How We Brew
The Hall Brewery
This 10 bbl (bbl stands for barrel which is equivalent of 36 UK gallons or 163.66 litres) brewery, situated in a converted stonemasonry/joinery shed at Thornbridge Hall was originally used by Marston Moor brewery in North Yorkshire until its installation at Thornbridge near the end of 2004. The brewhouse uses 20 barrel hot and cold liquor tanks and utilises a single temperature infusion mashing system. Our mash tun has a slotted false bottom and is sparged after the mash period is complete (usually 75-90 minutes), collected in a small underback (a vessel that acts as an intermediate between the mash tun and the kettle itself) and pumped into the kettle. The kettle has a gas fired internal coil and the wort is hopped and boiled for 75-90 minutes before being passed through a heat exchanger and aerated on its way to the fermenting vessels. Our very own strain of yeast is added and fermentation takes place over three to five days, after which the beer is chilled. The yeast is collected (for repitching) and the beer is transferred to a conditioning tank where maturation occurs prior to casking.
Installed and commissioned in 2009, this is all about using the best technology we can to continue making great beer. Loads of research culminated in an Italian manufactured 30 bbl/50 hL (hL means hectolitre, which is a commonly used brewing term meaning 100 litres) brewhouse with one piece of German kit. It is a four vessel system consisting of a mash kettle that allows us to do single infusion, step infusion or infusion-decoction mashing, a "Helios" Lauter Tun, a whirlpool kettle with a thermosiphon external calandria boiler and a fantastic Rolek Hopnik hopback that allows us to whack a bunch of hop characters into our brews. Our hot and cold liquor tanks are 15000 litres each and we have nine 30 bbl/50hL fermenters and four 60 bbl/100hL fermenters. We have a dual tank yeast propagation/storage system, a Seital centrifuge and a Vigo/Cimec bottling line that should allow us to bottle 1-2000 bottles an hour! We also have a semi-automated cask washer and cask filler. Well, that's the basic stuff...
So now for The Science!
Brewing begins with water. Our water supply is the same for both breweries, which is extremely important in giving us a consistent beer character no matter where the beer has been brewed. The water supply to Derbyshire is relatively soft, which is very beneficial for brewing, allowing us to modify the water through the addition of a variety of mineral salts. This lets us emulate water from different areas of the world. We may want to go with London style water for a Porter, Dublin style for a dry stout and Burton style for pale ale. Varying amounts of five different salts are used and these are Calcium Sulphate (gypsum), Calcium Chloride, Magnesium Sulphate (Epsom salt), Sodium Chloride (table salt) and Sodium Bicarbonate (baking soda).
What's the point of using these salts? As I'm sure you're aware, beer is mostly water, usually between 90 and 94%. Water is a great solvent and all of these brewing salts dissolve in water (to a greater or lesser degree) and undergo a process called dissociation. Water is a molecule that exhibits polarity, meaning that the hydrogen and oxygen molecules have charges. It is these charges that allow our brewing salts to break up into their ions. For example, if we were to dissolve sodium chloride in water, the positively charged sodium ions (Na+) would be attracted to the negatively charged oxygen ions (O2-) in the water and the negatively charged chloride ions (Cl-) would be attracted to the positively charged hydrogen ions (H+) in the water.
This is all fair and well, yet how does this help the brewer? The fact that these salts dissolve allows all of these ions to affect certain parts of the brewing process. Calcium is important during mashing. It boosts and stabilises enzyme activity during mashing (both by pH reduction and heat protection of enzymes), as well as aiding starch gelatinisation and lautering performance. It also binds with phosphate ions (from the malt) which facilitates a decrease in pH in the boiling of the wort, inhibits too much colour formation, aids the coagulation of proteins and effects bitterness extraction during boiling. It is also important in fermentation performance and the flocculation of yeast cells and helps prevent haze. A pretty important ion!
Magnesium also helps to lower the pH of the mash but more importantly, it is important as a cofactor for yeast during fermentation. It's important to be aware that magnesium ions can contribute harsh bitterness to a beer though, so these salts are always used with caution!
