Safety
What keeps it, and what does not
Lacto-fermentation is safe when the pH falls fast enough and unsafe when it does not. Everything below is a published threshold with the sentence it came from, and nothing below is a judgement that your jar is fine.
Last reviewed 6 September 2026
What this page is not
Read this first
Nothing on this site tells you that a preparation is safe. It publishes the parameters named sources have published, with the sentence each one came from, so that you can go and read the source. That is a different thing from an assessment, and the difference matters most exactly here.
If you want a procedure that has been tested and validated, use one: the USDA Complete Guide to Home Canning publishes tested methods for sauerkraut and for fermented dill pickles, and they are linked from those two pages on this site. A tested procedure is worth more than a table of parameters, and this site is a table of parameters.
Acid is the preservative, not salt
This is the sentence most popular accounts of pickling get backwards. Salt selects: it slows down the organisms that would rot the vegetable and gives the lactic acid bacteria, which tolerate it better, a head start. What actually preserves the food is the acid those bacteria then make.
The spoilage organisms can tolerate salt concentrations up to between 5 and 7%, therefore it is the acidic environment created by the lactobacilli that keep the spoilage bacteria at bay, rather than the addition of salt.
FAO Agricultural Services Bulletin 134, 5.6.2
Read the numbers in that sentence again. Spoilage organisms tolerate 5 to 7 percent salt. A sauerkraut is salted at 2 to 2.5 percent. The salt alone was never going to be enough, and it is not supposed to be: it is buying a few days for the fermentation to take the pH down.
Lactic acid bacteria produce lactic acid which reduces the pH (ie increases the acidity) to a level that prevents the growth of food poisoning organisms.
FAO Agricultural Services Bulletin 134, 6.3.5
The 4.6 threshold, and where it comes from
The number is in the Code of Federal Regulations, in the definition of an acid food:
(a) Acid foods means foods that have a natural pH of 4.6 or below.
21 CFR 114.3(a). The full section, unedited, is at /regulation/21-cfr-114-3.
A finished pH of 4.6 or below is the line below which Clostridium botulinum does not grow and produce toxin. It is the reason a fermented vegetable is shelf-stable at all and the reason a half-fermented one is not.
Two of the ferments on this site have a published terminal pH, and both are well below the line: sinki falls from 6.7 to 3.3, and gundruk reaches 4.0 by about the sixth day. Both are fermented without any salt at all, which tells you where the safety actually comes from.
What a lacto-ferment is, regulatorily
A lacto-fermented pickle is generally not an "acidified food" in the sense of 21 CFR 114, because that part covers low-acid foods to which acid is added. A ferment makes its own. The threshold is the same either way; the process requirements in that part are written for a different kind of operation. The scope language is quoted in full on the regulation pages rather than paraphrased, because paraphrasing a scope clause is how a reader ends up believing the opposite of it.
Temperature, with a consequence at each end
The USDA is the only source on this site that states a band and says what happens outside it in both directions. For sauerkraut:
Store fermenting cucumbers between 70° and 75°F. This is the optimum temperature for growth of the organisms necessary for fermentation.
USDA, via the National Center for Home Food Preservation
Below 60 F, the guide says, kraut may not ferment at all. Above 75 F it may go soft. For fermented dill pickles the acceptable range extends down to 55 F, at the cost of five to six weeks instead of three to four, and above 80 F the pickles go too soft during the fermentation.
The FAO bulletin describes the same window in Celsius and explains the mechanism: 18 to 22 C is the optimum for Leuconostoc mesenteroides, which is what starts the fermentation, and above 22 C the lactobacilli take over sooner than they should.
A variation of just a few degrees from this temperature alters the activity of the microbial process and affects the quality of the final product.
FAO Agricultural Services Bulletin 134, 5.6.2
The two scales are the same window described twice; there is a converter rather than a conversion printed in place of either source's own figure.
The brine floor
A brine gets weaker as it works. The vegetable gives up water and takes up salt, and the concentration falls. For commercially brined cucumbers the FAO bulletin puts a floor under it:
It is crucial that the salt concentration does not fall below 12%, otherwise conditions do not allow for fermentation.
FAO Agricultural Services Bulletin 134, 5.6.3. There is a calculator for putting it back.
That figure is for a strong-brine commercial cucumber process, not for a 2 percent household kraut, and the difference between those two regimes is one of the easiest things on this subject to get wrong. Read the parameter next to the tradition it belongs to.
Under the brine
Every source consulted for this site says the same thing in different words, and it is the single instruction with the highest ratio of importance to effort.
The cucumbers are submerged in the brine, ensuring that none float on the surface - this is essential to prevent spoilage.
FAO Agricultural Services Bulletin 134, 5.6.3
Keep cucumbers immersed in the brine.
USDA, via the National Center for Home Food Preservation
An insufficient level of juice to cover the kraut during the fermentation allows undesirable aerobic bacteria and yeasts to grow on the surface of the kraut, causing off flavours and discoloration.
FAO Agricultural Services Bulletin 134, 5.6.2
Botulism, honestly
Botulism from a properly acidified vegetable ferment is rare. It is not impossible, and the way it happens is not mysterious: the fermentation stalls or never starts, the pH never gets below 4.6, and the jar is then a sealed, moist, low-acid, oxygen-poor environment, which is precisely what Clostridium botulinum wants.
