Management of harvest and post-harvest of blueberries
As in other fruits, within the chain of management for the production of blueberry, the post-harvest stage of the fruit is a key point to reach the consumer with a quality product. Quality that is defined by a series of factors such as color, firmness, absence of damage, balance sweetness / acidity and aroma.
There is a wide range of varieties that today are cultivated commercially that can be differentiated in many aspects, including growth habit, production date, flavor, among others. It is also important to consider that the postharvest behavior may be different between varieties, since these may have a different metabolism in relation to respiration and ethylene production, susceptibility to decay, firmness to harvest and post-harvest, sugar / acid ratio , etc.
However, there is a common point for all of them, and is that they are characterized by being very perishable after harvest. Among the main causes of deterioration in cranberry are: rots, dehydration, loss of firmness, loss of appearance, development of disorders and sensory quality. Therefore, the challenge to arrive with a quality product is even greater under the conditions of countries like Chile, since the main consumer markets are distant (USA, Europe) so the fruit must maintain its integrity and quality for a prolonged period.
Cranberries show a climacteric respiratory behavior, characterized by a respiratory and ethylene rise during maturity. However, unlike other climacteric fruits, such as apples, blueberries must be harvested close to ripeness of consumption since the organoleptic attributes (flavor) do not improve after harvest. It is important to consider that varieties can have different levels of respiration. In addition, the respiratory rate is influenced, as in other fresh products, by the temperature.
Breath of cranberry fruit at different temperatures.
Due to the small size of the fruit, which translates into a greater relationship between surface area and volume, cranberries are more susceptible to water loss (or dehydration) than larger fruits such as apples. Perhaps one of the few practical advantages of this morphological feature is the shorter time required for cooling processes. On the other hand, the epidermis (skin) of the fruit is thin and very susceptible to mechanical damage and water loss. However, a morphological characteristic that contributes to diminish water loss is the wax content of the cuticle located on the epidermis. Therefore, the maintenance of this cuticle during the production chain has a cosmetic effect, both by contributing to the reduction of dehydration and the blooming of the fruit.
In general, blueberries do not show a large production of ethylene compared to other fruits. However, the rate of production of this hormone as well as the response to it is related to the variety. All the factors described above make that the post-harvest handling should be oriented mainly to the handling of temperature and relative humidity.
HARVEST MANAGEMENT
Quality of the fruit
Quality is defined by a series of factors that can be grouped into visible quality, organoleptic quality and nutritional quality. Visible quality refers to the appearance of the fruit, which in blueberries is defined as: (i) a uniform blue fruit, (ii) presence of wax on the surface of the fruit (known as bloom) that the consumer relates to a fresh fruit, (iii) absence of defects such as mechanical damage and decay, (iv) shape and size of the fruit, and (v) fruit with adequate firmness. The organoleptic quality is determined by an adequate content of sugars, acids and volatile compounds responsible for the characteristic aroma of the fruit. Therefore, all pre-harvest and post-harvest operations must be oriented to maximize the arrival of a quality product to the consumer. The quality indices normally used by the fresh fruit industry are: color, size, shape, absence of defects, firmness and flavor.
Maturity of the fruit
Much of the potential for post-harvest duration of the fruit (or quality maintenance) is defined at the time of harvest, especially for berries. The first factor to consider is the selection of the appropriate harvest moment, which for blueberries is defined by the color of the fruit. Despite its climacteric characteristic, blueberries must have a uniform blue color development to obtain a good quality fruit. Harvested fruits of red color, although they maintain a greater firmness and will develop a blue color after harvest, will have inferior organoleptic quality to a fruit harvested with an appropriate color. At this time, all precautions must be taken to reduce damage from blows and exposure to high temperatures, which will only be achieved with a good training of harvest personnel. Greater manipulation of the fruit will only contribute to causing damage and removing wax from the skin of the cranberry. If the harvest containers are overfilled, the compression damage causes a direct effect on the fruit and on the other hand it will hinder its subsequent cooling.
If the harvest is carried out directly in the export container, the fruit is subjected to less manipulation, which favors, among other things, the maintenance of the bloom, less compression damage and less exposure to contamination. Another important factor is to avoid exposing the fruit to high temperatures during harvesting, which is why rapid transport to packing is essential.
Color of the main fruit factor to consider at the time of harvest.
Crop management in blueberries to obtain quality fruit. A) Harvest trays, B) careful manipulation of the fruit.
Crop management in blueberries to obtain quality fruit. A) Careful handling of the fruit, B) and C) trays protected from the sun.
POST-HARVEST HANDLING
Handling of temperature and relative humidity
One of the most critical points for the extension of the post-harvest life of blueberries is the temperature, which must be managed from the orchard at the time of harvest by using shading or moving quickly to the packing places (packing) where there is a temperature control. If the harvest conditions do not allow a fast and frequent transfer of the fruits to the packing, it is recommended to cover the trays with materials that allow to reflect the sun avoiding the increase of temperature of the fruit.
The temperature has a direct relation with the metabolism of the fruit and with the life in poscosecha. During the harvest the fruits are generally under conditions of high ambient temperature, which causes them to be breathing at a high rate. In the breathing process oxygen is consumed (O2) and carbon dioxide (CO2) is produced in order to produce the energy necessary to maintain life; however, as by-products there is heat of respiration and water released to the medium. Breathing and above all increases in its rate can affect the quality of the fruit, for example product of the heat of respiration that increases the temperature, water loss occurs in the process and it is also possible to observe in many cases a drop in acidity because the acids are used as preferential substrates for the breathing process.
After the harvest and arrival at packing, efficient systems are needed to achieve a rapid removal of the field heat prior to storage and reach a temperature between 0 and 1 ° C, which is recommended for storage and transport.
