Is Thermus thermophilus the causal agent of pinking in cheese?
“Pinking” the development of a red / pink colour in many cheese-types has been known for many years and has been the attention of significant research interest for decades. Much of this research has been inconclusive although some association with lactobacilli has been suggested.
Recently Quigley et. al. (2016) used shotgun metagenomic sequencing to reveal the presence of bacteria corresponding to three phyla, Firmicutes, Actinobacteria, and Deinococcus-Thermus in 8 ‘continental’ cheeses exhibiting pinking. The phylum Deinococcus-Thermus was absent in control cheese.
Bacteria from the Deinococcus-Thermus grouping have the ability to produce carotenoid pigments and were suspected of involvement in the pinking phenomenon.
The authors' published some results of Raman Spectroscopy of pink areas from the defective cheeses (Figure 1) showing that the red and pinks areas associated with the defect corresponded to peaks of carotenoid compounds.
Figure 1. Results of Raman Spectroscopy of pink areas of defective cheeses. From Quigley et. al. (2016).
The red areas were found to be associated with peaks characteristic of lycopane (perhydro-transformed carotenoid from lycopene) which was absent from non-pink regions from the same cheese. The pink layer was also shown to be associated with a carotenoid salt.
The authors isolated Thermus thermophilus, a Gram-negative, non-pathogenic thermophile, often found in hot water from the cheese. I am not aware that T thermophilus has been identified in cheese before and is an unusual bacterium in many respects e.g. it has multiple copies of its genome that may help confer its extreme resistance to high temperatures. The concentration of T . thermophilus in cheese was surprisingly low, around 103 CFU/g on Castenholz medium.
Cheese manufactured with milk inoculated with the T. thermophilus isolate gave cheese with the pink defect suggesting that T. thermophilus was the causal agent of the pinking defect.
It will be interesting to see if further studies of this defect using growth media can confirm that T . thermophilus is the sole causal agent of the pink / red colour defect.
Quigley L., O’Sullivan D.J., Daly D., O’Sullivan O., Burdikova Z., Vana R., Beresford T.P., Ross R.P., Fitzgerald G.F., McSweeney P.L.H., Giblin L., Sheehan J.J., Cotter P.D.. (2016). Thermus and the pink discoloration defect in cheese. mSystems 1(3):e00023-16. doi:10.1128/mSystems.00023-16. This article can be downloaded from http://msystems.asm.org/content/msys/1/3/e00023-16.full.pdf at no charge.