Difference between revisions of "Residual Alkalinity illustrated"

From German brewing and more
Jump to: navigation, search
(Temperature dependency of the pH measurement)
 
(18 intermediate revisions by the same user not shown)
Line 1: Line 1:
This article tries to be yet another one that explains the concept of residual alkalinity and mash pH. For many all grain brewers this subject is the last frontier. For others, especially the ones cursed with very alkaline water, it is essential for brewing a wide range of styles.
+
{| style="width:800px"
 +
|
  
==Water and pH==
+
This used to be the “Understanding Mash pH” article. But over time some of the article’s contend had become outdated as I gained a much better understanding of mash pH.
[[Image:Water_chem_pH.gif|400px|right]]
+
  
Let's start with distilled water. Pure water contains only H2O molecules. Some of these molecules disassociate into a hydronium (H+) and a hydroxyde (OH-) ion. pH (see [http://en.wikipedia.org/wiki/PH Wikipedia pH]) is a logarithmic measure of the hydronium ion concentration in a solution. In a solution at 25 °C, a pH of 7 indicates neutrality (i.e. the pH of pure water) because water naturally dissociates into H<sup>+</sup> and OH<sup>&minus;</sup> ions with equal concentrations of 1&times;10<sup>&minus;7</sup> mol/L [Wikipedia]. The illustration on the right shows distilled water where 2 of the water molecules disassociated into 2H<sup>+</sup> and 2OH<sup>-</sup>:
+
A 3 part series was written to replace that article:
 +
* Part 1: [[An Overview of pH]]
 +
* Part 2: [[How pH affects brewing]]
 +
* Part 3: [[Mash pH control]]
  
H<sub>2</sub>0 <-> H<sup>+</sup> + OH<sup>-</sup>
+
What remains of the old article is a series of illustrations that visually explain the concept of residual alkalinity.
 
+
<br style="clear:both;"/>
+
  
 
==Alkalinity==
 
==Alkalinity==
 
[[Image:Water_chem_alkalinity.gif|400px|right]]
 
[[Image:Water_chem_alkalinity.gif|400px|right]]
  
When this water rains down from the atmosphere, it picks up CO<sub>2</sub> to form carbonoic acid. Even more CO<sub>2</sub> is picked up while the water trickles through the soil. This carbonic acid is then able to react with minerals like calcium to form calcium carbonate which, in water, is present in its disassociated form:
+
When this water rains down from the atmosphere, it picks up CO<sub>2</sub> to form carbonoic acid. Even more CO<sub>2</sub> is picked up while the water trickles through the soil. This carbonic acid is then able to react with minerals like calcium to form calcium oxide:
  
 
CaO + H<sub>2</sub>O + CO<sub>2</sub>O -> Ca<sup>2+</sup> + HCO<sub>3</sub><sup>-</sup> + HO<sup>-</sup>
 
CaO + H<sub>2</sub>O + CO<sub>2</sub>O -> Ca<sup>2+</sup> + HCO<sub>3</sub><sup>-</sup> + HO<sup>-</sup>
  
The result of this reaction are a bicarbonate (HCO<sub>3</sub><sup>-</sup>) and a hydroxyde (HO<sup>-</sup>) ion. Both of which can bind a hydronium ion (H<sup>+</sup>) and thus reduce the number of free hydronium ions. This results in an increased pH (less acidic) as well as in a buffering capacity. The latter is the ability of the solution to resist a change of pH (concentration of hydronium ions) even if additional hydronium ions are added (e.g. the addition of an acid). The bicarbonate ion (HCO<sub>3</sub><sup>-</sup>) reacts with the added hydronium ions (H<sup>+</sup>):
+
The result of this reaction are a bicarbonate (HCO<sub>3</sub><sup>-</sup>) and a hydroxide (HO<sup>-</sup>) ion. Both of which can bind a proton (H<sup>+</sup>) and thus reduce the number of free protons. This results in an increased pH (less acidic) as well as in an increased buffering capacity. The latter is the ability of the solution to resist a change of pH (concentration of protons) even if additional protons are added (e.g. through the addition of an acid). The bicarbonate ion (HCO<sub>3</sub><sup>-</sup>) reacts with the added protons (H<sup>+</sup>):
  
