Dehydration is one of the critical factors in the post-harvest of fruits, especially in cherries, whose small size and characteristics make them a fruit that easily shows symptoms associated with this phenomenon, such as the browning of the pedicel. This is a reality that represents a great challenge for both producers and the entire national fruit industry, which requires new and better tools to improve the condition of the arrival of cherries to distant markets such as the Asian continent.
Throughout the post-harvest process, cherries go through different stages, each of which experiences dehydration to a different degree. However, it is at harvest time that the greatest degree of dehydration of this delicate fruit is experienced due to the exposure of the freshly harvested fruit to the prevailing environmental conditions in the orchard, characterized by high temperatures and low relative humidity.
Vapor pressure deficit (VPD) is a direct indicator of the dehydration that fruit will experience when exposed to certain environmental conditions. Air temperature, relative humidity and fruit pulp temperature are the factors considered in the equation to determine the VPD at a given time.
In order to find solutions and new proposals to reduce cherry dehydration during this first stage of post-harvest, our R&D team at Johnson Industrial Projects monitored the vapor pressure deficit in a cherry orchard located in the O'Higgins region, in central Chile, which allowed us to identify its evolution throughout the day. In this way, we were able to identify the critical hours for the fruit and the most advisable times to carry out the harvest, always thinking about providing the best possible conditions for the freshly harvested cherry.
The study consisted of monitoring relative humidity, air temperature and cherry pulp temperature in the orchard throughout the day and night during the cherry harvest season, in order to identify the vapor pressure deficit (VPD) at each hour of the day. These measurements were made with temperature and humidity sensors with data loggers located in different locations within the orchard, which recorded the variations of these variables for 14 consecutive days.
From the information collected on ambient temperature, relative humidity and fruit pulp temperature, the vapor pressure deficit (VPD) and its evolution throughout the day were calculated using a psychrometric table. The results of these evaluations were graphed, giving rise to Figure 1, where the evolution of the VPD throughout the day in the orchard can be observed.
Figure 1. Evolution of the vapor pressure deficit (VPD) throughout the day in a cherry orchard during harvest season (orchard located in the town of El Abra, commune of Requínoa, O'Higgins Region).
The green box marks the time when the DPV is less damaging to the cherry crop, with values below 2 kPa. As can be seen, this time corresponds to the period between 6 am and 12 pm. After midday, the DPV reaches values that increase the dehydration of cherries and, therefore, should be avoided.
If these values are projected to the right, moving forward in the hours of the day, equivalent DPV values are observed again when the curve is decreasing. The night-time schedule in which the DPV is similar to that existing during the daytime cherry harvest has been marked with a blue box, which allows us to conclude that, if a night-time harvest is carried out, it should be carried out from 8:00 p.m. onwards.
This graph also identifies a critical period of the day that occurs between 2:00 p.m. and 7:00 p.m., when the DPV reaches its highest values. During this time period, the fruit, if harvested, would be exposed to the highest levels of dehydration.
The results of this study allow us to conclude that cherry harvesting in the morning, before 12:30, takes place under conditions of DPV less than 2 kPa, exposing the fruit to less dehydration. Because DPV conditions greater than 2 kPa are detrimental to the harvested cherry due to the high dehydration to which it is exposed, it is not advisable to harvest the cherry between 12:30 and 19:30 hours.
Finally, if a night-time harvest is considered part of the strategy, it must be carried out under the same DPV conditions as the daytime harvest, so it should not start before 8:00 p.m., and may be extended indefinitely during the night depending on logistics, availability of light and labor.
