Brewing microbiology – off-flavour identification: In beer quality control, which contaminant is classically responsible for producing a distinct “parsnip-like” odor and taste in finished beer?

Introduction / Context:
Sensory taints in beer often point directly to specific microbial contaminants. A classic exam favorite is the “parsnip-like” odor/taste descriptor. Knowing which organism produces this off-flavour helps brewers trace contamination sources, select appropriate microbiological tests, and refine CIP (clean-in-place) protocols.

Given Data / Assumptions:

• The flavour is described as parsnip-like rather than buttery, phenolic, sour, or sulfury.
• Organisms listed include Gram-negative beer spoilers and brewing yeasts/bacteria.
• Beer is a selective environment: low pH, ethanol, hop iso-alpha acids, CO2, low oxygen.

Concept / Approach:
Different spoilage microbes generate signature flavour notes. For example, Lactobacillus and Pediococcus cause lactic sourness and diacetyl; wild yeasts yield phenolic/clove or solventy aromas; Pectinatus/Megasphaera cause sulfidy/rancid notes in strict anaerobiosis. The parsnip-like taint has been historically associated with Flavobacterium proteus contamination, which can survive niche points in the cold side and impart distinctive vegetal notes inconsistent with normal beer character.

Step-by-Step Solution:
Match the sensory descriptor “parsnip-like” to known microbe–taint associations recorded in brewing quality control texts. Exclude organisms whose typical metabolites do not yield this note (e.g., diacetyl from lactic acid bacteria is buttery, not parsnip-like). Identify Flavobacterium proteus as the classic culprit for parsnip-like taint. Select the corresponding option.

Verification / Alternative check:
Brewhouse investigation correlates include recovery of yellow-pigmented, oxidase-positive Gram-negative rods from beer lines or packaging, with sensory panels repeatedly flagging a parsnip-vegetal nuance in affected batches. Corrective actions reduce the taint once the organism is eradicated.

Why Other Options Are Wrong:
Zymomonas anaerobia – linked to over-attenuation and acetaldehyde/fruity notes, not parsnip-like.

Lactobacillus pastorianus – produces lactic sourness and diacetyl (buttery).

S. carlsbergensis – the standard lager yeast; not a spoilage off-flavour source per se.

Pectinatus frisingensis – gives sulfidy/rancid and butyric-like faults, not parsnip.

Common Pitfalls:
Confusing grassy/hoppy notes with vegetal taints; remember this descriptor is contamination-linked, not a hop aroma.

Final Answer:
Flavobacterium proteus.