doublehelix wrote:Frankly, one good way (need a real lab) to study fines migration is to label (fluorescent/radio-tagging sized particles), then run an extraction. .
I'm back with a bit more info, after a couple of hours spent.
The good news is we probably don't need to go the fluorescence tagging route to confirm fines migration - unless it is to study the fines migration rate or to see it in real time.
So I pulled two espresso of 15g dose into 25g beverage. One of them is a normal espresso (pulled in 15g VST filter basket) ; the other is a 'filtered' espresso pulled with 2 layers of Aeropress filter in the VST filter basket. The filtered espresso has a slightly finer grind to prevent gusher. The samples were kept in a container for about 1 hour until they were viewed under the microscope. Multiple observations were done (shaked prior to sampling vs allowed to sit for 15-30 min and then sampled).
Here're some pictures to visualize what espresso fluid looks like at micron level (Yeah, these are the stuff that we stomach every day
Normal Espresso - unfiltered (without Aeropress paper filter):
The brown grains are coffee grinds/solids
The reason for two magnifications - low magnification was to have a bigger view field (more representative of the overall sample), and high magnification was to look at the details (more details but less representative)
Note: I had to vary the depth of focus in order to visualize either the emulsion droplets or the particles. In reality, they're probably suspended uniformly throughout the fluid.
Filtered Espresso - 2 layers of Aeropress paper filter
From my limited observation above, fines migration definitely do occur (in both filtered and non-filtered espresso), and they are one of the major components in espresso. What was more surprising to me was how much oil droplets/emulsion are there in the cup.
The biggest differences between the filtered & non-filtered espresso
i) More emulsion droplets are observed in the filtered samples (I've got more pictures unposted but the trend seems true, at least for these samples) - not sure exactly why but we can always theorize.
ii) There are *slightly* less coffee grinds/solids in the filtered espresso, especially the larger grinds at >30um. Aeropress seems to be quite effective at filtering fines larger than 20-30 micron.
i) By taking the top part and bottom part of the unfiltered espresso, I can observe >30 micron grinds in the bottom part (due to sedimentation), but not in the top parts (suspended grinds are normally <20 micron).
ii) The edges of the coffee particles appear to be more rounded, compared to the initial pictures I've taken in the OP(which has sharp edges). This could be due to swelling in water (ethanol was used previously), or the pressure/flow in espresso has smoothen/eroded the edges of the coffee particles.
Because of the more rounded feature of the particles, I believe, but can't confirm, those micron-grains at 1000x magnification are indeed micro-fines . They could potentially be emulsion droplets (but unlikely).
Also, to confirm what I am seeing are actually real, I tried syringe filter with 0.22um(I believe that's what VST uses) and 0.45um membrane filter. I couldn't see anything under the microscope at all for both filters (except for some noises). No grains, no emulsion, nothing. The VST refractometer measures only the dissolved solids
, but gives no information about the particulates/emulsion - nothing new but it's good to visualize the difference directly.
Syringe-filtered espresso - 0.22um filter
I guess what would be real interesting to do next, is to visualize the shots from an EK43 grinder and a flat burrs grinder.