Natural fruit ripening is a combination of physiological, biochemical, and molecular processes. It involves coordination of different metabolisms with activation and deactivation of various genes, which leads to changes in color, sugar content, acidity, texture, and aroma volatiles. The change in color during the fruit ripening process is a result of unmasking of pigments by degradation of chlorophyll, synthesis of different types of anthocyanins and their accumulation in vacuoles, and accumulation of carotenoids.
Production of complex mixture of volatile compounds, such as ocimene and myrcene, and degradation of bitter principles, flavonoids, tannins, and other related compounds enhance the flavor and aroma of the fruit. Sweetness increases because of increased gluconeogenesis (metabolic pathway that generates glucose), hydrolysis of polysaccharides, decreased acidity, and accumulation of sugars and organic acids. Furthermore, textural changes resulting in the softening of fruits occur due to enzyme-mitigated alteration in structure and composition of the cell wall. Through the above changes, fruit becomes ripe with distinctive characteristics: sweet, colored, soft, and palatable. The only safe and worldwide accepted method is using ethylene, which is a natural hormone for ripening when done under controlled temperature and relative humidity conditions.
Ethylene being a natural hormone does not pose any health hazard for consumers of the fruits. It is a de-greening agent, which can turn the peel from green to perfect yellow (in the case of bananas) and maintain the sweetness and aroma of the fruit, thus value addition in the fruit is possible as it looks more appealing. It has been known for a long time that treatment of unripe fruits with ethylene would merely stimulate natural ripening until the fruit itself starts producing ethylene in large quantities.
Technologies for ripening of fruits
Lack of easier and rapid methods for uniform ripening poses a major problem in the fruit industry. Almost all methods of ripening, either conventional or the modern chemical methods, come with their own merits and demerits. Normally, the number of days taken for edible ripening varies for different fruits and prevailing climatic conditions. For instance, it takes about 5 to 6 days for mangoes and 6 to 7 days for sapotas to ripen. Under natural conditions, ethylene, a ripening hormone produced by the plant plays a major physiological role in the ripening process. A simple technology practiced in households to trigger ripening is to keep un- ripened and ripened fruits together inside an air tight container. Since the already ripened fruits release ethylene, ripening will be faster.
Mango ripening in air tight rice bin
Another method is to place the fruits intended for ripening inside an air tight room and induce ripening through smoking inside smoke chambers. Smoke emanates acetylene gas. Several fruit traders follow this technique to achieve uniform ripening especially in edible fruits like banana and mango. But the major drawback of this method is that the fruits do not attain uniform colour and flavour. In addition, the persistence of smoke odour on the product impairs its quality. Spreading unripe fruits as layers over paddy husk or wheat straw for a week to ripen is another alternative.
Mango ripening using paddy straw
Another practice is that some farmers dip unripe mature fruits in 0.1 per cent ethrel solution (1 ml of ethrel solution in 1 litre of water) and wipe it dry. The fruits are then spread over a newspaper without touching each other and a thin cotton cloth is covered over this. In this method, the fruits will ripen within two days.
In one of the simple and harmless techniques, 10 ml of ethrel and 2 gm of sodium hydroxide pellets are mixed in five litres of water taken in a wide mouthed vessel. This vessel is placed inside the ripening chamber near the fruits and the room is sealed air tight. About a third of the room is filled with fruits leaving the remaining area for air circulation. Ripening of fruits takes place in about 12 to 24 hours. In order to reduce the cost of chemical, some ethylene releasing fruits such as papaya and banana can also kept in the same room. Ethylene gas filled in pressurized cans promote fruit ripening in 24-48 hours
Controlled degreening sometimes is carried out on citrus grown in tropics. Many citrus cultivars mature before green colour disappears from peel. Breakdown of chlorophyll and production of a rich orange colour require exposure to low temperature during maturation, and this explains why mature citrus frequently is sold green on markets in humid tropics, where even night temperatures may not drop much below 25oC. The ceiling of room is relatively high, allowing boxes to be stacked at least four boxes high. A false ceiling is added to provide for adequate air. Degreening is carried out in ripening rooms, with same ethylene concentrations as above. this process takes 2 to 3 days, and it is again necessary to ventilate daily to ensure that carbon dioxide levels do not exceed 1%. The most rapid degreening occurs at temperatures of 25 to 30oC but the best colour (concentration of peel carotenoids) occurs at 15 to 25oC.
Health hazards associated with these Chemicals
Eating artificially ripened mangoes causes stomach upset because the alkaline substance is an irritant that erodes the mucosal tissue in the stomach and disrupts intestinal function. Prolonged exposure to the chemical could lead to peptic ulcer.
In humans, acetylene is not acutely toxic if it is below the permissible levels whereas if it exceeds the limits then its inhalation can cause unconsciousness and it may affect the neurological system by inducing prolonged hypoxia i.e. deficiency of Oxygen.
Calcium carbide is a corrosive and dangerous chemical containing traces of arsenic and phosphorus hydride as impurities.
The early symptom of arsenic and phosphorus poisoning include diarrhoea (with or without blood), vomiting, thirst, weakness, burning sensation in abdomen and chest, difficulty in swallowing, irritation/burning sensation in eyes/skin, soar throat, cough, shortness in breathing and ulcer on skin etc.
Calcium carbide is known to have carcinogenic properties also.
Point to be noted
The industrial grade carbide gas popularly known as ‘masala’ is often used by some unscrupulous traders to release acetylene gas for artificial ripening of mango, banana, papaya, etc. Calcium carbide contains traces of arsenic and phosphorous which are harmful to humans and may cause dizziness, frequent thirst, weakness, difficulty in swallowing, vomiting, skin ulcers, etc. It is equally harmful to the handlers. There are also chances that calcium carbide may come in direct contact with fruits during application and leave arsenic residues and phosphorous on fruits and so the use of this chemical is banned in India. In order to discourage the use of the banned calcium carbide and the non-availability of an alternative ripening agent, the FSSAI has permitted the use of ethylene gas for ripening of fruits in India to vide notification dated 23 August 2016.
Sahil Sharma, Researcher, Department of food technology, Lovely Professional University Punjab & Independent Food Technologist