1. Why Cool Flowers?
2. Cooling of Flowers
3. Optimisation of Coolchain
4. Reference Sources
5. Acknowledgements

Rapid cooling of packaged cut flowers and foliage reduces the time they would otherwise endure higher temperatures and therefore helps to prolong quality and vase life.

Freshly cut flowers and foliage are highly perishable and deteriorate quickly when exposed to unfavourable environmental conditions such as adverse temperatures. Any technology which ensures that cut flowers reach the required low temperatures as soon as possible, and also maintains these optimal temperatures, is of considerable benefit to people involved in their production and sale. Rapid cooling is therefore the vital first step in the coolchain of cut flowers.

Why Cool Flowers?

The temperature of flowers and foliage at harvest is normally close to that of the ambient air. At this temperature, respiration activity is very high, and storage/vase life will be very short. Therefore it is always best to harvest flowers during the cooler parts of the day, either early morning or late afternoon. As a general rule, the faster field heat is removed from the flowers, the longer will be their storage potential.

Respiration:
Temperature directly affects the respiration rates of cut flowers and foliage. Respiration is a complex process involving many enzymatic reactions. The rates of these reactions, within the normal physiological temperature range, increases exponentially with increase in temperature. In fact, between 0 and 20°C, respiratory activity of carnations can increase 25 fold (Reid & Kofranek, 1980). Below or above the limits of this range (which will be different depending on particular flower crop), activity falls off due to a decline in enzyme activity.

High respiratory activity not only generates further heat around the product, but also uses up stored reserves within the flowers and foliage. This is particularly important for certain Protea and Leucadendron species that develop leaf blackening. This is thought to be a result of rapid depletion of stored carbohydrates from the leaves due to the high respiratory demand of the inflorescences at room temperature (see Proteaceae - Flower and Foliage Production ). Therefore, lowering product temperatures will decrease respiratory activity, which in turn will slow down the use of stored reserves and the generation of heat.

Ethylene:
Ethylene gas is a pollutant generated naturally by all vegetation, especially that which is cut and/or decaying, ripening fruit and vegetables, and some senescing flowers. It is also produced from coal gas, petroleum gas, and in exhaust fumes of internal combustion engines, such as those used in some forklifts, heating systems, etc.

Ethylene reduces the longevity of some flowers and foliage by causing rapid wilting of petals (e.g. carnations), shedding or shattering of petals (e.g. snapdragons, delphiniums), or other changes to petal tissues, such as loss or change of colour (e.g. orchids). Therefore, flowers which are sensitive to ethylene should not be held in the same coolstore as ethylene-producing fruit, vegetables or foliage, or be exposed to exhaust fumes (see Table 1).

Low temperatures can reduce both the rate of ethylene production and the sensitivity of flowers to it. For instance, carnations stored at 0°C would need to be exposed to higher ethylene concentrations for a longer duration before petal in-rolling resulted, whereas a shorter exposure and/or a lower concentration of ethylene at 30°C may be sufficient to cause damage.

Moisture loss:
Flowers can be considered essentially as fancy packages of water. The water has been costly to put into the flowers during production, and any water loss resulting in wilting directly affects returns to the grower. The difference between the humidity of the produce and the humidity of the air (the vapour pressure difference or deficit - VPD) is the driving force behind moisture loss. Even if saturated cool air is used to cool flowers, as long as the flowers remain warmer than the air, they will continue to lose water. Therefore it is important to cool flowers and foliage quickly to minimise the period when a VPD exists.

Both high relative humidity (RH) and low temperature are important in reducing moisture loss from foliage. At 0°C and 80% RH, moisture loss is twice that at 0°C and 90% RH. At identical RH, moisture loss can be halved through lowering the temperature by 10°C. Maintenance of optimum flower temperature (see Table 1) is therefore one way to reduce water loss and prevent wilting.

Airflow over the product is also important in determining the amount of moisture loss, especially for fragile products like flowers. Of course, a high airflow is required to ensure rapid removal of heat from the foliage, but its effects on moisture loss must also be considered. Packaging such as plastic sleeves that limit the loss of moisture from the flowers, will reduce wilting, but may in turn decrease cooling rates and increase likelihood of rots.

