In households around Australia, the debate continues: is tomato sauce best kept in the fridge or the pantry?
And what about other condiments, like jam, peanut butter and sauces?
To find out the answer once and for all, we spoke to two food scientists. Here's what they had to say.
Why tomato sauce is best kept in the fridge
When buying supermarket sauces and condiments, start by checking the label for storage instructions, says Rozita Spirovska Vaskoska, who leads the food microbiology team at CSIRO.
"Following the manufacturer's instructions on storage is recommended, as the food industry conducts shelf-life tests on their products to assure they remain safe," Dr Vaskoska says.
If you look at a bottle of tomato sauce, it's likely to contain a note to refrigerate after opening. You might be wondering why, considering that many people keep it at room temperature.
Dr Vaskoska says the recommendation is mainly about maintaining food quality rather than safety.
"Open tomato sauce is commonly stored at room temperature [without issues] in fast food restaurants and households for hours," she says.
"[However] as soon as you open a bottle of tomato sauce, there is some risk of growth of microorganisms, mostly spoilage microorganisms such as yeast or lactic acid bacteria."
When these sorts of microorganisms grow, they can affect the food's taste and cause the familiar smell of food going bad. However, Dr Vaskoska says they are unlikely to pose a significant health risk.
"These products are on the low side of risk compared to potentially hazardous foods: the foods that contain raw or cooked meat, for example," Dr Vaskoska says.
"Although [tomato sauce] does not have to be returned to the fridge in two hours as we do for potentially hazardous foods, tomato sauce still should go back to the fridge if that is what the label says."
What about other supermarket condiments?
Dr Vaskoska says most condiments available at supermarkets contain preservative ingredients such as salt, sugar, acid or spices, or are manufactured in a way to prevent the growth of microorganisms and mould.
Like tomato sauce, you should generally keep these items in the fridge after opening, Dr Vaskoska says.
This includes items such as jams, bottled sauces (like pasta sauce) and store-bought mayonnaise. Again, the label will tell you how to best store the product for safety and maximum shelf life.
"Condiments differ in their formulation and the nutrients they contain," Dr Vaskoska says.
"Condiments like mayonnaise, given the nutrients they contain, should always be refrigerated after opening."
She notes that homemade mayonnaise should never be stored out of the fridge as it is not subject to "pasteurisation, controlled acidity levels or food safety standards" that ensure shelf stability.
Why the fridge is generally safer than the pantry
Dr Vaskoska says pathogenic microorganisms such as E. coli or salmonella would only appear in store-bought condiments if there was some form of contamination during production or at home.
These pathogens are far more serious than spoilage microorganisms — they can cause food poisoning and lead to hospitalisation and even death in severe cases.
Senaka Ranadheera, a food scientist and associate professor at the University of Melbourne, says refrigeration reduces the risk of food poisoning as it hampers the growth of microorganisms.
"Lots of these pathogenic microorganisms get into our food through the oral-faecal route," he says.
"For example, lots of people do not have good toilet habits, they don't wash their hands after using the bathroom.
"Because of that, there's a risk that anyone can have certain pathogenic microorganisms in their hands."
For example, those microorganisms could be transmitted to a spoon, which could be transferred to a jar of mayonnaise, where they can grow.
"If you refrigerate, the growth is lower, so we can keep it to minimum level, but if you keep them at room temperature, then they grow very rapidly," Dr Ranadheera says.
Dr Ranadheera says microorganisms grow best in hot and humid conditions in the so-called temperature "danger zone".
"Between five and 60 degrees, many pathogenic and also spoilage microorganisms are very happy. That's the optimal temperature for growth and their reproduction," he says.
The items you can safely keep in your pantry after opening
Some condiments can be kept in the pantry even after they've been opened because of their properties or preservation methods.
An example would be a tabasco-style vinegar hot sauce, which is too acidic for microbial growth, Dr Vaskoska says.
Another good example is honey. Dr Ranadheera says honey's natural antimicrobial properties and low moisture content mean it can be safely stored at room temperature.
Many store-bought peanut butter spreads can also be stored in the pantry, Dr Ranadheera adds, as the high oil content and lack of moisture prevents the growth of most microorganisms.
For those reasons, many peanut butter labels recommend storage in a "cool, dark place".
Some of these items, such as peanut butter and hot sauce, may still lose quality over time and refrigeration may improve their shelf life.
When should you throw condiments out?
So, how can we know when to throw something out?
"In the case of lactic acid bacteria, there might be a smell -- a kind of spoiled milk smell. With the yeast as well, there could be some yeasty off flavours," Dr Vaskoska says.
"There could be visual signs of spoilage as well, such as mould appearing on the products."
Again, Dr Vaskoska says condiments are considered low risk in terms of food safety. And, even if a condiment is contaminated by a pathogen like salmonella, it's not something you can easily detect.
"The safety of the product cannot be determined on visual or sensory indication," Dr Vaskoska says.
Dr Vaskoska says most condiments will have a "best-before" label rather than a use-by date.
Unopened foods with a best-before label are safe to use past the listed date, Dr Vaskoska says, but they might have lost quality.
"If something has a use-by date then it should be thrown out [once the use-by date has passed]," she says.
