Saturday, January 25, 2014

Bacteria, Diffusion, and Kombucha...

Note: This program first aired January 25, 2014.

One thing you should know about me, if you ever happen to be in my kitchen, is that I love fermented foods. I love making them and I love eating them. My refrigerator is packed with jars of kim chee, pickled cucumbers and turnip, fermented salsa verde and hot sauce. I use naturally occurring bacteria to do my culinary bidding. The bacteria make the food more digestible and tasty and preserve it long into the winter. The bacteria in all of my ferments are, if everything goes well, invisible. I see and taste their effects and by products, but never the bacteria themselves. In the past year I’ve embarked on another fermentation adventure; I’ve started making kombucha. Kombucha a fermented beverage made from sweet tea. Its very different from the other ferments I make in that the bacterial culture is present in a very visual way. Kombucha is fermented using a symbiotic culture of bacteria and yeast (called a SCOBY), and it makes a bit thick rubbery mat that floats on top of the tea in your fermentation container. The sight of it doesn’t bother me at all, but some other people in my family can’t stand the sight of it, and will never drink kombucha because it comes from such a weird looking gelatinous mass.

The culture forms a large mass because one of the bacteria involved makes cellulose, otherwise it is likely that we might not see much evidence of these bacteria and yeasts, which is how some of us like it. If individual bacteria could get big enough for us to see, they might look just like the SCOBY, gelatinous, slimy, and nondescript, putty colored. But individual bacteria can’t get that big, and there is a good and simple physical reason why. It has to do with the ratio between surface area and volume, and the process of simple diffusion.

Diffusion is the phenomenon of particles moving from an area of high concentration to an area of low concentration. It is, for individual bacteria, a passive process, it requires no additional energy to make it occur. Following the laws of the universe, as long as nothing impedes it, diffusion will happen all on its own. We talk about it in terms of bacteria because it is how bacteria get their nutrition and get rid of their waste. Nutrients diffuse across the cell membrane into the bacteria, because they are more concentrated outside than they are inside the cell. Waste products move in the opposite direction for the same reason. The rate at which things diffuse is determined by several factors, including the concentration gradient and the mixing rate. The biggest factor though is surface area. The more surface area, the more surface there is for nutrients to pass through. That is a good thing. BUT, the more surface area, the more volume. In fact, the volume increases much faster than the surface area does. At a certain point, there isn’t enough surface area to have enough diffusion to support all the biological activity that is needed to make that much volume survive. Nutrients can’t diffuse in fast enough throughout the bacterial cell. Waste can’t get out fast enough. This is why individual bacterial cells are limited in size. Simple diffusion can not support the metabolic needs of the cell once it gets too large.

These are organisms that have been engineered by evolution to make natural processes further their very existences. Life is the antithesis of entropy, the physical process of increasing chaos that occurs in the absence of the input of energy. Life is matter organized and perpetuated, but in the case of these very small organisms, they are using a bit of physics to further their biological prerogative. Relying entirely on diffusion has created physical limits that the bacteria must live within, but for them the trade off has been worth it. They have been alive on Earth longer than anything else by billions of years, and there are more of them than anything else living.

In thinking about this, I’ve realized this is the same way I’ve come to feel about winter, or summer, or rain. I can fight the cold, or the heat, or the damp, or I can accept it, and even incorporate it into the yearly cycle of my life. Yes, the cold of winter creates a limitation in my life, my ability to lounge about in a sun dress and flip flops, but these natural phenomena serve the natural parts of me in some way. The cold slows me down and draws me in, the heat opens and relaxes me, the damp hydrates me, all things I need at one time or another. It’s a lesson from the invisibles of this world, let nature work for you. And take full advantage of the potential of anything that is going to happen anyway. Things are complicated enough as it is.


The issues of diffusion and bacteria are very nicely summarized at this referenced blog: