Introduction to Wine Fermentation:
Common Fermentation Challenges and Technological Solutions
by Paige Jarvis
Table of Contents:
- Wine Fermentation
- Wine Fermentation Challenges
- Wine Fermentation Analysis
- Wine Fermentation Conclusion
Understanding the science behind wine fermentation begins in your tanks and is crucial to successful wine production. Winemaking has existed for centuries resulting in a variety of different types of winemaking styles and wine products. However, each bottle of wine has a common story. Wine is heavily influenced by both physical and biological factors during the winemaking process, and these outside players have a strong impact on the overall wine quality.
Winegrowers and winemakers are continuously changing their practices according to scientific knowledge and advances. In this guide, Introduction to Wine Fermentation, you will learn the basics of wine fermentation, common winemaking challenges, the fundamental role of yeast and how technological advances are currently transforming the market.
As we know, winemaking is categorized into five stages: harvesting, crushing/pressing, fermentation, clarification and aging/bottling.¹ During the growing and harvesting stage, the unharvested grape is subject to non-tolerant changes in weather patterns and other microorganisms in the environment.
Unhealthy microorganisms can cluster around the grape stomata, ultimately resulting in damaged grapes. Filamentous fungi can infest the surface of the grape. Mold, mildew damage, rot can enter grape cracks and holes. Even before the grape enters fermentation, the grape has already been subject to unhealthy microbes. The vineyard environment inherently can affect the overall output of the wine fermentation process.² Winemakers must be aware of these common environmental challenges and how they inhibit the next phase in the process; fermentation.
After the process of collecting and crushing the grapes, winemaking moves into a core stage; fermentation. During wine fermentation, the wine juice is specially paired with wine yeast to convert glucose and fructose to ethanol. Fermentation begins once the sugar is added to the yeast, which results in carbon dioxide, alcohol and elements of other compounds including esters.³
The fermentation process is an active display of the microbial ecosystem of grapes and wine. Without the interplay of multiple microorganisms, fermentation couldn’t happen, and we’d have no wine. Wine fermentation has two key stages: primary and secondary, or the aerobic and anaerobic processes.
In the first three to five days of fermentation, seventy percent of fermentation typically takes place. While alcohol is produced in the aerobic stage, the majority of yeast cells’ energy is focused on reproducing, multiplying up to one hundred to two hundred times their original number.
The anaerobic stage of wine production typically lasts one-to-two weeks depending on the wine profile. Yeast cell activity slows each day, with there being fewer nutrients and sugars for the cells to eat and yeast cells coming under increasing levels of alcohol stress. The fermentation process is finished when fifty-five percent of the sugar is converted into alcohol and the remaining forty-five percent into carbon dioxide gas.
These molecular microbiological changes are part of the overall inner microbial ecosystem contributing to wine production. Ultimately, the fermentation process begins in the fermentation tank, where alcoholic fermentation is carried out by the yeast, or the starter culture.
Yeast in Wine Fermentation
Yeast in alcoholic fermentation is a central player. Wine yeast is commonly unicellular (one cell) organisms found in nature. Some of the commonly found yeasts are Saccharomyces, Kloeckera, Hanseniaspora, Candida, Hansenula, Pichia, and brettanomyces. Natural fermentation yeast strains are unpredictable; thus, many winemakers choose to utilize yeast strains in the form of a ‘starter culture’ or an active dry wine yeast to conduct alcoholic fermentation.
A starter culture is the primary living microorganism used to speed the fermentation process. Starter cultures are commonly found in all types of fermented foods, and in wine fermentation, the starter cultures of Saccharomyces and non-Saccharomyces, S. cerevisiae and S. bayanus, are generally the primary microorganism of the fermentation process. Together, they contribute to the overall wine aroma and quality that winemakers and oenologists alike seek.
Pure yeast in the active dry form has become the most cost-effective and efficient yeast form for wineries.Where active dry yeast aid winemakers in fermentation control, the overall process still needs regular analysis of the active yeast culture. Overall, by monitoring the viability and concentration of the yeast cells, a winemaker can better fine-tune fermentation parameters, resulting in a better fermentation outcome.
Wine Fermentation Challenges
Fermentation is a complex ecosystem of multiple variables. With so much variability, it comes to no surprise of the challenges that arise during fermentation. Sluggish and stuck fermentations are still common problems in the industry, and these types of fermentations are difficult to restart running the risk of spoilage from oxidation and bacterial contamination.
When sluggish and stuck fermentation occurs, the fermentation process has stopped prematurely or the rate of fermentation has significantly slowed. The commercial risk results in selling over-sweet wine with inferior quality. The occurrence often leads to wineries disposing of spoiled wine batches resulting in bottom line profit struggles and unamused winemakers.
As a good practice, a winemaker should be well aware of common factors that lead to a sluggish and stuck fermentation. Awareness of possible problems enables better quality and successful fermentation. Common triggers⁵ of stuck or sluggish fermentation include⁷:
Nitrogen deficiency is a common enabler of slowed fermentation rate that limits yeast growth. Fermentation can also slow due to a decrease in oxygen, but the challenge is ratified by aerating musts to supply additional oxygen and eliminate the deficit.
