MICROBES IN THE BOTTLE
Microbes to watch out for: Dekkera/
Brettanomyces, lactic acid bacteria; if
residual sugar is present, Saccharomyces
Danger signals: Change in color,
aroma, or flavor, CO2 production,
Monitoring methods: Sensory evaluation,
microscopic exam, culturing,
PCR-based genetic techniques.
To assess microbial stability before
bottling, there are four main questions
1. Does the wine have residual fermentable
sugar (over 0.5 g/L glucose
2. Does the wine have residual malic
acid if in pH range for MLF (this pH
3. Does the wine have Pediococcus?
4. Does the wine have Brettanomyces?
If the answer to ALL four questions
is no, the wine is likely to be microbially
stable even if not sterile-filtered
to remove all microbes. Surface film
yeasts can grow in the bottle for only a
few days, until dissolved oxygen is
depleted, and Acetobacter cannot grow
unless the closure is compromised.
1. Residual fermentable sugar
trumps everything else.
Most wine microbes will use residual
fermentable sugar if it is present.
Fructose can be an exception;
Saccharomyces is reluctant to use fructose
if the glucose/fructose ratio is less
than 1:6 to 1:10. Although it is risky to
count on it, and other yeasts and bacteria
can use fructose, Saccharomyces refermentation
is less likely in stuck wines
that have been depleted of glucose.
Traditionally, the lowest levels of
concern for Saccharomyces are 5 to 10
cells per BOTTLE (not per ml). Wines
with only 6 cells per bottle have fermented,
but other wines escaped
problems with 15 cells per bottle.
These are very low numbers, however,
and methods that do not detect
as few as 3 cells per bottle are not sensitive
Filtering 250 ml of wine for yeast
and 100 ml for bacteria, and culturing
the membranes, is the preferred
method for checking bottle sterility.
Genetic tests that require 10 cells/ml
for detection are not suitable even if an
entire bottle is filtered to concentrate
PCR tests can help monitor bottled
wines for microbes that are at high
enough levels, and
viable but non-culturable
(VBNC) microbes or