Away From the Bench

The world outside of the lab

Archive for the month “December, 2012”

Stinky Feet Beer

Cheese and beer go together like rama lama lama ka dinga de dinga dong.  Cheese is a great partner to beer and can provide a similar taste or a sharp contrast.  Historically, cheese and beer were both produced and eaten by monks and farmers.  Many pubs combine beer and cheese into dips or soups that are usually very delicious.  I am a beer lover, so when I popped open a bottle of Berliner Weisse aged on fresh blueberries from a local brewery, I was excited.  It poured out an interesting mauve color.  (This color description reminds me of another science story – but I’ll leave that for a future post).  Then, like a shovel to the head, it hit me.  It attacked my nose with tiny odor knives.  The smell.  Was.  Disgusting.  I expected a sour blueberry aroma, but instead I got cheese.  Stinky feet cheese.  My family used this term when the cheese my mom bought had a certain ‘odeur’ to it.  I clearly remember a few instances when I curled up my nose and reeled away from the table after taking a sniff of strong cheese.  I think this primal sensory memory was permanently burned into my brain.  I grew up thinking that this term was something my family had just come up with.  I had no idea it was a real thing.

Artist’s rendition of stinky feet cheese

There are several different organisms (bacteria and mold) that produce compounds to make cheese smell rank.  The bacteria that causes Limburger cheese to smell like a dirty foot party is Brevibacterium epidermidis.  Guess where this little guy was cultured from?  You guessed it – actual stinky feet (epidermis = skin).  This type of bacteria converts the amino acid methionine (found in foot sweat and dairy proteins) into gaseous methanethiol.  There are other places this methanethiol byproduct is lurking other than your cheese and your feet.  I learned this a few years ago from my friend Stef.  She made a comment about our pee smelling bad after we had asparagus at dinner.  I had no idea what she was talking about.  Turns out, the metabolism of asparagus produces a cocktail of volatile organic compounds that can cause your urine to have a sulfuric smell, and one of those compounds is methanethiol.  I apparently don’t have the single-nucleotide polymorphism (SNP, which is a DNA sequence variation) in my olfactory genes to detect it or am one of the few who may not produce ‘asparagus pee’ because my urine didn’t smell any different to me.  Another organism that is very sensitive to methanethiol is Anopheles gambiae, the principal type of mosquito that carries malaria.  Dr. Bart Knols and colleagues observed female mosquitoes were attracted to stinky feet – and created a mosquito trap baited with dirty socks (see Table 1).

Mosquito table2

CC copyright: Njiru BN, et al. (2006) Malaria Journal, 15:5-39.

Although I may not have mosquito-like senses when it comes to methanethiol, I’ve determined that I am pretty sensitive to isovaleric acid.  This fatty acid is produced on feet and in strong cheeses such as Durrus by the bacteria Staphylococcus epidermidis through metabolism of the amino acid leucine.  This bacteria can also grow out-of-control on your face and cause bad acne.  Whenever I smell isovaleric acid, you can bet that I’m calling the odor ‘stinky feet cheese’.  Similarly, another bacteria called Propionibacteria can be found on feet, in cheese (like Swiss), and on acne-laden skin.  This bacteria can produce isovaleric acid as well as propionic acid, which has a vinegar-like smell.  Who knew stinky feet and stinky cheese had so much (bacteria) in common?

So how did isovaleric acid get into my beer?  I’m pretty sure they didn’t brew it with dirty socks or pungent cheeses.  Hops contain three different types of alpha acids that create the bitter flavor in beer.  If the hops get old or are not stored correctly, one of those alpha acids, humulone, can oxidize and produce isovaleric acid.  You can even buy capsules of purified isovaleric acid to train yourself to identify this smell and taste in your beer.  Hops are usually boiled with the wort (the sugar-water produced from mashing the grain) to introduce the bitter flavor and the isovaleric acid byproducts will be drastically reduced.  However, Berliner Weisse wort may only be boiled for about a quarter of the time than as for other types of beer (or not even boiled at all).  This leaves the stinky, cheesy isovaleric acid in the beer.  The off-flavor is intensified by the low pH of the sour mash.  However, there are many Berliner Weisse beers that do not have this cheese flavor (and that are presumably boiled for a longer period).  The worst part was, the aftertaste of my beer had a strong cheese flavor…and it lingered.  Needless to say, I could only stomach two sips – and the second one was only to confirm its nastiness.  After chugging a glass of water, I found it quite unfortunate but necessary to dump the rest.

A few weeks after this beer was released and killed the noses and palates of its drinkers, the brewery admitted the mistake and recalled the beer.  Since then, they have brewed a new batch of the blueberry Berliner Weisse and also brewed a batch with raspberries.  I thoroughly enjoyed the raspberry beer and its tart, fruity flavor…and smell.  In fact, I might even pair the next bottle with a nice (mild) cheese.

Blue Lady

The second batch of blueberry Berliner Weiss

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