Olives straight from the tree are too bitter to eat. In the Greek method, ripe olives go straight into strong salt brine and stay
there for months while yeasts and lactic acid bacteria ferment them and the bitterness leaches out. At home that means a brine of about
6% salt for the first week, then a stronger one of about 12% of the water’s weight, kept at 16–27°C for at least two months. Olives are
a low-acid fruit, so the salt has to stay high from start to finish.
Why Olives Need Curing, and the Greek Way of Doing It
Fresh olives contain oleuropein, a bitter compound that makes them inedible off the tree
(Perpetuini et al., 2020). Table olives are one of the oldest fermented vegetables of the Mediterranean, and the main
producers today are Spain, Egypt, Turkey, Algeria, Italy, Greece and Portugal (same source).
The styles differ in how they remove the bitterness. Spanish-style green olives are debittered with lye before they ferment. In the
Greek style, olives go directly into brine and oleuropein is broken down by the enzymes of the microbes that grow on them
(Perpetuini et al., 2020). In Greece, the main varieties for this are Kalamata and Conservolea, placed in brine of
8–10% salt “or even higher according to local practices” with no debittering step first (Bonatsou et al., 2018). The
international trade standard calls these “natural olives”: olives “placed directly in brine in which they undergo complete or partial
fermentation” (International Olive Council). Brine curing is the home version of that method, and it needs nothing but olives,
salt, water and time.
Two Brines: Why the Salt Goes Up After a Week
University of California Cooperative Extension’s olive handout starts the olives in a milder brine for 7 days, then moves them to a
strong brine for at least two months (UCCE Master Food Preservers). Weighed out, its figures come to:
| Stage | UC figure per US gallon of water | Per litre of water | Salt as % of the water |
|---|---|---|---|
| First brine, 7 days | 8 oz pickling salt | 60 g | 6% |
| Strong brine, 2 months or more | 1 lb (1½ cups) pickling salt | 120 g | 12% (10.7% of the finished brine) |
Weigh the salt. The handout also gives cups (¾ cup for the first brine, 1½ cups for the strong one), but a cup of coarse or flaky
salt weighs much less than a cup of fine pickling salt, so the weight is the number to follow. Our salt and brine calculator does the conversion for any jar.
One reason to raise the salt: the olives soak it up. In a study of Kalamata olives held in 7% brine, the salt fell by 1.5–2.0 points in
the first 10 days, as low as 5%, and had to be topped up with dry salt for the rest of the six-month fermentation
(Bonatsou et al., 2018). UC doesn’t say why it uses two brines, but changing to the strong brine after a week keeps the
salt high in the same way. The strong brine is about one and a half times as salty as the brine for
fermented dill pickles (about 7–8% of the water) and four to six times the 2–3% most vegetable ferments use: an
olive is a dense, oily fruit that sits in its brine for months, not weeks.
What Happens in the Jar, and How Sour It Gets
Greek-style olives don’t sour as hard as cabbage or cucumbers. In the Kalamata study, yeasts dominated, lactic acid bacteria could
no longer be counted after 20 days, and the brine finished at pH 4.2 after 187 days (Bonatsou et al., 2018). The 4.6 line and how to test a brine are explained in our fermented food pH chart.
In a study of Conservolea olives in 8% salt brine and lower-sodium salt mixes, every brine had a vigorous lactic fermentation and
reached pH 3.9–4.2 (Panagou et al., 2011). For
comparison, the trade standard asks the brine of natural olives that aren’t heat-treated to have at least 6% salt and a pH of no more than 4.3, with at least 0.3%
acidity as lactic acid (International Olive Council).
So a finished jar sits only a little below the pH 4.6 line that food-safety rules use for acid foods, and the high salt is doing much
of the protecting. That’s why the method has no low-salt version. If you have a calibrated pH meter, a brine reading of 4.3 or lower
matches the commercial yardstick; a reading above 4.6 after two months means the batch didn’t ferment properly and should be thrown
away.
The longer olives stay in brine, the less bitter they get. For milder olives, UC suggests replacing the strong brine with a fresh batch
once a month for two or three months; otherwise ferment until they taste the way you like (UCCE Master Food Preservers).
Never Move Cured Olives Into Plain Water
In February 2004, 28 people who ate at one restaurant in Molise, Italy, fell ill with botulism; 15 were admitted to hospital. Only
the home-preserved green olives in salt water remained linked to illness after analysis. They had soaked in salt water for about 35
days, then been put in jars with the salt water replaced by fresh water, and neither the amount of salt nor the pH had been standardised. A jar from the
same batch had a pH of 6.2 (Cawthorne et al., 2005).
Olives in fresh water have neither the salt nor the acid that keeps Clostridium botulinum from growing. Keep cured olives in
their strong brine. To make them less salty for the table, soak only the olives you’re about to eat, keep them in the fridge, and eat them
soon.
Keeping Them: In Brine or in the Fridge
- In the fridge, in brine. This is the simplest choice for a home batch. UC also lists olives in oil for short refrigerated
storage; don’t keep olives in oil at room temperature. - In a very strong brine at room temperature. UC’s concentrated brine builds up to 6 cups (3¾ lb) of pickling salt per gallon of water
over four days, and olives kept this way “will remain good for 8 to 9 months” (UCCE Master Food Preservers). Before
eating, they need four changes of fresh water over two days; do that soak in the fridge and eat them soon after.
Don’t can them. UC says to “never can olives for which you do not have a research-tested processing time”, and its pressure-canning
process is for lye-cured olives; for brine-cured olives its table lists only brine and refrigeration
(UCCE Master Food Preservers). A boiling-water canner is never safe for olives, which are a low-acid food.
Soft Olives, Film on the Brine and Bulging Lids
Softening is mostly caused by microbes that break down pectin. Moulds and some bacteria growing on the surface of the brine can soften
olives within about two weeks, so keep the olives weighted under the brine, check the jars and remove any film. A storage brine of at
least 10% salt, a covered jar or the fridge helps prevent it (UCCE Master Food Preservers). Very ripe, large olives soften more easily (same source). A thin white film
is usually yeast; fuzzy or coloured mould means the batch goes in the bin (see our guide to kahm yeast and mould).
The ferment makes gas: check the jars regularly, and if a lid bulges, loosen it to let the gas out and close it again
(UCCE Master Food Preservers). Throw out, without tasting, any olives that turn slimy or mushy or smell rotten or putrid
instead of fruity and briny. As UC puts it: “When in doubt, throw it out.”
Dry salting is the other salt-only method in UC’s handout: small, fully ripe black olives mixed with 1 lb of salt for every 2 lb of olives
(1 kg salt per 2 kg) and stirred weekly for five to six weeks. They come out wrinkled, salty and slightly bitter, and keep up to six months
in the fridge (UCCE Master Food Preservers). Salt-curing fruit works the same way in preserved lemons;
for another long Southern European ferment, see brovada, Friulian turnips in grape pomace.