Effect of Active Coatings Containing Lippa citriodora Kunth. Essential Oil on Bacterial Diversity and Myofibrillar Proteins Degradation in Refrigerated Large Yellow Croaker

Abstract

The research evaluated the effects of locust bean gum (LBG) and sodium alginate (SA) active coatings containing 0.15, 0.30 or 0.60% lemon verbena (Lippa citriodora Kunth.) essential oil (LVEO) on the bacterial diversity and myofibrillar proteins (MPs) of large yellow croaker during refrigerated storage at 4 °C for 18 days. Variability in the dominant bacterial community in different samples on the 0, 9th and 18th day was observed. Pseudomonas and Shewanella were the two major genera identified during refrigerated storage. At the beginning, the richness of Pseudomonas was about 37.31% and increased for control (CK) samples during refrigerated storage, however, the LVEO-treated samples increased sharply from day 0 to the 9th day and then decreased. LBG-SA coatings containing LVEO treatments significantly delayed MPs oxidation by retarding the formation of free carbonyl compounds and maintaining higher sulfhydryl content, higher Ca²⁺-ATPase activity, better organized secondary (higher contents of α-helix and β-sheet) and tertiary structures during refrigerated storage. The transmission electron microscope (TEM) images showed that the integrity of the sarcomere was damaged; the boundaries of the H-, A-, and I-bands, Z-disk, and M-line were fuzzy in the CK samples at the end of storage. However, the LVEO-treated samples were still regular in appearance with distinct dark A-bands, light I-bands, and Z-disk. In brief, LBG-SA active coatings containing LVEO treatments suggested a feasible method for protecting the MPs of large yellow croaker during refrigerated storage.

Keywords: active coating; bacterial diversity; large yellow croaker; lemon verbena essential oil; myofibrillar proteins structure.

Figure 1

A schematic representation of preparation procedures.

Figure 2

Rarefaction curves of 16S ribosomal RNA genes sequencing of bacterial V4 region of the large yellow croaker samples during refrigerated storage at 4 °C. A: fresh; B: CK on the 9th day; C: LYC-0.15%LVEO on the 9th day; D: LYC-0.30%LVEO on the 9th day; E: LYC-0.60%LVEO on the 9th day; F: CK on the 18th day; G: LYC-0.15%LVEO on the 18th day; H: LYC-0.30%LVEO on the 18th day; I: LYC-0.60%LVEO on the 18th day.

Figure 3

Microbial community structure variation in different stages. The relative abundance of bacteria at the (A) phylum and (B) genus level was shown. Below, the top 10 abundances at the phylum and genus levels were merged into others. Each bar represents the relative abundance of each sample. Each color represents a particular phylum or genus. A: fresh; B: CK on the 9th day; C: LYC-0.15%LVEO on the 9th day; D: LYC-0.30%LVEO on the 9th day; E: LYC-0.60%LVEO on the 9th day; F: CK on the 18th day; G: LYC-0.15%LVEO on the 18th day; H: LYC-0.30%LVEO on the 18th day; I: LYC-0.60%LVEO on the 18th day.

Figure 4

Heatmap of the changes in the microbial communities of large yellow croaker samples during refrigerated storage at 4 °C. Select the bacteria whose relative abundances were in the top 35 at the genus level obtained in this study. A: fresh; B: CK on the 9th day; C: LYC-0.15%LVEO on the 9th day; D: LYC-0.30%LVEO on the 9th day; E: LYC-0.60%LVEO on the 9th day; F: CK on the 18th day; G: LYC-0.15%LVEO on the 18th day; H: LYC-0.30%LVEO on the 18th day; I: LYC-0.60%LVEO on the 18th day.

Figure 5

Changes in free carbonyl compounds contents (A), total sulfhydryl groups (B), Ca²⁺-ATPase activity (C), and surface hydrophobicity (D) of myofibrillar protein in large yellow croaker (Larimichthys crocea) during refrigerated storage. Bars represent the standard deviation (n = 3). (CK: large yellow croaker samples were packaged with FG/SA films without LVEO emulsion; LYC-0.15%LVEO: large yellow croaker samples were packaged with LBG/SA films containing 0.15% LVEO emulsion; LYC-0.30%LVEO: large yellow croaker samples were packaged with LBG/SA films containing 0.30% LVEO emulsion; and LYC-0.60%LVEO: large yellow croaker samples were packaged with LBG/SA films containing 0.60% LVEO emulsion).

Figure 6

Changes in intrinsic fluorescence intensity (IFI) of myofibrillar protein in large yellow croaker (Larimichthys crocea) during refrigerated storage. (CK: large yellow croaker samples were packaged with FG/SA films without LVEO emulsion; LYC-0.15%LVEO: large yellow croaker samples were packaged with LBG/SA films containing 0.15% LVEO emulsion; LYC-0.30%LVEO: large yellow croaker samples were packaged with LBG/SA films containing 0.30% LVEO emulsion; and LYC-0.60%LVEO: large yellow croaker samples were packaged with LBG/SA films containing 0.60% LVEO emulsion).

Figure 7

Transmission electron microscopy (TEM) micrographs of large yellow croaker (Larimichthys crocea) during refrigerated storage. (CK: large yellow croaker samples were packaged with FG/SA films without LVEO emulsion; LYC-0.15%LVEO: large yellow croaker samples were packaged with LBG/SA films containing 0.15% LVEO emulsion; LYC-0.30%LVEO: large yellow croaker samples were packaged with LBG/SA films containing 0.30% LVEO emulsion; and LYC-0.60%LVEO: large yellow croaker samples were packaged with LBG/SA films containing 0.60% LVEO emulsion).