Chapter 2: Cold Store Requirements

Table of 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 Fittings
   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 Forklifts
   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. 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 for 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 and maintain air and product temperatures inside the cold store, minimizing heat leakage from the outside.

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 outside joints 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 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 Insulation

¹ Insulation in cold store floors is not necessary if the underfloor structure is kept dry.
² Thicker insulation is required in the ceiling in warmer climates.

2.6 Cargo Protection

• Must be provided inside and outside the store at all times.
• An air lock structure should protect cargo from the elements and prevent heat leakage, maintaining the cold chain during loading/unloading.

2.7 Drip Trays

• Must be installed under cooling units to drain defrost water to the 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 to precool the product to the optimum storage temperature and maintain it according to product requirements. The cold store must be designed, built, and managed to precool and store the product under optimum conditions.

Key factors:

• 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).
• Consider additional heat sources: heat leakage, forklifts, air circulation fans, door openings.
• Factors affecting cold air circulation: packaging, stacking patterns, store dimensions.
• Required cooling rates, temperature, and relative humidity ranges.

The cold store should be capable of precooling the designed maximum intake volume within 24 hours (exceptions: grapes—72 hours; Granny Smith apples—5 days). It should also maintain the optimum storage temperature within ±0.5°C. Relative humidity (RH) 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 others must have 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 where it senses the average temperature in the immediate environment.
• Never place directly in front of an air fan or heat/cold source.
• For chilled produce, place 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 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.
• Recommended accuracy: ±0.25°C or better.

4.2.3 Thermocouples

• Created by fusing copper and constantan; electric current relates to temperature.
• Use only high-quality “Type T” wires and precise procedures for ±0.5°C accuracy.

4.2.4 Resistance Thermometers (Thermistors)

• Measure voltage drop related to temperature.
• Can be made small and robust, with accuracy better than ±0.1°C.
• More expensive than thermocouples.

4.2.5 Mechanical Bi-metal Strips or Coils

• Used in older controllers and chart recorders; not recommended.

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.
• Accuracy: ±0.5°C for general storage, ±0.25°C for 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 random positions; may not reflect true delivery/return air temperature.
• Accuracy: better than ±0.25°C at 0.0°C (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 (2x2x2 cm) from pure distilled water.
• Fill insulated containers with ice cubes; add a small amount of distilled water to form an ice-water mixture.
• Stir continuously to maintain constant temperature.
• The mixture must register 0.0°C on a certified standard thermometer.
• Submerge the sensor to be calibrated in the mixture, stirring slowly.
• Take the reading when stabilized.
• The difference between the standard thermometer (0.0°C) and the device gives the accuracy.
• The calibration factor is the difference between the readings and 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 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%, produce loses water, leading to desiccation, shriveling, weight loss, and reduced quality.
• Some products (e.g., leafy vegetables, flowers) require RH close to 100%; others (onions, garlic, flower bulbs) require around 70%.

5.3 Optimum RH

• Most fresh produce: 90–95% RH (±5% fluctuation).
• See Chapter 8 for product-specific RH requirements.

5.4 Measurement of RH

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

5.5 Factors Affecting RH

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

Correct RH maintenance requires:

• Proper cooling coil design (large surface area, correct coolant temperature).
• Airtight building procedures to minimize heat leakage.
• Management to avoid exceeding designed heat load.

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 outdoor use (exhaust gases are a health hazard).
• LPG: May be used in cold stores with approved exhaust gas purifier; not for confined spaces.

6.2 Battery Powered Forklifts

• No safety hazards or pollution; not suitable for uneven terrain.
• Ideal for loading/unloading in cold stores, vehicles, containers, and ships.

6.3 Hand Operated Pallet Jack

• Cost-effective for small volumes on level, solid surfaces.
• Can be used for loading vehicles and containers, but container floors may restrict movement.

7. Precooling and Storage

7.1 Precooling

• Rapidly remove field heat to slow respiration and ripening.
• RH is low during precooling due to temperature differences between product and cooling coil air.

7.1.1 Forced Air Cooling (FAC)

• Uses fans to force cold air through pallets/cartons.
• Switch off fans immediately when optimum product temperature is reached to prevent moisture loss and chilling injury.

7.1.2 Hydro Cooling

• Uses cold water shower; removes heat quickly without moisture loss.
• Not favored in South African fruit/vegetable industries.

7.1.3 Room Cooling

• Passive cooling by loading product into a cold store without active airflow.
• Not recommended due to slow cooling, temperature variation, and reduced RH.

7.2 Storage

• Maintain product within ±0.5°C of optimum storage temperature and RH of 90–95%.
• Keep doors closed; avoid loading warm product into a cold store with already cold product.
• Practice efficient stock management for traceability.

7.2.1 Regular Atmosphere (RA) Storage

• Standard atmosphere: ~22% oxygen, 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 permeable bags, creating lower oxygen and higher carbon dioxide concentrations inside the bag.

8. Temperature and RH Records

Accurate measurement and recordkeeping are essential for traceability and insurance.

8.1 Products Handled

• Keep records of products in/out of cold stores: type, volume/mass, packaging, dates, and times.

8.2 Product Temperature

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

8.3 Calibration of Instruments

• Calibrate all temperature instruments at least once a year (by an accredited organization).
• Mobile/handheld instruments: calibrate weekly during operation.
• Calibration must be done in pure melting ice at 0.0°C.
• Attach a sticker with the correction factor if not internally zero-calibrated.

8.4 Frequency of Temperature Readings

• Record product temperatures during loading, precooling, and storage to identify hot/cold spots.
• Air temperatures (delivery and return) should be recorded continuously (every 2 hours during precooling, at least every 8 hours during storage).
• Room air temperatures should be recorded in large stores, especially where air circulation is restricted.

8.5 Temperature Records

• Keep all temperature records for at least five years for traceability and insurance.
• Handwritten 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, combined with cost efficiency and traceability.

• Take multiple readings and use the average.
• Place fruit pulp temperature sensors in fruit at loading; calibrate and record regularly.
• Each forced air cooling tunnel must have at least two pulp sensors (four if pallets are stacked three high).
• For every three tunnels, have at least one pulp sensor in the front (face) on the outside.

9.1 Set Point Temperature

• The thermostat set point must ensure air off the cooling coil is controlled within ±0.5°C of the specified delivery air temperature.

9.2 Delivery Air Temperature (DAT)

• DAT is the temperature of air just before it contacts the product.
• May not vary by more than 0.5°C in various positions.
• Set the thermostat to deliver air within ±0.5°C of the specified carrying temperature.

Factors causing DAT variation:

• Wide thermostat control range
• Uneven air flow through the coil
• Faulty fans
• Long air path
• Obstructions or short-circuiting of air

9.3 Return Air Temperature (RAT)

• RAT is the temperature coming off the product.
• Can vary due to heat leakage, air short-circuiting, product temperature variation, poor air circulation, fruit ripening, or decay.

9.4 Space Air Temperature (SAT)

• SATs are air temperatures at various positions in the cargo space.
• Important for identifying hot spots and for low temperature insect sterilization procedures.

9.5 Product Storage Temperature

• The optimum product (pulp) temperature for storage/transport.
• For apples: -0.5°C.
• Factors causing variation: inefficient precooling, product physiology, packaging/stacking, DAT variation, cold air supply rates.

9.6 Actual Product Temperature

• The temperature of a single fruit, measured with a calibrated device.
• Take multiple readings from different fruit/positions for a reliable average.
• Product temperature should be within ±0.5°C of the specified optimum storage temperature (±0.25°C for cold sterilization treatments).