 
HCO<sub>3</sub><sup>-</sup> + H<sup>+</sup> -> H<sub>2</sub>O + CO<sub>2</sub>O
 
HCO<sub>3</sub><sup>-</sup> + H<sup>+</sup> -> H<sub>2</sub>O + CO<sub>2</sub>O
  
Forming water (H<sub>2</sub>O) and carbon dioxide (CO<sub>2</sub>O). This buffering capacity is called the alkalinity of the water and can be expressed as either ppm HCO<sub>3</sub> or ppm CaCO3. The latter is an equivalent concentration of dissolved chalk (CaCO<sub>3</sub>) that needs to be present to get a given concentration (ppm) of bicarbonate (HCO<sub>3</sub>).
+
Forming water (H<sub>2</sub>O) and carbon dioxide (CO<sub>2</sub>O). This buffering capacity is called the alkalinity of the water and can be expressed as either ppm HCO<sub>3</sub> or ppm CaCO3. The latter is an equivalent concentration of dissolved chalk (CaCO<sub>3</sub>) that hat provides the same alkalinity.
  
 
<br style="clear:both;"/>
 
<br style="clear:both;"/>
Line 28: Line 29:
 
[[Image:Water_chem_malt.gif|400px|right]]
 
[[Image:Water_chem_malt.gif|400px|right]]
  
When malt is added, the malt's phospates (mainly potassium phosphate K<sub>2</sub>HPO<sub>4</sub> [Fix,1999]) dissolve in the mash water:
+
When malt is added, the malt's phosphates (mainly potassium phosphate K<sub>2</sub>HPO<sub>4</sub> [Fix,1999]) dissolve in the mash water:
  
 
K<sub>2</sub>HPO4<sub>4</sub> -> 2K<sup>+</sup> + HPO<sub>4</sub><sup>2-</sup>
 
K<sub>2</sub>HPO4<sub>4</sub> -> 2K<sup>+</sup> + HPO<sub>4</sub><sup>2-</sup>
Line 36: Line 37:
 
[[Image:Water_chem_acidification.gif|400px|right]]
 
[[Image:Water_chem_acidification.gif|400px|right]]
  
 
+
Kolbach, a German brewing scientist, found that the malt's phosphates react with the calcium and magnesium ions from the mash water [Fix, 1999]:
Kolbach, a German brewing scientist, found that the malt's phosphates react with the calcium and magnesium ions [Fix, 1999]:
+
  
 
3Ca<sup>2+</sup> + 2HPO<sub>4</sub><sup>2-</sup> <-> 2H<sup>+</sup> + Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>
 
3Ca<sup>2+</sup> + 2HPO<sub>4</sub><sup>2-</sup> <-> 2H<sup>+</sup> + Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>
  
This reaction releases 2 hydronium ions (H<sup>+</sup>) as well as calcium phosphate (Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>) which is pretty much insoluble in wort. The important aspect however is the release of 2 hydronium ions which can react with the bicarbonate ions (HCO<sub>3</sub><sup>-</sup>) that are responsible for the water's alkalinity [Narziss, 2005]:
+
This reaction releases 2 protons (H<sup>+</sup>) as well as calcium phosphate (Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>) which is pretty much insoluble in wort and precipitates. The important aspect however is the release of 2 protons which can react with the bicarbonate ions (HCO<sub>3</sub><sup>-</sup>) that are responsible for the water's alkalinity [Narziss, 2005]:
  
