Low field nuclear magnetic resonance (LF-NMR) relaxometry in hake (Merluccius merluccius, L.) muscle after different freezing and storage conditions

Low field NMR T-2 transverse relaxation measurements were performed on muscle samples from sixty hake (Merluccius merluccius, L). Fish fillets from hake stored in ice for 3 and 14 days were subjected to different freezing methods (air blast, liquid nitrogen or walk-in freezer) and storage conditions (-20 and -10 degrees C for 5 days, 8 and 18 weeks). Distribution analysis of T-2 data of unfrozen muscle displayed a major band (T-21), accounting for 90-92% of the total signal, with a relaxation time centred at 47-60 ms and a broad band with protons of higher mobility between 300 and 800 ms, accounting for 3-5% of the signal. Upon freezing, T-21 became wider and an extra band appeared within the range 120-360 ms. Whereas no changes were detected at -20 degrees C, the T-21 time constant decreased during frozen storage at -10 degrees C in a similar way for all three freezing methods. The relative abundance of T-21 declined with storage time but differences were found as a function of freezing. Results are discussed in the light of morphological alterations and protein denaturation, and it is concluded that LF NMR relaxometry is sensitive to different freezing and frozen storage conditions which can have important implications for the quality of fish muscle. (C) 2013 Elsevier Ltd. All rights reserved.