As a distiller or someone planning a new distillery start-up, understanding the nuances of fermenting grapes is crucial for crafting exceptional brandy.
Even if you are not doing the process from field to bottle yourself – it pays to understand how wine makers operate and the kind of decisions that will influence what you have to work with.
This article delves into the essential aspects of this process, from selecting the right grapes to monitoring fermentation dynamics. By comprehending these them, you’ll be well-equipped to control the quality and flavour of your brandy.
Assessing and preparing grape juice (Must)
Brandy production is an art that begins long before distillation. The grape varieties, terroir, and fermentation conditions collectively contribute to the unique taste profile of each brandy. This means that the growing, harvest and fermentation phases are as crucial as the distillation itself.
Unlike neutral spirits, where fermentation serves mainly to produce alcohol, in brandy production it’s also about cultivating a broad set of flavours and aromas.
The first step in fermentation is the preparation of grape juice, also known as ‘must’. This involves assessing the juice for its sugar concentration, acidity, and nutritional content.
The sugar levels, measured in Brix, determine the potential alcohol content of the brandy, while the acidity (measure via its pH) impacts the fermentation dynamics and the final taste.
The ideal combination of the two – high sugar concentration and low pH – are unique to grape fermentation. This presents both challenges and opportunities for flavour development.
The first of those challenges is in the selection and preparation of the grapes. The choice of grape variety, harvesting at the correct ripeness, and gentle pressing are all critical to obtaining high-quality must. Moreover, what’s ideal for a distiller tends to be different to what is ideal for those making table wine.
Brix (sugar concentration):
In brandy production, the starting gravity of the grape juice (must) can easily reach 1.070 to 1.080 or higher, which corresponds to 19° Brix or higher.
This is significantly higher than the starting gravity for most whisky styles, which typically do not exceed 1.065-1.070 (16-17° Brix).
The high sugar concentration increases osmotic pressure on the yeast, potentially leading to greater ester production during fermentation.
pH levels
The starting pH of grape juice for brandy is often well under 4.0, (some are as low as 3.3). Compared to grain washes for whiskey, which typically start at around 5.2-5.8 and drop to about 4.0 as fermentation progresses, it’s far more acidic
This lower pH in grape juice helps to keep unwanted microorganisms at bay but adds stress to the fermentative yeasts.
The pH level is a critical factor to monitor as it influences yeast health and the overall fermentation process.
Fermentation times
The fermentation process for brandy can take considerably longer than for grain or molasses fermentations.
While grain fermentations for whiskey may complete within 48 to 72 hours, brandy fermentations can take weeks.
While this article is geared towards grape based brandy, In the case of traditional Calvados production, a French apple brandy from Normandy, fermentation can extend up to six months!
Nutrient management: The backbone of fermenting grapes
Both the health and activity of yeast needs to be monitored. Because of this, nutrient management is a critical tool in the distiller’s arsenal for crafting a brandy with desired characteristics.
Yeast Assimilable Nitrogen (YAN) levels are particularly important. Adequate YAN ensures a steady and complete fermentation, preventing ‘stuck’ fermentations and / or off-flavours developing. The ideal YAN range is typically between 150-200 mg/L.
Commonly, di-ammonium phosphate (DAP) is used to augment nitrogen levels where needed. However, DAP alone isn’t always enough. Yeast also requires other nutrients such as vitamins, minerals, and fatty acids for optimal growth and fermentation.
These can be supplemented through various nutrient preparations, including yeast hulls, which provide additional benefits like helping to adsorb undesirable compounds.
Balancing these nutrients is not just about ensuring fermentation completion and fulfilling the maximum potential yield. It’s also about influencing the final flavour profile. Nutrient levels can affect yeast metabolism, thereby impacting both the types and quantities of other compounds produced.
Selecting the right yeast for fermenting grapes
Just as it is with other spirit, the choice of yeast is a defining factor in brandy production.
Distillers must decide between pitching commercial yeast strains and / or allowing natural fermentation with wild yeasts present on the grapes. Each approach has its merits (which is why some chose both and stage their fermentation in a sequential manner). We explain more on this in our general article about Fermentation.
Commercial yeast strains
Brandy distillers often select commercial yeast strains that are specifically developed for fermenting grapes ahead of spirits production. An obvious advantage of off the peg commercial yeasts is their consistency and predictability in fermentation. They’ve been tested, there are guides for how to use them.
These strains vary in their characteristics and will be marketed and labeled with that in mind.
Some produce more terpene alcohols which add aromatic complexity, others are proficient in producing esters for fruity and floral notes. Meanwhile, some are better suited for high alcohol environments and maximising yield.
Natural (Wild) yeast strains
An alternative to pitching commercial yeasts is allowing the natural wild yeasts present on grapes to conduct the fermentation. The spontaneous fermentation method can yield a wide range of flavours due to the diversity of yeast species on grape skins.
Important species include:
Metschnikowia: Known for producing fruity aromas, although not capable of fermenting high sugar levels.
Torulaspora delbrueckii: Contributes to pleasant aromatic profiles and helps reduce acetic acid and acetaldehyde levels.
Brettanomyces: A genus with a controversial reputation, known for producing smoky and sometimes medicinal aromas.
Saccharomyces spp: Including Saccharomyces cerevisiae, these are the primary consumers of juice sugars and responsible for most of the alcohol production.
Using isolated wild yeasts is a modern approach in wine fermentation.
