Climate change spares no domain, and the production of spirits, especially their aging in barrels, is no exception. The influence of climate variations on the organoleptic properties and stability of finished products is becoming a key concern for quality-conscious producers.
To understand the impacts of climate change, it is essential to revisit the origin of the compounds that form the very essence of spirits. Since only volatile compounds from the fermented matter remain in the distillate after distillation, this first article focuses on the origin and organoleptic impact of these compounds.The second article will address the importance of mastering distillation to produce a high-quality spirit.
Finally, in a third part, we will explore the physico-chemical changes that spirits undergo during barrel aging.
This three-step exploration will help us understand the effects of climate change on the composition of barrel-aged spirits and suggest possible solutions to preserve their quality.
Origin of volatil compounds in fermented products
Organoleptic impact of these compounds
Families of compounds derived from raw materials and fermentation
Upon exiting the distillation process, a spirit can contain several hundred compounds. These molecules belong to different families originating from raw materials (fruits, vegetables, plants) that undergo various enzymatic, chemical, and biochemical transformations. These transformations begin during the initial processing of the raw materials and continue throughout the fermentation process [1 – p 547].
Compounds intrinsic to Raw Materials
Whether it’s fruits (grape, apple, etc.), cereals (barley, corn, rice, sorghum, etc.), sugarcane, agave, or other plants (beetroot, potato, etc.), these raw materials contain compounds that play a key role in fermentation and in the development of volatile aromatic compounds:
Importance of the sanitary condition on the formation of undesirable volatile compounds
The sanitary condition of the raw material plays a crucial role in the formation of undesirable volatile compounds. The presence of mold can lead to the degradation of cell walls, creating damp areas. These areas release nutrients and produce enzymes, creating conditions that are favorable for the proliferation of bacteria, enzymatic reactions, or even attacks from certain bacteria or native yeasts [2]. The consequences for the production of undesirable volatile compounds are numerous. Here are a few examples of compounds formed due to these alterations, which, when carried through distillation, will have a major impact on the quality of the spirit:
- Acetic acid (vinagar odor)
- Ethanal (oxidized apple odor)
- Sulfur Compounds (including H₂S, rotten egg odor)
It is therefore important to ensure proper protection of the raw material upstream, with appropriate and carefully managed phytosanitary treatments, and to harvest under good conditions, using dry ice during transport if necessary.
As a last resort, sorting to eliminate as much raw material with compromised sanitary conditions as possible is essential to maximize the chances of a well-controlled fermentation.
Control of preliminary treatments before fermentation
In general, regardless of the raw material used, it must be as “clean” as possible to avoid any contamination that could produce undesirable flavors, such as those from soil or plant debris. These contaminations can also lead to uncontrolled spontaneous fermentations, thereby compromising control over the fermentation process.
Certain steps, such as the germination or malting of barley, which are necessary to release the sugars from plants, must also be carefully managed to prevent the formation of undesirable volatile compounds:
- Sulfur Compounds such as H₂S (rotten egg odor) or Thiol (burnt rubber or cabbage odors).
- Ethanol: Associated with green apple aromas, but in excess, it can impart a harsh alcohol or solvent-like taste.
- Hexanal: Has the scent of freshly cut grass or leaves. In small amounts, it can add complexity, but in excess, it gives an unpleasant musty or vegetal sensation.
- Guaiacol and Creosol: Often linked to smoky, tar, or burnt wood aromas. In small quantities, they can be desirable in certain peated whiskies, but an excessive concentration can result in overly aggressive and unbalanced flavors.
- Phenol: Can impart medicinal or antiseptic aromas, which, in excess, are considered unpleasant.
For fruits, the contact time of the juice with the fruit skin, vegetal parts, or pits should be as short as possible to avoid the formation of undesirable compounds [3 – p234] such as:
- Hexanol, with a grassy odor.
- Methanol, which has regulated levels due to its toxicity.
- Ethyl Carbamate, a carcinogenic compound that can form right after distillation due to certain precursors present in the pits (such as hydrocyanic acid).
Equipment maintenance
Thorough cleaning, including both washing and disinfecting equipment in contact with raw materials, is essential to limit contamination by microorganisms that can grow by using raw material residues as nutrients. Certain microorganisms, like yeasts, have the ability to enter a dormant state to survive unfavorable conditions.
