We found from our previous studies on L - lactate sensor that to obtain a reliable analysis with the sensor a regular evaluation of the sensor performance is ...
L· IACTATE ELECTROCHEMICAL BIOSENSOR: PERFORMANCE EVALUATION AND THE APPLICATION IN MONITORING OF MILK MICROBIAL ATTACK Nwosu,T . N., Department of Chemistry , Obafemi Awolow o University,Nigeria. Moscone, D., Palleschi, G., and Mascini, M. Department of Science and Chemical Technologies, 11 University of Rome , Rome , Italy. W e found from our previous studies on L - lactate sensor that to obtain a reliable analysis wit h the sensor a regular evaluation of the sensor performance is essential(l, 2 ) Thi s observation is perhaps applicable to most sensors. Whe n this is related to cases where accurate and rapid lactate determination is critical, this need assumes a greater importance. In this article we report the procedure for the evaluation of L - lactate performance including a highlight on the potential of the sensor for monitoring milk microbial attack. The sensor calibration curve was evaluated in two media: in phosphate buffe r solution and in dilute milk sample containing calibrated L- lactate standard. By calibrating in milk sample medium , the calibration curve so obtained is considered to be representative of practical situation. The limits of detection of the sensor was established by extensiv e calibration in finely calibrated lactate standard solutions (0.001M calibration ). The upper and low limits of detection were found to be 0.002mmol/ l and 25mmole/ l respectively. The period of contact with sample solution after which the sensor signal begins to drift was evaluated and was found to be 45 - 50minutes. Interference effect s of commo n components of cow milk e.g, ascorbic acid, lactose, metal ions (Ca++, P042-) were studied and at the milk dilution factor of 1:200 no significant effect from any of these species were observed. pH effect was also negligible. Reproducibility and recovery studies were conducted in buffe r and milk sample randomly spiked wit h standard lactate. Percent recovery were calculated in two ways viz : from standard calibration curve and by comparing signal to concentration ratio of standard and unknow n lactate sample giving percent mean recovery error of +3% and -4% respectively. Storage conditions under which the sensor performs optimally were studied. Storage in phosphate buffe r solution, pH 7.4 in a refridgerator proved the best over all. Other characteristics of the sensor evaluated include response tim e and linear range of response. The normal level of lactate in fresh cow milk is about 1 - 2mole/l. Bacterial attack on milk material takes place at elevated temperature ( > 37oC ) and or at increased pH (< 8) converting some of the milk lactose to lactic acid. The application of L- lactate sensor for monitoring milk microbial attack was demonstrated in this study by changing these two parameters i.e, temperature and pH and monitoring the change in lactic acid produced with the sensor. Arising from the evaluation studies, some suggestions are putforward on how to improve the sensor performance during use. REFERENCES 1. R. Pilloton, T. N. Nwosu and M. Mascini, Anal. Lett. No. 21 (1988) 727. 2. R. Pilloton, T. N. Nwosu, M. Mascini, D. Mascone, and G. Palleschi, Proc. Nat. Conf. on Catalysis. Roma,(1988) 269-285. -249-
