Fermentation described here is the process of Kefir Grains consuming lactose (milk sugars) and producing Kefir (healthy gut bacteria and probiotics). Fermentation is used in alcohol production too but Kefir will not give you a buzz.
Kefir has two forms:
Water Water Kefir Grains must have non-mammal milk or sugar. Most commonly used is simple white sugar. I tried Water Kefir grains once but unless room temperature was above 74 degrees, nothing happened. The warmer the better but you'll need to research what the maximum temperature should be.
Milk Milk Kefir Grains must have mammal milk. If you're wondering whether Almond Milk will work, ask yourself which animal the milk came from. If the milk did not come from an animal it will not provide the nutrients that the grains need and your grains will starve and die. Milk Kefir Grains can thrive in cooler temperatures. I ferment for several hours at a room temperature between 68 to 72 degrees, depending on the season, and a few days in the refrigerator (it's cold, I don't know what the temperature is).
You can use milk grains in sugar water and you can use water grains in milk for a short while, but the grains will eventually starve. Think of it this way, you can eat lawn grass. But if you only ate lawn grass you would eventually starve. You also won't feel well and you won't do everything you should (because you'll become unhealthy and cranky) and similarly your kefir grains will also not do well and not produce what they should, etc.
When we provide the right food for the grains, the grains will consume their respective sugars and produce probiotics and healthy gut bacteria. During fermentation a gas called Carbon Dioxide is produced. Carbon Dioxide is the carbonation in carbonated beverages. When we have a store bought container of a carbonated beverage, the container is designed to hold a certain amount of pressure. Manufacturing has engineered containers to withstand the pressure of the beverage. When we ferment, we can't be sure that our containers are designed to capture and hold the carbon dioxide (without breaking) and so we want the gas to escape. We could just leave the top off, but then stuff can fall, crawl, or fly into it.
Wild Yeast! When we ferment we want to control the fermentation process by only using the type of grains/yeast we pre-select. When I make beer I use water, sugar, yeast, and a few other ingredients. As the yeast eats the sugar it produces alcohol. When my beer ferments I want to make sure nothing falls into the liquid while I let the carbon dioxide escape. It's not because I'm concerned about bugs, I can scoop those out and the alcohol might sanitize the liquid. The issue is something called Wild Yeast. Wild Yeast is everywhere. It's one of the things that causes things to turn nasty when they get old. If something lands in your Kefir during primary fermentation, it's possible that it will introduce unwanted yeast into your fermentation. This wild yeast will consume the sugars and produce something (we don't know what). The "something" can be anything from tasting crappy all the way to making you feel nauseous. Wild Yeasts can cause weird coloration too (pinks, yellows, etc). Wild yeast can produce all sorts of things you don't want to eat. It's easy to avoid wild yeast, just use reasonably clean utensils, don't towel dry the interior of your containers with unclean towels, and don't use any grains which fell on the counter or floor (and don't double dip when tasting).
Fermentation has two parts:
Primary - This just means that you combined all the ingredients and you are waiting for it to be "done". With Kefir "done" is when we believe that the liquid is no longer just milk, but also contains healthy gut bacteria and probiotics.
Secondary - After the Kefir is done and we have strained off the grains, we can use it / drink it or we can experiment with adding flavors. I like to add 3/4 cup Kirkland organic agave syrup to 1/2 gallon kefir. You can use any type of flavoring you wish our skip this part.