Inhibition of Escherichia coli O157:H7 and Listeria monocytognes growth in milk and cantaloupe juice by thymol nanoemulsions prepared with gelatin and lecithin

Abstract

Thymol is an effective antimicrobial and is to be encapsulated for use in liquid foods. Previously, novel thymol nanoemulsions have been prepared with the emulsifier combination of gelatin-A (acid-hydrolyzed) or -B (base-hydrolyzed) and lecithin. The objective of this study was to characterize the antimicrobial activity of thymol nanoemulsified by gelatin and soy lecithin, using milk and cantaloupe juice as two model food systems at 21 °C. Nanoemulsions were overall more effective than free thymol in both media. Compared to the bacteriostatic effect of free thymol, 4.5 g/L nanoemulsified thymol gradually reduced Listeria monocytogenes by 5 and 3 log CFU/mL in 2% reduced fat and full fat milk after 48 h, respectively. In skim milk, the nanoemulsion with gelatin-A was overall more effective against L. monocytogenes than that with gelatin-B. Conversely, the nanoemulsion with gelatin-B reduced Escherichia coli O157:H7 to below the detection limit after 8 h in 2% reduced fat milk and 48 h in full fat milk, contrasting with the bacteria recovery for the nanoemulsion with gelatin-A after 48 h. The improved antimicrobial activities of thymol nanoemulsions were supported by the quantified increase of thymol concentration in the serum of milk. The nanoemulsion prepared with gelatin-B was more effective than that of gelatin-A in cantaloupe juice. The slight difference in bacterial recovery after treatment by nanoemulsions prepared with two types of gelatin suggests the impacts of droplet surface properties on interactions with bacteria membrane. The studied nanoemulsions have great potential for use as novel antimicrobial preservatives to improve food safety.