Carbonate or bicarbonate ions are also very important in brewing. These play a major role with regard to the pH of the water. Remember that the enzymes that are present in the malted grains themselves all work at an optimum temperature and optimum pH, hence it is essential that these are balanced correctly. These ions tend to move the water more towards a high pH (alkaline) which works in the favour of dark beers, where the malts used often contain compounds that can be quite harsh or astringent. The ions help to balance out these acidic dark malt characters. However, in well-hopped beers, alkaline water tends to result in a rather intensely bitter beer. At Thornbridge we always take this into account, the biggest challenge is how to use salts in a dark, well-hopped beer!
Sodium affects the perceived flavour of the beer by enhancing its sweetness. Kind of weird when you think that we use salt on our food to increase its saltiness! The only thing is that you have to be careful when using too much of this in the presence of sulphate ions. Sulphates give crispness and dryness to the palate when used correctly, but can tend towards salty and harsh (and laxative!) if over-used. If you have too much of both sodium and sulphate ions in a beer you also get an unpleasant harshness, so again, it comes down to balance. Chlorides also give a rounded, full bodied beer and are beneficial when used in darker, sweeter beers. There are loads more ions that play minor roles in brewing, but I'm sure you want to learn a bit more about how we make the beer!
We've established how important water is for brewing, so now let's talk about malt. Malt begins its life in a field, generally as barley (though oats, rye, wheat and other grains are also malted). I'm sure you're all well aware that the barley grain is the seed/fruiting body of this grass and it is this little grain that makes beer possible. A barley grain is 70-85% carbohydrate. This is very important as carbohydrate means sugar and sugar means fermentation! As well as various types of carbohydrate, there are different amino acids, proteins, phenols (think tannins and polyphenols), vitamins, sulphur-containing molecules, even lipids.
Interestingly though is how this grain, growing happily away in a field in East Anglia, becomes malt. Firstly the grain is harvested and tested to make sure it is of the right grade to become brewers malt. Things such as the correct amount of nitrogen, the moisture content, even the grain size and germinative capacity are all measured prior to its selection. If it makes the grade, it is its first step on the road to becoming what every Hordeum vulgare (barley, to you and me) seed dreams of from sending out its first roots and shoots!
The barley is cleaned and graded prior to steeping in fresh water. Steeping is important as it is the maltster's job to trick the barley into germination. Through a series of steeping and air rests (remember, the grain isn't yet a plant and still needs oxygen and releases carbon dioxide), the moisture content of the grains increases. This allows certain parts of the grain, mainly the embryo (which will become the roots and shoots) and the endosperm (the starchy bit that becomes flour if you mill the barley) to absorb water, respire, use up any sugar reserves and then begin to release a crazy sounding compound called gibberellic acid. It is this that then stimulates another part of the barley cell wall, specifically called the Aleurone Layer... and this is what us as brewers are interested in.
The increase in moisture content allows the roots and shoots to begin growing. This is because enzymes begin to get released from the aleurone layer and these allow that starchy material I just mentioned to begin to be broken down into sugars. And just like us, the barley will convert sugar to energy... energy that is essential for the little rootlets and shootlets to break through the husk of the grain and attempt to grow into a nice, happy barley grass! Not so fast though... if all of this starch gets broken down by enzymes into sugars which are then used as an energy source for growth of a plant, then doesn't that mean that the brewer is going to have less sugar for his or her yeast to convert into delicious beer? Absolutely, so the growth of the rootlets and shoots (its fancy name is the Acrospire) are carefully monitored and this is one of the many balancing acts that is essential in the production of a great beer. All of this takes place in germination boxes, where the germinating barley grains are generally moved regularly so that all of those roots don't clump together and create one great seed mat!
There are a few things for the maltster to think about. Firstly, there needs to be enough enzymes released from the aleurone layer to allow a breakdown of carbohydrates, proteins and lipids in the actual brewing process. Secondly, the grain itself needs to be sufficiently broken down by enzymes so that when the brewer makes his mash, he gets lots of easily extractable sugars from the malt and finally there has to be enough colour and flavour precursors available that will give the malt its unique characteristics after it has been kilned/dried.
