Beer spoilers – diastatic activity: Which organism is a well-recognized potential spoilage yeast in beer due to its ability to ferment dextrins (over-attenuation)?

Introduction / Context:
Some yeasts possess extracellular glucoamylase activity that breaks down dextrins into fermentable sugars, causing over-attenuation, over-carbonation in package, and flavor instability. Identifying the key culprit prevents costly recalls.

Given Data / Assumptions:

• Beer contains residual dextrins after mashing/boiling/fermentation.
• Standard lager/ale strains do not over-attenuate dextrins.
• Diastatic activity is the distinguishing spoilage trait.

Concept / Approach:
Saccharomyces diastaticus (sometimes designated S. cerevisiae var. diastaticus) secretes amyloglucosidase allowing it to hydrolyze dextrins and continue fermenting in package. Symptoms include high CO2, gushing, and bone-dry beer. It is therefore a classic beer spoiler in QA manuals.

Step-by-Step Solution:
Identify the organism with extracellular dextrin-degrading enzymes. Exclude standard production yeasts that lack problematic diastatic phenotypes. Select S. diastaticus as the spoiler. Note that detection often uses STA1 gene assays and attenuation tests.

Verification / Alternative check:
Micro labs confirm by qPCR for STA1 and growth on media with dextrins as sole carbohydrate, showing continued fermentation versus controls.

Why Other Options Are Wrong:
S. carlsbergensis and S. cerevisiae are normal brewing yeasts; Lactococcus lactis is not a yeast; Brettanomyces is a spoiler in some contexts but the hallmark here is diastatic over-attenuation specific to S. diastaticus.

Common Pitfalls:
Confusing phenolic off-flavour from Brettanomyces with over-attenuation from diastaticus; these are different fault mechanisms.

Final Answer:
Saccharomyces diastaticus.