Quark and cheese are primarily casein-based in terms of protein content. The stomach’s digestion of casein results in the production of protein fragments (peptides) that have an unpleasant flavor, despite casein itself not having a bitter flavor.
In a study conducted by the Leibniz Institute for Food Systems Biology at the Technical University of Munich (LSB), this was established for the first time.
According to the study, bitter peptides can activate stomach cells’ ability to secrete acid by binding to their cellular bitter receptors. The research team hypothesized a mechanism that would support milk protein’s well-known ability to satisfy hunger.
The stomach is also involved in the hormonal regulation of hunger and satiety.
We know from our own and other studies, that bitter substances can stimulate gastric acid secretion, increase serotonin release from gastric cells, delay gastric emptying, as well as exert a satiating effect. Interestingly, protein building blocks such as the bitter-tasting amino acid L-arginine are also among the bitter substances with a satiety effect.”
Veronika Somoza, Principal Investigator and Director, Leibniz Institute for Food Systems Biology
This inspired the research team of Veronika Somoza to investigate if casein digestion in the stomach results in peptides that, like L-arginine, can have an immediate effect on gastric cells.
Enzymatic digestion releases bitter substances
Pigs were utilized as model animals by the researchers because they closely resemble humans in terms of their digestive systems. According to the study’s findings, the stomach’s normal casein digestion does in fact result in the production of physiologically significant levels of bitter peptides.
We were able to demonstrate this exemplarily for five peptides using state-of-the-art food chemistry analysis methods and with the help of sensory tests.”
Phil Richter, Study First Author and PhD Student, Research Group Metabolic Function & Biosignals, Leibniz Institute for Food Systems Biology
The team’s further molecular biology investigations also show that three of these peptides can increase acid output from human stomach cells in a cellular assay system. The peptides have lengths of six, eight, and seventeen amino acids.
Additionally, gene expression studies of the gastric cells and knock-down experiments indicate that the peptides act through two bitter receptor types that are unique to the gastric cells. The latter is also present on the tongue and is significant for how bitter items taste there.
Peptides and bitter receptors with regulatory potential
“Our results suggest that the bitter peptides released from casein already stimulate regulatory mechanisms in the stomach that are partly responsible for the satiating effect of milk protein. Two bitter receptors are probably also involved in these”, added Somoza.
The Leibniz director claims that to further examine the regulatory potential of such peptides along with that of extraoral bitter receptors, clinical trials are currently required.
Richter further stated, “If our hypothesis is confirmed, it would be conceivable in the future to target non-bitter, palatable proteins for foods from which more bitter but satiating peptides are released in the stomach. These could help regulate food intake and ultimately maintain a healthy body weight.”
Richter, P., et al. (2022). Bitter Peptides YFYPEL, VAPFPEVF, and YQEPVLGPVRGPFPIIV, Released during Gastric Digestion of Casein, Stimulate Mechanisms of Gastric Acid Secretion via Bitter Taste Receptors TAS2R16 and TAS2R38. Journal of Agricultural and Food Chemistry. doi.org/10.1021/acs.jafc.2c05228