some aspects of espresso extraction - 1

jim schulman

abstract

the paper presents and discusses data from three related aspects of espresso extraction:

the data sets used in this study are appended at the end.

how the shot taste changes as the yield, or total solids extracted from the puck, is varied.

how the yield can be controlled by varying basket geometry, dose and grind.

how these control input changes cause the yield changes.

introduction

some aromas that promise joy from a brewed cup of coffee elicit dread when smelled in a shot of espresso. why do so many great brewed coffees taste awful as espresso?

the espresso community has long established rules about this. roughly stated, coffee for espresso has to be low in acidity, not too lightly or darkly roasted, heavy bodied, and contain a significant percentage of dry processed beans. but despite everyone's experience with spectacularly acidic or bitter shots has developed, there is no convincing logical reason for this.

espresso is more concentrated than regular coffee; so strongly flavored coffees are said to become too strong as espresso shots. but the oils and crema of espresso buffer its extra strength. moreover, the bad taste of these coffees done as espresso seems less about it being too strong, and more about being unbalanced. the explanation for the unsuitability for espresso of some great brewing coffees may not be in espresso's strength, its extraction of lipids, or its creation of crema. rather, it may be that espresso brewing extracts flavors differently, and alters their balance in the little cup. this paper tries to get some answers to what this difference is, how it occurs, and how it can be controlled.

espresso taste at different levels of solubles yield

summary of results : under-extracted espresso tastes excessively sharp and acidic ( and salty ). properly extracted espresso has the sweetness to balance the acids and bitters. over-extracted espresso tastes dull and tarry, or just bitter-sweet.

coffee strength, solubles yield, and taste

everyone knows espresso is stronger than regular coffee; the same tablespoon of coffee brewed into six ounces of regular coffee is used to make one ounce of espresso. but this is not the whole story. not all the ground coffee goes into the cup or shot. in properly brewed coffee, about 18% to 22% of the ground coffee is dissolved into the water, while the rest is spent grounds; the range in espresso shots is wider, running from 15% to 25%. this proportion of ground coffee that ends up in the cup is called the solubles yield; a brew with too low solubles yield is called under-extracted, while one with too high solubles yield is called over-extracted.

it is important to understand that solubles yield and brew strength are two separate things. for instance, imagine that the six ounce cup of brewed coffee was extracted at 24%. its strength would be 24% of the coffee measure divided by 6 ounces or roughly 4% per ounce. if the same measure in a shot of espresso was extracted at 16%, all of it would still be in one ounce of water, so the under-extracted espresso would still have four times the strength of the over-extracted brewed cup(1).

if we imagine tasting the same amount of extraction in less and less water, we would notice the same aromas and tastes getting stronger and stronger. in an analogy, it's the volume getting louder, but the music staying the same. how about if we taste more and more extracted ground coffee in the same strength of brew? roasted coffee has over a thousand different flavor chemicals. some of these dissolve quickly, some more slowly. as the grounds become more extracted, the slower dissolving flavors become more prominent in the brew. so, in our analogy, it would be the volume staying the same, but the music gradually changing.

this paper is about the solubles yield and how it a affects the taste; it is not about brew strength. the brew strength of a standard espresso shot has about 120 parts per thousand coffee solids. the shots i made for this paper ranged from about 100 to 160 parts per one thousand, with weaker and stronger shots occurring at all solubles yield levels. ın the data analysis, the effect, if any, of the brew strength, is removed from the results.

how to taste for proper extraction

with a thousand flavor compounds to play with, no two coffees are, or ever will be, the same. so how can there be any way of knowing what extraction is best for what coffee? fortunately, these flavors can be grouped into a few large families, so that all the members of a given flavor family extract in similar ways. this work was done by ted lingle, who grouped the flavors by molecular weight, with the light weight ones dissolving quickly, and the heavy weight ones dissolving slowly :

there are two light weight, fast dissolving, families that are fully present even in under-extracted coffee.

i. fruit acids : they have fruity or floral aromas and flavors, crisp tastes in sweeter brews, and sour tastes in less sweet ones. these dissolve the fastest

ii. maillard compounds : they have the aromas and flavors of toasted grain, wood, tannins, or nuts; and tastes which are sharply bitter in less sweet brews, and warm, round, and malty in sweeter ones. these dissolve more slowly than the fruit acids, but will still all get into even the most under-extracted cup or shot

there are also two heavy weight, slow dissolving families which require high solubles yields to reach their full strength.

iii. caramels : they have caramel, vanilla or chocolate flavors and a sweet taste. since almost all sugars in green coffee are caramelized during the roast, these are the primary source of sweetness in coffee. dark caramels, which taste bitter-sweet, dissolve more slowly than light caramels, which taste more sugary. some light caramels will get into even lower extractions, but require higher extraction rates to be completely dissolved.

iv. dry distillates : they are reduced (burnt) caramels and maillard compounds that become dominant in dark roasts. they have the aromas and flavors of clove, tobacco, peat, or turpeny, a dully bitter, ashen taste in less sweet brews, and a bitter-sweet molasses taste in sweeter brews. these dissolve very slowly, but are tasteable at very low concentrations. their presence is usually a good reason to keep extraction levels fairly low.

this flavor classification ( 2 ) provides a road map to a balanced coffee extraction, either for brewing or in espresso. while describing the taste of coffee both accurately and in detail is an art; it is fairly easy to sort the tastes and smells into these four broad groups. it is even easier to tell if the coffee is under-extracted, properly extracted or over-extracted:

i. under-extracted : do the fruit acids taste sour, the maillard compounds sharply bitter ? it's under-extracted.

ii. properly extracted : do the fruit acids taste crisp, and the maillard compounds warm and round ? it's properly extracted.

iii. over-extracted : are dull or tarry tastes overwhelming everything else ? is it mostly bitter-sweet caramels and molasses with nothing else ? it's over-extracted.