In general, the rapid spoilage of fish after harvest is mainly due to different mechanisms, including (i) post mortem enzymic autolysis, (ii) microbial spoilage and (iii) oxidation of lipids.

Immediately after slaughter, the endogenous autolytic enzymes present in fish muscle become highly active and begin a proteolytic process which leads to protein decomposition and solubilization; peptides and free amino acids formed via autolysis, as well as biogenic amines formed through the action of decarboxylases, lead to fish spoilage ^[1][5].

Trimethylamine (TMA) is the main indicator of unpleasant ‘fishy smell’; it is a volatile nitrogenous base, produced post mortem in fish by the degradation of Trimethylamine Oxide (TMAO). TMA is below the detection limit in freshly caught fish, but some bacteria, such as Shewanella putrefaciens, Aeromonas spp., Enterobacteriaceae, Photobacterium phosphoreum, Vibrio spp., Micrococcus, Acinetobacter, Moraxella, use TMAO as an osmoregulant to avoid dehydration in marine environments and tissue waterlogging in fresh water by reducing TMAO to TMA, creating the ammonia-like off-flavours ^[2][4]. Other biological amines (BAs) are produced through microbial decarboxylation, including histamine, putrescine, cadaverine, spermidine and spermine ^[3][6]. Among BAs, histamine is the degradation product of histidine. Some histamine is produced by endogenous tissue enzymes in relatively small quantities, immediately after fish capture, while most histamine is produced by the bacterial flora ^[4][7]. The bacteria directly involved in the production of high levels of histamine are those possessing the enzyme histidine decarboxylase, such as P. phosphoreum, Enterobacteriaceae and Pseudomonadaceae (including Morganella morganii and Klebsiella pneumoniae, Proteus and Pseudomonas spp.) ^[5][6][7][8,9,10]. However, it is known that changes in the organoleptic characteristics of fish depend mainly on an increase in the microbial load. In particular, two groups of microorganisms can be distinguished in fish, namely, the indigenous or autochthonous microbiota and the exogenous or allochthonous microbiota. In fish from temperate or warm waters, the autochthonous microbial flora consists mainly of narrow aerobic or facultative aerobic mesophilic Gram-negative (Pseudomonas spp., Moraxella, Acinetobacter, Flavobacterium, Xanthomonas and Vibrio) and Gram-positive bacterial species (Bacillus, Corynebacterium, Micrococcus and Lactobacillus). Whereas, in cold-water fish, microbiota consists of Gram-negative species in the surface mucus (mainly Pseudomonas, Alteromonas, Photobacterium, and Shewanella) and Gram-positive species in the intestinal contents (Clostridium spp.) ^[8][9][10][11,12,13]. The exogenous microbiota consists of typically terrestrial microbial species, such as Enterococcus, Escherichia coli, Salmonella, Enterobacter, Klebsiella, Shigella and Yersinia. This type of contamination mainly affects fish living near the coast contaminated by sewage from large urban agglomerations ^[11][14]. Finally, particularly in fatty fish, chemical oxidation of lipids is a common cause of spoilage which leads to the formation of all those compounds conferring the characteristic rancid off-flavours to spoiled fatty fish. Spoilage can be also caused by lipid hydrolysis through lipolysis. Lipolytic enzymes can either be endogenous of the fish itself (present in the fish skin, blood and tissue) or can be the product of the psychrotrophic microorganism’s metabolism. Regardless the origin of the enzymes, the fatty acids formed during hydrolysis interact with sarcoplasmic and myofibrillar fish proteins, causing denaturation and texture changes ^[12][15]. Even though the spoilage causes of fish can be explained through the above-mentioned processes, in general, all these mechanisms progress simultaneously, accelerating the overall spoilage of these products.

