12 April 2019

Polyphenols from plants improve the quality of long-life milk


New research from Department of Food Science at University of Copenhagen (UCPH FOOD) shows that long-life milk will have an even longer shelf-life if small amounts of plant extract (polyphenols) are added, for example, from green tea. The discovery, which has been patented, can make the milk more stable and thus help prevent food waste.

Long-life milk is UHT treated, i.e. it is pasteurised at ultra-high temperature, which makes it possible to store the milk at room temperature for up to nine months. This type of milk is not widely consumed in Denmark, but it is very popular in Asia, Africa and Southern Europe, where the climate is warmer and it is therefore advantageous to be able to store the milk outside the refrigerator. Due to the high heat treatment, some aroma compounds develop in the milk, which are increasingly concentrated the longer the milk is stored, so that the quality of the milk deteriorates if it is stored for a long time.

“It is still microbiologically stable, so you can certainly drink it after nine months, but the taste will be considerably inferior and the milk turns brown and begins to clump,” says associate professor and leader of the research project Marianne Nissen Lund, UCPH FOOD. 

“The formation of the aroma compounds is due to reactions between sugars and proteins in the milk, which is followed by a cascade of reactions, collectively called Maillard reactions,” says Marianne Nissen Lund.

These Maillard reactions are the same chemical reactions that give the desired aroma of coffee after roasting and bread after baking, but in milk the aroma formation is usually undesirable. In addition, the proteins undergo physical changes during storage, which cause the milk to clump when the milk reaches a certain age. It has already been shown that the Maillard reactions can be inhibited very effectively by adding plant extract to the milk, but how the flavour would develop over a long period of time was unknown.

“Our study showed that the addition of very small levels of natural polyphenols from plants can reduce some of the undesirable flavours from the Maillard reactions and that the effect can still be seen one year after the product has been produced. Some of the undesirable aroma compounds were reduced by as much as 90%,” says Marianne Nissen Lund.

As a side effect, the researchers discovered that the clumps in the milk were also minimised. How much longer the milk could be stored is not yet known as the study only ran for a year – so at least a year. On the other hand, the polyphenols caused the milk to change colour in a more brownish direction, so more research remains to be done if we are to prevent clumps and unwanted aroma formation in the milk by adding plant extract in practice.

The University of Copenhagen and Arla Foods have patented the new method and the university has since sold its share of the patent to Arla.

Health effects examined

The researchers also discovered that the polyphenols bind to the milk proteins, so they will now examine what this means for the absorption of the proteins in the body.

“Initially, we are examining the absorption of proteins bound to polyphenols in mice. Basically, our hypothesis is that some of the milk proteins are not absorbed because the polyphenols are bound to them, but we do not know, and conversely, the polyphenols can actually improve the absorption of the milk proteins,” says Marianne Nissen Lund.

The results of this research are expected to be available in 2021.