Determination Of Tin And Lead In Canned Foodstuffs By Spectroscopic Methods

Abstract

The concentration of tin and lead in canned foodstuffs including vegetables, fruit juices, meat products and fish and baby foods (Nan, S-26 and Cerelac) are determined by spectrophotometric methods. The amounts of the two metals have also been determined in bottled and fresh foods for comparison. The analysis for tin has been done using two methods, that is, quercetin (3, 3', 4', 5, 7- pentahydroxyflavone reagent and catechol violet (Pyrocatechol Sulphonphthalein) reagent. The absorbance of the orage quercetin-tin complex in 96% ethanol was measure~1rat 437nm while the red-orange catechol-violet complex was measured at 555nm in aqueous solution. The catecholviolet method was found to be more sensitive than the quercetin method at concentrations below 1.0 mg kg-l but the two methods gave comparable results for the concentrations normally found in canned foods. Lead has been determined using dithizone (diphenylthiocarbazone) reagent and the absorbance of the red lead dithizonate complex dissolved in chloroform was measured at 516nm. Lead has also been determined using the atomic-absorption method and the two methods gave comparable results. Tomato products contained the highest concentration of tin ranging from 20 - 300 mg kg-1 while fruit juices, vegetables and fish contained 40.0 - 110.0, 7.0 - 55.0, 4.0 - 7.4 mg kg respectively. Comparison with fresh foods showed more than ten fold increase for the canned foodstuffs except fish which had a two-fold increase. The amount; of tin in each can indicates the extent of corrosion and this depends on the type of can (plain o lacquered), whether the food contains corrosion accelerators such as nitrate (NO-3) ion and also the shelf-life. In the lead determination, canned fish contained the highest concentration ranging from 0.70 - 1.42 mg kg-1 while tomato products, fruit juices -1 and vegetables contained 0.30 - 1.03, 0.20 - 0.60 mg kg respectively. No lead was detected in baby foods that is Nan, 8-26 and Cerelac. Comparison with fresh foods showed a four to six-fold increase for canned foods. The amount of lead in each category of food may be accounted for by the amount originally present and that introduced from the solder. Total lead in Kenyan tea was found to be 6.0 mg kg and the amount dissolving in an equivalent of one cup of tea (125 cm3) was found to be 0.02 mg. The estimated daily tin intake is 0.2- 17mg and tin poisoning has been observed with samples -1 containing 250 mg kg tin. For lead, the daily intake is estimated at 0.2 - 0.3 mg and levels in excess of 0.8mg per litre in blood give rise to lead poisoning. The World Health Organization -1 has recommended a limit of 250 mg kg of tin in canned foods and a statutory 2.0 mg kg for lead. Although the amount of tin in most of the foods analysed was found to be below, this can be dangerous occasionally if an old sample, is consumed. The nitrate in the foods can also be converted to nitrosoamines which are known to have carcinogenic effects. Again, although most of the foods were found to contain low quantities of lead, that is, below the recommended level, there should however be concern about canned fish and beverages such as tea whose lead content is high.