The objective of this study was to determine the effect of seasonal variation on milk composition and establish an algorithm to predict density based on milk composition to enable the calculation of season-based density conversion calculations. A total of 1035 raw whole milk samples were collected from morning and evening milking of 60 spring-calving individual cows of different genetic groups, namely Jersey, Elite HF (Holstein–Friesian) and National Average HF, once every two weeks for a period of 9 months (March–November, 2018). The average mean and standard deviation for milk compositional traits were 4.72 ± 1.30% fat, 3.85 ± 0.61% protein and 4.69 ± 0.30% lactose and density was estimated at 1.0308 ± 0.002 g/cm3. The density of the milk samples was evaluated using three methods: a portable density meter, DMA 35; a standard desktop version, DMA 4500M; and an Association of Official Agricultural Chemists (AOAC) method using 100-mL glass pycnometers. Statistical analysis using a linear mixed model showed a significant difference in density of milk samples (p < 0.05) across seasonal and compositional variations adjusted for the effects of days in milk, parity, the feeding treatment, the genetic group and the measurement technique. The mean density values and standard error of mean estimated for milk samples in each season, i.e., spring, summer and autumn were 1.0304 ± 0.00008 g/cm3, 1.0314 ± 0.00005 g/cm3 and 1.0309 ± 0.00007 g/cm3, respectively.

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The objective of this study was to determine the effect of temperature on whole milk density measured at four different temperatures: 5, 10, 15, and 20°C. A total of ninety-three individual milk samples were collected from morning milking of thirty-two Holstein Friesian dairy cows, of national average genetic merit, once every two weeks over a period of 4 weeks and were assessed by Fourier transform infrared spectroscopy for milk composition analysis. Density of the milk was evaluated using two different analytical methods: a portable density meter DMA35 and a standard desktop model DMA4500M (Anton Paar GmbH, UK). Milk density was analysed with a linear mixed model with the fixed effects of sampling period, temperature and analysis method; triple interaction of sampling period x analysis method x temperature; and the random effect of cow to account for repeated measures. The effect of temperature on milk density (ρ) was also evaluated including temperature (t) as covariate with linear and quadratic effects within each analytic method. The regression equation describing the curvature and density-temperature relationship for the DMA35 instrument was ρ = 1.0338−0.00017T−0.0000122T 2 (R 2 = 0.64), while it was ρ = 1.0334 + 0.000057T−0.00001T 2 (R 2 = 0.61) for DMA4500 instrument. The mean density determined with DMA4500 at 5°C was 1.0334 g cm −3 , with corresponding figures of 1.0330, 1.0320 and 1.0305 g cm −3 at 10, 15 and 20°C, respectively. The milk density values obtained in this study at specific temperatures will help to address any bias in weight-volume calculations and thus may also improve the financial and operational control for the dairy processors in Ireland and internationally.Keywords Density, payment, raw milk, temperature, whole milk. milk procured by dairy processing companies plays a vital role in the profitability of the business and is a key determinant of the value of milk (Lindmark-Månsson et al., 2003). A significant amount of research has been conducted globally to study the physico-chemical properties and variations in milk composition during the course of the year. Variations in composition of milk are dependent on a number of factors, such as season, lactation stage, health of cow, feeding regime and cow genetics (Heck et al., 2009;Kljajevic et al., 2018). As a result, the composition of milk and its associated functional properties can vary significantly throughout the year (Chen et al., 2014). This is particularly true where pasture-based feeding is practised, that is in New Zealand, Australia and Ireland. The associated changes in feeding pattern affect