Effect of edible active film of chitosan containing fish protein hydrolysate (FPH) on chemical and microbial properties of rainbow trout (Oncorhynchus mykiss) fillets during the refrigerated storage

Following extensive research on antibacterial and antioxidant properties of chitosan and hydrolyzed proteins and their satisfactory results, the use of these compounds as natural preservatives and good alternative to antibacterials and synthetic antioxidants in various nutrients is essential. The aim of the present study was to investigate the properties of chitosan coating containing FPH in the preservation of rainbow trout (Oncorhynchus mykiss) fillets at refrigerated temperatures.

The hydrolyzed protein powder (FPH) used in this study was produced by enzymatic hydrolysis of frame (skeleton with the meat attached to it) of common carp (Cyprinus carpio) with flavourzyme enzyme. Accordingly, this powder was added to the chitosan coating (2% w/v chitosan + 2% w/v FPH). In order to investigate antibacterial and antioxidant properties of chitosan coating containing FPH, rainbow trout fillets were coated with chitosan (treatment 2) and chitosan containing FPH (treatment 3). Then, these sample treatments and control (treatment 1) were subjected to chemical (PV, TVN-B, TBA, FFA and pH) and microbial (count of aerobic mesophilic and psychrophilic bacteria) tests on days 0, 4, 8, 12, 16 and 20 in refrigerated storage. This study was implemented in form of completely randomized design and data were analyzed by one-way ANOVA and significant differences between the means were tested by Duncan's test at 95 confidence level.

According to the chemical tests, TBA, TVN-B and FFA indices showed an increasing value during the refrigeration period significantly (P<0.05) while their trend was lower in treatment 3 compared to the treatments 1 and 2. TBA index for treatments 1, 2 and 3 in day 0 was 0.017, 0.015 and 0.014 mg MDA/kg fillet respectively that this amounts reached to 1.49, 0.99 and 0.52 mg MDA/kg in day 20. At the beginning of the preservation period, TVN-B index was calculated 13.36, 13.18 and 12.46 mgN/100gr fillet for treatments 1, 2 and 3, respectively. But these values changed to 43.36, 30.19 and 22.11 mgN/100gr fillet for mentioned treatments at the end of preservation period. FFA index was 0.16, 0.14 and 0.12 percentage of oleic acid for treatments 1, 2 and 3 in day 0 whereas after 20 days of storage, this index increased to 2.55, 1.76 and 0.98 percentage of oleic acid for mentioned treatments respectively. The PV index was significantly less in treatment 3 compared to the treatments 1 and 2 in days 12, 16 and 20 (2.72, 4.42 and 4.12 meq o2/kg lipid respectively) but continuous incremental trend was not recorded in this index with increasing preservation time, even the end of the experimental period (day 20), the index decreased in all of treatments compared to the 16th day. The results of pH changes showed the stability of this index in treatment 3 during the preservation period (pH~6.30). Meanwhile, in day 12, 16 and 20, the pH of treatment 3 was significantly less than treatments 1 and 2 (p<0.05). The bacterial load count of aerobic mesophilic and psychrophilic bacteria in treatments (while having an increasing trend during the preservation period) showed that in day 8, 12, 16 and 20, the bacterial levels of treatment 3 were significantly less than treatments 1 and 2 (p<0.05). In this study, adding FPH produced from common carp fish (with degree of hydrolysis 15.9%) to chitosan resulted in enhanced antioxidant and antibacterial properties of chitosan coating. So that, the film obtained from the combination of chitosan and FPH was much stronger barrier against lipids oxidation and bacterial proliferation in rainbow trout fillets (at refrigerated temperatures) than pure chitosan film.