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The direct method (quantified from the lettuce sampled after wastewater submersion) and indirect method (estimate of the concentration based on the retained wastewater on the lettuce leaf surfaces as described in Equation (3)) for determining the retention of E. coli were compared. The arithmetic mean E. coli count of the three composite samples from each lettuce type was used to represent the concentration of E. coli enumerated by the direct method. The composite samples were used in this comparison as they were considered to better represent the number of E. coli enumerated using the standard method for the microbiological examination of fresh produce employed by food regulatory agencies (FSANZ 2001; USFDA 2003). The data are shown in Table 2. There were no statistically significant differences in E. coli concentration for all three varieties of lettuce between the direct and indirect method of enumeration (p > 0.01).

Table 2

The comparison of E. coli concentration quantified by the direct method, enumerated from a 25 g composite sample of lettuce, and the indirect method, estimated from the E. coli concentration of the wastewater and the volume retained by the lettuce

Lettuce varietiesMicrobial wastewater quality (E. coli MPN/100 mL)E. coli concentration (MPN/100 g) (Mean ± S.D.)
Direct methodIndirect method
Iceberg 75.9 (n = 3) 6.7 ± 5.8 10.2 ± 3.3 
1,299.7 (n = 3) 221.3 ± 84.9 237.4 ± 57.4 
27,550 (n = 3) 2,470.7 ± 848.8 3,278.5 ± 600.4 
Cos 75.9 (n = 3) 20.3 ± 10.5 17.1 ± 2.9 
1,299.7 (n = 3) 303.3 ± 83.1 262.5 ± 15.8 
27,550 (n = 3) 5,395.7 ± 652.9 5,234.5 ± 1,360.6 
Oak leaf 75.9 (n = 3) 20.3 ± 10.5 32.0 ± 1.6 
1,299.7 (n = 3) 405.0 ± 29.8 510.8 ± 20.4 
27,550 (n = 3) 9,424.0 ± 658.2 11,470.0 ± 937.2 
Lettuce varietiesMicrobial wastewater quality (E. coli MPN/100 mL)E. coli concentration (MPN/100 g) (Mean ± S.D.)
Direct methodIndirect method
Iceberg 75.9 (n = 3) 6.7 ± 5.8 10.2 ± 3.3 
1,299.7 (n = 3) 221.3 ± 84.9 237.4 ± 57.4 
27,550 (n = 3) 2,470.7 ± 848.8 3,278.5 ± 600.4 
Cos 75.9 (n = 3) 20.3 ± 10.5 17.1 ± 2.9 
1,299.7 (n = 3) 303.3 ± 83.1 262.5 ± 15.8 
27,550 (n = 3) 5,395.7 ± 652.9 5,234.5 ± 1,360.6 
Oak leaf 75.9 (n = 3) 20.3 ± 10.5 32.0 ± 1.6 
1,299.7 (n = 3) 405.0 ± 29.8 510.8 ± 20.4 
27,550 (n = 3) 9,424.0 ± 658.2 11,470.0 ± 937.2 

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