So I mentioned the aleurone layer and the release of enzymes, all stimulated by the weird-sounding gibberellic acid. The enzymes released also sound like they belong in a science fiction sub-plot and are mainly: β-glucanase, α-amylase, proteases and β-amylase. They all have various roles in the breakdown of the cell walls and molecules within the barley grain and are also the main enzymes that brewers want to harness in our mash.
Before all of those carbohydrates have disappeared into plant growth, the maltster takes the green malt, as it is now called, and gradually decreases the moisture content through kilning. The grains usually have around 42-44% moisture at this stage, but we want nice, stable, dry malt to store in our breweries, so we need the moisture to decrease to around 3-5%. The thing is though, that if the grain is heated to quickly or too unevenly or with too high a temperature, then it can damage all of those enzymes I just mentioned – enzymes that are essential for us to make beer! Over a few stages the malt is dried and depending on what type of malt is wanted, the kilning temperature is increased or decreased at different moisture contents or for different times. It is here that you can get rich, caramel or chocolate malts or delicate pilsener or pale ale malts or black, roasted, charred malts. There are loads!
So we have the malt and we have the water. Now we have to get both of these together somehow... At our Riverside brewery we use a two roller mill to literally pop the husk and slightly squish up the crunchy contents of our fantastic Thomas Fawcett (based in Castleford, Yorkshire) malt . Again, we have to show restraint... too much crush and our malt becomes dust... too little and we don't crack the husk open enough, meaning that the grain won't absorb water as well or act as it should further down the process line. It is essential that we break open the malted grains but not damage the husk too much. This husk will act as nature's own filter bed in the lautering/sparging part of the process, so if it completely disintegrates, we'll end up with a load of problems!
Currently at our Riverside brewery, we use a step-infusion mash for most of our beers. The malt is milled the day before and stored in a last grist case in our malt room and then chain-conveyored through to a large malt hydrator known as a Steel's Masher. This fancy piece of equipment is where the milled malt and hot water are combined and mixed with a series of jets and baffles before entering the mash kettle itself as a porridge-like slurry. The initial mash comes in at around 62 – 63°C and once the mashing in is complete and mixed well by the giant agitator at the base of the vessel, the walls of the vessel are heated, via a steam jacket, and increase the mash temperature to around 65°C. This sits for a period of approximately one hour and is then heated more rapidly up to 72°C.
Now different breweries will all do different things during the mash. The enzymes that have carefully being protected and selected for by the maltster all come into their own in different ways. They use the pH of the mash to their advantage (helped by the water composition and salt additions that I explained way above) as well as the temperature. In fact, they all have their very own optimum temperature and pH ranges. Some breweries will have rests at certain temperatures to allow the enzymes in question to behave in such a way to give them the beer they want to brew.
In summary, the enzyme known as β-glucanase, which is responsible for breaking up glucans, long-chain carbohydrates that tend to have quite gummy characteristics (though are great for providing mouthfeel in a beer, so we don't want to get rid of all of them!) works between around 35-55°C. The proteinases and peptidases which are responsible for breaking down proteins into smaller units called peptides, and peptides into amino acids (which are essential for yeast growth later in fermentation) work around 47-52°C and continue through up to around 60°C. Again, we don't want these to completely break down everything – proteins are necessary for head formation in the beer and all brewers love to see a pint of their magic with a lovely firm, creamy head on it (yes, even brewers down South... let's just blame the pubs or customers or enter a long debate over how to serve a pint of beer shall we, hehe). Downstream, the residual proteins will interact with some of the molecules that come through from the hops, helping stabilise the foam. Again, care has to be taken as too much protein will also give the beer a haze... mind you, it will be a lot better for you! Between 55 and 60°C, the β-glucanase activity stops and up to 65°C the mighty α-amylase takes over, breaking down larger sugar units into the main malt sugar, aptly named maltose. Starch gelatinisation also finishes off around these temperatures, where the carbohydrate granules actually pop open, allowing the enzymes to easily get to these molecules. 65-70°C sees a bit of good old β-amylase action, chopping up starch into dextrin.
