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Five causes, and they are not equally likely. Work through them in this order and you will usually find it on the first one.
By Scooter M.·Published September 1, 2026
The short answer
Check the bottle seal first. A gasket that leaks slowly produces a completely flat batch, because the gas escapes as fast as the yeast makes it and pressure never builds. Only after that should you look at sugar, temperature, timing and yeast.
This is the most common cause and the least suspected, because people reason about the recipe rather than the hardware. A slow leak does not give you slightly less fizz. It gives you none, because carbonation requires pressure to build, and a leaking bottle never builds any.
The two-minute test: fill the bottle with plain water, close it, invert it over a sink and squeeze firmly. Bubbles or a hiss at the closure means the seal is not holding. For a slower leak, leave the inverted bottle standing on a paper towel for an hour.
Gaskets perish, go hard, crack and take permanent flat spots. EZ Cap states its swing tops are reusable and can be replaced as needed, which is exactly the right way to think about them: consumable. Gaskets and seals.
The yeast needs something to eat. A batch bottled from a very long first fermentation may have almost no residual sugar left, and if you also added nothing at bottling there is nothing to ferment.
Add one to two tablespoons of fruit juice or chopped fruit per 16 oz bottle, or half a teaspoon of plain sugar if you do not want to change the flavor. Start modest — over -carbonation is a messier problem than under-carbonation.
Bottle conditioning is temperature-driven in exactly the way the first fermentation is. Bottles left in a cool room take four or five days rather than one or two, and in a genuinely cold room they may not get there at all before you give up.
Move them somewhere warmer and give them another two days before concluding anything. Temperature is the same variable that governs the first ferment, and the same fixes apply.
One to five days is the published range and people frequently check at 24 hours, find it flat, and refrigerate. Cold effectively stops the process, so a batch refrigerated too early stays flat permanently.
The reliable answer is the plastic tester: fill one food-grade PET soda bottle from the same batch and squeeze it daily. When it goes hard, it is ready. Until it does, it is not.
A bottle whose closure is designed to be maintained and replaced, and a way to know whether the room the bottles are sitting in is warm enough to condition them.
EZ Cap
The only bottle here with a published pressure rating — 100 PSI — in amber glass, which is the combination a second ferment actually asks for.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
Craft a Brew
A four-inch liquid crystal strip that sticks to the glass and reads the liquid rather than the air, which is the distinction that matters.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
EZ Cap
The same swing top and amber glass at quart size, which turns a gallon batch into six bottles instead of ten and roughly halves the bottling job.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
The least common cause, and the one to consider only after the other four. Heavy straining removes yeast, and yeast is what does the work in the bottle. A very carefully filtered batch has less to ferment with.
If everything else checks out, strain less next time — or add a spoonful of the sediment from the bottom of the brewing vessel to each bottle.
| # | Check | Fix |
|---|---|---|
| 1 | Does the bottle hold water pressure when inverted and squeezed? | Replace the gasket, or use a crown cap |
| 2 | Did you add sugar, fruit or juice at bottling? | 1-2 tbsp juice per 16 oz bottle |
| 3 | Is the room where the bottles sit above 70 F? | Move them somewhere warmer |
| 4 | Have they had at least 3 days? | Wait. Use a plastic tester bottle. |
| 5 | Did you strain very heavily? | Strain less, or add a little sediment |
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Ranked on the published rubric. Every row jumps to that product's full write-up below; the button goes straight to Amazon.
