Seasonal variation in the composition of Irish manufacturing and retail milks. 2. Nitrogen fractions

Concentrations of total protein, casein, whey protein, non-protein nitrogen and urea were measured in three manufacturing and five retail milks, at 3-week intervals, over a 12-month period. Total protein concentrations in manufacturing and retail milk were similar during both the outdoor grazing (June to October) (33.4 g/kg), and indoor feeding (December to March) (30.2 g/kg) periods. Retail milk had a higher concentration of casein than manufacturing milk during both the outdoor (P < 0.001) and indoor (P < 0.01) periods (26.5 v. 25.6 g/kg and 24.1 v. 23.5 g/kg, respectively). Retail milk also had a higher (P < 0.001) casein number than manufacturing milk during both periods (79.7 v. 77.1% and 79.9 v. 77.1%, respectively). Whey protein concentration was lower (P < 0.001) in retail than in manufacturing milk during both the outdoor (4.8 v. 5.8 g/kg) and indoor (4.4 v. 5.4 g/kg) periods. While non-protein nitrogen (calculated as protein equivalent) was not affected by milk type during the indoor period (1.63 g/kg), it was higher (P < 0.01) in retail milk during the outdoor period (1.97 v. 1.91 g/kg). Urea levels were higher (P < 0.001) in retail milk during the indoor period (254 v. 229 mg/l). All the measured nitrogen fractions were present at higher concentrations in both types of milk during the outdoor period than during the indoor period.