From time to time I present a lecture called ‘VST:WTF?’. It’s a two hour long foray into the science of measuring coffee with a refractometer. More than a thousand Baristas have sat through it, and now I’d like to share portions of the presentation with you as well. Each week I’ll be taking a couple slides from the VST:WTF? lecture and explaining them in detail.
(Props to Mark Free of Everyday Coffee for the lecture name!)
If you’ve never used a refractometer, don’t know what VST is, or just have no idea what I’m talking about, don’t fret! You’ll be a master on the topic in no time.
Let’s start at the beginning.
VST stands for Voice Systems Technology. It’s a company founded 25 years ago and operated by Vince Fedele. Vince is one of the most intelligent and pragmatic humans I know, and is pretty much always right. Over the years VST, in its various incarnations, has invented and sold a lot of really cool things, some of which you’ve probably used. As an Apple Developer & Partner in the Jobsian era VST made the first USB product (floppy drive!), the first firewire hard drive, the first smart-charger for Li-Ion, Li-Polymer, NiMH and many other advanced battery and storage related peripherals. Later on VST developed a Level 3 high resolution fingerprint imaging scanner and software for the FBI.
JSYK: Level 1 fingerprinting looks at simple pattern, or ridge flow. Level 2 uses features like ridge ends, bifurcations, dots etc. Level 3 is kind of insane, mapping the fingerprint ridge width/shape and even your pores, and can be as accurate as a DNA test.
Needless to say, VST isn’t mucking around.
Lucky for us, in 2008 VST started releasing products aimed squarely at the coffee industry, releasing the world’s first coffee refractometer and related software designed specifically for coffee and espresso. At this very moment, the coffee industry was thrust out of the stone age and into the 21st century.
Now would be a good time to remind you that I’m not paid by VST for anything. Sensory Lab and St Ali sell their products, but so does everyone else, because they’re the best. Thanks go to Vince for helping me with the lecture over the years, and for fact-checking this particular post.
What’s a Refractometer?
A refractometer is a device that measures the deflection of light as it passes through something. You know the whole ‘aim below the fish when throwing a spear’ thing you learnt in that movie sometime? That’s refraction at work.
Once you measure how far the light was bent, it’s communicated as ‘refractive index’ or RI. RI is a super useful number in all kinds of fields like agriculture, pharmaceuticals, photography, gemology, process control, and now coffee (it’s also how your car’s wipers know if it’s raining. I thought that was magic, but science wins again).
This is VST’s LAB Coffee Refractometer. It’s the only refractometer I recommend if you want accurate and consistent results.
The RI (Refractive Index) of a brew is measured by placing a small sample of coffee on a refractometer’s prism. The refractometer will shine light at the sample through this prism. The light doesn’t pass through the coffee, it only touches the boundary layer between the glass and the liquid and bounces back. This phenomenon is called the ‘Total internal reflection’ (you know when you’re underwater the surface looks like a mirror? That thing.). A linear detector then receives the light, and sends a signal based on where it gets hit, which can be used to calculate RI. Here’s what it looks like from the side.
When dealing with coffee, there’s a specific relationship between RI and the Total Dissolved Solids (TDS or strength) of the coffee, but it varies over temperature.
One way to measure TDS is by dehydrating the brewed coffee in an oven and weighing what’s left, but it takes ages, requires incredibly accurate scales and costs way too much. Luckily, VST’s refractometer measurements are as accurate and repeatable as a traditional dehydration oven paired with scales accurate to +0.0001g. These kinds of accuracies were previously only possible with laboratory apparatus costing several tens of thousands of dollars. So don’t complain that it’s too expensive.
When a brew is stronger, it will bend the light more. If it’s weaker, it will bend the light less. This is super useful, because it means that with a coffee refractometer, we can measure the strength of a brew with exceptional accuracy, provided the refractometer employs a sensor with enough resolution, and the requisite accuracy and precision (ability to repeat the same measurement). To get meaningful, accurate results from coffee with a refractometer you need incredible levels of accuracy. This thing is far from a sundial.
Reminder: when talking about Strength and TDS, I’m referring to the proportion of the beverage that’s made up of dissolved coffee flavour; NOT the perceived intensity. eg. My espresso is 10%TDS but my filter coffee is 1.5%TDS. (For more on strength read this post.)
Once you know the strength of a coffee, you can also calculate how much flavour you extracted from the dry coffee grounds. This is called extraction yield. (for more on extraction read my first post here). Extraction is even more useful than just TDS on its own. In combination, TDS and Extraction can tell you an incredible amount about your coffee, your brewing equipment, your brewing method(s), your roasting and even your technique. Herein lies the incredible value of refractometers for coffee.
Next week I’ll start delving into some common concerns and misconceptions about refractometry. Have you heard or have any? Please share in the comments below and I’ll do my best to address them next week!
In the meantime, if you’re looking to get a refractometer, head to the US-based VST store or hunt down a local distributor from further down that page. I would recommend getting the refractometer, some spare syringes/filters and the iOS/Android app (which is now fully-featured with charts and recipe creation just like the desktop version).
Pink Floyd were obviously fans of refraction and dispersion also.