Grapes Microbes and Stuff



Professional winemakers always wash their grape receivers, crushers, etc. before any grapes are processed. The pros make sure everything is clean, but they seldom attempt to sterilize their crush equipment. On the other hand, the home winemaking literature is filled with statements such as "...first, assemble all the winemaking equipment and sterilize everything with a sulfite solution......" Have you ever wondered why the pros seem so indifferent about sterilizing their crush equipment?

On average, a ton of California wine grapes contains seventeen pounds of dirt, one mouse nest, 147 bees, 98 wasps, 1,014 earwigs, 1,833 ants, 10,899 leaf hoppers and seven pounds of bird droppings. Besides the above materials, the waxy coating on grapes contains many microorganisms. Grapes are crushed without washing, so crushed grapes contain many substances and microorganisms. Consequently, attempting to sterilize crush equipment seems a bit futile.

Grape juice is a hostile environment to most microbes because it has a low pH, a high sugar level and a high phenolic content. After fermentation, the alcohol content is also high, so wine is even less hospitable than juice. During fermentation, much of nitrogen (bird droppings) and protein materials (bugs) are consumed by the yeast. When fermentation is complete, the dirt, bee wings,earwig tails, etc. settle to the bottom of the fermenters, and when the wine is racked, much of the yeast and ugly stuff is left behind. Now, the new wine is becoming clean and pure.

Only a few yeasts and a few bacteria can grow in wine. No molds grow in wine. However, molds can grow in dilute wine solutions, so hoses, pumps and tanks must be washed and rinsed carefully to mold avoid contamination. The yeasts found in wine are primarily Saccharomyces (sugar loving), and these are the fermenting yeasts. Several popular strains of Saccharomyces cerevisiae include Montrachet, Epernay II, Pasteur Red, etc. The more alcohol tolerant yeasts, such as Prise de Mousse and Pasteur Champagne, are strains of Saccharomyces bayanus. Only a few other yeasts such as Schizo saccharomyces, Brettanomyces, Mycoderma and Flor yeast (film yeast) grow in wine, and these yeasts are generally considered wine spoilage organisms.

Wine bacteria are mostly limited to two major groups. Lactic acid bacteria belonging to the Lactobacillus, Leuconostoc and Pediococcus genera convert malic acid into lactic acid. Many of these lactic bacteria can also convert sugar directly into acetic acid. Fortunately for winemakers, the lactic bacteria are sensitive to sulfur dioxide, so they are relatively easy to control.

The second group of wine bacteria is the vinegar bacteria. These microbes convert ethyl alcohol into vinegar, and vinegar bacteria are one of the primary wine spoilage organisms. However, vinegar bacteria require oxygen to convert alcohol into vinegar. Consequently, wine is always stored in sealed containers, and the containers are always kept full to avoid vinegar formation.

The great French scientist Louis Pasteur observed that only a few types of microorganisms and no disease producing bacteria (pathogens) could grow in wine, and he wrote " is one of the most healthful and hygienic of beverages...." His comment may seem a bit paradoxical since wine starts as such a gross liquid. However, the action of the yeast and a few rackings make wine an unusually pure beverage.