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
Cheap Talk 
4 comments
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April 6, 2010 at 9:00 am
Alicia
What about things that get better with time up to a certain point and then begin to degrade. These might have polynomial curves. Things like wine, cheese, men ect
April 6, 2010 at 9:19 am
Ryan
I’ve always thought that 1 day was too young and 4-14 days after the roast date was optimal. My palate isn’t grand but I can tell the difference between 5 days off roast date and 2 months (maybe because I brew espresso also and the crema goes flat).
Where to you order from? I’ve had great luck with Ecco, Blue Bottle, and Espresso Vivace. I’ve also found that being in your Roasters shipping zone is helpful. I can get coffee from these Roasters 2 days after roast while it can be a week when it comes from Intelli.
April 6, 2010 at 10:54 am
Lones Smith
While I agree that a banana may look convex, if you hold it one way, I think that their ripening is somewhat concave. Look at the day 6 to day 7 drop-off:
April 13, 2010 at 8:10 am
Noto
My brother tells me marijuana is more like coffee than milk. Also, my guess is that the freshness curve for wine is quasi-concave (first non-decreasing, then non-increasing).