some aspects of espresso extraction - 3

solubles yield by dose and grind

summary of results :

for a given basket, grinder, and machine, higher doses lead to lower extractions, lower doses to higher extractions.

measuring solubles yield :

as usual, andy schecter had asked the right question and done the right thing. he asked "how do you know a ristretto is really stronger than a normale? maybe the ristretto has less of the puck in it." then he started weighing shots and pucks.

i was obsessing about the extraction process when he emailed me about his project, and ı realized he had figured out a way to get control over the solubles yield in extraction. the yield is the percentage reduction in puck weight as its constituents are dissolved into the espresso. ıf one weighs the puck before and after the shot, one can figure it out. if one does it over and over again, for different baskets, doses, coffees, shot times, shot weights, and puts the data through the statistical meat-grinder; one can detect the patterns and learn to control the extraction. i was off and running.

the puck weighing has some additional requirements. the puck is wet after the shot, and needs to be oven dried before weighing. fresh ground coffee holds some water, so a grind sample also needs to be oven dried and the initial puck weights have to be adjusted accordingly. finally, some grounds from the puck cling to the shower screen and group gasket after the shot; these need to be recovered with careful brushing.

there are two sources of error in the yield results. any ground coffee not recovered for the weighing would reduce the paks weight and register as increased yield. the paks moisture contains extracted coffee which is added to back to the paks weight while baking, thus registering as decreased yield. marino petracco, in the extraction chapter in ılly, cites 25% yields for italian dosing standards. my measurements were about 2% to 3% less and may be an artifact of this soggy puck error. however, since these sources of error are the same for every observation, the relationship between yield and its predictors is unaffected.

controlling solubles yield :

my observations ranged from 15% to 25% of puck weight extracted. there were two outliers, one at 12% and one at 26%, both coming from extractions well outside normal espresso time and volume limits. the next task was to find out which of the controllable shot parameters can be used to set the yield.

grind setting was not consistently related to yield except when using the same basket, coffee, and roast level. different coffees, roasts and baskets require different grind ranges.

the major predictor of yield is the weight of the puck divided by the hole area at the bottom of the basket (p/a). i measured the diameter of the hole-punched area at the bottom of the basket and squared it. this figure is not the actual hole area; but is directly proportional to it and easy to determine. the p/a measure can be simply understood as the depth of the puck in the basket; a fat or deep puck extracts less solubles than a thin or shallow puck. when the puck is conical rather than cylindrical, so that the top surface is larger than the surface against the filter holes; the p/a measure can be understood as the average length the water has to travel through the ground coffee. it is operationally identical to the depth of the puck in a perfectly cylindrical basket.

this straight line relation explained about 75% of the variations in yield, and the predictions had a standard error of 1.8% yield level. moreover, the formula using p/a remained the same for all baskets, coffees, and roast levels for the same machine. in the graphic below, which applies to the elektra semiautomatica group, an lm double basket dosed at the ıtalian standard of 12 to 14 grams would yield about 22% to 20%, and only 17% to 18% when dosed at the us standard of 17 to 19 grams.

andy's observations with his highly modified silvia also showed the p/a relation with the same predictive power. however, the slope of his observations is different from mine. this graph shows only the readings i took on the elektra.

changes in shot time and weight also affected the yield, but the effect, when shots were cut as they blonded, was surprisingly smal l( 6 ). adding these variables to the predictor equation only improves the model's overall accuracy by about 3%, and only reduces the standard error in the yield prediction from 1.71% to 1.65%. given the added complexity of the formula, it has no everyday use.

the first reaction ı got to this work is that low dose shots probably taste weak when compared to high dose shots. this is true when a poorly trained barista pulls a shot with roughly the same volume for both doses. if one attends to the color of the stream when stopping the shot, the solids concentration in the cup remains fairly constant regardless of dose. ın this experiment, in the 24 shots ı made for the taste test, there was no relation between dose and the amount of solubles in the cup. ın the graphic below, shot concentration is measured as the weight reduction of the puck (solids getting into the cup) divided by the weight of the shot. the unit is parts per thousand, and the dashed line shows the 120 parts per thousand standard.

in practice, on any given machine, one can use a simple straight line p/a formula to calculate the yield for any basket and coffee. however, since it is different from machine to machine, and maybe grinder to grinder, all baristas will have to work it out for their own setups. fortunately, it is not required to know the exact figures. one only needs to know that going to a lower dose and finer grind in the same basket increases the extraction. one can taste the shots, analyze their flavors using the taste model in the last section, and make dosing changes in the right direction until one has found the correct dose for every coffee (7).

why does it work this way? why does only the dose and basket shape matter a lot, while the shot time and volume don't matter much ? to answer these questions, one needs to learn more about how the coffee extracts during the course of an espresso shot.