13,700
edits
Changes
m
Normal The normal procedure for [[Sour_WortingWort_Souring|kettle souring]] techniques is to boil the wort a second time after the pH has been lowered in order to kill the ''Lactobacillus'' souring culture. The lactic acid fermentation by the ''Lactobacillus'' generally results in a wort pH of around 3.0 - 3.6, at which time the wort is boiled to kill the ''Lactobacillus''. In this case , yet another consideration is the effect of low pH on the decomposition of SMM into DMS. The [[Dimethyl_Sulfide#Mashing_and_Boiling|half-life table above]] demonstrates that a decrease of -0.3 pH increases the half-life by 5.5 minutes at 100°C. Unfortunately , we do not have data to show how the much lower pH values that are achieved after souring wort will effect affect the half-life of SMM <ref name="Scheuren2014"></ref>, but assuming the effect is linear then the conversion of SMM into DMS during the second boil would be greatly reduced. Assuming the effect of pH on SMM half-life is linear, and that every -0.3 pH increases the half-life by 5.5 minutes at 100°C, the half-life table could hypothetically be updated to include data that looks something like what we present in the first column below. This indicates that wort boiled after souring probably does not create significant amounts of DMS from SMM.
spelling
</code>
As expected, a high amount of SMM survives (920 µg/L) the 82°C heat pasteurization, and a relatively low amount of DMS is created (80 µg/L). The example in "Principles of Brewing Science: A Study of Serious Brewing Issues" uses the same total SMM value for the malt, but with a 90 minute boil 210 µg/L of SMM is left over after the boil, and 790 µg/L of DMS is created during the boil. During boiling the created DMS is evaporated off, but during heat pasteurization , the DMS is retained in the wort.
The next equation determines how much SMM is converted to DMS during cooling, and this amount is considered more important because it is mostly not evaporated off (especially in a closed cooling system). This equation, however, uses an average between the boiling temperature and the final chilling temperature. This model has proven to work well assuming normal brewing procedures which assume the wort is boiled, but cannot be used for wort that is not boiled. This is because the half-life of SSM is doubled for every 6°C cooler (see the [[Dimethyl_Sulfide#Mashing_and_Boiling|SMM half-life table]] above).
[https://www.facebook.com/mark.hammond.1253 Mark Hammond] from MTF used a computer program to model the conversion of SMM to DMS taking into account the [[Dimethyl_Sulfide#Mashing_and_Boiling|SMM half-life]] at different times and temperatures during various methods of the "no boil" process. Rather than using Fix's average half-life approach, numerical modeling divides the heating, boiling and cooling times into very small time steps (for the work below, a time step of 0.6 seconds was used), during which the temperature is approximated to be constant. The computer program calculates the amount of SMM converted to DMS during the time step, the amount of DMS volatilized during the time step, then plots the total DMS and SMM in the wort at the end of the time step. Finally, the program calculates a new temperature, which depends on whether the wort is heating up, being held at a constant temperature, or cooling. The program then loops through all the calculations at this new temperature, calculating all the same quantities for the next time step.
The new equations from [http://onlinelibrary.wiley.com/doi/10.1002/jib.301/full "Scheuren, Baldus, Methner and Dillenburge (2016): Evaporation behaviour of DMS in an aqueous solution at infinite dilution – a review"] were used to determine the evaporation of DMS during heating, boiling, and cooling. The evaporation of DMS during cooling assumes an open cooling system in order to demonstrate the voltilization volatilization of DMS at temperatures below 100°C. The DMS value will be retained in the wort at the point at which the wort is closed to open cooling (for example, the cooling wort can remain open so as to volatilize DMS during cooling until the wort temperature drops below the pasteurization temperature of 60°C). Hammond assumed a linear heating rate and used Newton's Law of Cooling with constants based on empirical data taken from his own homebrewing equipment. The volume graphs were determined by evaporation rates from the heat of vaporization for water coupled with empirical evaporation rates of Hammond's homebrewing equipment. By observing these estimations, it can be seen that no-boil or "raw ale", and wort boiled for short durations, Hammond predicts less DMS than what is predicted using the traditional model.
For the DMS amounts in the following graphs, Hammond calculated the mass of DMS to be 62/164 of a gram of DMS for every gram of SMM decomposed. Since we get one molecule of DMS (62 g/mole) from each molecule of SMM (164 g/mole), we don't get one for one mass of DMS for SMM. Keep that in mind when comparing the decline in the SMM concentration graph to the DMS concentration graph <ref name="hammond">Private correspondence between Mark Hammond and Dan Pixley. 03/15/2016 - 03/23/2016.</ref>.
[[File:DMS Pasteurization.png|none|thumb|500px|SMM conversion to DMS during a 20 minute heat up, 15 minute pasteurization at 82°C, and 60 minute cool down cooldown to 20°C. Graph created and provided by [https://www.facebook.com/mark.hammond.1253 Mark Hammond].]]
In the above computer generated graph based on the half-life rates of SMM, only ~60 µg/L of SMM is converted into very little DMS. Even if a closed cooling system were to be used, only ~6 µg/L of DMS would be retained. This demonstrates the very slow decomposition of SMM into DMS at below boiling temperatures.
Hammond also generated graphs for a 15 minute boil and a "0 minute" boil (wort heated to 100°C, then immediately cooled). These graphs are seen below:
[[File:DMS 15MinBoil.png|none|thumb|500px|SMM conversion to DMS during a 20 minute heat up, 15 minute boil at 100°C, and 60 minute cool down cooldown to 20°C. Graph created and provided by [https://www.facebook.com/mark.hammond.1253 Mark Hammond].]]
[[File:DMS 0MinBoil.png|none|thumb|500px|SMM conversion to DMS during a 20 minute heat up to boiling temperature (100°C), then immediate cooling for 60 minutes to 20°C ("0 minute" boil). Graph created and provided by [https://www.facebook.com/mark.hammond.1253 Mark Hammond].]]
For comparison sake, a graph of a 60 minute boil is shown below, again with an "open cooling" system that allows continued evaporation of DMS during the cooling period:
[[File:DMS 60Minute.png|none|thumb|500px|SMM conversion to DMS during a 20 minute heat up, 60 minute boil at 100°C, and 60 minute cool down cooldown to 20°C. Graph created and provided by [https://www.facebook.com/mark.hammond.1253 Mark Hammond].]]
In an open cooling system, which the graphs assume, DMS continues to volatilize off. In a closed cooling system, the points on the graphs where the boiling/heating stops and the cooling begins, are the points where DMS amounts will remain. If the cooling rate is slow to begin and the wort is held within the 90-100°C range for an extended period of time (for example in the case of whirlpooling), more DMS will continue to accumulate. Assuming the wort is quickly chilled, the table below shows a summary of the predicted DMS from the above charts:
===Kettle Souring and Effects of pH===
{| class="wikitable"