3. Innovative Preservation Methods Applied to Fish Products

The purpose of an optimal preservation method should be counteracting the causes of food deterioration maintaining its chemical (its composition), physical (its condition), organoleptic (taste, smell and colour) and nutritional (presence of proteins, fats and carbohydrates, vitamins, mineral salts and water) properties. Therefore, besides traditional preservation methods, the great challenge of modern food technology is to develop less aggressive preservation processes, which keep the product ‘natural’, although with a lower shelf-life. Non-thermal technologies are able to significantly inactivate microorganisms in food, extend shelf-life without significant changes in sensory perception and maintain the nutritional value of the processed food ^[13][16]. Among the main non-thermal inactivation techniques studied to be applied for fish products, non-thermal atmospheric plasma (NTAP), pulsed electric fields (PEF), pulsed light (PL), ultrasound (US) and electrolysed water (EW) are described in the following sections, the benefits and the limits of each method are highlighted and the potential positive or negative effect on the quality of the treated products is underlined.

3.1. Non-Thermal Atmospheric Plasma (NTAP)

3. Innovative Preservation Methods Applied to Fish Products

3.1. Non-Thermal Atmospheric Plasma (NTAP)

In physics and chemistry, the term plasma is used to denote the state of an ionised gas. Plasma is considered the fourth state of matter, alongside the liquid, solid and gaseous states. While the presence of plasma on Earth is relatively rare (with the exception of lightning and the aurora borealis), in the Universe, it constitutes more than 99% of known matter; the upper layers of the Earth’s atmosphere (ionosphere), the outer gaseous layers of the Sun and stars and interstellar space are plasmas (natural plasmas).

A plasma can be generated artificially by supplying a gas with sufficiently high energy by means of lasers, shock waves, electric arcs, or electric and magnetic fields (glow discharge). There are two types of plasma, thermal and non-thermal atmospheric plasma (NTAP), depending on the conditions under which it is generated. Plasma generated at ambient pressure and temperature is called cold plasma (CP), atmospheric cold plasma (ACP), or non-thermal atmospheric plasma (NTAP) and it differs from thermal plasma obtained at higher powers and pressures. To generate NTAP, any type of energy (electrical, thermal, optical, radioactive and electromagnetic) can be used to ionise gases, but mainly electrical and electromagnetic fields are used. Plasma has a neutral ionised gaseous form consisting of ions, free electrons, gas atoms and molecules, as well as UV photons depending on the process parameters and the gas used ^[14][17]. It is created after exposure of the gas to an electric field created between two electrodes (cathode and anode), separated by a small distance of 1 cm. The gases mainly used for plasma generation, which can also influence its properties, are oxygen (O₂), nitrogen (N₂), carbon dioxide (CO₂) and noble gases, individually or in combination for optimum results. Oxygen seems to be more effective than the other gases due to its ability to cause greater oxidation of nucleic acids and amino acids ^[15][18]. Plasma technology has proven to be a successful tool in both the food sector (for the decontamination of abiotic food surfaces, such as packaging materials, foods in their final packaging and various food products) and medical sector ^[16][17][19,20]. In the last decade, NTAP technology has been widely used for the preservation of various food products, such as meat ^[18][21] and fresh agricultural products ^[19][22], while its use in fish and seafood is still limited ^[20][21][22][23,24,25]. The effectiveness of this technology obviously depends on many factors, such as voltage, frequency, treatment time, working gas composition (WGC), post-treatment/exposure time and sample surface area ^[21][23][24,26]. The type and concentration of reactive species (RESPE) produced, such as reactive nitrogen species (RNS) and reactive oxygen species (ROS), including ozone, peroxide, singlet oxygen and different types of nitrogen oxides (NxOy), are mainly responsible for the inactivation of microorganisms and depend on the above parameters. However, this technology is only able to inactivate microorganisms on the surface of solid food, due to its poor penetration capacity. When a food has high microbial loads that form multiple layers of bacteria on the surface, the upper layers of cells protect those below and the decontamination effect is unfortunately not complete. Three mechanisms of action have been observed for the inactivation of microorganisms: (1) the direct disruption of the membrane or cell wall, with leakage of cellular components; (2) oxidative damage to membranes or intracellular components, such as proteins and carbohydrates; (3) damage to cellular DNA. NTAP technology has recently been proposed to inactivate many common pathogens in fish products (Staphyloccocus aureus, Listeria monocytogenes, Salmonella Typhimurium and Enteritidis, Clostridium perfringens, E. coli), various spoilage and spoilage microorganisms (Pseudomonas, hydrogen sulphide-producing microorganisms, Enterobacteriaceae), including yeasts and fungi (Cladosporium cladosporioides and Penicillium citrinum), standing up as a precious additional tool for the successfully decontamination of various food and seafood products.