In essence, specialist yeast producers take the most desirable wild yeasts from a specific location, isolate them and turn them into a powder. This ensures distilleries can pitch in the same dose and strains each time.
This method offers the benefits of mixed organism wild fermentation while minimising the risk of undesirable yeasts and bacteria. Moreover, because what’s been isolated is regionally specific and the process is custom for each producer, the mix is easily something that can be safeguarded as a proprietary blend.
As we mentioned above, it’s also possible to stage the ferment in a sequence and use the best of both worlds. Some producers allow wild yeast grows first, and once the fermentation process is underway, inoculate (pitch) in commercial strains to develop it from there. Wild yeast starts it off naturally, adding flavour and character, commercial yeast that is more tolerant to high alcohol environments finishes it off.
There’s no right or wrong approach, The focus should be on what is replicable, and consistently able to produce high quality results.
Managing the fermentation process
Fermentation management is a delicate balancing act. Yeast is typically added at a rate of about 2.5 million cells per mL of must, but this can vary based on the specific conditions of the fermentation.
As with other raw materials, temperature control is important when fermenting grapes; too high and you risk killing the yeast or producing unwanted flavours, too low and the fermentation might be sluggish or incomplete.
The fermentation time for brandy can vary widely as a result.
While some fermentations complete in about two weeks, others might take over a month. It depends on the grape, the region’s climate, the yeast and the dose added. Throughout this time, the distiller must monitor the process, ensuring the yeast remains healthy and active, and adjusting conditions as necessary to achieve the desired outcome.
Malolactic fermentation (MLF) is a secondary fermentation process that can significantly influence the flavour of brandy.
It involves the conversion of malic acid, which is harsh and tart, into lactic acid, which is softer and creamier. This process softens the acidity of the brandy but also adds complexity to the flavour profile.
The prevailing view among Cognac producers is that distilling wine before or after malolactic fermentation is generally acceptable. It just depends on what a producer is seeking from their distillate. That said, distillation should not occur while malolactic fermentation is actively underway.
As to how it happens – distillers have the option to initiate MLF using specific bacteria cultures or allow it to occur naturally.
The decision depends on the desired flavour profile and the level of control sought in the fermentation process. For instance, controlled MLF using selected bacterial strains can provide consistency, while natural MLF can introduce unique, location-specific characteristics to the brandy.
Is malo’ good or bad for Brandy makers?
The arguments made by distillers who prefer post-malolactic distillation, is that while acidity decreases during this process, the malic acid acts similarly to a sugar in the must, prompting a secondary fermentation. Once this is complete, the wine tends to be more stable, with a lower risk of subsequent yeast or bacterial activity.
Furthermore, some distillers regard malolactic fermentation as beneficial when the wine exhibits the flaw of containing ethanal. It’s possible for malolactic fermentation to consume up to two thirds of the ethanal present.
On the other hand, there are those who (like, Rémy Martin for example) opt to distil wine before malolactic fermentation. They believe this approach better aligns with producing an aromatic, full-bodied, and intense eau de vie.
For those looking inhibit malolactic fermentation, it’s advisable to chill wine tanks to 10 degrees Celsius post-fermentation. Conduct taste tests to determine which tanks are the most delicate and which possess the greatest potential for aromatic compounds. Based on that, prioritise the tank’s order to go through to distillation.
Time management post-fermentation: Contending with spoilage organisms
Once fermentation is complete in brandy production, time becomes a critical factor. This urgency is primarily due to the distiller’s approach to sulphur addition, which differs significantly from typical winemaking practices.
Winemakers commonly add sulphur dioxide (SO2) as an anti-microbial agent to prevent spoilage. However – In the context of distilling, the addition of sulphur is actively avoided.
We’ve written an article about it separately here delving into all the details – Sulphur Removal
Not having it in however, means that the presence of spoilage organisms like Acetobacter (which can turn alcohol into vinegar) and Brettanomyces (which can produce off-flavours) is a significant concern in grape fermentation.
These organisms can quickly ruin a batch if not properly managed. To prevent spoilage, strict sanitary practices are essential.
This includes thoroughly cleaning and sanitising all equipment, controlling the fermentation environment to discourage unwanted microbial growth, and monitoring the fermentation process closely. Sometimes, interventions like adjusting the pH or alcohol levels may be necessary to create conditions unfavourable for these spoilage organisms.
Most common spoilage organisms watch out for –
Acetobacter: Produces acetic acid and ethyl acetate, leading to vinegar-like aromas.
Kloeckera apiculata: Can produce significant amounts of ethyl acetate.
Brettanomyces spp.: Known for contributing various aromas, though less problematic in a distillery setting.
Lactic Acid Bacteria: Includes strains that can cause off-flavours like diacetyl and excessive ethyl acetate.
Final considerations
Making a liquid prepped for distillation is a complex interplay of science and art. It’s not just fermented grape juice!
From selecting the right yeast and managing nutrients to controlling fermentation conditions and dealing with potential spoilage, each step requires careful consideration and expertise.
Just like you can’t make great whisky without first making a great mash, you can’t make great brandy without first making great distilling wine.
Quality control in grape fermentation encompasses various factors. Continuous monitoring of fermentation parameters like sugar levels, pH, and temperature is crucial. Adjustments will need to be made throughout the process to ensure a consistent and high-quality product.
By mastering these elements, distillers set the foundation for producing brandies that are not only high in quality but also rich in character and tradition.
