First the basics — pH is a measure of hydrogen ions (H+) in solution. Water (H2O) exists as a mixture of H2O molecules and its component ions H+ and OH-. In solution, hydrogen ions (H+) are always associated with one or more water molecules and are nowadays usually written as H3O+ to reflect this fact. An H3O+ ion is called a hydronium ion. In pure water at 25 °C, there are roughly two H3O+ and OH- ions
for every billion water molecules.
This corresponds to a concentration
of 10-7 mol/L.
In 1909, a Danish scientist named Soren Sorenson devised the pH scale while working at the Carlsberg Brewery in Copenhagen. He devised a logarithmic scale that runs from 0–14 and represents the negative log of the hydronium ion concentration. For example, if the concentration of H3O+ ions in a solution was 10-5 mol/L, the pH of that solution would be 5. A pH value of 7 is considered neutral because, at 25 °C, the concentration of H30+ ions in solution equals the concentration of OH- ions. Values lower than 7 are acidic and higher are basic (or alkaline). Each single number change on the pH scale represents a 10-fold change in hydronium ion concentration. The addition of acids, which donate H3O+ ions to a solution, lower the pH of a solution. Bases (or alkalis), which donate OH- ions, raise the pH of solutions.
The pH of a solution can be measured using pH papers in which a paper strip is impregnated with a dye whose color is determined by the H3O+ ion concentration. Or it can be measured more accurately using a meter that measures the flow of electrons across a glass membrane. The flow is caused by the difference in electron concentration in the test solution and a standard solution inside the electrode.
Homebrewers have access to reasonably priced pH meters and they make a useful addition to your homebrewing tools, provided you plan to take steps to influence your beer's pH.
A pH meter should be calibrated each brewing session by presenting the electrode with at least two different standard buffer solutions. In most cases, we use standards of pH 4 and 7 (well, 4.01 and 7.01, really) to calibrate the pH meter. This is great for brewers, since we're really interested in the pH range of 4 to 8. This range encompasses typical pH values for mash, wort, all water used in brewing and the final product. Once the meter is 'trained" to recognize the pH of the standards, it can then identify the pH of an unknown solution. Due to the logarithmic nature of the scale, small differences in recorded values in the brewery represent huge differences in the actual hydronium ion content to the point where a change of 0.1 pH unit represents a doubling of the actual hydronium ion content.
Why is all this important? Well, almost every reaction occurring in our breweries is carried out by enzymes; the mash conversion, the reactions in fermentation and the maturation all require enzymes to work. Every enzyme has an ideal pH at which it operates most rapidly. Even those reactions dependent on chemistry rather than biochemistry (i.e. non-enzymatic reactions) such as hop resin isomerization, haze formation, color development or beer staling are influenced by pH. Finally our perception of a beer's flavor, after-taste and mouthfeel are influenced by the beer's pH.
A mash carried out with distilled water will end up with a mash pH in the 5.8–6.0 range due to the buffering capacity of weak organic acids and amino acids from the malt. Buffering capacity refers to the ability of some chemicals to resist a change in pH of a solution when acids or bases are added to the solution.
Calcium ions in the brewing water, however, react with phosphate ions in the malt to release H30+ ions and acidify the mash. Hence the presence of calcium ions can lower the mash pH to the 5.4–5.6 range. This drives the pH in the mash toward the optimal conditions for each of the two main mashing enzymes, alpha amylase (pH between 5.6–5.8) and beta amylase (pH between 5.4–5.6). Thus, the presence of calcium in the mash increases extract recovery from the malt. Most water contains calcium and so in a lot of cases there is enough present in the water for mashing. If not, calcium can be added to the mash in the form of calcium sulfate (CaSO4, gypsum) or calcium chloride (CaCl2), either by adding the salt directly to the mashing water or by mixing the solid material in with the milled malt prior to mashing.
Some brewers, particularly Ger-man brewers, feel that protein solubilization, breakdown and particularly amino acid production is an important factor in mashing and so indulge in a whole range of temperature rests. Some of the enzymes responsible for this action have optimal pH's far lower than simple calcium treatment will allow (provided they survive kilning, which in the case of British malt is unlikely). These brewers treat their mashes with acids in order to drive the pH lower still. Lactic, phosphoric, citric and even sulfuric acids may be used to drive the mash pH down to the 5.2–5.4 range.
Sparge water is another area where brewers need be concerned about pH. Sparge water is the hot (150–170 °F/66–77 °C) water sprayed onto the surface of the grain bed in the mash tun to replace the strong wort as it is drained from the bottom of the mash. It is strained through the grain bed and collected as it contains diluted wort. As the run off of wort from the mash tun nears its completion, the liquid being collected is quite close in composition to the water added to the grain bed above. It is very weak and contains little in the way of dissolved sugars, amino acids and buffering ability. Thus the pH can rise. If it rises above 6.0, it can extract some of the more undesirable compounds that would otherwise remain in the spent grain, namely polyphenols. Polyphenol (tannins) compounds are a part of beer haze and can provide harsh astringent beer flavors.
