Non-enzymatic browning in lime juice is accompanied by an increase in off flavour, which also limits shelf life. During evaporative concentration of lime juice, rise in temperature leads to browning pigment formation. Clarified lime juice remains free from non-enzymatic browning when subjected to vacuum evaporation at a temperature range of 30-50oC.
Ascorbic acid is responsible for the development of browning reactions in fruit juices, concentrates and in canned vegetables. Mixtures of ascorbic acid and amino acids develop brown colour more rapidly than mixtures of reducing sugars and amino acids. Dehydro-ascorbic acid highly reactive and can react with amino acids. In the decomposition of ascorbic acid or dehydro ascorbic acid furfural and ozone of L-xylose are formed which are highly reactive. The tamarind fruit pericarp (pulp) contains several phenolic compounds most of which are proanthocyanidins. In addition tamarind pulp contains tartaric acid and a small amount of ascorbic acid. All these compounds are easily prone to oxidation.
Oxidation of phenolic compounds leads to darkening of tamarind pulp. Darkening due to ageing and exposure to air also occurs in some kinds of wine and ketchup preparations. The speed at which they turn dark depends on the chemical composition of phenolic compounds present in the fruit ingredients.
In the case of tamarind darkening due to age, oxidation occurs primarily as a non-enzymatic chemical reaction. Refrigerator would significantly reduce the rate of oxidation and thus darkening of tamarind pulp.
Participants