Assessments of Microbial Activity in Wine

Linda F. BissonDepartment of Viticulture and Enology,

UCDDecember 13, 2013

Post-Fermentation Microbial Activity• Turbidity• CO2 Production

• Film Production• Polysaccharide Production• Positive Character Production• Negative Character Production

– Odor– Taste– Color– Sediment– Haze

Mechanisms to Assess Post-Fermentation Microbial Presence

• Microscopically• Appearance of Wine

– Turbidity– Effervescence– Films

• Analysis of Metabolic Activity

The Types of Impacts of Microbial Metabolites

• Positive character synthesis• Negative character synthesis• Modify existing character• Enhance or amplify perception of existing characters• Diminish or buffer perception of existing characters• Matrix effects: change in chemical properties

Positive Contributions to Aroma

• Direct synthesis• Release of bound molecules• Autolysis factors• Indirect (chemical) effects• Matrix effects

Types of Microbial Transformations of Flavor Compounds

• Primary roles– Production of flavor compounds de novo from

nutrients– Liberation of grape flavor components from

precursors• Secondary roles

– Provide chemical reactants – Enzymatic modification of grape/oak flavors– Impact Redox status and buffering capacity

Microbial Components Impacting Wine Flavor

• Metabolites• Enzymes• Catalysts• Mannoproteins and Polysaccharides

Direct Synthesis

• Esters• Alcohols• Aldehydes• S-volatiles• Acids

Release of Bound Molecules

• Thiols in Sauvignon blanc– Cysteine conjugates– Carbon sulfur lyase activity

• Terpenes– Β-Glucosidase activity

• Norisoprenoids– Β-Glucosidase activity

Autolysis Factors

• Release of active enzymes: hydrolases• Release of cellular macromolecular

components• Breakdown of cellular macromolecular

components

Indirect (Chemical) Effects

• Creation of reactive molecules– S-containing compounds– Diacetyl– Aldehydes

• Alteration of pH• Alteration of Redox status of juice

Matrix Effects

• Alteration of solution chemistry• Production of flavor enhancers

– Dimethyl sulfide

• Production of masking factors– Fusel family– Hydrogen sulfide

Post-Fermentation Microbiota

• ML Fermentation• Surface films• Brettanomyces spoilage• Yeast spoilage

Monitoring Lactic Acid Bacterial Activity in Wine

• Measure decrease in malate levels• Assess change in pH• Note presence of effervescence• Follow increase in wine turbidity• Evaluate aroma profile

Monitoring Lactic Acid Bacterial Activity in Wine

• Measure decrease in malate levels– Will tell you when it is done

• Assess change in pH– Something is happening, will not tell you it is complete

• Note presence of effervescence– Something is producing CO2

• Follow increase in suspended wine turbidity– Something is growing in the wine

• Evaluate aroma profile– Characteristic compounds of lactic acid bacteria

• Diacetyl• Acetic acid• Off-characters

ML Fermentation• Monitor disappearance of malate

– Paper chromatography– Enzymatic

• By hand• Automated

– Spectroscopy• UV/VIS• FTIR (Fourier Transform Infrared)

– Photometric analysis

Note on Measurements

• Some methods are direct– Interference is minimal– Misidentification of compound rare

• Some methods are indirect– Need to make sure peak is correctly being

identified– Interference can be a problem– Misidentification can occur if unexpected

components are present

Lactic Acid Bacteria Off-Odors

• Mousiness• “Poo” off- notes• Animal characteristics: fur, meaty• Floral taints: geranium• Metallic• Vomit• Rancidity

Mousiness

Several compounds(oxidation products of lysine) have been implicated in this off-character:

2,4,6-trimethyl-1,3,5-triazine

2-ethyl-3,4,5,6-tetrahydropyridine

2-acetyl-3,4,5,6-tetrahydropyridine

Lactic Acid Bacteria Off-Tastes

• Enhanced sourness• Bitterness

Monitoring Lactic Acid Bacterial Activity in Wine: Best Practices

• Microscopic analysis• Aroma analysis• Taste analysis• Monitor malate levels

Surface Films

• Bacteria or Yeast • Most Often Both• Off-Character Production• Film Microbiota Can Produce Copious

Amounts of Off-Characters Due to Availability of Oxygen

Brettanomyces

• Problem in barrels• Can monitor 4-ethyl phenol or 4 ethyl guaiacol

formation by nose or analytically• Some wines suppress odors from these

compounds; the mechanism is not understood• Monitor via microscopy; sometimes

Saccharomyces looks like Brettanomyces• Genomic/molecular monitoring of presence

Spoilage Yeasts

• Use residual sugar• Turbidity and effervescence in bottle• Zygosaccharomyces• Saccharomyces

Advice

• Need to know you have the potential for a problem before the off-character appears

• Some of these off-characters are transient• Some characters difficult to remove

– Blending out– Charcoal fining

• Need to be careful to NOT spread it around the winery