Vacuum cooling: the Best Postharvest for Cut Flowers
In this article factors that influence the longevity of flowers are described with respect to pre-cooling methods. The article is originally published by www.floriculture.co.ke
Comparison between cooling flowers methodes
Floriculture is an agricultural sector of worldwide importance and of paramount social and economical influence. Roses account to a large percentage of all flowers grown. After flowers are harvested, temperature is the one factor affecting them the most. This is the time to evaluate different cooling methods used in the postharvest of roses, by measuring their effects over floral longevity and other quality variables. The residual effects of passive, forced air and vacuum cooling methods were evaluated, after transport simulation. The test was performed at a flower-exporting farm. It was found that those flowers exposed to vacuum cooling showed the longest longevity while those that took forced air had the lowest.
The main cause of elimination of flowers was the presence of Botrytis (44%) and dormancy (35%). No significant differences in such causes were found among the various cooling treatments; however it was observed that those flowers that went through the passive and forced air cooling methods showed presence of Botrytis much sooner than those exposed to vacuum cooling. Furthermore bent neck in vacuum cooled flowers only were observed after day 12 while in the other treatments that happened within the first five days of the test. With regard to the quantity of stems affected by dehydration, no differences were found among all treatments, which refutes the common belief that vacuum cooling accelerates dehydration of flower stems.
Most flower farms are structured based on management, planning, production, personnel and post harvest divisions. The main objective of each of the above activities is the final obtainment of products of excellent quality. Post harvest is a very important phase in the production process since economic losses due to the reduction in percentage of exportable flowers and claims from the buyers are mostly ascribed to deficiencies in their post harvest.
The main problems related to the quality of flowers during the production phase are inappropriate harvest in length of stalks and opening cut stage, bent stems, mechanical damage and sanitation problems. Those related to post harvest are classification and bunch forming, deterioration, hydration and cold chain.
Fresh cut flowers are still live material
They are metabolically active and therefore subject to the same physiological processes as the plant. However, after being cut they deteriorate faster, under similar environmental conditions.
Thus, longevity of cut flowers is determined by the same factors that affect the growth of plants, such as temperature, humidity, water, light and availability of nutrients.
Transpiration is defined as loss of water from the plant in the form of vapour. Of all the water that a plant absorbs, less than 5% is kept for its growth and even less is bio chemically used. Although traditionally it has been said that transpiration is useful in order to control the temperature of the plant and to transport the nutrients taken by the roots, its usefulness is currently being questioned.
Transpiration rate is affected by relative humidity, temperature and air speed. When water loss out of the stem is higher than absorption, cell turgidity is reduced which leads to wilting and/or what is known as bent neck.
Respiration is the process in which the cell metabolites are oxidized and electrons are transferred through a series of carriers to the O2. H2O and CO2 are formed and the energy, released in several steps, is transduced into ATP.
Upon increasing the respiration rate in the plants, the nutrients’ reserves (photo-assimilated) are used faster and flowers’ lives are shortened. Temperature is a very important factor in plants, since very small changes can pose dramatic effects over their physiological processes. Generally speaking, such processes occur between two and four times faster if temperature is increased by 10ºC and two to four times more slowly if it is reduced by the same amount. Respiration rate affects quality of flowers much more than transpiration.
Among plant hormones, ethylene is the main factor exerting influence over their senescence. Flowers synthesize ethylene, especially before wilting and in most species, this causes senescence and abscission. Further, ethylene is involved in inducing changes in the permeability of cell membranes, including modifications in composition, physical and functional properties: a slight increase in the permeability of membranes is characteristic to those flowers in advanced stages of senescence.
Ethylene and temperature also interact, influencing the quality of flowers. In simple terms, as temperature increases, less ethylene is needed to cause damage. It is worth noting that ethylene levels as low as 0,03 mg L-1 are enough to cause problems in flowers kept at 20ºC (68ºF).
It is therefore obviously important to help keep the flowers at the right temperature (2-5ºC, 36- 41ºF) during their transportation and storage, in order to preclude quality problems caused by undesirable increases in the transpiration, respiration and ethylene release rates.
