Bitterness explored in cheese products

Fresh cheese products with a high calcium content (such as quark or skyr) can develop a bitter taste during their production, which may ead to rejection by the consumer. Research by the University of Hohenheim in Germany suggests that bitter peptides in calcium-rich milk products such as quark or skyr come from starter cultures.

Three research groups have investigated how this bitter taste can be prevented. For a long time, the calcium concentration was considered to be the trigger for the formation of so-called bitter peptides during fermentation. But it doesn’t seem to be that alone, the researchers concluded.

Rather, it is probably the interplay of calcium content and starter cultures. By selecting suitable mixtures of bacterial cultures for fermentation, it may be possible in the future to reduce the bitterness of the finished product or perhaps even avoid it entirely.

The project was funded by the Federal Ministry for Economic Affairs and Energy (BMWi) with a total of almost €450,000.

The so-called bitter peptides, which are responsible for the bitter aroma, are small protein fragments that are formed from larger proteins with the help of special enzymes, the peptidases.

“These fragments respond to our bitter sensors on the tongue. That’s why these foods are not spoiled, they just don’t taste so good to bitter-sensitive people,” explains professor Herbert Schmidt from the department of food microbiology and hygiene at the University of Hohenheim.

It is still unclear whether these peptidases come from the added lactic acid bacteria or come from the milk, if it is because they are produced by the cow and passed into the milk or because they get into the milk through contamination with other bacteria.

The question is, why are bitter peptides mainly found in products that have a high calcium content?

“The aim of our research project was to understand, how the bitter substances are released in the manufacturing process and which factors can reduce this,” professor Jörg Hinrichs from the dairy science and technology department says.

Fermented dairy products are created by the work of lactic acid bacteria. They are added to milk or milk concentrates as so-called starter cultures and convert the milk sugar into lactic acid and other substances.

Usually the manufacturers also add a small amount of rennet so that the milk coagulates better. The milk protein flocculates and can then be separated from the liquid part of the milk, the whey.

However, the lactic acid bacteria also need the protein in milk for their own nutrition. They break down larger proteins into smaller pieces so that they can be taken up into the cell. There the peptides are broken down even further and can then be used by the cells for their own protein metabolism. But if too many of these fragments remain in the dairy product, they can cause a bitter taste.

The formation of these bitter peptides during the fermentation process is a complex interaction between various milk components and the genes or enzymes of the bacteria. In order to better analyze these interactions, the scientists worked with various, in some cases newly developed, model systems.

A newly developed concentration and filtration process makes it possible, for example, to change and adjust the composition of the milk concentrate in a targeted manner before fermentation.

“In contrast to the classic manufacturing process, we first draw off the whey and then ferment the milk concentrate,” explains Hinrichs. “This method allows us to variably control the fat, calcium and protein content and thus simulate different compositions.”

Using this cream cheese model, the scientists analysed the effects of different fermentation conditions on the formation of bitter peptides, such as temperature and pH value as well as different bacterial cultures.

A number of lactic acid bacteria are used to make cream cheese products, most commonly Lactococcus lactis.

“The starter cultures used for fermentation are almost as diverse as the dairy products on the market,” says Schmidt.

In addition, as with almost all bacteria, there are different lineages with different properties, the so-called strains. “In one culture we found a mixture of 30 to 40 different strains,” Schmidt adds. “That doesn’t make the analysis and the targeted control of the fermentation process any easier.”

The scientists have nevertheless succeeded in identifying some of the genes, which play an important role in the formation and breakdown of the bitter peptides.

The focus of interest is the gene that is responsible for the formation of the peptidase. It was suggested that the activity of this peptidase gene could be regulated by calcium. New methods make it possible to record the activity of individual genes.

“It turned out that calcium has no influence on the activity of this gene. However, we know that the pH value has to drop below five within 24 hours during fermentation in order to reduce gene activity,” summarises Schmidt. “Of course, the easiest way would be, if we could breed strains, which don’t have this gene. Unfortunately, bacterial strains, which do not have the peptidase gene, cannot survive on their own. They need the other strains that produce the vital peptides for them.”

“We found a total of nine different regulation points for the enzyme activity. And depending on the bacterial strain, calcium can have a different effect. Sometimes it promotes enzyme activity and in another strain it reduces it,” notes professor Lutz Fischer from the department of biotechnology and enzyme science.

“In more recent work, however, there is a clear trend. The milk’s own enzymes from the cow, such as cathepsin and plasmin, are probably not the cause, Rather, it seems to be solely responsible for the added bacterial cultures, which are necessary for the fermentation of milk or milk concentrates,” he adds.

The challenge now is to put the different bacterial cultures together in such a way, so the bacteria can still perform their desired task, but, on the other hand, as few or none of the bitter peptides are contained in the finished product.

This is why the scientists’ recommendation to the manufacturers of starter cultures is to use enzymatic tests in addition to traditional culture and molecular biological techniques. According to the current state of knowledge, the formation of bitter peptides is mainly decided at the enzyme level.

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