Bacterial spoilage

What types of wine spoilage organisms are there?

• yeast - aerobic and fermentative

• bacteria - LAB, acetic acid bacteria (AAB) other

• fungi - botrytis other

What is the definition of wine microbial spoilage?

imparting an off-aroma or flavour to wine or cause a change in the physical appearance of a wine (e.g., haze)

What is the impact of growth of spoilage microorganisms on wine composition?

• microbiological faults diminish the acceptability of the finished wine therefore QUALITY

• changes occur in wine depending on which organism develops

What are some examples of bacteria involved in wine spoilage?

• LAB - Lactobacilli

• AAB

• Pediococcus

In the metabolism of hexoses, what is involved in homofermentative species of bacteria?

Hexoses fermented via glycolytic pathway with pyruvate being reduced by lactate dehydrogenase to lactic acid.

Almost entirely lactic acid (>80%), trace of acetic acid, ethanol and carbon dioxide.

2 ATP/hexose molecule

In the metabolism of hexoses, what is involved in heterofermentative species of bacteria?

Sugar metabolized via alternative pathway (phosphoketolase pathway).

Multiple products; acetic acid, carbon dioxide, glycerol, ethanol, acetaldehyde and lactic acid (<50%).

1 ATP/hexose molecule

How does acidification occur in wine spoilage?

• due to production of acetic acid and D-, DL- or L-lactic acid

• produced by heterofermmentative species of lactobacillus, lentilactobacillus and oenococcus from glucose (acetic acid plus L- or D-lactic respectively)

• produced from homofermentative species of lactobacillus, lactiplantibacillus and pediococcus from pentoses (no acetic acid produced, DL or L-lactic)

• wines with residual sugar (CRITICAL FACTOR) are at GREATEST risk (particularly wines setting on lees - potential nutrients)

• may also occur during AF if there is delay in onset of yeast activity

• generally also associated with high pH (CRITICAL FACTOR)

What is mannitol taint? And how is it formed?

• due to enzymatic reduction of fructose

• produced by heterofermmentative bacteria - e.g., L. brevis

  • associated with elevated acetic acid, D-lactic acid, n-propanol, 2-butanol

  • associated with ropiness and diacetylmorphine

• wine tastes viscous, acetate-esterish

OCCURS IN WINES WITH

HIGH RESIDUAL SUGARS AND HIGH pH

*CRITICAL FACTORS*

How does ropiness/increased viscosity occur in wine spoilage?

• due to production of extracellular polysaccharides - D-glucan: slimy texture

• important factors are:

  • strain (e.g., P damnosis, resistant to ethanol and SO2)

  • glucose

  • lack of agitation, high biomass and nitrogen availability

• trait is mediated by a plasmid, therefore STRAIN SPECIFIC

• associated with high acetic acid and mannitol taint and becomes evident after MLF

• controlled by pH and SO2

How is excessive diacetyl produced?

• buttery/butterscotch impact

• generally >4mg/L considered faulty

• produced during MLF either:

  • during hexose/pentose utilisation

  • during citric acid metabolism, during MLF by O. oeni

• may be removed by post-fermentation treatment with S. cerevisiae (or further aging on yeast lees)

How are mousy off-flavours produced?

• flavour reminiscent to aroma of caged mouse

• only detected after the wine has been swelled takes seconds to build - off-flavour lingers

• considerable variation in the sensitivity between individuals

• due to production of N-heterocyclic compounds

  • 2-acetyltetrahydropyridine - associated with bread and corn chip aroma

  • 2-ethyltetrahydopyridine

  • 2-acetylpyrroline - associated with jasmine rice aroma

• implicated bacteria include: L. brevis, L. cellobiosis, L. hilgardii

• requires ethanol, amino nitrogen: L-lusine and/or L-ornithine

• associated with high pH and low SO2 wines

• other microbes too

What is involved in the degradation of tartaric acid?

• associated with wines already spoiled and where other acids (citric, pyretic, malic) have been metabolised: results in DEACIDIFICATION

• rare spoilage as limited to a few strains of L. plantarum (Mech I) and L. brevis (Mech II)

• associated with high pH wines

• acetic acid (>3 g/L) produced

How is geranium off-odour produced?

• related to metabolism of sorbic acid by LAB

• sorbic acid is reduced to sorbinol by Oenococcus

• NOT pediococci or lactobacilli

• low odour threshold: 100 ng/L

• only in wine, as need ethanol

• only add sorbic acid to wines with LAB is not expected

• sensory threshold 100 ng/L

What is involved in glycerol catabolism?

• a principle yeast metabolite

• ranges in concentration (5-8 g/L)

• important sensory effects

• bacterial activity can breakdown glycerol; important consequences

  • LAB

  • AAB

What is the typical spoilage characteristic produced by LAB and glycerol?

Bitterness

important factors:

• strain

• low glu and fru

• oxygen (potentially)

• more likely in red wine

• pH and SO2

Generally speaking, what are AAB and what do they cause?

Gram negative rods

– Acetobacter

– Gluconobacter

– Gluconoacetobacter

cause high volatility acidity (acetic acid may react with ethanol, forming ethyl acetate)

KEY FACTOR: AEROBIC GROWTH

At what stages is AAB problematic?

1. on damaged grapes (particularly in association with botrytis)

2. primary fermentation (if sluggish)

3. post-primary fermentation processing (during conservation period of MLF) and storage

   • mousy off-flavour

What is the relationship between AAB and glycerol?

• AAB degrade glycerol via dihydroxyacetone

• sweet/etherish sensory properties, binds to SO2

• further reaction with proline to produce ‘crustlike’ sensory character

• not all species

• inhibited by ethanol, therefore, typically related other botrytis affected grapes