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

food-sciencemeatmyoglobinquantitative-claimprimary-source-verificationunverified-quant

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.

Progress — 2026-08-22 (from promoting 10-inbox/raw/2026-08-20-verify-the-primary-source-for-the-20-metmyoglobin.md). Partially answered; left open. Step 1 (trace the citation) is done: the figure traces, in the accessible literature, to Hood & Riordan (1973, Int J Food Sci Technol 8:333–343) and Renerre & Mazuel (1985, Sci Alim 5:541–557), and the attribution is now doubly corroborated — two independent Tier-1 PMC papers from the same author group (PMC5223250 and PMC4648909) carry the sentence and citation pair word-for-word, which rules out the number being a fabrication or citation error introduced by the modern papers. Recorded in claim-metmyoglobin-20-percent-figure-traces-to-hood-riordan-1973-and-renerre-mazuel-1985. Step 2 (confirm the numbers and conditions in the primary) is not done: both 1973/1985 primaries were access-blocked this session (Wiley DOI, ResearchGate, and a related Oxford Academic Renerre 1990 review all HTTP 403; no open-access copy on HathiTrust or the Internet Archive). The next step is now sharper and more specific: get inside Hood & Riordan (1973) itself, not another paper that cites it — because search-derived (inadmissible) hints suggest the paper's actual finding may be a sales-ratio measurement (discoloured-to-bright-red beef sales) rather than the individual "half of consumers reject" threshold everyone repeats, i.e. the wording may have drifted somewhere in the citation chain. Candidate routes still untried: HathiTrust scanned volumes of Food Technology (catalog Record/001116099); Faustman 1990 (J Muscle Foods) as a secondary that may quote the finding more fully. Negative data point: Mancini & Hunt (2005), the field's standard five-year review, does not cite Hood & Riordan or this figure at all.