# Mechanistic Insights into Fish Spoilage and Integrated Preservation Technologies
## 3 Preservation Technologies and Optimization
### Chemical and Biological Preservation
#### 3.2.4 Antimicrobial Polymer and Biopolymer Coatings  
Antimicrobial coatings are thin films applied to fish surfaces that combine film-forming polymers, such as poly(lactic acid) (PLA), polyethylene-vinyl alcohol (EVOH), or natural biopolymers, including chitosan, alginate, and gelatin, with active agents like metal nanoparticles or essential oils to inhibit spoilage micro-organisms [22; 23].  
Synthetic polymer coatings typically incorporate inorganic nanoparticles (for example, silver or zinc oxide) into a PLA or EVOH matrix. These nanoparticles continuously release antimicrobial ions and act as an oxygen barrier. Biopolymer coatings exploit chitosan's intrinsic antimicrobial properties or embed nanoemulsified essential oils. Once applied, these coatings adhere to the fish surface, control moisture and oxygen transfer, and provide sustained release of antimicrobial compounds that disrupt microbial cell membranes and inhibit enzyme activity.  
In cod fillets, PLA films containing zinc oxide nanoparticles reduced counts of _Pseudomonas_ and _Shewanella_ by 3 log CFU/g over 10 days while remaining transparent and mechanically stable [99]. Chitosan coatings with thyme-oil nanoemulsions achieved a 2.5 log CFU/g reduction in psychrotrophic bacteria and a 50% decrease in TBARS in trout over 12 days at 4 \({}^{\circ}\)C [95]. Alginate coatings loaded with silver nanoparticles extended carp fillet shelf life by 8 days by inhibiting both aerobic and anaerobic spoilage organisms [90].  
Future research should focus on adjusting polymer molecular weight and cross-link density to fine-tune antimicrobial release rates, developing biodegradable nanocariers such as starch or cyclodextrin inclusion complexes for safer delivery, and designing multi-layer films that integrate oxygen scavengers, moisture regulators, and freshness indicators to further enhance efficacy and consumer acceptance.  
Figure 5 shows that each chemical or biological intervention offers a distinct balance between oxidative protection and microbial control. Nanoemulsions deliver the greatest shelf-life extension (\(\approx\)10 days) by forming deep, oxygen-barrier coatings rich in antioxidants, but they achieve only moderate microbial kill (\(\approx\)1.5 log CFU/g). Essential-oil treatments provide an intermediate profile, extending freshness by \(\approx\)8 days while reducing spoilage bacteria by \(\approx\)2 log CFU/g through combined radical scavenging and membrane disruption. Ozonation excels at rapid microbial inactivation (\(\approx\)3 log CFU/g) yet yields the smallest shelf-life gain (\(\approx\)7 days) due to its propensity to generate secondary oxidation products. Polymer coatings match ozonation's microbial reduction (\(\approx\)3 log CFU/g) but also deliver a stronger barrier effect, extending shelf life by \(\approx\)8 days through reduced oxygen ingress and controlled release of embedded antimicrobials. These trends highlight the importance of selecting or combining methods according to the dominant spoilage pathway: nanoemulsions for high-fat, oxidation-sensitive species; ozonation or polymer coatings for lean, microbe-susceptible fish; and essential-oil systems when balanced control of both oxidation and microbial growth is needed.  
Figure 5: Comparison of shelf-life extension and microbial reduction for chemical and biological preservation methods.