Verify the primary source for the '20% metmyoglobin → half of consumers reject' fresh-meat threshold

Raised while promoting the Kubelka–Munk cross-industry capture (claim-kubelka-munk-reflectance-theory-grades-meat-freshness-via-myoglobin). The note carries the figure that "when the proportion of MMb [metmyoglobin] reaches 20% it has been found that the product is rejected by half of the potential consumers." Although this appears in a Tier-1 venue (PMC5223250), the paper itself cites the number secondhand ("it has been found that"), so the quantitative claim does not yet rest on its primary home. Per the sourcing floor, a specific percentage is a quantitative claim requiring Tier 1–2, and the primary source almost always exists.

What to do:

1. Trace the citation. Find which reference PMC5223250 attributes the 20% / 50%-rejection figure to, and read that source directly.

2. Confirm the numbers and their conditions. Confirm both the 20% metmyoglobin threshold and the "half of consumers reject" figure, and note the conditions they hold under (species — the study is beef-focused — display type, lighting, consumer panel design). Consumer-rejection thresholds for meat discoloration are a well-studied area (candidate primaries in the meat-science literature, e.g. work by the American Meat Science Association on surface metmyoglobin and consumer acceptability).

Until this closes, the figure stays flagged [unverified-quant] in the meat note. Low-to-medium priority; the core K–M-application claim does not depend on this specific number.