Brewhouse mash holding – unintended fermentations: If the mash in the brewhouse is held too long at permissive temperatures, which contaminant fermentations may occur and damage wort quality?

Introduction / Context:
Mashing schedules balance enzyme activity and microbial control. Prolonged holding, especially in the 30–50°C range or during slow cooling, invites contaminant growth that can sour or foul the wort prior to kettle boil.

Given Data / Assumptions:

• Extended dwell time before boil allows microbial proliferation.
• Available carbohydrates and amino acids support lactic and clostridial contaminants.
• pH and warm temperature can drive off-flavour production.

Concept / Approach:
Lactic acid bacteria (e.g., Lactobacillus, Pediococcus) generate lactic acidity and diacetyl, while Clostridium species can produce butyric acid and foul, rancid aromas. Together they cause sourness, butyric notes, and gas formation, compromising foam and flavor stability.

Step-by-Step Solution:
Identify likely spoilage guilds in warm, nutrient-rich mash. Relate LAB to lactic acid formation. Relate clostridia/anaerobes to butyric fermentation. Select the combined option “Both (a) and (b).”

Verification / Alternative check:
Pilot mashes intentionally held warm without boil back up these outcomes, with measured drops in pH and rise in butyric concentration, confirming the risk.

Why Other Options Are Wrong:
Citric/propionic fermentations are not the typical mash faults in brewing; the dominant spoilage pathways are lactic and butyric under these conditions.

Common Pitfalls:
Assuming the upcoming boil will fix all issues; volatile butyric and diacetyl precursors persist and degrade beer quality.

Final Answer:
Both (a) and (b).