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January 30, 2017 /
VST:WTF Part 1

VST:WTF?

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.

 

If you have found this useful and want to enjoy delicious coffee with the rest of the community – register for our monthly Superlatives coffee subscription. Or if you just want to keep up with every thing Barista Hustle – sign up to the Newsletter.

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Nic Witzke
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Nic Witzke

2 questions!
1) Thoughts or experience with Atago refractometers?

2) Ive heard that VST refractometers are very sensitive to sucrose.. Essentially, coffees that taste better at higher TDS readings can partially be explained by higher amounts of sucrose, while coffees tasting optimal at lower TDS likely have lower sucrose amounts. Thoughts?

Matt Perger
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Matt Perger

I’ll be covering this yes – in short, suspended (undissolved) solids change the reading.

FiDi Man
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FiDi Man

I have only ever used one on my brews – I am sure you are going to share, but why do you need to filter espresso through those little disks? Would that not effect the TDS?

Apostolis Paipatis
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Apostolis Paipatis

Very useful information Matt in order to understand the function and the usefulness of VST and i hope in the next hustles for more inside information, regarding also tha collaboration of the VST and Extract Mojo.

Jason Reed Miller
Guest
Jason Reed Miller

Hi Matt, I’m so glad you are writing about this now. We have been experimenting heavily with VST refractometer to determine a universal extraction yield to strength ratio that is delicious for all filter coffees. Do you think one exists?

Nic Witzke
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Nic Witzke

Socratic Coffee rules!
I was first introduced to Atago through Joe and his research. Im thinking about ordering one as well!
Thanks Mitchell.

Justin Dedini
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Justin Dedini

It seems pretty standard to aim for about 1.35% tds with a 20-21% range in extraction. However, I really love 1.40%tds with 20% extraction, though the resulting brew recipe may not always be economically viable for most cafes. Don’t be afraid to experiment a bit, and serve what tastes best to you 🙂

Kimberly Snyder
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Kimberly Snyder

“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.”
Why does temperature affect how much light is bent? What temperature is idea/accurate to measure the TDS, and how far can the temperature deviate from that golden number before it starts to affect the reading?
Thanks for the read. Big time follower!

Mitchell
Guest
Mitchell

Hi Nic!

Check this out. http://socraticcoffee.com/2015/05/measuring-total-dissolved-solids-a-refractometer-comparison-part-ii/

There has been debate over its accuracy, but from this group’s study, it’s comparable, if not almost comparable to the VST III. I just bought the atago (TDS), waiting for it to come through the mail.

AndyS
Guest
AndyS

The density of coffee varies with temperature, and this changes the way light is bent. The ideal temperature to measure TDS is room temperature. If your refractometer instrument has been in the room long enough to warm up or cool down to room temperature, the instrument has been calibrated with room temperature distilled water, and your coffee sample has been cooled to room temperature before placing it on the prism, you will get the most stable and most accurate readings.

Matt Perger
Guest
Matt Perger

Temperature affects density, which directly affects refractive index.
The best temperature for your sample is the exact same temperature as the prism. Any difference between the prism and the sample creates an error. Best best is to cool a portion of the sample in a room-temperature ceramic before placing it on the refractometer.

Matt Perger
Guest
Matt Perger

Definitely a no on the universal number. There’s a range, but you’ll always find coffee that are deviant.

Matt Perger
Guest
Matt Perger

Hey mate

1) I don’t believe they have the resolution required for precision, or the unit-to-unit consistency for accuracy.
2) Refractometers are very sensitive to anything that changes the refractive index. Sucrose included. I don’t see how this could be a problem though, as roasted coffee contains no sucrose!

Ep 2. V is for VST with Jono from Rosetta | CoffeeBrewmance
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Ep 2. V is for VST with Jono from Rosetta | CoffeeBrewmance

[…] What is VST, and why is it important read Matt Perger’s post: baristahustle.com/vstwtf-part-1 […]

Roman Nejedlý
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Roman Nejedlý

Yeah, that might be true. I’m asking, because the only thing we’ve found out is, that VST is able to “break” the beam to 1024 fragments and measure their TDS (and then show the average TDS…), whilst the others can do “only” 256. But I’m not sure if it’s the only “advantage”.

AndyS
Guest
AndyS

Matt: hilarious! I didn’t see your reply before I posted almost the exact same thing. 🙂

Nick Brenn
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Nick Brenn

From my conversation with Vince Fedele a few months ago, he explained how the components inside of the VST are equivalent to what you’d find in a lab-grade benchtop refractometer.

Roman Nejedlý
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Roman Nejedlý

Hi Matt, why do you think, VST refractometer is the best one? Compared with Atago coffee refractometer for example…

AndyS
Guest
AndyS

Hi Mitchell: I’ve used both the Atago refractometers (a little) and the current VST Lab units (a lot). I can’t comment on their relative accuracy, but in terms of materials and build quality the Atago feels light duty and the VST heavy duty. It has a sapphire prism which is very expensive, but almost impossible to scratch. After each measurement I just wipe it with a paper towel and water. The Atago has some sort of glass prism; you have to use a lint-free cloth or you risk scratching it. The Socratic guys tested what they could, but build quality… Read more »

tacvbo
Guest
tacvbo

Hi Roman! Why you don’t take a look at “Measuring Total Dissolved Solids: A Refractometer Comparison” by http://socraticcoffee.com/ It’s a very objective three articles series, comparing 3 coffee refractometers including the last two versions of VST and the Atago. It seems that there is little to no statistically significant difference between them but still you have to make your own conclusions based on the information they gather.

