For the pH-insensitive reference fluorophores a combination of 544 nm / 590 nm filters was used. The emitted fluorescence was measured after passing through a filter of 538 nm transmission maximum. The fluorescence intensity of the pH-sensitive indicator fluorophores was measured by applying an excitation filter with maximum transmission at 492 nm. The resulting layer thickness of the so created sensor was about 5 µm. The microtiter plates with sensors should eliminate these disadvantages.įor measurements with chemical optical pH sensors 10 µL of a solution containing pH-sensitive and pH-insensitive fluorophores were dropped into each well of a 96 well round bottom polystyrene plate (Greiner, Germany). The CINAC system is highly reliable but limited to 16 parallel experiments and quite labor intensive due to required cleaning and recalibration of the electrode. Results gained from the following fermentation experiments in microtiter plates with integrated pH sensors were compared with those from the established CINAC-method to verify the suitability of this new approach. Preliminary tests had shown, that the optical characteristics of the plates and especially milk had influence on the pH sensor signal, as the sensors were not optically isolated. Aim of this study was to develop a new method that allows screening of a large number of samples in microtiter plates by optical pH measurement. This acid production can be used to characterize milk fermentation and is therefore an important method to test the activity of starter cultures.
During milk fermentation organic acids, mainly lactic acid, are produced by the microorganisms lowering the pH value. Monitoring and screening starter cultures is of great importance in modern dairy technology to guarantee quality of products like e. This early study resulted in an improved SensorPlate - the HydroPlate. The new approach was compared to the established CINAC system. Differences in inoculum type and size of cultures of Lactococcus lactis and Streptococcus thermophilus could be clearly detected, even though there were disturbances caused by particles and fluorophores contained in the medium. Microtiter plates with integrated chemical optical pH sensors were tested for their suitability to monitor acid production during milk fermentation.
In this study an easy-to-use method to screen a large number of inoculum cultures as quality control for raw materials was developed. Starter cultures are used to guarantee good and reproducible quality of dairy products. Heinzle 1ġTechnical Biochemistry, Saarland University, Saarbrücken, GermanyĢDanisco Cultor Niebüll GmbH, Niebüll, GermanyģInstitute for Analytical Chemistry, Chemo- and Biosensors, University Regensburg, Regensburg, Germany