Improved analysis of Monascus pigments based on their pH-sensitive UV-Vis absorption and reactivity properties

ABSTRACT

Monascus pigments, a mixture of azaphilones mainly composed of red, orange and yellow pigments, are usually prepared in aqueous ethanol and analysed by ultraviolet-visible (UV-Vis) spectroscopy. The pH of aqueous ethanol used during sample preparation and analysis has never been considered a key parameter to control; however, this study shows that the UV-Vis spectra and colour characteristics of the six major pigments are strongly influenced by the pH of the solvent employed. In addition, the increase of solvent pH results in a remarkable increase of the amination reaction of orange pigments with amino compounds, and at higher pH (≥ 6.0) a significant amount of orange pigment derivatives rapidly form. The consequent impact of these pH-sensitive properties on pigment analysis is further discussed. Based on the presented results, we propose that the sample preparation and analysis of Monascus pigments should be uniformly performed at low pH (≤ 2.5) to avoid variations of UV-Vis spectra and the creation of artefacts due to the occurrence of amination reactions, and ensure an accurate analysis that truly reflects pigment characteristics in the samples.

GRAPHICAL ABSTRACT

KEYWORDS:

• Monascus pigments
• pH control
• UV-Vis analysis
• amination reaction

Acknowledgement

The authors gratefully acknowledge Professor Jianhua Ju and the South China Sea Institute of Oceanology for technical and instrumental assistance.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary-material

Supplemental data for this article can be accessed at http://dx.doi.org/10.1080/19440049.2016.1214289

Additional information

Funding

This work was supported by National Natural Science Foundation of China [grant number 31271925]; Special Project on the Integration of Industry, Education and Research of Guangdong Province, China [grant number 2013B090600015]; and Science and Technology Program of Guangzhou, China [grant number 2014J4100192].