The science of alcohol
The science of alcohol

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The science of alcohol

5 Flavours in alcoholic drinks

Alcoholic beverages come in a vast variety of flavours. Some of these are added deliberately to the drink (quite common for alcopops, fruit ciders and spirits), but in traditional drinks such as beer the flavours occur naturally, coming from the components – hops, grains, water and yeast. You first met these components of beer in Week 2. In this video, Danny Allwood describes some of the flavours associated with these components and where they come from.

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The beer is one of the most diverse drinks available. And the flavour profile of your beer is made up of what you put it in, in terms of your raw ingredients, and also the processes which you've put them through. So if we start at the beginning, the malts which we use, which make up what's known as the grain belt-- so the types of malt we use, the amount of malt we use, the mash temperature that we use, which determines how sweet or dry our finished beer will be, depending on how many-- or the amounts of residual sugars which are left in the beer, will all have an impact. If we use more dark malts, more coloured malts, we'll end up with caramel, biscuity flavours, roasty flavours, coffee-type flavours, and these are all present in the malt from the malting process. As soon as we've made wort, we then boil it, and that is another process. So those compounds which we've extracted from our malt into our wort can then undergo further chemical transformations. If you heat chemical compounds up, they can react with each other, and one of the classic examples in brewing is the Maillard reaction. And that's the reaction of amino acids with sugars. And you can see this when you bake a loaf of bread. It's the reason the bread goes brown on the outside, those are Maillard reactions. And the same thing occurs when we're brewing, it occurs in the malting process what we're making malt, and to a certain extent, it also happens in the boil. So although you've extracted these compounds from your raw ingredients, they then undergo further chemical transformations due to the processes that you're putting them through. Also in the boil, we're then going to add our hops. So the hops are contributing the alpha acids and the hop oils, which are giving us bitterness, in terms of the alpha acids, because they are summarised to the iso-alpha acids, which give us the bitter compound. And the hop oils, which are giving us the hoppy, aromatic, very sort of citrusy, herbal type flavours, but tend to boil off quite quickly, so we tend to use them later in the boil. Or, we can use a process called dry hopping, which is where we add dry hops after fermentation is complete, and those will tend to add very bright, citrusy, piney type flavours and aromas to the beer. Finally, we've got fermentation, where the yeast get to do its job. And the type of yeast that you use, whether it's a Belgian strain, or a German strain, or an English strain, will determine the type of compounds that are produced by the yeast. So principally, they're using fermentable sugars, which you produced in your mash, and they're turning them into alcohol and carbon dioxide. But they're also capable of processing other compounds which are found in your wort. And they may come from your hops or from your malts, and the yeast can process those into other compounds, as well. So the yeast are not just producing alcohol, they're also having a profound effect on the flavour. So the flavour of your beer will ultimately depend on what ingredients you choose to use and how you choose to use them, and how you do the processes of boiling, and fermentation, and things like that. So everything makes a contribution, and that's what makes beer one of the most diverse products available.
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Within the malt, the yeasts and sugars interact producing lots of different chemicals that contribute to flavour and aroma. Brewing temperature and yeast strain strongly affect this. The main compounds are esters and phenols. In general, more esters are produced when the fermentation step is warmer. Ale yeast prefers warmer temperatures compared to lager yeast and produces compounds such as isoamyl acetate (sweet candy banana), ethyl octanoate (apple skins) and ethyl hexanoate (anise). Not all compounds smell good, though. Warm fermentations can result in ethyl acetate (acetone), which is commonly used in nail polish remover! Other flavours that are yeast dependent are 4-ethylphenol (an earthy aroma) and 4-vinyl guaiacol (cloves).

How the brewer treats the grains during the malting process leads to many of beer’s prominent flavours. Toasting the grains and the drying process have a large effect and also affect the colour. For example, darker grains smell like candy floss due to the presence of maltol.

During the toasting process the amino acids and sugars react together in a caramelisation called the Maillard reaction. Literally hundreds of compounds can result from this with the most common flavours and aromas including caramel, chocolate and coffee.

Hops are some of the most recognisable flavour providers. Bitter compounds such as the humulones – known as alpha acids – are present in the hop flower. The higher their levels, the more bitter the beer will be. Varieties of hop differ in content and therefore bitterness.

The classic hop smell comes primarily from essential oils in hops. You saw in Week 2 that, for a ‘hoppy’ beer, more hops are added at the end of the brewing process. The most common oils are alpha-caryophyllene and humulatriene. These two compounds create the key pine, citrus and sage-like aromas that hoppy beers are famous for.

Consider the structure of humulone in Figure 7.

This shows the chemical structure of the alpha acid humulone.
Figure 7 The alpha acid humulone, found in hops
  • Is humulone likely to contribute to the taste or the smell of a beer?

  • Humulone is a large molecule with an RMM of 362.46 and as such is highly likely not to be volatile at room temperature. Therefore, it is more likely to contribute to taste rather than smell. Other smaller hops can be used to contribute to the smell of a beer.


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