Fermentability
Advanced all-grain brewers strive to exercise enough control over their brewing so they can manipulate their ingredients and techniques in order to achieve the qualities they desire. For example, once a brewer gets to know his (or her) system, hitting his target original specific gravity (OG) consistently is often a goal.
Somewhat more difficult — but just as important — is achieving a desired finishing specific gravity (FG). This affects several major characteristics of beer, including the alcohol content, mouthfeel and the perceived sweetness. Beers with a lower FG are less viscous and are generally perceived as less sweet; the opposite is true of beers with a higher FG. Beers with a greater difference between the OG and FG have higher alcohol content and are said to be more highly attenuated, that is they have greater fermentability.
By the numbers
Unfortunately, there is no easy formula for predicting the finishing gravity and the fermentability of a beer; there are only some very general rules of thumb. For example, beginning brewers are told to expect the FG points to be approximately one-quarter of the OG points. According to this rule of thumb, a beer with an OG of 1.048 will finish at about 1.012 (because one-fourth of 48 is 12).
The relationship between the OG and FG is called the apparent attenuation (AA) and the formula for it is:
AA = [(OG – FG) * 100]/OG
If a beer has an original gravity of 1.056 and a final gravity of 1.014, the apparent attenuation is 75%:
AA = [(56 – 14) * 100]/56 = 0.75
The attenuation is “apparent” (as opposed to “real”) because it does not take into account the lower density of ethanol (specific gravity 0.07893) compared to water (SG 1.000).
The determination of real attenuation is more complex. It requires accurately weighing a sample of beer (to a resolution of 0.01 g), distilling off (at a lower boiling point than water) the ethanol, adding water to achieve the original weight of the beer and finally measuring the specific gravity. For an approximate formula for typical homebrew gravities, the OG and FG values must first be converted to °Plato). Then, the formula is:
Real attenuation (RA) = [(OG(°P) – ((0.8114 * FG(°P)) + (0.1896 *OG(°P)))) * 100]/OG(°Plato)
If we use the approximate conversion of specific gravity to °Plato as specific gravity points divided by four, the result for our example OG of 1.056 (14 °P) and FG. of 1.014 (3.5 °P) is a real attenuation of 60.8 percent:
RA = [(14 – ((0.8114 * 3.5) + (0.1896 * 14))) * 100]/14 = [(14 – (2.84+2.65))*100]/14 = 60.8
The major reason for difficulty in predicting and controlling the attenuation of a beer is that the factors are rather complex and subtle. However, that does not make it impossible to do so. Armed with information about the ingredients and brewing techniques that affect fermentability, you can learn to greatly influence the outcome with regard to this important aspect of your beer.
Ingredients are the key
Two very important determinants of final gravity are grain (or extract or sugar) and yeast selection. Pale base malts and adjunct grains produce the most fermentable wort (wort that may ferment to an apparent attenuation of 70–80%), while darker specialty malts contribute to less fermentable worts. Caramel and crystal malts will yield wort that only ferments to 40–60% apparent attenuation. With dark roasted malts, this may be as low as 25%. On the other hand, simple brewing sugars have very high attenuation. In the case of table sugar (sucrose), the apparent attenuation can be more than 95%.
Due to the manufacturing process, malt extract is usually less fermentable than mashed grains. The apparent attenuation can vary in the range of 50–75% depending on the brand and color. If you brew with extract and wish to accurately predict fermentability, it’s worth consulting your supplier or the manufacturer for the typical value.
Yeast suppliers will give the range of apparent attenuation that a strain typically exhibits. The numbers are based on the assumption that all-malt worts — made from pale malt and only a small percentage of darker specialty malts — are being fermented. Additionally, they assume adequate pitching rates and aeration, as well as fermentation in the recommended temperature range. Some highly attenuative dried ale strains can achieve an apparent attenuation of 80% or more, while the least attenuative liquid strains can be in the low 60% range.
Inside the mash
Beyond the selection of ingredients, fermentability is largely a matter of controlling the mashing process. During mashing, the crushed grain is mixed with water to hydrate the proteins and starches and dissolve any sugars already present. Whether in a single temperature rest or during multiple steps, the starches are gelatinized (transformed to a semi-liquid state) and acted upon by various malt enzymes to convert them to sugars of varying complexity.
The two most important enzymes are alpha-amylase and beta-amylase. Alpha- amylase randomly breaks the long chains of starch molecules into smaller pieces. Beta-amylase works on the ends of the pieces and produces maltose, an easily fermentable disaccharide (sugar molecule in two parts), and so-called “beta limit dextrins.” The degree of complexity of the sugars is what determines those that can be metabolized by the yeast, thus contributing to fermentability, and those that will remain in the finished beer.
These processes and enzymes are dependent upon the temperature, which is why controlling the mash temperature is so important for all-grain brewers. There are additional, somewhat less significant factors, which we will examine later.