If you can imagine the starch molecules arranged like a big many-branched oak tree... the β-amylase works like some sort of rabid brewing wood-borer, breaking the twigs of the tree away but once it gets close to a branch, it struggles, it's just too difficult for it to break them down, leaving behind dextrins (which are also very important for the body and mouthfeel of a beer). The α-amylase is more random... kind of like the Tasmanian Devil with a pair of hedge-trimmers. It smashes up the branches, thus helping the β-amylase (our wood-borer) to nibble away at the twigs, giving us loads and loads of nice fermentable sugars as well as a few tasty long chain carbohydrates. Finally at around 70-72°C we also get another bunch of reactions beginning to occur, mainly the formation of glycoproteins, when sugar meets protein pretty much. These are also responsible for foam stability in the finished beer.
Whewwh! Right... we have a big grainy, sticky, wet vessel of barley porridge. All of the insides of our lovely malted barley have undergone some sort of conversion to yummy, Horlicks-smelling sugars and they're all trapped up within the husks, which are half-floating in this thick delicious gruel. So how do we get this sugary syrup away from the husks? Simple!
The mash is transferred from our mash kettle into another vessel called a Lauter Tun. The lauter tun that we use was especially developed by Velo, and is known as the Helios system. But more about that later... This vessel has a slotted false bottom, meaning that there are a series of narrow slits all around the plates on the base of the vessel that allow liquid to get through, yet husks to stay put. This is important as it acts as the final filter for this beautiful, sweet, syrupy liquid, which brewers refer to as wort, as it progresses on its way to be boiled and continue its transmogrification into beer.
Anyway... we have the mash in a new vessel and we need to start separating liquid from solid. The liquid begins draining off from under the slotted false bottom and is recirculated through a sight glass that lets us check for clarity. When we're happy that the wort is clear enough, we begin collecting it in the next vessel, called the Whirlpool Kettle, where the wort will eventually be boiled. As we collect the wort in this vessel, we begin a process called sparging in the lauter tun. The lauter tun has a series of large blades in it which actually cut through the bed of grains. These allow us to get all of the sugars out of the grain bed more easily, as well as evenly distributing the grain as it is first pumped into the lauter tun.
The Helios system that we use is slightly different to a standard lauter tun, in where the blades just spin from a central axis and forever cut the same circle. Instead of one motor to spin the blades, this one has two. One of them is slightly off-centre, so the rakes/blades will never cut the same circle twice... kind of like one of those spirograph things! This is good for us as it allows us to get a great amount of extract (sugary wort) without having to sparge for ages. This is important because of the temperature of the sparge water. It's usually around 76°C which helps slow down and eventually stop any enzyme activity, but at these higher temperatures you can actually draw out some of the harsher, more astringent molecules from the barley husks. Again, that word balance comes in to action... we need to get as much of the sugar from the grain bed, without too much of the astringency that can leach from all of the tannins, silicates and phenolic material that you tend to get in husks.
But after a couple of hours we achieve just that and have a nice whirlpool kettle full of delicious hot wort. We then recirculate this wort through something called an external calandria thermo siphon boiler... that's probably not it's real name, but it sounds pretty cool, so that's what I call it! You all know how a Thermos flask works. You fill it with your tea and there is a nice insulative layer surrounding it, keeping it nice and warm. It's all to do with fun things like reducing infrared radiation, thermal layers, vacuum flasks etc. etc. Now imagine the glass/silvery part of the Thermos flask was a pipe filled with wort and the nice insulative plastic bit was an even larger pipe around that. Then imagine you had a series of the glass/silvery bits (but they are now magically transformed into pipes) all bunched together in this big mega-giant thermos that you could actually pump steam through. Amazing! That is kind of what our boiler does! The wort is initially pumped through the series of small pipes with a jacket of steam surrounding them. The boiler itself is probably a good couple of metres long, so as the wort travels from the bottom to the top of the boiler unit, there is a change in its heat (warm water rises due to natural convection) and it actually starts drawing itself through without the aid of a pump. This is great for us! For one we don't have to pump the wort as much and that's always a good thing. The wort contains loads of long-chain molecules... all of those dextrins and glucans I mentioned earlier and these can get pretty messed up by the shearing action of a centrifugal (a spinny roundy) pump, hence effecting the body of the beer and we don't want to have read this far and learnt all that you've learnt to then go and mess things up!
