Professor Xu Hong, Nanjing University of Technology, et al.: Preparation of ε-polylysine antibacterial coating and its application in freshwater fish preservation

1.
Nuclear magnetic hydrogen spectroscopy results of ε-PL-AOT complex

As can be seen from Figure 2, the peak at δ 8.
024 is the proton peak (D,—C—NH3⁺)
attached to amino C.
The peak at δ 2.
936 is the proton peak (B,C—CH2—N)
on the methylene group linking C and N.
In addition, the sharp splitting peak at δ 0.
858 corresponds to the methyl proton on the AOT backbone and the methyl proton (H, C—CH3)
on the branched carbon chain, respectively.
In addition, through the ratio of the area of each proton peak, such as the area of the AOT main chain carbon submethyl proton peak (2.
301) is about twice the area of the ε-PL primary amine proton peak (1.
000), it can be seen that the ε-PL-AOT complex is formed
by the reaction of the same amount of substances.

2.
ε-PL-AOT coated functionalized antibacterial plastic wrap

ε-PL-AOT is a water-insoluble complex, so it is dissolved with ethanol and sprayed on the surface of PE plastic wrap, and then dried at room temperature, and the composite coating
is formed on the surface of plastic wrap after ethanol volatilization.
As shown in Figure 3, white deposits appear on the surface of the plastic wrap treated with ε-PL-AOT coating, but it does not affect the transparency of the plastic wrap itself, and the light transmittance is better
.

3.
Infrared spectroscopy results of ε-PL-AOT coating

As shown in Figure 4, the characteristic peak of PE/ε-PL-AOT sample at 1 046 ^cm-1 was the symmetrical telescopic vibration of the S=O bond in the AOT, indicating that the charge action between sulfonate and the ε-PL ammonia ion would cause the symmetrical telescopic vibration peak of the S=O bond to be blueshifted, which confirmed that ε-PL-AOT formed a stable coating
on the surface of plastic wrap.

4.
Analysis results of ε-PL-AOT coating elements

As shown in Table 1, compared with PE plastic wrap, the percentage of N and S elements on the surface of plastic wrap treated with ε-PL-AOT coating increased significantly, and the ε-PL-AOT composite formed a stable coating on the surface of the plastic wrap through the hydrophobic force between it and PE, so the structure contained a large amount of N and S, which confirmed the successful
construction of the PE film surface ε-PL-AOT coating 。 The surface of PE plastic wrap, PE/ε-PL and PE/AOT is dominated by C element, because PE plastic wrap is composed of polyethylene, the main element composition is C element, ε-PL and AOT are water-soluble, and a stable coating
cannot be formed on the surface of plastic wrap.
The XPS spectra composed of PE/ε-PL-AOT and its control coating surface elements are shown
in Figure 5.

5.
Surface hydrophilicity of ε-PL-AOT coating

As shown in Figure 6, because PE plastic wrap itself is a hydrophobic material, the surface contact angle is 113.
7°, and the contact angle of PE/AOT and PE/ε-PL is not significantly different from PE plastic wrap, which proves that the coating is washed off during the washing process and cannot be stably combined
with the film surface 。 However, the contact angle of ε-PL-AOT coating decreased to 39.
91°, and the hydrophilicity increased significantly, mainly because ε-PL-AOT interacted with the hydrophobic surface of PE through hydrophobic force to form a stable coating, and the hydrophobic alkane chain of AOT formed a stable bond with PE molecules, and ε-PL hydrophilic molecules were exposed to the upper end of the coating, showing high hydrophilicity
.

6.
Analysis results of mechanical properties of ε-PL-AOT coated antibacterial plastic wrap

As shown in Table 2, the addition of ε-PL-AOT had basically no effect on the thickness, tensile strength and elongation at break of antibacterial plastic wrap, among which, compared with PE plastic wrap, the thickness of antibacterial plastic wrap did not change; Although the tensile strength and elongation at break decreased slightly, the overall change was small, and the mechanical properties of antibacterial plastic wrap were less
affected.
In general, after adding ε-PL-AOT to PE plastic wrap, the coated antibacterial plastic wrap still has good mechanical properties
.

7.
Evaluation of antibacterial effect of ε-PL-AOT

Antibacterial effect of ε-PL-AOT antibacterial coating

As shown in Figure 7, the number of colonies of PE/ε-PL-AOT membrane-treated samples can be visually observed with the naked eye
.
Due to the lack of antibacterial function of PE membrane, the colony numbers of Escherichia coli and Staphylococcus aureus after 24 h were 6.
78(lg(CFU/g)) and 6.
87(lg(CFU/g)), respectively.
The number of bacteria on the surface of ε-PL-AOT antibacterial membrane was much lower than that of the control group, 2.
94 (lg(CFU/g)) and 2.
73 (lg(CFU/g)), respectively, which proved that the coating had broad-spectrum antibacterial function
.
The main sterilization mechanism is contact sterilization, and when the ε-PL molecules on the coating surface contact with bacteria, it can destroy the integrity of bacterial cell membranes, resulting in changes in cell membrane permeability, leakage of bacterial contents, and then lysis and death
.

ε-PL-AOT antibacterial coating on freshwater fish preservation

As shown in Figure 8, the number of colonies in the samples of the three treatment methods can be directly observed by the naked eye, the total number of bacterial colonies in the blank control group (no plastic wrap) and PE plastic wrap treatment group is higher than 1×10⁶CFU/g, according to GB 4789.
2-2016, the total number of colonies of first-class freshness of meat products is less than 1×10⁴CFU/g, which is spoiled meat
.
The PE/ε-PL-AOT antibacterial film treatment group showed obvious fresh-keeping effect, there were no visible colonies on the plate, and the total number of colonies was less than 1×10⁴CFU/g, which met the first-class freshness standard for meat products in GB 4789.
2-2016, and the above results showed that ε-PL-AOT antibacterial coating had a good application prospect
in the field of freshwater fish preservation at room temperature.

Conclusion