Hi,

After months of having good shots from my Silvia I've suddenly developed a channeling problem, but I haven't changed anything in my technique*.

The shots seem to start okay but then about half way through a very large air bubble develops in the center of the pour and I get twister channeling, resulting in undrinkably sharp tasting shots. Like I said, I haven't changed anything in my technique compared to what I've been doing for ages, not even the type of coffee, and it's only 7 days old (although the problems only started when I got this latest bag).

Tried a backflush and thorough cleaning, didn't help.... any ideas?

Thanks

EDIT - * that should probably say 'consciously changed'

Check out this thread about my donut extractions. Sounds rather familiar, although my Silvia has gotten rock steady, my two-group machine has gotten really finicky, and I hadn't changed anything either.
I've recently started downdosing and using a finer grind, but that hasn't stopped the spritzes. It has somewhat cured the donuts, but the coffees are more bitter and sharp, and the pucks are soggy and mushy, with a bit of soup on top when I pull the PF.
I'm thinking that the major change here has been the upward trend in the humidity, from almost nothing to nearer 80 - 100%. I'm not attuned to the changes, and I'm thinking that last year at this time I was still new to the machine and not so settled on my technique, so I just attributed the changes to bad technique. I'm not sure that we're having the same problem, but the tornado channeling with the center bubble (if I have this right) usually result from the center of the puck not producing as much as the outsides, similar to a donut, but we are having similar symptoms.

It is strange. It's something I saw when I first started with Silvia and I put it down to overdosing and bad distribution. But this is happening now with 14-15g doses and a good 7 months of experience.

It does seemed to be a 'delayed donut' as you say, faster flow around the center. Just stumped as to why it's happening out of the blue like this. Perhaps I've just become a bit blase towards distribution


A bad shot, but shows the bubble cone just before going off in a twister.... seems to guarantee a sharp unpleasant shot

I believe that is a side effect of a dead center (donut) extraction. Since there is no flow from the center of the basket, the sides eventually draw in but there is still an air pocket above that dead spot in the center of the basket.

cannonfodder wrote:I believe that is a side effect of a dead center (donut) extraction. Since there is no flow from the center of the basket, the sides eventually draw in but there is still an air pocket above that dead spot in the center of the basket.

S'kinda what we said, there Dave, but we expected the 'why' to come from a large caffeinated brain like yours! ; >

I'm getting a bit of the same, and downdosing and grinding finer and tamping lighter just seem to make the shots a bit more bitter, and to spritz uncontrollably. You folks that have experience with humidity, is that a possible explanation?

DC wrote:A bad shot, but shows the bubble cone just before going off in a twister.... seems to guarantee a sharp unpleasant shot

As Dave et al said, looks like a late stage donut extraction. All extraction cones "pucker up" shortly before blonding, but the exaggerated twister points to a low flow center. Ken posted this photo of "extraction interruptus," i.e., the resulting puck from an extraction ended at the first moment of beading:



So what are the possible causes of donut shots? A few of us chimed in the thread Donut Extractions with Good(?) Technique. My bet is on:

• Distribution / dosing - updosing with a downward tap mid-dose risks weighting the center despite redistribution. If you updose in this fashion, check that the distribution is even before tapping. Ideally little redistribution is necessary if you've rotated the portafilter while dosing (I believe it was Jason who said, "Focus on the landing" - good advice).
• Crashing against the screen - especially if you're updosing, this will lead to lower flow at the center and break the side adhesion.
• Brew pressure - while not a direct cause, it may be worth lowering the brew pressure 0.5 bar. Don't rely on the onboard gauge, they're not always accurate in absolute terms.
• Degassing coffee - not my favored diagnosis, but very fresh beans can "give up" early on an extraction.

Psyd wrote:You folks that have experience with humidity, is that a possible explanation?

Humidity would explain the need for a grind change, but not sudden and persistent donut extractions. I fought the troubles of changing humidity at the beach a couple years ago (link). Today I'd blame the coffee before the humidity (e.g., stale / degassing, bad blend mix).

HB wrote:Today I'd blame the coffee before the humidity (e.g., stale / degassing, bad blend mix).

*sigh* I wish I could get me some Rocket Classic right about now...
I've got some of that Paradise in the mail, though. That'll settle it!

To elaborate on my "bad bean" mix comment. Coffee beans have different hardness and thus grind differently, producing various amounts of fines. It can be problematic if a blend includes coffee beans widely separated on the fines production range and their proportions are off. It's an unlikely diagnosis, but it happens (Tim the head roaster at Counter Culture Coffee brought this up as a "diagnose the pour" puzzler).

Psyd wrote:*sigh* I wish I could get me some Rocket Classic right about now...
I've got some of that Paradise in the mail, though. That'll settle it!

I've been using this stuff in its place.

It's different, but similar and very good.

Espresso One: Cottonwood

HB wrote:To elaborate on my "bad bean" mix comment.

I was mostly teasing, but I meant to imply that this is a problem that has crossed blend lines. I've had the same issue with four previously problem free blends, from four different roasters, so it probably isn't the beans in my case.
In other news, I have been getting better and better pulls dosing 18g and messing with grind and tamp (reducing both a bit at at time) in an 18g basket. I'm not as pleased with the taste of the result as I am with the look and performance, so I'm contemplating returning to a slightly over-dosed basket and a thirty pound tamp. I am having fun seeing how the adjustment makes the cup taste different, and it's educating my palate along the way. I actually got dark chocolate and marzipan this morning! Previously I would get 'good' or 'not so good' as the descriptors, but lately I've been identifying wine tastes along with the marzipan and such.
I think I'm in the ristretto fan camp, though.