Climacteric fruits like mango, banana, papaya, sapota and custard apple are often harvested in a mature but unripe condition and then subsequently allowed to ripen by natural release of ripening hormone (ethylene) from the fruit. However, natural ripening in some fruits is a slow process, which leads to high weight loss, desiccation of fruits and uneven ripening. With the rapid development of fruit trade, artificial ripening has become essential and the methods practiced earlier by small traders are smoking and calcium carbide treatment.
Bad effects of calcium carbide
Fruits are still being commercially ripened with banned chemical like calcium carbide which after reaction with water vapour present in the surrounding atmosphere releases acetylene gas. Some traders ripen fruits like banana in enclosed chambers where large quantities of calcium carbide is put and water sprinkled before sealing the chambers. Though the released acetylene triggers ripening process in fruits, it is an inflammable gas involving risk of fire hazards. Calcium carbide is also put in small packets in the fruit boxes and in some cases sprinkled onto the fruit surface. However, calcium carbide contains chemical impurities such as arsenic hydride and phosphorus hydride that are highly carcinogenic compounds. Improper use of calcium carbide can therefore cause chemical contamination of fresh produce. Further fruits ripened with calcium carbide though develop attractive surface colour, are inferior in taste, flavour and spoil faster.
Government of India has banned the use of calcium carbide for ripening of fruits under PFA Act 8-44 AA, 1954.
Alternatives to calcium carbide
Ethrel or ethaphon (2-chloroethane phosphonic acid) is a commercially available plant growth regulator, which is a source of ethylene similar to that naturally released by fruits during ripening process. Although dipping of fruits in diluted ethrel solution is recommended for enhancing ripening, it is a cumbersome process and may cause some problems if commercially available ethrel contains chemical impurities. Further, Food Safety and Standard Authority of India (FSSAI) in its advise issued during May 2010 permits only the use of ethylene in gaseous form for artificial ripening of fruits. To overcome these problems ethylene gas is commercially used in modern ripening chambers which requires huge investment and is not economical for farmers or small traders. Catalytic reactors based ethylene generators are also available which produces ethylene gas using ethanol or methanol or ethrel.
An alternative simple method is standardized for enhancing the ripening process by exposing the fruits to ethylene gas released from ethrel/ethephon solution. This is a simple method wherein small quantity of alkali is added to ethrel to release the ethylene gas and the fruits are exposed to this liberated gas in air-tight portable plastic tents. In this method the fruits are placed in ventilated plastic crates inside air-tight plastic tents of known volume. Required/calculated quantity of ethrel is taken into a container and placed inside the tents to which required quantity of alkali (Sodium hydroxide) is added for releasing the ethylene gas from ethrel solution and the tents are sealed air tight immediately. A small battery operated fan can be placed inside the tent for uniform circulation of released ethylene gas. After 18-24 hours of exposure the fruits are taken out for completing the ripening process at room temperature or 18-24°C especially for Robusta banana. Using dessert coolers during ripening at room temperature would help to reduce the temperature and increase the relative humidity thereby reducing the weight loss.
Mango fruits exposed to 100 ppm ethylene gas for 24 hrs can be ripened in 5 days as compared to 10 days in non-treated fruits without adversely affecting the quality. Similarly banana bunches/hands exposed to 100 ppm ethylene gas for 18 hours can be ripened in 4 days at RT and 6 days at 20°C. Papaya fruits exposed to ethylene gas ripened with uniform surface colour and uniform firmness in 4 days at ambient temperature. Therefore, use of ethylene is suggested as a safe alternative to calcium carbide for ripening of climacteric fruits.