Last reviewed 1 Oct 2026 · 8 min read
Purpose of cold storage
Low temperature slows respiration, ripening, water loss, and the growth of micro-organisms, thus extending the storage life of fresh produce (days to months). Rule (Q₁₀): the rate of respiration roughly doubles to triples per 10 °C rise in temperature, so cooling quickly after harvest (pre-cooling) gives the biggest gain. India has a large number of cold stores (mostly for potato); the cold chain (pack house, pre-cooler, reefer vehicles, cold store, ripening room) is still developing.
Refrigeration cycle
Vapour-compression cycle
- Compression (compressor) — low-pressure vapour is compressed to high pressure and temperature (reciprocating, screw, scroll or centrifugal).
- Condensation (condenser) — vapour rejects heat to air/water and condenses (air-cooled, water-cooled, or evaporative condenser).
- Expansion (expansion valve/capillary) — pressure drops; the liquid partially flashes and cools.
- Evaporation (evaporator) — liquid refrigerant absorbs heat from the cold room and evaporates (the refrigeration effect); the vapour returns to the compressor.
Unit: 1 ton of refrigeration (TR) = 3.517 kW = 12,660 kJ/h (about 211 kJ/min) — the heat removed in melting one tonne of ice in 24 h.
Refrigerants
| Refrigerant | Remarks |
|---|---|
| R-717 (ammonia) | Excellent efficiency, large cold stores (potato); toxic and flammable; steel pipes |
| R-22 (HCFC) | Widely used earlier; being phased out (ozone-depleting) |
| R-134a, R-404A, R-407C, R-410A (HFCs) | Common in smaller systems; high global-warming potential (phase-down under the Kigali Amendment) |
| R-290 (propane), R-600a (isobutane) | Natural, low-GWP hydrocarbons, flammable (small systems) |
| R-744 (CO₂) | Natural; high pressure; cascade and transcritical systems |
(India has agreed to phase out HCFCs and phase down HFCs under the Montreal Protocol and its Kigali Amendment — see the current schedule.)
Systems for cold stores
- Direct expansion (DX) (refrigerant in the cooling coil) for small rooms; flooded or pump-recirculated ammonia for large stores; brine/glycol (secondary refrigerant) where leaks are a concern.
- Evaporators: forced-draught unit coolers (fans) with defrost (electric, hot gas, water).
- Controls: thermostat, humidistat, pressure controls, defrost timers, alarms.
Storage conditions
| Produce | Temperature (°C) | RH (%) | Approx. storage life |
|---|---|---|---|
| Apple | 0–4 | 90–95 | 3–6 months (longer in CA) |
| Potato (table) | 2–4 (some cold-induced sweetening occurs; reconditioning before use); processing potatoes 7–10 (to limit sugar build-up) | 90–95 | 5–8 months |
| Potato (seed) | 2–4 | 90–95 | 5–8 months |
| Onion (dry) | 0–2 | 65–70 | 6–8 months |
| Mango (mature green) | 12–13 | 85–90 | 2–4 weeks |
| Banana (green) | 13–14 | 85–95 | 2–4 weeks (chilling injury below about 12 °C) |
| Grapes | −1 to 0 | 90–95 | 1–3 months |
| Tomato (mature green) | 12–15 | 85–90 | 2–3 weeks |
| Leafy vegetables | 0–2 | 95–100 | 1–2 weeks |
| Citrus (orange, mandarin) | 3–8 | 85–90 | 1–3 months |
| Cabbage, carrot | 0–1 | 90–98 | 1–5 months |
(Values are indicative; follow crop-specific recommendations from the research institutes. Do not store ethylene producers (apple, banana, ripening fruit) with ethylene-sensitive crops (leafy vegetables, carrot).)
Chilling injury and freezing injury
- Chilling injury: damage to tropical and sub-tropical produce stored below a critical (non-freezing) temperature — pitting, discoloured skin, failure to ripen, increased decay (banana, mango, tomato, cucumber, pineapple).
- Freezing injury: ice crystals damage the tissue; the product turns watery on thawing; most fresh produce freezes at about −0.5 to −3 °C.
- Relative humidity: too low causes shrivel (water loss), too high promotes decay.
Pre-cooling
Rapid removal of field heat (to about 7/8 of the way down to the storage temperature within hours) before storage or transport.
| Method | Principle | Suitable for |
|---|---|---|
| Room cooling | Cold air circulating in the room; slow | General; limited |
| Forced-air (pressure) cooling | Fans draw cold air through vented boxes; 4–10 times faster than room cooling | Most fruits and vegetables (grapes, berries, tomato) |
| Hydrocooling | Chilled water showered/immersed | Sweet corn, carrot, mango, citrus |
| Vacuum cooling | Water evaporates at low pressure, cooling the product | Leafy vegetables (lettuce) |
| Package icing | Crushed ice in the package | Broccoli, sweet corn |
Cooling time is characterised by the half-cooling time and seven-eighths cooling time; (7/8 cooling time = 3 × half-cooling time).
Design of a cold room
| Element | Practice |
|---|---|
| Size and layout | Based on the quantity stored (including stacking height and bulk density); room dimensions with aisles; load bays and pre-cooling area; cold-room doors; air curtains |
| Insulation | PUF (polyurethane foam, thermal conductivity about 0.022–0.026 W/m·K) panels of about 100–150 mm; EPS (about 0.035); glass wool/mineral wool; vapour barrier on the warm side to prevent moisture ingress |
| Floor | Insulated and loaded with a protective slab; under-floor heating if freezing room to avoid ground heave |
| Air circulation | Fans and ducts for uniform temperature; stacking leaves flow gaps |
| Loading | Stack on pallets with ventilation spaces (a distance from walls of about 0.3 m); do not exceed the design loading rate (typically about 5–10 % of room capacity per day for the pull-down) |
| Safety | Emergency door release, alarms, ammonia detection, fire protection, back-up power |
Overall heat transfer coefficient of an insulated wall: when the insulation thickness is dominant.