Pitted olives are produced by machines capable of processing thousands of olives a minute. They are remarkably good at it, but not perfect, which explains why the occasional stone survives and why “pitted” should never be taken to mean “guaranteed stone-free”.
There is a particular sound that anyone who regularly eats pitted olives never wants to hear.
Crunch.
You have bought a tub of pitted Kalamata olives from the supermarket. You eat one, then another, then perhaps another dozen, and every one of them is exactly as promised: no stones.
That is precisely why the next one can cause trouble. After enough perfectly pitted olives, you stop treating them as fruit that once contained a hard stone and start treating them as something that doesn’t. You bite down normally — until, one day, you find the exception.
The first thought is usually not about olive-processing technology. It is more likely to be: Was that the stone or was that my tooth?
It’s easy at that point to be furious with the manufacturer or supermarket. You bought pitted olives, so surely finding a pit inside one means somebody has made a serious mistake.
The answer isn’t quite that simple.
Take a close look at the packaging and you may find a warning that seems almost absurd:
May contain pits or pit fragments.
Coles is particularly direct about it. Its pitted Kalamata olives are described as mechanically pitted, followed by a warning that an occasional pit or pit fragment may remain and that consumers should take care to avoid dental injury.
It raises an obvious question: how can we build machinery capable of removing stones from olives at industrial scale, yet still end up with the occasional one in the packet?
The answer begins with the olive itself.
Take a look at the next pitted olive you eat
Before putting it in your mouth, turn it over. A pitted olive carries the evidence of what has happened to it, with an opening where machinery has passed through the fruit and removed the hard stone that once occupied its centre.
It’s easy to imagine some fantastically sophisticated process going on inside an olive factory, but the basic idea is quite mechanical. The difficult part is doing it very, very quickly.
An olive is a drupe, the same broad type of fruit as a cherry or plum, with soft flesh on the outside and a hard stone within it.
The International Olive Council identifies the relationship between the two as one of the important characteristics of a table olive. A flesh-to-stone ratio of 5:1 is considered acceptable, and the stone should ideally separate readily from the flesh. Size, shape, skin and stage of ripeness also vary between olives and varieties.
In other words, an olive-processing machine isn’t handling ball bearings. It’s handling fruit.
Inside the pitting machine
A 2025 engineering study from the University of Seville gives us a remarkably detailed look at how commercial olive-pitting machinery works.
The olives first enter a feeder, where they are separated and carried individually in compartments towards the pitting mechanism. At the critical moment, the fruit meets a stone-detaching mechanism fitted with awls. Through a combination of rotary and linear movement, the machine pierces the olive and separates the stone from the flesh.
Then it has to do exactly the same thing to the next olive, and the one after that.
Modern machines described by the researchers can process 2,500 to 3,000 olives every minute, depending on the variety. At the upper end, that’s 50 olives every second.
Think about what the machine is being asked to do in that fraction of a second. Each olive has to arrive correctly, sit correctly in the carrier and meet the pitting mechanism in the right orientation. The stone has to come away from the flesh, the olive has to survive the process in saleable condition, and everything has to be ready for another piece of fruit about one-fiftieth of a second later.
It works most of the time, but occasionally an olive doesn’t cooperate.
The olive that gets through
This is where the explanation for that unexpected crunch becomes clearer.
Engineers who study these machines have identified several things that can go wrong. Two olives can enter together, fruit can break during feeding or a blockage can leave a carrier empty. More importantly for anyone eating the finished product, an olive can enter the pitting mechanism incorrectly oriented.
Researchers call one particular form of this the “boat error”. Instead of meeting the pitting mechanism in the intended orientation, the olive is pierced off-axis. The result can be damaged flesh, a stone that isn’t properly removed, or fragments of the stone left embedded in the olive.
Seen in that context, the supermarket warning doesn’t seem quite so strange. A machine may do its job successfully thousands of times while somewhere in that stream of irregular pieces of fruit, one olive behaves differently.
The next challenge is catching it before it reaches the consumer.
Finding the mistakes is an engineering problem of its own
Olive processors have quality-control methods specifically aimed at finding pitting failures. One traditional method described in the engineering research sounds wonderfully simple: flotation tanks can be used as part of destructive sampling to identify pitting defects.
The limitation is that sampling is not the same thing as examining every olive, which is one reason researchers are now experimenting with computer vision.
Cameras can watch olives as they travel through the machinery while software analyses their size and orientation. In the 2025 University of Seville study, 97 per cent of the Gordal olives analysed were correctly aligned before pitting, while 1.1 per cent showed critical misorientation.
Those figures need some context. The study involved Gordal olives rather than Kalamata olives, so they should not be applied to every olive-processing line. What they demonstrate is the problem engineers are trying to solve.
A companion 2025 study goes further, examining how machine vision might help identify the off-axis orientation that can lead to pulp damage and stone splinters.
Nor should a processing defect rate be confused with the number of stones consumers actually find in supermarket olives. There are quality-control processes between pitting and the finished product, and the research does not establish that one olive in every hundred bought at a supermarket will contain a stone.
If anything, everyday experience suggests how effective the entire process usually is. You can eat pitted olives for weeks or months without encountering a whole stone, which is also why the one that slips through can catch you completely unprepared.
The stone isn’t the only thing being removed
There is another part of the olive’s journey that is easy to overlook: you couldn’t simply pick most olives from a tree and serve them in a bowl.
Fresh olives contain oleuropein, which contributes much of their intense bitterness. Before they become the olives we recognise as food, that bitterness has to be reduced through processing.
Depending on the style, this can involve brining, repeated washing, fermentation or alkaline treatment. The methods vary enormously between countries, traditions and types of olive. Kalamata olives are commonly prepared as naturally ripened black olives and are often processed in brine.
By the time we encounter an olive in the supermarket, the agricultural product has already been through a surprisingly long transformation. Perhaps that is also why the word pitted carries more certainty in our minds than it should.
We tend to see it as a description of what the olive is. From a processing point of view, it makes more sense to think of it as a description of what has been done to the olive: it has been mechanically pitted.
The process is astonishingly fast and remarkably reliable, but it is not infallible.
So should we actually be careful eating pitted olives?
Yes — although there is no reason to be frightened of them or suspicious of every olive in the bowl. It simply makes sense to remain aware that a stone is possible.
This isn’t advice dreamt up by nervous lawyers. Australian supermarket packaging explicitly acknowledges the possibility. Coles warns that mechanically pitted olives may contain an occasional pit or fragments and specifically mentions the risk of dental injury.
Once you’ve seen how the machinery works, that warning makes much more sense. The extraordinary thing isn’t really that a stone occasionally survives; it’s that a machine can take a soft, irregular piece of fruit, remove a hard object buried inside it, leave the surrounding flesh largely intact and repeat the operation as many as 3,000 times in a minute, hour after hour.
Almost every olive we eat gives us good reason to trust that process.
Then comes the rogue one.
Crunch.
And suddenly you remember what used to be inside it.