Optical instrument for the rapid detection of microorganisms in dairy products

MicroFoss, an optical instrument capable of detecting metabolic changes due to microbial activity, was tested for the detection of various groups of microorganisms in dairy products. Optical changes in the growth medium are monitored in a semi-fluid zone that separates the liquid medium containing the sample. Raw and pasteurised milks were evaluated for total viable counts (TVC), coliform and Enterobacteriaceae. Yoghurt was evaluated for coliform, Enterobacteriaceae and yeast. The MicroFoss TVC method for both pasteurised and raw milk utilised pre-filled vials containing 8 mL of Nutrient Broth with Bromcresol purple with 2 mL of milk sample. In a comparison of the instrument TVC methodology to the standard plate count methodology, correlation coefficients R=-0.93 and R=-0.92 were obtained for raw and pasteurised milk, respectively. An advantage of using the MicroFoss method was that results are obtained in 2-11 h rather than 24-48 h, with samples with bacterial counts above the acceptable levels detecting in most cases within an 8 h shift. The MicroFoss methodology for the estimation of numbers of coliform and Enterobacteriaceae utilises pre-filled vials containing 5 mL of broth. In a comparison between the MicroFoss coliform methodology and the corresponding plate count methodology utilising Violet Red Bile Agar (VRBA), a correlation coefficient of R=-0.95 was obtained. An identical correlation coefficient of R=-0.95 was obtained for the comparison of the MicroFoss Enterobacteriaceae methodology and the corresponding plate count methodology utilising VRBA+1% glucose. In 60 pasteurised milk samples tested, only 2 samples were positive by the MicroFoss method, one of which had 1 cfu mL(-1) by the plate method while the other had <1 cfu mL(-1). Coliform or Enterobacteriaceae were isolated from the detecting vials demonstrating the higher sensitivity of MicroFoss. The MicroFoss method for the detection of the presence of yeast in yoghurt required a 1:10 dilution of the product followed by a pH adjustment. Presence of yeast in yoghurt and soft cheeses was always detected without any false positive results. Results were available in less than 40 h. (C) 2002 Elsevier Science Ltd. All rights reserved.