Black kraut defect in sauerkraut: What is the principal cause of the black kraut discoloration observed in some kraut fermentations?

Introduction / Context:
Black kraut is a dark discoloration defect that can develop during sauerkraut production. Understanding its multifactorial etiology helps processors fine-tune blanching, brining, oxygen control, and sanitation to maintain bright, appealing color.

Given Data / Assumptions:

• Plant tissues contain polyphenols and oxidative enzymes like polyphenol oxidase and peroxidase.
• Micro-organisms in fermentation can produce metabolites (e.g., sulfides) and alter redox conditions.
• Trace metals such as iron can catalyze pigmentation reactions.

Concept / Approach:
Discoloration usually arises when plant phenolics are oxidized enzymatically and when microbial by-products (including sulfides) interact with metals, forming dark complexes. Oxygen ingress accelerates both enzymatic browning and microbial surface growth. Thus, the defect reflects combined actions rather than a single cause.

Step-by-Step Solution:
Link plant enzymes to browning: polyphenol oxidase converts phenolics to quinones that polymerize into dark pigments.Add microbial contribution: certain bacteria and yeasts produce metabolites that react with metals or pigments, deepening color.Consider processing: inadequate acidification or oxygen exclusion intensifies both pathways.Conclude that both plant enzymes and micro-organisms together best explain black kraut.

Verification / Alternative check:
Preventive steps include rapid acid development, minimizing iron contamination, blanching or enzyme inactivation where appropriate, and strict anaerobic conditions during fermentation and storage.

Why Other Options Are Wrong:

• Plant enzymes alone or microbes alone fail to explain severity and variability across processes.
• Sunlight exposure is not the primary driver for the characteristic internal darkening.
• None of the above contradicts established multi-factor causation.

Common Pitfalls:
Assuming color is only chlorophyll-related; overlooking trace metal and redox interactions that intensify dark pigments.

Final Answer:
The combined actions of plant enzymes and micro-organisms.