Volatile acidity – What are the effects on my wine?August 22, 2019
Wine MicrobesDecember 19, 2019
Hazes and deposits in bottled wines
Throughout the long and intensive winemaking and bottling process many problems may arise resulting in a faulty or flawed wine. Sometimes these problems are easily avoidable with strict cellar sanitation and bottling regimes, however, on the odd occasion these faults and flaws sneak into wine despite the winemaker’s intervention. These flaws may arise in numerous forms, from hazes and deposits, to colour instabilities, off-aromas and odours and may even affect the taste of your finished product. We briefly touch on common hazes and deposits seen in bottled wines. These hazes and deposits are known to influence the turbidity and appearance of bottled wines, frequently rendering the wine unacceptable to the consumer.
Microbiological hazes and deposits Non-wine typical hazes and deposits
Growth of yeasts and bacteria in bottled wines that contain residual substrates (glucose, fructose, and malic) may cause turbidity and a sediment. The two fermentation yeasts, Saccharomyces and Zygosaccharomyces, can form a yeast lees deposit, bottle refermentation, increased turbidity and a sediment when allowed to grow in bottled wines with residual glucose and fructose. Zygosaccharomyces may even use sorbate as a growth substrate where growth occurs without the production of CO2, but with the production of a large, granular sediment. Common lactic acid bacteria, being Oenococcus, Lactobacillus and Pediococcus, are also capable of metabolizing residual substrates to produce granular sediments, CO2, and tartrate instabilities. Tartrate instabilities are due to the fermentation of malic acid resulting in a pH shift.
These hazes and deposits come from various sources.
Firstly, mould has become an ever-increasing problem in finished wines. As it is alcohol-intolerant and requires oxygen for growth it will not survive conditions present in wine. However, certain bottling line practices may result in floating mould particles within the wine, or mould growth under screwcaps. Mould particles floating within bottles are frequently due to bottles that have been rinsed for bottling but not used. Mould is then capable of growing in these moist bottles that are often stored in humid environments. When these bottles are rinsed before use, the mould is loosened but not always completely removed. Bottles are then filled, and mould is left as a floating particle within the bottle.
On the odd occasion, bottling lines are set to run at a pace that is too fast, causing full wine bottles to collide. This collision often results in wine being spilt. When screwcaps are placed on moist bottles mould growth may occur under screwcaps.
Although this mould will not continue to grow, it is visually displeasing to consumers and wine is rendered unacceptable to drink
Figure 1: Haze formation due to yeast refermentation in the bottle
Figure 2: Filter powder seen under the microscope.
Other sources of non-wine typical hazes and deposits include;
- Wine processing aids such as filter powder
- Filtering equipment such as fibres from disintegrating filter pads
- Bottling materials including cork dust and cardboard
Crystalline deposits are easily identifiable as they are heavy and quickly settle to the bottom of the bottle. These crystalline deposits can either be Potassium bitartrate crystals or Calcium tartrate crystals. Crystals have no impact on the taste or sensory characteristics of wine but are aesthetically displeasing to the consumer.
Potassium comes from grapes and accumulates during ripening.
Ripe grapes = higher potassium levels!
Potassium bitartrate crystals are usually all precipitated out of wine during the cold stabilization process as the saturation equilibrium is largely temperature dependent. Crystals may, therefore, be formed when wine is cooled below its stabilization temperature.
Over time, anthocyanin and tannin complexes may form and after many years may become large enough to precipitate out. The sediment is generally dark red to brown and granular, often adhering to the sides of the bottle.
Premature ageing and oxidation may result in tannin/anthocyanin complexes precipitating out more rapidly.
Figure 4:Microscopic image of Potassium bitartrate
All proteins in wines are derived from grapes and when not stabilized with bentonite, or when insufficiently stabilized, proteins may denature, agglomerate and form a visible haze. Protein hazes are cloudy/grainy and may settle out of the wine as a granular sediment. As proteins are not as heavy as crystals, protein precipitates may remain suspended in wine for some time before settling to the bottom.
Red wines contain tannin complexes that may bind to proteins throughout fermentation and maturation, these larger complexes precipitate out effectively removing proteins from red wines, making red wines less susceptible to protein instabilities. White wines and light red wines are, however, susceptible to protein instabilities. Bentonite fining is effective in removing heat-unstable proteins from wines.
Figure 3: Anthocyanin/tannin complexes seen under the microscope
Figure 6: Proteinaceous materials seen under the microscope
What can be done about my tartrate instability?
Calcium tartrate crystals precipitate over time and will often only be detected in bottled wines. Cold stabilization is not effective in removing crystals as calcium tartrate precipitation is not readily influenced by cold temperatures. Calcium tartrate instabilities are, therefore, more of a concern when calcium levels in wines are high, with calcium originating from grapes and wine processing aids. Where synthetic tartaric acid products are used, calcium tartrate instabilities may be observed, even where calcium levels are low. Addition of grape juice concentrate having a high calcium level to wine with low levels of calcium may also result in calcium tartrate instabilities.
Cold stabilization, electrodialysis and the addition of crystallization inhibitors all aid in the prevention of tartrate instabilities. However, as previously mentioned, cold stabilization is not effective in removing calcium tartrate precipitation. The addition of crystalline inhibitors is also known to be more effective in the prevention of potassium bitartrate crystals than calcium tartrate. Electrodialysis, on the other hand, removes calcium and thus removes the risk of future calcium tartrate instabilities, as well as, potassium bitartrate instabilities.
Figure 5: Microscopic image of calcium tartrate