A typical example of contamination is caused by yeasts of the genus Brettanomyces, which can proliferate on raw materials rich in polyphenols (such as red grapes or cider apples). These yeasts produce volatile compounds, particularly ethyl-phenols, responsible for unpleasant odors often described as barnyard smells. Brettanomyces yeast has a strong affinity for porous surfaces, especially wood, which is particularly difficult to disinfect thoroughly. In cider production, wooden presses equipped with burlap cloths are a known source of Brettanomyces contamination.
Control of alcoholic and bacterial fermentations to limit the production of undesirable volatile compounds
1- Alcoholic fermentation (AF)
This process is carried out by yeasts, with the primary goal of maximizing ethanol production. It primarily requires sugars, along with nitrogen. Stressed yeasts—due to suboptimal fermentation conditions (temperature, pH, nitrogen and amino nutrition, or insufficient aeration)—can generate various by-products, such as higher alcohols. This fermentation may be disrupted by bacterial or even yeast (e.g., Brettanomyces) contamination if fermentation conditions deteriorate, preventing complete sugar depletion.
In general, a rapid and complete fermentation is desired, achieved by inoculating with yeasts suited to the specific conditions of the raw material and climate, while maintaining a temperature suitable for the yeasts. Yeast suppliers can provide usage recommendations. Daily monitoring of the fermentation process (density and temperature) allows for timely intervention if there is a significant slowdown in the density decrease. Actions such as aeration, temperature adjustment (increase or decrease), nitrogen addition, or re-inoculation with more active yeasts can be taken as needed.
In certain cases, a longer fermentation period of several weeks may be sought to promote the development of aromatic compounds. In this scenario, the risk of bacterial deviation is high, making the quality of raw materials, as well as control, monitoring, and management of all fermentation parameters, even more critical.
2- Malolactic fermentation (MLF)
This secondary fermentation is carried out by lactic acid bacteria. Whether induced or spontaneous, it can only occur when the raw material contains malic acid, a compound found in many fruits (such as grapes and apples). This process converts malic acid into lactic acid, a milder acid, leading to a reduction in the product’s acidity. Although MLF often reduces the concentrations of so-called “primary” aromas, it may be sought after for certain specific spirit profiles, particularly those intended for extended aging in barrels (this aspect will be explored in the third article).
Among undesirable volatile by-products are aggressive compounds like acetic acid, ethyl acetate, and acrolein, whose organoleptic properties will be described in the following section. However, MLF can help reduce excess ethanal, a volatile compound that should be minimized in spirits [2][3].
If MLF is desired, it is essential to create conditions favorable to the development of lactic acid bacteria while minimizing the production of by-products from competing bacteria. For this purpose, a pH below 3.5, an alcoholic fermentation temperature between 18 and 22 °C, and a residual sugar level below 2 g/L are recommended.
It should be noted that polyphenols present in red grapes or certain varieties of cider apples can inhibit malolactic fermentation, as can an excess of fatty acids produced by yeasts [5 – Chapter 7 and Chapter 8].
Families of compounds obtained after fermentation and their main representatives
non-exhaustive list
Organoleptic impact of the main volatile compounds in Eaux-de-Vie and possible control of their production before distillation
This paragraph is dedicated to the primary volatile compounds that can be routinely analyzed in raw materials, whether through traditional analytical methods, micro-distillation techniques for tasting, or chromatographic techniques, with or without prior micro-distillation (see the final paragraph).
Presented in table format, it summarizes the origin and organoleptic impact in an eau-de-vie for each compound family, providing insights on controlling their production before distillation.
Maximum concentrations vary significantly depending on the type of spirit being produced. Therefore, it is essential to start by checking the regulations in the country where the product will be marketed, as well as any specifications of the appellation if applicable, and consult an expert to determine the appropriate concentration ranges to follow.
Legend : AF = Alocoholic Fermentation – MLA = Malolactic Fermentation
Quality control of fermented raw material
1- Control of the volatile compounds
As previously discussed, certain volatile compounds in fermented raw materials play a crucial role in the composition of future eaux-de-vie. Among these, the following compounds can be routinely analyzed: ethanal, ethyl acetate, higher alcohols, acrolein, ethyl butyrate, 1-butanol, 2-butanol, and ethyl lactate.
It is challenging to recommend specific concentrations of these compounds, as they depend on the type of eau-de-vie being produced and the type of distillation equipment used, as explained in the next article “Climate changes impact to quality and stability of spirits – Part 2” : Modification of Volatile Compound Composition During Distillation – Quality Criteria.”
