A survey of nitrate contents in Indonesian milk by enzymic analysis

In this paper, a rapid and simple enzymic method is described for the determination of nitrate in 32 fresh and five dry Indonesian milk samples, deproteinized by Carrez reagents. Interference from albumin, casein, lactose and chloride ions was controlled. The calibration graph was linear over the range l-12.5 micrograms/ml NO3-; r = 0.9998. The limits of detection and quantification were found to be 0.45 micrograms/ml NO3- and 1 microgram/ml NO3- respectively. Standard nitrate solutions (10 micrograms/ml NO3-) were used to evaluate the precision. The results showed an average of 10.1 micrograms/ml, a standard deviation of 0.3 and a relative standard deviation of 3.4%. Adequate agreement was found between results obtained by the enzymic method and those of the French official reduction/photometric reference method (AFNor). Good recoveries (100% +/- 5%) were found for nitrate added to milk. The nitrate levels were in the range 1-2.6 mg/kg NO3- for fresh milk and 1.1-18 mg/kg NO3- for dry milk. All the results are in good agreement with those previously published for UK and American milk.