On examination of the deposit or haze under the microscope, it is usually possible to classify the particles present as belonging to one of four main categories:
Crystalline deposits are characterised by relatively large (approximately 0.1mm – 1.0mm), regular, geometrically shaped particles. Crystals can usually be clearly observed using stereomicroscopy. Compounds typically observed as crystals in wine include potassium hydrogen tartrate, calcium tartrate, ellagic acid, calcium oxalate and flavonols, with the two forms of tartrate being the most common. Examples of crystal shapes include ‘boat’- and lens-shaped (potassium hydrogen tartrate), and rhomboid (calcium tartrate). While the shape of the crystals can give some indication as to their composition, further information can be gained by infra-red (IR) Spectroscopy, flame tests and spot chemical tests.
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Microbiological deposits in wine can usually be classified as either yeast or bacteria. Both types of cells are considerably smaller than the particles of crystalline deposits, requiring a much higher level of magnification (400x) for the cells to be clearly visible. In general, yeast cells are larger than bacterial cells (with dimensions in the range 3 – 12 µm) and are present only as single cells, sometimes with a new cell ‘budding’ from an original cell. Yeast cells typically found in wine can be spherical, ovoid, elongated, lens-shaped or lemon-shaped (apiculate).
Bacterial cells found in wine are usually spherical in shape (coccoid) or rod-shaped. They are typically smaller than yeast cells (dimensions around 1 – 3µm) and can be present as single cells, pairs, or chains of cells.
Mould is occasionally found growing on the surface of stored juice and concentrate, but cannot survive in wine. Its appearance is easily distinguished from yeast or bacteria, in that it resembles irregular filaments rather than distinct cells.
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Amorphous deposits usually have an irregular, granular appearance when examined using phase contrast microscopy, even at high magnification. Possible components of an amorphous deposit from wine include protein, metal/protein complexes, pigments, polysaccharides, polyphenolic compounds and condensed tannins. Little information can be gained from observing the appearance of amorphous deposits, although the presence of bright, shiny-looking spheres under phase contrast microscopy can suggest the presence of a metal. In general, further analysis of the deposit, using techniques such as spot chemical tests, Atomic Absorption Spectroscopy and Infra-red Spectroscopy is required to make an identification.
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Other deposits encompass foreign particles found in wine that do not conform to any of the three categories discussed above. These deposits can range from filter aids such as diatomaceous earth or filter fibres, fining agents such as bentonite, to chips of glass, pieces of amorphous silica, cork dust and cork coating.
No general description can be given for these foreign deposits, except that they do not conform to the characteristics of any of the deposit types previously discussed. Identification of unusual deposits can, therefore, be difficult. Some, like diatomaceous earth, are immediately obvious when observed under the microscope, while others, like cork coating material, have specific spot tests.
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