Hot topic: Black spot defect in Cheddar cheese linked to intramammary teat sealant
The aim of this study was to characterize a novel appearance defect in Cheddar cheese, referred to as the black spot defect (BSD), and identify its potential cause. BSD is characterized by the presence of small spherical black spots, ranging from 0.20 to 4.7 mm in diameter, distributed uniformly throughout the cheese mass. On average, about two spots per kilogram of cheese are observed. To date, BSD has only been identified in aged Cheddar cheese. Elemental analysis revealed that the BSD regions of the cheese contain elevated levels of bismuth, approximately 400 µg/g, a concentration that is about 2,500 times higher than the native bismuth levels typically found in cheese. Transmission electron microscopy of the BSD regions revealed the presence of amorphous solid structures and one-dimensional, hair-like formations—features not found in non-BSD areas. These “nanorod” structures are likely formed through the crystallization of bismuth(III) sulfide and are proposed to be the source of the black discoloration observed. Based on these findings, we hypothesize that bismuth salts, which may be entrained in the cheese curd, react with hydrogen sulfide produced during aging to form bismuth(III) sulfide. We further suggest that the presence of these bismuth salts may be due to residual bismuth subnitrate from a commercial intra-mammary teat sealant, which could unintentionally be incorporated into the cheese milk.