Effects of Sample Age on the Suitability of Fresh Milk Samples for Instrumental Composition Analysis

¹ Laboratory Services Division, University of Guelph, Guelph, ON, Canada N1G 2W1
² Department of Food Science, University of Guelph, Guelph, ON, Canada N1G 2W1

This investigation was undertaken by the Ontario Agriculture and Food Laboratory Services Centre to determine whether the officially acceptable time for testing milk composition could be extended from the currently allowed maximum of 5 d to at least 6 d. Samples of milk from producers were divided into aliquots and stored at 4°C in separate vials. Each day, one vial from each sample was tested on infrared instruments. 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 µm), protein (6.5 µm), lactose (9.6 µm), and fat B (3.5 µm) and estimates for fat, protein, lactose, and lactose plus other solids were recorded. The maximum sample age at testing was 16 d, and the total number of observations was about 1220. Small but statistically significant effects of age were found both within and beyond the currently accepted testing period of 2 to 5 d. However, there was no consistent effect of age on instrumental estimates of protein, fat, lactose, or lactose plus other solids in samples less than 10 d old. Signals and estimates for lactose were most affected by age.

Key Words: fresh milk samples • storage time • instrumental analyses • composition estimates

Submitted on January 13, 1997
Accepted on July 30, 1997

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