Chapter 2: Cold Store Requirements

Contents

1. Cold Store Registration
2. Cold Store Construction and Insulation Requirements
   • 2.1 Foundation
   • 2.2 Structure
   • 2.3 Vapour Barrier
   • 2.4 Doors and Fitting
   • 2.5 Insulation
   • 2.6 Cargo Protection
   • 2.7 Drip Trays
3. Refrigeration Capacity
4. Temperature Instrumentation Required
   • 4.1 Temperature Controlling Devices
     • 4.1.1 Position of Thermostat Sensor
     • 4.1.2 Accuracy of Temperature Control
   • 4.2 Temperature Sensing or Measuring Devices
     • 4.2.1 Thermometers
     • 4.2.2 Electronic Handheld Thermometers
     • 4.2.3 Thermocouples
     • 4.2.4 Resistance Thermometers
     • 4.2.5 Mechanical Bi-metal Strips or Coils
   • 4.3 Temperature Recording and Logging Devices
     • 4.3.1 Fixed Installations
     • 4.3.2 Mobile Data Loggers
   • 4.4 Calibration of Temperature Measuring, Controlling and Recording Devices
5. Relative Humidity
   • 5.1 Definition of RH
   • 5.2 Importance of RH During Storage
   • 5.3 Optimum RH
   • 5.4 Measurement of RH
   • 5.5 Factors Affecting RH
6. Pallet Handling Equipment
   • 6.1 Internal Combustion Powered Forklifts
   • 6.2 Battery Powered Forklift
   • 6.3 Hand Operated Pallet Jack
7. Precooling and Storage
   • 7.1 Precooling
     • 7.1.1 Forced Air Cooling
     • 7.1.2 Hydro Cooling
     • 7.1.3 Room Cooling
   • 7.2 Storage
     • 7.2.1 Air Storage
     • 7.2.2 Controlled Atmosphere Storage
     • 7.2.3 Modified Atmosphere Storage
8. Temperature and RH Records
   • 8.1 Products Handled
   • 8.2 Product Temperature
   • 8.3 Calibration of Instruments
   • 8.4 Frequency of Temperature Readings
   • 8.5 Temperature Records
9. Cold Store and Storage Management
   • 9.1 Set Point Temperature
   • 9.2 Delivery Air Temperature
   • 9.3 Return Air Temperature
   • 9.4 Space Air Temperature
   • 9.5 Product Storage Temperature
   • 9.6 Actual Product Temperature

1. Cold Store Registration

All cold stores used for the export of South African perishable products must be registered with the PPECB and annually re-inspected, as required by the PPECB Act and Regulations. Registration protects the product, the owner, and the consumer, and supports food safety and traceability. PPECB does not prescribe specific designs or materials but ensures an efficient cold chain to maintain consumer confidence in South African perishable products.

Definitions:

• Precooling: The process of removing field heat from the product and cooling it to the optimum storage temperature.
• Storage (Cold Storage): Keeping the product at the optimum temperature to achieve maximum storage life.

2. Cold Store Construction and Insulation Requirements

The main requirement is to efficiently reduce air and product temperatures during precooling and keep them constant during storage. Minimizing heat leakage from the outside environment is essential.

2.1 Foundation

• Must be well-drained and above the groundwater table to minimize heat leakage through the floor.

2.2 Structure

• Must be stable, without movement that could cause stress and cracks in the vapour barrier and insulation.

2.3 Vapour Barrier

• An efficient water vapour barrier must be installed and maintained on the warm side (outside) of the insulation.
• Outer skins (usually sheet metal) of sandwich panels serve as a barrier for water vapour ingress.
• All joints on the outside must be well sealed; inside joints may be left unsealed to allow trapped moisture to migrate to the inside atmosphere.

2.4 Doors and Fittings

• Must make a vapour-tight seal (gas-tight for controlled atmosphere cold stores).
• Door seals must be intact and seal all around.
• Pipes, cables, brackets, etc., must be sealed with appropriate sealant to prevent water vapour entering the insulation.
• Door locks must comply with all National Safety Requirements.

2.5 Insulation

• Must be the correct type and thickness to provide an efficient barrier to heat leakage and must be kept dry.
• Minimum specifications are given in Table 2.1.

