Hi, I’m Tristan, this is Magic Maillard Maple, and today we are thinking about the
science behind Beer.
[Music]
So like the rest of the UK we are on lockdown right now, but the sun is shining, it’s
a bank holiday, so we are going to break out the homebrew kit
and make ourselves a batch of Ale.
Specifically, we are going to make a batch of my “lemon porridge” IPA, so-called
because I include rolled oats as part of the mash, and use lots of Citra hops to give a
fresh Citrus flavour.
The written history of beer goes back to Ancient Egypt and Mesopotamia, and when you think
about it, the benefit of this beverage to these vast, growing, cereal farming civilisations
were enormous.
To make any alcoholic beverage, we need a food stuff naturally high in carbohydrate.
This could be sweet fruits, like apple or grapes, or could be a starchy cereal. Yeast
ferments sugars to (predominantly) ethanol and carbon dioxide, making a fizzy, flavoursome
drink that is calorie dense, and has a long shelf life. The alcohol content is typically
sufficient to decrease the growth of pathogenic bacteria, meaning that beer was a relatively
safe alternative in times and places where clean drinking water was not readily available.
Today, beer represents almost 40% of all UK alcohol sales value – a market that was
worth an estimated 46 billion pounds in 2017. However, unlike cider or Wine, beer is a difficult
beverage to make by accident – mainly because we need to convert the starch in the grain
to easily fermentable sugars. The first step of making beer, is the malting process.
Beer is typically made from “malted” barley. We start by steeping the grains in water,
to trick them into germinating. After we see the first shoots formed, known as chits, we
transfer the grains to a temperature-controlled germination vessel. During the malting stage,
the barley grains start to produce a wide range of hydrolytic enzymes, and the of outer
endosperm wall is broken down. Finally, we can place the grains in Kiln to halt any further
germination. Malted grain is a good source of proteins and reducing sugars, so by adjusting
kiln time and temperatures we can influence the degree of Maillard browning that takes
place. This makes a wide range of crystal and speciality malts possible, that can give
complexity to the odour, colour, flavour and viscosity of the beer. However, pale malts
made at low kiln temperatures are still the primary ingredient of most beers, as this
is the best source of intact amylase enzymes, that are needed in mashing.
During mashing, we combine milled malt, known as grist with warm water to make a solution
called “sweet wort”. We have two objectives here: first, we want to extract as much starch
from the grains as possible, and second, we wish to break that starch down to the mixture
of fermentable sugars and maltodextrins.
As with all cereal crops, like wheat, rice and corn, barley starch is comprised of two
types of molecule – amylose and amylopectin. Both of these consist of long chains of glucose,
joined by alpha links in the 1 and 4 position. The main difference is whilst amylose is linear
and helical, amylopectin branches every 20-30 units. This gives these starches very different
behaviours, that make them useful as thickeners and gelling agents in food.
At the start of the process, we will initially hold the water at about 38 degrees Celsius
– at this temperature, enzymes responsible for breaking down fibre and hazing proteins
have the greatest effect. We will then raise the temperature up to the so-called brewers
window – between 64- and 70-degrees C. This is where the amylases are most active.
We have two main amylases at work:
a-Amylase, operates best at 68-72 oC. This endo enzyme breaks the 1-4 bonds in starch
at random places – turning each individual chain into several shorter chains called maltodextrins.
These molecules thicken the wort, giving the final product body.
B amylase operates best at slightly lower temperatures. This exo-enzyme can only work
from one end of the starch chain, liberating a unit of maltose each time. These smaller
disaccharides can be directly used by yeast to make alcohol.
Therefore, as our mashing temperature rises above 65 degrees, we decrease the potential
fermentability of our wort, but increase the viscosity. Today, we will compromise by mashing
at 67 oC for an hour. After this we will “mash out” by raising the temperature to 75 degrees
– at this higher temperature, the after is a more efficient solvent for starch extraction,
and any extra will be largely converted to maltodextrins.
We then remove the grist form the wort, and drain. As there is still some starch left
in the grains, we will gently sparge using fresh water at 80oc. If we were to use boiling
water here, we would start to also extract some unwanted bitter
and burnt flavours from the grist.
