The many-sided role of carbon dioxide in postharvest practice

Relevance to practice

Elevated CO₂, often together with reduced O₂, is applied at various stages of the fresh supply chain. It is used for long-term storage (e.g. apples, pears, berries and kiwifruits), temporary storage or transport (e.g. strawberries, cherries and bananas) and modified atmosphere packaging of fresh-cut vegetables (e.g. lettuce). We experience many beneficial but unfortunately sometimes harmful effects of CO₂ in practical situations. A better understanding of the CO₂ mode of action helps to optimize its application for a better product quality.

Internal or external CO₂

High CO₂ can inhibit physiological processes, such as respiration and ethylene production. The concentrations of CO₂ to achieve such inhibition are much higher than the approximately 0.04 % in the ambient air. As CO₂ is a product of respiration, the CO₂ exchange between the fresh product and its environment is important to take into account. The CO₂ concentration inside bulky organs such as fruits and tubers, where the diffusion of gas is impeded, is often several % higher than the surrounding atmosphere.

This is especially the case at higher temperatures and advanced ripeness stages. Even when CO₂ is kept low in the surrounding, CO₂ can still be active due to its internal concentration. This can give a positive result, such as a longer shelf life. However, CO₂ injury can occur in certain situations, even when ventilation or other measures effectively remove CO₂ from the environment.

CO₂ in aqueous solution

In postharvest we usually consider CO₂ in its gaseous phase. It is good to realize that CO₂ is soluble in water. This is very relevant for horticultural products with water as the main component. As with other gases, the solubility of CO₂ in water increases as the temperature drops. Again relevant for horticultural products, with regard to cold storage temperatures.

When CO₂ dissolves in water, some of it is hydrated to form carbonic acid (H₂CO₃). Due to the dissociation of carbonic acid to bicarbonate (HCO₃^-) and hydrogen ion (H+), pH of living cells could drop when exposed to high CO₂. The intracellular pH values are normally regulated within narrow limits, but although not proven, acid stress may occur in fruits with high CO₂ levels.

The reaction of CO₂ with water can also be relevant on the exterior of cold products. When there is free moisture on the skin (e.g. condensation droplets) in an environment with high CO₂, the carbonic acid can lead to an etched appearance of the fruit skin. Although very exceptional, this is a possibility to take into account when assessing damages of the fruit skin. This damage can be distinguished from a physiological damage. An important difference is that it is less dependent on variety, maturity or fruit size.

Inhibition or stimulation of ethylene production

CO₂ is mostly associated with inhibition of ethylene production of fruits. But CO₂ can also promote ethylene production rate, depending on the commodity and the CO₂ concentration. It is known that CO₂ can stimulate ethylene production by activation of the enzyme ACC oxidase. This stimulation has usually a short-term effect. At a relatively high (internal) CO₂ concentration above 1% and/or prolonged exposure time, ethylene production is mostly reduced. This reduction is achieved by a different way of CO₂ action.

A special effect of CO₂: stomata in tulip bulbs

Sometimes remarkable CO₂ effects are found that deserve an explanation. This certainly applies to the fact that CO₂ can stimulate water loss of stored tulip bulbs. The actual mechanism of CO₂ action on this water loss is unknown. But in one of the earlier studies within our advisory and research programme, we demonstrated that the most likely explanation involves the stomata which are present on the tulip bulb scales. While stomata in leaves tend to close when CO₂ is increased (over low ranges), the situation for tulip bulbs may be quite different. We used scanning electron microscopy to study the stomata. It appeared that the stomata were blocked by a stomatal wax plug under normal gas conditions. When the bulbs were exposed to high CO₂, the wax plug disappeared and the water loss increased. We also found a similar effect after exposure to ethylene and low O₂.

Inhibition of mould growth

Harvested horticultural products may benefit from increased CO₂ concentrations by suppressing mold growth. The minimum effective concentration to suppress Botrytis rot is 10% or even much higher. Of course, physiological injury to the products due to excessive CO₂ should be prevented. When this happens, it can in turn even cause an increased incidence of decay.

Astringency in persimmon

Another special practice use of high CO₂ is the de-astringency treatment. Some persimmon cultivars produce astringent fruit, caused by soluble tannins, and cannot be consumed as such. These fruit need a postharvest treatment to remove astringency by changing tannins into the insoluble form. High CO₂ treatment (above 80%) is an effective method for astringency removal. The increase of acetaldehyde due to CO₂-treatment provokes the polymerization of soluble tannin, which becomes insoluble. The fruits lose the astringent taste thanks to these high CO₂ levels.

Conclusion

In the supply chain of fresh horticultural products, we strive for an optimal CO₂ concentration. To optimize the use of CO₂ we have to take into account the physiology of fresh products, the storage technique and the physical and chemical properties of CO₂. In our consultancy work, we apply scientific knowledge to optimize CO₂ conditions for a better product quality.