 
H<sup>+</sup> + HCO<sub>3</sub><sup>-</sup> ->  H<sub>2</sub>O + CO<sub>2</sub>
 
H<sup>+</sup> + HCO<sub>3</sub><sup>-</sup> ->  H<sub>2</sub>O + CO<sub>2</sub>
  
The result is a lowered alkalinity or buffering capacity of the water. If no more bicarbonate ions are present, the pH sinks due to the increase of hydronium (H<sup>+</sup>) ions. Kolbach's work found that not all of the waters calcium and magnesium ions take part in the above reaction. He found that only 2 out of 7 calcium ions and one out of 7 magnesium ions react with the malt's phospates to release hydronium ions. This resulted in the defition of residual alkalinity, which is the a measure of concentration of bicarbonates left over after the acidifying reaction between the malt's phospates and the water's calcium and magnesium has been taken into account: When the calcium, magnesium and bicarbonate concentrations can be expressed in an unit that is equivalent to the ion concentration (as opposed to the weight concentration) of these ions (like dH (German Hardness), the equation for the residual alkalinity simply is:
+
The result is a lowered alkalinity or buffering capacity of the water. If no more bicarbonate ions are present, the pH sinks due to the increase of protons (H<sup>+</sup>). Kolbach's work found that not all of the water's calcium and magnesium ions take part in the aforementioned reaction. He found that only 2 out of 7 calcium ions and one out of 7 magnesium ions react with the malt's phosphates to release protons. This resulted in the definition of residual alkalinity, which is a measure of the alkalinity left after the acidifying reaction between the malt's phosphates and the water's calcium and magnesium has been taken into account: When the calcium, magnesium and bicarbonate concentrations can be expressed in an unit that is equivalent to the ion concentration (as opposed to the weight concentration) of these ions (like dH (German Hardness) or mEq/l), the equation for the residual alkalinity simply is:
  
 
RA = KH - (CH + 0.5 * MH)/3.5
 
RA = KH - (CH + 0.5 * MH)/3.5
  
Where RA is the residual alkalinity in dH, KH the alkalinity (carbonate hardness), CH the Calcium Hardness and MH the Magnesium Hardness. But a water report ususally doesn't show these parameters as dH, especially not in a non-German country. In order to convert this formula such that it can be used with ppm as the unit, the molar weight of the carbonate, calcium and magnesium ions has to be taken into account. Though this is straight forward, it involves juggling a lot of numbers and since it doesn't help in understanding water chemistry we just use what others already did on this subject.
+
Where RA is the residual alkalinity in dH, KH the alkalinity (carbonate hardness), CH the Calcium Hardness and MH the Magnesium Hardness. But a water report usually doesn't show these parameters as dH, especially outside of Germany. In order to convert this formula such that it can be used with ppm as the unit, the molar weight of the carbonate, calcium and magnesium ions has to be taken into account. Though this is straight forward, it involves juggling a lot of numbers and since it doesn't help in understanding water chemistry i created a spread sheet for making these calculations: [http://braukaiser.com/documents/Kaiser_water_calculator.xls Kaiser_water_calculator.xls]
  
Palmer's book [Palmer, 2006] and how-to-brew.com [howtobrew.com], for example are great resources. Palmer developed a normograph that makes it easy to determine the residual alkalinity of brewing water graphically: [http://howtobrew.com/section3/chapter15-3.html howtobrew.com chapter 15]
 
 
==Malt and Beer Color==
 
Residual Alkalinity is only one factor in the final mash pH. Another important factor is the color of the malts that are used. Darker malts (base malts as well as specialty malts) contribute additional acidity other than the one described above. This acidity also counteracts the alkalinity of the water and needs to be taken into account to ensure that the resulting mash pH is in the preferred range of 5.2 - 5.5. This has been taken into account in Palmer's Normograph, where he gives a color scale on which the residual alkalinity target can be matched with the anticipated beer color.
 
 
== Temperature dependency of the pH measurement ==
 
 
The pH of a solution changes with its temperature. This may need to be taken into account when measuring the pH and determining what the pH target is. Some authors cite the pH ranges and targets at the actual reaction temperature (i.e. rest temperature) and others cite the pH values as if measured in a cooled sample. The latter makes sense since it is a good practice to cool a sample before measuring it with a pH meter. It also gives a common reference and allows for giving pH values without the need of giving a temperature as well. Unless otherwise noted, all pH values on this Wiki are pH values measured at room temperature (25 C)
 
 
The following table shows the pH error for different temperatures and pH values. The higher temperatures lead to lower actual pH values at these temperatures [EMD]:
 
 
{| border="1" cellspacing="0" cellpadding="5" align="center"
 
