Second Fermentation Explained: How to Get Natural Carbonation

What Second Fermentation Is

Second fermentation is the process of sealing a finished fermented beverage in an airtight container with a small amount of sugar or fruit. The remaining yeast and bacteria consume the sugar and produce carbon dioxide, but because the container is sealed, the gas dissolves into the liquid instead of escaping. The result is natural carbonation, the same kind of fizz you find in champagne and naturally carbonated mineral water.

Which Fermented Drinks Benefit from Second Fermentation

Kombucha, water kefir, tepache, and ginger bug sodas all respond well to second fermentation. Each of these beverages retains enough active cultures after their initial fermentation to produce significant carbonation when sealed with additional sugar. Dairy ferments like milk kefir can also be lightly carbonated, though the results are more subtle. Vegetable ferments like sauerkraut do not benefit from second fermentation.

How to Do It Safely

Safety is the biggest concern with second fermentation because sealed glass bottles under pressure can explode if carbonation builds too much. Use bottles designed for carbonation, such as swing-top Grolsch-style bottles or thick glass bottles with tight caps. Fill bottles leaving about an inch of headspace at the top. Add your sugar source, one to two teaspoons of sugar, a tablespoon of fruit juice, or a few pieces of fresh or dried fruit per 16-ounce bottle.

Timing and Temperature

Second fermentation typically takes one to three days at room temperature. Warmer temperatures speed up carbonation, while cooler temperatures slow it down. The exact timing depends on how active your cultures are, how much sugar you added, and the ambient temperature. After 24 hours, carefully open one bottle over the sink to check the carbonation level. If it is lightly fizzy, you might want another day. If it hisses forcefully, refrigerate all bottles immediately.

Burping Your Bottles

During second fermentation, it is wise to burp your bottles daily, especially in warm weather. Burping means briefly opening the cap to release excess pressure, then resealing. This prevents dangerous pressure buildup while still allowing carbonation to develop. Once you have experience with your specific setup and recipes, you will learn how much pressure your bottles build and can adjust your burping schedule accordingly.

Flavor Combinations for Carbonated Ferments

The flavoring possibilities are virtually endless. For kombucha, try ginger and lemon, mixed berry, mango and turmeric, or lavender and honey. For water kefir, grape juice, pomegranate, passion fruit, or coconut water all work beautifully. For ginger bug sodas, experiment with root beer flavors using sassafras and vanilla, or make fruit sodas with apple, cherry, or citrus juices. Each combination produces unique flavors that far surpass store-bought alternatives.