SURVIVAL OF ESCHERICHIA-COLI O157-H7 AFTER FREEZING AND THAWING IN-GROUND BEEF PATTIES

Survival of Escherichia coli O157:H7 strains QA 326, and ATCC 43889, 4 3894, and 43895 after freezing (-20 degrees C, 24 h) and thawing (4 de grees C for 12 h, 23 degrees C for 3 h, or microwave heating of 700 W for 120 s) in ground beef patties was determined by reference most pro bable number (MPN), hydrophobic grid membrane filter SD-39 agar, and s orbitol MacConkey agar (SMA) spread-plating methods. Populations decre ased from 0.62 to 2.52 log(10) CFU/g, with the extent varying signific antly by strain. Strain QA 326 populations almost always decreased the most, up to 1.87 log(10) CFU/g more than the least sensitive strain. Microwave heating was the most lethal thawing treatment for strain QA 326, and 4 degrees C thawing was the most lethal treatment for strain ATCC 43894. Thawing treatments varied in relative lethality for the ot her two strains. For strain QA 326 (4 degrees C and microwave thaw tre atments) and strain ATCC 43889 (4 and 23 degrees C thawing), the enume ration method significantly affected a population decrease. The SD-39 agar method best recovered strain QA 326 while the SD-39 agar method a nd the reference MPN method best recovered strain ATCC 43889 after 4 a nd 23 degrees C thawing, respectively. The greatest difference in popu lation decrease measured by any two methods was 0.58 log(10) CFU/g. Re sults showed (i) a wide range in freeze-thaw sensitivity among E. coli O157:H7 strains, (ii) no thawing method had consistently and signific antly greater lethality, and (iii) the reference MPN, SD-39 agar, and SMA methods differed Little in ability to enumerate E. coli O157:H7.