399. The estimation of vitamin A in butter: A critical study of methods

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Abstract
1. The Lovibond Tintometer has been found more reliable than either the Pulfrich Photometer or the Spekker Absorptiometer for the assessment of the blue colour from the Carr-Price test on butterfat unsaponifiable residues.2. Using a Beckman Spectrophotometer, vitamin A in butterfat unsaponifiable residues has been estimated also by the intensity of absorption at 325 mµ. From spectrophotometric readings at this wavelength and at 450 mµ, the irrelevant absorption, due both to carotenoid and non-carotenoid compounds, may be calculated as follows:Corrected density reading at 325 mµ = 9/10 (A – 12/100B),where A = density reading of the unsaponifiable residue in petrol ether at 325 mµ, B = density reading of the same solution at 450 mµ.3. The Morton & Stubbs equation for irrelevant absorption in fish-liver oils is not generally applicable to butterfat unsaponifiable residues.4. A comparison of the vitamin A content of butterfat samples by the Carr-Price test and by the direct spectrophotometric method has shown a reasonably good agreement between the two values.