Antimicrobial properties of nisin after glycation with lactose, maltodextrin and dextran and the thyme oil emulsions prepared thereof

• Nisin was glycated with lactose, maltodextrin, and dextran by dry heating method.
• Smaller saccharides and longer glycation led to lower antibacterial activity of nisin.
• Glycation did not improve antibacterial activity of nisin, either in TSB or in milk.
• Thyme oil and glycated nisin still had the additive antibacterial activity.
• The additive antibacterial activity in milk was reduced after emulsification.

To clarify the reported conflicting antimicrobial activities of nisin after glycation, nisin was glycated with lactose, maltodextrin, and dextran at 70 °C and 50% relative humidity for 1–24 h. Nisin before and after glycation was studied for the first time to prepare thyme oil emulsions. The activity of glycated nisin and the thyme oil emulsions was tested in both tryptic soy broth (TSB) and 2% reduced fat milk. Results showed that nisin glycated with a smaller saccharide for a longer duration had a higher degree of glycation and the reduced number of positive charges lowered its antibacterial activity. The emulsified thyme oil had an additive effect with either glycated or native nisin against Listeria monocytogenes Scott A and Bacillus subtilis in TSB and 2% reduced fat milk. However, emulsions were less effective against L. monocytogenes Scott A in milk than same units of native nisin and same concentration of free thyme oil, likely due to the reduced availability of thymol and carvacrol, the main components of thyme oil. These results showed that glycation of nisin cannot broaden its antimicrobial activity and nisin is not a good compound to prepare emulsions of essential oils.