Measuring coffee TDS

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PeetsFan
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#1: Post by PeetsFan »

I read about measuring total dissolved (coffee) solids, and it seems expensive and involved. I was wondering if a simpler approach is possible:
  • Weigh the ground coffee before pulling the shot
  • Weigh the finished espresso after pulling the shot
  • Dump ALL of the grounds onto a coffee filter or similar
  • Thoroughly dry the grounds
  • Weigh the dried grounds
Then, with the magic of arithmetic, you can calculate TDS. I'm bad at math, but I think it's:
Weight of: (pre-brewed coffee) - (post-brewed grounds) = Dissolved Solids
Weight of Dissolved Solids / Weight of Finished Espresso = %TDS

Example: 15 g in portafilter, 30g in cup, weight of dried grounds is 13.5g
Dissolved Solids weight is 1.5g
1.5g / 30g = 0.05, or 5% TDS.

Is this correct?
I always weigh my ground coffee and my finished shot anyway, so I would only need to dry and weigh my spent grounds. I think. It won't measure calcium or silica in the water, but I think the weight of the coffee in the water would utterly dwarf such values, and I don't care about minerals, I care about the coffee.

mountmustache
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#2: Post by mountmustache »

I think the problem is how do you "thoroughly dry grounds"? Any extra water left in the spent grounds will skew your results

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jpender

#3: Post by jpender »

PeetsFan wrote:Is this correct?
Not quite. If you do it perfectly you'll be measuring total solids. Because espresso is metal filtered and contains a significant amount of undissolved solids you'll get a higher number than the actual TDS (total dissolved solids).

You have to dry the grounds completely and keep them dry, and cooled, when you weigh them. That's a technical problem that isn't trivial to solve in a home kitchen.

In addition, the fresh grounds contain significant moisture that you must account for in order to do the calculation for total solids. So you really need to grind and dry a sample of fresh grounds as well in order to find out how wet the coffee is.

When you add up all of the uncertainties in this process you'll likely discover that it's difficult to get precise numbers out of it. And it's a lot of work, time-consuming and stinky work.

An alternative is to dry the liquid coffee, filtered or centrifuged to remove the suspended, undissolved solids. For that you need a better scale but nowadays one can be had for a relatively low price (under $50). Filtering or centrifuging adds cost of course. And it's still a tedious job, but at least not quite as stinky as drying grounds.

Better still: Buy an Atago refractometer. Or better better still: pony up for a VST refractometer. If the numbers are that important to you that's the way to go. If you're like me and just a hobbyist when it comes to TDS then goof around with drying grounds. It can't hurt. It will just stink.

PeetsFan (original poster)
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#4: Post by PeetsFan (original poster) »

jpender wrote:
Better still: Buy an Atago refractometer. Or better better still: pony up for a VST refractometer. If the numbers are that important to you that's the way to go. If you're like me and just a hobbyist when it comes to TDS then goof around with drying grounds. It can't hurt. It will just stink.
Thank you.
You've completely crushed my dreams, but thank you.

It's ok, really. I'm just a casual home espresso maker. No big deal. I'm not going to dig into anything that requires more that 0.1 gram of accuracy. In fact, I think I needed you to stop me from doing this.