A single tube system was employed in the electrochemical measurement. 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 portion (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