Sample preparation
Samples (75) were prepared by mixing various commercially-available milk, including evaporated, containing 0%, 0.5%, 1.5%, 2.0%, 3.2% and 7.5% of fat. When necessary, small quantities of water were also added. Concentrations of 0.5–3.9%, 2.2–4.9% and 3.4–7.0% were obtained for fat, protein and carbohydrates, respectively (a detailed composition of the samples, determined using the reference IR method, is presented in Table S1 of supplementary material). Each sample was divided into 3 portions and quantified by three independent analytical methods i.e. Raman, ATR and reference IR. Spectroscopic measurements were performed without any physical or chemical pretreatment of samples, except for maintaining the samples at 40 °C.