Dealing with ripening and ethylene

The application of low oxygen (O2) and high carbon dioxide (CO2) is used for various types of fruit and vegetables in controlled atmosphere (CA) storage and modified atmosphere (MA) packaging in order to extend shelf life and maintain quality. Low O2 and high CO2 can reduce ethylene production and sensitivity. O2 is a substrate for the production of ethylene, so a lower O2 content leads to less ethylene production. High CO2 also prevents ethylene production, although this effect is often smaller.

The prevention or inhibition of ethylene activity can be achieved by using 1-methylcyclopropene (1-MCP). This volatile organic compound works by blocking ethylene receptors in the fruit. As a result, it prevents the ripening of ethylene-sensitive fruit. 1-MCP is registered for field use in many countries, and is widely used commercially as a gas application mainly on apples and pears. It is also very effective on products such as banana and plum. With the correct use of 1-MCP, various quality benefits can be achieved, such as better retention of firmness during storage and in the supermarket, better retention of basic color and less chance of storage defects. Ethylene blockers are sometimes also applied before harvest. Sometimes these fruits receive an ethylene treatment upon arrival at the export destination to ripen evenly. For flowers, STS (silver thiosulphate) is used as an ethylene blocker. This is absorbed by the flowers through the water.

Fruits can be divided into two groups based on the ripening mechanism: climacteric and non-climacteric fruits.

Climacteric fruits continue to ripen after harvest. They show a peak in ethylene production during maturation, which is often accompanied by a peak in respiration. The sharp increase in climacteric ethylene production early in maturation is the beginning of change in color, aroma, texture, taste and other attributes. Examples of climacteric fruits are apple, avocado, banana, pear and tomato.

Ripening of non-climacteric fruits is often considered an ethylene-independent process. In non-climacteric fruits, respiration does not show a major change upon ripening and ethylene production remains at a low level. Examples are blueberries, cherry, grape and strawberry.

In climacteric fruits, ethylene can prevent or stimulate its own production, depending on the stage of development of the fruit.

In the first phase there is a low base production of ethylene. This phase is associated with terms such as preclimacteric phase, immature, negative feedback regulation and system 1 ethylene.

In the next stage, maturation, there is a high level of ethylene production. This large amount of ethylene is caused by self-stimulating ethylene production. This means that the presence of ethylene stimulates more ethylene production, a kind of chain reaction. This phase is associated with terms such as climacteric phase, maturation, positive feedback regulation and system 2 ethylene.

When climacteric fruit has reached a certain level of maturity, its ethylene production increases and the fruit begins to ripen. The start of the climacteric phase can be accelerated by harvesting the fruit (as with apples). Sometimes the fruit needs a cold period (storage) before the climacteric phase starts (for example in some pear varieties). For fruits such as avocado and banana, an external application of ethylene is used postharvest to initiate ripening or to achieve more uniform ripening in the batch