Cut flowers, usually refers to the flowers with stems and leaves, which are cut from flower plants and used in flower arrangements, flower baskets, bouquets, etc. Due to the biological characteristics of cut flowers, they are liable to get aging and decay during transportation, resulting in loss of commodity value before selling to the market. Therefore, the preservation of the packaging is extremely important.
Flower Aging Mechanism
When the flowers are cut from the plants, their living activity continues. Due to the loss of water and nutrient supplies, they will gradually age and fade. The root reasons for this are water imbalance and respiration of fresh cut flowers.
The imbalance of water refers to the fact that the water absorption of the stems and leaves after cutting from the plants is lower than the transpiration, so the balance is broken and the cut flowers would begin to fade due to water loss. Obviously, excessive water transpiration rate is the direct cause of water imbalance for cut flowers, which is closely related to the stomatal opening degree, air relative humidity and light conditions. The stomata are the gateway for gas exchange between plants and the external environment. They are also the channel for water transpiration, which are mainly distributed on the epidermis of leaves and stems of the plants. Different types of cut flowers have different degrees of water imbalance due to the distribution of stomata and the mechanism of closure regulation. Since the water vapor content in the fresh cut flower cells is close to saturation, so the relative humidity can be regarded as 99%. When the cut flowers are placed in a low humidity environment, owing to the large humidity difference, water transpiration of the cut flowers would be accelerated. Meanwhile, light is also a powerful catalyst for accelerating the transpiration of cut flowers. Strong light can not only promote the stomatal opening of cut flowers, but also increase the temperature of cut flowers and air, increasing the pressure difference of water vapor between inside and outside of the leaves and resulting in more serious water loss of the cut flowers. In addition to the effects of water transpiration, when water transportation and absorption are hindered, it may also affect the water balance of cut flowers. Generally, the blockage in the part of the wooden conduit caused by the growth of microorganisms such as bacteria and mold and the blockage of the incision and the physiological blockage caused by the secreted substances of the injured cells are the main reasons for the blockage of water absorption.
Respiration is a chemical process in which plants absorb O2 to oxidize and decompose the organic matter in cells to produce energy and generate CO2. When the flowers are cut, the respiration continues, converting the stored nutrients into energy, but because of the lack of photosynthesis, the nutrients would not be replenished and the cut flowers would gradually age.
In addition to the above-mentioned direct influences, the water imbalance and respiration of fresh cut flowers may also cause the increase of aging acid, ethylene and other senescence regulating hormones, and the damage of flower cell membrane mechanism. All those factors may induce and promote the decay of cut flowers.
Modern Packaging Technical Requirements for Fresh Cut Flowers
Based on the above analysis, the packaging technology of fresh cut flowers should not be limited to the traditional packaging formats, which integrate fresh-keeping technologies such as fresh flower preservation, temperature control technology and humidity control technology. In general, it can be studied from three aspects: (1) High water vapor barrier technique. Using packaging materials with high barrier to water vapor to create a high-humidity environment for transportation and storage of fresh cut flowers and reduce the water loss caused by excessive water vapor pressure difference. (2) Modified atmosphere packaging technique. Using modern packaging technology to create a low O2 and high CO2 air atmosphere for fresh cut flowers and reduce the frequency of respiration. (3) Chemical preservation technique. Using chemical means in the packaging technology to regulate the growth state of the plant and inhibit the production rate of senescence regulating hormone e.g. ethylene.
High Water Vapor Barrier Technique
The water vapor barrier property refers to the ability of water vapor to permeate from the higher concentration side to the lower concentration side of the material. The whole permeation process can be divided into several steps i.e. adsorption, dissolution, diffusion and desorption. The water molecules on the higher concentration side are adsorbed on the surface of the material, dissolved into the material and diffused inside and then desorbed from the lower concentration side. The duration of this process depends on the rate of dissolution and diffusion of the water vapor in the materials. The slower the rate is, the higher the barrier to water vapor is. Since the dissolution and permeation of water vapor are different in distinct materials, it is particularly important to select packaging materials with suitable moisture barrier properties.
We tested the water vapor transmission rate of several common packaging materials with Labthink’s water vapor transmission rate tester and obtained the following results.
It can be seen from the test results that LDPE has the highest water vapor transmission rate while the paper-aluminum-plastic composite material has the lowest water vapor transmission rate. The other two materials have little difference in moisture permeability. Considering the highest cost of paper-aluminum-plastic composite material among all those materials, HDPE and BOPP are more suitable moisture-resistant packaging materials for fresh cut flowers. In practical applications, the plastic material can be used as an inner layer of the packaging for cut flowers, or as an interlayer between the inner and outer layer of the corrugated cardboard, so that the fresh-keeping effect will be greatly improved.
Modified Atmosphere Packaging Technique
According to the above analysis, the respiration will accelerate the decay process of fresh cut flowers and it is necessary to take certain measures to inhibit the respiration of fresh cut flowers. Modified atmosphere (MA) technique is an important branch of modern packaging technology, which uses a variety of methods to create a high CO2 and low O2 environment in the package, thereby reducing the respiration of the cut flower. The most-commonly used method is the natural oxygen reduction method, which relies on the respiration of cut flowers, through which CO2 content will be raised while the O2 content will be decreased. This method is simplified in process and low in cost, but it requires film with unique permeation effect. On one hand, the film should have high CO2 barrier property to maintain a high level of CO2 surrounding the cut flower. On the other hand, it should have proper O2 permeability to avoid the influence on the life of flower caused by extremely low concentration of O2. It is known in one literature that, a polypropylene FG film was developed in Japan, which has significant performance on moisturizing and barrier property. However, it should be noted that different cut flowers have different respiratory rates due to differences in their biological characteristics. How to achieve dynamic balance between modified atmosphere and barrier property is still the focus of future research.
Chemical Preservation Technique
Chemical preservation uses a variety of chemical preservatives to maintain the life of cut flowers, which can be generally classified into three types: 1, energy supply type, such as sucrose, fructose or glucose, providing nutrient source for cut flowers, regulating water balance and maintaining normal cell permeation, which is conducive to the quality improvement of cut flowers 2. Water balance improvement type such as adding a bactericide e.g. hydroxyquinoline sulfate to the organic acid which can lower the pH value of the solution can effectively inhibit the blockage of the water conduit caused by bacterial growth and promote the water absorption of flowering branch. The addition of inorganic salts such as K+, NiSO4, CoCl2 can increase the turgor of the petal cells, which is conducive to the maintenance of the water balance of the cut flowers. 3. Mature hormone-inhibiting type, mainly ethylene inhibitors and growth regulators, such as Ag+, STS, AOA, ethanol, and cytokinin, which have a significant effect on slowing down the senescence process of cut flowers.
In practical operations, it is recommended to effectively combine the chemical preservation technique with packaging fresh-keeping technique to improve the comprehensive preservation effect of cut flowers. The bactericide, the ethylene inhibitor may be applied to the inner layer of the corrugated cardboard or the inner layer of plastic packaging. Or the bactericide and ethylene inhibitor may be stirred into the plastic raw material to produce plastic bag as the inner packaging of fresh cut flower to improve the preservation effect.
Fresh cut flowers are the most delicate and hardest to store in cash crops. How to reduce the loss of cut flowers during storage and transportation is an important issue for fresh-keeping of cut flowers. This article provides some tips on the fresh-keeping of fresh cut flowers from the perspectives of modern packaging technology, which may be helpful to relative practitioners in this industry.