However, as a guideline, here are some suggested maximum values for a product with an alcohol content of 10% vol.:
- Ethanal: < 50mg/L
- Ethyl acetate: < 80mg/l
- Superior alcohols: < 500mg/l
- Acroleine: absence
- Ethyl Butyrate, Butanol-1 et Butanol-2: < 5mg/l
- Ethyl Lactate: < 100mg/L
These standards may be much more restrictive for certain appellations, such as Cognac and Armagnac, or for eaux-de-vie that will be marketed as “white,” making them more sensitive to compounds that can alter the perception of aromatic components.
For analysis methods, refer to the last paragraph of this article.
2- Control of physicochemical classical parameters
It is also essential to consider certain classic parameters, such as alcohol content, residual sugars, pH, total SO₂, volatile acidity, and, if necessary, monitoring malolactic fermentation through malic acid and lactic acid analysis.
As a guideline, and for the majority of fermented raw materials, here are some recommended levels for these parameters:
- The higher the alcohol content, the less concentrated the distilled product will be in aromas. A recommended alcohol level is between 6% vol. and 10% vol. for better concentration.
- The more residual sugars, the lower the alcohol yield, which increases the risk of bacterial contamination before distillation. Additionally, the risk of sugar caramelization during distillation should not be overlooked, as it may cause a “burnt” taste associated with furfuryl compounds (furfural, 5-methylfurfural, or 5-hydroxymethylfurfural) and pyrazines, which can impart a yellow tint and an almond note to the distillate. The sugar level should be below 5 g/l.
- The lower the pH, the more stable the fermented product will be while awaiting distillation, allowing for better aroma production during distillation. Ideally, the pH should be below 4. For example, above pH 4, certain bacteria, such as Zymomonas, may produce ethanal in cider. This defect, known as “raspberry-like,” can render such products unsuitable for distillation.
- The lower the total SO₂, the less likely ethanal production and the formation of undesirable sulfur compounds will be. A maximum of 20 mg/l is recommended.
- The higher the volatile acidity, the sooner the product should be distilled. Generally, it is recommended not to exceed 0.6 g/l. Acetic acid, the main component of volatile acidity, can transform into ethyl acetate through esterification. If the distillation yield of acetic acid is low, the yield for ethyl acetate can be significant. If in doubt, monitor the rise in ethyl acetate levels during storage before distillation. A maximum of 50 mg/l is recommended.
- Malolactic fermentation is considered to have started when lactic acid exceeds 0.3 g/l and is complete when malic acid falls below 0.3 g/l. This process can begin and then stall if temperatures drop below 10-12°C. Caution is advised, as fermentation may resume if temperatures rise above 10-12°C.
3- Control through tasting
Tasting the fermented raw material will complement analytical control to highlight certain volatile compounds that are difficult or too costly to analyze, such as stable odors caused by certain volatile phenols.
However, do not hesitate to assess the impact of distillation on certain odors. While some may seem unpleasant in the fermented raw material, copper can act as a decontaminant during distillation. By comparing tastings of fermented raw materials with those of the distilled eaux-de-vie, you will learn to recognize undesirable odors.
Conservation of fermented raw materials before distillation – How to avoid using sulfur dioxide (SO₂)
To avoid deterioration of your fermented product and eliminate the need for SO₂, it is essential to start with the healthiest possible raw material, transported to the processing and fermentation site as quickly as possible. The use of inert gases, such as CO₂, nitrogen, or argon, can help limit oxidation during these transfers.
After fermentation, storage conditions should support optimal preservation, using impeccably clean containers, kept at a low temperature (<12°C), and without racking to prevent the loss of CO₂, a natural preservative against bacterial contamination. A reductive environment will be beneficial for good preservation. In a reductive environment, it is not uncommon to see bacteria develop that cause the “ropy wine” effect. This phenomenon is characterized by a viscous texture and oily appearance due to the production of polysaccharides by lactic bacteria. Generally, these bacterial colonies do not significantly contribute to the production of undesirable odors.
The analytical equipment necessary for quality control
1- Analyses of classic analytical parameters
These analyses can be performed in-house with appropriate standard equipment. Contact the company DUJARDIN-SALLERON which can study your needs and suggest equipment tailored to your activity and constraints, whether budgetary or related to personnel availability.
There is a method capable of simultaneously measuring numerous parameters in musts or fermented products: mid-infrared analysis (FTIR devices). Currently, these are limited in the types of matrices they can analyze. The investment is more costly than traditional analytical equipment, but considering their productivity, reproducibility, and precision, the cost-effectiveness is worth examining. For further information, contact the following companies: FOSS, DUJARDIN-SALLERON, ANTON-PAAR.
