Winemaking: More secrets of fermentation

More on the magical microbes that play a vital part in turning grapes into wine; this time it's the turn of the lactic acid bacteria. Caroline Gilby MW explains.

Winemaking: more secrets of fermentation
Beaujolais Nouveau epitomises the carbonic maceration style.

There are two more winemaking processes to look at to demystify the facts behind those technical words that so often get bandied about in the wine world. First of all, I will attempt to cut through the rubbish that is often talked about carbonic maceration (or macération carbonique as it is commonly called because it is so associated with Beaujolais in France). Then, I will go onto the 'malolactic fermentation', (often abbreviated to 'malo') another fermentation process in winemaking where bacteria takes centre stage.

Carbonic maceration

Carbonic maceration has almost certainly happened by accident since grapes were first turned into wine but was more formally 'invented' in 1934.The process depends on having intact berries - which usually means hand harvested whole bunches.

Both machine harvesting and destemming bunches cause too much damage and therefore leakage of juice for this process to work. The bunches are then put into an oxygen-free atmosphere in an enclosed fermentation tank, and this forces the grape cells to respire in an anaerobic way as there is no oxygen to fuel normal respiration.

Normal respiration (or breathing to you and me) uses oxygen to completely break down sugar to provide energy and produces carbon dioxide and water as by-products, but when there's not enough oxygen, cells have a metabolic shortcut so that they can still produce energy.

Anaerobic breathing - what's that?

To use a human analogy, when you are sitting at your desk reading this, you are breathing in enough oxygen to sit there comfortably, sustaining all your energy needs. However if you get up and sprint for a train, you quickly run out of energy and get muscle pains and maybe a stitch. This is because you aren't getting enough oxygen for this higher energy demand so your body goes into shortcut mode or anaerobic respiration. The by-product of this is lactic acid which collects in your muscles giving that heavy leg feeling and that stitch (though but luckily this can be broken down later on).

Plant cells have a similar metabolic short cut, but instead of lactic acid, they produce ethanol as a by-product and it is this phenomenon that carbonic maceration relies upon. In practical terms, what happens is that the winemaker fills the vat up with carbon dioxide to exclude all oxygen before adding the whole bunches, or with so-called 'semi-carbonic maceration' the weight of the bunches crushes some of the berries at the bottom of the tank.

These then start to ferment normally creating an atmosphere of carbon dioxide in the tank and thus anaerobic conditions. (Just in case you didn't know, carbon dioxide is a suffocating gas that suppresses the breathing reflex and, as it is denser than air, it will collect in winery vats and low corners of wine cellars. It can be fatal too - a young winemaker died in Spain a few weeks ago).

The grape cells are thus forced into anaerobic respiration and alcohol is produced within the whole berries. Unfortunately, alcohol is toxic to plant cells, and once it accumulates to about 1.5 to 2%, the grape cells start to die and the berries eventually collapse. In the meantime, colour pigments (or anthocyanins) dissolve out of the skins into the pulp, but very little tannin, and various fruity esters (particular ones that give flavours of cherry, raspberry and even banana) are produced.

…enter the yeasts
Finally as the berries collapse, or are crushed deliberately by the winemaker, yeast takes over and finishes the fermentation as normal. The end result is typically bright fruity wines with relatively low tannin levels that are easy to drink young - Beaujolais Nouveau is the epitome of this style, but many producers in and out of Beaujolais use a semi-carbonic maceration to increase levels of fruitiness and drinkability. A good example is Duboeuf's Gamay Vin de Pays de L'Ardeche.

Malolactic magic

Once upon a time, winemakers noticed that bubbles appeared in their wine in springtime and thought that this was simply the young wine reacting 'in sympathy' to the rising of the sap as the vines came back into growth after lying dormant through the winter. Now we know that it is caused by a type of fermentation that needs warmer conditions to take place - such as warmer weather in springtime.

