Yes, grape juice can ferment without the addition of commercial yeast, but it cannot ferment without the action of microorganisms. Fermentation is a biological process where sugars are chemically converted into other compounds, most commonly ethanol (alcohol) and carbon dioxide, by microbes. When grape juice is left to sit, “spontaneous fermentation” begins, driven by the natural population of microbes present on the grapes and in the environment. This natural process was the original method used for thousands of years to create wine, contrasting with the modern technique of inoculating sterile juice with a specific, cultured strain of yeast.
The Basic Requirements for Fermentation
Fermentation requires three fundamental components: a sugar source, water, and an agent to process the sugar. Grape juice naturally provides the first two, containing glucose and fructose dissolved in water, making it a highly hospitable medium for microbial growth. The initial stage of alcoholic fermentation begins when microorganisms start to consume these sugars.
Environmental conditions surrounding the juice determine the speed and success of the process. Temperature is a primary factor, as most fermenting microorganisms thrive within a moderate range, typically between 62°F and 80°F (17°C and 27°C). The process operates under anaerobic conditions, meaning that once dissolved oxygen is consumed, the conversion of sugar to alcohol continues without the presence of air.
The Role of Wild Microbes
The agents that cause spontaneous fermentation are various microorganisms collectively referred to as “wild yeast” and bacteria. These microbes reside on the surface of grape skins, particularly in the waxy layer known as the bloom, and are also present in the winery environment. Once the grapes are crushed, these organisms are introduced directly into the nutrient-rich juice, or must.
The primary organism responsible for converting sugar into alcohol is the yeast Saccharomyces cerevisiae, which often becomes the dominant species. However, the initial phases are often dominated by non-Saccharomyces strains like Hanseniaspora or Metschnikowia, which contribute unique flavor compounds. Commercial winemakers often prefer adding a cultured yeast strain because it provides consistency and reliability, ensuring the fermentation finishes without stopping prematurely.
Understanding Spontaneous Reactions
Not all spontaneous microbial activity in grape juice leads to the production of alcohol; other reactions can occur, often resulting in spoilage. Uncontrolled fermentation is characterized by a microbial succession, where different organisms thrive at various stages of the process. If the primary yeast fails to dominate, or if oxygen is introduced later, bacteria can take over.
One common spontaneous reaction involves acetic acid bacteria, such as Acetobacter and Gluconobacter species, which convert alcohol and residual sugars into acetic acid, commonly known as vinegar. These bacteria thrive in the presence of oxygen, making them a risk if the liquid is exposed to air. Lactic acid bacteria (Lactobacillus or Oenococcus) can also impact the flavor profile, sometimes causing a secondary fermentation that converts sharp malic acid into milder lactic acid, or producing off-flavors.
Controlling or Preventing Fermentation
Controlling the microbial environment is paramount for managing grape juice, whether to make wine or keep it fresh. To stop or prevent spontaneous fermentation, the goal is to inhibit or eliminate the active microorganisms. Refrigeration is a simple, effective method, as cold temperatures significantly slow or stop the metabolic activity of yeast and bacteria.
Pasteurization, which involves briefly heating the juice to a temperature sufficient to kill the microbes, is another common technique for long-term preservation. For those intending to make wine later, adding sulfur dioxide (sulfite) can temporarily suppress the wild yeast and bacteria present in the juice. This chemical control allows the user to later introduce a specific, cultured yeast strain, ensuring a more predictable and clean alcoholic fermentation.