At its core, vegetable fermentation is native microbes (bacteria that live in the soil and come out of the soil on the peels/skins of vegetables) eating the sugars that are naturally present in those same vegetables. I often talk and write about giving those microbes the “right conditions” so that they can vigorously feast and make wonderful fermented vegetables for you to eat.
What I usually mean by the “right conditions” is the proper temperature (room) and the proper contact with air (little to none). Other than that, there isn’t much more that your vegetables need to become tangy, tasty, probiotic ferments. One thing that does help the process along, however, is making those sugars a bit more available to our friends the lactic acid bacteria. That’s why we chop vegetables. It’s also why the size of the piece is important.
One citation I love from Sandor Katz’ The Art of Fermentation concerns how the good bacteria grow from a teensy fraction of the total bacterial population on a live plant, to being the dominant strain during fermentation. On living plants, lactic acid bacteria may only account for “0.1% to 1% of the total microbial population,” the study, from the FEMS Microbiology Review, states. While they’re there, they may help protect the live plant from bad bacteria, but it’s once the plant is harvested, the real fun begins.
The act of harvesting the plants breaks down some of the cell walls and
gets the party started makes nutrients more available to bacteria. For the first couple days of fermentation, the good guys (lactic acid bacteria) and the bad guys (pathogenic bacteria or bacteria or enzymes that break down plant material rather than make it tasty and healthy) battle it out, fighting over the limited resources available. The pH is still too high for the bad guys to have been killed, but after a few days of fermentation, the lactic acid bacteria have lowered the pH, the bad guys lose, and your lactic acid bacteria are able to live long and prosper.
But that key, kickstarting ingredient is the freeing up of the plant nutrients through harvesting. When we chop vegetables, we do more of that. We give the bacteria food to eat. I like to think of chopping my vegetables for fermentation as giving the LAB a fridge full of freshly prepared salads versus giving them a fridge full of whole vegetables. Of course they can do the work required to eat the whole vegetables, but it’s so much easier to eat those already-made salads.
I frequently ferment whole vegetables, and there are traditional vegetable ferments made from whole vegetables, but I do considered them to be next-level vegetable fermentation. More salt is needed at the start, since salt is another thing that gives lactic acid bacteria a leg up versus its bacterial competitors in the early days of fermentation, and they take waaaaaay loooooonger to ferment than their sliced counterparts.
The other side of that coin is making very small pieces (grating, for instance). When very sweet vegetables (certain carrots and beets come to mind) are shredded into very small pieces, you get a little bit too much sugar availability. That actually gives a little bit of an advantage to the native yeasts that are in there, and while they will eventually acidify into pickles, the brine can be a bit viscous (and in my opinion, unpleasant). This is a very common issue with beet kvass if my email inbox is any indication.
So, if you’re just getting started with vegetable fermentation, try an average sweetness vegetable and do a little bit of slicing or chopping for best results. My go to for first timers? Radish Pickles.
Do you like to ferment whole vegetables? Had the slimy beet or carrot brine? Share your chopping-related story in the comments.
*The study linked to above is lots of fun. Most of it is easily understandable for those without a science background (not always the case with microbiology texts). Just keep in mind that it was published over 20 years ago and some of the questions that are raised therein are things we now know.