In order to diminish the application limitations of essential oils (EOs) as natural antimicrobial components in the food industry, novel antimicrobial materials were designed and prepared by immobilization of thymol derivatives on silica particles with different morphologies (hollow mesoporous silica nanoparticles, MCM-41, amorphous silica). The structural characteristics of antimicrobial materials were estimated by FESEM, FT-IR, TGA, N2 adsorption-desorption, and small-angle XRD, and the results revealed that both mesoporous silica nanoparticles maintained the orderly structures and had good immobilization yield. Furthermore, the antibacterial performance tests showed that mesoporous silica nanoparticles greatly enhanced the antimicrobial activity of thymol against two representative foodborne bacteria (Escherichia coli and Staphylococcus aureus), and the application of the antimicrobial support was tested in apple juices inoculated with E. coli. The MBC of functionalized mesoporous silica supports was established to be below 0.1 mg/mL against E. coli and S. aureus, which is much lower than that of free thymol (0.3 mg/mL and 0.5 mg/mL against E. coli and S. aureus, respectively). In addition, at a range from 0.05 mg/mL to 0.2 mg/mL, immobilized hollow mesoporous silica nanoparticles (HMSNs) can inhibit the growth of E. coli in apple juice and maintain good sensory properties during 7 days of storage.
In recent years, silica nanomaterials have been widely studied as carriers in the field of antibacterial activity in food. Therefore, it is a promising but challenging proposition to construct responsive antibacterial materials with food safety and controllable release capabilities using silica nanomaterials. In this paper, a pH-responsive self-gated antibacterial material is reported, which uses mesoporous silica nanomaterials as a carrier and achieves self-gating of the antibacterial agent through pH-sensitive imine bonds. This is the first study in the field of food antibacterial materials to achieve self-gating through the chemical bond of the antibacterial material itself. The prepared antibacterial material can effectively sense changes in pH values caused by the growth of foodborne pathogens and choose whether to release antibacterial substances and at what rate. The development of this antibacterial material does not introduce other components, ensuring food safety. In addition, carrying mesoporous silica nanomaterials can also effectively enhance the inhibitory ability of the active substance.
This research provided a novel enzyme-responsive antimicrobial carrier aiming at overcoming the volatile loss of active antibacterial components, by employing mesoporous silica nanoparticles (MCM-41) as the matrix of encapsulation and Zein as the molecular gate. Since Zein could be consumed by bacteria, Zein-functionalized MCM-41 acted as an enzyme-responsive gate and improved the controlled-release capacity. The results showed that the amount of capsaicin released from Zein-functionalized MCM-41 without bacteria was quite low compared with the essential oils liberated with bacteria. This validated that the delivery of capsaicin was hampered by Zein and the existence of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) promoted the release of encapsulated cargo. The release rate of capsaicin in Zein-functionalized MCM-41 climbed with the growth velocity of bacteria. These functions were realized in the form of controlled diffusion of essential oils encapsulated in MCM-41 by electrostatic interaction, and Zein was performed by both covalent bonding interaction and electrostatic interaction. Zein-functionalized MCM-41 was 2.4 times more effective in killing E. coli and 1.2 times more effective in inhibiting S. aureus than an equal amount of free capsaicin, and possessed a long-lasting antibacterial activity. The responsive antimicrobial material might be used as a promising preservative in the food industry for antimicrobial activity enhancement.
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