Glucose/Fructose ratio or the high-concentrate of sugar in grape musts can inhibit yeast growth and slow fermentation.
Lack of temperature management leads to sluggish fermentation. At temperatures of 29 °C / 85 °F, the high temperatures inhibit the ethanol tolerance of yeast resulting in separating yeast growth patterns.
Nutritional imbalance is a common indicator of the lack of yeast viability. Well-balanced nutrition is key to assuring that the yeast is viable and optimized for the yeast aroma biosynthesis and release.
Yeast rehydration and handling are often mishandled. It’s key to a successful alcoholic fermentation as a pivotal phase in the survival and efficiency of the wine yeast. If done improperly, more than half of the viable yeast can die.
Healthy fermentation occurs from maintaining, analyzing and improving causes due to stuck or sluggish fermentation. Common culprits include an imbalance in nutrients, oxygen, pH, sugar, wild yeast, temperature, fructose, and cell population.⁶ For example, if the temperature is too cool, yeast cells are not able to activate and ferment sugars and will lie dormant within your batch.
Wine Fermentation Analysis
Fermentation is the fundamental process of turning grape juice into wine, and because of this, the process should be carefully monitored. Monitoring comes before, during, and after fermentation – and winemakers should measure and watch several key parameters. By carefully watching parameters, winemakers can greatly reduce the effects of common wine fermentation challenges. Common triggers of sluggish or stuck fermentation can be closely monitored and the quality effects reduced. In this guide, the fermentation maintenance of yeast cells is explored.
Fermentation Analysis of Yeast Cells
Winemakers carefully monitor the parameters of their yeast culture by conducting regular wine fermentation analysis. Two examples of the analysis involve watching cell concentration regularly and tracking overall parameters consistently during fermentation. For example, the fermentation stage of pitching analysis involves watching cell concentration. Consistent concentration is important to achieve similar quality production from wine batch to wine batch.
Furthermore, a sampling analysis is done during the fermentation proper and the end of fermentation. At each sampling test, consistent harvesting and re-pitching practices yield better fermentation reads and results over multiple samples. The purpose is to test the parameters of the fermentation control and carefully track yeast viability, the budding yeast cells, and yeast concentration.
A game-changing method for understanding what is happening during fermentation is counting yeast cells, which allows you to determine the concentration of yeast within your batch. Yeast concentration, again, is an important parameter to constantly watch. Traditionally, this portion of the fermentation analysis is conducted with a counting chamber (or a hemocytometer) to find the “pitching rate” (or initial cell density).
A hemocytometer is used for direct cell counting. In the process, a calibrated grid is placed over the culture chamber at which point the individual counts the number of cells per grid square by looking through a microscope.⁸ Because the process is done with a human touch, statistics dictate at least 20 grid squares must be counted and averaged.
As we know, a winemaker has to use a ‘pitching rate’ to maintain wine quality standards. Efficient fermentations are not possible without consistent and adequate pitch rates. The quantity of yeast has a direct effect to the finished product; from flavor and aroma profile to clarification.
The historic methods of measuring yeast viability and concentration are relatively bulky, tedious and expensive. Traditionally, the hemocytometer has been used to perform the fermentation analysis at consistent intervals (by hand) during testing. However, the traditional method isn’t feasible for modern-day winemakers. The pitching process is time-consuming, combined with the high costs of lab technician time, expensive equipment and requirement of multiple tests of the same capacity to reach an accurate outcome.
Automated Yeast Cell Counting
Automated yeast cell counting is a viable method to improve the counting process, counting accuracy, testing time and variability among samples and collect analytical data centrally. It provides a fast but also accurate and precise measurement of yeast cells.
Modern automated yeast counting involves the use of a microscope, optoelectronic image sensor chip, and other forms of advanced technology like a user interface to display to reconstruct the holographic shadows captured by the image sensor chip. The microscope automatically classifies live and dead cells in a yeast sample stained with methylene blue.
Wine Fermentation Conclusion
Fermentation is a complicated and fundamental phase of the winemaking process. For years, winemakers and ecologists have looked to perfect the process by understanding the common challenges that appear – and ratifying them with new scientific approaches. Yeast concentration or yeast counting is a commonly known wine quality analysis process that winemakers undergo. An often tedious and costly task, new advancements in yeast counting are changing the game by improving the process, accuracy, testing time and variability of the samples. When you incorporate modern technological advancements into wine production, your winery steps into the threshold of better wine quality and production cost reduction.
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1 The 5 Stages of the Wine Making Process. https://altovineyards.net/5-stages-wine-making-process/
2 David Mills. Wine Fermentation
3 Jeff Chorniak. Wine Fermentation 101. https://winemakermag.com/article/wine-fermentation
4 Wine Yeast. https://www.extension.iastate.edu/wine/wine-yeast
7Stuck Fermentations. https://www.winemak-in.com/en/dossiers/551/stuck-fermentations-causes-and-cures
5Kenneth C. Fugelsang. Wine Microbiology.
6 JR. Morris. Fermentations: Problems, Solutions and Prevention.
8 Hemocytometer. https://www.sciencedirect.com/topics/engineering/hemocytometer