Table 2.1: Minimum Required Thickness for Polystyrene and Polyurethane Insulation

Type of Store & Position	Polystyrene (mm)	Polyurethane (mm)
Cold store Floor	Not necessary¹	Not necessary¹
Cold store Walls	100–150	75–100
Cold store Ceiling	100–150²	100–150²
Deep freeze Floor	150	100
Deep freeze Walls	150–200	100–150
Deep freeze Ceiling	150–200²	100–150²

¹ If the underfloor structure is kept dry.
² Thicker insulation required in the ceiling in warmer climates.

2.6 Cargo Protection

• Must be provided inside and outside the store facility at all times.
• An air lock structure should be fitted to protect cargo and maintain the cold chain during loading/unloading.

2.7 Drip Trays

• Must be installed under cooling units to drain defrost water outside via a U-tube or water trap.
• Pipes transporting coolant must be well insulated to prevent condensation dripping onto cargo or the floor.

3. Refrigeration Capacity

Refrigeration capacity is the ability of the cooling installation to precool the product to the optimum storage temperature and maintain it according to specific product requirements.

Key considerations:

• Intake volumes and product temperatures must be known to calculate the heat load to be removed within a specified precooling time (e.g., 24 hours).
• Additional heat sources: heat leakage, forklifts, air circulation fans, door openings.
• Factors affecting cold air circulation: packaging, stacking patterns, cold store dimensions.
• Cooling rates, temperature, and relative humidity ranges to be maintained.

The cold store should be capable of precooling the designed maximum intake volume within 24 hours (exceptions: grapes up to 72 hours, Granny Smith apples up to 5 days). It should also maintain the optimum storage temperature within ±0.5°C. Relative humidity (RH) is also a design criterion and must be maintained as follows:

• During precooling: minimum RH of 70% for very short periods only.
• During storage: average RH of at least 90±5%.
• Exceptions: onions, garlic, and certain flower bulbs at 70% RH.

4. Temperature Instrumentation Required

Cold stores must be fitted with temperature controlling devices (thermostats), sensing devices (thermometers), and recording/logging devices. Stores hired out or handling products for other exporters must also have suitable calibrated temperature data loggers.

4.1 Temperature Controlling Devices

• Thermostats control temperature at a pre-selected value (set point, SP).

4.1.1 Position of Thermostat Sensor

• Install the sensor to sense the average temperature in the immediate environment.
• Never place directly in front of an air circulating fan or heat/cold source.
• For chilled produce: about 3 meters in front of the cooling coils or where air first contacts the product.

4.1.2 Accuracy of Temperature Control

• Temperature must be controlled within ±0.5°C of the set point (SP).
• Some cold treatment protocols require ±0.25°C accuracy.

4.2 Temperature Sensing or Measuring Devices

4.2.1 Thermometers

• Mechanical dial types are not recommended due to large graduations and lower accuracy.

4.2.2 Electronic Handheld Thermometers

• Quick response, fine graduations (usually 0.1°C).
• Correction factor may be needed for calibration deviations.
• Accuracy of ±0.25°C or better is recommended.

4.2.3 Thermocouples

• Created by fusing copper and constantan; generates a current related to temperature.
• Only high-quality “Type T” wires should be used.
• Procedures must ensure accuracy of at least ±0.5°C.

4.2.4 Resistance Thermometers (Thermistors)

• Measure voltage drop related to temperature.
• Can be made small and robust, often in a spear point probe.
• Accuracy better than ±0.1°C, reusable, but more expensive than thermocouples.

4.2.5 Mechanical Bi-metal Strips or Coils

• Used in older controllers and chart recorders; largely replaced by electronic equipment.

4.3 Temperature Recording and Logging Devices

4.3.1 Fixed Installations

• Installed in cold stores, vehicles, containers, and ships.
• Must be regularly calibrated in melting ice at 0.0°C by an accredited institution.
• Accuracy must not exceed ±0.5°C (general) or ±0.25°C (cold treatment).
• Recording frequency: <2 hours during unstable conditions, ≤8 hours when stable. Continuous recording is recommended.

4.3.2 Mobile Data Loggers

• Self-contained, battery-powered, recommended for accuracy and reliability.
• Usually record air temperature at the sensor position; external sensors can record product temperature.
• Used to record temperatures at random positions within pallets and refrigerated space.
• Accuracy usually better than ±0.25°C at 0.0°C but must be calibrated in melting ice.
• Recording intervals should be as short as possible, considering memory and battery life.