The failure modes that produce that outcome are the ones on this page. Too cold to ferment. Too little of the sugar the bacteria need. Salt so heavy it inhibits the lactobacilli as well as the spoilage organisms. A lid clamped down early on a batch that never acidified. The USDA states the requirement as a property of the finished product rather than of the process:
There must be a minimum, uniform level of acid throughout the mixed product to prevent the growth of botulinum bacteria.
USDA, via the National Center for Home Food Preservation
If you intend to store a ferment sealed and unrefrigerated, measure the pH. Strips are adequate to tell 3.4 from 4.6; a meter is better. Both are on the equipment page, which is the only page on this site where anything is sold, and it is deliberately not this one.
Defects, and what causes them
The USDA publishes a defects table for fermented pickles. It is transcribed here whole rather than sampled, because a defect list with the awkward rows removed is a list that flatters the process.
Soft or slippery pickles. (If spoilage is evident, do not eat.)
| Cause | Prevention |
|---|---|
| Salt brine too weak during fermentation. | Maintain salt concentration specified in recipe. |
| Cucumbers stored at too high a temperature during fermentation. | Store fermenting cucumbers between 70° and 75°F. This is the optimum temperature for growth of the organisms necessary for fermentation. |
| Insufficient amount of brine. | Keep cucumbers immersed in the brine. |
| Pickles not processed properly (to destroy microorganisms). | Process pickles in canner after filling jars. |
| Moldy garlic or spices. | Always use fresh spices. |
| Blossom ends not removed from cucumbers. | Slice at least 1/16th inch off blossom end of cucumbers and discard. |
Strong, bitter taste
| Cause | Prevention |
|---|---|
| Spices cooked too long in vinegar, or too many spices used. | Follow directions for amount of spices to use and the boiling time. |
| Vinegar too strong. | Use vinegar of the proper strength (5% acidity). |
| Dry weather. | No prevention. Bitter taste is usually in the peel or skin of fruits and vegetables. |
| Using salt substitutes. | Potassium chloride, the ingredient in most of these, causes bitterness. |
Hollow Pickles
| Cause | Prevention |
|---|---|
| Cucumbers too large for brining. | Use smaller cucumbers for brining. |
| Improper fermentation. | Keep brine proper strength and the product well covered. Cure until fermentation is complete. |
| Long lapse of time between harvest and brining. | Fermentation process should be started within 24 hours after harvesting cucumbers. |
| Growth defect of cucumber. | None. During washing, hollow cucumbers usually float. Remove and use for relishes instead of fermented pickles. |
Shriveled Pickles
| Cause | Prevention |
|---|---|
| Placing cucumbers in too strong brine, too heavy syrup, or too strong vinegar. | Follow a reliable recipe. Use amounts of salt and sugar called for in a recipe, and vinegar that is 5% acidity. |
| Long lapse of time between harvest and brining. | Brine (start fermentation) within 24 hours after harvesting cucumbers. |
| Overcooking or overprocessing. | Follow a reliable recipe exactly. |
| Dry weather. | No prevention. Bitter taste is usually in the peel or skin of fruits and vegetables. |
Scum on the brine surfaces while curing cucumbers.
| Cause | Prevention |
|---|---|
| Wild yeasts and bacteria that feed on the acid thus reducing the concentration if allowed to accumulate. | Remove scum as often as needed. |
Dark or discolored pickles. (If brass, copper or zinc utensils and brining equipment were used, do not use pickles.)
| Cause | Prevention |
|---|---|
| Minerals in hard water. | Use soft water. |
| Ground spices used. | Use whole spices. |
| Spices left in jars of pickles. | Place spices loosely in cheesecloth bag so they can be removed before canning. |
| Brass, iron, copper or zinc utensils used. | Use food-grade unchipped enamelware, glass, stainless steel, or stoneware utensils. |
| Iodized salt used. | Use canning or pickling salt. |
Spotted, dull, or faded color
| Cause | Prevention |
|---|---|
| Cucumbers not well cured (brined). | Use brine of proper concentration. Complete fermentation process. |
| Excessive exposure to light. | Store processed jars in a dark, dry cool place. |
| Cucumber of poor quality. | Use produce of optimum quality, and grown under proper conditions (weather, soil, etc.) |
White sediment in jar.
| Cause | Prevention |
|---|---|
| Bacteria cause this during fermentation. | None. |
| Salt contains an anti-caking agent or other additives. | Use canning or pickling salt. |
When to throw it out
The USDA's own sentence
Caution: If the pickles become soft, slimy, or develop a disagreeable odor, discard them.
USDA, via the National Center for Home Food Preservation
Kahm yeast, the white film that forms on an exposed surface, is not the same thing and the guide treats it separately: remove it as often as needed. Mould is not kahm yeast. A fuzzy coloured growth, an off odour, a slimy texture or a jar that never soured are all reasons to stop.
Which salt, and why it is not a detail
Use canning or pickling salt.
USDA, via the National Center for Home Food Preservation
Since flake salt varies in density, it is not recommended for making pickled and fermented foods.
USDA, via the National Center for Home Food Preservation
The FAO bulletin lists what the common impurities do, which is a more useful answer than "use good salt": lime lowers the acidity of the finished product and shortens its life, iron blackens the vegetable, magnesium makes it bitter, carbonates soften it, and anti-caking agents cloud the brine.
For pickling any variety of common salt is suitable as long as it is pure.
FAO Agricultural Services Bulletin 134, 5.6
The density point in the second USDA sentence is the reason this site's calculators ask for a weight rather than a volume, and the reason it does not convert the USDA's tablespoons into a percentage. Flake salt and fine salt differ by nearly a factor of two by volume, and a factor of two on the salt is not a rounding error.