Jason Reed Miller
Guest
Jason Reed Miller

Agreed!

Not only is 1.40% TDS with 20% EY very delicious to me, but I have found that, depending on the grinder being used, 1.40% with 20% EY is potentially a more “forgiving” place to dial in to!

Jason Reed Miller
Guest
Jason Reed Miller

Agreed. We were looking to find a recipe that is right in the center of the range so that it would be near the peak deliciousness for every coffee. We have had good results for the many, but have also had coffees that require a different recipe to be delicious. Here’s another burning question: I’ve been doing all experimentation on EK43 and Baratza Forte. I’ve found that extraction yields above 20% on the Forte tend to taste a little over, whereas many times I’ve gone all the way to 22% on the EK43 with very sweet and full results. Do… Read more »

Max Müller
Guest
Max Müller

Hi Matt, thanks again for that Hustle! Can’t wait for the upcoming VST parts to be online! Is there a possibility to buy your complete lecture as video, containing all the details? Thanks, Max.

Patrik Rami
Guest
Patrik Rami

Hi Matt! One concern with refractometry I came across was to do with evenness of the brew. Any brew will be essentially a mix of coffee grounds that were extracted a lot and some that were extracted less (especially with most espresso grinders). If the TDS reading is but an average, it may be easy to misinterpret the reading. Obviously, the solution would be to work on an more even extraction which is a topic of many hustles, but you won’t read that off of the TDS readings. Secondly, I learned that most of the time, when you mess up… Read more »

Ronnie
Guest
Ronnie

The looks to be the same as the MISCO Palm Abbe, but the patented equations that are programmed into it are the result of empirical analysis done by comparison to another established method (probably oven drying). The 1,024 figure would be the number of elements in the array, which you can think of like pixels for resolution. The math is very complex and uses both data inputs (refractive index and temperature) to achieve a reading with a warrantied accuracy of +/- .03% TDS. This level of accuracy is not possible by a linear correlation to a Brix reading or any… Read more »

AndyS
Guest
AndyS

Hi Patrik: I’m not Matt, but you raise two important issues. Taking your second point first, it is true that uneven extractions result in lower extraction yields. This is because the extraction rate of solids from coffee grounds decreases rapidly as more and more water is introduced. IOW, when water is added to grounds a lot is extracted at first, but very quickly the amount of solids available for extraction goes down. If the extraction is uneven it means that some areas of the coffee bed are “starved” for water while others have a surplus of water. The grounds that… Read more »

Jono Niclair
Guest
Jono Niclair

Hi Matt 🙂 I recently purchased the VST Coffee Tools iPhone App and realised after refracting a few brews that my extraction % were a bit different than what I would usually get when inputing the details into the desktop application (which has already been setup by the roastery I work in). I then had a look through the app settings which were all set to default and realised there are a lot of variables that could produce different extraction % readings. Please suggest best settings for both ‘Espresso’ and ‘Coffee’ modes within the iPhone app. Thanks!

stereobooster
Guest
stereobooster

Matt have you seen https://publiclab.org/notes/warren/08-06-2013/brainstorming-coffee-analytics-system-for-coffee-shops. I believe refractometers should be much more affordable than they are now

Examining the Impact of Particulate Matter in Refractometry for Coffee Assessment – Socratic Coffee
Guest
Examining the Impact of Particulate Matter in Refractometry for Coffee Assessment – Socratic Coffee

[…] the principle of refractive index (RI). For a clear, coffee-related explanation, we suggest reading this explanation. To think of it simply, a refractometer consists of a light emitter and a light […]

doublehelix
Guest
doublehelix

FWIW, for espresso, I went cheap and have been using a Milwaukee digital refractometer that costs ~$100. I’m mainly interested in establishing a “relative vector” regarding how my extractions are pulling, in place of “absolute numbers.” A couple of things to keep in mind when using this refractometer: 1-Calibrate with distilled water; also good to use a standard sucrose (table sugar) solution 2-Automatic temperature compensation, provided by the instrument, works–allow sufficient time for your sample to equilibrate with the prism 3-The Milwaukee refractometer does sport a flint glass prism, use soft tissues to wipe clean (I use cheap optical wipes)… Read more »

Ronnie
Guest
Ronnie

Never tried the Atago though. Accuracy, ease of use, durability, and a robust software sweet are really all that matters to me. VST has all of them, so it’s gold to me.

Kamil
Guest
Kamil

There is a mistake in first picture. “real position of fish” should be “apparent position”. According to watching a glass of water with a straw insite. eg.comment image

Matt Perger
Guest
Matt Perger

That’s spectrometry. Very different to refractometry.

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