Monoazo dyes as probes for bulk and local viscosity: Potential applications in semi-solid foods

Monoazo dyes are widely used as colorants in semi-solid foods due to their strong absorption and high stability. These molecules, however, also have the properties of molecular rotors whose fluorescence emission is sensitive to solution viscosity. They thus have the potential to act as intrinsic probes of the rheological properties of liquid and semi-solid foods during processing and storage using inexpensive spectroscopic techniques. The use of environment-sensitive fluorophores such as azo dyes as sensors for local and bulk viscosity in food systems is proposed. In contrast to conventional measurements using viscometers, this fast methodology can be applied to small amounts of sample, avoids wall slippage, does not imply the disruption of the sample's microstructure during its loading in commonly used rheometers and can be easily implemented in an on-line device. A correlation between bulk viscosity and fluorescence intensity of a variety of monoazo dyes has been established in model systems including glycerol, glycerol-water, concentrated sucrose and hydrocolloid solutions. The emission intensity of the dye increased as the viscosity of the solution increased. This observation can be explained by a restriction by viscosity of the rate of rotational motion around the nitrogen-nitrogen bond of the azo group, a motion that efficiently quenches the excited state. The studied probes offer promising perspectives for on-line and minimally disturbing rheology-based quality control measurements in food products.