Inclement weather and lengthy fermentation time risks in natural yeast fermentations

I have previously characterized the risks associated with natural-yeast fermentations thusly: (i) stuck fermentations; (ii) yeasts washed off grapes during inclement weather; (iii) spoilage yeast contamination; (iv) lengthy fermentation times; and (v) persistence of negative characteristics.  I have begun a process of exploring these perceived risks in greater detail beginning with a post on spoilage yeast contamination and continuing with a subsequent post on the risks of sluggish and stuck fermentations.  In this post I will examine the risks associated with (i) inclement weather and (ii) lengthy fermentation times.

Yeasts and Inclement Weather

Regardless of the source (bird droppings, stoamch of bees, etc.), Saccharomyces yeasts are present in very small quantities on the grapes exiting the vineyard at harvest; according to Bisson and Butzke (Diagnosis and Rectification of Stuck and Sluggish Fermentations, American Journal of Enology and Viticulture 51(2), 2000), as low as 100 viable cell/ml.  With this low initial level of Saccharomyces, it is understandable that there would be some concern that rain could separate the grape from its precious cargo and, conceptually, leave the must laying around in the tanks pining for  a long lost suitor. There are two issues with this scenario.

First, if the rain is heavy and persistent enough, the greater risk is for the development of rot and the mold and bacteria which accompany it.  These molds and bacteria can make their way into the must if care is not exercised and proliferate during the lag phase with an associated wine-spoilage risk.  Secondly, there is a much greater yeast population in the winery than there ever was on the grape at any time during its residence in the vineyard. According to Bisson (Introduction to Wine Production, Viticulture and Enology, Section 3, Lecture 11, enologyaccess.org), yeast cell population in the winery is 102 cells/ml early in the vintage and 106 cells/ml late in the harvest as cells build up on the winery equipment.  These cells can more than make up for any cells washed off the grapes during a rainstorm.

Lengthy Fermentation Times

Longer fermentation times can result from (Bisson): (i) long lag before the onset of fermentation; (ii) normal start but a slowdown during fermentation; (iii) sluggishness throughout the process; and (iv) arrested fermentation.  Natural fermentations do have longer lag times because of the growth requirements placed on the yeast populations.  For example, the maximal yeast density during fermentation is 108 cells/ml while most inoculations are 106 cells/ml.  It requires seven generations (and 24 to 35 hours) to bridge the gap (Bisson).  To this we must add 12 to 24 hours for the yeasts to adjust to the must environment.  In the case of a natural ferment, the growth requirements are more intense.  To get from 100 cells/ml to 106 cells/ml will require 13 generations and to this must be added the time to maximal yeast population.  Even if the must is colonized by winery-resident yeasts, the growth curve is still steep.

It is true that lengthier fermentation times are associated with natural yeast fermentations but the overall lengthening of the wine production process might be viewed by the natural-yeast practitioner as an essential characteristic and requirement in realizing a more complex end product.


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