Fruits and vegetables are living entities and continue to sustain a number of metabolic processes such as respiration and transpiration, even after harvest. Fruit development involves three different phases: Growth, Maturity and Senescence.
The Ripening process occurs during the maturation phase. Fruits undergo many structural and biochemical changes during ripening such as firmness and color changes involving chlorophyll reduction, increase in carotenoids or anthocyanins, sugars, and biosynthesis of volatile organic compounds (VOCs). It is during the ripening process that fruits develop their characteristic taste and aroma and become fit for human consumption.
Based on the ripening behaviour, fruits are classified as:
• Climacteric: These enter ‘climacteric’ phase after harvest and continue to ripen. The ethylene biosynthesis increases corresponding to respiration pattern. These include apple, banana, mango, plum, sapota, kiwi.
• Non- Climacteric: These fruits do not ripen further once harvested. They produce very little or no ethylene and do not respond to ethylene treatments. Examples are litchi, orange, grapes, strawberry, watermelon.
Ethylene is the ripening hormone and regulates all the changes that occur during ripening. The responses to the endogenously produced or exogenously applied ethylene are numerous. Ethylene stimulates the Ripening of fruit, Pigment synthesis (e.g. anthocyanin), Chlorophyll destruction, increased respiration and yellowing. Most of these responses can prove to be beneficial or detrimental depending on the requirement. For instance, chlorophyll loss or yellowing is highly undesirable in green vegetables, but is useful in degreening of citrus fruits.
Ripening turns into senescence when these biochemical processes start deterioration of fruit leading to aging and death of tissues. The loss of firmness or softening of fruits is a very important quality parameter. The softening is due to cell wall degradation induced from several enzymes that are synergistically activated. Ethylene plays a crucial role in regulating these genes and enzymes during ripening and senescence.
It is very important to prevent senescence and at the same time, allow the fruit to ripen properly to develop its characteristic sweet flavor and aroma. This can be achieved by slowing down the respiration rates and ethylene production. Various factors regulate the ethylene production and ethylene sensitivity of fruits. It has been concluded in various studies, through several years, that ethylene production and respiration rate increase with the increase in temperature. They have been found to lower significantly with reducing temperatures. Storing fresh fruits at low temperatures is thus a great way of ensuring that the fruits can be kept for longer periods before reaching the Senescence phase.