With forced air cooling strategies it is possible to reduce the orchard temperature to storage temperatures (0-1 ° C) in less than 1 h. It has been shown that blueberries cooled to 1,5 ° C in 2 h had a lower level of rot after storage than those cooled to the same temperature but in 48 h. The cold control can reach the packing line, where a pre-cold tunnel can be incorporated that allows obtaining fruit with a temperature close to 0 ° C at the end of the line. Another important point to take into account to make forced air cooling efficient are the perforations in the packaging materials, as well as their orientation to favor the flow of cold air. Another form of cooling used is in a conventional passive chamber. However, the heat removal is slow and inefficient, since the fruits located in the center of containers or pallets receive inadequate cooling, also generating condensation conditions by releasing warm air to the fruits outside that are at a lower temperature.
Once the rapid cooling has been carried out and the fruit has reached its optimum storage-transport temperature, it is important to maintain the cold chain to avoid temperature increases. This is how to avoid thermal breaks, an ideal operation for blueberries considers the tasks of prefrío (forced air), packing in a refrigerated environment and then storage-transport at a constant temperature at 0 ° C, condition that must be maintained until its final reception.
In general, blueberries are very susceptible to water loss, which negatively affects the appearance of the fruit since "wrinkling" is observed. For this reason it is critical to keep the fruit at the recommended temperature and humidity to reduce the deficit of steam pressure and dehydration. Along with the use of low temperature, blueberries should be stored with a high relative humidity (95% to 0 ° C), a condition that will help to reduce the water loss of the fruit. With good harvest management, rapid cooling and storage at 0 ° C, under conditions of relative humidity between 90 and 95%, the blueberries have a minimum duration of 14 days.
Pallet with boxes of blueberries packed for export in cold chamber to 0 ° C.
Use of controlled and modified atmospheres
With the use of low temperature (0 ° C) as the basis for handling postharvest blueberries, a series of technologies have been evaluated to extend their postharvest life. The most used, modified atmosphere (AM) and controlled (AC), are based on the modification of the composition of gases (O2 and CO2) during storage and / or transport. In both techniques, the main effect on the physiology of the fruit is the decrease in metabolic activity as well as the control of fungi. Among the potential benefits of these technologies can be mentioned a reduction of dehydration (mainly AM) and lower development of rots, as long as they are used correctly. The levels of gases achieved through the use of AM are dependent on characteristics of the fruit (respiratory rate, temperature), the cover or film (permeability mainly), and environment (temperature). On the contrary, in AC the levels of gases to be used are maintained and / or adjusted automatically during the entire storage of the fruit, which makes it independent of the factors mentioned for AM.
One option that is used in AM to reach the final gas concentration more quickly is to perform an initial injection, which is subsequently maintained through the respiration of the fruit and characteristics of the film (active modified atmosphere).
For bilberry, the concentrations that have shown advantages in the extension of postharvest are 2-5% of O2 and 10-15% of CO2 to 0 ° C. The effects of high CO2 pass basically through the control of pathogens such as Botrytis, concentrations greater than 10% have shown to be efficient in the control of pathogens.
One of the factors that determines which concentrations to use to achieve maximum postharvest benefit is the susceptibility of a particular variety to low levels of O2 and high levels of CO2. Low levels of O2 (<2%) or high levels of CO2 (25%) can develop metabolic processes that result in the development of strange flavors or aromas in the fruit, browning or discoloration and a higher incidence of rot, which is undoubtedly the cause of rejection at the time of sale.
The development and severity of the aforementioned problems are partly determined by the CO2 and O2 concentrations reached, the time of exposure to them, and the susceptibility that the variety may present. One of the main factors to be successful with AM is the maintenance of an appropriate temperature throughout the chain, otherwise the aforementioned detrimental processes will be accelerated. Furthermore, it is important to consider the varietal factor, as respiratory rates vary depending on the variety. An additional effect of the use of AM is to reduce the loss of humidity, however, if excessive condensation occurs it can increase the problems of rotting.
Among the precautions to be considered when working with AM are: the cooling processes of the product are slower, the film should not be damaged since otherwise the beneficial atmosphere will be lost, and the use of films with low permeability to water vapor since this will generate a high relative humidity that will benefit the development of pathogens.
Incidence of decay
Of the main problems in the post-harvest of blueberries, the development of decay undoubtedly occupies a preponderant place. Of the pathogens that frequently attack these fruits we have botrytis (Botrytis cinerea), anthracnose (Colletotrichum sp.) and rhizopus (Rhizopus sp.).
However, the main fungal problem in post-harvest blueberries is botrytis. Although good temperature management can reduce the incidence of this fungus, they can not slow down its development, since it is capable of developing even at 0 ° C. The use of high CO2 in AC or AM management is also able to reduce the level of incidence of the pathogen, but without a doubt all these postharvest strategies must be supported by a good handling of the pre-harvest and harvest; This is how the application of fungicides at critical moments of pre-harvest infection, such as flowering, helps reduce post-harvest incidence levels, and crops must be avoided on days with high humidity or free water.
The development of other pathogens such as rhizopus during post-harvest is often associated with poor handling in temperature, and lack of hygiene during the harvesting and packaging processes.
It is mentioned that the presence of ethylene during storage can stimulate the growth of Botrytis cinerea, organism that causes decay. However, in general no direct benefits have been observed in the fruit when using products to reduce the synthesis or action of ethylene.
Rots observed in blueberries throughout the cold storage.
Source: Institute of Agricultural Research
The content of this article was prepared by Bruno Defilippi, Agronomist, PhD. Paula Robledo, Agronomist, Cecilia Becerra, Agronomist, www.inia.cl and it was revised and republished by Portalfruticola.com
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