! TEMP / pH !! 2 !! 3 !! 4 !! 5 !! 6 !! 7 !! 8 !! 9 !! 10 !! 11 !! 12
 
|-
 
| 15 ° C || .15 || .12 || .09 || .06 || .03 || 0 || .03 || .06 || .09 || .12 || .15
 
|-
 
 
|}
 
|}
 
 
TEMP / pH 2 3 4 5 6 7 8 9 10 11 12
 
5 ° C .30 .24 .18 .12 .06 0 .06 .12 .18 .24 .30
 
15 ° C .15 .12 .09 .06 .03 0 .03 .06 .09 .12 .15
 
25 ° C 0 0 0 0 0 0 0 0 0 0 0
 
35 ° C .15 .12 .09 .06 .03 0 .03 .06 .09 .12 .15
 
45 ° C .30 .24 .18 .12 .06 0 .06 .12 .18 .24 .30
 
55 ° C .45 .36 .27 .18 .09 0 .09 .18 .27 .36 .45
 
65 ° C .60 .48 .36 .24 .12 0 .12 .24 .36 .48 .60
 
75 ° C .75 .60 .45 .30 .15 0 .15 .30 .45 .60 .75
 
85 ° C .90 .72 .54 .36 .18 0 .18 .36 .54 .72 .90
 
 
==Measuring Mash pH==
 
 
Though it is oftentimes good enough to target a particular residual alkalinity for a particular style of beer, only a measurement of the mash pH can tell how well this target has been achieved. The home brewer has 3 options to choose from:
 
 
===Litmus paper===
 
 
[[Image:Economy-ph-strips.jpg|right|frame|Litmuspaper test strips [northernbrewer.com]]]
 
 
Litmus paper contains a dye that changes color when exposed to an acid or base. The extent of the color change is matched against a scale to determine the pH of the sample. Litmus paper, generally known as pH test strips, are a cheap way of determining the mash pH. But because they operate over a fairly large pH range and the dye tends to run into the sample they are hard to read. But they are sufficent to check if the mash pH is somewhat close to the anticipated target.
 
 
====Pro====
 
* cheap ($4 for a pack of 100 strips)
 
* no maintanance
 
 
====Con====
 
* difficult to read (+/ 0.5 pH)
 
* dye will run
 
 
<br style="clear:both;"/>
 
 
===precision test strips===
 
 
[[Image:Colorphast.jpg|right|frame|colorpHast precision pH test strips [northernbrewer.com]]]
 
 
EMD Chemicals makes pH test strips (colorpHast) that also use a pH sensitive dye, but this dye will not run like litmus paper. It also shows a stronger change of color over a narrow pH range which makes them easier to read and more precise than litmus paper. But a comparison against a pH meter has shown that these stips can show a systematic error that was also confirmed by EMD Chemicals: [[colorpHast_vs_pH_meter|colorpHastStrips vs. a pH meter]]
 
 
Note that these strips have to be read in natural or tungsten light. Fluorecent light will make the reading appear in a different color. This can be a problem if you tend to brew late at night and have energy saving light bulbs.
 
 
====Pro====
 
* fairly easy to read with a reasonable accuracy of +/- 0.3 pH
 
* no maintanance
 
 
====Con====
 
* a little bit on the pricy side ($30 for 100 strips), but they can be cut in smaller strips to make the package last longer
 
* they can have a systematic error which makes it difficult to determine the pH with a higher accuracy
 
 
<br style="clear:both;"/>
 
 
===pH meter===
 
 
[[Image:PHep.jpg|right|frame|digital pH meter [northernbrewer.com]]]
 
 
A pH meter ([http://en.wikipedia.org/wiki/PH_meter Wikipedia: pH meter]) convertes the pH difference beween the sample liquid and a reference liquid, which is inside a bulb at the tip of its probe, into a voltage difference that can be measured and converted into a pH reading. These instruments are able to measure pH very precisly. But this precision comes at a price: not only are pH meters relatively expensive, their electrodes have only a limited lifetime of 1-2 years when cared for well. They also require constant maintanance like calibration and the tip of the electrode needs to be stored wet.
 
 
====Pro====
 
* very accurate (+/- 0.01 pH for some models)
 
* easy to read
 
 
====Con====
 
* an average meter costs between $80 - $130
 
* a replacement electrode costs about $50-$60 and may need to be replaced every 1-2 years
 
* calibration necessary
 
 
<br style="clear:both;"/>
 
 
===What to get?===
 
 
The ColorpHast pH measuring strips are the best value for the price. They can be cut into smaller strips to make them last and are sufficiently precice and easy to read for all grain brewing. But keep in mind that they can be off by a few 10th pH units and when you experience mash and sparge pH related off flavors, you may want to adjust the value read by a few 10th pH units.
 
