Ferment in glass, food-grade plastic, stainless steel or a stoneware crock with a sound, lead-free glaze. Don’t use copper, brass,
iron, zinc, galvanized metal or aluminium, and don’t use a glazed pot unless you know it doesn’t leach lead. Those rules come from
extension guidance on fermenting and pickling (Penn State Extension; NCHFP), explained below. The
reason behind all of them is the same: a ferment becomes acidic and sits in its container for weeks, and acid pulls metals out of the
wrong surfaces.
Why a Ferment Is the Hardest Test for a Container
Lead, copper and zinc leach into acidic food, and lead leaches more the longer the food stays in contact. The US Food Code, FDA’s
model rules for restaurants and other food businesses, sets its limits for leachable lead in ceramics by how long food stays in a piece
and what kind of food it holds. Cups, mugs and pitchers get the strictest limit: they hold food longer, and a pitcher may hold
fruit juice. It keeps copper and brass away from food below pH 6, such as vinegar, fruit juice or wine, and galvanized metal away from
acidic food, noting that zinc leaching increases with acidity (FDA Food Code 2022). A leaflet from Contra Costa Public Health and Get the Lead Out Bay Area puts
it plainly for dishware: the longer food is stored in it, the more lead gets into the food, and acidic foods and drinks leach lead out
much faster (Contra Costa Public Health leaflet).
A crock of sauerkraut is acidic food held for weeks: at 70–75°F (21–23°C) it takes about three to four weeks to ferment fully, and
up to six at 60–65°F (15–18°C) (Penn State Extension). So a container that seems fine for serving a salad can still leach
into a ferment.
Lead From Glazed Pots: Cases From Fermented Drinks
Three published cases show what happens when a fermented drink sits in a glazed vessel:
- A married couple in Australia needed chelation therapy for lead poisoning after drinking kombucha for six months that they brewed
in a ceramic pot; the doctors proposed that acids in the tea drew lead out of the glaze (Phan et al., 1998). - In France, two adults with unexplained abdominal pain or anaemia were traced to lead-glazed earthenware jugs, one used to keep
kefir and the other kombucha (Sabouraud et al., 2009). - In Spain, public health investigators in the Balearic Islands traced lead poisoning in a family, including two children aged 8 and
9 with no symptoms, to kombucha made in ceramic vessels and sold in ceramic bottles. Two batches still fermenting at the maker’s held
0.95 mg of lead per kg (14 days in) and 7.1 mg/kg (19 days in). The bottled kombucha held 47 mg/kg, and lead migrating from the ceramic
bottles measured 5.8 to 73 mg per litre. The investigators named the bottles as the source (Munilla García et al., 2023).
The Spanish case has a second lesson. The vessel the 19-day batch fermented in passed Spain’s legal limit for lead migration (0.41 mg
per litre against a limit of 1.5), and the maker of the vessels and bottles had told the kombucha makers the glaze was lead free, in a signed
document for the bottles. Yet that batch held 7.1 mg/kg (the authors suggest the SCOBY may also have carried lead over from the glaze in
earlier batches), and they called for the migration limits to be reviewed (Munilla García et al., 2023). Passing a short
laboratory test, or carrying a lead-free promise, doesn’t make a glazed pot safe for ferments. That is also why our kombucha guide says to brew in glass.
How to Check a Crock You Already Own
FDA says a properly made and fired glaze binds its lead, so any that reaches food is insignificant. The trouble is pottery that wasn’t
fired properly (FDA). FDA lists the kinds of ceramicware most likely to leach lead:
- handmade with a crude look or an irregular shape
- antique
- damaged or excessively worn
- bought from a flea market or street vendor, or from a maker you can’t identify
- brightly decorated in orange, red or yellow, because lead is often used to make those colours stronger
A crock made in a commercial factory for everyday use, and not on that list, is much less likely to have a problem. An old or worn
factory crock is still on it. A “lead free” label isn’t proof either: health authorities have reported to FDA traditional pottery
labelled lead free that leached as much lead as lead-glazed ware, and FDA notes that potters who switched glazes may still fire in old
kilns once used for lead (FDA). The Food Code adds that small potters often can’t control the firing of lead glazes
as well as commercial makers (FDA Food Code).
If your crock fits that list, FDA suggests these steps (FDA):
- Test it. Lead-testing kits from hardware stores use a swab rubbed on the surface that touches food. In most kits, the swab changes
colour if lead leaches onto it. If it does, FDA strongly advises against using the piece for food. Only a positive result tells you
much: the kits were designed for paint, they don’t detect lead beneath the surface of a dish, and a negative result doesn’t ensure a
dish is safe (Contra Costa Public Health leaflet; CPSC). - Look underneath. Pottery made only for decoration may be stamped “Not for Food Use—May Poison Food”. Don’t use it for food.
- If you can’t test it or trace it to a reliable maker, consider not using it for food at all.
