Since the cherries are harvested from the tree until they reach the destination market, the fruit is exposed to water losses that occur in the different stages of the logistics process, from harvest to maritime transport, being in the stages before packaging in a modified atmosphere bag. where the greatest impact occurs.
In the particular case of cherries, the organ in which the loss of water is first evident is the pedicel; This is a herbaceous tissue with a thinner epidermis than that of the fruit and due to its high surface/volume ratio it is more sensitive to dehydration. The green and turgid appearance of the cherry pedicel is considered an indicator of the degree of freshness of the fruit (Linke et al., 2010), while, due to the loss of water, it turns brown, thin, which gives the fruits an aged appearance, reducing their commercial value (Candan et al., 2017).
In the orchard, the climatic conditions of temperature and humidity will influence this loss of water, in addition, the prolonged waiting times for hydrocooling will negatively affect the appearance of the pedicel. The product that is exposed to the heat of the sun and low relative humidity will suffer a significant initial loss of water, the effects of which cannot be reversed with any postharvest procedure or technology.
The dehydration and browning of the pedicel are the result of the loss of water, in this process the integrity of the membrane is affected, with the subsequent oxidation of compounds inside the cells, which as a result give the brown coloration (Linke et al. ., 2010). Environmental factors such as relative humidity (RH) and temperature are elements that affect the Vapor Pressure Deficit (VPD), which expresses the ability of the atmosphere to extract water from tissues (Knoche, 2015).
Strategies to minimize water loss at the harvest stage include the following measures:
1. Harvest in the morning
Normally the cherry harvest starts early in the morning (6:00 AM) and lasts until 2:00 PM. The objective of this measure is to prevent the fruit from being exposed to high temperatures for a long time. It is generally recommended that the permanence of the fruit in the field is not more than 4 to 6 hours.
Graph 1. Influence of environmental conditions at harvest on the dehydration of the fruit. Source: Rodriguez, 2014.
Graph 2. Dehydration in cherries Van at 20°C and 70% RH. Zoffoli Fountain, 2000
The loss of water is usually expressed as weight loss of a tissue and, in the case of cherries, to see the effect of dehydration of the pedicel, the fruit is weighed as a whole. Graphs 1 and 2 show the effect of exposure time after harvest at a condition of 20ºC and HR of 60% and 70%, respectively; increases in weight loss of 60% are observed every 4 hours, reaching critical levels at 6 hours under these conditions (Zoffoli, 2000).
2. I use physical barriers to protect the fruit from the sun
The use of physical barriers makes it possible to avoid a rise in the temperature of the harvested fruit and, depending on the material used, also allows a better humidity differential to be maintained around the fruit.
In studies carried out by Kupferman (1998) it was observed that the fruit kept in the shade had a lower pulp temperature than the fruit exposed to the sun, in addition to showing fewer brown pedicels (Table 1). Exposure to the sun for more hours will result in a higher fruit temperature and more damage to the pedicels.
Table 1. Effect of waiting time and storage conditions on pulp temperature and condition of pedicels after storage at 0ºC.
3. Blankets or reflective carp
The reflective cover or tent is an element used to protect the fruit in bins, being used alone or in combination with sponges.
In a study conducted by Schick et al. (2002), where he used a three-layer Mylar™ cover (layer in contact with the product, corresponded to a silver; the middle layer is a standard woven polyethylene material and the outer part was a glossy white cover), managed to maintain or slightly lower the temperature of the fruit at harvest and increased the relative humidity to 100% around the fruit. Meanwhile, the fruit that was kept in the shade, covered by cardboard or conventional canvas, increased its temperature and decreased its relative humidity, as observed in graphs 3 and 4.
Graphs 3 and 4. Flesh temperature and ambient relative humidity at 3 inches below top of fruit surface for cherries covered with reflective mulching vs. bins without mulching under tree shade. Source: Schick and Toivonen (2002).
4. Wet sponges
In our country, wet sponges are used, either individual (tote by tote protecting the first run) or complete sponges to cover the surface of the bins; with this, direct sun is avoided on the fruit and moisture is added, the latter is achieved by adding chlorinated water to ensure safety. This work is often done poorly, where humidity is not implemented properly and ends up using dry materials.