A matter of degrees
In order for the enzymes to function effectively, the starches first must be gelatinized. For barley, this occurs at a temperature range of 140–149 °F (60–65 °C); the range for wheat is 136–147 °F (58–64 °C). Some other adjunct grains (corn and rice, for example) have higher gelatinization temperatures, which is why they must be used in either flaked (pre-gelatinized) form or boiled in a separate cereal mash. Beta-amylase is active at the same temperature range as barley gelatinization (140–149 °F/60–65 °C). At higher temperatures, it quickly begins to be denatured and is much less effective. Alpha-amylase begins to work at 140 °F (60 °C), but is most effective in the range of 149–158 °F (65–70 °C), above which it too begins to be denatured.
Note that there is some overlap in these temperature ranges, which allows for considerable manipulation of the mash schedule. Beta-amylase, which is more active at a lower temperature range, produces less complex and more fermentable sugars. Alpha-amylase, which is more active at a higher temperature range and produces more complex, less fermentable sugars.
You may sense a general trend emerging: lower mash temperatures favor beta-amylase, which results in higher fermentability, while higher temperatures favor alpha-amylase, resulting in less fermentable wort. But the starches must be gelatinized (and beta-amylase activity is partially dependent on alpha-amylase activity). Accordingly, there is a minimum temperature of about 148 °F (64 °C) at which gelatinization has occurred and alpha-amylase becomes sufficiently active. Likewise, beyond a maximum temperature of around 158 °F (70 °C), neither enzyme is effective for very long.
For the single temperature infusion mashes, a temperature of 148 °F (64 °C) will produce more simple sugars and a more fermentable wort, while a rest at 158 °F (70 °C) will result in more complex sugars and less fermentable wort. There is also a temperature that tends to equalize the effect of both alpha- and beta-amylase, which is why many single infusion mash recipes recommend a temperature rest at 152–153 °F (66 °C). This attempts to strike a balance between the enzymes and produces wort of “average” fermentability. Actual mashes performed under test conditions show a decrease in apparent attenuation of more than 6% between wort mashed at 148 °F (64 °C) and 158 °F (70 °C).
Although not strictly necessary with modern malts, it may sometimes be desirable to do a step mash, in order to maximize efficiency (the extraction of total sugars, as opposed to fermentability) and the effectiveness of both alpha- and beta-amylase. Popular step mashes often include rests at 140 °F (60 °C) and 158 °F (70 °C), the bottom and top of the active temperature ranges for both enzymes and the minimum temperature for barley starch gelatinization.
Beyond temperature
While the mash temperature is clearly the most important factor in controlling fermentability, there are others. The time for the rests during mashing also affects fermentability. Modern well-modified and highly enzymatic malts are known for their quick conversion of starches to sugars. Tests prove that the great majority of starches are converted in the first 5–10 minutes as they are hydrated and gelatinized and brought to the correct temperature. This has led some brewers to conclude there is little benefit from mashing for a much longer period of time. However, conversion does not tell the entire story. There is evidence that beta-amylase takes somewhat longer to work than alpha-amylase. Therefore a longer rest (up to 90 minutes) will encourage greater fermentability, although the effect seems to be much less than that of the temperature itself. Common conversion rests for mashing with modern malts are 30–60 minutes.
Additionally with multi-step mashes, varying the difference in times between the beta- and alpha-amylase-favoring rests will change fermentability slightly. If you are seeking more fermentable wort it may be worth increasing the beta rest and decreasing the alpha rest time.
A minor factor is the mash thickness, the ratio of water to grain. There are limits, however. The grain becomes fully hydrated at a water/grain ratio of about 1.0 quart per pound (2.1 L/kg), which should be considered a minimum, and the enzymes become too dilute and less effective at more than about 2.0 qts./lb. (4.2 L/kg). Within this range the effects of the mash thickness on fermentability are believed to be small, and other considerations such as mash tun capacity, ease of mixing and stirring, and ease of increasing the temperature should take precedence.
One final issue is the mash pH. Research has shown that beta-amylase is favored at a somewhat higher pH (~5.5) than alpha-amylase, which has an optimum pH of 5.0. This might suggest increased fermentability with a higher mash pH, but in general, as long as the mash pH is within the recommended range of 5.2–5.6 there are no problems.
Not the least is the yeast
Yeast management and fermentation procedures also affect the FG a brewer achieves. Every good brewer knows the importance of pitching a sufficient population of healthy yeast. The classic ale pitching rate for commercial breweries is one million cells per milliliter of wort per °Plato. However, there is some anecdotal evidence from Belgian brewers that slight underpitching (up to 25% less than the optimal population) can actually increase fermentability. The problem is that very few homebrewers have the ability to accurately count yeast. Given the consequences of severe underpitching, it is better to err on the side of pitching more yeast than needed than too little.
Ensuring that the wort is well-aerated when the yeast is pitched can increase attenuation. Optimal aeration involves more than merely stirring the wort or shaking the fermenter. An aeration or oxygenation system is a good investment. (See the December 2005 installment of the Advanced Brewing column for more information.)
Increasing fermentation temperature can also increase attenuation by increasing yeast metabolism. However, warmer fermentations also increase the production of various flavor compounds and that may or may not be appropriate for the style being brewed. One possible method of achieving higher attenuation, while overcoming this problem is gradually increasing the temperature as fermentation begins to subside.
