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ABSTRACT
The objective of this work was to characterize a novel appearance defect found in Cheddar cheese, heretofore referred to as black spot defect (BSD), and to determine an etiology. Uniformly distributed throughout the cheese mass, BSD appears as small spherical black spots from 0.20 to 4.7 mm in diameter and at an average frequency of about 2 spots per kg of cheese. To date, BSD has only been found in aged Cheddar cheese. Selected elemental analysis found the BSD region in cheese to have average concentrations of the element bismuth of approximately 400 µg/g, representing an approximately 2,500-fold increase over native levels of bismuth in cheese. Transmission electron microscopy analysis of the BSD region revealed amorphous solid structures and one-dimensional hair-like structures, neither of which was present in non-BSD regions. Such amorphous "nanorod" structures can be formed by the crystallization of bismuth III sulfide and are proposed to be a source of black discoloration. We hypothesize that localized bismuth salts entrained within the cheese curd react with hydrogen sulfide generated during aging to generate bismuth III sulfide. We further propose that the presence of localized bismuth salt precursor results from residual levels of a commercial intramammary teat sealant containing bismuth subnitrate that becomes unintentionally entrained within the cheese milk.
Key words: cheese, black spot, Cheddar, teat sealant
A recently discovered visual defect in cheese has caused some concern within the cheese manufacturing industry. Although not apparent until some degree of cheese aging has transpired, this defect, heretofore referred to as black spot defect (BSD), can result in a dramatic reduction in the value of affected cheeses. The Dairy Grading Branch of the USDA Agricultural Marketing Services has deemed cheese with BSD as follows, "No U.S. Grade Assigned Due to Foreign Material," making the product ineligible for either USDA grade certification or purchase under a government procurement program. Black spot defect has not been observed in cheese milk or in younger cheeses, whereas aged Cheddar cheese appears to be most susceptible. Although this work does not include specific studies to suggest a time of onset for BSD, based on the history of dozens of contributed samples received, we speculate that it takes several months for BSD to become manifest in a typical Cheddar...