If you think shiny red apples and bright yellow mangoes are a veritable source of vitamins and minerals, it’s time to think again. Brilliant skin colour does lend a luscious look, but in reality such fruits may be dishing out poison instead of nutrients.

No, this is not to do with traces of harmful pesticides that remain on fruits even after they are harvested. Scientists are pointing fingers at certain chemicals that traders use as artificial ripening agents.

A study by two researchers at the Bidhan Chandra Krishi Vishwavidyalaya in Nadia in West Bengal has shown that unscrupulous traders often use a chemical called calcium carbide to artificially ripen fruits such as apples, bananas, mangoes and papayas. According to the study, which appeared in the December 25 issue of Current Science, calcium carbide — even though banned recently — is widely used for ripening fruits, putting the health of consumers at risk. Apart from releasing noxious gases like phosphene and carbon monoxide, it breaks down the organic composition of vitamins and other micronutrients.

“Calcium carbide only changes the skin colour; inside the fruit remains raw,” says Md Wasim Siddiqui of the department of post harvest technology of horticultural crops at the agricultural university. “The more immature the fruit, the higher the calcium carbide required to ‘ripen’ it. This makes the fruit tasteless, unhealthy and slightly toxic.” Siddiqui, together with his senior R.S. Dhua, also showed that chronic exposure to the chemical could lead to peptic ulcers.

Besides, industrial grade calcium carbide (which most fruit traders use) contains arsenic and phosphorus hydride, which are hazardous to the human body.

“The government has recently banned the use of calcium carbide as a ripening agent under the Prevention of Food Adulteration Act of 1954. Illegal use attracts a fine of Rs 1,000 and up to one year in jail,” cautions S. Rajarathnam, head of fruit and vegetable technology at the Mysore-based Central Food Technological Research Institute.

When it comes to ripening, fruits are of two types: climacteric and non-climacteric. While the former can ripen even after they have been plucked, the latter can ripen only on the plant, and hence have a shorter shelf life.

The fact that climacteric fruits can be harvested while they are still raw helps in their transportation to far-flung places. Once there, vendors use certain chemicals for ripening them. The ideal ripening agents are those that release ethylene. “Ethylene is natural fruit hormone and hence kicks off normal process of fruit ripening,” says Rajarathnam.

Calcium carbide, on the other hand, releases acetylene, which is an analogous of ethylene, and hence provokes the same effects, says Siddiqui.

There are several reasons fruit traders prefer calcium carbide to ethylene-releasing ripening agents such as ethrel or ethephon whose usage is permitted. One, calcium carbide is cheaper than the other chemicals. And two, the process involving the latter is cumbersome. The fruits have to be either dipped in a solution containing a portion of these chemicals or fumes of these chemicals have to be passed through them, explains Siddiqui.

Calcium carbide, on the other hand, is easy to use. The vendor only needs to keep a small quantity of the chemical in an airtight chamber or room where the fruits are stored.

However, fruits treated with the ethylene-releasing chemicals have a more acceptable colour than naturally ripened fruits and also a longer shelf life than those ripened with calcium carbide, observes Siddiqui.

“Consumers are not aware of the risk they may be undertaking when they make purchases based on the colour of fruits. They often forget that the natural colour of certain varieties of mangoes and bananas even after ripening is green,” points out Siddiqui. For example, mangoes of the Langra, Himsagar and Fazli varieties, Cavendish bananas and some varieties of tomato are not yellowish or red when ripe. But when chemically treated, their peel becomes either bright yellow or red.

According to the scientists, it is not difficult to identify fruits that have been artificially ripened. For instance, if all the bananas in a bunch are of a uniform colour, it is more likely that they have been artificially ripened. Naturally ripened fruits are not uniformly yellow; rather, they are of mixed green and yellow hues.

Similarly, it is not advisable to buy fruits when they arrive in the market before their actual season. “One can be almost sure that they have been artificially ripened,” says Siddiqui.

So vitamins or poison? The choice surely isn’t difficult.