Transport Considerations
Among the factors compromising the temperature ranges through handling and transportation of plant materials, it was found that air circulation between boxes and inside them is usually deficient. The consequence of the above is a very poor control over temperature. Once harvested, flowers are faced by adverse weather conditions, mainly high temperatures and low relative humidity, which will affect their post harvest quality. Some farms hydrate their flowers inside their greenhouses, while most carry them dry to their post harvest rooms.
Furthermore, some of the farms receive the flowers in the post harvest rooms where temperature averages 19.1ºC (66.4ºF) but most of the farms use some form of precooling. The above implies that the care given to flower post harvest management is not the best possible, at this stage it is advisable to keep cut roses at 0-5ºC (32-41ºF), in order to guarantee length of vase life.
When transporting them to the airport, some of the fleets used are not equipped with cooling systems. Upon arriving into the airlines warehouses at the airport, while x-ray scanned and built into pallets, flowers are kept at least one hour out of the cold rooms. which is not convenient.
Pre-cooling methods used in cut flower post harvest facilities. The most important aspect in preserving the quality of fresh cut flowers is to have them cooled down as soon as possible after harvest and also kept within the right temperature range (2-5ºC/36-41ºF) up to their distribution. The purpose of cooling is to remove the field heat by lowering the temperature at harvest time to the ideal for transportation.
The above benefits the flowers by reducing their respiration and transpiration rates and therefore precluding damages due to dehydration. Also, ethylene release is reduced, delaying maturity during transportation and distribution and increasing vase life. However, the quality of stored flowers will never be as good as that of fresh ones. Some inconveniences of stored flowers are loss of vase life, difficulty in blooming, discoloration of petals, yellowing of foliage, and increased incidence of diseases like Botrytis.
Once flowers are packed, they require a longer period of time for cooling, wherefore the importance of pre-cooling. The high temperatures in the greenhouses and packing areas increase the respiration rates of the flower stems, making obvious the need to take urgent steps to avoid such situations or at least to minimize their impact.
The three pre-cooling methods used in postharvest are Passive air, forced air and vacuum cooling.
- Floral longevity
Differences between treatments are noticeable. The increase in the longevity in vacuum cooling over forced air and passive air is significant statistically, five hours in which flowers are taken out of the cold chain, upon simulating transportation, might affect the longevity of all stems.
- Water Uptake
Therefore, the highest water loss is caused by the forced air method. Vacuum cooling has the lowest.
- Flower opening stages and discoloration of flowers
No differences is found among all treatments in the floral opening stages variable and an average grade of 4.5 is observed at day 15. Neither are differences in colour between treatments found, and in all cases the flowers show a gradual discoloration from red –brown to purple in all stems.
Shrinkage Variables
After evaluation of vase life no significant differences between the shrinkage variables is found. It is necessary to stress and emphasize the differences that are noticeable during the process of the evaluation.
Botrytis
Flowers cooled by forced air surpass the 50% level of eliminated stems due to Botrytis around day 6. In the case of passive air, that happen around day 8 while in vacuum cooling it takes between 10 and 11 days allows the longest vase life, in such variable.
Bent neck
All three methods reached the 50% shrinkage at day 12 while 75% of stems were eliminated between days 13 and 14. It is necessary to note however that those flowers exposed to vacuum cooling only showed their first bent neck after day 12, which increases their ornamental value. Dehydration Flowers reach 50% and 75% shrinkage levels between days 13 and 14. Even though it is expected to find earlier symptoms of dehydration on the vacuum cooled ones.
CONCLUSION
- Vacuum cooling resulted in the longest longevity of all tested floral stems.
- In spite of what was expected, vacuum cooled flowers did not show any more symptoms of dehydration than the other cooling methods.
- Forced air statistically showed the least floral longevity and higher water uptake when compared to vacuum cooling. It also showed the earliest bent neck and dehydration symptoms and the highest incidence of Botrytis.
- No statistic differences were found between vacuum and passive cooling in any of the evaluated variables, which is a good argument when choosing between them.
