Abstract
The performances of three non-destructive sensors, based on different principles, bioelectrical impedance analysis (BIA), near-infrared spectroscopy (NIR) and time domain reflectometry (TDR), were studied to discriminate between unfrozen and frozen-thawed fish. Bigeye tuna (Thunnus obesus) was selected as a model to evaluate these technologies. The addition of water and additives is usual in the fish industry, thus, in order to have a wide range of possible commercial conditions, some samples were injected with different water solutions (based on different concentrations of salt, polyphosphates and a protein hydrolysate solution). Three different models, based on partial least squares discriminant analysis (PLS-DA), were developed for each technology. This is a linear classification method that combines the properties of partial least squares (PLS) regression with the classification power of a discriminant technique. The results obtained in the evaluation of the test set were satisfactory for all the sensors, giving NIR the best performance (accuracy = 0.91, error rate = 0.10). Nevertheless, the classification accomplished with BIA and TDR data resulted also satisfactory and almost equally as good, with accuracies of 0.88 and 0.86 and error rates of 0.14 and 0.15, respectively. This work opens new possibilities to discriminate between unfrozen and frozen-thawed fish samples with different non-destructive alternatives, regardless of whether or not they have added water.
Highlights
Than the frozen-thawed samples (63.80 ± 3.72%) (F = 51.28, p = 0.00), indicating a water loss phenomenon during the freezing-thawing process. Despite this water loss, the moisture/protein ratio was statistically different between the control (2.71) and injected samples (2.85) in thawed state (F = 42.32, p = 0.00), showing that added water was still present in injected samples after they were frozen and thawed
Since the calibration and validation datasets were different for each sensor, the moisture, protein and fat values of each dataset are different
The freezing process applied to fish causes the formation of ice crystals within the muscle tissue
Summary
Freezing and frozen storage has been widely used by the fish industry for extending the storage life of this perishable foodstuff. Unfrozen seafood products are still highly demanded in most countries [1], having usually higher market prices than frozen ones [2]. Due to its high price and because in most cases consumers are unlikely to perceive visually the differences after thawing, unfrozen fish has been shown to be vulnerable to adulteration and fraudulent mislabeling involving replacement by frozen-thawed products [3,4]. The addition of water and additives in fish is a common practice in the fish industry, which could be used together with the freezing and frozen storage to improve the quality of seafood products [5] and to avoid drip loss [6]. If the amount of added water is less than the 5% of the weight of the product, it is not necessary to be indicated on the label [7]
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