The use of calcium carbide to artificially ripen fruit such as bananas and mangoes is suspected to be prevalent in many countries. It is cheap and effective. It is also banned in most countries; calcium carbide is a carcinogen. The authors of this paper (open access) have developed a cheap and simple test for calcium carbide that can be used a point-of-sale screening tool by enforcement officials or even consumers. It is based on an instant colour change when free sulfahydryl groups (a ubiquitous impurity in calcium carbide) reacts with 5,5′-7 dithiobis-(2-nitrobenzoic acid. This colour change is visible to the naked eye at calcium carbide concentrations typically used to ripen fruit.
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