Crystallization and demineralization phenomena in washed-rind cheese

The Microbial Diversity on the Surface of Smear-Ripened Cheeses and Its Impact on Cheese Quality and Safety

Smear-ripened cheeses are characterized by a viscous, red-orange surface smear on their rind. It is the complex surface microbiota on the cheese rind that is responsible for the characteristic appearance of this cheese type, but also for the wide range of flavors and textures of the many varieties of smear-ripened cheeses. The surface smear microbiota also represents an important line of defense against the colonization with undesirable microorganisms through various types of interaction, such as competitive exclusion or production of antimicrobial substances. Predominant members of the surface smear microbiota are salt-tolerant yeast and bacteria of the phyla Actinobacteria, Firmicutes, and Proteobacteria. In the past, classical culture-based approaches already shed light on the composition and succession of microorganisms and their individual contribution to the typicity of this cheese type. However, during the last decade, the introduction and application of novel molecular approaches with high-resolution power provided further in-depth analysis and, thus, a much more detailed view of the composition, structure, and diversity of the cheese smear microbiota. This led to abundant novel knowledge, such as the identification of so far unknown community members. Hence, this review is summarizing the current knowledge of the diversity of the surface smear microbiota and its contribution to the quality and safety of smear-ripened cheese. If the succession or composition of the surface-smear microbiota is disturbed, cheese smear defects might occur, which may promote food safety issues. Hence, the discussion of cheese smear defects in the context of an increased understanding of the intricate surface smear ecosystem in this review may not only help in troubleshooting and quality control but also paves the way for innovations that can lead to safer, more consistent, and higher-quality smear-ripened cheeses.

Invited review: Crystals in cheese: More than a curiosity

Scientific interest in cheese crystals extends back more than a century. However, starting around the 1970s, industry interest, and interest on the part of cheese scientists, grew dramatically as changes in cheesemaking technology and market changes caused the presence of crystals in the marketplace to increase; advanced analytical capabilities enabled new crystalline species to be identified, their origins and causative factors to be elucidated, and their contributions to cheese texture to be better understood. It is now evident that a host of organic- and inorganic-based crystals occur in natural cheeses. Some crystals form preferentially at the surface of rindless or rinded cheeses, others in the irregular openings or spherical eyes that occur within the body of some cheeses, and still others embedded within the cheese matrix. It is also evident that crystals may profoundly influence cheese texture, both as a direct consequence of their abundance, size, shape, and hardness, and as an indirect result of cascading physiochemical events initiated by crystal formation. Consumer response to increased incidence of crystals in the marketplace has been mixed. On the one hand, surface crystals of calcium lactate pentahydrate on Cheddar cheese came to be viewed quite negatively in some markets, often being mistaken for mold growth and spoilage. This triggered industry concern and led to considerable research to determine the underlying causes and to develop strategies to limit or prevent calcium lactate pentahydrate formation. At the same time, other forms of crystallization increasingly came to be viewed as positive features in the growing market dedicated to artisanal and traditional cheeses, giving rise to a bifurcated consumer response to cheese crystals that is evident today. Traditional artisanal cheesemakers perhaps have the most to gain from advances in cheese-crystal research. Traditional artisanal cheeses rely heavily on stories that are weaved around their identity to create uniqueness and add value. A challenge and opportunity for these cheesemakers in the United States and globally will be to translate the fascinating science of their cheese crystals into engaging narratives that capture the imagination, add value to their cheese, and enhance the enjoyment of their cheese by consumers.

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.