Juicy Bits ...

On this page, we'll discuss the initial steps in turning grapes into wine.

Extracting the Grape Juice

Different components of the grape give off attributes to the resulting wine. Once you understand the physiology of the grape, you quickly see what impact certain winemaking techniques have on the flavour and structure of the wine.

Getting juice out of the grapes means breaking the skin and membrane surrounding the berries. With ripe fruit, that's not too difficult. Sometimes this happens in the containers after picking, or in the trucks during transport to the winery. The sheer weight of those on top of the pile crush those at the bottom. The juice that runs freely from this step is called [appropriately ...] 'free-run' juice. This is the best quality juice for making premium wines.

Despite images of naked feet tromping all over the grapes, in the majority of cases machines are next used to crush or press them to release more juice. Crushing and destemming are particularly severe treatments; raising the level of phenolics in the juice from shredded stalks and split seeds. However, crushing the grapes expedites the process of juice extraction - thus is more cost effective.

Pressing whole bunches of grapes is generally reserved for premium wines - eg champagne. This way the juice is extracted without picking up a lot of phenolic compounds.

In most cases, grapes are pressed a number of times to extract as much juice as possible. With each pressing the quality of juice squeezed out drops in quality.

Why? Well, the longer juice is in contact with the skin, stalks and seeds, the greater the opportunity for tannins and bitterness to permeate the juice. Granted, sometimes higher tannin is what the winemaker is aiming for - more so with reds than whites.

Think about the above-right diagram. If bitterness comes from seeds, you won't want to crush them too much in the process, would you?

The first pressing is usually gentle, and is often added to the free-run juice. This makes the premium wine.

Second pressings, you get the next best thing.

Third pressings ... you guessed it - low value, low quality wine - suitable for cheap casks.

The crushed grapes and juice are called "Must".

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Must Management

With the majority of wine types or styles, the aim is to minimise damage or alteration to the grape varietal's flavours and aromas. When a grape is intact, the skin acts as a barrier to oxygen and microbes in the outside world. Once the skin is broken - it's a free-for-all! And if the grapes are warm, enzymatic and oxidative activity accelerates.

Management of must differs between red and white wines. With whites, it's removed to avoid skin contact affecting the wine. Sometimes the berries aren't even crushed - they're just pressed gentle to extract the juice.

With reds, however, exposure to tannins and other phenolics might be just what the winemaker wants to add complexity to his wine. Of course, as the colour of the grapes is in the skin, must contact is a prerequisite for making red wine!

The winemaker has a job on his hands protecting his precious raw materials from disaster.

Firstly, the temperature needs to be controlled - more so in warmer regions, and particularly for delicate white wines. Preferably, must should remain between 10^oC to 18^oC.

Such is the importance of must temperature for delicate whites - eg Rieslings - they are often picked early in the morning before the sun even gets a chance to heat them.

Cooling off the must is done either in tanks with outer 'cooling jackets' or via heat exchange - ie must is fed through a tube within another tube containing chilled water or brine.

Another method of minimising must exposure to oxygen is the use of inert gas "blankets" to fill head-space in tanks and containers.

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Fermenting the Grape Juice

The sugars in grape juice are converted into alcohol with the use of yeasts.

Wild yeasts are already present on the skins of grapes, but using wild yeasts alone to convert sugars is a bit like tight-rope walking without a safety net. It's not for the faint-hearted winemakers - and hats off to those who try it and succeed!

Probably the biggest problem using natural yeasts is that they're not alcohol-tolerant! As the alcohol content rises in the wine, the yeast dies off. This can leave stuck ferments that are difficult to restart and result in poor quality, oxidised, volatile wine with broad yeasty characters.

Not only are they poor fermenters, wild yeasts give off vile aromas and flavours. Some argue, though, that these dubious qualities can also make a wine interesting! Thus sometimes they are considered desirable for an initial ferment .

Winemakers rely on consistency and reliability in yeast strains to produce good wines.

Famous cheese and chocolate-makers, the Swiss, are responsible for the Saccharomyces yeasts used around the world for winemaking. Saccharomyces is alcohol tolerant.

Basically, they grow the yeasts - adding nutrients and oxygen before freeze-drying. Cans of the dried yeast are then rehydrated at the winery and tossed into the must to start fermentation.

Yeast comes in different strains, and are chosen based on the style of wine being made. When making aromatic white wines, for example, the strain of yeast chosen must ferment in cold termperatures, and preferably give off fruity esters rather than hydrogen sulphide or acetic characters. For fuller-bodied wines, the winemaker looks for a strong, consistent, alcohol-tolerant variety with low-foaming quality [in the case of barrel fermentation].

It's important to note, though, that not all sugar gets converted in all wines. Sometimes residual sugar is a necessary component of the wine type or style. More on that in other winemaking pages ...

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Last Modified: Wednesday, April 11, 2007