How rapid pre-cooling can protect quality and extend vase life

For the global cut-flower industry, temperature is one of the most important factors determining how much usable vase life remains when flowers finally reach the consumer. A flower harvested in Kenya, Ethiopia or the Netherlands may pass through a packing facility, refrigerated transport, an airport, an international flight, an auction or distribution centre, a wholesaler and finally a florist or supermarket before reaching the customer. Throughout that journey, the biological clock continues to run. This is why the speed at which field heat is removed immediately after harvest can be just as important as the temperature at which flowers are subsequently stored, Cold Change shares.

Why rapid cooling matters
Cut flowers remain living plant material after harvesting. They continue to respire and lose moisture. Higher temperatures accelerate respiration and can increase water loss, carbohydrate consumption, microbial development and the rate at which flowers progress towards senescence. Cold storage slows these processes considerably. However, there is an important distinction between placing warm flowers into a cold room and rapidly pre-cooling those flowers before cold storage. A conventional cold room is primarily designed to maintain already-cooled products at a low temperature. When large quantities of warm flowers are placed inside, removing heat from the center of boxes and pallets can take many hours, and during this period the flowers continue to respire at an elevated rate. Rapid pre-cooling aims to remove this field and packing heat before long-term refrigerated storage and transportation begins.

Vacuum cooling
Vacuum cooling provides a different method of removing heat. Rather than relying primarily on cold air gradually travelling through boxes and around individual flowers, a vacuum cooler reduces the pressure inside a sealed chamber. As pressure falls, the boiling point of water also falls. A very small quantity of moisture within and around the product evaporates at low temperature, and the energy required for this evaporation is taken from the product itself in the form of heat. The result can be extremely rapid and relatively uniform cooling throughout a palletised load. For suitable horticultural products, temperatures can potentially be reduced from typical post-harvest temperatures to cold-chain temperatures in a fraction of the time required by conventional room cooling.

Pre-cooling followed by cold storage
Vacuum cooling should therefore not necessarily be viewed as a replacement for refrigerated storage. The two technologies perform different jobs: vacuum cooling removes heat rapidly, while cold storage maintains the low temperature afterwards. For exporters, the combination can be particularly valuable, since flowers can be rapidly brought towards their required storage temperature and then transferred into the existing cold chain. This reduces the period during which freshly harvested flowers remain warm.

The potential effect on vase life
Research into cut-flower handling has repeatedly demonstrated the importance of rapid temperature reduction after harvest. The exact vase life achieved cannot be expressed as a single number because it depends on cultivar, harvest maturity, growing conditions, hydration, handling, disease pressure, storage duration and transportation conditions. Nevertheless, the difference between poorly cooled flowers and flowers that are rapidly pre-cooled and subsequently maintained within an effective cold chain can be substantial. To illustrate the type of improvement that effective post-harvest temperature management can potentially produce: roses might last approximately 4–7 days without effective rapid cooling, approximately 7–12 days with refrigerated storage alone, and potentially 10–18 or more days with rapid pre-cooling combined with a refrigerated cold chain. Carnations might range from approximately 7–10 days without rapid cooling, to approximately 14–21 days with refrigerated storage, to potentially 18–28 or more days with pre-cooling and cold chain combined. Chrysanthemums follow a similar pattern, moving from approximately 7–14 days, to approximately 14–21 days, to potentially 18–28 or more days. Gerbera might last approximately 4–7 days without rapid cooling, approximately 7–12 days with refrigeration, and potentially 10–18 days with the combined approach. Lilies could range from approximately 5–8 days, to approximately 7–14 days, to potentially 10–18 or more days. Tulips might move from approximately 4–7 days, to approximately 7–10 days, to potentially 10–14 or more days. Alstroemeria could range from approximately 7–10 days without rapid cooling, to approximately 10–14 days with refrigeration, to potentially 14–21 or more days with pre-cooling and cold chain combined.

These figures should be treated as indicative ranges rather than guaranteed results. The important point is the relationship between temperature and remaining post-harvest life: every unnecessary hour spent warm consumes part of the flower’s potential marketable life. For an international exporter, even a relatively small improvement after arrival can have considerable commercial value.

Roses and the African export industry
Roses are particularly important because East Africa has developed into one of the world’s major production regions for export cut flowers. Kenya and Ethiopia supply enormous quantities of roses and other flowers to European and international markets. The logistics involved make post-harvest temperature management especially important, since flowers may be harvested under relatively warm conditions before travelling thousands of kilometres to the final market. Rapidly removing heat close to the point of harvest therefore has the potential to preserve more of the flower’s available post-harvest life for the customer rather than consuming it during the first stages of the supply chain.

