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How to preserve your own olives safely

When it comes to beating botulism, some preserving practices are better than others.

QUESTION: I’ve started preserving olives using a heavy dose of salt in water and turning them for three weeks until they’re shrunken in their watery liquid. However, I haven’t stored them in the refrigerator. After reading your article on botulinum and olives, I’m wondering if this is risky.

ANSWER: The good news is you’re quite safe if you’ve been using a heavy dose of salt in the water containing your olives, although mould might grow if they’re not kept in the fridge.

The main issue is ensuring you prevent any clostridium botulinum present in the olives from producing their toxin – one of the most lethal substances known to humans.

Botulinum toxins block nerve function and can lead to respiratory and muscular paralysis. Any suspected case of botulism is treated as a medical emergency and food manufacturers take care to prevent any clostridium botulinum in our food from producing these devastating poisons.

But, as pointed out in this column on September 21, many cases of botulism are caused by home-prepared vegetables stored in oil or water. Between 1994 and 1998, more than 100 cases of botulism in Italy were traced back to home-prepared vegetables preserved in this way.

It’s important, then, that home cooks who are preserving vegetables in oil or water understand the food safety protocols to follow. First, remember that clostridium botulinum is a bacterium that produces its dangerous toxins under low-oxygen conditions. So, as soon as you store vegetables in oil or water, you’ve created low-oxygen conditions perfect for the toxins to be produced if the bacteria are present.

The second key point is that clostridium botulinum is commonly found in soil and water, which means produce that comes in close contact with soil, such as mushrooms and many vegetables, can contain the bacterium. That’s not usually a problem because, ordinarily, you would store your mushrooms or vegetables in the fridge for a short time before cooking them.

However, if you’re going to preserve and store vegetables such as olives for an extended period, it’s crucial that you prepare them carefully to reduce the risk of food-borne illnesses. And in this case, you have found the perfect solution: a saline solution.

Steve Flint, a professor of food safety and microbiology at Massey University, says the concentrated saline will draw water out of the olives so they shrink. The salt then acts as a preservative.

It’s important that you use enough salt, Flint says. A concentration of 10% salt is needed to ensure no risk from botulism. In such a solution, it is safe to store your olives for extended periods either inside or outside the fridge.

QUESTION: I buy olives in brine and then drain and store them in olive oil in the fridge for up to a year. Is it safe to continue doing this?

ANSWER: The short answer is no. “Draining the brine and keeping the olives in oil has some risk associated with it,” Flint says. “The bacterium that causes botulism, if present, can grow in an environment containing oil.”

Instead, it would be safer to keep the olives in brine containing 10% salt, he says. “The period for safe storage in a solution containing more than 10% salt will be indefinite, as no unhealthy bacteria can grow.”

The only thing that may happen over that extended period is some spoilage of the food caused by enzymatic deterioration, and potentially the growth of moulds.

This article was first published in the October 26, 2019 issue of the New Zealand Listener.