Cold storage and controlled-atmosphere storage
How cold stores and controlled or modified atmospheres keep produce, storage conditions for major crops, cold chain breaks, and a refrigeration load sum.
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Apples picked in autumn are sold crisp the following summer. Grapes from Chile reach Europe weeks after harvest, and Kenyan green beans are on British shelves within two days of picking. None of this would be possible without the cold chain, the unbroken sequence of cooling, refrigerated storage and refrigerated transport, and without controlled-atmosphere storage, which slows produce even further by changing the air around it.
Cold storage, the cold chain and controlled atmospheres are core topics in post-harvest technology and agricultural engineering.
Principles
Low temperature slows respiration, ripening, water loss, sprouting and the growth of decay organisms. Each product has an optimum: as cold as possible without freezing (most produce freezes at about −0.5 to −2 °C) or chilling injury, combined with the right relative humidity to limit water loss.
| Product | Temperature (°C) | Relative humidity (%) | Approximate storage life |
|---|---|---|---|
| Apple | −1 to 4 (variety-dependent) | 90–95 | 3–8 months, longer in CA |
| Pear | −1 to 0 | 90–95 | 2–7 months |
| Grape | −1 to 0 | 90–95 | 1–6 months, often with sulphur dioxide pads |
| Orange | 3–9 | 85–90 | 1–3 months |
| Banana (green) | 13–14 | 90–95 | 1–4 weeks |
| Mango | 10–13 | 85–90 | 2–3 weeks |
| Tomato (mature green) | 12–15 | 90–95 | 1–3 weeks |
| Potato (seed) | 2–4 | 95–98 | 5–10 months |
| Potato (processing) | 8–12 | 95–98 | 2–10 months, with sprout control |
| Onion (dry) | 0 | 65–70 | 1–8 months |
| Leafy greens, broccoli | 0 | 95–100 | 1–3 weeks |
| Carrot (topped) | 0 | 98–100 | 4–6 months |
Potatoes for frying and chips are stored warmer because below about 8 °C they convert starch to sugars (cold-induced sweetening), which darkens fries and increases acrylamide formation.
The cold chain
A cold chain links:
- Precooling soon after harvest, removing field heat.
- Cold storage at the packhouse or a distribution hub.
- Refrigerated transport: insulated trucks, rail wagons and reefer containers for sea freight.
- Distribution centres, retail display and the consumer's refrigerator.
A chain is only as good as its weakest link. Temperature data loggers and, increasingly, wireless sensors record temperature through the chain.
In many low- and middle-income countries the chain is broken or absent, especially for smallholders. Options include evaporative coolers (such as brick-and-sand chambers, which keep produce cooler and more humid than ambient air with no electricity), solar-powered cold rooms, shared community cold stores, and small cold rooms using modified air conditioners.
Storage management
- Hygiene: clean and sanitise rooms between seasons; remove rotting produce promptly.
- Air circulation around and through stacks, with enough space between pallets and walls.
- Humidity control: humidifiers or wet floors where needed, and avoiding coils that run too cold and dry the air.
- Stock rotation: first in, first out.
- Compatibility: products stored together should share temperature and humidity needs, and ethylene producers should be kept away from ethylene-sensitive products. Odours also transfer, for example from onions to apples.
Controlled and modified atmospheres
Lowering oxygen and raising carbon dioxide slows respiration and ethylene action, on top of the effect of cold.
- Controlled-atmosphere (CA) storage: gas-tight rooms where O2 and CO2 are measured and controlled continuously. F. Kidd and C. West developed "gas storage" of apples in England in the 1920s, and the first commercial CA stores opened there in 1929. Typical apple conditions are about 1 to 3 per cent O2 and 1 to 5 per cent CO2 at 0 to 3 °C, depending on the variety.
- Ultra-low oxygen (ULO) storage uses O2 below about 1 per cent.
- Dynamic CA (DCA) lowers O2 to just above the level at which the fruit starts fermenting, detected by chlorophyll fluorescence, ethanol or the respiratory quotient, and adjusts it continuously.
- Modified-atmosphere packaging (MAP): the produce's own respiration, and films of chosen permeability, create a low-O2, high-CO2 atmosphere inside a package. Used for cut salads, berries, and many fresh-cut products.
- 1-MCP blocks ethylene action and is used with cold and CA storage, especially for apples.
Too little O2 or too much CO2 causes fermentation, off-flavours and internal browning, so limits are set carefully for each variety.
How much refrigeration does a cold store need?
A store receives 10 tonnes of apples a day at 25 °C and must cool them to 0 °C within 24 hours. The specific heat of apples is about 3.6 kJ per kg per °C.
- Product (field heat) load = mass × specific heat × temperature drop = 10,000 × 3.6 × 25 = 900,000 kJ.
- Spread over 24 hours (86,400 s): 900,000 ÷ 86,400 ≈ 10.4 kW.
- Respiration heat: at roughly 20 mg CO2 per kg per hour averaged during cooling, and about 10.7 J per mg CO2: 10,000 × 20 × 10.7 ÷ 3,600 ≈ 0.6 kW.
- Adding heat through walls, doors, fans, lights and people, often another 30 to 50 per cent, the total is around 15 kW.
In refrigeration tons (1 TR = 3.517 kW), that is about 4 TR. Designers add a safety margin and size the equipment for the hottest days of the harvest season.
Key points
- Store each product as cold as it tolerates, without freezing or chilling injury, at the right humidity.
- Chill-sensitive produce (banana, mango, tomato) stays above about 10–13 °C; potatoes for processing above about 8 °C.
- The cold chain runs from precooling through storage, transport and retail; breaks shorten shelf life.
- CA storage lowers O2 (about 1–3 per cent) and raises CO2 (about 1–5 per cent); ULO and DCA go further; MAP creates atmospheres in packages.
- Refrigeration load = m × c × ΔT ÷ time, plus respiration and building loads; 1 TR = 3.517 kW.
What the exams ask
- School-leaving and entrance papers: why produce is refrigerated, and storage conditions for common fruits and vegetables.
- Degree and research-fellowship entrances: optimum temperatures and humidities, chilling injury, CA and MAP principles, mixed-load compatibility, and evaporative coolers.
- Agricultural engineering and professional examinations: cold-store design, refrigeration load calculations, insulation, and cold chain management.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why not store all fruit at 0 °C?
Tropical and subtropical produce suffers chilling injury at low temperatures, showing pitting, browning and failure to ripen. Each crop has its own safe minimum.
Is controlled-atmosphere fruit less nutritious?
CA mainly slows ripening and ageing. Fruit from good CA storage often retains firmness, acidity and many nutrients better than fruit kept in air for the same time.
Do evaporative coolers really work?
In hot, dry weather they can lower temperatures by about 10 °C and keep humidity high, often doubling the shelf life of vegetables. They are much less effective in humid weather.
Sources
- Kader, A. A. (ed.) Postharvest Technology of Horticultural Crops, 3rd edition. University of California ANR Publication 3311, 2002.
- Gross, K. C., Wang, C. Y. and Saltveit, M. (eds) The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. USDA Agriculture Handbook 66, 2016.
- Thompson, A. K., Prange, R. K., Bancroft, R. and Puttongsiri, T. Controlled Atmosphere Storage of Fruit and Vegetables, 3rd edition. CABI, 2018.
- ASHRAE. ASHRAE Handbook: Refrigeration. Chapters on commodity storage requirements and thermal properties of foods. ASHRAE, 2022.
- FAO. The State of Food and Agriculture 2019: Moving Forward on Food Loss and Waste Reduction. FAO, Rome, 2019.
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