EOs are complex mixtures of volatile organic compounds (VOCs) produced as secondary metabolites in plants and frequently responsible for the characteristic odor of plants
[56]. They are characterized by two or three major VOCs at fairly high concentrations (20–70%) compared to other VOCs
[57]. Some EOs have antimicrobial and antioxidant properties and an increasing demand for natural preservatives has led to EOs as potential alternatives for antimicrobials and antioxidants
[58]. EOs have been proved to be effective antimicrobials against some foodborne pathogens including
S. Typhimurium,
E.
coli O157: H7,
Campylobacter, L.
monocytogenes,
S.
aureus, and others. Studies show that the efficacy of EOs depends on chemical structure, concentration, matching the antimicrobial activity spectrum with the target microorganism(s), interactions with the food matrix, and application method
[59].
It has been observed that some EOs show inhibitory effect on membrane integrity against the tested food-borne pathogenic bacteria
[60][61][62][63] (Figure 1). Intracellular material leakage is a general phenomenon results in cell death. The hydrophobic nature of EOs could interfere with bacterial lipid membrane resulting in increased permeability of the cell constituents
[64][65], which is in agreement with other phenolic compounds
[66][67][68][69]. So far, most studies concerning the antimicrobial action mode of EOs have been carried out on bacteria, while less is known about their effects on molds and yeast. Gram-positive bacteria are generally more susceptible than Gram-negative ones
[70]. The cell wall lipopolysaccharides (LPS) of Gram-negative bacteria can create a barrier toward macromolecules and hydrophobic compounds, preventing active compounds in EOs reaching to cytoplasmic membrane
[71]. The combinations of EOs with other natural preservatives or even other chemical ones also show positive effects.
Figure 1. (
a) Bulk EOs and different types of EO delivery systems, including nanoemulsion, liposomes, and biopolymer films; (
b) Proposed common mechanisms of action and target sites of essential oils (EOs) or EO delivery systems on bacterial cell
[72].
Plant extracts have broad application prospects in fish preservation. The antimicrobial activities of plant extracts may be attributed to the combined effects of polyphenols adsorption to bacterial membrane with membrane disruption and subsequent cellular contents leakage, and the generation of hydroperoxides from polyphenols
[73][74][75][76]. Plant extracts also show antifungal activities, antioxidant, antimutagenic activities, and inhibit lipid oxidation in food
[77][78][79][80]. Numerous studies have been done in-vitro to evaluate the antimicrobial activities of plant extracts; however, only a few studies are available for fish preservation as the antimicrobial activity of plant extracts does not produce as marked inhibition as many of the chemical preservatives in fish. The plant crude extracts generally contain flavonoids in the form of glycosides, in which the sugar presenting in them decreases the effectiveness against some food-borne pathogens
[81][82].
Natural wood smoke is a suspension of vapors, liquid droplets, and solid particles and produced by controlled wood smoldering without oxygen or at reduced oxygen levels. Different woods’ smoke have different antimicrobial properties as the woods generate different levels of antimicrobials, such as organic acids, phenols, and carbonyls during pyrolysis. Now, more than 20 different kinds natural wood smoke including redwood, black walnut, hickory, birch, white oak, aspen, chestnut, and cherry have been evaluated the antimicrobial properties against
A. hydrophila, and
S. aureus [83]. Additionally, the smoke treatment could increase redness of the fish muscle and stabilized it during frozen storage
[84]. It should be noted that wood smoke contains some harmful compounds, such as polycyclic aromatic hydrocarbons (PAHs)
[85][86]. Since the 1970s, liquid smoke has been developed and become popular resulting from the concern of potentially carcinogenic benzopyrenes
[87]. Liquid smoke preparations can be either incorporated as a surface additive during post thermal processing or a formula ingredient during batter mixing to reduce or eliminate food-borne pathogens as well as impart desired smoky flavor to the products. Many studies have been focused on the use of liquid smoke as a postlethality dip or spray treatment to reduce or eliminate food-borne pathogens on fish products. Antimicrobial efficacy of liquid smoke can be enhanced by vacuum-packaging, essential oils, and NaCl
[88][89][90][91][92]. Also, wood smoke can be converted into nanocapsules using chitosan and surface contact area could be increased, which delayed microbial growth in fish fillets at cold storage conditions
[93][94].
As natural sources of bioactive compounds, algae and mushrooms have a wide range of biological activities including antimicrobial, antioxidant, antiviral, anti-inflammatory, and other health promoting benefits
[56][95][96][97][98][99]. Among the major bioactive ingredients of algae and mushrooms with demonstrated antimicrobial activities, proteins, antioxidants (polyphenols, flavonoids, and carotenoids), polyunsaturated fatty acids, and polysaccharides are the most important ones
[100]. Until now, the antimicrobial potential of algae and mushrooms has been generally tested in vitro, providing reliable quantitative estimates of minimum inhibitory concentration (MIC) values for many samples
[100][101]. Compounds reported to be present in algae included phlorotannins, terpenoids, phenolic compounds, acrylic acid, steroids, cyclic polysulphides, halogenated ketones and alkanes, and fatty acids that act as bactericidal agents
[102]. The presence of these compounds suggests alternative mechanisms for antimicrobial action. For example, phlorotannins could inhibit the oxidative phosphorylation and bind with bacterial proteins including enzymes and cell membranes, leading to cell lysis
[103]. The mechanisms of sulphated polysaccharides and algal polysaccharides may be related to glycoprotein receptors on the cell surface of polysaccharides which bind with compounds in the cell wall, cytoplasmic membrane, and DNA, increasing the cytoplasmic membrane permeability, protein leakage, and binding of bacterial DNA
[104]. The antimicrobial activities of mushrooms may be related to a variety of secondary metabolites with biological activity, such as gallic acids, some phenols, volatile compounds, free fatty acids, and their derivatives
[105]. Considering the wide biodiversity of mushrooms, they could easily become accessible sources of natural preservatives. However, few studies have evaluated the antimicrobial activities of algae and mushrooms in fish preservation.
Saponins are natural glycosides compounds in some plants showing promising results as a broad-spectrum antimicrobial and antifungal activities
[106][107]. The antifungal activity of saponins interacts with cytoplasmic membrane sterols, the ergosterol, can provoke pores and loss of membrane integrity, resulting in cell death
[108].
Flavonoids are ubiquitous in photosynthesizing cells and are commonly found in some plant parts. They exhibit broad-spectrum antimicrobial activities due to the ability to form complexes with extracellular and soluble proteins as well as with bacterial membranes
[109][110][111]. Flavonoids have antimicrobial activities against bacteria and the hydroxyls at special sites on the aromatic rings of flavonoids improve the activity. However, the methylation of the active hydroxyl groups generally decreases the activity. The hydrophobic substituents such as prenyl groups, alkylamino chains, alkyl chains, and nitrogen or oxygen containing heterocyclic moieties usually enhance the activity for all the flavonoids
[112]. As a whole, it is necessary to further investigate their potential use as food preservatives as people are increasingly interested in finding more natural alternatives.
3.3. Animal-Derived Compounds
At present, many animal-derived antimicrobial compounds have also been used for fish preservation. Such examples include chitosan from shellfish, lactoperoxidase, and lactoferrin from milk, and lysozymes from hen eggs
[113]. However, one of the major problems associated with animal-derived antimicrobials is their allergen risk; the sources of such ingredients are often allergen-containing foods including shellfish, milk, and egg
[114].