Technical note: Milk composition in mice-Methodological aspects and effects of mouse strain and lactation day

Analysis in individual mouse milk samples is restricted by small sample volumes and hindered by high fat contents. Miniaturized methods were developed for the analysis of dry matter (DM), crude fat, crude protein (CP), and lactose in individual samples of <= 200 mu L of fresh or previously frozen mouse milk and used to compare milk from the mouse strain DU6, the largest growth-selected mouse line worldwide, with unselected mice (CON) on lactation d 3, 14, and 18. Individual milk samples were collected by means of a self-constructed milking machine. Aliquots of 10 mu L of milk were used to measure DM [ coefficient of variation (CV) < 2.1%], which was subsequently used to analyze nitrogen for calculation of CP (CV 2.7%). Crude fat was determined in 100 mu L via a miniaturized Rose-Gottlieb method (CV 2.8%). An HPLC protocol was used to analyze lactose in 20 mu L of diluted whey (CV 5.3%). The miniaturized methods gave similar results compared with conventional approaches. Homogenization was the most important factor affecting milk composition and its reproducibility. Milk storage at -20 degrees C had no effect on composition. Irrespective of the mouse strain, maximum values of 45.5% DM, 29.8% fat, and 12.7% CP were observed at d 14. The greatest lactose contents were found on d 18 (2.41%). Milk lactose concentration at d 3 was lower in DU6 (1.13 +/- 0.10%) than CON (1.67 +/- 0.18%). The method provides an accurate assessment of mouse milk composition.