One beer please

by Vivienne Baillie Gerritsen

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Beer has been around for thousands of years. Beer foam has not. Beer foam, as indeed beer haze, is one of today's hot topics in the world of beer brewing. Besides a beer's taste of course. What a beer should look like - once served in a glass - has become paramount for a brand's commercialization. And that is why there has been much bustle around the chemistry at work in such a process. It has been known for a while now that a number of proteins, or more correctly polypeptides, are involved in foam formation but it wasn't possible to pinpoint which protein was more involved than another. Finally, it appears that one barley protein has managed to wriggle out of the crowd. And what might that protein be? Lipid Transfer Protein 1 or LTP1.

« No one really knows what the aspect of beer was thousands of years ago. But historians agree that foam was most probably not part of the beverage's aesthetics. »

First, a little history. The first traces of beer stretch back 6'000 years at the time of the Sumerians. It is thought that the process of fermentation was discovered quite by chance. Probably when a chunk of bread, or some grain, was left to dampen and, in doing so, fermented and produced an inebriating pulp. In fact, baked bread may well have become the way an essential ingredient of beer was stored and transported.

When the Sumerian empire collapsed, the Babylonians moved in and continued the tradition of beer brewing. And they are known to have brewed over twenty different types of beer! It was the woman's job to brew, as it was to make bread in those days. The Sumerian priestesses were the first to brew beer and this is probably where the connection between religion and beer stems from. Besides the fact, that it was widely thought and accepted that the intoxicating side effects of beer could only be of divine origin. Beer possessed a spirit since it could possess that of the consumer.

Beer tradition spread to the west - as so many traditions did. When the Romans came along, they brought with them the culture of wine and beer was nudged into the more 'barbaric' spheres. Though, as a souvenir of their passage, they graciously gave beer the name it has today, as 'beer' comes from the Latin 'bibere' for 'drink'.

In Germany however, beer became and remains to this day hugely popular. As in many Nordic countries. In the middle ages, numerous monasteries brewed their own beer devising methods and tastes, which gradually became more and more sophisticated. Monks could well have taken to beer brewing because it gave them something substantial to put into their stomachs during periods of fast. It has been reported that, in some monasteries, monks had the permission to drain up to 5 liters of beer per day!

[ Drinking Beer, Andriy Khomyk]

Drinking beer, Andriy Khomyk

Courtesy of the artist

No one really knows what the aspect of beer could have been in those days. But historians do agree that foam was not part of the beverage's aesthetics and that there was probably very little, if none, at all. When barley is put through the brewing process, many of its proteins end up in the final product, i.e. beer as we know it, although very few of them in their natural state. Indeed, the great majority are split into smaller polypeptides. LTP1, however, is one of the lucky ones and though it is denatured, it does manage to survive the brewing and is found in the end result with its sequence still quite intact. In fact, it is thought that it is precisely the denaturation of LTP1 that is one of the secrets of beer foam. In vivo, LTP1 occurs as a small globular protein, with a central and conical hydrophobic core, which stretches from one end of the molecule to the other. A C-terminal stretch of amino acids of undetermined structure acts as a lid.

« LTP1 survives brewing and its denaturation may well be one of the secrets of beer foam formation.»

In the process of beer brewing, LTP1 loses its 3D structure but it hates the water it is forced to rub shoulders with. So what does it do? It grabs hold of a bubble of CO2 - a side product of barley fermentation - and then rises to the surface of the liquid by holding on for dear life. In effect, LTP1 proteins form a coat around every beer bubble. And what is a bubble's enemy? Grease. LTP1 just loves it. A dirty glass, lipstick, crisps - you name it - is a beer foam's worst enemy. When LTP1 encounters grease, it is dissolved immediately. And the bubble bursts. Result? The foam disappears.

Naturally, the primordial role of LTP1 in barley is not for the pleasure of bar tenders and the maintenance of beer foam. However, besides this pleasing coincidence for the consumer's eye, no one really knows what LTP1's function in vivo is. In vitro, it can lodge lipids and various fatty acids in its central core, quite comfortably. So it was thought that LTP1 was probably involved in lipid transport across membranes and perhaps even membrane biogenesis. The structure of the protein seems to be quite expandable and could take on various functions depending on its ligand. What is more, LTP1s are located in the cell wall and it has been suggested that LTP1 could have a role in cutin monomer transport; indeed, there is a large concentration of LTP1 in the surface wax. However, it is now known that LTP1 is in fact secreted, which leads to other hypotheses.

The discovery that barley LTP1 actually undergoes a lipid posttranslational modification in vivo offered yet another intriguing hypothesis. Barley LTP1s have been reported to play a role in plant defense. Though how, no one could explain. The lipid which is bound to LTP1 could have antimicrobial activity toward fungi and bacteria. In the event of defense, it would interfere with the pathogen's membrane either still bound to the protein or indeed released.

So, besides the fact that LTP1 function remains quite unresolved, research around its role in maintaining beer foam - as around the role of many other proteins which float in a glass of beer - is thriving. What if the quantity of LTP1 were increased? The more LTP1, the more the foam. Needless to say, much has been done to find ways of increasing the amounts of LTP1. Especially as it is known that its presence in barley is quite dependent on the weather; the wetter the summer, the less LTP1 in barley...

One of the more 'natural' ways to introduce larger quantities of LTP1 in beer would be to harvest barley which produces large amounts of the protein. Another far less natural procedure would be to introduce the LTP1 gene into a yeast genome. This has already been done by one German team... Though brewers have shown some interest, it will not be a smooth affair to make beer consumer's swallow genetically modified beer... So, if you like a beer with a consistent head, the most harmless and easiest way to go about it is to make sure that your glass is squeaky clean and your lips grease free!

References
1. Kapp G.R., Bamforth C.W.
The foaming properties of proteins isolated from barley
J. Sci. Food Agric. 82:1276-1281(2002)

2. Lindorff-Larsen K., Lerche M.H., Poulsen F.M., Roepstorff P.
Barley lipid transfer protein, LTP1, contains a new type of lipid-like post-translational modification
J. Biol. Chem. 276:33547-33553(2001)
PMID: 11435437

3. Lerche M.H., Poulsen F.M.
Solution structure of barley lipid transfer protein complexed with palmitate. Two different binding modes of palmitate in the homologous maize and barley nonspecific lipid transfer proteins
Protein Sci. 7:2490-2498(1998)
PMID: 9865943
Swiss-Prot cross references
Lipid Transfer Protein (LTP1), Hordeum vulgare (Barley): P07597
SwissProt
Protein Spotlight (ISSN 1424-4721) is a monthly review written by the Swiss-Prot team of the SIB Swiss Institute of Bioinformatics. Spotlight articles describe a specific protein or family of proteins on an informal tone. Follow us: Subscribe · Twitter · Facebook