 
A pH meter is recommended if you have other than mashing uses for it. Because of its precision it can also be used to monitor the pH of the fermentation and finished beer. A rise in the beer pH for example can be an indication of autolysis due to the release of the more basic/alkaline contents of yeast cells into the beer.
 
 
==Means of adjusting the mash pH==
 
 
When the mash pH has been measured and determined to be out of range, there are several methods available to fix this problem:
 
 
===Lowering Mash pH===
 
 
====Calcium and Magnesium additions====
 
 
[[Image:Gypsum.jpg|right|frame|Gypsum [morebeer.com]]]
 
 
According to the equation above, calcium and magnesium will reduce the mash pH if they are added as non carbonate/bicarbonate salts (Chalk CaCO3 does not lower the pH because it increases the alkalinity of the water). But since not all of the calcium and magnesium ions react with the malt's phosphates, the amount that can be added to counteract the alkalinity of the water is limited by the calcium and magnesium levels appropriate for a particular style of beer. Another factor to be considered is, that Ca amd Mg need to be added as salts which will also contribute sulfates (calcium sulfate (Gypsum, CaSO4) and magnesium sulfate (Epsom salt, MgSO4)) and/or chlorides (calcium chloride (CaCl)). To high of sulphate and chloride concentrations may also lead to undesirable beer flavors. Palmer suggests the following ranges for Ca, Mg, SO4 and Cl in brewing water [Palmer, 2005]:
 
*Calcium: 50 - 150 ppm
 
*Magnesium: 10 - 30 ppm (high levels taste sour/bitter)
 
*Sulfate: 50 - 150 ppm (accentuates hop bitterness, but high concentrations (>400) it is harsh and unpleasant)
 
*Chloride: 0 - 2500 ppm (Excessive concentrations can lead to chlorophenol off-flavors)
 
 
The affect of the Sulfates on hop bitterness are one reason why many brewers prefer calcium chloride (CaCl) additions over gypsum (calcium sulfate, CaSO4) additions for mash acidification.
 
 
<br style="clear:both;"/>
 
 
====Foodgrade Acid Additions====
 
[[Image:Phosphoric_acid.jpg|right|frame|Phosphoric acid [morebeer.com]]]
 
 
Acid additions work by contributing H+ ions to the mash. These ions react with the bicarbonate ions to lower the alkalinity or simply increase the H+ concentration to lower the pH of the mash. Pretty much any foodgrade acid, from phosphoric acid to vinegar (acedic acid), can be used in the mash. But brewers commonly only use phosphoric acid, sufuric acid or lactic acid. These acids provide flavors that are most compatible with beer. Like salts, acids will also contribute ions othert than the needed hydronium ions (H+) to the mash:
 
*phosphoric acid: phosphates
 
*sulfuric acid: sulfates
 
*lactic acid: lactates
 
 
<br style="clear:both;"/>
 
====Sour Mash and Wort====
 
Because of the [http://en.wikipedia.org/wiki/Reinheitsgebot Reinheitsgebot] (German purity law for beer) German brewers are not allowed to use acid or salts in their mash. But when they tried to brew the now popular light colored lagers with alkaline water they failed until they added soured mash to the mash. The sour mash is a mash that has been partially fermented by lactic acid bacteria which are available in abundance on the malt and are therefore not a violation of the RHG. In order to do a sour mash, the brewer mashes some grains to convert the starch into sugars. Then the mash is cooled to 80 *F (35*C) and some crushed malt is added to innoculate the mash with fresh lactic bacteria. This mash then sits overnight and starts fermenting. The next day it can be added to the regular mash and its lactic acid will serve to balance the alkalinity and lower the pH.
 
 
The big problem with sour mashing is it's labor intensity and inconsistency. Because the bacteria on the malt is not a pure culture of lacto bazillus other flavor compounds can be created in the sour mash which may carry over into the finished beer.
 
 
Wort can also be soured to be used for mash acidification as well as acidification during the boil.
 
