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How much impact has having a coffee bag being opened vs. sealed on preserving the aroma of beans over 1 month?

I am deliberating between 250 gram and 500 gram packages, with the former being 4% more expensive per kilo then the latter.

250 grams lasts me about a month (allows for one 8-gram espresso equivalent every day), so buying the larger bag, in the second month I would be using beans which are stored one month longer in a opened bag then if I would be buying the smaller bag.

Does it make sense to opt for the smaller 1-month bags in this usage pattern, or is the difference going to be infinitesimal?

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    Possible duplicate of When does coffee go off? – MTSan Oct 16 '17 at 13:36
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    Short answer is, yes. Cost and freshness (so, taste) is a trade-off. – MTSan Oct 16 '17 at 13:37
  • 4% is pretty minuscule, I'd stick with the smaller size. Roasted coffee only stays fresh for about 2 weeks, so you definitely don't want to be using the same bag for 2 months. – Evan Nowak Oct 22 '17 at 4:27
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Trading off taste difference against price depends on how much each of them matters to you.

Storing an open bag of beans in the cabinet for 8 weeks probably does matter to taste, while a 4% price difference is easily in the noise.

Taste Difference

It seems fair to assume you care about taste differences since you're asking. So for that side of the comparison, it’s sufficient to ask whether you can taste any difference, and that’s fun and easy to test. See this answer on how to do a triangle test.

Here's an experimental design for coffee bean storage duration: Weigh out the coffee into small plastic bags, put half the bags in the freezer and half the bags in the cabinet, then at different durations, defrost a frozen bag while still sealed (so the beans don't get water condensation from the air), and compare the coffee it makes against a bag from the cabinet. I did this with medium roast beans listed as “high body” and “bold acidity” so degradation would be readily noticeable.

After 2 days, we could not distinguish the frozen from cabinet-stored beans. (That rules out the hypothesis that freezing hurts beans, at least when defrosted in the bag.) After 4 weeks, we still could not distinguish them. After 12 weeks, all tasters could tell, and with high confidence.

Caveats: That was just one kind of bean, made just one way, with 5 tasters who are no cross-section of the world’s population. But each taste test got “statistically significant” results for a hypothesis picked in advance. Good enough for me.

I doubled the duration between each test, hence I didn't test 8 weeks. The taste impact of 8 weeks storage is still in play. Do try the test at 4 and 8 weeks with your favorite coffee. For discussion’s sake, let’s call it likely.

Side note: If you want to stretch out the freshness of a 2 month supply, it’s viable to freeze some of it in small bags.

Price Difference

Perception studies have a notion of “just noticeable difference” (JND). A smaller difference than that is unlikely to be noticeable (not the same as “measurable”). A starting assumption for a new domain is JND = 1dB (as it is for audio volume) which is about 26%, but a quick web search gets us a JND threshold for price from Marketing As I know it:

Why is it a 10% off sale fails to get your attention while a 50% off sale brings in consumers from the surrounding counties? The reason the 10% sale does not excite is that it is below the JND. Research has proven that the JND for price tends to be between 20 to 25%.

In context, a % comparison (whatever the percentage amount) assumes you have a budget or reference in mind for that expense. If the % difference blows your coffee budget, you could compensate by drinking less often, or better yet by making more of your coffee at home than from cafes. If you think of coffee as part of your overall food & drink budget, maybe you could make up a large price difference by drinking less soda or eating less meat (both of which are health and environmental improvements).

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Perhaps a better way to frame your question is to ask if you are willing to pay an extra 4% to have coffee fresher by one month. For me that's an easy one - absolutely I would pay an extra 4% to have fresher coffee.

There are all kinds of opinions about how long freshly roasted coffee maintains its freshness, but I've never seen anyone opine that roasted coffee beans can stay fresh for over one month without a noticeable decline in taste. I home roast my coffee and it rarely gets to be over two weeks old before it's gone. But ultimately that's a question for you and your taste buds and your wallet.

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