Photoacoustic (PA) spectroscopy and optothermal window (OW) technique were used to explore their potential to detect red beet added as a colorant to tomato ketchup. The associated changes of colour resulting in the changes of absorbance (and hence of PA and OT signals) were monitored in the 500 nm region corresponding to the absorption maximum of lycopene. Both methods were shown capable of quantifying about 1% of red beet (by mass) in the mixture of ketchup and red beet.
Bicanic, D. D., Westra, E., Setters, J., van Houten, S., Huberts, D., & Colic-Baric, I. (2005). Photoacoustic and optothermal studies of tomato ketchup adulterated by the Red Beet (Beta vulgaris). Journal de Physique IV France, 125, 807-811. https://doi.org/10.1051/jp4:2005125186