There has been concern over the fruit ripening process, especially when it comes to the artificially ripened fruits.
This is a valid concern not only in Kenya but also globally, as fruits are consumed best when they are ripe, with the exception of a few ones like apples and some varieties of pears.
For proper ripening, most fruits are harvested mature-green, when physiological growth, which refers to cell division, cell multiplication and cell differentiation, is complete.
Thanks to recent advances in technology, it is now possible to hasten the ripening of fruits using ethylene, a hormone produced naturally or by the combustion of gases.
It is, however, important to note that ripening does not refer only to change in colour, usually from green to yellow to red or even black, depending on the fruit, as commonly understood. Colour change just happens to be the main apparent indicator of ripening.
There are two types of fruits on the basis of ripening. Fruits that ripen after harvesting at maturity are known as climacteric and fruits that will not ripen after harvesting even at maturity, are known as non-climacteric.
Non-climacteric fruits have to be allowed to ripen while still attached to the plant. The best known non-climacteric fruits are citrus and the pineapple.
Most of all the other commonly eaten fruits are climacteric and this group includes the banana, mango, passion fruit, papaya, avocado and the tomato.
When climacteric fruits are harvested, two things are very important. The system that ensures protection from attack by pests when attached to the plant is curtailed, making the fruits more vulnerable.
Secondly, the fruit is still living and continues with respiration, which finally leads to ripening at nominal rate for some time after which the rate increases tremendously and heralds the onset of ripening.
Thereafter, the ripening is automatic and takes only a day or two. The increase in respiration is paralleled by the production of ethylene, a ripening hormone produced naturally by plants such as apples, avocados, bananas, melons, peaches, pears, and tomatoes.
Ethylene can, however, be produced artificially through the combustion of gases.
The period between harvesting and the onset of ripening can be contracted through addition of exogenous ethylene to shorten the ripening period.
Ethylene is a growth/maturity hormone, which is naturally produced by all vegetative plants. Under normal temperatures and pressures, ethylene exists in gaseous form. Maturity and, therefore, ripening of non-climacteric fruits is enhanced by spraying in the field.
The banana is the most ripened and consumed fruit in Kenya. To produce high quality ripe bananas, caution must be taken to avoid blemishing during harvesting and post-harvest handling, principally during transportation and cutting.
If any latex falls on the individual bananas, this should be removed soon by washing with water. Blemished parts or parts with latex on the banana do not ripen to yellow colour. These are the dark spots often seen on locally vended ripe bananas.
Hence, in preparation for ripening, the banana bunches or the cut hands (clusters attached to the stem) can be dipped in a solution of an approved pesticide at the recommended concentration, then drip-dried.
This action prevents crown rot or stem end rot, which provides an entry point for bacteria and fungi. The bananas should then be packed in an enclosure (to prevent easy escape of gas) with ethylene gas till colour breaks.
With this process, ripening is instantaneous. The gassing process takes between 20—24 hours, mostly depending on the temperature.
In natural ripening in an enclosure like an underground pit, the ethylene produced accumulates and acts like added ethylene.
The source of ethylene can be liquefied gas in a tank, or from ripening fruits. Ethylene production can also be induced in the ripening fruits by introducing injury.
Fruits fight injury by producing ethylene as a healing hormone and this helps to speed up the onset of ripening.
However, injured fruits lose their appearance quality on ripening. A banana bunch can, however, be safely ripened faster by injuring the pseudo-stem, better still by drilling a hole on the stem and putting in salt or sugar.
Many hydro-carbons mimic ethylene in the ripening of fruits, including acetylene, propylene and the hydrocarbons produced by partial combustion of carbon-rich substances like waxes and candles. The commonly used hydrocarbon in the ripening of fruits next to ethylene is acetylene.
The best and easiest to way to produce acetylene is the reaction of calcium carbide with water (which produces acetylene and calcium hydroxide) in an enclosure, then channelling the acetylene to the bananas.
Between 2011 and 2014, the University of Nairobi’s Department of Food Science, Nutrition and Technology participated in the Kenya Horticultural Competitiveness Project (KHCP) of the USAID (United States Agency for International Aid), where we demonstrated accelerated ripening of bananas using smoke from burning candles to farmers in Nyeri, Meru and Kisii.
Besides accelerated ripening, the other advantage of using ethylene and similar gases in fruit ripening is that it promotes even ripening.
The method is also advantageous when handling huge volumes of produce and most importantly, fruit so ripened is perfectly safe for human consumption.
Imungi is a Professor of Food Chemistry, Department of Food Science, Nutrition and Technology at University of Nairobi [email protected]