EFFECTS OF SAMPLE AGE ON THE SUITABILITY OF FRESH MILK SAMPLES FOR INSTRUMENTAL COMPOSITION ANALYSIS

This investigation was undertaken by the Ontario Agriculture and Food Laboratory Services Centre to determine whether the officially accepta ble time for testing milk composition could be extended from the curre ntly allowed maximum of 5 d to at least 6 d. Samples of milk from prod ucers were divided into aliquots and stored at 4 degrees C in separate vials. Each day, one vial from each sample was tested on infrared ins truments. Daily testing continued until the physical properties of the sample made it impossible to introduce the sample to the instrument. Signals for fat A (5.7 mu m), protein (6.5 mu m), lactose (9.6 eta m), and fat B (3.5 mu m) and estimates for fat, protein, lactose, and lac tose plus other solids were recorded. The maximum sample age at testin g was 16 d, and the total number of observations was about 1220. Small but statistically significant effects of age were found both within a nd beyond the currently accepted testing period of 2 to 5 d. However, there was no consistent effect of age on instrumental estimates of pro tein, fat, lactose, or lactose plus other solids in samples less than 10 d old. Signals and estimates for lactose were most affected by age.