The science of alcohol
The science of alcohol

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The science of alcohol

6 Spoiled beer

We can all dream of the perfect pint but just like any food that has been left out too long, or not prepared properly, beer can go off. Drinking it will not be pleasant or in some cases it will even be bad for you.

In this video Danny Allwood discusses his thoughts on spoiled beer.

Download this video clip.Video player: soa_1_w3_s6_vid_spoiledbeer.mp4
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So like any process, sometimes brewing goes wrong. And the result of this is what brewers call off flavours. These are undesirable flavours that you get in finished beer which shouldn't be there. They were never intended to be there. So usually, this is a problem with the process or with storage of the beer. So some examples are things like dimethyl sulphide, which is really, it's a component in malt. And if you don't boil the water vigorously enough, you won't remove the dimethyl sulphide, and the end result is that your beer tastes a little bit like cooked cabbage. It's not particularly pleasant. A little bit of dimethyl sulphide is OK in some beer styles, like lagers, but generally it's not a desirable thing. Another common mistake that happens in brewing is temperature control. So the yeast are very temperature-dependent. And if you allow your fermentation to get too warm, you can produce compounds like fusel alcohols, which taste very hot, very sort of alcoholic flavour, and esters, which are very, very fruity. So esters are characteristic of ale styles, but too much esters and it becomes too characteristic. So, another common issue is infection. Now, we are deliberately infecting our beer, if you like, with yeast. We're infecting it with the yeast that we want to brew the style that we want to brew. But wild yeast can get in, and these may have a different flavour profile to the yeast that you intended, or bacteria can get in. And certain bacteria can cause-- can produce butyric acid, which is the smell of vomit. They can produce lactic acid, they can produce acetic acid, which is vinegar. And if your beer tastes sour, unpleasant, vinegary, infection's usually the issue. And that's caused by poor sanitation, poor hygiene practises. The other main arena in which off flavours can get in is in storage of the beer. And two of the most common ones are oxidation, where oxygen has got into the beer, it's reacted with some of the compounds that have come from the hops and things like that, and the resulting flavour is one of stale, cardboard-type unpleasantness. So if your beer is not airtight, it can go off with oxidation. The final one is light strike, or skunked beer. This is beer that's stored in clear or green glass bottles. Compounds that have come from the hops react with UV light, which get through the glass in the bottle, and you produce the compound which is also produced by skunks in skunk spray. So your beer will smell and taste of skunk spray. Some manufacturers who package their beer in clear bottles will recommend that you put a lime in the top of it, and to some degree, that's the mask that flavour of skunk spray. Some commercial beers have this as a particular characteristic which consumers have come to expect. But, in most brewed beers, it's considered a fault. So, all being well, brewing will produce some very nice beer, but things can go wrong, and usually there are ways of fixing it. Normally, it involves leaving the yeast to do their job and tidy up after themselves.
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The brewing industry uses the term ‘off flavours’ to describe any undesirable flavours which arise in a finished beer.

  • From the video can you recall from what stages in the brewing process off flavours are most likely to arise?

  • Boiling, fermentation and storage are the main processes in which undesirable flavours can be introduced into the finished beer. However, poor sanitation at any stage in the process can lead to contamination with bacteria. When bacteria carry out the reactions needed to sustain life, compounds are produced which can lead to off flavours such as acetic acid (giving rise to the taste of vinegar, as in corked wine).


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