Proficiency Testing (PT) is an external assessment program to measure the performance of a laboratory. Freeze drying is a well-known... Show moreProficiency Testing (PT) is an external assessment program to measure the performance of a laboratory. Freeze drying is a well-known dehydration process to produce food products or bio-products with high quality. In this study, freeze dried PT samples containing bacterial strains were prepared and examined for retention of the viability of the bacteria. Ground beef and skim milk were inoculated with bacteria related to food safety, either with or without cryoprotectants, and then freeze dried. The effects of trehalose solution, milk-based mixture, and microbial freeze drying buffer on lyophilized E. coli strains recovery in ground beef were determined. The freeze dried samples were packaged in heat sealable polyethylene terephthalate bags, and stored at either 24 °C or 5 °C in dark. The viability of the freeze dried bacteria was tested during storage. Gram-positive bacteria survived better than Gram-negative bacteria during lyophilization and storage. Skim milk provided better protection of bacteria than ground beef during lyophilization and storage. None of the cryoprotectants examined improved survival of E. coli strains in ground beef during lyophilization and storage. The viability of a cocktail of Gram-positive and Gram-negative bacteria freeze dried in skim milk was stable over 60 days storage at 5 °C in dark. M.S. in Food Process Engineering, May 2014 Show less
