FAQ and Documentation for the SMPH IBU Calculator

Background:

• What is the SMPH model?
  The SMPH model is a method for estimating IBUs in finished beer. This method accounts for the contributions to the IBU from both isomerized alpha acids (IAAs) and auxiliary bittering compounds (ABCs). It also accounts for changes to IBUs from the hopping rate, hop form (cones or pellets), a hop stand or whirlpool, wort cooling time, wort gravity, wort clarity, wort pH, fermentation, krausen loss, and dry hopping. The techniques used in this method are described in the blog posts "A Summary of Factors Affecting IBUs" and "IBUs and the SMPH Model". This model is based on my own experiments (including more than 300 measured IBU values) and work previously published by Val Peacock, Tom Shellhammer, and Mark Malowicki. While this model would not have been possible without their previous work, they had no knowledge of, or input on, its development, and so the name SMPH is not an acronym but simply a sequence of four letters.


• How accurate is this model?
  On a test set of 18 measured IBU values from finished beer, the SMPH model had a root-mean-square error of 2.5 IBUs. Other models (Tinseth, Rager, Garetz, and mIBU) had error more than 4 times higher on the same data. More detail can be found in the blog post "IBUs and the SMPH Model".


• I'm used to the Tinseth model. How does SMPH compare?
  The Tinseth model was developed using IBUs from pre-fermentation wort, not beer. Fermentation will reduce IBUs, and so the Tinseth model will generally predict values that are larger than SMPH values and also larger than IBUs in finished beer. There are many other factors that are accounted for in the SMPH model but not in the Tinseth model. In particular, other IBU models only account for alpha-acid isomerization; they do not account for the auxiliary bittering compounds that influence both bitterness and measured IBU values. Therefore, there is no good way to translate or convert between IBUs predicted by the Tinseth model and IBUs predicted by SMPH.


• I'm using a recipe that says the beer will have 50 IBUs. Where did that number come from?
  Many recipes provide IBU estimates from the Tinseth formula because it is widely known. Other recipes might give estimates from the Rager or Garetz models, perhaps the SMPH model, or (ideally) IBUs measured by a laboratory on a beer produced by this recipe. If the recipe doesn't specify where the IBU number came from, there is no way to know. It would be very helpful if recipes put the model name next to the IBU value, e.g. "50 IBUs (Tinseth)".


• Is this calculator free? Are you tracking any data?
  The calculators on this site are (and always will be) completely free to use. No data (e.g. IP address or other identifying information, usage, history, or clicks) is tracked. There is no advertising, thanks to no-cost hosting by GitHub. I earn no money from this website.


• How can I save or load a recipe?
  To save settings to a text file or load them from a file, use the "Save" and "Load" buttons near the bottom of the page. Saved recipes are only stored on your local computer or device.

Inputs:

• Wow, why are there so many inputs?
  In the course of many experiments looking at measured IBU values, I've found that pretty much every step of the brewing process seems to have some effect on IBUs. A model that uses only information about the hops, wort volume, and the specific gravity often won't be able to give a good prediction. However, the default values should be reasonable for most homebrewing situations. You should only need to change values if they don't look correct to you.


• Why are some values in a greyish-red font?
  Some default values are shown in grayish-red. These values depend on other inputs, and are shown in this color in order to indicate that they may change if other input values change. For example, the temperature-decay defaults are based on the wort volume, kettle diameter, and kettle opening diameter. The defaults should yield reasonable results for most homebrewing scenarios. If you set a value that is currently grayish-red, the color will change to black and it will no longer change depending on the other input values.


• How do I get back to a default value?
  To get back to a default value, enter 'd'. To clear all inputs and reset the page to default values, use the "clear" button at the bottom of the page.


• Why does it ask for kettle diameter and kettle opening diameter?
  The kettle diameter, the opening diameter, and the volume are used to predict how quickly the wort will cool after flameout. The rate at which the wort cools will affect the final IBU prediction. For many homebrewers, the kettle diameter and the opening diameter are the same (with an uncovered kettle).


