I'm writing a book about distilling for the technical distiller and am toying with the idea of putting all of the info here, asking for feedback and then publishing it later as well. Let me know what you think of this idea if you want down in the comments...but until I decide, I'll throw some material at the wall and see what the splatter looks like.
Theory of Fermentation
I like to start with the definitions so here’s fermentation: Fermentation is metabolism that occurs in the absence of oxygen. Back into the wayback machine for some highschool chemistry/biology...Respiration is breathing, right? Aerobic metabolism means consuming energy/carbon sources while oxygen is present. It’s what you do. Anaerobic respiration is a very ancient metabolic process that microbes make use of. There didn't used to be much oxygen around after all.
Categorizing microbes for their respiration/metabolic styles we get:
1. Obligate aerobes: organisms that can only utilize aerobic metabolic pathways. They die in the absence of oxygen (or go dormant if they’re tough).
2. Obligate anaerobes: organisms that can only utilize metabolic pathways that do not require the presence of oxygen. They die or go dormant in the presence of oxygen.
3. Facultative aerobes: These can survive in either environment but prefer the aerobic and are more efficient there.
4. Facultative anaerobes: These can survive in either environment but prefer the anaerobic and are more efficient there.
Yeast are facultative aerobes. They thrive in an oxygen environment but if the oxygen gets used up they switch modes and make the stuff they need to make through other means. I’m not going to go through an extensive review of metabolism but I will give some background here as it’s easy to overlook the purpose of metabolism, and an understanding of the purpose of these metabolic pathways helps the distiller to trouble shoot and improve yield if you know the motivations of your actors.
The primary purposes of metabolism are twofold: 1. to produce energy, 2. to produce the materials necessary to build and repair cell tissues. Metabolism takes a carbon energy source, strips it down to component parts and uses pieces of it for these two purposes. Sugar is a great carbon and energy source. Breaking the bonds of that ring structure makes electrons available for the cell’s machinery to convert to stored energy.
Aerobic respiration among yeasts is used for reproduction, or biomass production. With oxygen present the yeast cells will reproduce quickly adding yeast cells fast. If no oxygen is added to the environment they will consume all of it and when it is gone switch to anaerobic metabolism. If you bubble air or oxygen through the media, they will continue building biomass. This is the essential of yeast propagation. You have a carbon source (sugar), you add yeast cells and continuously bubble air or oxygen through the media and build an extremely high biomass which can be used to pitch or inoculate a beer or mash. When brewers collect yeast from the bottoms of their fermenters, they are debilitated because of the stressors they are subject to during the fermentation process. Our brewer pulls out a clump of yeast, oxygenates the mash he is pitching into in order to encourage the debilitated yeast culture to propagate (prop up) to a higher cell concentration for the next round of fermentation, and then allows the mash to go anaerobic after the yeast culture is healthy and thriving. Once the substrate is void of oxygen, anaerobic metabolism occurs and that's where the fun happens.
The yeast cell life cycle in a fermentation cycle is characterized by a collection of important stressors.
1. Osmotic stress
2. Solvent or alcohol stress
3. Nutrient scarcity
4. Competitive inhibition
Remember osmosis? Osmosis is the tendency for a solute (substance that is dissolved in solution) or solvent (substance that is doing the dissolving) to travel from a position of higher concentration to a position of lower concentration through a membrane. The yeast cell wall functions osmotically and the term solvent stress describes the stress the yeast experiences at the beginning of the fermentation when there is a relatively high concentration of sugars in the aqueous media. That is to say there’s a lot of sugar that wants to push into the cell because the membrane facilitates the movement of material in or out of the cell to balance concentrations on either side of itself. If too much sugar passes the membrane into the cell it can damage the cell.
High gravity brewing or mashing involves fermenting from a brix of 20 or higher. This is a stressful situation for a yeast cell, and although I'm not totally familiar with the mechanisms yeast use to survive this particular stress, I know that ordinary yeasts will not ferment to completion in a high gravity environment. Ordinarily what happens is that the yeast makes it through the initial fermentation and then stalls without consuming all of the sugars because other stressors build up and they just give up. If the yeast is not equipped for high gravity or there are other stressors present such as a dearth of nutrients, the yeast will be debilitated by the effort of dealing with the high gravity and may stall out later in the fermentation.
Solvent or alcohol stress basically defines for the inevitable fact that our yeast are committing suicide by excreting substances that are ultimately toxic to itself. Alcohols damage cell walls as the concentration climbs. Yeasts are constantly being bred which are resistant to higher concentrations of alcohol but it’s pretty uncommon to go over 16% alcohol without some extreme measures.
Nutrient scarcity can define either the condition that develops as the yeast consume all of the sugars and nutrients in the media and deplete it over time, or a condition that comes from improper mash management at the beginning. There are entire books on the trace mineral content of water for mashing and PhDs have been written on the proper concentration of Nitrogen and minerals in a yeast nutrient.
Competitive inhibition is what happens when some other microbe comes to the party and does something to the environment that inhibits yeast activity. A very smart man and someone dear to me used to say that yeast are the water buffalo of the microbial world. They’re big and dumb and do their job just fine as long as you can keep them from walking off the cliff. Any microbe will have mechanisms for gaining an edge over its competitors. The most common inhibitor in the microbial arsenal is acid production. Lactic acid bacteria (LAB) pump out tons of lactic acid (the same acid that gives many cheeses their tangy appeal) to lower the ph of the environment so that other microbes struggle.
This is one of the most effective competitive tactics. LAB are great at consuming dextrins that yeast cannot so a great argument for the understanding of metabolic pathways such as these is that if your mash goes sour and little alcohol is produced, it often means that you are not getting complete saccharification. You are not breaking your starches down completely, leaving dextrins intact. Yeast can’t metabolize them quickly but LAB can, so they are guzzling the dextrins pumping a ton of lactic into the media stalling out the big slow yeast who give up like spoiled 4 year olds on a pout.
I'm stopping here for now. Stay tuned dear reader.