Regional Differences in Nitrogen Fractions in California Herd Milks

Concentrations of protein calculated from total Kjeldahl nitrogen and total Kjeldahl nitrogen corrected for the nonprotein nitrogen were determined in samples of herd milk collected from dairy farm tanks in California's San Joaquin Valley (45 samples) and from the area north of San Francisco Bay (55 samples). Means for the San Joaquin Valley and North Bay Area were for protein determined from nitrogen corrected for nonprotein nitrogen 3.15 and 3.61%, for total nitrogen 522 and 598 mg/100 g milk, and for nonprotein nitrogen 28.6 and 32.6 mg/100 g. Nonprotein nitrogen was 5.5% of the total nitrogen in the North Bay Area and 5.5% of total nitrogen in the San Joaquin Valley. Regional differences were statistically significant. Protein content of milk and dairy products should be determined from Kjeldahl total nitrogen concentration corrected for the nonprotein nitrogen, and instrumental procedures, such as infrared and dyebinding, should be calibrated to actual protein content excluding nonprotein nitrogen.