So we are now boiling the wort. Why? Where to begin!! Boiling sterilizes the wort, and this is really important, especially as we are going to add our own strain of brewing yeast and don't want any microorganisms or wild yeast to get into the wort and give us a completely different flavoured beer.
It also acts to completely inactivate any enzymes that may have made it through the lautering. We don't want the enzymes to keep working, breaking down sugars and proteins etc. We want our wort to be fermentable, but not too fermentable! A bit of residual sugar is always important to balance bitterness (unless you are going for a really dry beer of course).
Next is the precipitation of protein, mostly as a substance known as trub, which is actually a conglomeration of proteins, lipids, polyphenols etc. We don't want too much of this carrying over into the finished wort – it can give us haze issues as well as potential off-flavours in the beer.
Next up is colour and flavour formation. You should know that when you cook a steak in a fry pan, it tends to brown up and gain a lot more flavour. This is to do with a series of reactions called Maillard reactions, Amadori rearrangements and Strecker degradation. What, I hear you say? Pretty much you get a bunch of reactions occurring in the presence of heat, especially when there are sugars and proteins present that result in yummy caramelised, roasty and even nutty flavours. If you boil for an extremely long period of time, you get even more of these characters coming through! Coupled with all of those speciality malts I mentioned earlier, you can see why those darker beers can sometimes taste like they do!
Time to introduce one of our favourite things at Thornbridge, the mighty hop. The hop is a plant, weirdly in the same family as Cannabis and even Nettles that we use to add spice and fruitiness and intensely flavoured brilliance to our brews. Oh yeah, and let's not forget the bitterness! The part of the long, far-reaching vine (or bine as it is called in hop-land) that brewers are interested in is the flower, sometimes referred to as the cone. It is this that is jam-packed with loads of wonderful resins and essential oils. Specifically it is a fraction referred to as the α-acid that the brewer is often interested in. It is this set of compounds that undergoes an interesting chemical reaction when boiled called isomerisation. In other words, the molecule changes shape, but retains the same amount of atoms. Hence our α-acids become iso-α-acids and with this change they become more soluble in water and become bitter to the taste. We also get all of those other deliciously aromatic oils released into the boiling wort. Unfortunately, due to the nature of aromatic substances, they are often quite volatile and easily escape into the atmosphere, but we have another trick up our sleeve to fix that later...
Even though boiling can drive off some of the nice aromas that can come from hops, it also helps to drive off stuff that we don't want carrying through into the finished wort. Compounds like dimethyl sulphide, which has an almost baked bean, cooked vegetable, sweetcorn character to it is never desirable in your pint and it also removes some aldehydes that could potentially cause off flavours through oxidation downstream.
and Wort Boiling again
Finally, boiling helps to concentrate the wort. Just like when you make a delicious gravy to go with your Yorkshire puds for Sunday lunch, this concentration helps intensify flavours and that has to be a good thing! Boiling also helps to lower the pH of wort.
So back to the hops. Stop yawning!
We add hops at various stages during the brewing process. In our Whirlpool Kettle, we use pellet hops. These are simply hops that have been ground and pelletized and are used here because they work perfectly with the whirlpool action of this vessel. While the beer is boiling, and more specifically at the end of the boil, the hopped wort is pumped through a tangential inlet in the vessel, creating a whirlpool motion. This allows all of the bits of trub and hop to accumulate in a cone in the middle of the vessel, hence acting as a clarifying stage in the process. One thing you don't want as a load of lumpy proteins and bits of hop flower floating in your glass of beer, so this is one of those things that helps to stop this from happening. It is here that we also add seaweed. There are two main types of seaweed used in the copper, where they are called copper finings. We use a Pacific Seaweed (Euchema cottonii) product, but another popular copper fining product is Irish Moss (Chondrus crispus) and it works by allowing proteins to clump together then undergo a process called precipitation (yes, just like what water does when it goes from vapour to rain... I guess we can say we make it rain proteins!!).