When moist air is cooled, a point is reached at which the vapour pressure of water reaches the maximum for that temperature, and the water will then condense onto the surfaces of the produce or packaging. This temperature is called the dewpoint temperature. Condensation can lead to development of rots and accelerated warming of the flowers, and weakening of cardboard packaging. Excessive fluctuations in coolroom temperature around the dewpoint temperature will cause condensation to occur and should therefore be avoided.

Physical damage:
Bruises and wounds to cut flowers and foliage often cause increased ethylene production, which may increase respiration and heat production, and increase moisture loss. Prompt cooling to low temperatures slows these processes, reducing the effects of physical damage. The best options of course are to ensure that systems are in place throughout the handling chain to reduce the likelihood of damage, and to ensure that no damaged product is packed.

Harvesting during the hotter times of the day can lead to many quality problems, especially wilting of the flowers, accelerated opening and senescence, and damage to the buds, e.g. Oriental lilies can develop brown depressions on the top surface of the buds during storage if harvested during hot weather and placed immediately into the coolstore.

Chilling or freezing damage:
The optimum storage temperature for most flowers lies between 0° and 7°C, depending on the particular crop. The air temperatures in the coolstore should be checked to ensure the thermostat has been correctly set, as temperatures below -1 to -0.5°C may freeze or discolour the flowers, or may inhibit later bud opening. Many semi-tropical and tropical flowers, and some temperate crops, will not require cooling to these low temperatures, and may in fact store better at 10-12°C, making it essential that growers identify the optimum storage temperature for each of their crops (Table 1). Holding these flowers at too low temperatures may cause deterioration through chilling injury (e.g. heliconias, orchids, nerines). This may result in poor opening of the buds after storage, and rapid loss of quality, such as discolouration of the flower parts, watersoaking of the petals, and flower collapse.

Table 1: Optimum storage temperatures (°C) of a range of common flower crops grown in NZ, and an indication of whether they are sensitive to ethylene.

Cooling of Flowers

Cooling rate will be determined by the mass of flowers and other produce in the store (the load), the cooling capacity of the particular store and the ventilation around the flowers. Therefore, if the flowers are packed in boxes or bunched in buckets, the movement of cool air around them will be lowered, and the time required for cooling will often be greatly increased. Also, cooling is often uneven due to improper orientation of bunches or packaging throughout the consignment. Furthermore, it may take several days to cool a pallet of packed boxes of warm flowers in a conventional coolstore, and before they have been adequately cooled, the heat generated will have caused considerable deterioration. In some cases, the cooling capacity of the coolstore may not be sufficient to cool the flowers to the desired temperature.

Therefore, the most important requirement for a successful and efficient cooling operation for flowers is unrestricted passage for the air past the flowers. Sufficient air must pass from the coolstore through the box of flowers via the ventilation holes. It is essential that holes are not blocked internally (by foam, tissue or plastic wrappers) or externally (by tape or incorrect alignment of lids or adjoining boxes). Packing that inhibits uniform air movement through the box such as sheets of tissue between layers of flowers should be eliminated.

Although flowers may be cooled rapidly, they also heat up quickly during packing and will continue to generate heat during transport unless the coolchain is maintained. Any temperature rise will of course be determined by the ambient temperatures encountered during handling and transportation (e.g. on the tarmac in Tokyo), and the adequacy of the coolchain (package and container insulation, presence of icepacks, container refrigeration, etc.).

Forced-air cooling:
Forced-air cooling is a method of rapidly cooling packed boxes of produce to ensure they leave the grower’s packhouse and/or the exporter’s depot at low temperatures. The procedure is also a convenient means of quickly removing the field heat from produce that is to be stored. While unpacked flowers cool quite rapidly due to their large surface area to volume ratio (generally less than 1 hour), forced air cooling can greatly accelerate cooling rates when flowers are packed tightly into boxes. Packaged flowers can take about 1 day to cool just sitting in a coolstore, but forced air cooling can reduce this to about 1 hour. As the capital expense is not that great (i.e. a fan and piece of canvas), more use should be made of forced air coolers for cut flower consignments being exported from New Zealand.

This method of cooling operates by a fan drawing refrigerated air through a packed box of flowers, increasing the air movement over the warm flowers, thereby quickly lowering flower temperatures (Figure 1). Forced-air cooling is probably not necessary for the volumes of flowers produced by individual growers in New Zealand, but may be useful for larger exporters or freight forwarders who are moving large volumes of product.

