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BSU helps TV presenter Jimmy Doherty unwrap the mystery of sour milk

Tuesday, 14 January, 2025

Have you ever wondered why sour milk is considered bad, while soured cream is a staple in many kitchens? The latest episode of Channel 4’s Food Unwrapped set out to answer this question, with a little help from Bath Spa University.  

In the episode, which debuted on Friday 10 January, co-host Jimmy Doherty took a trip up the drive at Newton Park to meet Dr Matt Baker, Course Leader for BSc Biology and Senior Lecturer in Human Biology and Nutrition at the University’s science laboratory. The first part of the segment involved explaining the science behind what happens to milk when it starts to go off, and Matt was ready with samples of milk and petri dishes. 

Matt explained:   

“We let some semi-skimmed milk sit in the fridge for around 10-15 days, taking samples every so often and then growing them up on petri dishes to see what bacteria were there. I explained what effect they would have on the milk as it curdled, and we got Jimmy to do a sniff test on the milk samples.” 

After Jimmy had a whiff of some particularly unpleasant-smelling samples of milk, Matt explained that bacteria growing in them makes milk start to turn and creates nasty smells. Even though milk is pasteurised at the dairy, trace amounts of bacteria survive, and multiply over time. 

“In addition to that, you’ve probably taken a cheeky swig from the carton every so often, which means you’re introducing some extra bacteria into the carton as well,” Matt said. 

There are two main types of bacteria that contribute to milk’s transformation from a vital addition to any cuppa to something best tipped down the sink. To demonstrate how quickly these bacteria can grow, Matt prepared petri dishes containing casein, a protein found in milk, and lactose, which is a sugar found in milk. Holding the dishes up to the light, viewers could see masses of bacteria from the milk growing in them.  

Summing up, Jimmy explained that most of the sourness comes from the bacteria eating the sugar in the milk and turning it into lactic acid. As the sugar-eating bacteria runs out of sugar and begins to die, that’s when the protein-eating bacteria take over and finish the job, creating what Jimmy called “a horrible, stinky mess.” 

After learning the science behind what makes milk go bad, Jimmy was off on his travels again, further into Somerset to learn how to make soured cream at the Yeo Valley dairy. But Matt was happy to have helped explain why we definitely don’t want to consume bad milk, as well as featuring some of the excellent work our scientists get up to in BSU’s lab. 

Matt said: 

“What viewers see in the episode was the result of several weeks of work with the Unwrapped production team, myself, and our excellent technical staff, particularly our tech team leader, Sarah Bagwell who did all the hard work in the lab culturing the bacteria. The best bit is these are our main teaching labs, so these are the labs that our students get to use every day.” 

Food Unwrapped is available to watch now on Channel 4.