Viability and Spoilage Issues
How long do fermented foods remain edible?
Short answer: Indefinitely! Well, actually, it depends. But, if stored properly, fermented foods can last indefinitely, as confirmed by the USDA, on page 7, in a 1917 "Farmer's Bulletin." More recently, the USDA states that fermented foods can last at least a year, even without refrigeration (see section 14.3.1)--a fact that we could have garnered from historical evidence, which is discussed below. Seems like the USDA is getting a bit more cautious in its old age, but I'm goin' with "indefinitely." Of course, how does a fermented food not get eaten within a year!? Oh. I actually have a couple examples below. One example has to do with Captain James Cook. I saved that historical nugget for last.
Want It a Bit More Sour?
Leaving your ferment out at room temperature will cause further fermentation, which will result in the production of more lactic acid, which means more sourness. But extended time (weeks, at least) at room temperature or above may also result in the food texture becoming a bit softer, as the bacteria and naturally-occurring enzymes in the food slowly soften the cell walls. Depending on the length of time out of the fridge, and the temperature, the food may also darken just a bit. None of this affects edibility.
Proper Storage: In the Fridge; Reduce Headspace
How do you "properly" store our fermented foods? Simple answer: in the fridge. If your fermented food is stored in the fridge, the lid is on firm (but not super tight), and the brine covers the food, reaching almost to the top of the container (with little headspace--a new container, basically), then a conservative estimate is that our fermentation can last at least a year.
The cooler refrigerator temperatures also greatly slow down microbial growth and enzymatic activity. This means fermentation is slowed to a crawl, keeping your fermented food tasting almost exactly as the day you bought it. The cold temperatures also slow and hinder growth of yeasts or molds on your ferment. Even though the fridge slows fermentation down to a crawl, the food does still contain living bacteria slooooowly doing their thing. Therefore, just to be safe, it is best not to screw on the top extremely tight, especially if the jar is full, or almost full. If the pressure gets too great from CO2 produced by the fermenting bacteria, you want it to be able to escape.
To reduce the chance of spoilage, if you have eaten more than a quarter of a jar of ferment, and do not plan on eating the rest within a week, let's say, or longer, you can put what's left into a smaller, clean container. This reduces the amount of headspace, which reduces the amount of air, which reduces the chance of spoilage.
Now for a couple "what if's" that you may run into.
"Something" is Growing on the Surface of the Ferment
What if you left a jar of our fermented foods in the pantry or on the counter by mistake. Or perhaps you forgot a container of fermented food in the corner of your fridge. Either way, you find the jar a few days, weeks, or maybe months later. In such a circumstance, there is a chance that you might find a film of either yeast or mold on the brine's surface, or in the headspace of the jar. The yeasts and molds are on the surface because they need air to multiply/regenerate.* These yeasts and mold sit on the surface, breaking down, and feeding, on the lactic acid in the brine. (Pederson at 71, 176). And if there is a lot of headspace in the jar, oxygen can diffuse into the top portion of the brine and even the food, oxidizing the food and allowing aerobic yeast--especially the microaerophilic ones--to grow down to that level of oxygen diffusion. (Id. at 78-79).
*Note: That said, some yeasts are "facultive," meaning they can undergo both aerobic (with oxygen) and anaerobic (without oxygen) respiration. All molds, however, require oxygen. (Pederson at 43, 61-62, 78-79).
The chances of yeast or mold growth happening to refrigerated food is very small. And chances are less still if the jar is unopened and/or there is almost no headspace (air space) within the jar. In other words, if yeast or mold growth does happen, it will almost always happen to an open, partially filled, un-refrigerated container. But, don't despair, some or all of the fermented food might still be perfectly fine. Let's take a closer look.
The yeast that grows on the surface of ferments has a certain name in English: kahm yeast. This is--no surprise--a German term, meaning "mold" or "white film." As used in English-speaking fermenting circles, however, it refers only to a harmless yeast (not mold) that forms a white/whitish film on fermenting products.
Less often, there may actually be mold on top of the brine. Mold has a fuzzy look, as opposed to the filmy look of the yeast. Now, things get a little "fuzzy" here as to what to do. Generally speaking, some molds can be harmful, others can be harmless. Those that grow on ferments, however, are apparently harmless. Here is why I say that.
