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Hot or cold, how do you take your milk?

Hot or cold, how do you take your milk?

A recent study by Teagasc examined how the temperature of milk we drink affects its digestion in the stomach. The researchers used a laboratory system to mimic the digestion of whole milk and applied medical imaging techniques (Magnetic Resonance Imaging or MRI) to monitor the process.

When we drink hot or cold milk straight out of the fridge, it changes the temperature of the stomach’s content from the normal body temperature of 37°C, at least for a few minutes. This temperature difference can affect the action of the pepsin, the main enzyme responsible for breaking down milk during digestion in the stomach. The study aimed to understand how this may impact milk digestion.

The milk proteins, casein and whey protein, form a milk curd in the stomach, a process called coagulation. Project leader André Brodkorb says: “This is very similar to making cheese from milk. However, in the stomach we have the action of acid and pepsin that forms the curd”. MRI results clearly showed that when hot milk is consumed, proteins in the milk coagulated faster compared to when cold milk is consumed. This happens early in the digestion process due to better enzyme activity at higher temperatures. However, there was an unexpected observation regarding the movement of fat in the stomach. When cold milk is consumed, the milk fat tends to rise to the top, similar to creaming. In contrast, when hot milk is consumed, fat tends to stay at the bottom of the stomach bottom of the stomach. If such effects (observed in the laboratory digestive system) persist in the human body, they may result in a quicker transfer of nutrients from the stomach to the small intestine where absorption into the bloodstream occurs.

The use of Magnetic Resonance Imaging (MRI) at the INRAE Research Centre in Rennes, France, allowed us to observe digestion without invasive methods. This medical imaging technique helped us see how fats and proteins behaved during digestion, providing crucial data that would have been difficult to get otherwise.

So what does that mean for us? Should we drink milk hot or cold? “The in vitro results are really interesting but it is still difficult to answer”, says Dr Daniela Freitas, Teagasc SFI-IRC researcher. “We would need human volunteers to lay still for up to 90 minutes in the MRI tube, because this is how long it would take a glass of milk to pass from the stomach to the intestines”.

preparing a simulated digestion for MRI

From left to right, Stephane Quellec (INRAE, France), Conor Fitzpatrick (Teagasc) and Jiajun Feng (PhD student, INRAE, France) preparing a simulated digestion for MRI.

The Ulysses Ireland-France initiative by the Embassy of France in Ireland and the Irish Research Council facilitated the exchange of researchers: Teagasc PhD student Conor Fitzpatrick and Dr Daniela Freitas travelled to Rennes in Brittany, France while PhD student Jiajun Feng travelled from France to the Teagasc Food Research Centre, Moorepark, in Ireland. Conor Fitzpatrick remarked: “This research fits in well with the overall research objective of the SFI/DAFM funded Vistamilk program: Collecting large amounts of data to better understand milk from source all the way to the gut.” He appreciated the chance to work with internationally recognised French researchers, learn advanced imaging techniques and gained experience on his PhD journey.

The results of this study were published in the high impact journal Food Hydrocolloids under the title "Monitoring the effect of consumption temperature of whole milk using magnetic resonance imaging."