Measuring/Hitting FG, System Designs, & Temp Impact on pH
Q. I monitor my fermentation with a Tilt hydrometer, but back it up with a standard hydrometer. The two are always a little off but are good checks. My final gravity (FG) is always high, no matter what style I am brewing. If the target is 1.018, I’m usually finishing at 1.022. Although I calibrated my Tilt, the hydrometer reads 2–4 gravity points higher for final gravity even when adjusted for temperature. What am I doing wrong to always have higher gravity at the end of fermentation?
Barney Heller
North Wales, Pennsylvania
A. Well, Barney, this question touches on two separate pain points in brewing — measurement challenges (calibration) and final gravity issues.
One of my brewing touchstones is to always give instruments a serious side-eye. I don’t recall when I began questioning instruments, but know that mistrust is an asset. You have two instruments that are supposed to measure the same thing and have two different results. You have two options: Compare your Tilt and your hydrometer against standards (and when you say you calibrated the TIlt hydrometer, I’m guessing this is what you have already done) or add a third instrument to the party. Although the second option is not a terrible idea, unless the third instrument has been certified all you will do is add more confusion to things. So, what about bumping these up against a standard?
The gold standard for specific gravity is pure water with a density of 1.000 kg/L or a specific gravity of 1.000 (SG is unitless as it compares the density of one liquid to that of water). For many instruments, a single-point calibration is insufficient and a second or third calibration standard is required. Examples of multi-point calibrations include pH, temperature, and mass. This is also true of density, but once a hydrometer of a given length and weight is calibrated over a range using at least two calibration standards, the calibrated scale can be replicated. The takeaway is that you have completed the first step in sleuthing out the measurement by dropping your hydrometer and your Tilt into pure water and measuring the density. They both should read 1.000 at the water temperature your hydrometer is calibrated (your Tilt has a built-in correction).
My distrust of instruments is generally related to devices with “black boxes” that bring in some sort of input and return a value. Measurement errors often result from something awry with the black box input. This could be a dirty sensor, something touching a sensor, or interference with moving parts. The Tilt is a clever device where density is determined by the angle that the Tilt device floats in liquid. As density drops, so does the Tilt device. And as the Tilt hydrometer sinks, it becomes more vertical. Drop the same Tilt hydrometer into a high-gravity wort, and it will lean more horizontal.
Both of your devices have simple measuring principles, although the inner workings of the Tilt are nifty. And both devices will be affected by deposits on the surface that change the weight of the device; make sure they are both clean. My money is on the Tilt for being correct and your hydrometer for being off. I guess this is a good time to mention that you are probably not the problem.
Hydrometers rely on the proper placement of a slip of paper for proper calibration. Misplacement by a couple of millimeters in a short hydrometer can result in significant errors. This is why it is critical to always test hydrometers in standard solutions. For those of us using sets of tall hydrometers with relatively narrow ranges, for example 1.000–1.034 SG, 1.032–1.068 SG, and 1.065–1.101 SG (or 0–8.5 °P, 8–16.5 °P, and 16–24 °P), calibration is easier said than done. Suffice to say, don’t trust a hydrometer further than you can drop it before first checking it out.
Missing your FG is a deep topic that I will simply dip my toe into. For starters, the FG of a brew has a lot to do with malt, mashing, and yeast. Change any of these things and expect a change in FG. But then there is the published FG. What does this mean? Is it a value plucked from the performance of a single batch of beer or is it the average FG of many, many brews of the same recipe? Here is the thing with FG . . . it usually contributes less to body and flavor than brewers think. The one exception to this is when a beer finishes high because of unfermented sugars that are sweet.
Details aside, if you want a drier beer, there are a few easy things to try. The first is extending your mash temperature in the 149 °F (65 °C) range. Sixty minutes is long enough to produce highly fermentable wort. Another thing to consider is to back off specialty malt additions, like crystal and caramel malts, that boost FG. And then there is yeast strain; yeast strains that are either unable to ferment maltotriose or those that do so poorly will leave higher finish gravities compared to strains that do ferment maltotriose. For the latter, most lager strains and ale strains like Chico gobble up maltotriose like nobody’s business.
Q. By reading one of your explanations on “simplifying brewing” I understand that you use a Grainfather all-in-one brewing system. I have used the Grainfather G30 for about eight years. From the very beginning I was upset with the non-homogeneity of temperature during mashing. Because the temperature measurement position is located under the false bottom, I have concluded that the wort atop of the grains is much cooler (confirmed by measurements with an external thermometer).
