Temperature management pears
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 pears. Cooler temperatures extend the storage life by, among other things, reducing respiration, water loss, colour change, and decay. The produce should however not be stored below their critical temperature, as chilling injury may then occur. 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 should be controlled throughout the whole fresh supply chain.
Optimal storage temperature
Attention points before the pears enter the storage room
Pears 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. 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 pears in the middle of the stack.
The storage room should be at the optimal temperature during a number of days before the first pears enter. In this way, more refrigeration capacity is available to cool down the pears when room loading begins. The setpoint for the air temperature during room loading varies per pear variety and harvest maturity. A maturity and quality check of the pears is highly recommended to refine decisions about temperature management.
Factors determining optimal storage temperature
Different pear varieties. Photo by Artem Samokhvalov/Shutterstock.com
Pear varietyThe optimal storage temperature depends on the pear variety. It is usually in the range between -1 °C and 0 °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 per pear variety. Some varieties even require specific cooling down procedures. Stabilizing product temperature during storage period is most effective to minimize moisture loss. Minimizing differences in product temperature at different positions in the room will reduce local moisture loss as well.
Illustration of cold store. Illustration by Ipajoel/Shutterstock.com
Storage technologyThe optimal storage temperature is usually the same for Refrigerated Air (RA) storage as for Controlled Atmosphere (CA) storage. Special temperature protocols can be required when the ripening inhibitor 1-MCP is applied in the storage room.
Harvest maturity plays an important role. Photo by Zoran Zeremski/Shutterstock.com
Harvest maturity and qualityLists with the recommended storage temperature per pear 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. An example is that pears with low sugar levels have a higher freezing risk, therefore these may need a slightly higher storage temperature. More detailed advice is given, for example, in the Storage Advice Tool or by storage advisors.
Heat sources in a storage room
Pears in the field. Photo by symbiot/Shutterstock.com
Field heatPears 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 pears, thus reduces the risk of shriveling and reduces loss of saleable weight. When pears enter the storage room, the so-called 'field-heat' must be quickly removed by sufficient cooling capacity in the storage room. A high level of air circulation is therefore necessary. It may take several days for the pears to reach an optimal internal temperature. The refrigeration system runs almost continuously during the period when the rooms are filled with harvested pears.
Fresh products generate heat because of their metabolism. Photo by WFBR
Respiration heatAlthough the pear respiration slows down at low temperature, it will continue throughout the life of the pear. This respiration means a continuous heat source by the pear itself. To keep the pears at their optimum internal temperature, this heat must be removed by cooling. Storage operators use temperature probes inside some of the pears to measure the pear temperature and set the right room control temperature.
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 capacity. Good insulation of the building is therefore important. But also shadow (by trees), a light coloured building surface, ventilation of heat inside the building, or even building cooling will help.
Evaporator fan. Photo by WUR
Heat production of evaporator fansEvaporator fans support the heat exchange over the cooler surface and distribute heat from storage room / product to the evaporator. All power of the fans needed must be removed by the cooling. Selecting the right settings for fan operation will help temperature distribution, energy use and product quality.
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 act as small heat source. Photo by WFBR
Less important 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. For the cooling operation, the control sensor that measures the air temperature at a fixed position is important. 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 sensorsThe use of product sensors (with probe inside the product) 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 (preferrably < 0.5 °C). The warmest place of the storage room is mostly underneath the evaporator of the cooler. While the coolest place is along the air flow towards the opposite side of the room.
Handheld sensor. Photo by WFBR
Hand held 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?
Pears are often stored below 0 °CPears won't freeze at temperatures just below 0 °C due to the high sugar content in the fruits.