Feeding the Good, as it applies to Sourdough Bread and Our Home Microbiome

“Feed the good” is of course not just a trademark for Pedigree dog food, but a philosophy of how to selectively cultivate many things in life.  One of our staff has gotten into baking bread this year as a healthy alternative to store-bought bread (which has who-knows-what in it).  His bread “starter” is about 6 months old, which he normally maintains by refrigerating between feedings and baking about once a week.  He recently kept it about 10-14 days in the fridge between feedings, and it may have started to grow some mold on top.  In an effort to salvage it, he scraped off the top and got back to feeding it, switching containers as well.  It seemed to bounce right back into a healthy gooey starter.  What is going on in this little ecosystem, and what allowed the yeasts to overcome the mold?  

First of all, it helps to understand what is in a bread starter.  You may know that bread can be “leavened” (using yeast or another agent to make the bread rise) or “unleavened” (absence of a leavening agent).  The “starter” is like an incubator for leavened bread, and it contains two important classes of microbes: yeasts and Lactic Acid bacteria (LAB).  These wild yeasts and bacteria are floating around your own kitchen right now and are on your hands, so with management, you can enjoy their nutritional benefits.

Yeast gets a bad rap sometimes.  It’s in the fungus family, and when it’s given free reign in our guts or skin or other places, overgrowth can be harmful.  Yet yeast contributes much to our foods and nutrition when it’s kept in check.  During reproduction, most yeasts like Saccharomyces cerevisiae (common baker’s yeast)  convert simple carbohydrates (sugars) to carbon dioxide and ethanol in a process known as alcoholic fermentation. The production of carbon dioxide creates gas bubbles, which expand the dough. When baked at a high temperature, these bubbles expand further as more and more carbon dioxide is produced until the yeasts die off, resulting in that airy, spongy loaf we call bread. (The Science of Sourdough Starters)

The LAB can coexist with the yeast because they digest different parts of the flour, which contains simple sugars like glucose, fructose and maltose.  Some yeasts like C. milleri and S. cerevisiae prefer glucose and fructose. Meanwhile, LAB such as L. sanfranciscensis (this is the kind of bacteria that gives San Francisco sourdough that tangy flavor) prefer maltose, so they don’t compete for food.  Also, instead of producing carbon dioxide and ethanol like yeast, LAB produce lactic acid and sometimes acetic acid.  (Carbon dioxide in water is also acidic).   These acids lower the pH of the whole starter biome to between 3.5 and 5, so that other, bad bacteria and fungus cannot survive (with correct maintenance).   

Now, maintenance is all-important for keeping a healthy bread starter.  There are two main steps: 1) Discard some starter before each feeding to get rid of waste that the microorganisms produce as they consume nutrients—and keep the starter from taking over your space.  2)  “Feed” it with fresh flour and water.  (Technique #92: Nurture a Sourdough Starter for Old-World Bread)  When the starter is at room temperature, this warm temperature causes the microbes to grow at a rapid rate, so that maintaining it must be done 1 to 2 times daily.  If the starter is moved to the refrigerator, growth slows down substantially for easier maintenance.  

When our staffer delayed feeding the starter, most likely what happened is the LAB and yeasts started to die off, allowing the pH of the biome to rise.  With a higher pH, other fungus (and possibly bacteria) were able to start growing on top, where oxygen was available.  By scraping off the top, discarding most of the starter and placing some in a clean container with new food (flour) and water, he was able to restore the yeast-LAB balance. 

Can we apply this selective cultivation elsewhere in our homes?  For sure!  First, though, we need to recognize what factors allow harmful microbes to grow.  Here’s our short list:

• Moisture: Humidity above 60% and wet materials will cause mold growth–it’s just a matter of time.  

• Use mini-humidity sensors to monitor the relative humidity in different rooms

• Make a point of routinely checking your entire home for leaks or leaks waiting to happen.  

• Leave doors on mold-prone areas (like the shower, washing machine, dishwasher) cracked open to the ambient air so that they dry out between use.

• Remove wet clothing/bedding promptly and clean and dry it out if possible.

• Dust, wood, paper, leather and natural fibers like cotton can all provide food for mold.  Of course, our homes are made up of the latter substances but dust is one thing we can eliminate!  Also, paper and cardboard quickly absorb ambient moisture and attracts destructive insects like termites.  

• Do your best to get rid of stacks of paper and magazines

• Avoid storing items in cardboard boxes (plastic “totes” are better).

• Don’t buy furniture or flooring made of particle board or medium-density fiberboard (MDF) if possible, as they absorb water much more quickly than plywood or solid wood.

• High temperatures accelerate mold growth.  Even if you don’t have air conditioning, ventilation in the early morning and late evening can reduce temperatures in the home.

• Anti-bacterial cleansers sometimes “accidentally” allow bad bacteria to grow by killing the healthy bacteria (which compete with them) on our hands and body.  Most healthcare services (including the FDA) agree that plain soap and water and the proper hand-washing technique is sufficient for good hygiene.  (5 Reasons to Stop Using Antibacterial Soaps) 

What if there’s a leak or high humidity and the mold does take over a corner of your home?  Well, just like the bread starter, you don’t have to throw everything out (demolish the room or home) and start over.  By removing the mold and any damaged structure and cleaning thoroughly, you can go back to cultivating the anti-mold environment outlined above, with maybe some more safeguards in place to prevent the leaks (see our articles here and here).  If we think of our homes as places to cultivate a good microbiome that doesn’t allow the bad stuff to grow, it becomes as comforting as walking into that delicious bakery–no mold here!

Photo by Angelo Pantazis on Unsplash