Home Brewing. The fermentation


fermentation_cWhen the wort has been collected in the fermentation vessel, and cool, it will be necessary to dilute it down to the proper original gravity by topping up the fermentation vessel with water until the desired gravity is reached. The higher the original gravity of the wort, the higher will be the strength and body of the finished beer. Any recipe can be adjusted to provide a stronger or weaker version of the same beer by either using more or less ingredients, or by topping up the fermentation vessel with more or less water as appropriate.

Read also:

Ingredients for brewing: grain, sugar, malt extract

Traditional Commercial Brewing

Home Brewing. Beginners Start Here

Home Brewing. What should You Know about the Hops

All You should Know about Brewery Yeast

Home Brewing. Water and Water Treatment

Home Brewing. Mashing and Sparging

Wort aeration

It is essential that the wort contains a certain amount of dissolved oxygen for the early stages of yeast growth. When the wort is cool, measures should be taken to ensure that an adequate supply of dissolved oxygen exist in the wort. Some home-brew authors stress the importance of keeping air out of contact with wort almost to the point of paranoia, with the result that finds itself with a deficiency of oxygen during its first formative hours. The reason for his paranoia is the fear of contamination with air-borne bacteria, and there is a risk, but the risk is very small. The fact is that we must introduce air into solution at some point prior to, or just after, pitching the yeast. However, once the yeast has established itself and reverted to anaerobic respiration, air should not be allowed to get into solution again. This is important, not only from the aspect of bacterial contamination, but also to ensure efficient alcoholic fermentation by tne yeast, and to reduce the possibility of the beer degrading by various oxidisation processes during maturation and storage.

There are various methods of admitting oxygen into solution. Probably the simplest is to pour the wort from one fermentation bin to another a few times, generating plenty of violent, mechanical, sloshing action. An alternative method is to open the tap on one vessel and allow the wort to flow slowly, via an airborne drop, into another vessel below.

The wort should be cool before this operation is performed because the solubility of oxygen is greater at low temperatures.

Pitching the yeast

Once the wort has cooled to about 25°C it is safe to pitch the yeast. A yeast starter solution should be made up a few days before the main brew is started to ensure that a sufficiently large volume of actively fermenting yeast is available for pitching. The entire contents of the starter bottle should be emptied into the wort and stirred in. The greater the volume of yeast used for pitching the faster and surer will be fermentation. Commercial breweries use up to 25 per cent of the yeast recovered from a previous brew to pitch the next. This reduces the lag phase, ie the time lag between pitching and the yeast beginning efficient alcoholic fermentation.

Fermentation progress

A good deal can be learned about how fermentation is progressing by observing the surface of the wort. A perfect description of the appearance of the yeast head during the various stages of fermentation is afforded by the contemporary description of 1902 which I have used as a header to this text.

The yeast should begin to collect on the surface of the wort within a few hours of pitching, and the "dirty snowdrift" stage should be reached eight to ten hours after pitching. The first head of yeast carries up debris in the form of dark flocculum and scum. It is normal practice to skim off the debris to prevent it tainting the beer, taking care to leave as much of the clean yeast as possible undisturbed.

About twenty hours after pitching, the "cauliflower heads" or "stalks" should be formed, followed about eight hours later by the "ragged cliffs". Thirty-six hours after pitching the ragged appearance of the mass should be subsiding, and finally, within forty-eight hours the "billows of silky foam" should be evident, although perhaps not quite as dramatic as the contemporary description suggests.

This final stage indicates that the yeast has fully adapted to anaerobic respiration, yeast growth has diminished to a minimum and the yeast is now is working flat out producing alcohol. The foaming is caused by the copious amount of CO2 produced. If the dropping system of fermentation is being employed, it is at this stage that the fermenting wort is transferred or "dropped" into the secondary vessel.

The above description is representative only, the actual appearance and the timing of the various stages is very dependant upon wort composition, original gravity, and fermentation temperature. It is important with a genuine top working brewer's yeast that a thick fluffy head of yeast is formed on the surface of the wort, and I mean a thick fluffy head; two inches thick or more! If this is not achieved there is something wrong with either your yeast or your brewing technique.

It should be borne in mind that most beer kits use hop extracts as their source of bitterness and that some strains of yeast will not perform well if the sole source of hop products is in the form of extracts. No one knows for sure why this should be so, but it is undoubtedly the result of hundreds of years of training our yeast to like hopped beer. The only real solution, if you feel that you are suffering from poor yeast performance for this reason, is to use whole hops for part of your bitterness.

During the course of fermentation, the specific gravity of the wort will progressively fall as the sugars in the wort are converted to alcohol and carbon dioxide. The specific gravity of the wort should be frequently measured to ensure that fermentation is proceeding in a satisfactory manner.

The specific gravity at which fermentation is complete is called final gravity, and this varies depending upon the beer being made. With an all-malt beer this will be at about 25 per cent of the original gravity, but a beer which has a high proportion of cane sugar in its make-up will ferment to a lower gravity than a 100 per cent malt beer. Typically, a bitter with an original gravity of 1040 will ferment down to a final gravity of between about 1008 to 1010. Final gravity has been reached when the specific gravity of the beer remains more or less static for a period of twenty-four hours.

It is not necessary or desirable to fit the lid to the fermentation bin during fermentation. It is important that the fermentation be allowed to breathe. If for any reason the surface of the wort becomes exposed, or is in danger of becoming exposed, then it is probably wise to lay the lid loosely on the top of the bin to protect the wort from air-borne bacteria, but it should be removed when there is a good yeast head covering the wort. During the latter stages of fermentation, when activity is subsiding, a lid can be fitted if desired.

Fermentation temperature

The temperature at which fermentation is conducted has a great effect upon the quality of the finished beer. Maintenance of the correct fermentation temperature is important if consistent flavour and quality are desired. Unfortunately, the control of fermentation temperature within life limits is one of the most difficult problems facing the home brewer. If the temperature is too high, a racing fermentation and astringent off-flavours may result, whereas if the temperature is too low, poor yeast performance may occur, which usually shows itself by the yeast losing its buoyancy and sinking into the wort, giving the appearance of reverting to bottom working.

In my experience the ideal temperature is the minimum temperature at which good yeast performance is achieved. This varies between different types of yeast but is usually around 18°C; although to aim for 20°C is probably about right. The maximum range of temperature recommended for top working brewer's yeast is between the limits of 18°C to 22°C.

It should be borne in mind that the yeast provides many by-products other than ethanol and carbon dioxide. These have been mentioned elsewhere, but can be thought of as flavour compounds. These flavour compounds have an important bearing on the final outcome of the finished beer, but are most apparent when fermentation is conducted at a high temperature and least apparent when fermentation is conducted at a low temperature. It therefore follows that a beer which has been brewed at a very low temperature may be somewhat bland in flavour, whereas a beer that has been brewed at a high temperature may have flavour compounds that are somewhat overpowering or even unpleasant. Obviously a sensible balance must be achieved.

Energy, in the form of heat, is evolved in the course of fermentation which would normally cause the temperature of the wort to gradually rise. However, in our home-brewing environment the ratio of volume of wort to the surface area of our vessel is such that this tempera rise is not usually apparent. We can take advantage of this evolved heat to increase the temperature of fermentation, if desired, by lagging our vessel with a blanket or other insulating material.

When the temperature has risen to its maximum we can cool the wort by removing the lagging, and/or moving the vessel to a cooler place as appropriate. A number of traditional commercial breweries pitch their yeast when the wort has cooled to about 16 C, and allow the temperature of the wort to rise naturally to a maximum of about 22 C about half way through fermentation, at which point they turn on the cooling water and gradually pull the temperature down so that it is back to 16 C at the end of fermentation.

Home brewers normally control the temperature of fermentation by moving the fermentation vessel to a warmer or colder location as appropriate. Unfortunately it is very much easier to raise the temperature of fermentation than it is to lower it. Raising the temperature can be accomplished by means of a brewer's fermentation heater, by standing the vessel on a wine-maker's heated mat, or by lagging the vessel if the yeast is still actively fermenting. In the winter there is always somewhere in the house warm enough for satisfactory fermentation. Lowering the temperature, particularly in warm weather, is an entirely different matter. Ideally one should not brew in high summer because "summer brewed ale doth not keep" (Brakspear) but it is probably fairly safe to brew on the cooler days of summer, particularly if you do not intend to keep the beer for very long! Frankly, the last thing that I want to do in sweltering weather is slave over a hot wort copper.

Cooling can be accomplished by standing the vessel in a bath of cold water, or by means of an attemperator, a coil of copper or plastic piping submerged in the wort with cold water flowing through it. The flow can be adjusted to maintain the desired temperature. The twin-bin wort cooler described in the chapter on equipment is also useful as an attemperator.