4.4 Calibration of Temperature Measuring, Controlling, and Recording Devices

• Prepare ice cubes from pure distilled water.
• Fill insulated containers with ice cubes and a small amount of distilled water to form an ice-water mixture.
• Stir continuously to ensure a constant temperature.
• The mixture must register 0.0°C on a certified standard thermometer.
• Submerge the sensor to be calibrated and take the reading when stabilized.
• The difference between the standard and device reading gives the accuracy.
• The calibration factor must be applied if the device cannot be adjusted to 0.0°C.

5. Relative Humidity (RH)

5.1 Definition of RH

RH is the ratio of water vapour present in the air relative to the maximum amount that can be present at the same temperature and pressure, expressed as a percentage.

5.2 Importance of RH During Cold Storage

• Fresh produce contains a large amount of water; RH in intercellular spaces is 100%.
• Water vapour moves from high to low RH; if atmospheric RH is less than 100%, water is lost from the product, causing desiccation and quality loss.
• Some products can absorb moisture if the atmosphere is oversaturated.

5.3 Optimum RH

• Most fresh produce: 90–95% RH (±5% fluctuation).
• Leafy vegetables and flowers: close to 100% RH.
• Onions, garlic, some flower bulbs: ~70% RH.

5.4 Measurement of RH

• Psychrometer (wet and dry bulb thermometer) and psychrometric chart are reliable.
• Electronic RH meters are increasingly accurate.
• Follow manufacturer instructions and consider temperature, pressure, and air distribution.

5.5 Factors Affecting RH

• Temperature fluctuations and gradients.
• Large product heat load.
• Heat leakage (open doors, bad seals, fans, forklifts).
• Packaging material absorbing moisture.
• Low cooling coil surface temperatures causing condensation and ice.

Correct RH maintenance requires:

• Sufficient cooling coil surface area.
• Airtight building structure.
• Proper management to avoid exceeding heat load capacity.

6. Pallet Handling Equipment

Heavy loads require special handling equipment. Consider efficiency, cost-effectiveness, and safety.

6.1 Internal Combustion Powered Forklifts

• Can move heavy loads quickly over uneven terrain.
• Diesel: Only for open air use, never in cold stores or confined spaces.
• LPG: May be used in cold stores with approved exhaust gas purifier, not in confined spaces like containers or ship decks.

6.2 Battery Powered Forklift

• No human or food safety hazards, does not pollute storage atmosphere.
• Not efficient for uneven terrain but ideal for cold stores, vehicles, containers, and ship decks.

6.3 Hand Operated Pallet Jack

• Effective for small volumes on level, solid surfaces.
• Cost-effective for small operations; can be used for loading vehicles and containers, but T-bar floors may restrict maneuverability.

7. Precooling and Storage

7.1 Precooling

• Removes field heat and cools product to optimum storage temperature as quickly as possible.
• RH is very low during precooling due to temperature differences between product and cooling coil air.

7.1.1 Forced Air Cooling (FAC) / Pressure Cooling

• Uses fans to create low pressure across the pallet, forcing cold air through cartons.
• Fans must be switched off immediately when optimum product temperature is reached to prevent moisture loss and chilling/freezing injury.

7.1.2 Hydro Cooling

• Uses cold water shower to remove heat quickly without moisture loss.
• Not favored by South African fruit and vegetable industries.

7.1.3 Room Cooling

• Loading product into a cold store without active airflow is not recommended.
• Takes too long, results in temperature variation and reduced RH.

7.2 Storage

• Product must be kept within ±0.5°C of optimum storage temperature and at 90–95% RH.
• Temperature must be kept at optimum with a maximum fluctuation of ±5.0%.
• Doors must be kept closed; no warm product should be loaded into a store with already cold product.
• Efficient stock management is required for traceability.

7.2.1 Regular Atmosphere (RA) Storage

• Standard cold storage with ~22% oxygen and 0.03% carbon dioxide.

7.2.2 Controlled Atmosphere (CA) Storage

• Oxygen and carbon dioxide are accurately controlled and maintained.