5. Perforated plastic caps
In recent seasons, macro-perforated plastic caps for bins have been tested, which would increase the relative humidity (RH) inside the bin.
In order to have more practical information on the effects of using different fruit protection products, the Trío Kimün team carried out evaluations in the Pencahue and Quinta Morza area, recording relative humidity, ambient temperature, and pulp under the element protector during the harvest and storage period.
The materials used were: a well-wet individual sponge, a dry individual sponge, a reflective tent, a complete sponge from wet bins, and plastic caps (Figures 1 to 4). The sensors to measure relative humidity (RH) and temperature (T°) were placed in the second run of totes in each bin evaluated, leaving one tote below and another on the sampled box.
6. Records of environmental conditions
Graph 5. HR with different protective materials evaluated in the areas of Quinta Morza and Pencahue from harvest to collection center in the orchard.
As can be seen, from graph 5, all the materials used increased the percentage of relative humidity in the environment under them, with the individual dry sponge being the one that produced the least effect, increasing from 40 to 75%, while the rest of the materials makes it up to 80 to 90%. Well-moistened individual sponges had the highest moisture levels (over 85%).
Graph 6. Decrease in air temperature (A) and pulp (B) with different protective materials evaluated in the areas of Quinta Morza and Pencahue from harvest to collection center in the orchard.
The air temperature with all the protective covers decreased from 1ºC to 3ºC, with the individual well-wet sponge and reflective tent showing the greatest decreases (Graph 6A).
As for pulp temperature, the well-wet individual sponge and the reflective tent managed to reduce the temperature by 0.8ºC, a large sponge in bins rather maintained; the individual dry sponge and plastic cap in the room between the rows increase the temperature, but already in the collection center it decreases, reaching values similar to those of harvest.
Graph 7. Weight loss/hour according to protective material.
As observed in graph 7, there are statistical differences between the different protection materials, where the fruit with the wet individual sponge presented lower weight loss per hour (0.05%/hour), followed by the reflective tent (0.107 %/hour). The dry individual sponge presented the highest values of weight loss (0.161%/hour).
The above data is directly related to the environmental information collected; the material with the least weight loss is the one that shows the highest RH and the lowest air and pulp temperatures, while the reflective tent, despite lowering the temperature, contributes approximately 5% less RH.
Materials such as plastic caps and large sponges for bins showed less weight loss than dry sponges, but due to the limited data, they were not included in the statistical analysis.
The use of protective elements or barriers at the time of the cherry harvest positively influence the final condition of the product, improving the conditions of RH and T° inside the bins. For this reason, it is important to use them immediately after harvest when the bins remain between the rows and the fruit is not exposed to direct sunlight. In the case of using sponges, this must imply the use of adequate humidity, which must also ensure safety, otherwise it is better to use materials such as reflective tents, taking the same precautions to occupy them in the inter-row space immediately after harvest. while the bins are being completed. Furthermore, it should never be forgotten that the best way to minimize these losses is to reduce the times between harvest and hydrocooling.
The proper choice and use of these elements will directly impact the results of weight loss.
Candan, A. P., Romero, S., and Jara, G. 2007. Use of modified atmospheres in cherries cv. Lapins. Institute of Nutrition and Food Technology, EEA Alto Valle, 11.
Knoche, M., Athoo, T. O., Winkler, A., & Brüggenwirth, M. (2015). Postharvest osmotic dehydration of pedicels of sweet cherry fruit. Postharvest Biology and Technology, 108, 86-90.
Linke, M., Herppich, W. B., & Geyer, M. (2010). Green peduncles may indicate postharvest freshness of sweet cherries. Postharvest biology and technology, 58(2), 135-141.
Rodríguez, J. 2014. Postharvest handling with an impact on quality and condition of cherries. Foundation for Fruit Development (FDF), PDT Cerezos.
Schick, J. L., & Toivonen, P. M. (2002). Reflective tarps at harvest reduce stem browning and improve fruit quality of cherries during subsequent storage. Postharvest Biology and Technology, 25(1), 117-121.
Zoffoli, J.P. 2000. Critical Evaluation of Postharvest Management of Cherries. Pontifical Catholic University of Chile. https://www.agrotechnologia.es/index_htm_files/Cerezas%20y%20Durofel.pdf