Effect of freezing and frozen storage on microstructure of Mozzarella and pizza cheeses

Scanning electron microscopy was used to assess the effect of aging before (2, 7, and 14 days at 7 degrees C) or tempering after (1, 7, and 14 days at 7 degrees C) freezing, and frozen storage (1 and 4 weeks at -20 degrees C) on protein matrix of pasta filata Mozzarella and non-pasta filata pizza cheeses using unfrozen samples as controls. Pores and ruptures of reticular structure were observed in frozen-stored pasta filata Mozzarella cheese protein matrix, but cracks and clumps of bacteria were found in frozen-stored non-pasta filata pizza cheese. No obvious differences were discerriable between the microstructures of pasta filata Mozzarella cheeses frozen stored I and 4 weeks. Formation of the reticular structure in frozen-stored pasta filata Mozzarella cheese progressed during tempering. Microstructure of non-pasta filata pizza cheese frozen stored for 4 weeks contained more extensive Cracking and more areas of clumps of bacteria than that was frozen stored for I week. Aging of cheese before frozen storage was considered responsible for microstructural cracking; fewer cracks were found in the frozen-stored cheese tempered I and 2 weeks, but the clumps of bacteria were still observed. (C) 2008 Published by Elsevier Ltd on behalf of Swiss Society of Food Science and Technology.