Microbial quality of bagged baby spinach and romaine lettuce: effects of top versus bottom sampling.
Contamination with Escherichia coli O157:H7 and Salmonella have called into question the safety and microbial quality of bagged ready-to-eat leafy greens. This study expands on previous findings that these goods have high total bacteria counts (TBC) and coliform counts,  variation in counts among different lots, that Escherichia coli is present, and disparities in counts when bags are top or bottom sampled. Nearly 100 bags of baby spinach and hearts of romaine lettuce from a single brand were subjected to both top and bottom sampling. Product was blended,  and a portion serially diluted and plated to obtain TBC. Total coliform and E. coli levels were estimated by the most-probable-number (MPN) technique with ColiComplete discs. Top-sampled TBC from bags of baby spinach (48 bags, 13 different lots) ranged from 3.9 to 8.1 log CFU/g and  bottom-sampled TBC ranged from 4.0 to 8.2 log CFU/g, with 52% of the bags (or 39% of the lots) producing TBC higher in bottom samples. For hearts of romaine (47 bags from 19 different lots), top-sampled bags had TBC ranging from 2.4 to 7.0 log, and bottom-sampled bags had TBC from 3.3 to 7.3  log, with 64% of the bags (or 63% of the lots) showing higher TBC in bottom samples. However, we are unable to reject the hypothesis that the top and bottom samples from either commodity contain the same TBC (P ≥ 0.08). No E. coli was detected and total coliform bacteria counts  were, with few exceptions, ≥210 MPN/g, irrespective of TBC. In general, lots with the most number of days before the printed "use-by" date had lower TBC. However, the R(2) values for either baby spinach (0.4085) or hearts of romaine (0.2946) suggest that age  might not be a very good predictor of higher TBC. TBC varied widely between lots and even more so within same-lot samples, as indicated by the sum of squares results. This finding, along with higher TBC in bottom samples, suggests further consideration when a microbiological sampling scheme  of bagged produce is designed.
Division of Microbiology, Office of Regulatory Science, Office of Food Defense, Communication, and Emergency Response, U.S. Food and Drug Administration, College Park, Maryland 20740, USA. julie.kase@fda.hhs.gov
Julie A
JA
Kase
Stacey
S
Borenstein
Robert J
RJ
Blodgett
Peter C H
PC
Feng
D015169:Colony Count, Microbial / Q000379:methods*; D003257:Consumer Product Safety; D019453:Escherichia coli O157 / Q000254:growth & development / Q000302:isolation & purification*; D005506:Food Contamination / Q000032:analysis*; D005516:Food Microbiology; D059022:Food Safety; D006801:Humans; D018545:Lactuca / Q000382:microbiology*; D012475:Salmonella / Q000254:growth & development / Q000302:isolation & purification*; D018724:Spinacia oleracea / Q000382:microbiology*