I know the easy answer is flow rate, but which variables control the flow rate?

Early results with my new Silvia have me a bit perplexed about the relationship between grind, dose, tamp and shot volume.

Most of the time, my doubles end up at around 1.5 oz or less. I stop when the shot begins to blond. If I grind finer, the shot takes longer, but the volume is about the same -- 1.5 oz or less. If I grind coarser, the shot takes less time, but the volume is about the same -- 1.5 oz or less. Tamping seems to affect the dwell time, but doesn't seem to affect volume all that much.

This leaves dose and pressure. If I want to get closer to a 2 oz double, should I increase the dose? If dose is the main controlling factor, then if I'm trying to go to a 14g dose with an SO varietal, am I stuck with a 1.5 oz volume? Could the result I'm seeing be due to Silvia's pressure being a tad too high? (I'm working on sealing my new PF pressure gauge and should be able to determine the pressure soon.)

In a related, newbie question (please pardon my ignorance), are my 1.5 oz shots effectively ristrettos?

It's an excellent question, with lots of answers and question marks.

The simplest is when there is a flow meter (full auto) or you cut shots by volume itself, in which case, the volume is whatever you want.

If you cut the shots on a stop watch, then the volume depends only one the average rate of flow. This will make it a function of grind, coffee dose, and the basket geometry. Fix the dose and basket, and changing the grind, changes the volume.

But if you cut shots by blonding, it gets complicated; so this question is excellent (and lots of fun).

My feel is that if you tighten the grind, keeping everything else the same, the shots take longer and have less volume reaching the same blonding point. This is based on what used to be known as Al's Rule, that ristrettos should pull longer than normal sized shots.

You're observing the same volume to the same blonding point regardless of grind or shot time. This contradicts the standard account; but it could be true. Nobody's ever really measured a blonding point.

The blonding point is a stand in (aka instrumental variable) for extraction -- as you extract more and more from the puck, the flow gets lighter and lighter, so the same color means same extraction. It could be that for some coffees, this only depends on the volume, and not the time.

However, since nobody's managed to actually put a number on the degree of blonding or flow color, we've never really correlated it to extraction.

From what you say and from what I've seen on this site (e.g., When did this espresso extraction go blond?), determining the point of blonding is something of a subjective exercise, which makes this more difficult.

The Golden Rule doesn't adress blonding. It only says that you should get a certain volume in a certain amount of time. And in Perfecting the Naked Extraction Dan says:

Although stopping the extraction when blonding ensues will generally get you the best shot possible for a particular extraction, correcting for an improper rate of extraction leads to the road of better espresso. If the extraction is too slow, the espresso may be bitter with a dark brown crema; if the extraction is too fast, the espresso tends to be sour and the crema uniformly cinnamon colored. Taste is your ultimate guide to correcting errors in grind, dosing, and temperature. Applied consistently, getting good shots is mostly a matter of technique.

I had to read this several times, but I take it to mean that right now I'm optimizing my shots by stopping just as they go blond, but that my shots would taste better if I adjusted grind, dose and temperature for the Golden Rule volume in the Golden Rule time (1.5-1.9 oz in 22.5-27.5 seconds, at least according to the Italians.)

But what do I do if the shots go blond well before the target volume and/or time? They certainly won't taste better in that case.

dgreen wrote:But what do I do if the shots go blond well before the target volume and/or time? They certainly won't taste better in that case

If your shots go blond much too early (well before you're even in the ballpark of target volume) that means that you have ground much too coarsely or there's significant "channeling" if the grind was fine enough, that is, water is not percolating through the grains and extracting the good stuff but instead finding a place or several places to squirt through.

So you refine the grind and make sure you're distributing the coffee in the basket properly.

If you're confident that grind and distribution are good, and you still see egregious or much too rapid blonding, you might have your machine checked out: if the brew-pressure is sky-high, channeling will result no matter how perfect the grind and distribution.

dgreen wrote:are my 1.5 oz shots effectively ristrettos?

Ristretto and Normale and Lungo are basically designations for how "thick" or "viscous" or "dense" the coffee is; the names reflect water-to-coffee-solids ratios, not volumes, even if there is a strong correlation with volume given typical dose, pressure, etc. You could have two 1oz cups, one ristretto, one normale, even if both baskets had been dosed equally; the brew ratio would depend on all the variables that factored into each extraction. You wouldn't really know what to call a cup that had been pulled early because it was blonding early. The names have meaning only when things are going right. Even if you say to us, it was a 14g dose and there was 1.5oz in the cup after 20 seconds, we wouldn't know how the beans had been ground, and whether the flow had been constant or balky for those 20 seconds. We couldn't assess the quality of the extraction from the dose, the duration, and the resulting volume without making assumptions.

As I mentioned in a couple of other threads, I lowered the brew pressure on Silvia from about 10 bar to about 9 bar. I also spent a lot of time (and coffee), trying to precisely dial in a 14g dose of Ethiopian Ademe Bedane SO from Terroir. I worked hard on keeping the doses within .1-.2 grams of each other. Another thing I did was get a Tamp Mat so I could ensure that my tamps are as consistent as possible. I realized that I was varying tamp pressure quite a bit from one shot to the next, experimenting with different tamp pressures, and felt it would be a good idea to eliminate this variance and settle on one consistent tamp pressure so I could focus on the differences between various grind settings.

The results were twofold: First and foremost, the shots taste better. They're now landing between the too-sour and too-bitter range. It's hard to say, but I think lowering the brew pressure helped. It also helped to pull shot after shot making small adjustments to the grind until the shots tasted better. Second, I can pull two nearly identical shots in a row if I don't change anything (i.e., I'm finally acheiving some consistency, at least with a particular coffee at today's humidity.)

The volume is still running about 1.5 oz, but I'd say on the high side of that. I'm thinking that given my other parameters, 14g of this particular coffee isn't going to produce more volume unless I let it blond longer, then it will be bitter. I recall that Ken Fox, in his famous Basket Overloading thread, was pulling 1.5 oz shots with 14g of SO coffee.