 
<br style="clear:both;"/>
 
 
====Acid Malt====
 
[[Image:Malt.jpg|right|frame|Acid malt [morebeer.com]]]
 
 
From Narziss [Narziss, 2005]:
 
<blockquote>
 
Sour malt (a.k.a acid malt) is used as an addition to the grist to enhance the acidity during the mash. This affects the wort pH and to a much lesser extent the beer pH. The active component of this specialty malt is lactic acid. This can be formed by the lacttic acid bacteria, that are present on the malt, within 24 hours when the grain is steeped in 45 - 48 *C (113 - 120 *F) water. [...] The soured malt is carefully dried and then kilned at a high temperature to kill the bacteria. With a lactic acid content of 2-4 % the water extract has a pH of 3.8. Such a water extract can also be used to acidify the wort during boiling
 
</blockquote>
 
 
Acid malt has the same affect as adding lactic acid to the mash. It is generally easier to handle because it can easily be added by weight. The data sheet for Weyermann acid malt states that each addintional percent of acid malt in the grist lowers the mash pH by about 0.1 pH units. This assumes that any residual alkalinity has already been balanced.
 
 
<br style="clear:both;"/>
 
 
====Acid rest====
 
 
An acid rest is a low temperature mash rest during which the malt's enzymes dismantle phytin to phytic acid [Noonan, 1996]. This reaction is different to the mash acidification that stems from the reaction of the malt's phospates with the calcium and magnesium ions. The optimal temperature for the acid rest is 95 *F (35 *C) and is most effective in undermodified pale malts. It is also the first rest in the classical tripple decoction mash. But since modern malts don't require such intensive mash schedules anymore the acid rest is rarely used anymore.
 
 
===Increasing Mash pH===
 
 
When brewing dark beers with fairly soft water, the acidity of the dark malts can result in a mash pH that is to low for proper enzymatic activity and eventually proper beer pH. In these cases it is necessary to increase the mash pH.
 
 
This mash pH increase can be accomplished by the addition of bicarbonate salts like sodium bicarbonate (baking soda, NaHCO3) or calcium carbonate (chalk, CaCO3). These salts increase the alkalinity of the water which consumes H+ ions and increases the mash pH. Large amounts of sodium are undesirable in brewing water which is why chalk is preferred over baking soda.
 
 
===Five Star's 5.2 buffer===
 
 
Five Star Chemical Company ([http://http://www.fivestarchemicals.com/products.asp www.fivestarchemicals.com]) makes a pH buffer called 5<sup>2</sup> from food grade phosphate buffers that can be added to the mash and will lock the mash pH at 5.2. It works by providing a buffer that is stronger than the buffering capacity of the mash and thus overrides it's pH. It is a very simple solution that works for most brewing waters and can give the brewer a consistent mash pH without even the need for mash pH measurements
 
 
It's effectiveness is limited when used with very alkaline water because its buffer capacity, when used at the recommended dosage of 2oz / 31 gal, is not sufficient to overcome the waters alkaline buffer. At this point more  5<sup>2</sup> needs to be added which can result in off flavors due to the increased mineral content of the water. Conversely, when brewing with softer waters, less than the recommended dosage of  5<sup>2</sup> can be used to reduce the mineral additions to the water.
 
 
 
==When to treat the mash pH ?==
 
 
The mash pH should be measured shortly after dough-in. If a pH correction needs to be made to the mash it should be done immediately because mash pH also affects the enzymatic activity. When adding salts or acids to the mash, they should be added in small dosages and the mash pH needs to be measured after each dosage has been added and well mixed with the rest of the mash. The latter is a very important step that can be difficult if the mash has a thick consistency.
 
 
All necessary mash treatments should be recorded so that they can be done as water treatments (or in the case of acid malt as additions top the grist) before dough-in when the beer is brewed a second time. In this case no mash pH adjustment should be necessary after dough-in.
 
 
Using Palmer's nomograph and a water analysis of the brewing water, potentially necessary water and mash treatments can already be planned before the beer is brewed the first time which avoids surprises after dough-in.
 
