How water, barley, hops and yeast are processed to perfection

Kevin Lyons

Beer brewing is probably as old as – or almost as old as – the human cultivation and consumption of cereal crops. It’s not impossible to imagine that the first “brew” was accidental, and was simply discovered mid-fermentation by an intrigued group of early agriculturalists: perhaps in a forgotten grain storage container which had been left open and unattended in a damp or flooded area for the appropriate period of time. After all, wild yeasts – tiny, unicellular fungi which can act as little carbon-dioxide-emitting, alcohol-producing factories when exposed to wet sprouted grain – are ubiquitous in nature: growing in both terrestrial and marine environments, and carried over great distances through the air. This is not to say that the resultant gloop from the “brew” would have been even remotely palatable or beer-like in any way, but an event like this may have provided an impetus for exploratory brewing some time during the Neolithic Revolution (approximately 10,000 years ago). Although the origins of beer are disputed, the strongest evidence we have for early beer consumption comes from Sumer, the southernmost region of ancient Mesopotamia (modern-day Iraq and Kuwait), in the form of brownish calcium-oxalate-containing ‘beerstone’ residues – indicative of beer storage – left on the inside of a 5,000–6,000-year-old pottery jug (Michel et al., 1993).

Nowadays, many beer drinkers are perhaps enticed by the (mostly) pleasant psychoactive effects of beer drinking. However, historically, the benefits of boozing extended far beyond mere worldly pleasure. Living in an early human settlement, and faced with a contaminated water supply – brewing beer would have been an effective way to produce a safe, drinkable product, free of disease-causing microbes (thanks to the inhibitory effects of the beer’s alcohol content, high acidity and low oxygen content on the growth of many microbial pathogens). Of course, simply boiling the water over a fire may also have done the trick – but where’s the fun in that.

Up until the early- to mid-1800s, beer brewing was thought to be a purely chemical process; Louis Pasteur soon put that right (as he did many other issues) by discovering the role of yeast in fermentation. In the late 1800s, a scientist and brewer at the Carlsberg laboratories in Denmark – Emil Christian Hansen – discovered that the yeast being used by the brewery was, in fact, a mixture of several different species. Having verified this, he set about isolating and propagating one individual species, so as to standardize the quality and consistency of the beer being produced. He named this Saccharomyces carlsbergensis; however, the species was later found to be identical to a species previously discovered by the German scientist Max Reess. Hence, Reess’ name, Saccharomyces pastorianus (dubbed in honour of Louis Pasteur) is more commonly used today.

The Ingredients


The main ingredient of beer is water, which varies in composition and flavour throughout the world – an important consideration if a large-scale international brewing company is aiming for global consistency.


The second most abundant ingredient is barley (Hordeum vulgare), which is a member of the grass family, and is – from a human perspective – one of the most important plants on Earth. Satellite-derived land cover data and agricultural census data suggest that the global area distribution of barley is the fourth largest of all major food crops (behind wheat, rice and maize), covering approximately 9% of all the world’s croplands (Leff et al., 2004). Beer brewing utilizes the sprouted grains (i.e. seeds) of the barley plant – but more on that later.


Hops are the conical flowers of the female hop plant (Humulus lupulus). These enhance the taste of the finished product by imparting a variety of bitter, zesty, or citric flavours. However, we need not assume that early brewers added hops to their beer merely to improve the flavour. Flavonoids, produced by hops, have been proven to have anticarcinogenic, antioxidant, anti-inflammatory and estrogenic effects – but most importantly, they also have antimicrobial properties which reduce the likelihood of spoilage.


Now that we’ve got the relatively boring ingredients out of the way, we can talk about the real game-changer: yeast. A non-scientist looking through a microscope at some brewer’s yeast might think that the circular blobs more closely resemble bacteria than human cells. However, looks can be deceiving. Yeasts are eukaryotes, just like humans.

Yeasts were probably the first microbes ever to be studied by humans (we like alcohol), and hence were integral to the early development of microbiology and biochemistry (Barnett, 2003). No doubt hundreds of books could be – and have been – written about the applications of yeast in food and drink production, as well as all the medical advances made possible by yeast research. Most people are perhaps aware of the role of yeast in making bread, beer and wine (among other things). Fewer people may have heard of yeasts being used as model organisms for the investigation of processes relating to cancer, aging, evolution, the eukaryotic cell cycle, apoptosis and more.

The yeasts used in beer brewing are typically not selected at random from the environment (I’ll discuss one exception to this later). There are approximately 1,500 known yeast species, but only a handful of these are used in modern beer brewing. These yeast species have become domesticated and adapted to the brewing process, much like the domestication of animals and plants during the Neolithic Revolution. Other species of yeast – Cryptococcus neoformans, for example – can cause serious infections in humans, and are best avoided by brewers and non-brewers alike.

Ale or Lager


Virtually all brewing yeasts come from the genus Saccharomyces (from the Greek – meaning ‘sugar fungus’). Of these, Saccharomyces cerevisiae (ale yeast or baker’s yeast) and Saccharomyces pastorianus (lager yeast) are arguably the most important.

In addition to its use in winemaking and baking, S. cerevisiae is used to produce ales, porters, stouts, wheat beer and many regional German beers. It is a top-fermenter – meaning that the yeast cells rise to the top during fermentation and can be scraped off the surface and re-pitched into the next brew. However, like human cells, the yeast cells age and change over time, and so there is a limit to the number of times a single batch of yeast can be re-pitched. S. cerevisiae ferments best at relatively warm temperatures – approximately 18–25°C.