• What is "(Partial Boil) Added Water"?
  The field for "(Partial Boil) Added Water" is for the scenario where you add water to the wort after the boil, which is called a partial boil. This can have a large impact on IBUs if the alpha-acid solubility limit is exceeded.


• What is the "Global IBU Scaling Factor"?
  The global IBU scaling factor is a "catch all" scaling factor. If you use this model on a regular basis and find that the predicted IBUs are always consistently too high or too low compared with measured IBUs, you can change the global scaling factor to account for this. In this case, let me know so that I can better understand (and maybe fix) why the predictions are consistently off for your setup.


• What is "Default Hop Form"?
  The calculator will predict different IBUs for whole cones or for pellets. While each addition can be individually assigned to one of these two hop forms, the Default Hop Form selection will set the form of hops for every hop addition that is not individually assigned.


• Does this calculator support hop extract (CO2 extract)?
  No; currently only whole cones and pellets are supported.


• What is the "freshness factor" for hops?
  As hops sit in packaging after harvest, the alpha acids and beta acids become oxidized, which impacts IBUs. The rate at which this oxidation takes place depends on the storage conditions. The "freshness factor" represents how much oxidation has happened during hop storage. A value of 1.0 represents well-preserved hops that have not undergone any oxidation. The smaller the value, the less well-preserved or fresh the hops are. If you have a specific estimate of the freshness factor (or want to use the default), use this field to specify hop alpha-acid degradation. If you have details of the hop storage conditions, from which the freshness factor can be estimated, then use the "storage conditions" radio button to specify these conditions and see the estimated freshness factor.


• What is the "pellet factor" for hops?
  Hop pellets produce more IBUs than whole cones. This is not due to an increase in the rate of alpha-acid isomerization, but to an increase in the oxidation of alpha acids when hops are added to the boiling wort. It has been estimated that with pellets, the oxidation of alpha acids (when added to the boil) is double that of whole cones. This is why the default pellet factor is 2.0. (The default value is set to 1.0 for cones, because this factor is not relevant when using cones.) This effect on IBUs appears to be variety-specific. If you have a different estimate for the increase in alpha-acid oxidization with pellets, you can specify that with the pellet factor input box.


• How can I specify flameout or post-flameout hop additions?
  For "boil or steep time" with kettle additions, specify the number of minutes before flameout. A negative value indicates time after flameout. For example, a value of 10 means that the hops are added at 10 minutes before flameout (during the boil). A value of 0 indicates a flameout addition. A value of -10 means that the hops are added to the hot wort at 10 minutes after flameout. Post-flameout kettle additions can be made during a whirlpool or hop stand, or while the wort is being force cooled. For dry-hop additions, specify the number of days of dry hopping.


• Does this calculator support first-wort hopping?
  Unfortunately not.


• How do I know what values to use for post-boil temperature decrease?
  The defaults should work well in most cases. If you want to determine a value specific to your system, then follow the instructions in the blog post "An On-Line Calculator for the mIBU Technique" for using the Colby College website for curve fitting. The exponential decay function is probably somewhat more accurate than the linear function, but the linear function is easier to conceptualize (e.g. a decrease of 1.74°F per minute).


• How does the whirlpool and/or hop stand work?
  Specify the duration of the whirlpool and/or hop stand in the input box. If you check the box to "quickly cool, then hold at this temperature", the model will use the cooling rate parameter specified by the immersion chiller (whether or not you're really using an immersion chiller) to cool the wort to the specified temperature before starting the whirlpool or hop stand. If this box is not checked, then the starting temperature at the beginning of the whirlpool and/or hop stand is boiling. The temperature of the wort will then decrease (according to the post-boil wort temperature decay function) for the specified duration. During this time, IBUs will increase (with the amount of increase dependent on the wort temperature). When the whirlpool and/or hop stand is done, forced cooling will proceed with the specified method and rate of cooling.


