Day 179

Fridays: so many facts to learn from Mark Benzaquen, so little padding for my derriere.

In Sensory Evaluation, Mark covered some statistical analysis of test results, including calculations to see if our test results are statistically significant. There was a parade of formulae involving σ and δ and χ.


(As I may have mentioned, stats were never my friends in high school.)

Brain liquification having commenced, we moved on to Chemistry, where we reviewed the various analytical tests we can run on wort inside the brewhouse: starch conversion, specific gravity, pH, smell and taste, and colour and clarity, as well as dissolved oxygen in the aerated wort.

If we wanted to know the absolute maximum attenuation we could expect during fermentation–that is, how much sugar the yeast will gobble up–we could take a sample of aerated wort from the fermenting tank, ferment it for 48 hours at room temperature, then measure the specific gravity. It would taste awful from all the esters and off-flavours produced by the warm fermentation, but we would have a good idea of the attenuation to expect from our more temperature-controlled fermentation in the fermenting tank.

Those are the tests we can run in the brewhouse, and they tend to be fairly approximate. However, if we wanted further analysis, or more accurate numbers, we can also send samples of our raw ingreidents, wort and beer to the lab for analysis of:

  • pH of raw materials and wort
  • Specific gravity
  • Bitterness units
  • Colour on a quantifiable scale such as Standard Reference Method (SRM) or European Brewing Convention (EBC)
  • Free Amino Nitrogen (FAN) or Total Nitrogen Content (TNC)
  • degree of attenuation

For each of these, we also learned something about the exact tests that are run on the samples, such as the Kjeldahl method of measuring Total Nitrogen Content.

On to Microbiology, where we went over the chain of reactions the yeast uses in order to convert 1 molecule of glucose or fructose into 2 molecules of alcohol and 2 molecules of CO2.

We had already covered this in Intro to Brewing in the fall, but that had been a hasty 20-minute explanation with some hastily scrawled diagrams that created more questions than answers. Today, Mark took his time to explain the processes, using many good analogies to illustrate his points.

One analogy in particular made a particular process much clearer to me. Several times during the reaction chain, there’s a ATP to ADP side reaction that I never understood very well. (Recall that I did not take Biology in high school.)  Mark explained that adenosine triphosphate (ATP) is a kind of battery. When a phosphate ion from an ATP molecule is taken away and forced onto the main reaction molecule, it changes the very energetic adenosine triphosphate (tri = three phosphates) to less energetic adenosine disphosphate (di = two phosphates). Meanwhile, back in the main reaction chain, the addition of the phosphate molecule and its energy to the subject molecule has the effect of adding tension and energy, much like winding up a spring. I found the analogy of winding up the spring made the various processes a lot more comprehensible.

So, without going into a lot of detail, here’s what the yeast does:

  1. It transports glucose and fructose through its memberane, and each molecule is “tagged”, much like passing through a security gate and getting a visitor’s badge. [Once the yeast can’t find any more glucose or fructose, it will seek out maltose (2 glucose molecules), and then perhaps even maltotriose (3 glucose molecules). The maltose and maltotriose have to be broken down into individual glucose units before the process can continue.]
  2. Each tagged glucose or fructose molecule then undergoes a 10-step conversion process, called “glycolysis” or “the EMP Pathway”. At the end of this process, the yeast cell has charged up 4 ATP “batteries” and created two molecules of pyruvate (pyruvic acid).

OK, here’s the thing. Pyruvate is toxic and can’t be excreted. If the yeast doesn’t find some way to turn it into something than can be excreted, the yeast cell will sicken and die.

If the yeast has any oxygen (as it does at the start of fermentation, because you oxygenated the wort), it pulls the pyruvate into its mitochondria. Then through three processes called acetyl CoA formation, the Krebs citric acid cycle, and oxidative phosphorylation, it uses oxygen to convert every two molecules of pyruvate into water as well as charging up 36 or 38 ATP batteries (the number dependent on the type of yeast). This is a lot of stored energy, and the yeast uses it, plus lipid acids and other tools it might find lying around its cell, to create another yeast cell. This is what happens during the very active growth phase of fermentation: the creation of lots more yeast.

However, once all the oxygen in the wort is used up, the yeast then has to find a different way to get rid of the pyruvate. Without oxygen, it falls back on a 3-step process called ethanol synthesis, which converts each molecule of pyruvate into a molecule of ethanol and a molecule of  CO2, both of which can be excreted. This doesn’t produce enough excess energy to form new yeast cells, but it does produce enough energy to keep the yeast cell alive. By producing alcohol and CO2, the yeast cell is simply surviving, much to our eventual gratification.

There are a number of other minor synthesis pathways that produce aromas, flavours and off-flavours unique to each strain of yeast. However, the main reaction paths are the ones that matter the most to brewers.

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