Doctoral Dissertations

Date of Award

5-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Food Science and Technology

Major Professor

Qixin Zhong

Committee Members

P. Michael Davidson, Svetlana Zivanovic, Jun Lin

Abstract

As natural and effective antimicrobials, essential oils (EOs) have been increasingly investigated to control foodborne pathogens and enhance food safety. It is usually difficult to achieve high antimicrobial efficacy when directly incorporating EOs in food systems due to their low water solubility. Thus, a variety of EO delivery systems have been developed. Nanoemulsions have shown many benefits to encapsulate EOs. However, most EO nanoemulsions are currently prepared with synthetic surfactants.

The overall goal of this research was to prepare EO nanoemulsions with generallyrecognized- as-safe (GRAS) emulsifying agents to enhance their antimicrobial activity in foods. The first group of transparent nanoemulsions containing thymol, the major component in thyme oil, was prepared using whey protein-maltodextrin conjugate produced through the Maillard reaction using shear homogenization, with propylene glycol as a co-surfactant. Compared to free thymol, nanoemulsions showed significantly enhanced antilisterial activity in milk, which was attributed to the increased thymol solubility in milk and synergistic antimicrobial activity with propylene glycol. The second group of stable and translucent thymol nanoemulsions were prepared using the combination of gelatin and lecithin that formed complexes with the improved emulsifying activity than individual emulsifiers. The nanoemulsions were consistently more effective than free thymol against Listeria monocytogenes and Escherichia coli O157:H7 in milk and cantaloupe juice. To improve the emulsifying capacity, the last group of nanoemulsions with thyme oil was studied using the combination of sodium caseinate (NaCas) and lecithin. Coadsorption of NaCas and lecithin on oil droplets emulsified a higher amount of thyme oil as nanoemulsions than individual emulsifiers, reduced the occurrence of creaming, coalescence, and Ostwald ripening, and resulted in a long term storage stability. The nanoemulsified thyme oil demonstrated similar or better antimicrobial activity than free thyme oil in tryptic soy broth and 2% reduced fat milk. Therefore, the studied nanoemulsions prepared with GRAS emulsifiers have great potential for use as novel antimicrobial preservatives to improve food safety.

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