What would you plan to do with a carton of UHT milk that you bought over four years ago during Covid?
A professor at University College Cork (UCC) has conducted an experiment with a carton of UHT milk that he bought four years ago.
Prof. Alan Kelly bought a carton of the milk in 2020, which he said was “right at the start” of the Covid-19 pandemic.
He did not open it until yesterday (Wednesday, March 13), when he was conducting an experiment with a group of students.
Detailing the experiment on X, Prof. Kelly said: “When opened, it smelled perfect, and I am absolutely certain the product was sterile.
“The intense UHT heating had killed all bacteria years ago. However, the contents looked more whey than milk.
“We poured off the whey gently, and in the bottom third of the carton a gel appeared,” he added.
Prof. Kelly explained that the original milk had “neatly and cleanly separated into curds and whey”.
He added that the curd was “quite strong and cohesive”.
Explaining the process the UHT milk undertook over the four years, Prof. Kelly said that when the milk was raw over four years ago, a bacterium called pseudomonas grew in it.
The UCC professor said it was then “destroyed” when the milk was UHT-treated.
The process of ultra heat treating (UHT) is a food processing technology that sterilises liquid food, most commonly milk, by heating it over a very short time period to kill microbial spores.
“However, it had first produced an enzyme which is essentially indestructible, a bit like rennet,” he added.
Rennet is an enzyme-based key ingredient in cheese making, either from the stomach of young ruminants or plant-based.
Thus, the whole experiment consisted of what Prof. Kelly called “the world’s slowest cheese-making” that resulted in the creation of what he said was “basically a rennet gel”.
Summing up the experiment as “milk, destabilised by the enzyme ghosts of long-dead bacteria,” Prof. Kelly said it was “a very nice illustration of one of my favourite dairy science stories”.