Temperature management apples
An optimal temperature is mostly the first requirement to maintain a good quality of horticultural produce after harvest. Refrigeration is an effective method of preserving the quality of apples. Cooler temperatures extend the storage life by, among other things, reducing respiration, water loss, colour change, and decay. The apples should however not be stored below their critical temperature. This would result in risks on various quality problems such as weakened tissue leading to discoloration and increased risk of decay. The optimum temperature varies per commodity and cultivar and should be controlled throughout the whole fresh supply chain.
Optimal storage temperature
Attention points before the apples enter the storage room
Apples should be cooled rapidly after harvest. Ensure that the product ends up as soon as possible in a refrigerated area after harvest. Optimally within a few hours after harvest, and never longer than 24 hours. Never leave the product outside with thinking the heat will pull out during the colder night. The temperature difference between fruit and outside temperature is mostly not big enough. Moreover, there is outside often not enough air circulation for an effective cooling of the apples in the middle of the stack.
The storage room should be pre-cooled before the first apples enter. In this way, more refrigeration capacity is available to cool down the apples when room loading begins. The setpoint for the air temperature during room loading varies per apple variety, harvest maturity and intended storage technology (Refrigerated Air or Controlled Air). A maturity and quality check of the apples is highly recommended to refine decisions about temperature management.
Factors determining optimal storage temperature
Many different apple cultivars. Photo by Terrance Emerson/Shutterstock.com
Apple varietyThe optimal storage temperature depends on the apple variety. It is usually in the range between 0.5 °C and 4 °C. A colder temperature slows down ripening, but some varieties develop disorders such as 'low temperature breakdown' which is an internal browning. At higher temperatures, the ripening goes faster. Also the risks on too much moisture loss (leading to shriveling) and pathogen attacks (rots) increase.
Month calendar. Illustration by Oakozhan/Shutterstock.com
Storage periodThe optimal storage temperature may vary within the storage period. For some varieties, the temperature should be higher during the first weeks. It is especially the skin of these varieties that is susceptible to damage in this first period of cooling. An initial slightly higher or variable temperature at the beginning of storage can also help to reduce 'watercore'. This water-soaked and glassy fruit flesh could later lead to brown damage.
Illustration of cold store. Illustration by Ipajoel/Shutterstock.com
Storage technologyThe optimal storage temperature depends on the storage technology. The temperature in Refrigerated Air (RA) storage is often a bit lower (e.g. 0.5 °C lower) than in Controlled Atmosphere (CA) storage. Special temperature protocols can be required when the ripening inhibitor 1-MCP is applied in the storage room.
Temperature differs depending on the location in the storage room. Photo by WUR
Temperature differencesBecause of the design of the cooling system in the storage room, it is important to be aware of product temperature differences in the storage room. In optimized situation with right cooling design, operation and a right stacking, the difference between product temperature at the coldest position and the warmest position will be less than 0.5 degree. Perform regular measurements to check the temperature differences in your storage room.
A group of good looking apples. Photo by SaGa Studio/Shutterstock.com
Harvest maturity and qualityLists with the recommended storage temperature per apple variety can be found, for example, on internet sources. These temperatures are often based on an optimal harvest maturity and quality. However, be aware of necessary temperature adjustments if harvest maturity or quality is not optimal. More detailed advice is given, for example, in the Storage Advice Tool or by storage advisors.
Heat sources in a storage room
Crates with apples in the field. Photo by WFBR
Field heatApples at harvest are usually much warmer than their ideal temperature for subsequent storage. Fast cooling is necessary to delay ripening, senescence and rot development. It also reduces water loss of apples, thus reduces the risk of shriveling and reduces loss of saleable weight. When apples enter the storage room, the so-called 'field-heat' must be quickly removed by sufficient cooling in the storage room. Forced air circulation is therefore necessary. It may take several days for the apples to reach an optimal internal temperature. The refrigeration system runs almost continuously during the period when the rooms are being filled with apple bins from the orchard.
Apples respiring. Photo by WFBR
Respiration heatAlthough the apple respiration slows down at cold temperature, it will continue throughout the life of the apple. This respiration means a continuous heat source by the apple itself. To keep the apples at their optimum internal temperature, this heat must be removed by cooling. Storage operators use temperature probes inside some of the apples to measure the apple temperature and steer the cooling process.
The building is a source of radiation heat. Photo by WFBR
Radiation heatHeat transfer through the walls from the outside to the inside of the storage room can be considerable. Think of sun radiation. When a cold room is situated in a warmer environment, heat will be transmitted from outside to inside the chamber. This increases the need for cooling. Good insulation of the building is therefore important. But also shadow (by trees) and a light coloured building surface help.
Warmer outside air can enter a room with an open door. Photo by WFBR
Air exchange with warmer outside airAnother heat source in the storage room is the air exchange with warmer outside air. Think of the ventilation system and the doorways. This heat contributes especially during the filling time of the room. So don't leave the door open unnecessarily. In rooms where doors are often opened, it is worth checking whether the number (and length) of openings, can be reduced.
Lights can serve as small heat source. Photo by WFBR
Smaller heat sourcesIn the room, heat can come from equipment such as defrost equipment and fan motors. The cooling system can often be optimized to reduce these heat sources. Some smaller sources contribute as well to a higher temperature in the room, such as people working in the room, forklifts, and lighting. Turn off the lights in the room when you are not inside.
Temperature sensors in a storage room
Control sensor. Photo by WFBR
Control sensorThe storage room contains a number of temperature sensors. The most important one is the control sensor that measures the air temperature at a fixed position. This sensor is commonly placed in the air flow on the suction side of the evaporator. It is used to regulate the on/off cycling of the cooling.
Product sensor. Photo by WFBR
Product sensorThe use of product sensors (with probe inside the apple) are strongly advised at least at the coldest position and at the warmest position in the room. There are always some differences in product temperatures in the cold store. The warmest place of the storage room is mostly underneath the cooling system. While the coolest place is along the air flow towards the opposite side of the room.
Handheld sensor. Photo by WFBR
Handheld sensorA handheld thermometer must be available. This should be used for routine product temperature control. In rooms under Controlled Atmosphere (CA) conditions this can be done via the control window. In this case, the safety instructions must be known and followed.
Did you know?
The optimal storage temperature for apple is not necessarily the same over the total storage period.Some apple varieties should be gradually cooled at the beginning of storage to prevent skin damage. Or they need to be warmed up a few degrees at the end of storage to avoid bruising during the subsequent sorting.