In the kettle, pH plays a vital role in the various chemical reactions. The pH drops during wort boiling, again due to the reaction between calcium and phosphates. This is why some brewers wait to add gypsum in the kettle where the solid calcium salts are more soluble, relying on there being enough calcium already dissolved in the water to take care of the needs of the mash. Protein coagulation and hence hot break (or trub) formation is improved at lower pH's. This improves the beer's clarity and reduces its susceptibility to chill haze. However, lower pH's reduce the efficiency of hop alpha acid isomerization and solubility, which optimally occurs at pH values that are higher than those found in most kettles. The same applies to the reactions that darken color. Lower pH's result in less color pickup than higher ones.
Dimethyl sulfide (DMS) — the compound responsible for the corn, tomato, or stewed vegetable flavor in beer — is also influenced by pH. The conversion of the pre-cursor during boiling is slower at lower pH's.
Another factor influenced by pH during boiling is the action of kettle finings or Irish moss. They too have an optimal pH at which they are most effective. Luckily it's around 5.3, where we expect our boiled wort pH to end up anyway. If you drop the wort pH in the kettle to 5.0, you need to use 50% more kettle finings for the same result. Below 4.4 they do not work at all. There are many German brewers who feel it is also necessary to reduce the pH of the wort in the kettle to influence the final beer pH, something that simply acidifying the mash cannot adequately achieve. This will involve direct addition of acid to the kettle to drive the pH down to 5.1-5.2. Of course, this reduces hop extraction efficiency but does provide additional benefits associated with having a low final beer pH, one of which is a "cleaner" perception of the bitterness.
During fermentation, the pH of the wort drops rapidly. In the first 24 hours, the pH should fall from 5.3 down to 4.3. This is due to the rapid consumption by yeast of buffering capacity (i.e. amino acids) and the related production of acidic material such as organic acids. A slow fermentation or a long lag phase from your yeast will delay, or slow this drop and could result in beer flavor problems. The main issue would be in the ability of contamination in the form of spoilage organisms to gain a foothold while the pH is higher than 4.4.
As you can imagine, the multitude of biochemical reactions occurring in a fermentation vessel with the multiple billions of enzyme reactions occurring every second the pH is likely to have a profound influence on the rates. Remember also that quoted values for enzyme pH's refer to optimal rates and that enzymes are still active on either side of their optimal pH figure.
One simple example is the production, conversion and ultimate reduction of diacetyl, the buttery or butterscotch flavored by-product of a fermentation. Lower pHs (between 4.2 and 4.4) tend to favor the removal of diacetyl. For most beers made from American malt, or German-style lager beers, the final beer pH will be in the region of 4.2–4.4. For English-style beers, and cask conditioned beer in particular, the final beer pH will invariably be in the 3.9–4.1 range.
A combination of factors probably account for this, including additional buffering from higher protein malts in lager brewing, more rapid fermentations in ales, water salts in ale brewing and the tendency to use kettle sugars in ale brewing which offer no buffering. As beer matures, the pH will begin to rise slightly as yeast cells autolyse and release their contents into the beer. In fact, one test for yeast health involves measuring the pH difference between a slurry of yeast and the liquid around it. It can also be shown that reduced beer pH improves head retention in finished beer. Brewers who utilize isinglass finings should be aware that the action of finings is pH dependent. The action relies on electrical attractions between oppositely charged components and the degree to which a charge is expressed is dependent on pH. In general, high beer pH adversely affects fining action.
In general, the following benefits are known to be achieved by having the pH fall in the correct range throughout the process. The extraction of material into solution in the mash tun will be enhanced, the separation of the wort from the spent grain will be better. Hot break will be better and so later beer clarification will be easier. The beer will be less prone to contamination from spoilage organisms and maturation will be more successful.
Finally, a whole range of research has been done on the way pH influences a beer's sensory qualities. Lower beer pH (4.2–4.3) in lager brewing is said to provide a rounder, fuller, softer flavor. When brewing British-style ales, a pH on the lower end of the appropriate range (3.9–4.1) produces beer with a sharper, crisper, and fresher flavor. As pH drops below 4.0 (as it may in British-style bitters brewed with British malt) the beer tastes sharper and more acidic and the perception of astringency is enhanced. Hop bitterness is said to be more pleasant and less harsh and lingering, although it may be perceived at a higher level than measured. The beer's foam is more stable and finer. The excess of hydrogen ions in the mash and throughout the process is also likely to influence the beer's balance between its reducing power and its oxidizing potential, especially given that it is the OH- ion that is heavily implicated in these redox reactions. The OH- ion is a highly reactive ion that readily accepts a hydrogen ion. Lipid material in the mash readily gives up hydrogen causing it to begin the pathway that leads to staling aldehydes. Thus a lower mash pH will theoretically improve a beer's flavor stability in regard to oxidation. However, Japanese research shows that natural antioxidant protection from sulfite and polyphenols is reduced at lower pH's, so once again the debate rages on.
As a homebrewer what does all this mean and how can you influence any of this? You Should be aware of the flavor effects yeast autolysis has on your homebrew. Essentially though, brewing with all malt, good water, healthy yeast and clean equipment will take care of most of this on its own.Steve Parkes is the owner and lead instructor of the American Brewers Guild and head brewer at Otter Creek Brewing in Middlebury, Vermont.