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.
Choosing the Right Bottles for Second Fermentation
Bottle selection is a safety consideration, not just an aesthetic one. During second fermentation, carbon dioxide builds pressure inside sealed containers. Standard mason jars and thin glass bottles are not designed for internal pressure and can crack or shatter. The safest options are swing-top bottles like those made by Grolsch or Bormioli Rocco, which are designed to handle carbonation pressure and have a seal that can release if pressure becomes excessive.
Recycled commercial kombucha or beer bottles also work well because they were manufactured to contain carbonated beverages. Plastic soda bottles are a beginner-friendly option because you can squeeze them to feel how much pressure has built up. When the bottle feels firm and does not compress easily, carbonation is ready. This tactile feedback eliminates the guesswork that glass bottles require.
Regardless of which bottles you choose, always leave at least one inch of headspace at the top. This air space gives the carbon dioxide room to compress before the pressure becomes dangerous. Filling bottles to the brim leaves no buffer zone and significantly increases the risk of explosions or aggressive overflow when you open them.
Controlling Carbonation Levels
The amount of carbonation your second fermentation produces depends on three factors: the amount of sugar available, the temperature, and the time. More sugar means more carbon dioxide. Higher temperatures mean faster gas production. Longer time means more total gas is produced before you refrigerate.
For light, gentle carbonation similar to naturally sparkling water, use a minimal amount of sugar. Half a teaspoon of sugar or a tablespoon of fruit juice per 16 ounce bottle produces subtle fizz after two days at room temperature. This is a good starting point for beginners who are cautious about pressure buildup.
For vigorous, champagne-like carbonation, use one to two teaspoons of sugar or two to three tablespoons of fruit juice per 16 ounce bottle. Ferment at room temperature for two to three days. Be especially careful opening these bottles. Point the cap away from your face, hold the bottle at an angle, and open slowly, releasing pressure gradually. Having a glass ready to catch any overflow is wise.
Temperature is your emergency valve for controlling carbonation. If bottles are becoming too pressurized too quickly, move them to the refrigerator immediately. Cold dramatically slows yeast activity and stops further gas production within a few hours. You can always bring bottles back to room temperature to continue building carbonation if you refrigerated too early. This stop-start approach gives you precise control over the final carbonation level.