An ALDI shopper has left many stunned after noticing a curious detail on two similar milk buys.
Sharing on a Facebook group, the ALDI fan pointed out that the expiry dates on cardboard cartons of fresh chilled milk are significantly longer than those on plastic cartons of milk.
An image posted by the shopper showed that a one-litre of Farmdale Light Milk – packaged in a cardboard carton – has a best before date of October 14, while a plastic carton of the same variety of milk has an expiry date of September 23.
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Batch codes on the two milks – purchased on the same day – show that they were produced just one day apart.
This means that there is a 17-day difference in the two expiry dates, meaning chilled milk in cardboard cartons can last much longer.
In her post, the shopper reached out for help in working out why there’s such a significant difference.
“OK mums can someone tell me why the milk in the cardboard carton has such a longer use by date than the plastic bottle milk?” she asked on the Aldi Mums page.
“These two were bought on the same day – plastic 23 September and cardboard 14 of October. The ingredients and labels read the same.
“And why can’t we buy larger cardboard carton like say a two-litre? I’m sure we used to be able to once upon a time.
“Yes, I’m that old!”
Some Facebook users were quick to pass on their knowledge of the subject, revealing the little-known reason.
“Because milk in plastic bottles allows light to penetrate and encourage growth of microbes in the milk, whereas in cardboard it’s less likely,” one responded. “Hence the longer shelf or in this case fridge life.”
Another added: “I was coming to say this. I asked my local milk supplier and this is the answer I received.”
A third responded: “It’s because light gets in plastic bottles which encourages growth of microbes, giving it a shorter ‘life span.’”
One more noted: “Even milk packaged on the same day will have a longer life in the carton, because it doesn’t allow light exposure which encourages growth of bacteria.”
Many were “blown away” by the reasoning, saying they had always been curious about the difference in expiry dates.
“Such a light bulb moment for me. I’ve always wondered this,” said one.
Another said: “Makes sense! We found that milk in cardboard tastes so much better when heated and frothed in a coffee!
“The milk in plastic containers tasted funky a couple of days after it was opened.”
A third wrote: “Thank you for asking this! I’ve always wondered too.”
7NEWS.com.au has reached out to ALDI for comment.
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