Why does fresh tamarind become dark in colour after some months, but not when stored in air-conditioned godowns?

The tamarind fruit pericarp (pulp) contains several phenolic compounds most of which are proantho cyanidins. In addition, tamarind pulp also has ascorbic acid and tartaric acid in high amounts. All these compounds are easily prone to oxidation (which is why they act as `antioxidants' protecting cellular components from damage by oxygen radicals). Oxidation of phenolic compounds leads to darkening of the pulp. This is the reason why many fruits and vegetables including tamarind turn dark when exposed to air.

Darkening due to aging and exposure to air also occurs in some kinds of wine, mustards and ketchup preparations. The speed at which they turn dark depends on the chemical composition of phenolic compounds present in the fruit/ingredients. Additionally, the presence of oxidative enzymes may also contribute to the process. In the case of tamarind darkening due to age, oxidation occurs primarily as a non-enzymatic chemical reaction. 

As a general rule of thumb, there is a 50 per cent reduction in the rate of chemical reactions for every 10{+0}C reduction in temperature. So, refrigeration would significantly reduce the rate of oxidation and thus darkening of tamarind pulp. Enzyme mediated oxidations are also reduced under refrigerated conditions due to optimal temperature requirements of most enzymes.

Source: thehindu.com