טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKrepker Maksym
SubjectDevelopment and Characterization of Polymer Systems with
Antimicrobial Activity
DepartmentDepartment of Biotechnology and Food Engineering
Supervisor Professor Ester H. Segal


Abstract

Significant research is directed toward the development of active food packaging in which naturally-derived antimicrobials, such as essential oils (EOs), are incorporated into polymers. EOs offer a unique combination of a broad and potent antimicrobial activity, a Generally Recognized as Safe (GRAS) status in the U.S, and volatile nature. The latter presents a major challenge in their incorporation into polymers by conventional high-temperature processing techniques, and also result in undesirable organoleptic impact. Moreover, interactions of EOs with various food components may interfere with their antimicrobial activity and require higher EO dosages. Consequently, the challenge is preserving the EOs during the high-temperature polymer processing, and minimizing EO concentrations to reduce their organoleptic effect, while maintaining high antimicrobial activity.

This study presents an innovative approach for overcoming these challenges by using Halloysite nanotubes (HNTs), naturally-occurring hollow tubular clay nanoparticles, as functional carriers for EOs and exploiting synergistic interactions between several EOs. The HNTs entrap the EOs, protect them and allow their preservation during high-temperature compounding processes, while synergistic combinations of EOs enhance their efficacy at lower concentrations minimizing undesired organoleptic effects. Carvacrol, used as model EO or its synergistic mixtures with thymol, is loaded into the HNTs via shear mixing and ultrasonication processes. This step imparts enhanced thermal stability to the sensitive EOs and allows its high-temperature melt compounding with different polymers i.e. low-density polyethylene (LDPE) and polypropylene (PP). The LDPE nanocomposites containing HNTs/carvacrol hybrids exhibit outstanding antimicrobial properties against Escherichia coli (E. coli), Listeria innocua and a wide range of fungal molds. The use of HNTs hinders carvacrol losses during processing, but also its out-diffusion from the nanocomposites, resulting in prolonged antimicrobial activity. The antimicrobial effectiveness of these nanocomposites is also successfully demonstrated in complex model food systems such as soft white cheese and bread. Employing the synergistic interactions between carvacrol and thymol allows to reduce the total EO content (by at least 25% in comparison to the use of the individual EOs) within the polymer, while achieving highly-potent antimicrobial activity against E. coli is in both synthetic growth medium and in sensitive foods e.g. hummus. Next, we employed a forced assembly co-extrusion technology to produce carvacrol-containing multilayered films of LDPE and ethylene-vinyl alcohol copolymer.  The latter is used owing to its barrier properties and the resulting multilayers exhibit a unique combination of advantageous properties: outstanding antimicrobial activity, low oxygen transmission rates and carvacrol diffusivity (by >2 orders of magnitude in comparison to reference films, produced by conventional cast extrusion).

In the PP-based films, strong interactions between the PP matrix and carvacrol play a role in retaining the highly volatile carvacrol within the PP matrix during processing at elevated temperatures, and the HNTs were found to affect the properties of the PP/(HNTs/carvacrol) films in terms of their structure, mechanical properties and carvacrol release profile.

This research presents new bioactive polymer nanocomposites with an immense potential for active food packaging applications. This type of antimicrobial packaging allows to minimize the risks of microbial contaminations and extend food shelf life in the global effort to reduce food losses and wastage.