Diagnostic Key Normal Start: Abrupt Arrest
Normal Start: Abrupt Arrest
1. Did temperature fluctuation occur?
Solution: abrupt arrests of fermentation are not normal and indicate that a situation arose quickly to which the yeast could not adapt. Abrupt arrests are often found with temperature shift at high ethanol concentrations. Adaptation to ethanol forces changes in the plasma membrane that limit temperature tolerance. Ethanol tolerance also demands a constant supply of energy in the form of ATP to remove protons from the cytoplasm. A temperature shock can irreversibly damage the cells and the arrest fermentation immediately upon receiving the shock. Even if the temperature shock is in the permissive range that the cells can tolerate if other stressors are present they simply may not have enough resources to deal with multiple inhibitory situations simultaneously. Adaptation is just not possible. If this has occurred then the fermenting wine should be removed from the yeast lees and reinoculated with an adapted inoculant.
2. Were any additions or compositional adjustments made?
Solution: abrupt arrest can also accompany modification of the fermenting chemical environment. Additions of sulfite to a culture that is struggling to complete fermentation may lead to arrest of the fermentation. If acetaldehyde becomes sequestered by SO2 and no other metabolic mechanism is available for the regeneration of oxidized co-factors metabolism may simply arrest. Acid adjustments late in fermentation can also lead to arrest of the culture depending upon medium pH. In this case the addition is enhancing proton flux into the cells that may exceed the efflux capacity of the cell. Blending a fermentation that has not completed with one that is at a different stage may also occasionally lead to arrest of fermentation depending upon how different the ferments are compositionally. Sometimes such changes are made in anticipation of stimulating the subsequent malolactic fermentation. Problems usually arise when the yeast is struggling to maintain tolerance to ethanol. If the fermentation is robust the yeast will tolerate disruptions well. If a compositional adjustment has resulted in an abrupt arrest the fermentation should be reinoculated as the abruptness of the arrest indicates generally a loss of culture viability.
3. What was the pH?
Solution: yeast will lower the pH during normal growth and metabolism and are usually fine at typical wine pH values. If the pH drops below 2.8 the yeast will cease growth and metabolism and will lose viability. If low pH fermentations are desired, it is important to mitigate any other types of stresses that will occur such as nutrient limitations or temperature extremes. If the pH has dropped to an inhibitory level the pH should be increased and a new adapted inoculant used to restart the fermentation.
4. Are other microbes present?
Solution: the introduction of large population numbers of other organisms can inhibit fermentation progression. This inhibition can be abrupt if the organisms and their inhibitory end products are introduced in high concentrations. ML starters prepared in the winery are often used at a high inoculation rate and can contain wild lactobacilli that have produced high concentrations of inhibitory organic acids that are not diluted below inhibitory concentrations when added. For an abrupt arrest to occur, the change needs to be dramatic. The slow build up of a competing microbial population will lead to a gradually slower arrest but not an abrupt one. If other microbes were added and caused the arrest reinoculation with a bacterial-tolerant yeast strain should be considered. However if inhibitory concentrations of organic acids are present, any new inoculant will also be inhibited. Yeast ghost addition may be effective in stripping inhibitory compounds, but it is best to always evaluate the microbial composition of ML starters to make sure they are actually dominated by Oenococcus and not an inhibitory ML bacterium.