**1. Introduction**

Worldwide fisheries production and global per capita fish consumption have highly incremented in recent years [1]. The industrialization of the fish sector has brought not only a huge development but also an increase in the number of by-products generated during fish processing [2]. It is estimated that more than 70% of total fish captures are processed, of the processed products, about 50% result in solid waste and by-products [3]. These by-products are usually composed of viscera, heads, cut-offs, skin and fish that is damaged or unsuitable for human consumption [4]. Moreover, an additional source of by-products is represented by unwanted, non-targeted fish species (by-catches) that cannot be commercialized for direct human consumption [5]. These large quantities of unused fish products create serious pollution and environmental problems. Therefore, their correct reuse must become a priority for fish-processing countries and companies [6].

Most of these by-products should not be considered waste or less valuable materials [7], as they have grea<sup>t</sup> potential to be reused for higher-value applications [8]. Due to their high nutritive value, it is possible to give them a second life [7]. These secondary products

**Citation:** Nieto-Ortega, S.; Olabarrieta, I.; Saitua, E.; Arana, G.; Foti, G.; Melado-Herreros, Á. Improvement of Oil Valorization Extracted from Fish By-Products Using a Handheld near Infrared Spectrometer Coupled with Chemometrics. *Foods* **2022**, *11*, 1092. https://doi.org/10.3390/ foods11081092

Academic Editors: Marco Poian, Francesco Caponio and Antonio Piga

Received: 25 February 2022 Accepted: 8 April 2022 Published: 10 April 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

can be processed into products such as fish sausages, pâté, cakes, gelatin, soups, sauces or snacks (i.e., the consumption of small fish bones with a minimum amount of meat as snacks, which is actually done in some countries [9]).

The production of omega-3 ( ω-3)-rich fish oils represents an opportunity for valorizing fish by-products [3] and to achieve the zero-waste goal. The estimated amount of oil present in fish by-products varies from 2% to 30% of the total composition, depending on many factors, such as the fat content of the fish species and the distribution of fat in the fish body [10]. Fish oil is usually a good source of long-chain polyunsaturated fatty acids (PUFAs) [11], in particular docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) [12]. It can be reused to generate products of high added value for the pharmaceutical industry and as raw material for food supplements [13]. Therefore, the characterization of the fatty acids (FAs) profile of fish oils is essential because EPA and DHA levels determine the destination of the product and therefore its market value [12]. Such characterization is crucial when fish oil is obtained from canning industry by-products, where it is mixed with vegetable oils, which may change the oil FAs profiles, reducing the ω-3 proportion.

Nowadays, the most common technique used to analyze the FAs profile of fish oil is gas chromatography with a flame ionization detector (GC-FID) [14], a complex technique that is relatively slow and generates toxic waste [15]. Thus, a simpler and faster technique capable of providing a response in real time would allow companies to quickly assess the FAs profile of oil and determine its most convenient destination. In this sense, near-infrared spectroscopy (NIRS) represents a valid alternative to GC-FID or other more traditional methods, as it is a rapid, non-destructive technique [16,17] that has been used in recent years in industry for quality control and process monitoring [18–20]. Furthermore, recent advances have allowed for a significant reduction in the size and cost of such devices, making them suitable for on-site determination [21].

This is not the first time that NIR has been used for the evaluation of the lipid profile of fish derivates. Some authors, such as Bekhit et al. and van der Merwe et al. [22,23], have studied NIR to analyze PUFAs in ω-3 supplements. Others, including dos Santos et al., analyzed the ω-3 and omega-6 ( ω-6) content directly in fish fillets [24]. Other techniques, such as FT-NIR, were used by Karunathilaka et al. and Cascant et al. [14,25] to analyze omega-3 supplements and salmon. Only a few authors have used NIR to directly analyze fish oils [11] or used portable spectrometers [26]. Most research has been developed with big laboratory equipment and/or using processed fish pharmaceutical supplements or fish fillets, which prevents their use for this application in an actual industrial environment in the short term. More efforts are still needed to elevate the low technology readiness levels (TRLs) of such studies to be useful for the by-products industry. To the best of our knowledge, this is the first study demonstrating the scalability to industrial TRLs of NIR technology for measurement of the lipid profile of fish oil directly extracted from fish by-products.

Therefore, the principal objective of this study is to assess the potential of a portable device based on NIRS in combination with a partial least square regression (PLSR) analysis to characterize the FAs profile of fish oils in a rapid and non-destructive way. Thus, the device was not only calibrated to determine the ω-3 and ω-6 content but also to measure the complete fish oil profile, determining the saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) levels. The main objective of this work was to demonstrate the high level of maturity of a handheld NIR spectroscopy sensor in combination with chemometrics for the rapid characterization of fish oil in the fish by-product industry. This technique could enable a fast and accurate classification of processed products in the appropriate market category with economic benefits for the company and increased efficiency.
