A single tube system was employed in the electrochemical mea- surement. 75 mM Lactate and 5.5 mM NAD + (Nicotinamide adenine dinucleotide) were added as substrates. An aliquot (20 ml) of raw milk sample was separated by centrifugation during 20 min at 20 °C into 3 fractions: butterfat, milk serum, and casein proteins phases respectively. Then an aliquot (1 ml each) of the middle por- tion (milk serum containing LDH) was transferred into the reaction cell to react with the substrates. After the optimized experimen- tal condition was determined, the electrochemical sensor was per- formed at 42 °C, 0.5 V, with an adjustment of pH value to 9.0 ±0.1