What You Need To Know About Solubility and Making Coffee

This article discusses how solubility data is interpreted and its meaning when applied to quality control. The basis for the following discussion is the theory that, when there is a distinction between filter and espresso roasting, the meaningfulness of cupping solubility data differs. Furthermore, that cupping solubility data is insufficient as an indication of optimum espresso roast development.

I hope that this promotes discussion into solubility, its applications to roasters and its meaning for baristas.

At Assembly our objective, in all processes preceding the service of brewed coffee, is to optimise the inherent flavour capacity of a given green bean and to do so with absolute consistency. The meaningful interpretation of solubility data is a significant indication of how well this is achieved within the context of quality control. It is also an important consideration for baristas when making selections from roasted espresso samples.

The format for this article is as follows:

• An introduction to the topic of solubility and its relevance to the process of roasting, brewing and analysing coffee
• A discussion of the distinction between espresso and filter roasting referring to the variation in byproducts created and, thus, soluble material
• A comparison of espresso and cupping brewing as a basis for explaining inconsistencies that may arise in quality control
• Concluding remarks and proposals for drawing more meaningful insight from solubility data

This article is based on the premise that there is a distinction between roasting processes for espresso and filter coffee. Roasting for espresso is implemented at Assembly in order to balance values we have identified within the coffee drinking public without obscuring the inherent characteristics of a given coffee.

“Bean genetics and the roasting process have significantly more influence on the potential solubles yield of a coffee”

Solubility in coffee

Solubility is the capability of a substance (solute) to dissolve into another substance (solvent) to produce a solution. In the context of coffee brewing this means the capacity for solid roasted coffee to dissolve in water forming a solution of dissolved coffee and water. As ‘green’ coffee will not dissolve readily in water it is roasted to create a more efficient solute. Variables in the roasting process will determine the degree of solubility inherent in the roasted coffee.

A lot of conversations in coffee focus on solubles yield. This describes the percentage of solid coffee which has been dissolved into a beverage. For example, a particular brew may have a solubles yield of 20%, meaning that 20% of the coffee has been dissolved or extracted. 'Positive soluble material' is commonly used to refer to desirable flavour characteristics created by roasting coffee. Inversely, 'negative soluble potential' refers to a coffee of high solubility but consisting of undesired flavour characteristics.

From a barista’s perspective the solubles yield has traditionally been used as an indication of brew method efficiency but the inherent capacity for a given coffee to be ‘soluble’ is more effected by other factors prior to brewing. For example bean genetics and the roasting process have significantly more influence on the potential solubles yield of a coffee.

Roasting also determines the make up of soluble material on a spectrum of volatile to stable. For example one roasted coffee might possess a higher ratio of volatile (enzymatic) byproducts as a result of the roasting process while another may contain a higher percentage of stable (maillard/sugar browning) soluble byproducts.

“Given the stability of the soluble compounds in an espresso roast it is likely that the delicate brewing conditions of a cupping brew will not be sufficient to access all the soluble matter”

Roasting for espresso

Roasting for espresso is a practice commonly implemented by roasters for a variety of reasons warranting separate discussion. At Assembly we roast espresso to optimise the flavour capacity of a given bean and to best suit the needs of the cafes serving it. Currently this manifests itself in roast profiles which are designed to preserve balance while softening acidity and enhancing sweetness. Roasting in this way means marginally more development of the green coffee and therefore different characteristics within roasted bean.

Some of the enzymatic properties are deliberately degraded as the bean becomes more developed as more maillard compounds are created the bean fibre becomes less elastic. Ultimately the ratio of soluble matter which is present in the bean moves from volatile to more stable. As the bean becomes more brittle, it will also result in different mixes of particle sizes once ground which may effect particle suspension but not necessarily solubility. This is however a separate matter.

The diagram below illustrates the spectrum of compounds which can be created via the roasting process.

Specialty Coffee Association of America

Espresso and cupping brewing

Evaluating both consistency and optimum solubility usually begins by cupping coffee. Here it is proposed that when filter and espresso roasts differ so too does the meaningfulness of cupping solubility data.

Coffees which have been roasted for espresso, relative to filter roasts, possess a higher percentage of maillard reaction compounds and have undergone more bean fibre manipulation. In other words, the soluble matter which exists in a coffee roasted for espresso is less volatile then that of a filter roast. This means it is less readily extracted.

“High cupping solubility readings are not necessarily indicative of optimum roast development of espresso. Solubility data instead should be assessed relative to the degree of consistency maintained”

Given the stability of the soluble compounds in an espresso roast it is likely that the delicate brewing conditions of a cupping brew will not be sufficient to access all the soluble matter. In this case potentially only a portion of the soluble matter, which is of amore volatile nature, will dissolve. Using intense pressure however, an espresso brew, will more easily access all of the soluble material in the coffee and hence, result in a more accurate representation of roast development. The potential for maximum extraction in an espresso brew will be dependent on specific parameters of the brewing technique. Furthermore as a result of greater bean fibre manipulation in espresso roasts ground coffee will produce greater discrepancies in particle size. This will further restrict the extraction potential in a bowl by way of particle suspension. This can be explained by the presence of fines, that don't necessarily constitute any soluble matter, but will saturate the brewing water.

Alternatively, a filter roast which inherently possesses more elastic bean fibre a higher concentration of volatile, enzymatic compounds is more readily dissolved and, thus, can be expected to extract at high percentages in the delicate brewing conditions of a cupping bowl. In our experiences, these coffees have limited extraction potential in espresso brewing. It is hypothesised here that the higher percentage of volatile compounds are lost under intense brewing conditions and there is simply less stable soluble matter available to extract as a result of lighter roasting.

Conclusions

The conclusion proposed here is that high cupping solubility readings are not necessarily indicative of optimum roast development of espresso. Solubility data instead should be assessed relative to the degree of consistency maintained.

We employ the following criteria as part of the Quality Control Programme at Assembly:

• Extraction of filter in a cupping bowl as an indicator of consistency among roasts and optimum roast development.
• Extraction of espresso roasts in a cupping bowl as an indicator only of roast consistency.
• Extraction of espresso roasts in espresso brewing as an indicator of optimum roast development.
• Espresso roasts outside of reasonable parameters in a cupping bowl are indicative of potential roasting fault and flagger for further testing.
• Below .9% reflects a potentially overdeveloped or unevenly developed coffee
• Above 1.3% reflects potentially an underdeveloped coffee and/or baked - uneven roasts

Specific to Espresso:

• Below .9% reflects a potentially with overdeveloped and underdevelopment or unevenly developed coffee
• Above 1.3% reflects potentially an underdeveloped coffee and/or baked - uneven roasts

Originally published: 24 Oct 2015