Fresh-cut melon quality during storage: An NMR study of water transverse relaxation time Joana F. Fundo a , Ana L. Amaro a , Ana Raquel Madureira a , Alexandra Carvalho b , Gabriel Feio b , Cristina L.M. Silva a,⇑ , Mafalda A.C. Quintas a,c a CBQF – Centro de Biotecnologia e Química Fina – Laboratório Associado, Escola Superior de Biotecnologia, Universidade Católica Portuguesa/Porto, Rua Arquiteto Lobão Vital, Apartado 2511, 4202-401 Porto, Portugal b CENIMAT-I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal c IBB – Institute for Biotechnology and Bioengineering, Centre of Biological Engineering, Universidade do Minho, Braga, Portugal article info Article history: Received 3 August 2014 Received in revised form 10 February 2015 Accepted 3 March 2015 Available online 31 March 2015 Keywords: Fresh-cut melon storage Quality parameters Molecular mobility NMR abstract Molecular mobility is a fundamental parameter which reflects the dynamic properties of food compo- nents and contributes to food degradation reactions comprehension. Fresh-cut fruits have become an important food market segment. However, processing of fruits promotes faster its physiological deterioration, biochemical changes and microbial degradation. The purpose of this work was to use NMR methodology as a tool to evaluate fresh-cut fruit quality, during storage at refrigerated conditions. The fresh-cut melon transverse relaxation time (T 2 ) was measured for a period of 7 days of storage at 5 °C. The relationship between the obtained values, microstructure and quality parameters was investigated. In general, results show the existence of one class of water fluidity in the system, the one present in cells after processing. T 2 , a measure of this fluidity, is affected by the processing and storage time. Also, it is possible to find a close relationships between T 2 and quality parameters of total colour difference (TCD), firmness and a w . As T 2 increases TCD also increases, while firmness and a w decrease. These results highlight the usefulness of NMR methodology application in food science. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Stability of biological systems, including foods, depends strongly on molecular mobility (Roudaut et al., 2004) and water ‘‘availability’’. This availability is a manifestation of how freely water molecules can participate in reactions, namely degradation reactions (Ruan and Chen, 1998). Understanding, the integral con- cept of changes in location and mobility of water is particularly important considering that water molecular mobility profoundly influences the chemical, physical, and microbiological quality of foods (Vittadini et al., 2003). Water activity has been recognised, for a long time, as a primary guideline for safety and quality control of foods (Labuza, 1977). However, the limitation of this measurement has been expressed (Hills et al., 1996; Mathlouthi, 2001; Slade and Levine, 1991), since it is based on the assumption that foods are in its equilibrium state, being indifferent to the solute–solute and solute–water interac- tions, factors that have deep impact on food system’s reaction kinetics (Mathlouthi, 2001; Ruan and Chen, 1998). In recent years, nuclear magnetic resonance spectroscopy has evolved to become a powerful tool to probe the structure and dynamics of food constituents in solid state. Specifically, 1 H NMR has been used to investigate water dynamics and physical struc- ture of foods through analysis of nuclear magnetisation relaxation times (Fundo et al., 2014; Li et al., 2000). In these measurements, the samples are submitted to a static magnetic field and the pro- tons are excited by means of a radiofrequency pulse. The analysis of the signal emitted while the samples return to equilibrium (FID) allows the determination of the spin–lattice or longitudinal (T 1 ), and spin–spin or transverse (T 2 ) relaxation times. The latter is related with the mobility of the protons in the sample matrix (Fundo et al., 2014). This methodology has been applied in complex food systems such as crackers (Yan et al., 1996), wheat starch (Choi and Kerr, 2003), chicken meat (Li et al., 2000), carrots (Rutledge, 2001), kiwi fruit (Tylewicz et al., 2011) and even model bread crust (Chen et al., 1997). Foods and biological materials consist largely of water and macromolecules rich in protons and, since water protons are major contributors to the proton relaxation, the interactions between water and macromolecules represent the most important factors affecting the proton relaxation process (Ludescher et al., 2001). http://dx.doi.org/10.1016/j.jfoodeng.2015.03.028 0260-8774/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +351 22 5580058; fax: +351 22 5090351. E-mail address: clsilva@porto.ucp.pt (C.L.M. Silva). Journal of Food Engineering 167 (2015) 71–76 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng