If I have a jug of milk that is close to its expiration date and another, newer and unopened, jug of milk I will use up the old milk before opening the new one.

But if I have a batch of coffee that was roasted 2 weeks ago and a new, fresher batch comes in, I will open the new batch and save the old batch to be used up after the newer one is done.

The difference derives from shape of their expiration curves. Milk stays relatively fresh for a while and then rapidly deteriorates. It’s freshness curve is concave. Coffee quality deteriorates quickly after roasting and then stays relatively constant after that. 2 month old coffee is just as agreeable as 5 days old but both are much worse than 1 day old. Coffee’s freshness curve is convex.

The shape of the expiration curve determines whether you like or dislike mean-preserving spreads in the age profile of your stash. Convexity means you would choose 1/2-new 1/2-old over all-medium. Concavity means you have the opposite preference.

What are the expiration curves of other things?

Convex: eggs, bananas, significant others (except mine of course, she gets fresher with age.)

Concave: vegetables, bread, co-authors, this blog post