ohwhen wrote:Anecdotally, coffee does taste a lot when maintained at a drinking temp. I've also also always assumed the soluble bits of coffee being over extracted is what caused it. You can taste it very easily at coffee shops who don't do fresh batch brew in the afternoon.
In metal filtered coffee like espresso or moka you typically get something like 5% of the solids as undissolved, filterable solids. Probably some of those solids contain soluble material. But how much? For virgin coffee particles the upper limit of solubility is usually taken to be around 30%. So those numbers could be used as upper limits to what could be expected. For example, suppose that much soluble material was in a brew initially at 1.50% TDS. If it all dissolved the TDS would rise to 1.52%.
Years ago, after Vince Fedele of VST suggested that refractometer measurements can change over time due to soluble material dissolving in the cup, I did an experiment with my Aeropress. I brewed a batch (paper filtered) and split it. One part of the batch I syringe filtered (0.7um) and the other part I left alone. I measured the solids in both immediately and then put them each into closed containers. I repeated the measurements several times over a period of ten days, syringe filtering a portion of the unfiltered sample each time. The syringe filtered samples consistently measured at 1.33% TDS. The unfiltered sample measured 1.36-1.37% total solids. So that meant that this particular Aeropress brew was 2-3% undissolved solids.
Whatever the composition of the undissolved material it never added to the TDS, at least within the sensitivity of my measurement. Perhaps it added 0.005% TDS? Would that affect the taste? I think that's speculative.
One thing that is well known to occur with coffee is a chemical change that leads to increased perception of acidity and bitterness. I have read that it's the conversion of chlorogenic acid into other acids. It's the reaction that is responsible for the battery acid like coffee you get when a pot is left on a hot plate for a long time. This must also be happening, albeit to a lesser extent, as coffee cools down. There will also be loss of aromatics as coffee sits.
DamianWarS wrote:I would be interesting to just cup methods of keeping a brew warm via various reheating and insulation methods along with fresh, but perhaps this has a bit too digressed of a topic.
Yes, I think you're right. One can imagine numerous, tedious experiments tasting old, reheated coffee.
One thing is for sure: The coffee I brewed first and reheated in the microwave tasted better than the fresher coffee I made second. So whatever is going on it isn't enough to ruin the coffee, at least on that time scale (~10 min).