3.2. Pulsed Electric Fields (PEF)

PEF is an emerging technology that involves the delivery of short high-power electrical pulses (microsecond) to a product placed in a treatment chamber, confined between electrodes. The process produces modest thermal increases without causing any effect in the product. The application of an external electric field to biological cells (animal, plant or microbial) causes damage to the cell membrane. To date, a number of theoretical models have been suggested, but there is still no clear evidence of the mechanism of action at the cellular level. The most accepted theory is the electromechanical model introduced by Zimmermann et al. ^[24][46], which considers the cell membrane to be a capacitor with a low dielectric constant. Free charges of opposite polarity are present on both sides of a membrane (inner and outer), resulting in a natural transmembrane potential. Exposure to an electric field induces accumulation of charges inside and outside the cell across the membrane and thus an increase in transmembrane potential. Although the technique was known 50 years ago, PEF can be still considered an emerging technology, because its industrial applications are recent. Initially, the use of PEF in food processing has attracted great interest especially as a method to improve the extraction of specific components or improve drying efficiency ^[25][26][55,56]. The technology is, in fact, well suited for the extraction of high-value compounds from fish by-products, as it destroys only the biological cells in the food matrix with a high extraction efficiency, also compared to other methods ^[27][57]. Surprising results were recovered by using PEF to extract proteins from mussels ^[28][58], calcium and chondroitin sulphate from fishbones ^[27][29][57,59] and proteins from abalone viscera ^[30][60]. However, more recently, the use of PEF has been suggested also as a novel preservation method, due to its capacity to rapidly inactivate microorganisms, producing foods with great nutritional and sensory quality ^[31][32][33][61,62,63]. It has been also shown that using PEF in combination with other non-thermal technologies such as UV irradiation, microwaves, high-intensity light pulses (HILP) and high hydrostatic pressure (HHP) increased microbial inactivation ^[34][35][64,65].

3.3. Pulsed Light (PL)

Pulsed light (PL) is a non-thermal technology, approved by the FDA (Food and Drug Administration), which involves the emission of short flashes of light in a broad spectrum ^[36][68]. PL technology was first used in the medical field to sterilise medical devices and then in water purification processes; recently, it has also found new applications in air sanitation. In 1996, the FDA approved the use of PL technology for food production, processing and handling processes ^[37][69]. It is recommended to use the xenon lamp with surface emission of wavelengths between 200 and 1100 nm, with a cumulative treatment not exceeding 12 J/cm² and a pulse width not exceeding 2 ms. In the food industry, pulsed light technology is mainly used for ready-to-eat products, meat and fish products, or dairy products, which are subject to rapid spoilage and require delicate preservation measures. The decontamination effect of PL treatments is mainly due to the photochemical changes caused by UV-C radiation on microbial DNA, in combination with the photothermal and photophysical damage caused to cells by water vaporisation and membrane destruction ^[38][70]. The effectiveness of this method has been recognised against Gram-positive and -negative bacteria, as well as fungal spores, and the lethal effect is greater than UV treatment applied in continuous. In particular, it has been shown that Gram-positive bacteria are more resistant than Gram-negative bacteria and fungal spores show higher resistance than bacteria ^[39][71], although Gómez-López et al. ^[40][72] reported opposite results. However, each microorganism has a different sensitivity to treatment and this may be related to differences in the composition of the bacterial cell wall and their protective and repair mechanisms against damage ^[39][71]. The potential of pulsed light treatment depends on many factors, such as the exposure time, variations in the power of the UV source (which affects the electromagnetic wavelength), the presence of particles that can protect microorganisms from UV and the ability of the microorganisms to resist the radiation during exposure. High power, long treatment time and the closer distance between target and flash lamp cause an increase in microbial reduction but a consequent loss of quality, so it is necessary to find the optimal treatment conditions to improve microbiological safety without affecting food quality ^[41][42][73,74].

