338. The ferric thiocyanate method of estimating peroxide in the fat of butter, milk and dried milk

Abstract

INTRODUCTIONIn butter, milk, and dried milk, flavour deterioration accompanies a slight degree of oxidation of the butterfat, and for the study of oxidative deterioration of these products a method is required which measures fat oxidation at a very early stage. In this laboratory it has been found that the iodometric peroxide method of Lea(i), although slightly more sensitive than the fat aldehyde test of Schibsted(2), registers measurable values only at about the stage of oxidation of butterfat when flavour changes first appear. The work of Chapman & McFarlane(3) and of Lips et al.W has directed attention to the value of the ferric thiocyanate peroxide test in the study of oxidation of the fat in dairy products, and it was hoped to use this method in studying oxidation before flavour changes could be detected. Difficulties were encountered in the low solubility of fat in 96 % acetone, and in the rapid oxidation of ferrous salts in this solvent. Modifications have been made to the test, and in its final form it has proved sufficiently sensitive to follow the oxidation of the fat of milk from the time it leaves the udder. METHOD Reagents (1) Benzene-methanolThe solvent is a mixture of 70 vol. of benzene and 30 vol. of methanol. A.R. quality benzene is redistilled and the first three 20 ml. lots are used to rinse the receiving vessel. Synthetic methanol is dried by refluxing during 4 hr. with bright magnesium ribbon (5 g./l.), and is then distilled. The moisture content of the mixed solvent should not exceed 0-5%. This may be checked with the Karl Fischer reagent(5).As the benzene-methanol mixture is cheap, its recovery is probably not economical. When necessary it may be recovered by adjusting the acidity with a saturated solution of potassium hydroxide in methanol to give a reading of pH 7 with the glass electrode (to avoid distillation of ammonia or thiocyanic acid), refluxing during 4-5 hr. with 20-mesh anhydrous calcium sulphate, and then distilling. The moisture content of the recovered reagent should be checked and the j?H value determined and adjusted if necessary to between 6-0 and 7-0. The proportion of benzene and methanol may be corrected by adjusting the specific gravity to 0-850 at 20° C.(2) Ferrous chloride solution Hydrated barium chloride (0-4 g.) dissolved in 50 ml. water is added slowly, with stirring, to 0-5 g. hydrated ferrous sulphate dissolved in 50 ml. water, and 2 ml. of 10 N hydrochloric acid is then added. All reagents should be free of ferric iron. The precipitate G. LOFTUS HILLS AND C. C. THIEL 341 of barium sulphate is allowed to settle, and the clear solution is then decanted into a bottle protected from light. The solution is sufficiently stable to be usable for about a week; the extent of oxidation may be judged by noting the colour obtained when 1 drop of 30% ammonium thiocyanate is added to 1 ml. of the ferrous chloride solution. A solution containing one-fifth this concentration of ferrous salt, but the same concentration of hydrochloric acid, should be used for peroxide values b...

Studies on the manufacture of sweet cream starter butter: I. Effect of cream-holding temperature

“…The method used was the modification by of the method of Pien, Baisse & Martin (1937). The tests used for examination of the butterfats from the stored butters were the ferric thiocyanate peroxide test as described by Loftus Hills & Thiel (1946), the Schibsted fat aldehyde test as modified by Mummery (1947), and the normal fat acidity test.…”

I. Some After-Effects in Fats Irradiated with High-Energy Electrons and X Rays

“…The latter can be measured in several ways; that used in all the following experiments has made use of their ability to oxidise ferrous ions to ferric in benzene-methanol solution at +50°C, the ferric ions being estimated as the purple colour of ferric thiocyanate (Loftus-Hills and Thiel, 1946). The results are expressed as "peroxide values", i.e.…”

L'entreposage des beurres en frigorifique. Influence de la teneur en cuivre sur l'aptitude a la conservation

“…et au cours des deux dernières années suivant la technique de LOFT US HILLS[4], nous nous sommes contentés de reprendre…”