A very simple method of maintaining a low fermentation temperature in hot weather is to wrap a bath towel around the bin and ensure that it is kept soaking wet. The evaporation of the water from the towel will maintain a temperature a few degrees lower than ambient. If the bin, complete with towel, is stood in a large tea tray, and the tray is kept topped up with water the water will run up the towel by capillary action and ensure that it is always wet. ft will be most efficient if the bin is stood in well-ventilated place.

Fermentation pH

The acidity of the wort (pH) also has an important bearing on yeast activity. However, if we are brewing using established techniques, the pH of the wort should automatically be correct, and we should not need to concern ourselves with the subject any further. If one is consistently having difficulties with yeast performance then it may be worth checking wort pH at the beginning of fermentation to see that things are okay, but otherwise we can safely forget it. The ideal starting acidity for fermentation is around pH 5.3, but anything below about pH 5.8 is satisfactory. Starting pH is stressed because the pH of the wort falls very quickly during the course of fermentation, reaching a lower level of about pH 4 towards the end.

Yeast skimming

Many home brewing books recommend frequent yeast skimming. I do not go along with this. I do not believe in continually interfering with the ale. When one considers the amount of effort that is expended at every stage of beer production to ensure that conditions are correct for the optimum working of our yeast, such that we get a thick rocky head on the surface of our wort, it is rather amusing to learn that as soon as the thick yeast head is formed many people immediately skim it off again! While the yeast is sitting on top of the wort it is doing its job properly and protecting the wort from air-borne bacteria.

The only time that I skim yeast is if it is in danger of spilling over the sides of the bin, or if there are undesirable things on the surface of it. Of course, any muck that is brought up with the initial crop of yeast should be skimmed off, and anything else that looks undesirable should be removed. You will also be forced to do some skimming if the yeast is in danger of overflowing, but apart from this you can safely leave the yeast to its own devices until either you drop the beer into a secondary vessel, or until it is approaching the end of fermentation.

Towards the end of fermentation the yeast should be skimmed if it is in danger of collapsing into the beer, or the beer can be racked into a secondary vessel or cask leaving the yeast and sediment behind.

Single vessel fermentation

It is usual home-brewing practice to ferment down to final gravity in a single fermentation vessel and then cask the beer directly from this. This is by far the easiest and most straight-forward home-brewing method and many thousands of gallons of perfectly good home-brewed beer has been made in this way. Indeed, several well-known commercial breweries cask their beers straight from the fermentation vessel without as much as an intermediate racking tank. The only precautions necessary when using this method is to try to ensure that the dirty yeast head does not collapse into the beer at the end of fermentation. At the end of fermentation the yeast tends to loose its buoyancy and sink into the beer. A cleaner and more stable beer will be produced if this can be prevented. This simply entails skimming the yeast off the surface of the beer at the end of fermentation, or siphoning the beer into a cask or separate holding vessel leaving the yeast head behind.

The dropping system

The dropping system is an alternative to the single vessel fermentation system. With this system, when the beer has fermented a day or two, it is "dropped" into a secondary fermentation vessel leaving the dirty primary yeast head, mutant yeast cells, trub, and other debris behind. Fermentation is completed in the secondary vessel. It was the practice with the best breweries of old to utilize the dropping system; the famous Burton Union System is a modification of this system.

The advantages of the dropping system are that the dirty primary yeast head and the debris on the bottom of the vessel gets left behind, and the yeast gets thoroughly roused. However, a good many active yeast cells are also left behind on the bottom of the vessel, and some highly flocculant yeast strains require re-aeration to encourage the yeast to multiply and restore the yeast count to reasonable levels. It is this re-aeration that is the stumbling block for many home brewers due to the fear of airborne infection. It is true that re-aeration goes against the normal rules of sound home-brewed beer, but it is my view that the advantages of the dropping system outweigh its risks.

It is also true that some home brewers have problems with the dropping system and find that fermentation slows down considerably after dropping. It is probably true that the performance of some yeast strains is improved by dropping whereas other yeast strains object to it. I have always used the dropping system and have never had difficulty.