The next stage is to boil the wort. The main reason for this is to sterilise the liquid
ahead of pitching our yeast, but we get the added befits include decreasing pH, the coagulation
of some of the hazing proteins and the formation of a wider range of colours and flavours.
We also need to boil to wort so as to extract compounds from another major ingredient: hops.
Hops are the dried flowers of Humulus lupus, but we can also use Humulus japonicus, and
yunnanensis. These flowers were originally added to beer to improve its keeping quality,
but also provides a desirable bitter flavour, and flowery citrus notes. Hops contain three
major groups of compounds that give flavour and aroma to the beer.
Alpha acids provide the bulk of the bitterness in beer. Boiling hops improves the extraction
of these compounds from hop resin, and also drives their isomerisation into much more
bitter forms. The longer hops are boiled, the more bitter the resulting beer will be.
Beta acids are poorly water soluble, and as such contribute more to the aroma of beer
than to it’s bitterness, at least when using fresh hops. However, as these compounds are
oxidised, they become more water soluble and also more bitter, meaning that they can cause
brews to become more bitter if stored for an extended time.
Whilst hops only contain up to about 3% essential oil, this provides a wealth of flowery, herby
and citrus flavours and aromas. As these compounds are volatile, they are lost on extended boiling.
As such, whilst many traditional “real ale” recipes only require hops adding at the beginning
of the boil, “craft beer” recipes will also call for hops to be added at the end,
or even for dryhopping – this is where we would add the hops after wort had cooled.
The later we add hops to the boil, the more of the volatile content will be retained.
About 10 minutes before boiling is finished, we will also add some Irish Moss - this is
the common name for Chondrus crispus, a type of red algae that is a rich source of kappa
carrageenan. This is soluble in water above 60 degrees C, and when added to bitterwort
which is about pH 5, will form a negatively charged molecule. At this same pH, many remaining
hazing proteins in the wort will be positively charged. These two species will then be attracted
to each other, and they’ll form a complex with an overall neutral charge. This means
that the complex will drop out of solution – and we get less hazing in the resulting beer.
With our homebrewing set up, we don’t have the option to whirl the bitterwort to remove
sediment – luckily by using a hop basket, we can at least keep the larger chucks of
debris away from the beer. We can then cool the beer with a counter-current chiller – this
runs off a garden hose, and then we can then pump the bitterwort into a sterilised fermentation
vessel. Probably the most important rule for home brewing is to keep everything scrupulously
clean, and to sterilise everything that will make contact with the cooled bitterwort.
I’ll then check the original gravity of the beer using a hydrometer, and then I’ll
whisk the cooled beer with a beer paddle to get lots of air into the solution. Yeast is
a facultative anaerobe, it does not need oxygen to convert sugars to alcohol. However, it
does need oxygen to create cholesterol, and cholesterol is a vital part of the yeast cell
membrane. Ergo, by adding extra oxygen to the wort, we can speed up the fermentation
time, because it will be easier for the yeast cells to replicate.
After placing the fermentation vessel somewhere reasonably warm, we can pitch the yeast. This
time I’ve picked a freeze-dried California ale yeast, which will get the job done in
less than a week. Different yeasts can tolerate different final alcohol levels, and will also
produce a different profile of congener compounds that add to the flavour and aroma of the beer
– for example a Saison would be made using a yeast that would also contribute a range
of esters which would add a fruity note to the beer.
We’ll then seal the unit and place an airlock filled with dilute vodka in the top – this
allows the beer to vent carbon dioxide without allowing natural yeasts or bacteria from the
outside to contaminate the fermenting beer.
When the fermentation is complete, we can take a final gravity reading. By using this
equation, we see that the beer has achieved 5% alcohol by volume. We will then add a small
quantity of priming sugar – I prefer to use honey, and then transfer the beer to sterilised
amber bottles. This small amount of sugar will be used by the live yeast left in the
beer to undergo a secondary fermentation. This is what’s going to make the beer fizzy.
The beer will be ready to drink in about a week, though leaving for a few more weeks
will give a beer that has a more developed flavour and even better clarification.
Well, there we are – I hope you enjoyed learning some of the science behind homebrew.
We are off to enjoy the weather, and now a quiet pint. See you all next time!