Tea and coffee are widely consumed beverages across the world and they are rich sources of various polyphenols. Polyphenols are responsible for the bitterness and astringency of beverages and are also well known to impart antioxidant properties which is beneficial against several oxidative stress related diseases like cancer, cardiovascular diseases, and aging. On the other hand, proteins are also known to display many important roles in several physiological activities. Polyphenols can interact with proteins through hydrophobic or hydrophilic interactions, leading to the formation of soluble or insoluble complexes. According to recent studies, this complex formation can affect the bioavailability and beneficiary properties of both the individual components, in either way. For example, polyphenol-protein complex formation can reduce or enhance the antioxidant activity of polyphenols; similarly it can also affect the digestion ability of several digestive enzymes present in our body. Surprisingly, no review article has been published recently which has focused on the progress in this area, despite numerous articles having appeared in this field. This review summarizes the recent trends and patterns (2005 onwards) in polyphenol-protein interaction studies focusing on the characterization of the complex, the effect of this complex formation on tea and coffee taste, antioxidant properties and the digestive system.

The photophysical properties of commercially available dye Acridine Orange (AO) describe an excellent probe for selective and sensitive detection of caffeine in aqueous solution. AO exists in monomer-dimer equilibrium in water. AO monomer is a fluorophore, but the dimer is not. Addition of caffeine to the AO leads to a shift in the monomer-dimer equilibrium toward the direction of AO monomer (fluorophore) and results in enhancement of AO fluorescence intensity. Enhancement of AO fluorescence intensity in the presence of caffeine has been treated as a signal for caffeine sensor. Furthermore, the caffeine-induced shift in AO monomer-dimer equilibrium is attributed to the binding of caffeine with AO monomer, and the binding constant was higher at a low pH range (pH ~2) compared to pH ~7, which results in superior caffeine sensitivity at pH ~2. Finally, caffeine content in commercial tea beverages has been evaluated and compared with the value obtained with a standard HPLC method.