Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
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Summer 2011
(Dr. Sibylle Krieger-Weber, Personal communication)
average ethanol levels in table wines are increasing. Thus ethanol tolerance is an important consideration when selecting a bacteria strain. Ethanol affects the growth of the bacteria, but not malic degradation. Malic degradation, however, will not begin until the bacteria cell population reaches 1x106 cells/mL. Inoculation rates must be respected for a rapid start.
Addition of sulfur dioxide is an integral part of current winemaking practices. O. oeni is more sensitive to the anti-microbial effects of SO2 than other lactic acid bacteria (Lactobacillus spp. and Pediococcus spp) or yeast. It is not always recognized that S. cerevisiae can produce significant amounts of SO2 on its own. This was initially discovered by Fornachon (1968).
The amount of SO2 produced is highly dependent upon the strain of yeast and composition of the medium. Yeast strains are classed as low, medium, or high SO2 producers. The amount of SO2 produced can range from ~20 ppm to in excess of 90 ppm. It is essential to know the SO2 levels (both free and total) in a wine.
Fatty acids can be produced by yeast. Their production is dependent upon the must composition, grape cultivar, yeast strain, fermentation temperature, and winemaking practices. Short and medium chain fatty acids (hexanoic, octanoic, and decanoic acid) are reported to inhibit bacteria and yeast. These acids can target and alter the bacterial membrane, resulting in limited growth and reduced ability of the bacteria to degrade the malic acid (Table I).
It has also been demonstrated that fatty acids work in synergy with ethanol and pH, resulting in even more challenging conditions. On a positive note, it is possible to remove fatty acids via adsorption by yeast ghosts (hulls).
A number of yeast-derived metabolites can exert an inhibitory effect on ML bacteria. Some can be measured (ethanol and SO2), while others are more challenging to assess (fatty acids). Not all malolactic fermentation inhibition can be attributed to the production of yeastbased metabolites.
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The topic of malolactic fermentation always leads to lengthy conversations due to the different formulations available (direct addition, 1-step preparations, or standard build-up cultures), different inoculation protocols, and yeast strain compatibility with ML bacteria.
The winemaker must meet the challenges faced by O. oeni. Such challenges include when the ethanol level increases above 14%, pH values are below 3.2, free sulfur dioxide levels are above 10 ppm, total sulfur dioxide levels are above 30 ppm, or the temperature is below 16C.2
The conditions for success, although well documented and understood by winemakers, can still result in problems. These parameters must be viewed in combination (such as SO2 and pH) and not individually (SO2 or pH).
A recent focus has been on the isolation of strains with wider environmental limits. It is now possible to successfully achieve MLF in wines in excess of 16% alcohol. In addition to attempting to predict the success of MLF using the aforementioned parameters (alcohol, pH, SO2 concentration, temperature), we must also consider the initial malic acid level, the polyphenolics content, and the organism(s) that may be present and their metabolites.
Enological organisms and their relationships
There are currently approximately 150 diverse enological strains of Saccharomyces cerevisiae that have been isolated from the natural environment for a winemaker to choose from, plus 30 strains of Oenococcus oeni. If we extrapolate the number of combinations that can exist we would have 4,500. With so many combinations of yeast and bacteria that may be present (excluding native organisms), how can we predict the relationship between organisms and the resultant outcome of such relationships in our wines?
The relationships known to exist between S. cerevisiae and O. oeni include amensalism, predation, commensalism, mutualism, and protocol-operation.2 Put more simply, the relationships can be described as inhibitory, stimulatory, or neutral.
Inhibition of O. oeni by wine strains of S. cerevisiae
Ethanol was the first yeast metabolite reported as being antagonistic toward O. oeni. It is generally accepted that average ethanol levels in table wines are increasing. Thus ethanol tolerance is an important consideration when selecting a bacteria strain.