Destemming and whole bunch fermentation explained

Friday, 1 May, 2020
Natasha Jacka
Natasha Jacka, a final year Oenology student at Elsenburg, writes about destemming and whole bunch fermentation in winemaking.

Natasha Jacka is a final year Oenology student at Elsenburg College. She completed the Chef's Course at Silwood Cookery school, where she graduated with the Top Student award. She spent some years working in fine-dining restaurants and developed a fascination with wine. After working as a cellar intern at Cape Point Vineyards in 2017, she enrolled at Elsenburg. She is passionate about biodynamic farming and would like to make wine using sustainable methods.

Natasha wrote an insightful essay on the topic of 'destemming and whole bunch fermentation' for her course. Below is a summary of her essay and the PDF to the full essay.

Destemming can be defined as the process where the stalks are removed from the whole berry clusters. In the past, wines were made without stem removal or crushing processes. While today, the majority of red grapes are destemmed, increasingly there are winemakers who are opting to use stems during the winemaking process. There are several factors which impact the decision to destem.

Stem removal will prevent the development of green ‘cut grass’ or ‘herbal’ aromas in the wine as these are characters that can be attributed to the use of stems. Wines produced from grapes which have been destemmed have lower tannin concentrations. In instances where stems are removed, the wines may be described as having ‘dark red’ and ‘purple fruit’. Visually wines without stems have a far better colour intensity as the stems are responsible for removing a small amount of colour. In cooler regions the stems are more likely to remain unripe or take a longer time to ripen. Also cultivars with ‘greenness’ will have higher levels of methoxypyrazines in cooler areas. Greenness is one problem that is most associated with stems.

Even though the use of stems is increasing in popularity it will never be used in the region of Bordeaux. Cabernet Sauvignon, Cabernet Franc and Merlot all have varying levels of methoxypyrazines, giving the wines a green character especially if the grapes are picked too early. The use of stems on these cultivars could risk exaggerating green flavours already present.Therefore destemming in Bordeaux has become standard procedure.

Destemming may be beneficial to limit the uptake of phenolics in certain cultivars, such as Cabernet Sauvignon, where the phenolic content of the wine will already be fairly high due to the large skin to juice ratio. In California, producers avoid the use of stems with varieties such as Cabernet Sauvignon and Zinfandel. There is sufficient tannins present in these varieties and the stems are not needed to add additional tannin or increase the phenolic concentration.

In most of the regions in Chile, destemming grapes is common practice. This is especially prevalent in areas where mostly Cabernet Sauvignon and Merlot are cultivated. Powerful and robust wines with high tannin concentrations are produced. To produce such a powerful wine, there must be fairly high extraction taking place during fermentation. This will increase the risk of extracting astringent tannins from the stems and undesirable vegetal characters. For these same reasons, the majority regions of Argentina will destem their grapes. Powerful, tannic wines are produced predominantly from the Malbec grape.

Stems contain water and are rich in potassium. Destemming prevents the addition of water from the stems which will dilute the alcohol of the wine and it will prevent potassium from being secreted which would otherwise lower the acidity.

Destemming results in a wine with a lower tannin concentration as the stems release significant concentrations of phenolic compounds and tannins into the wine during fermentation and pressing.

Destemming will result in wines with greater colour intensity. In the Douro Valley of Portugal the grapes are destemmed followed by crushing in the production of Port. The destemming and crushing of the grapes allows for the rapid development of colour. It is crucial that no colour is lost because the fermentation process is shortened as spirits is added to halt the fermentation once a specific sugar concentration has been reached. Also because the fermentation has been shortened, extraction in the form of punch downs must happen extremely regularly to extract sufficient tannin and colour before the fermentation is stopped. This will result in green, harsh and astringent characters being present in the wine if stems were used.

In conclusion, the decision to destem is determined by several factors. These include: climatic conditions, cultivar and wine style produced.

Whole bunch

The process of whole bunch fermentation is a traditional method of red wine fermentation where the grape berries do not undergo destemming. Whole clusters are used for fermentation. This method generally involves some slight carbonic maceration especially in the early stages of fermentation. The popularity for using whole bunch fermentation has increased and winemakers may use either a certain percentage of stems or 100% whole bunch fermentation. The trend currently being pursued by winemakers is to create fresh, elegant and complex wines. This is accompanied by the preference for neutral oak, concrete tanks and amphorae.

If 100% of the grapes are whole bunches there should still be some juice in the bottom of the fermenter. As this starts to ferment the released carbon dioxide will protect the remaining fruit from oxidation until fermentation fully begins. The most common way to incorporate a percentage of whole bunches in a fermentation is to place the whole bunches in the bottom of the fermenter and place the crushed and destemmed berries on top. As the fermentation progresses an increasing amount of berries will be crushed as punch downs commence. It is crucial to determine the maturity level of the stems to be used. During ripening there is a transition of unripe, green stems to ripe, woody stems and finally to overripe brittle stems.

The chemical composition of the wine as well as the flavour profile will be altered due to the usage of stems. Whole bunch fermentation is most common in Burgundy and proves very successful on the cultivar Pinot Noir. Wines made from Pinot Noir usually have low tannin concentrations however this can be increased through the use of stems as tannin is leached from the stems into the must. This will increase the structure and ageing potential of the wine. In cooler vintages when acids are high the use of stems slightly lowers the wines acidity. Potassium present in the stems is leached into the must during fermentation. It combines with tartaric acid present and then precipitates acid out of the wine. The stems also decrease the negative effects of fungus such as Botrytis cinerea which may be present on grapes, especially in cooler regions. The presence of stems results in the oxidation enzyme laccase, produced by Botrytis cinerea, to be inhibited. This ultimately results in wines less flawed by rot.

The technique of whole bunch fermentation is used by traditionalists in Northern and Southern Rhône to make elegant Syrah’s. The tannin extracted from stems is finer, chalkier tannin provided that over extraction does not occur. For this reason whole bunch fermentation is also favoured by producers in Swartland, South Africa. Due to the extremely warm climatic conditions of the Swartland, the fruit ripens quickly. It can therefore be extremely beneficial to retain the stems during fermentation because they will add freshness and complexity to the wine. The alcohol concentration will be lowered as the stems leach water.

In conclusion, ripe stems are extremely desirable as they will impart structure and complexity without imparting undesirable green characters. The climate of the vineyard determines how the stems ripen and therefore determines whether stems should be used or if grapes should be destemmed.

Multiple factors are crucial when determining the percentage of whole bunch to use. This includes: the ripeness of the grapes, the degree of lignification of the stems and the presence of rot on the grapes. If the grapes are extremely ripe a greater percentage of whole bunch should be used as the stems will add freshness and vibrancy to the wine. If the stems are unripe a lower percentage of whole bunch should be used to prevent green characters from being imparted into the wine. Whole bunch is extremely successful on cultivars such as Pinot Noir and Syrah. It increases the tannins content and structure of Pinot Noir and adds complexity, vibrancy and elegance to wines made using this winemaking technique.

For the full essay in pdf format, click HERE.

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Natasha Jacka, final year Oenology student
Natasha Jacka, final year Oenology student

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