Microbes and food 1. Menu - you are what you eat
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Wine bottles
Picture 1.15a Wine on sale in the supermarket.

1.15 Wine
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What is it?
Wine is an alcoholic drink. It has been made for many centuries by fermenting a variety of fruits. Most wine is made from grapes. Fruit juice contains simple carbohydrates that the yeasts can start to break down straightaway, without any other processing.
Alcoholic drinks include wine, cider and beer. They are made by fermentation during a process called brewing. Most fruit and vegetables can be used to make an alcoholic drink because they contain carbohydrates such as starch and glucose sugar. When the carbohydrate is fermented by yeast, one of the products is alcohol.

Glucose -> alcohol + carbon dioxide + water

Generally grapes are used to make wine, apples for making cider and cereals such as barley or wheat for brewing beer.

How is it produced?
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When they are harvested, the grapes are crushed. This is the beginning of the sequence. Roll over the titles (starting with Crushing) to find out more.
Picking grapes photo
Picture 1.15b Harvesting the grapes for wine.
Bottling wine photo
Picture 1.15c Bottling the wine.
Crushing
Once the stalks have been removed, the grapes are crushed to a ‘must’ of pulp and skins. All grapes have white juices. Red wine is made by leaving the skins of red grapes in contact with the must for a while during fermentation to release the pigments.
The skins and fruit fibres are separated out.
Traditionally yeasts and bacteria living on the grape skins started the fermentation. Nowadays the must is heated or treated with sulphur dioxide to kill the naturally present microbes before a starter culture of wine yeast with known characteristics is added. The juice is fermented at 20-28ºC for 3-5 days.

The amount of sugar in the must, the fermentation period and the alcohol tolerance of the yeast affect the sweetness of the wine and the alcohol content. It can range from 10-18%. A dry wine results if all the sugar is fermented.

The large amount of carbon dioxide formed during fermentation escapes through a special one-way valve that stops any dirt or unwanted micro-organisms getting into the vat.
The wine is run off into a settling vat or barrels. The yeast that has grown during the fermentation falls out as a sediment known as ‘lees’ and is removed. The sulphur dioxide treatment is repeated. A secondary malo-lactic fermentation by bacteria often takes place at this stage, which reduces the wine’s sharp-tasting malic acid content and contributes to the flavour of the final product.
The wine is stored in barrels or vats at low temperature for a year or more to allow the flavours to develop. Complex chemical changes take place during ageing. Sometimes wine is aged only in the bottle.
Any remaining microbial growth and other particles are removed by filtration or by adding special compounds called fining agents.

The wine is filled into sterile bottles, corks or screw caps are fitted and plastic or lead seals placed over the top to keep out the air.

The wine continues to mature in the bottle. White wine is usually sold without much ageing, but red wine is often kept for several years to enable the full flavours to develop.

To make sparkling wines such as champagne, the fermentation is allowed to continue in the bottle. Sediment that collects in the neck of the bottle is removed by freezing that area, and removing the cork which 'disgorges' the accumulation. The bottles are refilled with clear sparkling wine which has been disgorged, corked and labelled.

If you use a kit to make wine at home, the fruit juice is provided as a concentrated syrup. You have to dilute this and add yeast to it.

How does it spoil?
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As wine is the product of an active fermentation, there are several ways it can deteriorate. If the excess yeast cells are not removed and the sugar content is too high, fermentation can continue after bottling, resulting in a ‘prickly’ wine, the corks blowing out or even the bottle exploding! A type of yeast called Zygosaccharomyces can also spoil wine. Red wine spoilage is often due to acetic acid bacteria. Residual sulphur dioxide, along with strict attention to hygiene during production, help to control most spoilage microbes. Storing wine bottles on their side also helps to prevent spoilage because any oxygen getting in through the cork is not concentrated at the top of the bottle, where the levels might be high enough to encourage bacterial growth.

The type of bottle closure actually affects the quality of the wine. Traditionally, corks have been used. Most are made from the bark of the cork oak tree which is ideal because it is flexible and does not allow oxygen or moisture through. Some stoppers are made from glued cork chips, although increasingly plastic ones are becoming favoured. This is because as cork is a living thing, it does vary in quality and moulds can sometimes grow in it which contaminate the wine. ‘Corked’ wine has a bad smell and tastes horrible. Some wine producers have even moved over to screw caps which completely prevent microbial contamination.

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Can it be harmful?
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As with all alcoholic beverages, harmful microbes are unlikely to grow in wine due to the concentration of ethanol present and the acidity of the drink. There is far more danger likely from the alcohol itself, which can be toxic to the body if too much is consumed, damaging the heart, liver and brain.
Warning
Wine is an alcoholic drink. Alcohol can be dangerous to health.