My earlier article looked at the primary alcoholic fermentation, which is brought about by the action of yeast, but in many wines there is a second fermentation, this time mediated by certain strains of bacteria. These Lactic Acid Bacteria (from the genera Leuconostoc, Lactobacillus and Pediococcus) use malic acid as an energy source, and produce lactic acid as the main by-product.

Malic acid is sharp and tastes of apples, and it happens to get its name from the apple family (whose Latin name is Malus), though malic acid is also the second most important acid found in grapes, especially when grown in cooler climates. The malolactic fermentation turns this sharp acid into the softer-tasting lactic acid (the gentler acid found in milk) and producing tell-tale bubbles of carbon dioxide as it does so. The result is that the wine becomes softer to taste, and at the same time, any wine undergoing this process, will gain mouth-feel and texture and may sometimes develop creamy buttery characters too.

Once upon a time, this process was rather random, depending on the lucky arrival of the right bacteria, plus the right conditions of temperature and levels of sulphur dioxide in the young wine. Nowadays, winemakers have a much better idea of how to control it and can buy packets of the right bacteria if necessary.

Which wines undergo malolactic fermentation?

Michael Brajkovich with his homemade 'ear trumpet' used for detecting when the malolactic fermentations start and finish.
Michael Brajkovich with his homemade 'ear trumpet' used for detecting when the malolactic fermentations start and finish.

Pretty much all red wine undergoes malolactic fermentation (it tastes really odd if it doesn't). However for white wines, winemakers can make a choice depending on the style of the wine they are making. For crisp fresh aromatic wines (sauvignon blanc for instance) usually there will be no malolactic, while rounder, richer creamier (and often oak-fermented) wines (chardonnay for example) may undergo full malolactic fermentation, or even a partial one, with the aim of combining freshness with texture and richness. The Society's Exhibition New Zealand Chardonnay from Kumeu River is a great example of barrel-fermented chardonnay undergoing complete malolactic fermentation (MLF); winemaker Michael Brajkovich explains what he likes about MLF:

'We use 100% MLF for all our chardonnay wines. The species of lactic acid bacteria that is used for just about everyone's MLF is Leuconostoc oenos, which has been renamed Oenococcus oenii. This species has a very good tolerance of low pH, and does not produce very much in the way of spoilage.

'If any Lactobacillus or Pediococcus species grow in your wine, then you have a real problem because of the high volatile acidity. Fortunately they normally will not tolerate low pH, or high SO2. One of the main by-products of MLF is diacetyl, which has a buttery smell. Diacetyl is very important to the flavour and aroma component in milk, butter, cheese and ice-cream. In wine, it can tend to dominate the aroma and mask other characters. These days we prefer to minimise the impact of diacetyl, and we do this by some simple strategies:

'Firstly, we wait for as long as possible after MLF is complete before we add any SO2. This is typically 2-4 weeks, which allows some of the diacetyl to disappear. Secondly, we keep the wine in contact with the yeast lees for as long as possible after the MLF. Yeast cells, even when dead, have a remarkable ability to consume diacetyl, so the longer on lees the less the buttery character.

Most of our chardonnays will have four to five months on lees post MLF, which really cleans them up beautifully, and the fruit can appear again from behind the veil of butter.

'The reason we use 100% MLF is to de-acidify the wine and make it more balanced. Our grapes are harvested at quite a high acid level, typically 8.5 – 9.5 g/l as tartaric. Even though we are a long way north in New Zealand, the proximity of the oceans to the west and the east really keeps the climate cool and the acids high.

Winemaking today involves a lot of science - but for the winemaker there is a real art too, in deciding exactly how to apply all the science to produce the best possible result, and make the most balanced and enjoyable wine.

Caroline Gilby MW

Guest Writer

Caroline Gilby MW

Caroline Gilby is a Master of Wine and a scientist by training. She is a wine writer with a passion for the wines of Central and Eastern Europe and contributes to several wine books, magazines and websites.

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