Washing won’t help: “no amount of washing, boiling, or other process can remove lead from pottery” (FDA). A crock
that tests positive, is stamped “Not for Food Use” or is cracked is for decoration only, with or without a liner.
For a crock whose glaze
you simply don’t know, Penn State’s general fallback for any doubtful container is a liner. Use a food-grade plastic bag, such as a
bag sold for roasting or brining turkeys, so the food never touches the glaze. Never use a garbage bag or trash liner
(Penn State Extension; NCHFP). The same guide says not to use lead-glazed crocks at all, so treat the bag
as a fallback, not a fix. Use a new, intact bag for each batch. If it tears or leaks so that brine reaches the glaze, don’t eat that
batch. Glass or a food-grade bucket is the simpler choice.
Babies, young children and unborn babies can be harmed by amounts of lead that cause no obvious symptoms
(FDA). If a child in your house has been eating or drinking from pottery like this, FDA’s advice is to stop using it
and ask your healthcare provider about a blood lead test: “It’s a simple test” (FDA). If you’re pregnant and have
been using it, stop and tell your provider.
Food-Grade Plastic Buckets and Bags
Plastic is fine if it is food-grade. NCHFP calls food-grade plastic and glass containers excellent substitutes for stone crocks, and
Penn State points out that the 5-gallon plastic buckets restaurants receive ingredients in make ideal fermentation containers
(NCHFP; Penn State Extension). Illinois Extension suggests glass or BPA-free food-grade plastic for small
batches (Illinois Extension). If you can’t tell whether a bucket is food-grade, treat it as if it isn’t.
NCHFP allows a 1- to 3-gallon non-food-grade container only if you line it with a clean food-grade plastic bag, and both guides say
never to use garbage bags or trash liners: “Be certain that foods contact only food-grade plastics” (NCHFP;
Penn State Extension). Plastic can hold on to smells from earlier batches, which our page on
storing fermented foods covers along with containers for the fridge.
Stainless Steel Yes; Copper, Brass, Iron, Zinc and Aluminium No
Stainless steel is fine. NCHFP lists it with glass, stoneware and food-grade unchipped enamelware as suitable
(NCHFP), and Penn State’s vinegar guide includes it too (Penn State Extension). The chip
matters: under the enamel is metal.
The metals to keep away from a ferment, and what each does:
- Copper and brass leach into acidic food. Brass is an alloy of copper and zinc that also contains lead, and all three can leach out of
it (FDA Food Code). - Galvanized steel is coated with zinc, which acid turns into zinc salts the body absorbs. Most reported zinc poisonings involve food
stored in galvanized containers (FDA Food Code). - Iron is on Penn State’s list of containers not to use, and iron utensils darken pickles (Penn State Extension;
NCHFP). - Aluminium and other bare metals corrode in acid, so Penn State’s vinegar guide rules them out for making or storing vinegar
(Penn State Extension); see our homemade vinegar guide. - Taps on drink dispensers count too. Continuous-brew jars for kombucha and vinegar often
come with a tap. Choose one of food-grade plastic, glass or stainless steel, not brass or a metal you can’t identify
(FDA Food Code). For the same reason, don’t keep kombucha or vinegar in lead crystal decanters, which wine and spirits
are known to draw lead from (Phan et al., 1998).
If pickles darken after brass, copper or zinc utensils or brining equipment were used, NCHFP’s instruction is not to eat them
(NCHFP). Other causes of dark pickles, such as hard water, ground spices and iodized salt, are in our
fermentation troubleshooting guide.
Glass Jars, Crocks and How Much Each Holds
Size the container to the vegetables: about 1 gallon (3.8 litres) of capacity for every 5 pounds (2.3 kg) of fresh vegetables, so a
5-gallon crock takes about 25 pounds (11 kg) of cabbage or cucumbers (NCHFP; Penn State Extension). Leave
3 to 4 inches (7.5–10 cm) at the top, and keep the vegetables 1 to 2 inches (2.5–5 cm) under the brine (Penn State Extension;
NCHFP).
Mason jars work but lose more batches. NCHFP calls quart and half-gallon jars “an acceptable practice” that “may result in more
spoilage losses”, and Penn State explains why: it is harder to keep the cabbage under the brine in a jar (NCHFP;
Penn State Extension). In a wide crock or bucket, hold the vegetables down with a plate weighed down with water-filled jars,
or with a sealed food-grade bag of brine (NCHFP; Penn State Extension). The method is the same as in our
sauerkraut recipe. Whole heads, as in our kiseli kupus, need a crock or barrel, because they
won’t fit in a jar.
An airlock is optional. Illinois Extension recommends one to keep air out but notes you can ferment without it, as long as the
vegetables stay under the brine (Illinois Extension). Whatever you use, wash the container, plate and weights in hot,
soapy water and rinse them with very hot water first (NCHFP).