Day 186

Another hard day at the office: we had to drink beer ALL MORNING.

On the downside, most of the beer–a bland and inoffensive (i.e. flavourless) popular brand–was spiked with off-flavours, and the beer that wasn’t spiked was, well, bland, inoffensive and flavourless.

We had covered a variety of off-flavours with Roger Mittag in the first-semester Sensory Evaluation class, but now it was time to explore four of them more slowly and discuss where in the brewing process they had occurred. This morning we tasted beer spiked with dimethyl sulphate (DMS), diacetyl, phenolic and metallic off-flavours.

With each off-flavour, we were able to directly compare it to a glass of undoctored beer.

After we were finished making notes on each, Mark Benzaquen gave us all five again in a random order–the four off-flavours plus the plain beer–and we had to decide which was which. I got three out of the five, only mixing up the phenolic and diacetyl, which for reasons unknown, my taste buds were not tasting at all this morning. Huh.

On to Microbiology, and a continuation of the various processes that yeast uses to produce various compounds in addition to alcohol. For instance, yeast wants to build more yeast cells. Part of the blueprint for a new yeast cell calls for 20 different amino acids, the basic building blocks of DNA and proteins. The yeast cell is able to quickly grab eight different amino acids provided by malt, and by using tools and materials it finds lying around its cell, it is also able to build the other twelve. Well, almost. It can build most of the structure of the new amino acid it requires, only lacking the amine group (NH3) that is common to every amino acid. This partial structure that the yeast cell has built is called an oxoacid or ketoacid, and is actually toxic to the yeast cell. No problem. The yeast cell deals with its little Frankenstein monster by ripping the amine group from some hapless amino acid that was just hanging around the place with nothing to do, then it welds that amine group onto its almost-finished construction project. Ta-dah! The yeast cell has successfully constructed a complete amino acid, which it can then use in to build a new yeast cell.

But wait! When the yeast ripped the amine group from the innocent amino acid, that bystander itself became a toxic ketoacid. To deal with its own toxic waste, the yeast cell can decarboxylate the ketoacid (remove a CO2 molecule) to form an aldehyde, which is slightly less toxic than the original ketoacid. If the aldehyde level inside the yeast cell stays fairly low, then the yeast cell will wait for it to be slowly excreted. However, if the aldehyde level inside the yeast cell reaches dangerous levels, the yeast cell takes the further step of using the aldehyde molecule to create a fusel alcohol–an alcohol molecule that is larger and more complex than ethanol. Fusel alcohols can be excreted very quickly, so the yeast quickly restores a healthy balance inside itself.

I won’t go into detail about the other processes we covered, but the yeast cell has various strategies that result in the production (or in some cases, the overproduction) of esters (fruity or spicy aromas), dimethyl sulphide (a cooked vegetable or creamed corn aroma), hydrogen sulphide (rotten eggs), sulphur dioxide (burnt matches), and diacetyl (butterscotch taste).

We also covered how to minimize these if they are considered off-flavours, or maximize them if the brewer wants to add the flavour to the beer. (For instance, although most fusel alcohols don’t add much to the flavour, they can add to the body of the beer, giving a bigger, fuller mouthfeel.)

If that wasn’t enough for the day, in Chemistry we went over most of this again, but from a chemical rather than biological point of view.

It is at this point that I usually say, “Whew! It’s the weekend.” However, in another amazing confluence of events, we have four projects due next week in Equipment (Tuesday), Chemistry (Tuesday), Packaging (Thursday) and Practical (Friday). It looks like I will have to skip St. Patrick’s Day this weekend (and the attendant loss of productivity on the following day) and get down to work.

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