2- Analyses of volatile compounds
The method consists of gas chromatography analysis coupled with a flame ionization detector (GC-FID). If the matrix does not allow for direct injection analysis, a micro-distillation can be considered to analyze the micro-distillate.
The equipment is quite expensive and requires a solid understanding of this analytical method.
To determine when it is preferable to equip in-house or outsource these analyses, contact EC Consulting.
I would like to thank the experts for their contributions to the writing of this articles:
Ludwig VANNERON, Marc GIBOULOT
Bibliography
[1] “Volatile compounds in Foods and Beverages” – edited by Henk MAARSE – 1991
P 547 : “Distilled Beverages” Lalli NYKANEN and Irma NYKANEN – FERRARI G., ROULLAND C., 2009. “Qualité de la matière première : nouvelles avancées”. Journée Technique de la Station Viticole du BNIC, 10-09-2009
[2] “The microbial ecology of wine grape berries” – A. Barata, M. Malfeito-Ferreira, V. Loureiro – International Journal of Food Microbiology – Volume 153, Issue 3, 15 February 2012, Pages 243-259
https://www.sciencedirect.com/science/article/abs/pii/S0168160511006878
[3] “Elaboration et connaissance des spiritueux – Recherche de la Qualité, Tradition et Innovation” – Roger CANTAGREL – Edition BNIC – 1er Symposium international de Cognac du 11 au 15 mai 1992.
p234 : “La qualité des eaux-de-vie de fruits en relation avec celle de la matière première (prunes, pêches, abricots) ” – R. CAMPEANU, MARA JONESCU, VALERIA IONITA, I GAVRIILESCU – ICVV – ROUMANIE.
[4] Réglementation et série d’articles sur le Carbamate d’éthyle
– RECOMMANDATION (UE) 2016/22 DE LA COMMISSION du 7 janvier 2016, concernant la prévention et la réduction de la contamination des eaux-de-vie de fruits à noyaux et des eaux-de-vie de marc de fruits à noyaux par le carbamate d’éthyle.
https://eur-lex.europa.eu/legal-content/FR/TXT/PDF/?uri=CELEX:32016H0022&from=FRA
“Le carbamate d’éthyle dans les boissons alcoolisées et les vinaigres” – boissons avril 2018 au 31 mars 2019 – Gouvernement Canadien – Chimie alimentaire – Études ciblées – Rapport final
https://inspection.canada.ca/fr/salubrite-alimentaire-lindustrie/chimie-microbiologie-alimentaires/rapports-danalyse-articles-revues-sa/carbamate-dethyle- -alcoolisee
– “Assessment of Ethyl Carbamate Contamination in Cachaça” (Brazilian Sugar Cane Spirit) – Aline M. Bortoletto and André R. Alcarde – Academic Editor: Dimitrios Zabaras – 31 October 2016 – Beverages
https://www.researchgate.net/publication/309604429_Assessment_of_Ethyl_Carbamate_Contamination_in_Cachaca_Brazilian_Sugar_Cane_Spirit
– Dossier carbamate d’éthyle et spiritueux : “quelles solutions pour l’exportation du rhum et de la cachaça ?” – Jérôme SAVOYE – 30 Nov 2020 – My Spirit Factory
https://www.myspiritfactory.com/blog/articles/dossier-carbamate-d-ethyle-et-spiritueux-quelles-solutions-pour-l-exportation-du-rhum-et-de-la-cachaca
[5] Le Paysan vigneron – “ETHANAL ET FERMENTATION MALOLACTIQUE ” – 24/07/2012
https://lepaysanvigneron.com/des-avanc/
[6] ” Les eaux-de-vie traditionnelle d’origine viticole ” LAVOISIER – 27/07/2007 – Alain BERTRAND
– Chapitre 7 ” Influence de l’anhydride sulfureux et de la lie sur la qualité du Brandy distillé en chaudière “
– Chapitre 8 “Teneurs élevées en esters d’acides gras à longue chaîne dans les eaux-de-vie d’Armagnac ” – Eric HERVE, Marie-Claude SEGUR et Alain BERTRAND
[7] “Les composés d’arôme du rhum traditionnel blanc : nature, propriétés sensorielles et voies de formation” – Julie Coustel, Pierre Giampaoli, Martine Decloux – Ind. Alim. Agr. 124 (7/8) 20-29 (2007)
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