• How do I know what value to use for forced cooling?
  The defaults should work well in most cases. If you want to determine a value specific to your system and you're using an immersion chiller or ice bath, then follow the instructions in the blog post "An On-Line Calculator for the mIBU Technique" for using the Colby College website for curve fitting. In this case, use an exponential function (i.e. a exp(-bx) + c) and fix parameter c at 68°F (20°C) for an immersion chiller or 105.53°F (40.85°C) for an ice bath. The absolute value of b is the exponential decay factor for the SMPH model. This parameter is the same when using metric or US customary units. If you're using a counterflow chiller, then the model assumes that once the wort has entered the chiller, it is quickly cooled. You only need to specify how quickly wort enters this chiller (in gallons or litres per minute).

Outputs:

• What is the Utilization output?
  Utilization is the amount of isomerized alpha acids (IAAs) in the beer divided by the amount of alpha acids added to the wort. (Losses along the way are often ignored; in this case, utilization can also be described as the relative concentration of IAAs to alpha acids.) This number is multiplied by 100 to convert from a ratio to a percent. Older IBU models relied primarily on utilization to estimate IBUs, but it is important to also consider the effect of auxiliary bittering compounds.


• What is the IAA output?
  This is the estimated concentration of isomerized alpha acids in the beer, in part-per-million (ppm) or mg/L.


• What does IAA_eq mean?
  The auxiliary bittering compounds (ABCs) can be measured in ppm, but this concentration is not as useful as the contribution of the compound to the IBU. Because ABCs absorb light differently than IAAs at 275 nm when measuring IBUs, the contribution of a compound to the IBU will be different from its concentration. On this page, an output noted with (IAA_eq) is the concentration of that compound in ppm multiplied by a scaling factor that reflects the light absorption of that compound relative to IAAs. (In other words, the concentration is in "IAA equivalent" units.) Then, according to the formula from Val Peacock, the IBU value from a hop addition is the sum of the IAA concentration and the ABC concentrations (expressed in IAA equivalent units), multiplied by 50/69.68.


• If I don't model dry-hop additions, I get 38.6 IAAs from one kettle addition. If I model dry-hop additions, I get 31.1 IAAs from the same kettle addition, even though nothing else has changed. Why?
  Dry hopping can sometimes reduce the concentration of IAAs in the beer. This effect is more pronounced with high-IBU beers.


• Why is the value of Total IBUs greater than zero even with no hop additions?
  Malt polyphenols are one of the auxiliary bittering compounds. Even with no hop additions, the malt polyphenols can contribute several IBUs.


• What is "Bitterness Intensity"?
  The IBU scales linearly with the concentrations of IAAs and ABCs. Bitterness, like most perceptual phenomena, does not increase linearly with the strength of the stimulus (as noted by Fechner's law). Therefore, there is a divergence from the linear relationship between IBU values and the perception of bitterness, starting at about 60 IBUs. There is still a strong correlation between IBUs and perceived bitterness, even at high IBUs. Christina Hahn has developed a quadratic equation to map between IBUs and perceived bitterness, accounting for this non-linearity. The SMPH calculator includes Hahn's perceived bitterness value (or "bitterness intensity") as an additional output.


• What is "Wort Forced-Cooling Time"?
  For immersion and ice-bath forced cooling, the approximate time to cool the wort to 140°F (60°C) is shown at the bottom of the output. This is not the time required to cool the wort to pitching temperature, but an approximation of how long it takes until alpha-acid utilization stops. This value is provided because it's a metric for how effective your forced-cooling system is.

Copyright © 2018-2023 John-Paul Hosom, all rights reserved. All product and company names are trademarks™ or registered® trademarks of their respective holders. Use of these names does not imply any affiliation with, endorsement of, or endorsement by them.

While I hope that you find this page useful, I make no guarantees about the accuracy or suitability of the results. Predicting IBUs is a bit of a "black art", because there are so many variables and there is so much variability. The only way to really know the IBU level of a beer is to have it professionally tested, which is something I highly recommend. Cheers!