 
==Sources==
 
:[Wikipedia] [http://en.wikipedia.org/wiki Wikipedia.org]
 
:[Narziss, 2005] Prof. Dr. agr. Ludwig  Narziss, Prof. Dr.-Ing. habil. Werner Back, Technische Universitaet Muenchen (Fakultaet fuer Brauwesen, Weihenstephan), ''Abriss der Bierbrauerei''. WILEY-VCH Verlags GmbH Weinheim Germany, 2005
 
:[Fix, 1999] George J. Fix Ph.D, ''Principles of Brewing Science'', Brewers Publications, Boulder CO, 1999
 
:[Palmer, 2006] John J. Palmer, ''How to Brew'', Brewers Publications, Boulder CO, 2006
 
:[Noonan, 1996] Gregory J. Noonan, ''New Brewing Lager Beer'', Brewers Publications, Boulder CO, 1996
 
:[EMD] EMD Chemicals, [http://www.readycult.com/analytics/literature/pH_Temp_Error_Table.pdf pH Error as a Function of Temperature]
 

Latest revision as of 02:24, 27 February 2011

This used to be the “Understanding Mash pH” article. But over time some of the article’s contend had become outdated as I gained a much better understanding of mash pH.

A 3 part series was written to replace that article:

What remains of the old article is a series of illustrations that visually explain the concept of residual alkalinity.

Alkalinity

Water chem alkalinity.gif

When this water rains down from the atmosphere, it picks up CO2 to form carbonoic acid. Even more CO2 is picked up while the water trickles through the soil. This carbonic acid is then able to react with minerals like calcium to form calcium oxide:

CaO + H2O + CO2O -> Ca2+ + HCO3- + HO-

The result of this reaction are a bicarbonate (HCO3-) and a hydroxide (HO-) ion. Both of which can bind a proton (H+) and thus reduce the number of free protons. This results in an increased pH (less acidic) as well as in an increased buffering capacity. The latter is the ability of the solution to resist a change of pH (concentration of protons) even if additional protons are added (e.g. through the addition of an acid). The bicarbonate ion (HCO3-) reacts with the added protons (H+):

HCO3- + H+ -> H2O + CO2O

Forming water (H2O) and carbon dioxide (CO2O). This buffering capacity is called the alkalinity of the water and can be expressed as either ppm HCO3 or ppm CaCO3. The latter is an equivalent concentration of dissolved chalk (CaCO3) that hat provides the same alkalinity.


Adding Malt

Water chem malt.gif

When malt is added, the malt's phosphates (mainly potassium phosphate K2HPO4 [Fix,1999]) dissolve in the mash water:

K2HPO44 -> 2K+ + HPO42-


Residual Alkalinity

Water chem acidification.gif

Kolbach, a German brewing scientist, found that the malt's phosphates react with the calcium and magnesium ions from the mash water [Fix, 1999]:

3Ca2+ + 2HPO42- <-> 2H+ + Ca3(PO4)2

This reaction releases 2 protons (H+) as well as calcium phosphate (Ca3(PO4)2) which is pretty much insoluble in wort and precipitates. The important aspect however is the release of 2 protons which can react with the bicarbonate ions (HCO3-) that are responsible for the water's alkalinity [Narziss, 2005]:

H+ + HCO3- -> H2O + CO2

The result is a lowered alkalinity or buffering capacity of the water. If no more bicarbonate ions are present, the pH sinks due to the increase of protons (H+). Kolbach's work found that not all of the water's calcium and magnesium ions take part in the aforementioned reaction. He found that only 2 out of 7 calcium ions and one out of 7 magnesium ions react with the malt's phosphates to release protons. This resulted in the definition of residual alkalinity, which is a measure of the alkalinity left after the acidifying reaction between the malt's phosphates and the water's calcium and magnesium has been taken into account: When the calcium, magnesium and bicarbonate concentrations can be expressed in an unit that is equivalent to the ion concentration (as opposed to the weight concentration) of these ions (like dH (German Hardness) or mEq/l), the equation for the residual alkalinity simply is:

RA = KH - (CH + 0.5 * MH)/3.5

Where RA is the residual alkalinity in dH, KH the alkalinity (carbonate hardness), CH the Calcium Hardness and MH the Magnesium Hardness. But a water report usually doesn't show these parameters as dH, especially outside of Germany. In order to convert this formula such that it can be used with ppm as the unit, the molar weight of the carbonate, calcium and magnesium ions has to be taken into account. Though this is straight forward, it involves juggling a lot of numbers and since it doesn't help in understanding water chemistry i created a spread sheet for making these calculations: Kaiser_water_calculator.xls