Evaluation of Escherichia coli contamination on fresh vegetables using fluorescence spectroscopy

Food safety becomes a major public concern in addition to the food quality. Among major bacterial diseases related to public health, Escherichia coli is most common and may cause illness or death once infected through polluted fresh-to-eat fruits and vegetables. Comprehensive inspection of E. Coli contamination on whole surface area of all fruits and vegetables cannot be conducted by current standard laboratory methods because the procedures of bacterial culture are time-consuming and labor-intensive. In this study, nondestructive fluorescence spectroscopy was applied to the inspection of E. Coli contamination on fresh-to-eat vegetables. A fluorescence spectrophotometer was used to measure the fluorescence intensity from the surface of Romaine lettuce (Lactuca sativa L var. longifolia) samples. Excitation wavelengths were set at 200-550 nm with 5 nm interval while emission wavelengths at 200-900 nm with 5 nm interval. The measurements provided a three-dimensional data cube, and could be used to study the biomaterial's surface properties by both analyses of discrimination and quantification. An excitation Emission Matrix (EEM) was established, fluorescence fingerprints were analyzed by Mahalanobis distance, and fluorescence peak points were identified. The results showed that the effective fluorescence intensity could be achieved when excitation wavelengths were at 200-300 nm and emission wavelengths at 300-400 nm. The contaminated lettuce samples could be successfully discriminated using principal component analysis (PCA) method. The successful rates of PLS-DA model were more than 80%. Multiple linear regression (MLR) analysis gave a correlation coefficient better than 0.8 for lettuces with different E. Coli contamination conditions. The work showed that fluorescence can be applied to rapid inspection of E. Coli contamination on fresh vegetables to assure the food safety.