If complying with BTA 2002 is the objective, neither the Act nor FDA guidelines stipulate definitive threshold levels for any ingredient, at least not that we have found. But the reality of a continuous process is that there is always a remnant of any previous ingredients in the pipes, silos, or bulk tanks until that vessel is cleaned. So for strict compliance with the Act (which does carry criminal penalties for non-compliance), every lot that could potentially have comingled with subsequent batches needs to be identified. The only acceptable cut-off is a cleaning of the vessel, destruction of its contents, and re-filling with fresh lots.
If “reasonable” recall is the objective; where product may be substandard, slightly contaminated, but certainly non-lethal, a certain amount of common sense may be applied to recall traceability. This becomes a risk management tradeoff of expected impact against costs. If the liquid sugar used a month ago had a bad odor, perhaps recalling any product made yesterday from the same tank the tainted sugar occupied 400,000 gallons ago may be overkill. But if that bad odor was a fast growing lethal biological, it could certainly still be in the tank – maybe even in greater concentration… To comply with BTA 2002, a company must be able to track any contamination, fully within 24 hours.
Intuitively, for solids, one would expect that if we had a 300,000 pound silo, and lot 1 was 100,000 pounds, followed by lot 2 at 50,000 pounds, and lot 10 at 70,000 pounds, one could derive what went into subsequent lots on some kind of a FIFO (First In First Out) basis. (Perhaps with some small overlap for comingling at the imaginary line between lots.) So once we had consumed 100,000 pounds, the assumed current lot would be lot 2; once we had consumed another 50,000 pounds, lot 10 would be in play.
However, this is not necessarily true. For solid storage systems that release contents from the bottom of the vessel, product tends to flow in a conical funnel: newer lots flow down the middle while previous lots hug the sides of the silo. Therefore, we might find that production after 75,000 pounds contains elements of both Lots 1 AND 2 (long before the 100,000 pound line). Also, production quantities after 160,000 pounds may still have some quantity of Lot 1 AND 2 as well. It is NOT true that only lot 10 is flowing after 150,000 pounds…all three may be in use.
Still, depending upon the ingredient and environmental conditions in the silo, one could come up with a mathematical model that could provide a realistic estimate, but it would only be accurate under controlled conditions.
This is compounded in the case of liquid or gaseous ingredients. They comingle easily and completely. Any math assumed would have to know the exact quantity in the bulk tank when any additions were made and could only estimate dilution levels.
So in reality, any product consumed after any receipt will contain traces of any lot from the same vessel up to that receipt, back to the last cleaning, perhaps 3 years ago!
But it is not all bad news! An alert on an ingredient does not necessarily require a recall of everything made in the last 3 years. We can state categorically that a lot made on Jan 14, 2012 cannot possibly contain even a miniscule trace of any lot received on Jan 15, 2012. That is simply not possible.
It may be possible to interpret some regulations pertaining to continuous processes to exclude more previous lots, but any conclusion must be carefully considered, and defensible; and presumably specific to the process in question. A silo that contains 40 different lots, that is filled daily, but “emptied” over a month, will have a different mathematical algorithm than a silo that contains 3 lots and effectively empties 4 times a day.
It is more likely that a terrorist chemical or biological contamination will be in a minor ingredient that is added discretely in smaller quantity to the process (or from the water supply which virtually nobody traces in lot traceability). Smaller, concentrated quantities are easier to acquire, move, and hide. Knowing what bulk ingredients are definitely NOT in a lot is substantially better than just guessing from paperwork… especially when there are only 24 hours with which to respond with complete information (as few as 4 hours if you are a co-packer).