Don't worry (too much) about the growth of surface yeast or mold
What do the experts have to say?
First, we have the late, Carl S. Pederson, Professor Emeritus at Cornell. He was a prolific researcher and a "giant" in the field of microbiology who was world-renowned for his research into food fermentation, especially sauerkraut. In his book, Microbiology of Food Fermentation, he was fairly dismissive of yeast and mold growths, stating:
Contact with air permits growth of yeasts and molds on the vegetable surfaces and generally results in softening, darkening, and development of undesirable flavors that may diffuse throughout the entire mass of sauerkraut. Only a few individuals are conditioned to desire this flavor development.
(Pederson at 167).
In regards to pickle fermentation (also lactic acid fermentation):
[If] the brine is not covered to exclude air, a filmy yeast growth will often occur on the surface. The growth is undesirable since the yeasts utilize the lactic acid, neutralize the brine, and make possible the growth of other organisms. Softening may result from such growth.
(Id. at 176).
In a paper devoted solely to sauerkraut fermentation, with focus on large-scale, commercial production, Pederson mentioned the following:
There are some individuals who prefer the strong-flavored, "cheesy" kraut produced by surface yeast fermentation following, regular acid fermentation. . .
In some factories, wooden covers were made to fit a vat with little exposed surface. Even these were inadequate because yeasts, molds, and insects developed. Some [kraut] packers did not realize that these surface microorganisms destroy the acid and cause distinct off-flavors and odors.
Essentially, he says these yeasts and molds are bad because they will give off-tastes, if they reach the food. He even seems to suggest that you can eat the affected food, if you find the off-tastes appealing! (No thank you!) Also, he makes clear that if yeasts and molds are left on long enough, they will eat the lactic acid, raising the pH of the brine, which allows other spoilage organisms to survive and grow. The bad result being that the food eventually becomes soft, in addition to acquiring off-tastes. In sum, I am assuming that if, during his 40 years of studying sauerkraut and food fermentations, he had knowledge that these molds or yeasts were serious health hazards, he would have mentioned such dangers rather than suggesting sauerkraut infused with these yeasts and molds is an acquired taste.
Second, we have the makers of Ball canning jars who put out a book on canning and preserving. The book is called, The All New Ball Book of Canning and Preserving : Over 350 of the Best Canned, Jammed, Pickled, and Preserved Recipes by The Jarden Ball, by Jarden Home Brands,* 2016. Here is what they say:
Foods kept at proper [fermenting] temperatures and well beneath their brine will still attract yeasts. The most common is Kahm yeast, a white film that is completely safe and can be scraped off from time to time. Exposure to air may also cause mold growth. Small amounts of mold growth along the rim of your jar are normal and can be cleaned off as soon as you see it. If your ferment [i.e., fermented food] is not weighted beneath the brine, it will attract other yeasts or mold that are not safe for consumption.
In other words, remove the yeasts or mold. If you have actual food that is above the brine, i.e., exposed to air, and this food has yeast of mold on it, then, of course, discard the infected food. The food below should be safe, however, having been protected by the brine barrier.
*Note: Here is some non-important trivia: The brand, "Ball," for glass jars, was part of the Ball Corporation, the current iteration of a U.S. company, founded in 1880, that used to make glass canning jars for over 200 years. The Ball Corporation has since sold their glass manufacturing facilities accompanying name, which are now owned by the Jarden Group, part of a Luxembourg conglomerate.