The Netherlands
The Netherlands remains at the centre of the international flower trade, both as a producer and as a global distribution hub. Large quantities of roses, tulips, chrysanthemums, lilies, gerberas and other varieties pass through sophisticated Dutch growing, auction, packing and distribution operations. Modern flower logistics already rely heavily on refrigeration. The next question for operators is increasingly not simply how cold the storage room is, but how quickly the product itself reaches the required temperature. A cold room at 2°C does not mean that the centre of a newly arrived pallet of flowers is at 2°C, and that distinction is important.

Cooling pallets rather than just cooling the room
This is one of the fundamental challenges with conventional room cooling. Air surrounding the pallet may reach the required temperature relatively quickly while flowers deep inside packaging remain considerably warmer. Boxes, sleeves and tightly packed pallets can restrict airflow and create temperature gradients throughout the load. Vacuum cooling largely avoids dependence on cold air penetrating from the outside towards the center. Because the pressure reduction occurs throughout the chamber and product load, cooling can be much more uniform. For large commercial flower packhouses processing hundreds or thousands of stems every hour, that uniformity can be particularly useful.

Cooling time
Depending on flower type, packaging, starting temperature, final temperature and equipment configuration, vacuum pre-cooling can potentially reduce a cooling process that might otherwise require several hours to a matter of tens of minutes. This has implications beyond flower quality: a faster cooling cycle means flowers can potentially move more quickly from harvest and packing into refrigerated storage and onward distribution. For high-throughput exporters, this can reduce bottlenecks between harvesting, packing and dispatch.

Energy consumption
Energy efficiency is also becoming increasingly important to growers and exporters. A conventional cooling system has to refrigerate large volumes of air and then transfer that cooling through packaging into the product. Rapid pre-cooling approaches the problem differently by removing heat directly from the product through evaporative cooling under reduced pressure. The precise energy comparison depends heavily on the facility, ambient conditions, refrigeration plant, chamber design, product load and target temperature, so it would be misleading to claim one universal percentage energy saving. However, shortening the period required to remove field heat can significantly change the energy profile of a post-harvest facility, particularly where large volumes of warm product are processed every day. The most useful comparison for growers is therefore energy consumed per pallet or kilogram of flowers brought from harvest temperature to the required cold-chain temperature, rather than simply comparing the electrical rating of individual machines.

Water loss
Because vacuum cooling works through evaporation, moisture loss is an obvious consideration for flowers. Typically only a relatively small percentage of moisture needs to evaporate to produce a substantial temperature reduction. For flowers, appropriate hydration and process control are therefore important. Depending on the product and packaging system, humidification or controlled water application can also be incorporated into the cooling process. The objective is to achieve the required temperature reduction while maintaining flower quality and hydration.

© Cold Change Group

Suitable cut flowers
Vacuum cooling may be considered for a range of commercially important cut flowers and foliage, including roses, chrysanthemums, carnations, gerberas, lilies, tulips, alstroemeria, gypsophila, lisianthus, hydrangea, mixed bouquets and decorative foliage. Different varieties require different cooling protocols, so commercial systems need to allow operators to control pressure, cooling endpoint and cycle parameters for individual products.

Why this matters for exporters
The economic value of rapid cooling becomes particularly clear when flowers are being exported internationally. Consider a flower with a potential vase life of two weeks under ideal conditions: if a significant proportion of that biological life is consumed during harvesting, packing, cooling, transportation and distribution, considerably less remains for the retailer and consumer. Rapid pre-cooling cannot reverse ageing that has already occurred; its value lies in slowing deterioration as early as possible. The objective is therefore not simply to make flowers cold, but to preserve as much of their potential post-harvest life as possible.

Larger commercial operations
Industrial vacuum cooling technology is already established in other areas of fresh produce processing, particularly products where extremely rapid removal of field heat is required. Commercial systems can accommodate anything from individual pallets to large multi-pallet loads. For major flower growers and exporters, large chambers can be integrated directly into packing and dispatch operations so that complete pallet loads are cooled before entering refrigerated storage. Automation can allow different recipes to be stored for individual flower varieties, with the system controlling pressure and cooling cycles automatically.

A changing cold chain
The global flower industry has spent decades improving refrigerated storage and transportation. The next opportunity is increasingly focused on what happens before the flowers enter that cold chain. A refrigerated truck cannot restore vase life already lost while flowers remained warm after harvesting, and neither can a cold room. The earlier respiration and moisture loss are slowed, the greater the proportion of potential flower life that can be preserved for the later stages of the supply chain. For growers in major production regions such as Kenya, Ethiopia and the Netherlands, this makes rapid pre-cooling an increasingly interesting component of modern post-harvest handling. As international supply chains become longer and retailers demand greater consistency and longer shelf life, controlling the first few hours after harvest may ultimately prove just as important as controlling the thousands of kilometres that follow.

For more information:
Sean Roche
Cold Change Group
Whatsapp: +44 7912691713
Tel: +447768795866
www.coldchange.com