The only thing I appreciate more than receiving great questions is receiving amazing follow-ups! Last week, Tom asked about blends and I ended up taking my usual scenic route to explain that a tasty blend demands a compromise. On one hand, it features the best individual profiles for the component coffees. On the other, it ensures these separate parts share a level of solubility. So Tom’s logical next question: Do you measure solubility?
Yes! We have both direct and indirect measurements for solubility. Like many in the industry, we rely on refractometers. I realize how fun this is starting to sound but rather than prattling on about the technical, I am going to lean towards simplicity and speak generally. A refractometer is a device that calculates light deflection as it passes through an object (see all this non-technical fun?). There are specific iterations of these designed for coffee that measure the bending of light (off the margin stratum between coffee and the device’s glass). A hefty brew will bend that light like a eighties wrestler; while a weaker brew will politely allow the light to merge in its lane. To get a visual sense of this machine (pictured below), a droplet of coffee is put into the chamber and the screen (or app) then provides a set of numbers.
Refractometers provide several key insights. We know ‘extraction’ or the amount of grounds dissolved into the water. These are expressed as a percentage of the yield. We also have the ‘TDS’ or the ‘total dissolved solids’ which references strength. The more solids within the brew, the more concentrated the cup. These two numbers – yield and strength – give direct information on solubility and by extension development of roast. If you need to grind fine to reach appropriate yield, then the roast profile needs some work. So onto the more indirect measurements…
In our ‘staging area’, each batch of green is weighed to one one-hundredth of a pound before roasting. Through comparison to the final roasted weight, we establish a percentage of weight loss. While most of the change is due to moisture evaporation, there is also the break down of elements like cellulose. The higher this percent, the more porous the bean structure – translating to increases in measurable solubility.
At the cupping table, a sure sign of undevelopment is no ‘crust‘ forms after contact with hot water. It signals that the extraction is struggling, again pointing to low solubility (one note: old coffee regardless of roast level will lack a crust). Another one of my other favorites checks and balances: cracking open a bean! A well developed coffee will have a uniform colour across the diameter. If you see lighter edges with a darker core, the roaster failed to properly and evenly apply heat. When ground, those areas will be difficult to extract. The most foreboding sign: you cannot crack the bean at all! Send it back, it is not done yet.
All of these varied measurements would be indirect as their unique criterias motion towards a generalized, hazy sense of solubility. Unlike a refractometer, which presents direct information. However, we have not yet discussed the greatest tool: taste! Given that solubility relies on development, drinking a cup will tell you a great deal. If you ever have grassy notes or the aggressive sour notes of unripe fruit, the coffee lacks development and will be insoluble. At the other end of this would be a dark roast. If to escape the mouth burn of char, you find yourself grinding more and more coarse – you will know the coffee is very soluble.
Not all coffees are created equal due to varietal, density, moisture content, growing conditions, processing, and even shipping. Trusting your roaster and then figuring out a ‘baseline’ on a grinder (akin to the 0 on our parameters scale) is very useful for being able to measure solubility. As we *grind* to a halt here, more to come next week!
Lee Knuttila 