Evaluation of unidentified structural features in hard, aged cheeses and soft, washed rind cheeses by powder X-ray diffractometry

Abstract Text: Hard, aged cheeses and soft, washed rind cheeses sometimes develop structural features that can be detected visually and texturally by mouthfeel, but which are incompletely characterized.  The objectives of this research were to evaluate visible spherical features embedded within the bodies of aged Parmigiano Reggiano and Gouda cheeses, and granular features at the surfaces of three artisanal washed rind cheeses using powder X-ray diffractometry (PXRD).  All cheese samples were purchased from local retail sources. Discolored spherical features up to ca. 5mm in diameter, which were readily visible against the darker cheese matrix, were extricated from the matrix of Parmigiano Reggiano and Gouda cheeses, defatted in acetone, ground to a powder and analyzed by PXRD.  Samples of the cheese matrix that surrounded the spheres were also prepared similarly and analyzed by PXRD for comparison.  Granular features at the surface of washed rind cheeses were scraped off, defatted in acetone, ground and analyzed by PXRD.  The resulting X-ray diffraction patterns were compared with those in a database of over one million known crystals to establish crystal identities. The discolored spherical features from both Parmigiano Reggiano and Gouda cheeses diffracted x-rays in a manner characteristic of leucine; however, the surrounding cheese matrix did not diffract x-rays.  The formation of numerous large (5 mm diameter) crystalline leucine entities may have important implications for the rheological properties of Parmigiano Reggiano and Gouda cheeses, which warrant further study. The washed rind cheeses exhibited surface grittiness that was perceptible in the mouth.  Surface scrapings from three different washed rind cheese varieties yielded x-ray diffraction patterns that were all unique from one another.  Two cheeses produced diffraction patterns that could not be identified, whereas the third displayed the presence of calcite crystals.  None of the surface scrapings contained appreciable quantities of previously documented cheese crystals such as brushite, tyrosine or calcium lactate.  Due to the outward migration of calcium in washed rind cheeses, it is possible that the unidentified diffraction patterns represent crystalline calcium salts in novel forms that have not been documented.  Based on anecdotal reports from cheesemakers and cheese mongers, we hypothesize that these surface crystals are responsible for distinctive visual and mouthfeel characteristics that appeal to consumers of artisanal washed rind cheeses.

Keywords: cheese, crystal, X-ray