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.
Consumer acceptance of food products and food companies’ reputations relies on obtaining items with consistent textural and rheological characteristics and preserving these properties along the food’s shelf life. These characteristics are particularly important and difficult to measure in semi-solid foods, i.e., structured materials that present both solid and fluid-like behaviour such as emulsions and suspensions (1, 2). Therefore, rapid and accurate monitoring of the overall quality and safety of manufactured and processed foods has been a perennial concern of the food industry. The identification and development of instrumental techniques that allow testing specific physical properties of food materials, such as rheological properties, contri