Crops and varieties
Quality and temperature
If products are kept too cold, cold damage can occur. Examples are discolorations or too little flavor development.
For a number of products, such as blueberries and cabbages, temperatures just above zero (0.5 to 1 °C) are ideal. But some varieties of apples, fruiting vegetables such as tomatoes and cucumbers, and tropical fruits such as mango and banana are sensitive to cold. They must be kept at higher temperatures, 4 or 14 °C, depending on the product.
Damage because of too low temperature
Apples with chilling injury. Photo by WUR
Chilling injuryChilling Injury is caused by storage below the optimum storage temperature. Symptoms of chilling injury are surface pits, watery flesh tissue, uneven ripening, skin discoloration, off-taste and higher susceptibility to postharvest decay. These symptoms usually show up once the product is transferred from cold to ambient temperature. The damage is irreversible and therefore problematic! Damage depends on the duration and depth of the temperature. In general, mature-green fruits are more sensitive than riper fruits.
Pears with freezing damage. Photo by WUR
Freezing damageFresh fruit and vegetables are rich in water, thus susceptible to freezing. The products may still look fine at freezing temperatures but become watery, soft and brownish after heating up. This product damage is very serious. The freezing point is not always at 0 °C. Thanks to dissolved substances in the fruit juice (especially sugars), the freezing point can be slightly lower. For this reason, pears are often successfully stored at -0.5 to -1.0 °C without freezing damage of the product.
Cold storage rooms
Most cold stores are designed to maintain a constant low temperature. The temperature sensor, which measures the air temperature in the cold store and allows for adjustment, is normally suspended in the return air stream that exits at the evaporator. In addition, it is wise to measure the product temperature (internally), this provides valuable insights into, for example, temperature differences in a cold store.
Optimizing the air circulation can minimize the temperature differences in the cold store.
It is important to realize that a reefer container and a refrigerated truck are designed to maintain, not lower, the temperature of the fresh product. It is therefore important to pre-cool the product before placing it in the reefer container or truck.
The load must be stacked in such a way that air circulation is sufficient everywhere, so that the temperature differences of the load remain small. Temperature loggers can be used to review the temperature profiles of the cargo afterwards.
The optimum temperature may differ between the products. With mixed loads of these products, the temperature is therefore not optimal for all products. This need not be a problem for short-term storage. If mixed loads cannot be avoided, fruit and vegetables can be combined whose optimum temperature does not differ too much. Most tropical products are sensitive to cold and cannot be combined with products that absolutely need cold temperatures, such as strawberries and leafy vegetables. A problem that can also occur with short-term storage is that a product can be affected by the transfer of odors that are emitted by another product. Humidity and ethylene are also important.
An infrared thermometer measures the surface temperature. Photo by WUR.
Types of thermometers
Various measuring instruments are available for control measurements of the temperature.
With a penetration thermometer, the core temperature of the product can be determined via a penetration measurement. Insertion thermometers with an extra-long insertion sensor are available to measure the core temperature in, for example, bulk bags. The surface temperature of the product is measured without contact with an infrared thermometer. A warning is in order here. The surface temperature quickly adapts to the ambient temperature and can then be significantly higher or lower than the core temperature of the product.
An example of logged temperature data. Photo by WUR.
Various types of loggers are used to record the temperature development during transport. The data from these loggers can be used to determine whether the temperature has been (approximately) at the set point. Loggers can differ in accuracy, resolution (number of decimals), and can be used once or multiple times. There are also more and more possibilities for real-time access to the data. With container transport, shipping companies are also increasingly willing to share their data in real time. The location of the logger is important. When it is placed at a different location than the control sensor, a deviation from the setpoint will often occur. Small temperature differences within the room cannot be avoided.
Significant temperature differences can occur within a reefer and also within a pallet. Photo by WUR.
Location of temperature measurement
The temperature of a load in a reefer or refrigerated truck is not always evenly distributed. This depends on the path the cold air stream travels. Preferably, the coldest and warmest spot is measured. In a reefer, the product in the front pallets is usually colder than in the back near the door. Temperature differences can also occur within a pallet. If in doubt, measure at several locations.
A simple calibration approach with ice. WUR's photo
Calibration of temperature sensors
Because the right temperature is so important for product quality, you need to be sure that the temperature sensors give a reliable reading. Therefore, the sensors should be calibrated regularly (e.g. once a quarter). The procedure is briefly as follows:
1. Prepare a mixture of water and crushed ice in a small container (for example a can or small bucket).
2. Allow this mixture to stabilise for at least a few minutes. Then insert the temperature sensor into this mixture.
3. Stir and check that the sensor reaches 0°C (32 °F) exactly.
Measuring the flesh temperature of pineapple. Photo from WUR
How to measure flesh temperature
1. Stick the thermometer probe a few centimeters deep into the fruit.
2. Repeat this after minimum 30 seconds and now read the temperature. This second measurement is important, because you can bring in heat from the warm thermometer probe into the fruit the first time.
3. Always register the temperature for later reference.
Examples temperature problems in practice
Did you know?
Tropical fruits are stored at higher temperatures than fruits from cooler climatesTropical fruit such as banana and papaya can not be stored as cold as fruits from cooler climates. The optimal storage temperature can vary from 13 °C (green bananas) to temperatures below 0 °C (e.g. pears or cabbages).
Some fruit require a period of cold before they can ripenSome fruit, for instance some pear varieties, require a period a cold storage before they can ripen properly. The cold storage temperature then mimics the colder temperature during autumn, which may be required as a signal for the fruit to ripen at all.
Forced-air cooling is a common method to quickly reduce flesh temperatureAir circulation in the storage room helps to cool the product. Therefore, forced-air cooling is a common method to quickly reduce the temperature of fresh products to the optimal.
In the Netherlands, the nutrition center provides advice to consumers about the storage of fruit and vegetables
The nutrition center in the Netherlands provides advice to consumers about the storage of fruit and vegetables via a storage guide