Clearly Problematic

Sulphur compounds are nasty things. Their presence is rarely welcome in beer, with perhaps the exception of small levels of dimethyl sulphide (DMS) in specific lager types. Hydrogen sulphide is another offender, being responsible for the smell of rotten eggs and the funkiness of primary fermentation. Yeast produce it at quite high levels during the frenzy of fermentation, but it is thankfully scrubbed from the beer by rapidly rising carbon dioxide. The classic exception to higher than normal levels of sulphur in beer is the 'Burton Snatch', famous in beer brewed with Burton water in the Midlands of England. It is caused by a very high sulphate concentration in the brewing water and has become a distinguishing feature of the style.

My personal sulphuric bugbear is the odour and flavour from light struck hops, more commonly known as skunking. It irritates me because it is the most easily preventable of all the invasive sulphur tints that plague beer - just stick the bloody beer in a brown bottle. The chemistry of skunking can be seen below. I love organic chemistry, but it's not for everyone and I am sorry for inflicting it on those who do not share my interest. The photolytic reaction to the left/down is the cause of my woes, the result of which is isopentyl mercaptan - the classic skunky smell. The top reaction running left to right is some clever brewing scientist's bright idea to prevent the breakdown of the hop alpha acid and remove the production of unpleasant sulphur compounds. These products are called reduced alpha acids (reduction is a chemical process involving the donation of electrons to a molecule). These reduced compounds do not suffer from light strike and also have the added bonus of providing more bitterness than an equivalent measure of standard alpha acids. Curiously they also promote far better foam stability, but at the risk of the foam looking rocky and artificial.

I stopped buying beer in clear bottles quite some time ago because of the unpleasant smell and flavour it suffers from. This is shame because there are quite a number of good beers sold in clear glass bottles. The difference between a beer in its intended state and the sorry condition it reaches us in clear glass has been highlighted to me on a number of occasions. The first was Marston's Old Empire IPA, an unremarkable beer in the bottle that tastes just like every other light struck beer, but a pint of it from cask at the GBBF proved entirely different and not just because of the cask/bottle divide. The most striking difference I have experienced was Bishop's Finger from a bottle that had been kept under wraps away from mischievous photons. It is a true strong malty richly hopped ale in this condition, but tastes like every other ale in a clear bottle when exposed to light. Happily I can say the same for this particular bottle of 1698 because the people at CAMRA must have kept it from the light during processing, and I, hopeful that they had done so, stashed it in the cupboard in the hope that it might be free from light struck off flavours. Thankfully it has not a hint of skunking. Instead it is a wonderful strong ale with full, sticky body and satisfying minerally English hops. I do not know why Shepherd Neame put all their beer in clear bottles. It seems crazy to me, especially with the generous hopping that is typical in most of their ale.

As an aside, in the above photolytic chemical reaction it is the presence of the oxygen atom and double bond that is responsible for light strike. Oxygen is very electronegative which means it really pulls on the electrons in the bond with the lower carbon atom. This weakens the bond and allows the lower fraction of the molecule to be cleaved off by an energetic photon. This same electronegativity is responsible for all life on earth in as much as it is the reason why a small molecule like water is a liquid at ambient temperature. So I'm left with a dilemma; all life on earth or skunk free beer?

Black Lightning & the DMS Issue

This quarter's CAMRA Beer Club belatedly arrived yesterday, just as I was starting to get a little concerned about it. A previous delivery had gone missing in the post so I get a little nervous when it's overdue. This month's motley crew of real ale look quite promising. Among them is Entire Stout by the Hopback Brewery of Wiltshire. So effective is the marketing of their golden ales it is very strange indeed to see a black beer coming from one of their bottles. I have enjoyed Summer Lightning many times in the past both bottled in Ireland and on cask in England and each time thought it a wonderful beer. I was surprised when Tim said he noted a distinct DMS note to it, and swiftly commented that DMS in unlikely in a beer brewed with pale malts. I had to eat my words shortly afterwards when I tried a bottle out of curiosity and was met with a strong vegetal note, typical of high DMS levels.

I might take a minute to jot down a few notes on dimethyl sulphide (DMS) to clarify why it is unusual to note it in some beer rather than others. It is a sulphur compound as the name suggests, and like most sulphur compounds is quite unpleasant smelling. It is a common feature of some lagers because lager malt contains large amounts of the precursor to DMS, S-methyl methionine (SMM), but the levels are low enough to add a distinct character to the lager which is often desirable. SMM is broken down into DMS by heat which in brewing occurs during the boil and during kilning in the production of malt. Thankfully DMS is quite volatile so it is driven off to the atmosphere during these processes. Pale ale malt is kilned at a high temperature compared to lager malt and as a result most of the DMS is driven off leaving very little in the finished malt. This explains my surprise that a golden ale brewed presumably with pale ale malt would have a strong DMS element to its character. The smell of DMS appears to vary with its concentration. Bamforth describes it as 'cat urine' which I can agree with because this is the distinct smell I get from Heineken, particularly in bottles. This might sound like I'm slagging Heineken off, but in fact a high DMS aspect in a lager suggests that it is made with a great deal of malt, rather than cheaper adjuncts that might thin out the body. The other smell commonly used is cooked corn, which I must confess I have never experienced. Another common smell is a rotting vegetable like odour, clearly outlined to me during a brewing course I undertook during which a concentrated DMS solution was passed around for us to smell. It had the unmistakable odour of cabbage left to rot in the drain of a sink after the washing up is done.

Getting back to the beer in hand, it's a nicely bitter stout with some chocolate in there somewhere, which really comes through during the swig when your nose is buried in the glass. The roasted barely is clearly evident to me at the moment because of late I have been drinking my own dark beer which is without any roasted barley at all. The bottle states it is suitable for vegans because it is not fined and has no residue of marine swim bladders in the form of isling glass. It seems common enough practice for brewers not to fine or filter their stouts, the Porterhouse don't bother either because stouts will hide any haze issues. All the better for us because nothing is stripped from the beer. It might not be so good for a work colleague of mine with a yeast sensitivity who asked me a few years ago if there was any yeast in commonly available draught beers. In my then ignorance, I told her no, it is all filtered, except for wheat beer, so she'd be safe enough. Let's hope she doesn't get a hankering for decent stout anytime soon...