Among the myriad incongruities that one gets to see in wonderful New York City, the labeling of the sell by date on milk containers is one of the more esoteric ones. Many readers of Natural Selections would have observed milk containers with a baffling warning along the lines of “Sell by July 13, In NYC July 11.” Some might even have been bothered by this mysterious inscription. Indeed, I noticed this oddity many times, but usually greeted it with the stoicism that New Yorkers are fabled for. I would have happily lived my life in ignorance had I not had the desire to perform a public service by apprising the Rockefeller community of the reason behind this bizarre phenomenon.

Having grand visions of public service, I let my mind wander and come up with potential explanations. Are New Yorkers very picky about the quality of milk they put in their coffee? Perhaps cat owners in NYC are a finicky lot? Or is the level of contaminants in the air particularly high? I was even willing to plate out samples of milk to see if some strange organisms showed up. Realizing that my thesis committee might not share my enthusiasm for plating out milk samples, I exercised the relatively mundane option of seeking answers through Google searches. It turned out to be a wise decision, as the answer lay in some arcane dating regulations established by the municipal Department of Health in NYC. Incidentally, the Google searches also revealed that the Rockefeller Institute, in its early days, had awarded grants for the study of bacterial contamination in the milk supply of NYC.

New York City is one of the few places in the US with a separate dating system for milk that limits its sale to 96 hours after 6 a.m. on the day after pasteurization. In most other places, there are no regulations governing the labeling of milk and it is left to the milk producer to decide on a sell by date (usually 6-10 days after pasteurization). The general justification for the stricter limits in NYC is that milk often lies unrefrigerated on sidewalks or in delivery trucks for long periods of time.

So, therein lies the solution to the great milk labeling mystery. The next time you are wandering in the dairy aisle of a supermarket, you may now look at the sell by dates in a state of enlightenment.