It's been a while since my last post - I was not quite prepared for how busy I would be teaching 3 classes, doing research and planning a wedding would be. But, the wedding is done! The semester is almost over! And my research is rife with blogging possibilities... Plus, I have a new obsession that ties together my loves of microbiology, immunology, food, health and beer: Fermentation.
In reality, this is not a brand new interest; I've been interested in the microbiology of beer brewing for about as long as I've been drinking beer (which is incidentally around the same time I started grad school), but my new obsession started with this book - The Art of Fermentation by Sandor Katz. I may do a full review of this book in a later post, but the short version: amazing. It's a detailed look at fermentation from culture (in the sociologic sense) to cultures (in the microbiological sense), and includes history, observation and a fair bit of very accurate science thrown in. There were numerous points in reading this book that I feared it was about to veer off into pseudoscience or overwrought conclusions as books about food and health are want to do, but in every instance, Katz cited real science and drew nuanced, well-supported conclusions.
At the same time, the book captures the excitement and wonder of using unseen microbes to transform food into delicious and healthy food. So much so that I almost immediately bought a couple of mason jars and started making sauerkraut. That was about 5 weeks ago - I'm on batch 3 already, and just started my first mead.
There are a couple of different ways to define fermentation. In a biochemical sense, it's the metabolism of sugar in the absence of oxygen. Some colloquial definitions refer to any actions of microbial metabolism on stuff. I'll mostly stick to the biochemical definition, both because I'm a biochemist by training, and also because that's where a lot of the magic happens in the production of fermented food products.
All living things, from bacteria to human cells, break down the chemical bonds in sugar and convert them into energy. The first step in this process is called "glycolysis" (literally: splitting glucose), where a molecule of sugar is split into smaller molecules, and the energy contained in the chemical bonds is harvested to produce cellular fuel. Glycolysis doesn't generate a ton of energy, but in the presence of oxygen*, the products of glycolysis can be broken down further, extracting a great deal more. Animals cannot survive with the meager energy gains from glycolysis alone (your need to breathe might have told you as much), but many microbes are happy to eek out an anaerobic existence without oxygen. But there's a biochemical problem they need to solve.
The breakdown of a molecule of glucose during glycolysis yields 2 molecules of pyruvate, a little cellular energy in the form of ATP, and reduces another molecule called NAD+ into NADH. This last step is the problem: NADH is a store of energy, and under aerobic (oxygen-using) metabolism, NADH is converted back into NAD+ as it's energy is recovered, freeing it up for another round of glycolysis. Without oxygen, NADH builds up, and NAD+ becomes scarce. Fermentation reactions replenish NAD+, and result in the conversion of pyruvate into the characteristic products that make foods wonderful and/or delicious: lactic acid or ethanol.
Lactic acid fermentation: kraut and kimchi
Most people are familiar with the fact that beer, wine and other alcoholic beverages are the result of microbial fermentation, largely performed by the yeast Saccharomyces ceravesiae. But here, I want to begin to talk about sauerkraut and lactic acid fermentation. The biochemical principal is precisely the same as for alcohol fermentation - cells need to liberate NAD+ for use in glycolysis - but instead of producing ethanol (CH3CH2OH) as a biproduct, the Lactobacillus bacteria responsible for the sour in sauerkraut produce lactic acid (CH3CH(OH)COOH).
The lactic acid in fermenting sauerkraut does two important things. First, it makes the environment inhospitable to a lot of disease-causing microbes. Fermentation has been used for millennia to preserve food in the absence of refrigeration - a jar of fermenting kraut can sit on your kitchen counter for months without going bad. Second, the acid is responsible for the delicious (to some) sour flavor.
Recipes abound online, but here are the basics: 1) Cut up cabbage. 2) Add some salt. 3) Press out water and sumberge cabbage. 4) Wait.
No need to add any bacteria - so long as the cabbage you use hasn't been irradiated, it will contain abundant Lactobacillus bacteria. The salt will pull water from the plant cells, and discourage growth of the bacteria you want to avoid. Submerging the cabbage in this liquid will encourage anaerobic growth, and lead to the production of lactic acid and sour flavor. Other microbes (like molds) can't grow in the acidic, anaerobic environment.
For my first batch, I started off small, but I included some hot peppers, carrots and onions along with the red cabbage (this is maybe closer to kimchi than sauerkraut). I only let it ferment for about 4 days, so it wasn't very sour, but the peppers gave it a nice bite. My second batch was a bit more ambitious - two large and two small mason jars filled with a fairly large cabbage, garlic, carrots, beats, shallots and onions - all from our local farm share. I let this batch ferment on the counter for about 12 days, pressing out the CO2 bubbles that formed every couple of days - the resulting kraut had a delicious sour flavor that my wife and I have been enjoying for weeks.
Katz's book is filled with exciting recipes and things to try. I already started a mead (more on that in a future post), and have plans for making kombucha, ginger beer, pickles and of course, more kraut/kimchi. I'm sure I will quickly run out of space, money and time, but it all seems worth it. My plan - plow whatever money I make from blogging into fermentation, and then write about the science (and the delicious results).
Let me know if you have any ideas to try, or if you too are a fermentation enthusiast and have recipes to try!
* Some organisms can use other molecules in place of oxygen as "terminal electron acceptors," but this is rare.