Characterization and identification of surface crystals on smear-ripened cheese by polarized light microscopy

Surface crystallization and radial demineralization of Ca, P, and Mg occur in smear-ripened cheese. Furthermore, crystals of ikaite, struvite, calcite, and brushite have been identified in cheese smears by powder X-ray diffractometry (PXRD), and ikaite and struvite exist in smears as single crystals. Polarized light microscopy (PLM) is a simple, inexpensive, and well-established method in geology to detect and identify single crystals. However, use of PLM to identify cheese crystals has not been reported previously. The specific objectives of this research were (1) to identify crystals in cheese smears using selected PLM criteria; (2) to compare identification by PLM against PXRD; and (3) to develop and evaluate a novel treatment for smear material to improve crystal analyses by both PLM and PXRD. Duplicate wheels of 4 cheeses produced by different manufacturers were obtained from retail sources. Scrapings of surface smears were prepared and analyzed by PLM and PXRD by previously described methods. Crystals were categorized by PLM based on angle of extinction (AE), birefringence behavior under crossed polarizers and quartz filters, and size and shape (circularity) by image analysis. Crystals observed by PLM fell almost exclusively into 2 readily differentiated groups based on birefringence behavior and estimated angle of extinction. Group 1 (n = 18) were highly birefringent with AE = 88-92°, whereas group 2 (n = 28) had no birefringence with AE = 13-26°. Group 2 crystals were significantly larger and more circular than group 1 crystals. Group 1 and 2 were identified as struvite and ikaite, respectively, based on known birefringence and AE characteristics. Struvite was identified in all 4 cheeses by PLM but in only 3 cheeses by PXRD. Ikaite was identified in 3 cheeses by PLM but in only 2 cheeses by PXRD. These discrepancies occurred because the smear scrapings from 1 cheese contained excessive amorphous matter that caused extreme background noise, potentially obscuring diffractogram peaks that may have been present. To minimize noise, smear scrapings were dispersed in aqueous NaOH (pH 10) before analyses, which resulted in consistent results by PXRD and PLM. The method also rendered high-quality images by PLM. Data suggest that PLM may offer a simple and inexpensive means to identify struvite, ikaite, and possibly other single crystals in cheese smears.