Third, we have advice from Sandor Katz. Sandor is the modern-day "fermentation whisperer" who has traveled the world learning about fermented foods in various cultures. He has written a couple of best-selling books on fermentation, and travels the world promoting fermented delicacies. Here is what he has to say about yeasts and molds that might have the audacity to grown on your fermented food:
The meeting at this boundary of the nutritious vegetable juices and the air encourages a rich biodiversity, where molds and yeasts frequently develop. Surface growth is common and normal; it should be removed, but it is not cause for alarm and it does not ruin your fermenting vegetables. *** The Kahm yeast layer is beige in color, with a dramatic texture, something like waves or a plate of spaghetti. Mold typically develops as a white film at first. It is not important to be able to distinguish between Kahm yeasts and molds. Surface discoloration can be caused by simple oxidation as well. Any growth or discoloration that develops on the surface of your fermenting vegetables should be removed. *** If this happens, remove most of it, as much as you can, and don’t worry. As long as the mold is white it is not harmful. If other color molds start to grow, do not eat them. Bright colors often indicate sporulation, the molds’ reproductive stage. To prevent spreading the spores, gently lift the entire mold mass from your ferment. Fortunately, the occasional colorful molds I have encountered (always preceded by white molds) were cohesive and easily removed in their entirety. The longer you allow a mold to grow on the surface of your ferment, the deeper its mycelia penetrate. Molds can digest pectins, leading to mushy vegetables. Eventually, the vegetables can come to taste like mold. Molds can also digest lactic acid, lowering the acidity of the ferment that enables it to be effectively preserved. Remove mold or other surface growth, as best you can, as soon as you notice it. After removing mold, evaluate the texture of the underlying vegetables. If the vegetables near the surface have been softened by mold growth, remove and discard.
(Katz at 195-97). (Available online, here.)
Sandor says essentially the same thing as Pederson and an international canning jar company whose roots go back over 225 years.
So what does all of this expert advice tell us?
What To Do If You Find Some
So if you see yeasts or molds on the brine or on the top, inside of the jar, don't worry yet. All is not always lost!
First, determine if there is enough fermented food remaining to even bother salvaging. If only an inch or two of food is left, and the food itself is covered in yeast or mold, it's easier to just toss all of it. If, however, only the brine has spotty colonies of yeast or mold, you can probably pour off the brine and eat the food underneath.
Second, if you have decided that there is enough to salvage (e.g., at least half of a jar), you now need to determine the extent of the spoilage.
Let's say you caught it early, e.g., thin film of Kahm yeast, spotty colonies of mold or yeast. Remove the unwanted guests as best you can. Either skim them off with a spoon, or just completely pour off the top layer of brine. If some of the actual food in the forlorn jar was above the brine, and is covered in mold and yeast, then scrape of this layer of food also and toss it, obviously. If you want to continue storing the food--after having removed the yeast or mold, and perhaps eaten some more--I would then scrub the jar lid with warm soap and water, and then, with a paper towel--slightly moist with hydrogen peroxide--wipe down any exposed surfaces within the jar and also the threads where the jar lid screws on. At that point, you can add new brine (ratio of 2 tablespoons of table or sea salt per quart of water), and fill all the way to the top, eliminating any headspace, i.e., air. (Remember that yeasts, and especially molds, need oxygen.) Or you can just transfer the remaining food to a smaller container, which also eliminates the headspace, adding brine as needed to fill to the top. Then store in fridge.
Let's say you caught the mold and yeast late, however, and the amount of yeast and/or mold are extensive, e.g., thick layer, large colonies, covering the entire surface of the brine, inside of the jar, etc. Two issues arise.
First, since the yeast or mold were growing for a relatively long time, then the yeast or mold might have reached the food below the brine as well, or at least imparted off-flavors as their by-products settled onto the food below, depending on the depth of the brine.
Second, because these yeasts or molds feed on lactic acid, they likely raised the pH of the brine, at least in the top areas, allowing spoilage organisms (yeast and unwanted bacteria) to grow in the brine. This higher pH is bad for our buddies, the fermenting, probiotic microorganisms, and, unfortunately, good for the bad, spoilage microorganisms. In such cases, the food at the top may be spoiled. Do a "look-and-smell-and-small-taste test." If the food is especially soft or mushy, or if it is discolored, and/or has an "off" or rotten smell, scrape off this layer of food, and discard. The layers beneath may be untouched and fine. Eat the untouched food right away, or--as described above--clean the jar, or move to smaller container, and store jar in the fridge.
If the growth of the yeast or mold has been very extensive where significant amounts/layers of food have been affected, just throw out the entire batch. Although Pederson says some people might like these "off-flavors," chances are you won't. Where these yeasts and molds have been active for a long time--months usually ("long" depends on temperature and other factors)--they likely changed the pH of the entire jar, disrupting the stable, lactic acid environment in all layers and parts of the food.
