719. Observations by fluorescence microscopy of casein in milk, curd and cheese

Abstract

Fluorescence microscopy was applied to the examination of casein in milk, curd and cheese, to check or support the picture obtained by electron microscopy. The best fluorochroming effect was shown by thioflavine T, acridine orange, coriphosphine and basic fuchsin.In milk, larger casein particles (0·1–0·8 μ in diameter) were rendered visible as minute nearly spherical corpuscles in lively Brownian movement. The sediment obtained on centrifuging the whole milk at about 900 g contained predominantly large particles (average diameter 0·;8 μ). On acid or rennet coagulation the casein particles aggregated to flocs, which finally formed a three-dimensional framework.From curd or cheese freezing-microtome sections (10 μ thick) were examined. The curds which did not possess the fibrous (‘chicken-breast’) texture, characteristic for wellcheddared curd, under the fluorescence microscope displayed a granular structure, whereas curds with a pronounced fibrous texture showed a network of thin micro-fibres, particularly in sections cut along the direction of macro-fibres.The sections (10 μ or less thick) of curd and cheese on the surface of water slowly spread out. The spreading was accompanied either by conversion of the form of casein from granular into fibrous, or by stretching of the micro-fibres in the already existing network.The grain boundaries in curd and cheese (Cheddar, Emmental, Edam) were almost free of fat. No grain boundaries existed in process cheese.