When boiling begins, the first lot of hop pellets are put into the kettle. We estimate how much utilisation of the alpha acids we will get based on our system. It's usually between 27-30% here. Oh yeah, forgot to mention! The alpha (α) acids in hops are measured by the hop producers and expressed as a percentage by weight. For example, the Nelson Sauvin hop we use in our Kipling is usually around 12-13% α-acid. We figure out how much bitterness we want in the beer based on this. We know the utilisation our brewhouse gives through experience and we calculate how much bitterness each hop addition will give at various stages throughout the boil. We generally do two additions in the whirlpool kettle, the first yielding 27-30% as mentioned above and the second, added 30 minutes into the boil, yielding less, maybe 22-27%. The second addition yields less for a couple of reasons. Firstly, as the α-acids isomerise during the boil, they begin to reach a saturation point. In other words, as you get more of these in the wort, then the isomerisation rate begins to decrease. Also, the second addition of hops is boiled for less time. This isomerisation into the bittering substances is dependent on time, temperature and also the wort strength in terms of the amount of sugars (or gravity as we tend to call it). Confused?
Let's talk about the last addition now. For our final addition of hops, we use flower (or cone) hops. We do this because we think that flowers give a fantastic aroma and flavour to our beer at this stage. Some people say that pellets retain aroma a lot better than flowers due to the dimensions of a pellet and the fact that the inside of the pellet is a lot less prone to oxidation. Others swear that flowers give a better character than pellets. A lot of research has been done on this and it really comes down to the brewer's personal choice. We love opening a bag of hops and rubbing the flowers between our hands. This warms up all of the essential oils and allows the beautiful fragrance of the hop to come forth. It gives us an idea of how these characters will translate into a beer and is essential to us developing and improving our beer recipes. That's one reason why we love flowers, they're a lot easier to rub and smell than the pellets are!!!
The final hop addition is usually massive, especially if we're brewing a hoppy beer like Jaipur where around 70% of the hops used in the beer are added at the last stage. The more hops you use, the more intense the flavour and aroma! It is here where our German vessel, the Rolec Hopnik comes into play. This vessel is a Hopback and the purpose of this is to hold the flower hops and act something like a giant tea-bag for us to infuse our wort with all that hoppy goodness. The inside of the vessel is double walled. The first wall is a stainless steel mesh, which stops the hops from getting through into the finished hopped wort and the outer wall is solid stainless steel. The wort is pumped from the whirlpool kettle into the hopback where it steeps with the hops, then the hopped wort drains through the mesh, goes through a heat exchanger which cools the wort and heads on its way to the fermenting vessel.
The great thing about hops is that there are around 300 different essential oils that can be combined and utilised by the brewer in an infinite number of ways. This is why craft beers from every brewery will always taste that tiny bit different.
So the wort is all hopped up and on its way to begin fermentation. We cool it down by running it through a plate heat exchanger. This is effectively a series of plates providing a really large surface area for the wort to travel through. On the other side of the plates is chilled water which takes the heat from the wort and is then returned to our hot water tank as part of an energy recovery system. The cooled wort is then oxygenated. We either use sterile air, food grade oxygen or both to achieve this. We add oxygen to the wort for one main reason – for yeast growth.
Now we enter the world of the Yeast. We have both Theodor Schwann (who effectively discovered that yeast was alive) and Louis Pasteur (whose studies further showed the significance of yeast and other microbes in fermentation and the spoilage of beer) to thank for this little unicellular microorganism and its use in brewing. Before this discovery, it was unknown how fermentations actually occurred. Brewers knew that yeast was an important part of the process, they just didn't know what it really did or how it did it! We've come a long way since then and due to the work of brewery laboratories and research scientists, we understand a lot more about the function of yeast cells and what actually occurs when wort becomes beer.
