Lesson 13: Water Quality & Advanced Brew Science - The Chemistry of Coffee

From the KoffyKraft notes

Learning Goal

Understand the impact of water quality on brewing. Learn about hardness, alkalinity, and how to achieve ideal water for coffee extraction. Explore advanced brew science concepts: the coffee brewing control chart, TDS measurements, and how grind particle distribution affects flavor. By the end, you'll know how to tweak or source water for optimal taste, interpret advanced measurement data if available, and appreciate the chemistry side of coffee brewing that often separates a good cup from a great cup.

Core Concept Explanation

Coffee is ~98% water, so water matters tremendously . Also, extraction is essentially a chemical process happening in water. At an advanced level, paying attention to water can boost your consistency and flavor clarity:

Water Hardness & Composition: The Specialty Coffee Association's recommended water for brewing has the following ballpark: Total Hardness ~50-175 ppm CaCO3, Alkalinity ~40 ppm, pH ~7, with certain calcium and magnesium content (they actually contribute to extraction flavor) . In simpler terms: you want some minerals in water (pure distilled water is actually not great for extraction - it can under-extract and taste flat ), but not too much (extremely hard water with lots of calcium will not extract well either and leaves scale in equipment). Moderately soft water with some hardness is best. Many find around 100 ppm hardness with a mix of calcium and magnesium yields excellent results. Magnesium particularly is noted to extract slightly more flavor (some coffee scientists suggest Mg2+ extracts more fruity acids, Ca2+ more bitterness - interesting if true). If your tap water is hard (lots of limescale, >200 ppm), consider diluting with distilled or using filters (carbon filters won't remove hardness fully, but ion-exchange filters can soften). If your water is too soft or distilled, consider adding minerals. Products like Third Wave Water exist (packets of minerals to add to distilled water to coffee-perfect levels). Or DIY: some people add a pinch of epsom salt (magnesium sulfate) and baking soda to distilled water in precise amounts - not necessary for everyone, but a known hack in coffee geek circles .

Impact on Taste: Hard water with high bicarbonate (temporary hardness) can make coffee taste chalky, dull, or bitter because it overbuffers acids - flattening brightness. Very soft or distilled water can make coffee overly sharp or sour and lacking body, because it under-extracts some components. Ideal water brings balance - allows good extraction of desirable flavors. Also, freshness of water matters: use fresh cold water that hasn't been sitting boiled - boiling drives off CO2 and can change pH slightly; reboiled water sometimes tastes flat due to lack of dissolved gases (though minor, but some connoisseurs always use freshly drawn water). Practically: if you can use a filtered water (like from a fridge filter or Brita) plus maybe cut with some spring water, you often get a decent mix. If you want to experiment, try brewing one cup with pure distilled water and one with a mineral water (one that's not super hard, maybe a spring water with moderate minerals). The difference can be surprising - often the distilled brew is more sour and weak. There is a famous anecdote: brewing the same beans with different waters can taste like totally different coffees. Some competition baristas bring their own water to events to ensure flavor consistency.

Brewing Control Chart: We touched on measuring extraction. The classic Coffee Brewing Control Chart (originated by Lockhart in the 1950s) plots TDS (strength) vs Extraction Yield . It has a central square called the "ideal" or Gold Cup zone around 1.2-1.4% TDS and 18-22% extraction . It's mainly used for drip/filter coffee. If you ever measure your brew with a refractometer: say your V60 coffee has 1.30% TDS and you know you brewed 30g into 500g water yielding ~475g coffee (accounting for absorption), you could calculate extraction: EY% = TDS% * (Brew Output / Dose). There are tools/apps that do it. E.g., 1.30% * (475/30) = 1.30% * 15.83 = ~20.6% extraction - smack in the middle of ideal . If your coffee tasted weak and indeed you found TDS 1.0%, extraction maybe 18% - possibly slightly under. The chart helps diagnose: too low TDS and low extraction -> under-dosed or under-extracted; high TDS but low extraction -> maybe you brewed too strong (not enough water) but didn't extract fully; etc. Without a refractometer, you can still conceptually use it by taste and knowing your ratios. E.g., if you always use 1:16 and it tastes good, you're likely in good zone. If you try 1:20 and it's thin, you moved out of the ideal strength even if extraction might be fine.

Grind Particle Distribution: Advanced brewers sometimes use laser particle size analyzers or at least think in terms of distribution curves. What matters to you: a grinder that produces a narrow distribution of particle sizes (more uniform) yields more even extraction (less bitter and sour mix). If your grinder produces a lot of fines (dust) and boulders, your extraction will have both under- and over-extracted elements. Burr alignment, sharpness, and design all affect this. For instance, conical vs flat burr debate: flat burrs often have tighter distributions (less bimodal). Conicals might produce more fines which can add body but also muddle flavor. As a home barista, you might notice that as your burrs dull, your brews get slightly more bitter (more fines = over-extracting). The takeaway: grinder quality is perhaps the most important equipment factor for advanced brewing after water. Upgrading from a $50 grinder to a $300 grinder can dramatically increase clarity of flavor because of better particle uniformity. If you cannot upgrade, you can mitigate issues by techniques: sifting out fines or boulders (there are sieves like Kruve sifter, though that's quite geeky and reduces brew yield), or using the Rao spin trick in pour-overs to even out extraction despite some fines . Also, be aware of static and clumping (WDT can help in espresso, but in filter, just give the grinds a shake to break clumps when wetting).

