I think I posted some early stuff on the refractometer thread back when. Fresh AeroPress diluted coffee at ~1.17 may go up to 1.25% in 4 days. I'm going by memory, as I'm still in the process of assessing my backup state after my data loss incident. The strength/time charts may be one of the casualties.
I recall having a cup of Starbucks brewed (paper filtered) started out at like 1.44% (going by memory) and after 6 days seemed to stabilize at 1.53-4% or so. I recall also doing some evaporation measurements, with little change in the sample residual - but it looks like in my spare time I'll have to re-do this.
So roughly speaking you've measured a change of about +0.1% TDS in 4-6 days? And did you see the same magnitude of change with syringe filtered samples?
And you are correct - using a refractometer to measure stored samples over more than a day or two is not really accurate. Which makes me wonder - the old way of certifying SCAA gold cup was to brew coffee using the prescribed method (brewing control chart), then send a sample in for testing. I believe that if they were using dehydration, this may work. But if they are using correlated refractometry now (since it's become the de-facto strength measurement method) then the strength measurements may be artificially high by the time they receive samples.
I looked at the form on the SCAA website. It appears that both submitting verification samples and an onsite analysis that includes determination of %TDS are required. So perhaps they've seen the effect you're describing?
jpender Senior Member Joined: 11 Jul 2011 Posts: 705 Location: California Expertise: I like coffee
Grinder: OE LIDO Vac Pot: S/S Moka Pot Drip: Aeropress
Posted Sat Feb 16, 2013, 2:52pm Subject: Re: Brewed Coffee Stability
I find that refrigeration does help. Hot coffee turns quickly for me - maybe in 30 minutes or less there are taste differences, even if there is no change in %TDS. Hot metal-filtered coffee that is kept hot (as in press pot brew in a thermos) turns quicker.
However, I can brew an AeroPress at double strength or more into a room temp mug, then put plastic wrap on top, and immediately into the fridge. It will keep to morning for sure, and responds very well to hot water dilution and/or microwaving.
I also have great results putting a french press brew into a good old paper coffee cup and let it cool naturally. The fines settle to the bottom of the cup, the temperature by the time I'm reaching the fines is probably approaching 90°F but the taste can be outstanding.
The only difference here is what we do with the heat.
I think the key is exposure to heat - you need the heat to extract, but the heat will then go on to convert flavor components (probably chlorogenic acids) in the coffee to worse tasting compounds. I have a few friends that were wondering how to improve their thermos coffee, and I suggested they make it the night before and refrigerate it before putting it in the thermos. Then, when they want coffee, pour some out and microwave it. So far, there seems to be anecdotal consensus that this does in fact make a better cup the next morning and later into the day. Try it - you might like it.
First impression: Cold, refrigerated coffee smells and looks nasty. Second impression: Reheated it wasn't bad. Third impression: Something wasn't quite right with it; not sure how to put my finger on it. Fourth impression: The refrigerated coffee seemed a little darker.
What I did: I carefully measured coffee, water, temperature, time and brewed a cup and immediatedly sealed and refrigerated it. The next morning I brewed a nearly identical cup and reheated the refrigerated coffee to within a degree or two of the fresh cup. Then I tasted them side by side. Not a blind tasting so maybe I lied to myself.
Although the two brews should have been very close I did not measure the strengths. Perhaps a small difference in concentration accounts for the subtle difference in color. But I wonder if the time in the refrigerator had something to do with it.
Sometimes I have to get up painfully early and hit the road. I was hoping this would be a good way to save time. I'm not sure it is.
edit (18 Feb) - I tried this again and the results were more favorable. I tasted the two cups (refrigerated/reheated versus fresh brewed) blind and I couldn't tell which was which. In order to keep their identities secret from myself I didn't even look at the coffee. The reason was that the reheated brew looked markedly different. Straight out of the refrigerator I noticed all of these short (~3mm) stringy precipitates floating disgustingly in the jar. I've seen something similar in liquid coffee undergoing oven dehydration and wondered what was coalescing. Most of these disappeared when the coffee was reheated but some "stuff" remained floating on the surface. It tasted fine though.
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