Zero allergen is defined as the lowest limit of the test to detect the allergen. I heard that this is around 5, 10 or 20 ppm depending on the allergen and the substrate. I would start by following FAARP's guidance. (FAARP is the Food Allergy Research and Resource Program at the University of Nebraska-Lincoln.)
Hi Hank Major,
It may have been an incomplete hearsay. This whole area is highly complex, both technically and linguistically.
Here is another version -
By default, the [probably US] law requires use of a “zero threshold,” meaning that any detectable level of a major food allergen triggers the labeling requirement. Consequently, industry relies on analytical results of “none detected” to provide assurance of allergen management.
However, there are sound scientific data to show that the safe level of allergens is higher than zero and that there are “thresholds” below which an allergen does not constitute a food safety hazard.
Allergen Management, 2015.pdf 1.11MB
The above quote is also a simplification IMO, scientifically and conceptually, eg define "safe" ? Why only "major" food allergens ?.
For an allergic consumer there is no safe limit for the allergens that they are sensitised to. The only successful management strategy is avoidance of those allergens.
However, to enable risk-based management of precautionary labelling for cross-contact allergens, the Allergen Bureau, in collaboration with the Food Allergy Research & Resource Program (FARRP) of the University of Nebraska (USA) & the Netherlands Organization for Applied Scientific Research (TNO), established the VITAL Scientific Expert Panel (VSEP). Using data from clinical food challenge studies, the VSEP has identified reference doses for the common food allergens at which 95-99% of the allergic population are unlikely react (1,2). These reference doses have been adopted as the basis for the action levels now used in the VITAL® Program Version 2.0
Food AllergenFAQs, ANZ,Current.pdf 630.07KB
JFI, Some USFDA Variations -
(Note the "voluntary")
@Barbara - the practical answer to yr OP is that, afaik, FS standards typically expect cleaning data showing nil detection for the relevant allergen when using the ("accepted") maximum sensitivity kit available.
You might need to check for false-positive also.