| # | Product | Best for | Score | Price |
|---|---|---|---|---|
| 1 | Score8.3 EZ Cap Swing Top Bottles, Amber Glass, 16 oz (Case of 12) The only bottle here with a published pressure rating — 100 PSI — in amber glass, which is the combination a second ferment actually asks for. | Second ferment, one serving at a time | 8.3out of 10 | |
| 2 | Score7.4 Craft a Brew Fermometer Adhesive Strip Thermometer A four-inch liquid crystal strip that sticks to the glass and reads the liquid rather than the air, which is the distinction that matters. | Reading the jar, not the room | 7.4out of 10 | |
| 3 | Score7.8 EZ Cap Swing Top Bottles, Amber Glass, 32 oz (Case of 12) The same swing top and amber glass at quart size, which turns a gallon batch into six bottles instead of ten and roughly halves the bottling job. | Bottling a gallon in six moves | 7.8out of 10 | |
| 4 | Score8.0 Bormioli Rocco Swing Top Bottle, 33.75 oz (1 L) Italian soda-lime glass with a published height and diameter and a hermetic clamp seal — the bottle that looks like a bottle, with the dimensions to check it fits a fridge door. | A bottle you would put on a table | 8.0out of 10 |
#1 · Second ferment, one serving at a time
The only bottle here with a published pressure rating — 100 PSI — in amber glass, which is the combination a second ferment actually asks for.
Who it is for. Anyone doing a second ferment who wants a bottle with a number behind it rather than a shape that looks strong.
Source: MoreBeer published EZ Cap product specification (capacity, pressure rating, case contents, crown cap compatibility), morebeer.com/products/ez-cap-swing-top-bottles-amber-glass-16-oz-case-12, fetched 2026-09-01
Skip it if. You want to see the color of the finished drink. Amber hides it; the clear version of the same bottle does not.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
#2 · Reading the jar, not the room
A four-inch liquid crystal strip that sticks to the glass and reads the liquid rather than the air, which is the distinction that matters.
Who it is for. Every brewer. This is the cheapest way to stop guessing what temperature the batch is at.
Source: Craft a Brew published Fermometer specification (size, display units, compatible surfaces, operating principle), craftabrew.com/products/fermometer-adhesive-strip-thermometer, fetched 2026-09-01. Comparable-strip ranges from retailer listings, cited as comparisons only.
Skip it if. You want a logged, numeric record. A strip is a glance, not a measurement you can write down with confidence.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
#3 · Bottling a gallon in six moves
The same swing top and amber glass at quart size, which turns a gallon batch into six bottles instead of ten and roughly halves the bottling job.
Who it is for. An established brewer bottling a gallon a week who is tired of filling ten bottles.
Source: MoreBeer published EZ Cap 32 oz amber listing, same product line as the specified 16 oz, morebeer.com/products/ez-cap-swing-top-bottles-amber-glass-32-oz-case-12, retrieved 2026-09-01
Skip it if. You are new to second ferment. Learn what your kitchen does to carbonation in 16 oz bottles, where the consequences are smaller.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
#4 · A bottle you would put on a table
Italian soda-lime glass with a published height and diameter and a hermetic clamp seal — the bottle that looks like a bottle, with the dimensions to check it fits a fridge door.
Who it is for. Someone bottling a lightly carbonated kombucha for the table, or storing finished kombucha in the fridge rather than conditioning it on a counter.
Source: Bormioli Rocco published Swing bottle specification (capacity, 12.13 in height, 3.5 in diameter, soda-lime glass, hermetic seal, care instructions), retailer specification listings retrieved 2026-09-01
Skip it if. You are chasing hard carbonation. Without a stated pressure rating, a bottle you are deliberately building pressure inside is a bottle you are guessing about — use the rated EZ Cap instead.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
Every specification cited on this page came from one of these documents, retrieved on the date shown against the product it belongs to.
The recipe, the timeline, the failures and the safety questions — answered directly, in the order a home brewer meets them.
Why a perished gasket is the most common cause of a flat second ferment, how to test a seal in two minutes, and when to replace.
What the second ferment is, how much sugar to add, how long it takes, and how to know when to refrigerate without opening every bottle.
Which swing tops publish a pressure rating, which are storage bottles wearing the same shape, and how to tell them apart before you fill them.
The 75-85 F band, what happens on either side of it, why the jar is not the room, and how to fix a kitchen that sits too cold.