I saw that the grain tube in the new Grainfather model has perforations on its cylindrical surface, so I am updating my system with a new basket. Though I question whether part of the wort will not be flowing through the grain with this design and the efficiency will be severely reduced. What is your opinion on this?
Luiz Rebouças
Via email
A. Luiz, thank you for the great question. I do brew using a Grainfather G30 and am familiar with how the original system is designed, as well as the new basket. For those of you who are not familiar with these systems, there are two main parts of the Grainfather and other systems based on the same basic all-in-one design (see diagrams shown in Figure 1).
The brew kettle is heated from the bottom using an electric heating element positioned on the bottom of the kettle from the exterior. When looking down into the kettle, the heater is not visible. When used for mashing, a smaller mash basket is inserted into the kettle to hold the mash. In the original basket shown on the left in Figure 1, wort flows down through the bottom screen, into the pump and is returned to the mash basket onto the top screen. Wort pooling above the top screen flows directly to the bottom of the vessel through an overflow pipe to prevent the pump from exerting too much pull on the mash and from starving after all wort outside of the basket has been pumped to the top.
In my experience, the original design works best when using coarsely milled malt or more finely milled malt in conjunction with rice hulls because the mash bed is more permeable. The issues I have experienced with the original design are variable yields, occasional long wort collection times, and difficulty with uniform mash temperature. This sounds like your experience.
One thing that works for me is to start my mash at about 140 °F (60 °C), periodically stir for about 15 minutes, then install the top screen, and start the recirculation pump and the mash profile. If I am using a single mash temperature, I start my mash at about 149 °F (65 °C), periodically stir for 15 minutes, install the top screen, and start the pump and simply set my mash temperature at 149 °F (65 °C) to maintain temperature. Mash stirring during the beginning of the mash really helps with thorough hydration of the malt while also moving things around to improve extraction. I spent my commercial brewing days using stirred mashes and really like the yield improvement and increased consistency between batches that stirring provides.
To answer your questions, I contacted Aaron Hyde with RahrBSG to get some information about the new basket design used in all new Grainfather systems. Aaron is currently RahrBSG’s Director of Product and Portfolio and the former General Manager of Portfolio and Strategy for Bevie, the New Zealand-based company that produces the Grainfather. The basket redesign was Aaron’s brainchild.
“I suspected side perforations would improve temperature control because the perforations improve wort flow through the mash, even when thick and sticky, which is why the overflow pipework on the old system was needed.” Aaron also felt that adding side perforations would not decrease efficiency because liquid tends to flow down through the grain bed during draining. In practice, users of the new design report higher yields in comparison to the old design and find the new design to perform more consistently from brew-to-brew.
One thing to consider is sparging technique. Some brewers like to keep a small volume of water above the mash bed during sparging and time additions or the flow rate of continuous sparge additions to maintain a consistent level of water. With the new design, that method would indeed result in water flowing out of the side perforations. The best approach to sparging is to add sparge water in batches until it just begins to pool. After a couple of minutes of draining, add more sparge water.
The larger models are equipped with a sight tube showing wort volume in the kettle, while the G30 does not have this feature. I use a calibrated wooden stick (flat yard stick purchased at the hardware store) with my G30 to monitor how much wort I have collected and use this information to gauge when more sparge water is needed (to use this stick, I slip it between the kettle and grain basket wall and look for the top of the wetted portion). For example, if adding sparge water in 2-quart (2-L) increments, waiting for the kettle volume to increase by 2-quarts (2-L) indicates when the next addition can be made.
I do think that the questions posed make sense, but at the end of the day, the improved liquid flow through the bed during wort recirculation outweigh the small volume of wort flowing outward from the perforations.
Q. I am watching John Palmer’s water presentation on the BYO website and he got into pH a little bit. I have always been confused about the change in pH when taking a sample. If I am understanding what John is saying, mash pH is 0.3 lower than the pH meter reading at room temperature or below? If that is the case, to ensure my mash pH is 5.2, the reading on my pH meter should be 5.5, correct?
Sometimes when I take a sample, I put it into an ice bath to quickly cool it down. If I am not careful, sometimes the temperature drops down to ~63 °F (17 °C) or so. What impact does measuring at this temperature have on calculating the mash pH?