I mentioned oxygen earlier. This is used initially by the yeast to grow. We add a certain amount of yeast to our wort. Pretty much, the stronger the beer will be, the more yeast we add initially. Let's take Jaipur, for example, where we add around 7 – 8 million cells of yeast per millilitre of finished wort. A lot of yeast! The yeast is floating around in a super rich environment of nutrients, mostly in the form of sugars, amino acids and peptides that the cells start voraciously feeding on (well absorbing through their cell walls at any rate) and undergoing a process called budding. This is where the yeast cell (referred to as the mother cell) creates another yeast cell (referred to as the daughter cell). Something called Yeast Viability is important here. This is one of the first laboratory checks we do on the yeast. We take a sample of the yeast that is going to be added, or pitched, into the wort, we count and calculate how many cells are present in one gram of yeast (not all by eye of course... it's usually between 500 and 2500 million cells in a gram!) and we also stain the sample with a special dye called Methylene Blue. If the yeast cells are healthy, an enzyme present inside of the cell will change the dye from blue to colourless. We calculate the percentage of cells that are able to do this and express it as Yeast Viability. We really want this to be greater than 90% otherwise we're just adding dead yeast to the wort and this can potentially cause off-flavours.
So we have yeast, oxygen and wort. The yeast is increasing in biomass (usually by anything up to 5 times) by a process called budding and using up nutrients to get its own nitrogen and carbohydrates and fatty acids and anything else it needs to grow. But what happens when the oxygen is all used up? This is when the yeast switches its mode of metabolism from aerobic to anaerobic. Anaerobic means in the absence of oxygen and it is here that the yeast begins the actual fermentation and sugars are converted to carbon dioxide and alcohol. Without boring you all to tears (I'm sure I can see some already... right at the corner of your eyes...) effectively, sugar is absorbed into the cell and broken down by a series of enzymes into the aforementioned carbon dioxide and alcohol. As well as this, there are a bunch of side reactions which both use and yield energy for the yeast cell and also give an interesting array of flavours to the finished beer. Fruity esters or flavour active ketones (such as the rich,intensely buttery diacetyl) or sulphury compounds or even heady, harsh, fruity or bitter fusel alcohols. They all blend together to provide the final beer flavour. Some of these are good and sought after, others not so much and a brewer puts all of his or her training and understanding into keeping some of these metabolic by-products out of the finished brew.
It is a ridiculously complex interaction between a living cell and the liquid it has been placed in. So many variables have to be taken into consideration: temperature, time, wort composition, oxygen content, the amount of particulates in the wort, yeast pitching rate, yeast viability and vitality, yeast strain, the activity of the yeast throughout the fermentation, flocculation capability, the list goes on and on.
We monitor the fermentation throughout by recording both the pH of the beer (aiming for a final pH of between 4 and 4.4 depending on the brew) and the gravity of the beer. The gravity is pretty much a way that we can see how much sugar we have present in the wort by measuring its density. We then look at a ratio of the density of our sample with the density of water. What? Okay... there are two ways we measure density at Thornbridge. One way is with a special little glass-bulbed measuring stick called hydrometer. This is weighed out and graduated exactly in its manufacture, so that we can place it into a measuring cylinder of our wort or beer and it will float (it is a weighted glass bulb). It shows us on a scale what density the wort/beer is and at this time we measure the temperature of the liquid also. We need to do this as the density of a liquid is dependent on temperature. We adjust the final reading from a little chart according to the temperature and just like that, we have a gravity reading!
There is of course an easier way. We also have a flash little machine called an automated density meter. We filter our sample, put it into the machine which has a crazy little measuring device called an oscillating U-tube, and this gives us a reading in just a few seconds! A lot more simple!!!
So by measuring this gravity, we then build up a fermentation profile for each of our beers. This will show us how the yeast is converting the sugar to alcohol and CO2 over a period of time. We control the fermentation by using temperature, generally large external jackets on the fermenters that run very cold glycol through them. This allows us to maintain or increase the fermentation temperature as is necessary. This is really important as yeast tends to give off a lot of heat when it ferments. If we were to leave the fermentation alone with no temperature control, the fermentation may get up to as hot as 30°C instead of the 18 – 22°C that we usually use for our fermentations. The other reason that this is important is that if the fermentation is too hot, we can get a lot of off-flavour development in the beer and the fermentation might be so quick that the yeast doesn't get a chance to aid in the maturation of beer flavours as they would in a normal fermentation.
