Food Structure


A study was undertaken to investigate the influence of buffer composition, pH and glutaraldehyde fixation time on the appearance of electron dense granules in yoghurt. Yoghurt particles were fixed in 3.5% glutaraldehyde and postfixed in 2% osmium tetroxide in verona! acetate or phosphate buffer. Thin sections were examined unstained with an electron microscope equipped with a scanning transmission electron microscope module and energy dispersive X- ray analyser.

Electron dense granules appeared whenever glutaraldehyde and osmium tetroxide were utilized sequentially, irrespective of the type of buffer, pH (5.0 vs 6.75), or glutaraldehyde fixation time (2 or 24 h). Granules were absent if glutaraldehyde was used alone. Granules were generally located around the outer edge of casein particles, fat globules and bacteria. X- ray microanalysis of these granules detected the presence of 89 - 100% osmium (Os) and 0- 11% chlorine (Cl) on a weight percent basis . Granules were removed by treatment with periodic acid or hydrogen peroxide. It appears that the presence of osmium tetroxide is a prerequisite for granule formation and their appearance is not always dependent on the use of phosphate buffer as has been suggested in previous research. The small quantity and variability in Cl content precluded this element from being considered a factor in granule formation. The most probable source of Cl is the embedding medium. Results from this study suggest that granules are fixation artifacts consisting of a complex of glutaraldehyde and osmium tetroxide, however the structure of the complex and mechanism of formation are still unknown.

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