Molecular mobility changes after high-temperature, short-time pasteurization: An extended time-domain nuclear magnetic resonance screening of ewe milk

Abstract

This study presents an extensive investigation on the effect of pasteurization on raw whole ewe milk. Milk samples have been analyzed, throughout lactation (from February to July), by time-domain nuclear magnetic resonance (TD-NMR), collecting the characteristic TD-NMR relaxation parameters, proton longitudinal and transverse relaxation times (¹H T₁ and T₂). Collected data aim at integrating previous NMR works, mainly focusing on dairy model systems (casein and whey proteins solutions and gels, reconstituted skim milk) and cheese, with specific reference to the effect of heat treatments. Whole ewe milk, from a single flock (Sarda sheep breed), was daily analyzed both as untreated (raw) and heat treated with a laboratory-scaled high-temperature, short-time treatment (72°C for 15 and 20 s). Moreover, molecular dynamics in milk were investigated by TD-NMR in different periods of lactation for the first time. As a consequence of high-temperature short-time treatment, ¹H T₁ and T₂ consistently shifted to lower values with respect to raw counterparts. Statistical analysis indicated a significant decrease of T₂ in treated samples, to an extent dependent on the heat treatment duration. A subset of dedicated experiments demonstrated that the observed T₂ shift is largely ascribable to protein molecular rearrangements and, to a lesser extent, to the interaction of fat globules with proteins or other nonfat components (or both). In light of the crucial importance of detecting the application of a heat treatment to milk, the results reported here suggest TD-NMR relaxation parameters were able to describe heat-induced changes in molecular dynamics and interactions of milk components in a water-rich environment. The use of TD-NMR can be considered a potential suitable technique for quality control and assurance practices in the dairy industry. Upon statistical validation of methods, the application of TD-NMR in the dairy industry would take advantage of its low cost, reliability, and robustness.

Keywords: heat treatment; sheep milk; time-domain nuclear magnetic resonance.