We begin with an Original Gravity (OG) and end with a Final Gravity (FG) and this drop in gravity is also used to calculate the alcohol content, usually expressed as Alcohol by Volume (AbV). Once the beer hits its designated gravity, the beer is then chilled down to between 4 and 6°C and the yeast is removed from the tank to be re-used in subsequent brews. The beer is left for at least 5 days after which we run it through our centrifuge. This acts to remove the yeast from the beer through a mechanical separation (by spinning the beer like crazy!) and we can adjust the amount of yeast we want carried over into the finished beer. This is important as sufficient yeast is needed in the cask to allow secondary fermentation (or cask conditioning) to occur. This is where any residual sugar that has survived the initial fermentation is slowly fermented out by the residual yeast while in its final package, in this case the cask. If we are going to bottle the beer, it is centrifuged so that we get a lot less residual yeast. It is then cold conditioned after which fresh yeast and priming sugar are added, bottling takes place and then the bottles are warm-refermented to add condition (CO2 content).
When putting the beer into cask we add a compound called isinglass, a very pure source of collagen protein that helps the yeast cells bind together. The collagen has a positive charge and yeast generally tend to be negative. This allows flocs of yeast cells to form and as these increase in weight they then precipitate out on to the bottom of the cask. This is one of the reasons why a beer needs to be stillaged (put somewhere where the cask can be effectively tapped and the beer drawn off) for at least 48 hours prior to being served. During this time, the cask is also vented, usually via a small piece of porous wood called a spile. This venting of CO2 can sometimes stir up the precipitated yeast cells at the base of the cask and can also result in far too much head when the pint is pulled at the bar(if not vented early enough), so it is essential to do this a good while before the cask is ready to serve. We don't want to waste beer now! Once the cask has been broached, it is essential to finish it within 3 days. The problem is that for every pint you draw at the bar, this means an equivalent volume of air is entering the cask. The beer begins a slow process of oxidation (which for some beers can mean a change in flavour, perhaps from an intense, hoppy character to a more muted, soft, fruity hop note) but also, due to the nature of a bar cellar, it also means the ingress of any wild yeasts or spoilage microorganisms that are present everywhere. This will effectively begin to make the beer spoil, although again, there are quite a few factors that can affect this. The strength of the beer plays a role, if it has a higher AbV it will be more difficult for spoilage microbes to survive. Even the amount of hops used may play a role, if it is a very heavily hopped beer, they natural antibacterial character of the hops may also reduce the spoilage potential. One thing to remember, even though you've probably pulled out this excuse on many an occasion, is that you cannot get ill from beer. Due to factors such as the pH and the alcohol content, it is just not the right environment to harbour pathogenic microorganisms. That's right, you can't blame the massive hangover on a dodgy pint or the fact that you thought it was okay to eat the leftover kebab sitting on your bedside table when you woke up at midday the next day and were then ill on a dodgy pint, you can't blame any crook guts on a dodgy pint (well, except for when you drink ridiculously copious amounts of them... that's poisoning alright, but of a different kind!). The fact of the matter is that for centuries, around the world, beer has been brewed for that exact reason. It metamorphosises non-potable, potentially contaminated water from a stomach-ache waiting to happen into a clean, safe product suitable for drinking. Through the action of boiling and fermentation and a drop in pH and the creation of alcohol, it makes it all okay!
So there we have it. From humble beginnings in a little two vessel brewhouse sitting in a converted shed in a Derbyshire Country House right through to the fantastic technology that we are using today, that's the story of how we brew our beer at Thornbridge. If you've managed to stay awake through the whole thing, then well done! Hopefully you now know a little about what we do here and why we do it. It doesn't necessarily always run as smooth as it should, but that would just take the fun out of it. Happy drinking!