Antimicrobial evaluation of red, phytoalexin-rich sorghum food biocolorant

<div><p>Sorghum (<i>Sorghum bicolor</i>) extract is traditionally used as red biocolorant in West Africa to colour foods, among which <i>wagashi</i>, a soft cheese. This biocolorant is a source of the phytoalexin apigeninidin and phenolic acids, and users claim that it has preservative effects next to its colouring properties. If such a claim can be scientifically substantiated, it adds a valuable functional property to this natural red colorant, thereby increasing its potential applications in the food industry. Hence, the present study evaluated the antimicrobial properties of dye sorghum extracts using challenge tests in broth and <i>wagashi</i> as a model of a popular food application. The alkaline extract and hot aqueous extract were used for dyeing <i>wagashi</i> by 87.7% and 12.3% of the traders, respectively. The dyeing procedure is perceived as a preservation strategy, and is also a means to maximise the revenues. However, results demonstrated that the application of sorghum biocolorant on <i>wagashi</i> had no inhibitory effect on the growth of fungi (<i>Penicillium chrysogenum</i>, <i>Cladosporium macrocarpum</i>) and <i>Escherichia coli</i> O157:H7. Furthermore, sorghum biocolorant in broth had no effect on growth of <i>Listeria monocytogenes</i> and <i>Escherichia coli</i> O157:H7. Consequently, the commonly used extracts for colouring soft West-African cheese did not show a preservative effect. In addition, dyeing did not affect the physico-chemical properties of <i>wagashi</i>. Still, the red colour hampered visual detection of microbial growth, thus clarifying the preservative effect reported by users.</p>