When using the dropping system in the home-brewing environment, fermentation is started in the primary vessel and is dropped into a secondary vessel after the yeast has stopped multi-plying and has fully adapted to anaerobic conditions. This corresponds to the "billows of silky bubbles" stage of fermentation mentioned above, when the yeast head levels out to an even mass of foam. If everything is going normally this will be about 36 hours after pitching, but in any case the beer should be dropped after about 48 hours.

A hydrometer cannot indicate when the yeast has gone anaerobic, but can be used as an indicator that things are progressing satisfactory. For those who prefer to use gravity as an indicator of when to drop, then the beer can be dropped when about half the fermentable material has been used up. With a 1040 beer this is at about 1025, assuming a final gravity of 1010. If the final gravity is not known then simply multiply the original gravity in degrees by 0.63, ie for a 1060 beer: 60 x 0.63 = 37.5, therefore 1038 or thereabouts will be a suitable point to drop. The danger of providing little sums such as this gives the false impression that a high degree of precision is required. A high degree of precision is not required; the exact point at which the wort is transferred is not critical. The only requirement is to ensure that there is sufficient fermentable material remaining for a vigorous fermentation to continue in the secondary vessel. If much more than half the fermentable matter has been used up things will be beginning to slow down. For this reason it is better to drop early than late. Dropping between 36 and 48 hours, even later if you do not happen to be around at the time, is usually good enough, it is not really necessary to resort to fussy measurements or calculations.

It is safe to run the beer from the primary vessel to the secondary vessel via the tap because re-aeration is required. A scavenger tube, as described in the chapter on equipment, may make life easier. Alternatively, the beer can be siphoned from one vessel to another and re-aeration performed using the measures mentioned at the beginning of this chapter. Care should be taken to leave as much yeast and sediment as possible behind in the primary vessel. A new protective head will soon form on the surface of the beer. The lid to the fermentation bin can be loosely fitted until the new protective head has formed, after which point it should be removed.


Sometimes a yeast will find it difficult to ferment a particular beer and fermentation will unaccountably stop, or at least slow down considerably. Rousing often stimulates a yeast back into action. Rousing normally refers to the practice of stirring the head of yeast into the beer to stimulate it back into action, but sometimes the wort is re-oxygenated as well.

Some strains of yeast flocculate early and spend all of their time sitting on top of the wort when they should be in suspension doing some work. These yeasts must be frequently roused by stirring them back into suspension. They are the types of yeast that I euphemistically term northern yeasts and are quite troublesome in the home-brew environment.

Rousing always has the danger of introducing airborne bacteria and should, in my view, be reserved for emergency corrective measures only.

Final gravity

Final gravity is the specific gravity at which the fermentation stops or, more correctly, reduces to a tick-over due to the majority of the fermentable material being exhausted. All of the recipes given in this book are provided with an approximate final gravity, but it must be borne in mind that these published final gravities are indeed approximate. They are provided to give the brewer some idea of what to expect and they should not be taken as being precise. There are a number of factors which affect the actual final gravity, such as the mash temperature, fermentability of the ingredients, the type of yeast, and original gravity, not to mention the accuracy of the hydrometer employed.

Actual final gravity has been reached when fermentation has abated at a specific gravity close to what we would expect for the type of beer being brewed; about twenty-five per cent of original gravity is a rough approximation. Fermentation can be considered to be abated when t specific gravity remains static for twelve to twenty-four hours. However, there is no need to really precise about such matters. It does not matter if the beer is racked into the cask a few degrees early.


When fermentation has stopped or when the specific gravity has fallen close to final gravity, it is time for casking. Some brewers transfer the beer directly to the barrel, others transfer to beer to an intermediate vessel first, fit a close-fitting lid, and leave it to stand in a cool place for a period of twelve to twenty-four hours in order to encourage more yeast to settle out.

At the end of fermentation the yeast is in danger of losing its buoyancy and dropping into the beer. It is desirable that the beer is transferred, or the yeast slummed off the surface, before this happens. If the yeast is skimmed off, a close-fitting lid should be fitted to protect the beer from airborne contamination.


Read also:

Ingredients for brewing: grain, sugar, malt extract

Traditional Commercial Brewing

Home Brewing. Beginners Start Here

Home Brewing. What should You Know about the Hops

All You should Know about Brewery Yeast

Home Brewing. Water and Water Treatment

Home Brewing. Mashing and Sparging


-1 #2 anonymus 2012-09-13 04:09 Quote
+1 #1 remy hair 2011-08-16 20:44 it is useful Quote

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