Brewing Variables Interplay: At this stage, you likely realize all variables connect: grind, water, ratio, time, temperature. Changing one often requires adjusting another. Advanced coffee brewing is about understanding these interactions deeply. For example, if you use cooler water, you might need a finer grind or longer time to achieve same extraction. If you switch to a very high elevation Ethiopian coffee that's dense, you might grind finer or use hotter water to extract its flavors. Recognize patterns: high-density beans often need more energy (heat, time) than low-density (darker roasts or low elevation). With experience, you'll start predicting adjustments. This scientific approach ultimately empowers you to brew any coffee to its potential.

Exercise: Water Experiment

This will illustrate water's effect. Brew a small cup of coffee (say a 10 oz/300 mL pourover) with two different waters:

Water A: use distilled or RO water (or the softest water you have access to).

Water B: use a mineral water or tap water that's been filtered but still has minerals (or add a pinch of clean baking soda and a tiny pinch of epsom salt to distilled to create your own brew water-like 0.1g baking soda + 0.2g epsom in 1 liter, which yields about 50 ppm alkalinity, 30 ppm hardness roughly).

Use the same coffee, same grind, same brew method side by side. Taste them. Most likely, Water A (distilled) brew will be sharper, possibly more sour or empty in body, whereas Water B brew might taste fuller, sweeter, more balanced. This drives home water's role. If you have strips or a kit to test hardness, test your tap vs ideal. You might find, for instance, your tap is 250 ppm hardness - no wonder your brews sometimes have chalky notes. Then maybe try cutting tap 1:1 with distilled and see if brew improves.

Optional Advanced Exercise: If you have access to a refractometer (some coffee enthusiasts invest in Atago or VST refractometers), measure a brew's TDS and calculate extraction. If not, you can also try to estimate: a well-extracted cup should have certain telltale signs in taste - balanced sweetness and clarity. Try intentionally under-extracting a brew (e.g., super coarse grind or short contact) and see if you can identify under-extraction by taste (likely yes: sour, thin). Then try an intentional slight over-extraction (too fine or long brew) to taste the difference (bitter, drying). This sensory practice aligns with what the measurements would tell you.

Quiz (Self-Check)

What are two key minerals in water that affect coffee extraction, and how do they generally impact flavor?

Why is pure distilled water not recommended for optimal coffee brewing?

On the coffee brewing control chart, what does a point representing 1.0% TDS and 16% extraction indicate about the brew? (Under/over extracted? Weak/strong?)

How might an inconsistent grind particle size distribution show up in the taste of a pour-over coffee?

Answers

1. Calcium (Ca2+) and Magnesium (Mg2+) are two important hardness minerals. They help extract flavor compounds. Generally, some hardness (Ca/Mg) improves extraction yield and can enhance flavor (Mg in particular is noted to boost perception of acidity/sweetness). If either is too high, though, it can extract too many bitter compounds or mute acidity. Bicarbonate (HCO3-, alkalinity) is also key - it buffers acids; a little is good to balance acidity, but too much can dull the coffee and make it taste flat or chalky . (Your answer might mention alkalinity too, which is fine). In short: need some Ca/Mg for good extraction, but not too much. 2. Distilled water lacks minerals, so it under-extracts coffee. You get a lower extraction yield and the coffee can taste sour, overly acidic, or lacking body because there are no mineral ions to help bind with flavor compounds . Additionally, without buffering, the brew can be overly sharp. Coffee brewed with distilled often tastes "hollow" or "empty." Thus, completely pure water isn't ideal - coffee brewing needs some mineral content for optimal flavor . 3. 1.0% TDS and 16% extraction is under the ideal range in both strength and extraction . It indicates the brew is weak (low strength) and under-extracted. This could happen if you used too much water (so it's thin) and a coarse grind/short brew (so not much was extracted). The coffee likely tastes under-extracted (sour) and also watery. 4. If grind is inconsistent (lots of fines and boulders), the taste will often be muddled: you might get both some bitterness from over-extracted fines and some sourness or emptiness from under-extracted large particles. The brew might lack clarity - flavors won't be distinct. You could experience more astringency (from over-extracted tiny particles) and also perhaps a weak body (since boulders didn't contribute enough). Essentially, uneven grind = uneven extraction , leading to a cup that can simultaneously have harsh and sour elements and not a clean flavor profile. If you notice a mix of off-tastes, grind inconsistency is a likely culprit.

Reflection

Water and science can feel technical. Do you find this aspect interesting or overwhelming? Write down if you would take steps to alter your water or if you might in the future. Perhaps you'll stick to filtered tap for convenience - that's fine. Or maybe you're inspired to try mixing your own ideal water for a special coffee you have. Note your plan. Also, reflect on a time your coffee at home tasted amazing and a time it tasted bad - could water have been a factor (e.g., did you use bottled water that time it was great)? The next time you are dialing in a brew and it's not working, consider the water variable too. Additionally, think about how far you want to go with measuring. Some enjoy geeking out with refractometers and charts; others prefer to rely on sensory tuning. There's no right or wrong - just tools available. Write a sentence about your approach to advanced measurements: are you content to trust your taste, or do numbers fascinate you? This will guide how you integrate these scientific aspects into your coffee routine.