Figure 1: Small forced-air cooler for flowers

Refrigeration:
The performance of a cooling operation is generally expressed in half-cooling times. This is the time taken for the flowers to cool down to half the difference between the initial flower temperature and that of the air in the coolroom. Figure 2 shows a typical cooling curve with a half-time of 20 minutes.

The rate of cooling becomes very slow as the temperature of the flowers approaches the temperature of the cooling air. Consequently, cooling is generally considered complete after three half-cooling times (i.e. at seven-eighths cool, or after 60 minutes in this example).

From the slope of the graph in Figure 2, we can determine that the average cooling rate during the first half-cooling time is twice that of the second half-time, and in the second half-time it is twice that of the third. As half the total heat is removed during the first half-time, the refrigeration capacity must be adequate to maintain cold temperatures during this initial period.

Most existing coolstores will have sufficient refrigeration capacity to handle small loads, but larger operations should be either checked for capacity or designed for a specific operation by a consultant engineer.

Insufficient refrigeration capacity will increase the cooling time because the air temperature will rise, which in turn will warm up other flowers in the coolroom. Also, a lower relative humidity may result, which will tend to increase moisture loss. To reduce drying, the refrigeration plant should be designed to give as high a relative humidity as practicable (at least 85% RH). "Air wash" coolstores, where the air is cooled and saturated by chilled water, giving close to 96% RH, are well suited for cooling and holding of cut flowers.

Measurements:
It is advisable to check your coolroom for efficiency by regularly monitoring the flower temperatures. This can be carried out by using a thermometer or temperature probe to measure temperatures in several cartons throughout the coolroom.

Figure 2: Hypothetical cooling curve for cut flowers, showing seven-eighths cooling in 1 hour. Coolroom air temperature is 0°C

Optimisation of Coolchain

• Growers should harvest flowers from the field or glasshouse during the coolest part of the day (preferably morning), and transport them back to the packhouse within 1 hour.
• If possible flowers should be immediately graded, bunched and sleeved, placed in buckets of water or preservative, and transferred to the coolstore for several hours or overnight.
• If immediate grading is not possible, flowers are best held in the coolstore, or if not available, the coolest part of the packhouse, until they can be dealt with.
• Check coolstore performance frequently, as often the coolstore may not be calibrated to cool the product to its optimum temperature, or may not have the capability to cool large volumes of product.
• Ensure that flowers are not bunched too tightly into cartons or buckets during cooling, thereby hindering airflow.
• Flowers for export should then be taken to the freight forwarders that night or the next morning, where they may be repackaged, recooled, and loaded into a container ready for shipping.
• Some exporters demand that the packaging and container contain icepacks and/or insulation materials, so check the requirements with your agent.
• During transit to overseas destinations, flowers may experience a range of temperatures from below freezing to over 40°C, depending on the temperature control in the hold and whether they are left sitting on the tarmac ready for phytosanitary inspections or further transit. Fortunately, these temperature fluctuations are seldom encountered, and recently the number of insurance claims has dropped.
• Because of the possibility of such temperature abuse however, it is important that growers ensure that only top quality flowers are sent.
• It is also important that exporters have the product cooled as rapidly as possible on arrival at the export destination (this is done regularly for example by some NZ exporters with depots in Los Angeles).

Reference Sources

Halevy, A.H. and Mayak, S. 1979. Senescence and postharvest of cut flowers. Horticultural Reviews 1: 204-236.

Halevy, A.H. and Mayak, S. 1981. Senescence and postharvest of cut flowers. Horticultural Reviews 3: 59-143.

Reid, M.S. and Kofranek, A.M., 1980. Postharvest physiology of cut flowers. Chronica Horticulturae, 20(2): 25-27.

Kader, A.A. (Ed.). 1992. Postharvest technology of horticultural crops. Second edition. University of California, Division of Agriculture and Natural Resources, Publication #3311. pp 53-68; 201-209.

Acknowledgements

C. Downs (Crop & Food Research, Levin) and B. McDonald (Industrial Research, Auckland) for preparation of the original Aglink, and N. Lallu (HortResearch, Auckland) and B. McDonald for editing of the revised HortFact.

Prepared for HortNET - June 1998