ASCORBIC ACID IN DEHYDRATED FOODS

Abstract

THERE is much evidence that titration against 2,6-dichlorophenolindophenol, provided it is done rapidly and in acid solution¹, gives results accurately agreeing with the biological estimate of ascorbic acid in common foodstuffs, including fruits and vegetables (both raw and as generally cooked), and in various animal tissues². Even for most manufactured articles, such as jams, tinned fruits and various other processed foods, the chemical test has been shown to be generally applicable². In a few exceptional instances, however, interferences from non-specific reductants may have to be allowed for. Such substances occur in caramelized and fermented products (for example, in malt extracts, or the products obtained by alkaline-heat treatment Of sugars, in materials such as oats or pea mush after heat treatment, and in yeast and fermented juices)³. The substances known to interfere include stannous and ferrous salts, as well as sulphites, sulphides and thiosulphates, among inorganic substances, and re-ductones and reductic acid among organic compounds. Substances of the former class, however, are not normally present in foodstuffs, and moreover they can be fairly easily differentiated if desired. Thus, thiosulphate and sulphydry^l compounds may be removed by precipitation with mercuric acetate⁴, sulphite by treatment with acetone⁵, or sulphides by conversion to hydrogen sulphide and removal in vacua. Lugg' has recently recommended the use of formaldehyde to differentiate ascorbic acid from sulphides, sulphites, thiosulphates, quinones, sulphydry^l compounds and reductones. It will be recalled that glutathione does not interfere under the conditions of the test', and that free cysteine is not normally a constituent of foodstuffs and in any event reacts so much more slowly that its presence would not be a source of serious error.