by Dr Malcolm Dodd of Cold Storage Solutions & Samadis
South Africa: Post harvest benefits of removing ethylene from cold storage environment of avocados
The avocado fruit does not ripen whilst still hanging on the tree so there is a picking window for the fruit. The correct maturity can be established by determining the moisture content of the fruit and picking according to the guidelines set out in the SAAGA Picking and Packing guidelines. Fruit maturity plays a very important role in determining fruit quality. Immature product leads to uneven ripening of fruit which makes it unacceptable from a consumer’s point of view. Fruit maturity also plays an important role in the sensitivity of fruit for low storage temperatures during shipping to overseas markets. Correct moisture determination is also extremely important for the determination of the correct temperature management system.
The main technology available to manage this high perishability is refrigeration and its application in the cold chain. The challenge with Avocado is the fact that chilling damage to the fruit tissues occurs at a relatively high (<8˚C) temperatures which do not slow down the respiration rate as much as lower temperatures would. Lower temperatures slow down the respiration rate of the fruit and thus help delay the onset of the climacteric ripening process. This is characterised by a preceding surge in the natural production of ethylene. Once this gas is introduced into the storage environment it stimulates the spontaneous ripening in fruit that have not reached the climacteric rise in respiration rate. This then creates the production of additional amounts of ethylene by other fruits. Thus an auto-catalytic spiral is started and unless this is controlled, the fruit will all be over-ripe by the time they reach the market. Therefore it is very important that the product is pre-cooled to the correct set point (2-4˚C for ripe fruit or 5-13˚C for green fruit) as soon as possible after harvest.
Additionally the relative humidity of the storage air throughout the cold chain should be maintained between 90 and 95%. Once the fruit has been cooled to the desired set point they should be kept at that temperature and relative humidity for the remainder of their post harvest life up until the point of sale.
Any unseen injury to the fruit and below the surface fungal infection will spontaneously cause the production of ethylene. As explained, this ethylene accelerates the ripening within that particular fruit as well as additionally stimulating the ripening in fruits that had not yet reached that stage of physiological maturity. It makes sense then to try and reduce the production of further ethylene. The best way to achieve this, over and above managing the temperatures and disease control, is to manage the ethylene levels around the fruit. This can be achieved by an organic chemical process called “scrubbing”. This process occurs spontaneously when air containing ethylene is passed over specialised clay particles that adsorb the ethylene onto them leaving an organically disposable residue. Thus the ethylene is removed from the storage air and is unable to stimulate the production of additional ethylene from pre-climacteric fruit by pushing them into the climacteric phase of respiration and thus ripening. Making use of ethylene scrubbers in the cold stores and shipping containers used to store and transport avocados will remove any ethylene that is in the storage air and the negative impact that this has on fruit quality. In addition the scrubbers will remove from the air the volatile gaseous acetylaldehydes that give the trigger signal to the growth of fungal spores that are circulating in the storage air. Hence fungal growth is limited.
The benefits of managing and controlling ethylene to maintain avocado quality is evidenced by the success that has been achieved with the ethylene blocker chemical 1 MCP. Fruit firmness is maintained and disorders reduced with the use of this product. However, one of the down sides to the use of an ethylene blocker is that it is often times too effective, causing the fruit to remain “sleepy” and not ripen up properly or causes it to ripen unevenly. This is not the case when simple ethylene scrubbing techniques are used because once the fruit is removed from an ethylene scrubbed/cleaned environment the natural ripening processes recommence spontaneously.
The prime initial benefit of this natural ethylene scrubbing as a process is that the ripening rate of the fruit is slowed down and the likelihood of fungal infection removed. The result will be avocado in the market place that look great, is uniformly ripe and tastes good.
Ethylene scrubbing is a relatively low cost and economic process that is organic and safe. A small investment in ethylene scrubbing technology in the cold store and also in shipping containers will have a high return on investment as the fruit will all be marketable at the highest value possible.
It is of interest to note that South Africa’s export rivals from South America are very active in the use of natural ethylene scrubbing especially in the avocado, mango and plum industries. The main shipping lines out of Latin America using CA containers to export avocado fit Bioconservacion ethylene scrubbing filters as standard practice .
For more information:
Dax Rowlands
Samapro Trading
Samadis (Pty) Ltd
Tel: 0027 82 794 5772
Email: [email protected]