3.4. Ultrasound (US)

Ultrasound is one of the innovative non-thermal techniques that is proving to be very successful in the food sector where it is actually used for freezing, cutting, drying, homogenisation, degassing, foaming, filtration and extraction processes. More recently, it has been also proposed as an alternative to heat treatments to control microbial growth ^{[43][44][45][46]}[76,77,78,79]. Ultrasonic waves used in the food industry are low energy, high frequency (16–100 kHz) waves. Any type of system used for US production consists of three parts: (1) a current generator that supplies electricity at the desired frequency to the transducer; (2) a transducer or converter, which converts electrical energy into mechanical vibrations (pressure waves) that are conveyed into a probe; (3) a probe that amplifies the vibration produced forming the sonication site that can be continuous or discontinuous. Typical ultrasonic systems are the ultrasonic bath, ultrasonic probes, parallel vibrating plates and radial vibrating systems. The mechanism behind sonication is the well-known phenomenon of cavitation, i.e., the repeated creation of microbubbles inside a liquid, followed by their implosion. The pressure resulting from these implosions causes the main bactericidal effect of ultrasound, which consists of a thinning of cell membranes, localised heating and production of free radicals ^{[43][44][45][46]}[76,77,78,79]. The effectiveness of the treatment depends on several factors, such as type of microorganism treated, amplitude of the ultrasonic waves, exposure/contact time, volume and composition of the food to be treated and temperature of the treatment. The literature reports that Gram-positive cells are more resistant to ultrasound than Gram-negative cells and this may be due to the structure of the cell wall. In addition, vegetative cells are more susceptible to bacterial spores. To make the action of ultrasound on microorganisms more effective, sonication is often combined with other treatments. It is common to use mild heat treatments (thermo-sonics), high pressures (mano-sonics), or both (mano–thermo-sonics) ^[47][80].

3.5. Electrolysed Water (EW)

Among the relatively new proposals, electrolytic water (EW) is attracting interest as a non-thermal technique in the food industry and agriculture. Similar to all the innovative methods mentioned above, EW is safer and more effective than traditional chemical agents, to which microorganisms are becoming increasingly resistant. In fact, it is considered as a new non-thermal and environmentally friendly sanitiser. The antimicrobial activity of EW has been widely demonstrated against various food-borne microorganisms, such as Pseudomonas aeruginosa ^[48][49][89,90], S. aureus ^[49][90], E. coli O157: H7 ^[50][91], S. Typhimurium ^[51][92], L. monocytogenes ^[50][52][91,93], Campylobacter jejuni ^[52][93] and V. parahaemolyticus ^[53][94]. It is also effective against spores, fungi and viruses present in food, environment and food processing plants. The antimicrobial activity and mechanism of action of EW against bacteria are not yet fully described. However, it is known that chlorine and reactive oxygen can break down the microbial cell membrane and cause oxidative DNA damage. EW has various applications in the food industry; one of the most significant is in the seafood industry, although limited effectiveness in decreasing the bacterial load on seafood at room temperature has been demonstrated. The treatment of fish products with various types of EW highlights great results for microbiological quality, but also good results in inhibiting pH changes, formation of total volatile basic nitrogen (TVB-N) and activity of the enzyme polyphenol oxidase (PPO).