The comparative effect of novel Pelargonium essential oils and their corresponding hydrosols as antimicrobial agents in a model food system

Essential oils and their corresponding hydrosols, obtained after distillation of various scented Pelargonium (Geraniaceae) leaves were assessed for their antimicrobial activity in a model food system. Both the essential oils and hydrosols were used at 1000 ppm in broccoli soup, previously inoculated with Enterobacter aerogenes (at 10(5) cfu g(-1)) and Staphylococcus aureus (at 10(4) cfu g(-1)). The results showed a complete inhibition of S. aureus in the broccoli soup by the essential oils of 'Sweet Mimosa', 'Mabel Grey', P. graveolens, 'Atomic Snowflake', 'Royal Oak', 'Attar of Roses' and a lesser effect by 'Chocolate Peppermint' and 'Clorinda'; the hydrosols, however, had a potentiating effect on the bacterial population in the food. Both extracts showed a complete inhibition of S. aureus in the Maximum Recovery Diluent (MRD). Antibacterial activity against E. aerogenes in the broccoli soup was generally very much reduced: only the essential oil of 'Mabel Grey' showed complete inhibition and virtually no reductions in colonies were seen with the other essential oils; the hydrosols again caused an increase in bacterial colonies. All the essential oils, bar Chocolate Peppermint showed complete inhibition of E. aerogenes in MRD, but the hydrosols showed no effect. The results strongly suggest that the residual hydrosols from distillation of these plant essential oils have no potential as antibacterial agents in foods, in contrast to most of the essential oils, which show potential against some micro-organisms, but only in some food systems. The problem of food component interference and its possible management is discussed.