Nutrition
Keep botulism at bay using the acid test or a salt solution when home-preserving.
Keep botulism at bay using the acid test or a salt solution when homepreserving.
Most bacteria are unable to grow in foods with a water activity below 0.9.
Question:
I read your warnings about botulism. I make a two parts olive oil to one part apple cider vinegar salad dressing and put chopped garlic in it, then store it at room temperature for lengthy periods. Does the vinegar make it safe?
Answer:
Acidity is our friend when it comes to staving off botulism and many other pathogenic bacteria. But as soon as you mix oil with vinegar, the resulting mixed solution probably removes that safety guarantee. To explain, the bacterium Clostridium botulinum produces a toxin that causes the rare and potentially fatal illness botulism. The botulinum toxin inhibits a neurotransmitter within our nervous system, leading to paralysis, starting from the muscles of the face and then spreading towards the limbs. In severe cases, botulism leads to paralysis of the breathing muscles, respiratory failure and death.
Because it is so toxic, food manufacturers must take all practical steps to prevent any botulinum bacteria present from growing and forming their toxin. And judging from New Zealand’s track record with botulism cases, they’re clearly doing a good job.
“Strains of Clostridium botulinum producing type A, B, E or F toxins (those that cause botulism in humans) have never been detected in commercially produced food or drink in New Zealand,” says Fiona ThomsonCarter, director of food science and risk assessment at New Zealand Food Safety.
What’s more, there haven’t been any outbreaks of human botulism in New Zealand, and only two isolated incidents in recent decades, Thomson-Carter says. “One, in 1984, was associated with improperly home-prepared toroi (watercress and boiled mussels), and the other, in 2014, was associated with packaged, heat-and-eat risotto that had not been refrigerated and was consumed three months after its use-by date.”
Still, we should take care and keep prevention in mind with any home-preserved foods. Refrigeration temperatures, combined with salt content and/or acidic conditions, will prevent the growth of the botulinum bacteria and formation of their toxin, along with a variety of other pathogenic bacteria.
Apple cider vinegar should generate
a pH of less than 4.6, which will ensure the product is safe, says Steve Flint, professor of food safety and microbiology at Massey University.
In commercial salad dressings, an emulsion is generated, so the oil is suspended in an aqueous acidic phase such as vinegar or lemon juice. But in your home-made salad dressing, you’ve no doubt noted that the oil and vinegar separate soon after shaking or mixing. In this situation, there is the possibility of Clostridium botulinum growing, particularly if the garlic is sitting in the oil in the container – so there is some risk, says Flint.
However, the garlic would be safe in 100% vinegar. So, you could add it to apple cider vinegar and store it for extended periods, then decant and mix some with olive oil, quite safely, each time you use it.
Question:
I’d like to preserve some of our fruit, such as lemons and persimmons. You mentioned using a 10% salt solution to reduce the risk of botulism. How do I make that?
Answer:
Adding salt to water results in a reaction that effectively binds the water molecules, making them less available for any pathogenic bacteria.
The water activity (AW) of a food is a measure of the availability of water for the metabolic activity and growth of micro-organisms. Pure water, for example, has an AW of 1.0. Most bacteria are unable to grow in foods with an AW below 0.9. Most yeasts and moulds that cause spoilage can’t grow in foods with an AW below 0.8.
“A 10% salt solution will generate a water activity of about 0.75,” Flint says. That’s enough to make life difficult for Clostridium botulinum, and any other pathogenic bacteria present.
Make your own 10% salt solution by placing 10 grams of salt in a measuring container, adding a little water to dissolve the salt, then topping up the water to 100ml.
Email your nutrition questions to nutrition@listener.co.nz