Naho Nakazawa

Freshness estimation of intact frozen fish using fluorescence spectroscopy and chemometrics of excitation-emission matrix.

The current study attempted to provide a convenient, non-invasive and time-saving method to estimate the freshness of intact horse mackerel (Trachurus japonicus) fish in a frozen state using autofluorescence spectroscopy in tandem with multivariate analysis of fluorescence excitation-emission matrices (EEM). The extracted fluorescence data from different freshness conditions were pretreated, masked and reorganized to resolve fish fluorescence spectra from overlapping signals and scattering profiles for detecting and characterizing freshness changes. The real freshness values of the examined fish samples were then traditionally determined by the hard chemical analysis using the high performance liquid chromatography (HPLC) method and expressed as K-values. The fluorescence EEM data and the real freshness values were modeled using partial least square (PLS) regression and a novel algorithm was proposed to identify the ideal combinations of excitation and emission wavelengths being used as perfect predictors. The results revealed that freshness of frozen fish could be accurately predicted with R(2) of 0.89 and root mean square error estimated by cross validation (RMSECV) of 9.66%. This work substantially demonstrated that the autofluorescence spectroscopy associated with the proposed technical approaches has a high potential in non-destructive sensing of fish freshness in the frozen state.

Effects of Chilled Storage, Freezing Rates, and Frozen Storage Temperature on Lipid Oxidation in Meat Blocks from Cultured Bluefin Tuna Thunnus thynnus

ABSTRACT The effects of a short chilled storage period before freezing, frozen storage temperature, and freezing rate on lipid oxidation of bluefin tuna (Thunnus thynnus) meat during frozen storage were investigated. After 12-months storage, all samples had increased in peroxide value though they were less at the lower temperatures (−45 and −60°C). Peroxide values in all samples stored at −20°C increased after 3 months storage, particularly those processed and stored 51 h after harvest. The lowest increase in peroxide value occurred in the samples frozen rapidly 3 h after harvest. Vitamin E levels decreased faster during frozen storage at −20°C. There were no apparent differences in levels of triacylglycerides nor in n-3 fatty acid levels between treatments, storage periods, and storage temperatures. After 12-months storage, headspace oxidative volatiles were highest in samples stored at −20°C and lowest in those stored at −60°C. Lipid oxidation in tuna meat stored at −45°C is similar to that at −60°C, and rapid freezing rather than slow freezing should be used.

Changes in protein properties and tissue histology of tuna meat as affected by salting and subsequent freezing

The effects of salting and subsequent freezing on the physicochemical and histological properties of frozen-thawed tuna meat were investigated. Salting facilitated the microstructural recovery as indicated by the decrease or disappearance of intracellular holes. The yield of the 0.5 M and 1 M salted samples increased by 20% which was evaluated by the mass ratio of products to raw material. Morphological transformation from ice columns to spherical or ellipsoidal ice crystals was tentatively attributed to the extraction/solubilization of myofibrillar proteins, contributing to increased water-holding capacity. However, increased thawing loss and centrifuging loss after thawing were observed in the 2 M and 3 M salted samples with large ice crystals and enlarged extracellular spaces. These modifications were closely associated with the changes in protein properties. In conclusion, enhanced water-holding capacity, high yield, and good freezing stability can be achieved by optimal salting.