One thing to keep in mind, when deciding whether to toss it or save it, is that these yeasts and molds that grow on brines and fermented foods have been growing on ferments for thousands of years--since fermenting began. Peasants, farmers, and other fermenters throughout the centuries did not discard an entire cask, crock, or barrel of sauerkraut, for example--and with it all of the hard work and precious food calories--because they found some mold or yeast scum on top of the brine or the edge of the vessel. They simply removed it. In fact--due to the relatively recent discovery of microorganisms, and even more recent understanding about the secrets of fermentation--food, beer, and wine fermenters through the ages had no idea what really caused the chemical changes in the food and liquids they were eating and drinking. Bacteria are invisible to the naked eye. But colonies of yeast and mold are not. Therefore, they often thought that the yeast and mold growths on top of the brine actually did the fermenting! (Pederson at 154).
All of the above information is to assure you that, most of the time, you do not have to throw out the entire jar of ferment, if you are inclined to salvage what you can. To be clear, however, if you feel more comfortable discarding a jar of food at the first sign of mold or yeast then, obviously, you can. That's a personal choice. In fact, we are more than happy to supply you with a new jar!
Fermented Foods Lasted for How Many Years!?
To finish up this piece, let's look at some interesting anecdotal and historical examples. Remember that, for many, many centuries, sauerkraut and other fermented vegetables were made in the fall as a method to preserve the summer and fall harvest. These fermented foods were stored--without refrigeration--in barrels or crocks in root cellars and basements, and eaten throughout the fall, winter, and early spring months. So we know fermented foods can last at least 6 months.
One example of a lacto-fermented food lasting for years is given by Sandor Katz, in his book, The Art of Fermentation. Katz describes enjoying three-year-old kimchi--sauerkraut's spicy, Korean cousin--that was stored in a barrel, in a basement, on a Vermont farm.
Finally, here is the interesting historical reference I teased at the beginning of this piece. This example shows that that sauerkraut, for example, can last for years, even in high temperatures and less-than-sanitary conditions.
Captain James Cook led two expeditions to the Pacific Ocean, in the late 1700s. One purpose of these expeditions was to find a cure for scurvy, with various food items being tested. One of the many food items loaded onto the ship was 7,860 pounds of sauerkraut. The ship, H.M.S. Endeavour, set sail on August 25, 1778, and returned on July 13, 1771. Upon his return, Cook reported as follows, to the Victualling Board, which was the arm of the British Navy tasked with outfitting ships with food and drink:
I am to acquaint you that Sour Kroutt together with the many other Antiscorbutics [anti-scurvy foods] my Lords Commrs of the Admiralty were pleased to order to be put on board did so effectually preserve the People from a Scorbutic Taint [scurvy] that no one dangerous case hapned in that disorder during the whole voyage, and it is the Surgeons, Officers and my opinion that Sour Kroutt had a great Share in it and that it will always be found extreamly beneficial to seamen when they are obliged to live long upon a Salt diet; it has the good quality not to loose any part of its Efficacy by Keeping, we used the last of it in September last after having been above two years on board & it was then as good as at the first. (Emphasis added. Spelling as in original.)
To summarize, Cook set sail, in August of 1778, with almost four tons of sauerkraut contained in casks. Over two years later, in September of 1780, his crew finished the last cask of it, and it was "then as good as at the first." In other words, he reported that, over two years later, the sauerkraut was in perfect condition! This was without refrigeration, through mostly hot climates, on a less-than-sanitary ship that sailed around the world, leaving England, sailing around the bottom of South America, to Tahiti, New Zealand, Australia, around the bottom of Africa, and then back to England. If that does not indicate that sauerkraut, and fermented foods, generally, are "good indefinitely," then I don't know what does!
(Naturally fermented, unpasteurized foods are naturally preserved by salt and fermentation by-products, such as lactic acid and antimicrobials, produced by the fermenting bacteria. For more information, read our FAQ, How Does Fermentation Preserve Food?)
Pederson, Carl S. Microbiology of Food Fermentations, 2nd ed. AVI Publishing Co., Inc., 1979.