Iran ranks seventh in the world for kiwi production, with over 3200 MT produced in 2012. Kiwifruit can be stored for over 6 months under appropriate conditions, but fruit softening and fruit rots (Botrytis cinerea) can cause severe losses during cold storage, transit, distribution and retail. Kiwifruit is a climacteric fruit very sensitive even at very low concentrations of ethylene (5-10 ppb). In fact, the capacity of Botrytis cinerea to produce ethylene has been linked to the pathogenesis of the mould. As a matter of fact, it is well know in the industry that ethylene removal is a must in kiwifruit storage either alone or in combination with other postharvest technologies. In this article, we discuss different aspects related to the application and benefits of the technology of ethylene removal by chemisorption in kiwifruit.
Kiwifruit [Actinidia deliciosa] is a member of the family Actinidiaceae originating from south-east Asia. It was introduced from China into New Zealand in 1904 and, from there, its cultivation rapidly expanded to other continents. It is a relatively recent commercially-grown fruit, with significant quantities being produced only after the 1980’s. Iran ranks seventh in the world for kiwi production, with over 3200 MT produced in 2012 (FAO, 2012).
Kiwifruit can be stored for over 6 months under appropriate conditions (Mitchell et al., 1990; Feng et al., 2006). However, fruit softening and fruit rots can cause severe losses during cold storage, transit, distribution and retail. The most important cause of decay in kiwifruit after harvesting is grey mould caused by Botrytis cinerea Micheli ex. Pers (Spadaro et al., 2010).
Kiwifruit is a climacteric fruit. Ethylene has been known for many years to initiate the ripening of climacteric fruits whether the ethylene is derived endogenously or from exogenous sources. Kiwifruit produces very low amounts of ethylene when stored at the right conditions of maturity, temperature and atmosphere (< 0.1 ml/kg*h), but it is very sensitive to the presence of ethylene even at very low concentrations. As little as 5-10 ppb of ethylene will induce fruit softening (Wills et al., 2001). Ethylene exposure is also involved in the development of a number of physiological disorders such as hard-core, white-inclusions and pericarp translucency.
It is also well-know that decay-causing infections may not develop until the tissue ripens or ages because the fruit becomes less resistant to the invading pathogen. Ethylene is not only produced by higher plants. Other microorganisms such as fungi or bacteria are able to produce ethylene during pathogenesis. The capacity of Botrytis cinerea to produce ethylene has been reported (Qadir et al., 1996). According to these authors, the fact that high ethylene production occurs with such small amounts of mycelia suggests a possible role for fungal produced ethylene in Botrytis cinerea pathogenesis of sensitive fruit such as kiwifruit.
Niklis et al. (1997) reported the following evidence: in a commercial kiwifruit coldstore where a number of Botrytis-rotted fruit are distributed within the stored fruits, besides the fruits that will be infected around these nests, the infected kiwifruit will produce enough ethylene to cause early ripening of the healthy fruit in the same storage room. Botrytis infestation in several storage buns or boxes may induce severe economic losses of the total stored fruit in one big storage room.
According to Wills et al. (2001), intervention to limit the ethylene action should be warranted for the most sensitive species such as kiwifruit.
Ethylene Removal in Kiwi Storage
It is widely accepted that ethylene removal is an essential measure in proper storage of kiwifruit. This gas can be removed from the cold-stores and during shipments by different techniques: mainly, catalytic converters and chemisorption. The use of controlled atmosphere and also Modified Atmosphere Packaging (MAP) packaging together with ethylene removal has been proven to be a very effective combination to maximise postharvest life (Mitchell et al., 1990; Kader, 1997). Even the application of the plant growth regulator 1-Metylcyclorpopene (1-MCP) has been proven to be more effective when combined with the ethylene removal (Bertolini, 2014)
In a coldstore, the fruit is releases ethylene steadily. To maintain ethylene levels below the established threshold, the media must be capable of eliminating ethylene gas faster than the fruit emission ratio. This is especially important in kiwi storage, because of the high sensitivity of the fruit at very low levels of the gas. The media must also have an ethylene absorption capacity high enough to protect the fruit during the whole period of conservation (Wills et al., 2004).
Aside from the technical properties of the media, the equipment design needs to be adequate for the system to be effective. In coldstores, the system’s ventilation should force the air through the media at a flow rate equivalent to the volume of the cold store so that the air is completely renewed at least once every hour. For fruit transport and distribution, other systems such as filters installed in the rerurn grid or individual sachets placed in each fruit box can be used. The media never comes into contact with the fruit, leaves no residue, and is compatible with organic produce.
Tables 1 and 2 show the efficacy of the commercial media Bi-OnÒ (Bioconservacion SA, Spain) in extending the life of kiwifruit (Oyarzun et al., 2004). According to the authors, the Bi-OnÒ sachets had an important effect on the ripening delay of kiwi fruits during the cold storage at 0ºC (softening and ºBrix content). Bi-OnÒ is an extruded media made of natural clays and potassium permanganate with an ethylene absorption capacity of 4.5 ml C2H4/kg (as tested following the method B-3150-14 – registered at the Intellectual Property Register as Scientific Work, Spain-). Bi-OnÒ media is also being used successfully in Iranian commercial colds stores since more than 10 years.
It is generally accepted that ethylene removal is still an essential intervention in kiwifruit storage to maximize postharvest life and effectively reduce losses. Ethylene removal by chemisorption is a technology that can be easily applied in the cold storage, transit, distribution and retail. It is also effective when used in combination with other technologies such as Controlled Atmospheres, MAP packaging and 1-MCP.
Effective chemisorption systems must be capable of eliminating ethylene gas faster than the fruit emission ratio in order to maintain ethylene under the safe levels of 5-10 ppb during the whole period.