Rick Bray
via email
A. I think the best way to explain this is to start with a brief discussion about pH and why temperature affects it. pH is a measure of hydrogen ion concentration using a logarithmic scale, where pH = -log [H+]. The pH of pure water is 7.0 at 77 °F (25 °C) because the concentration of hydrogen ions is 10-7 moles per liter — noted as [10-7] using standard chemistry shorthand — because of the equilibrium of water with its dissociated ions as shown below:
H2O H++ OH–
The equilibrium of molecules is governed by an equilibrium constant at a specific temperature. The equilibrium constant of water (written as Kw) is 10-14 at 77 °F (25 °C). As temperature increases above 77 °F (25 °C), or standard temperature used in chemistry, dissociation increases as does the concentration of hydrogen ions. Because pH is defined as the -log [H+], an increase in [H+] corresponds to a lower pH. Acidic solutions have a higher concentration of hydrogen ions than pure water and bases have lower hydrogen ion concentrations compared to pure water.
The graph shown in Figure 2 illustrates that water pH ranges from 7.5 to 6.1 over the temperature range from 32–212 °F (0–100 °C).
Using water as the topic of discussion, Figure 2 shows that water with pH 6.5 measured at 140 °F (60 °C) will increase to pH 7.0 when cooled to 77 °F (25 °C). However, this assumption becomes invalid if there is anything in the water that acts as a pH buffer. Buffers are systems of organic acids that can bind hydrogen ions through their own equilibria. For example, carbon dioxide readily dissolves in water and exists in three forms — carbon dioxide, bicarbonate, and carbonate, as shown in the following equation:
H2O + CO2 HCO3– + H+ CO3–2+ 2H+
Back to the assumption that water at 140 °F (60 °C) with pH 6.5 has a pH of 7.0 at 77 °F (25 °C). This is a poor assumption because the atmosphere contains about 0.04% carbon dioxide. Mashes contain much more buffering compounds compared to the small amount of carbon dioxide contributed by the atmosphere. These buffers include proteins, amino acids, phosphates, and nucleic acids from malt, plus carbonate from brewing water. To further complicate things, calcium and magnesium from brewing water both cause a reduction in mash pH because they react with malt compounds. In practical terms, this means that the mash system is heavily buffered and that changes in mash pH as a function of temperature are not as big as changes in pure water pH.
Life is full of approximations. The typical thumb is about an inch (2.5 cm) wide. A stone fetched from a pile of standard stones weighs 14 pounds (6.4 kg). A hand is 4 inches (10 cm) measured from thumb to opposite side of palm. And mash pH drops by about 0.30 pH units when cooled from mash to room temperature. One thing we know about these approximations is that they are indeed approximate!
The best way to consistently monitor mash pH is to either cool it to 68 °F (20 °C) — not 77 °F (25 °C) because biochemists use a different set of rules than physical chemists — or measure mash pH hot. If you prefer measuring mash pH at 68 °F (20 °C), you should use published pH ranges that are associated with cooled samples for your target range. Although the ranges vary by source, 5.45–5.65 at 68 °F (20 °C) agrees with textbook information. Some references, most notably Malting & Brewing Science by Hough, Briggs, Stevens, and Young, provide mash pH at mash temperature and at room temperature. The true confusion with this subject comes from the lack of temperature reference in nearly all published data about mash pH. Given the well-known effect that temperature has on pH, it’s appalling that brewing scientists and academics have omitted this important detail.
Hopefully the background about pH and temperature is useful. Now let’s apply this information to your specific questions. You correctly understand what John is saying. The mash pH is lower than the pH measured in a cooled sample. Is it 0.3 pH units lower? The only way to know is to measure the pH at two temperatures because the mash buffering systems are too numerous and variable to predict the temperature effect.
Yes, if you are targeting pH 5.2 for your mash pH, then you want your reading to be higher when measuring a cooled sample. I will come back to this in a moment.
If you cool your sample to 63 °F (17 °C) instead of 68 °F (20 °C), you cannot use the same approximation for the offset. Instead of the difference being ~0.3 pH units, it may be closer to 0.32 pH units. Is this difference going to change your beer? Probably not, unless you are brewing the same beer many times a year on a commercial scale.
Now that I have answered your questions, let’s muddy things up a bit! pH 5.2 likely became a target for mash pH because of the following excerpt from Malting & Brewing Science:“ An infusion mash is best carried out at pH 5.2–5.4. Consequently, the pH in the cooled wort will be 5.5–5.8.” I think the first sentence became part of the homebrewing zeitgeist while the values in the second sentence were forgotten! I suggest changing your target pH at 68 °F (20 °C) to be in the 5.5–5.8 range.