7.2.3 Modified Atmosphere (MA) Storage

• Product is packed in a differentially permeable bag, creating lower oxygen and higher carbon dioxide concentrations inside the bag.

8. Temperature and RH Records

Accurate measurement, management, and recordkeeping are essential for product value, traceability, and insurance.

8.1 Products Handled

• Keep accurate records of products taken in and out of cold stores (type, volume/mass, packaging, dates, times).
• Compliance with traceability requirements is essential.

8.2 Product Temperature

• Take accurate product temperatures of representative samples at loading in and out.
• Use reliable, calibrated instruments and apply correction factors as needed.

8.3 Calibration of Instruments

• All temperature instruments, recorders, and data loggers must be calibrated at least once a year by an accredited organization.
• Mobile and handheld instruments must be calibrated at least once a week during operation.
• Calibration must be done in pure melting ice at 0.0°C.
• Attach a sticker stating the correction factor if the instrument cannot be zeroed.

8.4 Frequency of Temperature Readings

• Product temperatures must be taken and recorded during loading in/out, precooling, and storage to identify hot and cold spots.
• Air temperatures (DAT and RAT) should be recorded continuously.
• Record at 2-hour intervals during precooling and at least every 8 hours during storage.
• Room air temperatures should also be recorded in larger stores.

8.5 Temperature Records

• Keep accurate records of all temperatures for at least five years for traceability and insurance.
• Handwritten DAT and RAT records are not acceptable for insurance claims.

9. Cold Store and Storage Management

Effective management requires understanding plant physiology, temperature dynamics, heat transfer, air characteristics, and more. Multiple readings should be taken and averaged.

• Place fruit pulp temperature sensors in fruit at loading.
• Number of sensors depends on product type, consignment volume, and client requirements.
• Sensors must be regularly calibrated and readings recorded.

Sensor Placement:

• Each forced air cooling tunnel: minimum two pulp sensors; if pallets are stacked three high, four sensors to reach the furthest point.
• For every three tunnels: at least one pulp sensor in the front (face) on the outside of the tunnel.

A well-managed cold store, even if below average, can outperform a better facility with poor management. Remember, the cold store itself does not cool the product—the cold circulating air does.

9.1 Set Point Temperature

• The required thermostat setting to ensure air off the cooling coil is controlled within ±0.5°C of the specified delivery air temperature.

9.2 Delivery Air Temperature (DAT)

• The temperature of air just before it contacts the product.
• DAT may not vary by more than 0.5°C in various positions to ensure even product temperature.
• Set point must ensure air is delivered within ±0.5°C of the specified carrying temperature.

Factors causing DAT variation:

• Too wide a thermostat control range
• Uneven air flow through the cooling coil (clogging/icing)
• Faulty fans
• Long air path from cooler to cargo
• Obstructions in air delivery plenum/ducting
• Short-circuiting of air, mixing cold and warm air

9.3 Return Air Temperature (RAT)

• The temperature coming off the product.
• RAT can vary considerably in different positions.

Factors causing RAT variation:

• Heat leakage
• Short-circuiting of cold air through open spaces
• Variation in product temperature through the load
• Poor air circulation
• Fruit ripening/decay generating heat

9.4 Space Air Temperature (SAT)

• Air temperatures at various positions in the cargo space.
• SATs are important indicators of hot spots and are considered in low temperature insect sterilization procedures.

9.5 Product Storage Temperature

The product storage temperature (carrying temperature) is the optimum pulp temperature for best storage/transport with minimal quality losses.
For apples, this is -0.5°C (all apples in the consignment should be at -0.5°C).

Factors causing variation:

• Inefficient precooling
• Variation in product physiology (maturity, ripeness, age, cultivar)
• Packaging/stacking patterns affecting air flow
• Variation in DAT over time/positions
• Variation in cold air supply rates

9.6 Actual Product Temperature

The actual product temperature is measured on a single fruit using a calibrated thermometer, thermocouple, or thermistor.

Best practices:

• Use a calibrated device and correct procedure.
• Take multiple readings from different fruit and positions for a reliable average.
• Note the range between coldest and warmest readings and their positions.
• Product temperature should be within ±0.5°C of the specified optimum.
• For cold sterilization (steri) treatments, the range must not exceed ±0.25°C from the specified carrying temperature.