The Crabtree effect – that is, the fact that some yeasts (including S. cerevisiae and S. pastorianus) utilize the relatively low-efficiency process of fermentation in the presence of oxygen and high external glucose concentrations, as a means of powering themselves and producing additional biomass, rather than the more efficient process of aerobic respiration (used by most other yeast species) – is something that has puzzled scientists for some time. Given that carbon dioxide and ethanol are produced by Saccharomyces species as waste-products of fermentation, some people have argued that the antiseptic nature of ethanol may have given the yeasts a competitive advantage over neighbouring microbes by inhibiting their growth. This would have enabled the yeasts to take the winner’s share of the spoils – e.g. rotting fruit.

The lager yeast, S. pastorianus (a.k.a. Saccharomyces carlsbergensis) – unlike S. cerevisiae – is a cryotolerant (cold-tolerating) bottom-fermenting yeast. First thought to be used in 15th century Bavaria, it is now used to produce pilsners, bocks and many other regional German beers. Due to their genetic similarities, it was known for some time that S. pastorianus was a hybrid between S. cerevisiae and another yeast species. In 2011, a new environmental yeast, Saccharomyces eubayanus, was discovered growing on Nothofagus (southern beech) trees in Patagonia, South America. When sequenced, the genomic DNA of S. eubayanus was found to be over 99% similar to the non-S. cerevisiae portion of the genomic DNA of S. pastorianus (Libkind et al., 2011). Thus the lost parent was found.

The Brewing Method


Modern brewing can be divided into six major stages: malting, mashing, wort preparation, primary fermentation, secondary fermentation, and bottling.

Due to the fact that yeasts cannot obtain nutrients directly from closed, dry barley grains, some water must be added to allow the grains to begin sprouting. This process – called ‘malting’ – enables the release of starch stored in the barley grains, and activates several enzymes; the grains are then roasted to develop their flavour. During the ‘mashing’ step, the roasted grains are mixed with water and heated. This allows the enzymes released from the barley grains during malting to break down the starch into simple, fermentable sugars. The sweet brownish-yellow liquid produced in this step is called ‘wort’ and is perfect yeast food. However, it is also perfect food for spoilage micro-organisms, and so must be boiled to prevent their growth. Hops are boiled with the beer for approximately 1 hour. The wort must then be cooled, filtered and aerated before the yeast can be added.

The ‘primary fermentation’ stage is a two-step process. First, the yeast cells replicate quickly and increase their numbers. Then, as the oxygen is depleted, the yeast begin fermenting, and they continue to produce ethanol and carbon dioxide from the simple sugars in the wort, until the ethanol concentration rises above a certain level. At this point, the yeast enter into a hibernation-like state known as ‘stationary phase’, in which they clump together in a process known as flocculation. In an ale brew, the clumped S. cerevisiae cells rise to the top. In a lager brew, the clumped S. pastorianus cells settle to the bottom. This clumping process enables easy separation of the beer from the yeast cells; however, some yeast cells will remain suspended in the beer, and continue to break down sugars by low-level fermentation. This ‘secondary fermentation’ stage is important – and may be prolonged for several weeks – as it enables the remaining yeast cells to eliminate diacetyl (which tastes like butterscotch) and acetaldehyde (which tastes like green apples) produced during the primary fermentation, as these compounds are not desirable in most beers. Further settling at low temperatures allows removal of the remaining yeast. Carbon dioxide is naturally produced by yeast during fermentation. However, additional carbon dioxide may be added just before bottling to ensure the appropriate level of bubbliness.

Strange Brew

Not all brewing processes follow the same general method. The production of Belgian ‘lambic beer’ – known in the US as ‘spontaneous ale’ – does not involve pure yeast cultures at all; rather the boiled wort is exposed to the open air, allowing a combination of wild yeast (e.g. Bretannomyces) and bacteria (e.g. Lactobacillus delbrükii, Pediococcus damnosus) to grow in it. Unsurprisingly, the outcome depends very much on the organisms, brewing location and season, and the resulting beers can be “dry, vinous and cidery – often with a sour aftertaste”. I’ve even seen a lambic beer described as tasting like a “wet dog in a phone booth” – whatever that means.

Find out more

In 2012, the American Academy for Microbiology (the honorific branch of the American Society for Microbiology) released a so-called ‘FAQ Report’ entitled If the Yeast Ain’t Happy, Ain’t Nobody Happy: The Microbiology of Beer. This report aims to provide the answers to all of the ‘frequently asked questions’ (FAQs) on the topic of beer brewing, and covers – in greater depth – most of the material included in this article. If you are an inquisitive alcoholic, Irish, or an insufferable craft-brewing hipster, this report is for you. If you’d like the short version – check out these two informative posters; and if you’d prefer to absorb your brewing knowledge via YouTube – you can do that too.


  1. Leff, B., Ramankutty, N., and Foley, J.A. (2004) Geographic distribution of major crops across the world. Global Biogeochemical Cycles. 18(1), GB1009
  2. Michel, R.H., McGovern, P.E., Badler, V.R. (1993) The First Wine & Beer. Analytical Chemistry. 65(8), 408A-413A
  3. Karabín M., Hudcová T., Jelínek L., and Dostálek P. (2015) Biotransformations and biological activities of hop flavonoids, Biotechnology Advances. 33(6), 1, 1063–1090
  4. Barnett, J.A. (2003) Beginnings of microbiology and biochemistry: the contribution of yeast research. Microbiology. 149, 557–567
  5. Libkind, , Hittinger, C.T., Valério, E., Gonçalves, C., Dover, J., Johnston, M., Gonçalves, P. and Sampaio, J.P. (2011) Microbe domestication and the identification of the wild genetic stock of lager-brewing yeast. Proceedings of the National Academy of Sciences of the United States of America. 108(35), 14539–14544