Structure and Dynamics of Milk Proteins Interacting with Caffeine and Espresso Determined by Two-Dimensional Infrared Spectroscopy

A molecular picture of milk and coffee interactions can help understand the effect of milk on the taste and nutritional properties of coffee beverages. Here, we study the structure and dynamics of milk proteins in the presence of caffeine with two-dimensional infrared (2D IR) spectroscopy. We present 2D IR spectra of milk, mixtures with caffeine, and espresso in the amide I region. While conventional vibrational spectroscopies face challenges in spectral interpretation due to the overlapping vibrational modes of milk proteins and caffeine, our study demonstrates that 2D IR effectively separates these modes by distributing them across two dimensions, enhancing the reliability of the analysis. This study highlights the utility of 2D IR as an effective analytical tool for probing the complexities inherent to food and beverages such as cappuccino samples. The experimental results indicate that the structure of milk proteins remains unchanged when interacting with caffeine in isolation or even real espresso. It has been reported that caffeine can slow down the water network. Our data now shows that the dynamics of milk proteins is unaffected by a less dynamic water network. The presence of caffeine or espresso components does not significantly influence the structural integrity or dynamic behavior of milk proteins on picosecond to femtosecond time scales.