MINERALS IN FOOD: IDENTIFICATION OF CRYSTAL PHASES ON THE SURFACE OF WASHED RIND CHEESE

Crystals in cheese are known to affect the quality of cheese, but little work has been undertaken to characterize those crystals. Crystals of ikaite (CaCO₃•6H₂O) and struvite (NH₄MgPO₄•6H₂O), not previously (ikaite) or rarely (struvite) identified in food products, were observed in the surface biofilms of washed rind cheeses. The biofilm is a 1 mm thick layer containing yeast, mold, bacteria, and extracellular material that is characteristic of washed rind cheese; these cheeses are periodically scrubbed with a brine to encourage biofilm growth. Two varieties of Vermont washed rind cheese were sampled. The biofilms and the underlying cheese were examined by petrographic microscopy (PM) and powder X-ray diffractometry. Crystals were observed by PM throughout the biofilm but not elsewhere in the cheese wheel. The biofilm of one cheese contained both ikaite and struvite in comparable quantities, whereas the other cheese biofilm contained only ikaite. Single crystal X-ray studies of these crystals yielded superior crystal structure determinations.

Struvite frequently occurs in wastewater treatment plants where it forms a scale on the interior of pipes. In the cheese biofilm, most of the necessary ingredients for struvite formation (PO₄, NH₄, and high pH) are present; however Mg concentrations tend to be low. Consequently, additional Mg may be introduced in the wash brine, thereby encouraging struvite formation. Ikaite is a cold-water marine mineral that is metastable and slowly decomposes to calcite or vaterite when removed from the formation environment; this phenomenon was observed when ikaite crystals were removed from the cheese biofilms. It has been assumed that ikaite requires sub-zero temperatures and elevated pressures to form at the expense of more stable calcium carbonate phases. The cheeses in this study were cave-aged at approximately 10°C and stored at 4°C after packaging, and at no point in the cheese aging and storage process were the putative P & T conditions for ikaite formation met. The presence of these mineral phases in cheese biofilms is noteworthy because it represents the first time that they have been identified in cheese and because they affect the quality and consumer acceptability of washed rind cheese. Minerals in food products represents an important new area of study.