Jonk wrote:There's a video on that as well 8)

http://www.youtube.com/watch?v=yqgKlqAUM9g

James Hoffmann to the rescue! LOL

I like cold coffee. Sometimes it's quite nice, even a little better than when it's hot. But that's not the question here.

I thought that he wouldn't able to really tell the difference, except for maybe the steam wand thing. But he didn't heat the samples back up to drinking temperature. He heated them above that level, assuming that's what most people would do. And he's probably right about that but it avoids the question: Can you reheat coffee to *drinking* temperature and not harm it? That's what I did in my test.

He didn't answer that question.

jpender wrote: James Hoffmann to the rescue! LOL

I like cold coffee. Sometimes it's quite nice, even a little better than when it's hot. But that's not the question here.

I thought that he wouldn't able to really tell the difference, except for maybe the steam wand thing. But he didn't heat the samples back up to drinking temperature. He heated them above that level, assuming that's what most people would do. And he's probably right about that but it avoids the question: Can you reheat coffee to *drinking* temperature and not harm it? That's what I did in my test.

He didn't answer that question.

I don't think it's an issue of harming it but that reheated coffee inately will taste different. When coffee is hot there is still undissolved coffee stuff in the cup and the hot coffee will continue to dissolve and things like astrgency may start to come out which is common as coffee cools. If you reheat coffee you will have more dissolve solids then in a fresh brew so it will probably be stronger. Not to mention effects of evaporation will also make it stronger. Two brews back to back made with the same coffee/process, one fresh and one reheated I suspect will not taste the same.

DamianWarS wrote:When coffee is hot there is still undissolved coffee stuff in the cup and the hot coffee will continue to dissolve and things like astrgency may start to come out which is common as coffee cools. If you reheat coffee you will have more dissolve solids then in a fresh brew so it will probably be stronger. Not to mention effects of evaporation will also make it stronger.

It sounds like you're describing the effects of heat and time, not reheating itself. If what you're saying is happening then a coffee maintained at drinking temperature would suffer the same changes, maybe more so since it would be hotter on average over time than a cup that cools and is then warmed up.

Wouldn't the presence of undissolved solids depend on the filtration method?

I'm approaching reheating as a consequence to heat and time. reheating via microwave may impact taste however I see heat and time having a greater impact than the reheating process itself (via microwave). obviously better filtration is going to have fewer undissolved solids, but it still has some and it still changes as the brew cools. also, evaporation (effect of heat and time) will also have an impact. cupping alone with limited equipment is going to mean more compromises, I would think using thermal carafes/tumblers would be a better solution but of course, that doesn't change the undissolved solids problem. 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.

jpender wrote:It sounds like you're describing the effects of heat and time, not reheating itself. If what you're saying is happening then a coffee maintained at drinking temperature would suffer the same changes, maybe more so since it would be hotter on average over time than a cup that cools and is then warmed up.

Wouldn't the presence of undissolved solids depend on the filtration method?

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. The microwave seems to mess up the taste but I'm not sure if it's for the same reasons or not.

I do think filtration method changes but have had issues with a range of brewing methods. Years ago, I would make a french press pot in the morning and take it to work and it would taste awful. I switched to chemex at some point. It helped but still tasted awful.

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).

jpender wrote: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).

I would suggest the coffee preheated dissolved more stuff than the fresh one and any volatile compounds would probably be depleted. the preheated coffee maybe tasted better but I'll bet the fresh coffee smelt better.

jpender wrote: For each of these tests I cooled the Aeropress down to 16°C by submerging it in a water bath and then dried it completely. That way for each test the Aeropress started at the same temperature.

I'm a little confused why you cool your aeropress so much. You must live in a cold house (I'm jealous, my wife makes me keep it near 23°C/72°F

This would account for you larger difference between preheating and not.

I agree, though that James result of the two methods being essentially equal ... is unbelievable. He should have done this a couple of time to confirm. My own experiment shows that it makes a 1-2° difference, but the slope of the temperature curves are roughly the same. (Unlike your flat preheat line). I used 11 grams of room temp coffee in my experiments. I very simply used a room temp Aeropress from the cabinet for the first round, then did a preheated the second round.

I still agree with James recommendation to forgo the preheat. The extra effort for preheating isnt worth it. The Aeropress really shines when its stripped down to its basic operation. It's such a simple and fast method.

Just a quick reply about thermal carafes/tumblers. In my experience there will still be differences to a fresh brew. Perhaps you could wait for the brews to equalize somewhat but then they've both deteriorated a bit.

The easiest thing is to compare cold coffee.. But it's not a good indicator of which cup was best hot.

Rickpatbrown wrote:I'm a little confused why you cool your aeropress so much. You must live in a cold house (I'm jealous, my wife makes me keep it near 23°C/72°F

This would account for you larger difference between preheating and not.

It's usually cool here in the mornings and the windows are open. We don't run the heater in April. The ambient temperature was actually 17-18°C when I did the first test but the tap water was 16°C so I just went with that.

A warmer Aeropress would result in a smaller difference. In a sense your Aeropress was mildly preheated relative to mine. It's definitely one of the variables. Coffee dose and temperature of the coffee are also going to have some effect. Kettle temperature too -- not everybody lives close to sea level. And while the Aeropress retains heat quite well the time between preheating and brewing will also have an effect.

Rickpatbrown wrote:My own experiment shows that it makes a 1-2° difference, but the slope of the temperature curves are roughly the same. (Unlike your flat preheat line). I used 11 grams of room temp coffee in my experiments.

Interesting that you didn't get the flattened profile. I used 15g of coffee. Maybe that forms a better insulating cap of grounds? Or maybe it's related to the grind?

Rickpatbrown wrote:I still agree with James recommendation to forgo the preheat. The extra effort for preheating isnt worth it. The Aeropress really shines when its stripped down to its basic operation. It's such a simple and fast method.

I used to use mine daily but now it's mostly a travel brewer (which means I hadn't used it since January 2020). When I brew in a hotel room I don't preheat. I don't weigh the grounds. I don't measure the water temperature or the water amount. I don't time the brew.