivity. Hence, the storage of single commodities is less complex than the storage of multiple commodities; however, the latter can still be the more viable option depending on external circumstances. Besides damages due to mismatches in temperature, also odors can be transmitted and ripening and decay can be affected through the exposure to ethylene, which can result in changes in color, flavor and texture. Further, only high quality produce should be ‘allowed’ in the cold storage and produce should be sorted accordingly[7]
Examples of odor transfers which should be avoided:
apples/pears with celery, cabbage, carrots, potatoes or onions
celery with onions or carrots
citrus with strongly scented vegetables
pears/apples with potatoes à former acquire unpleasant taste
green pepper will taint pineapples
onions, nuts, citrus, potatoes should be stored separately
Examples of ethylene producing and ethylene sensitive products:
Ethylene producing: e.g. apples, avocado, bananas, pears, peaches, plums, tomatoes
Ethylene sensitive produce: e.g. lettuce, cucumbers, carrots, potatoes, sweet potatoes
Stacking
Adequate airflow and the even distribution of cold air need to be ensured[7]. It is important to note that air always takes the path where resistance is lowest, and hence partly or unevenly filled storage facilities will have poor cooling rates[9]. The following points should be considered:A gap of at least 8 cm between walls and the floor, and the stacks of produce should be kept in order to ensure air flow
Well-ventilated storage boxes/containers/crates will improve cooling speed, such as PVC crates or ventilated boxes made of cardboard[9]
adequate space in between storage pallets should be about 4-6 inches[9]
Operation and maintenance
Cold storage is a very costly and energy intensive undertaking and therefore the door to the cold storage should be opened as few times as possible and that also in the early morning or evening hours[10]. Also the usage of lighting and fans should be kept at a minimum[11]. Further points should further be considered:
Logbook: operators should keep track of incoming and outgoing, as well as produced stored, including anticipated arrival time, quantity, quality, anticipated storage time, storage conditions required, sales price on day stored, actual sales price, energy consumed, etc. of new produce[7] [10].
Inventory: an up-to-date inventory should be kept including information about location within cold storage, type, quantity and harvest date of produce, use of pre-cooling method, date produce entered the facility, etc.
Loading: in general, loading rates should be 4-5% of total capacity. The cold storage should be divided in areas for short-term and longer-term storage space [9] [10]
Hygiene
In order to prevent pathogen built-up within the facility, strict hygiene measures are pivotal. All hygiene measures have to take necessary safety measures for staff into consideration.
Clean thoroughly at the end of each season: wash walls and floor with sanitizers, such as hypochlorite solution[7]
Use of ozone generators to prevent molds and fungi[7]
Regular ventilation to avoid built-up of ethylene, carbon dioxide and odors[7]
Further Information
References
- ↑ FAO (2004) Manual for the preparation and sale of fruits and vegetables – From field to markets, Rome: FAO, Available from: http://www.fao.org/docrep/008/y4893e/y4893e00.HTM
- ↑ University of California (2011) Compatibility Chart for Short-term Transport or Storage. Accessible from: http://postharvest.ucdavis.edu/produce_information/CompatibilityChart/
- ↑ University of California (2012) Produce Facts in English. Available from: http://postharvest.ucdavis.edu/producefacts/
- ↑ 4.0 4.1 4.2 4.3 GIZ-HERA (2011) Modern Energy Services for Modern Agriculture – A Review of Smallholder Farming in Developing Countries. Available from: http://www.water-energy-food.org/documents/giz/energy-services-for-modern-agriculture.pdf
- ↑ Practical Action (2003) Evaporative Cooling, Available from: http://practicalaction.org/evaporative-cooling-1
- ↑ 6.0 6.1 Practical Action (2010) Refrigeration Developing Countries. [Online] Available from: http://practicalaction.org/refrigeration-in-developing-countries-1
- ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 7.12 7.13 7.14 7.15 FAO (2009) Horticultural Chain Management for Countries of Asia and the Pacific Region. [Online] Available from: http://www.fao.org/3/a-i0782e/ Cite error: Invalid
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- ↑ 9.0 9.1 9.2 9.3 National Horticulture Board (2010) Cold Storage for Fresh Horticulture Produce Requiring Pre-cooling before Storage. Haryana: Cold Chain Development Centre National Horticulture Board
- ↑ 10.0 10.1 10.2 SUSTAIN (n.d.) Operational Manual for Cold Storage
- ↑ USAID (2009) Empowering Agriculture – Energy Options for Horticulture. [Online] Available from: http://ucce.ucdavis.edu/files/datastore/234-1386.pdf
