Rapid enumeration of microorganisms in foods by the direct epifluorescent filter technique

Abstract

Filtration of stomachered food suspensions through nylon filters (pore size, 5 .mu.m) removed most of the food debris without affecting the recovery of microorganisms; 2-10 ml of these prefiltered suspensions could be filtered in the direct epifluorescent filter technique (DEFT). The technique takes < 30 min to complete and has a lower sensitivity of <60,000 microorganisms/g for all products examined. Vegetative bacterial cells, spores, fungal hyphae and yeasts could be distinguished with the technique. For fresh meat and fish, the DEFT count of prefiltered suspensions agreed well with the plate count of unfiltered suspensions over the range of 104-1010/g (correlation coefficient of 0.91). For frozen meat and fish and frozen vegetables, the 2 counting methods had correlation coefficients of 0.87 and 0.66, respectively. The poor correlation for frozen vegetables was due to the inclusion in the DEFT count of nonviable bacteria killed by the blanching process used to inactivate enzymes. Good agreement was obtained between the prefiltered DEFT count and unfiltered plate count for cooked meats, cream doughnut and whole peppers. Possible reasons for the poor agreement between the DEFT count and plate count for certain products are discussed.