1. Environmental Microbial and Food Safety Laboratory, Agricultural
Research Service, ‡Food Quality Laboratory, Agricultural Research Service, and ∥Electron and
Confocal Microscope Unit, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland 20705, United States
2. Department of Nutrition and Food Science, and ⊥Department of
Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States