The consumption of natural products has become a public health problem, since these medicinal
teas are prepared using natural plants without an effective hygienic and sanitary control. The
aim of this study was to assess the effects of gamma radiation, on the microbial burden of two
medicinal plants: Melissa officinalis and Lippia citriodora.
Dried samples of the two plants were irradiated at a Co-60 experimental equipment. The applied
gamma radiation doses were 1, 3, and 5 kGy at a dose rate of 1.34 kGy/h. Non-irradiated
samples followed all the experiments. Bacterial and fungal counts were assessed before and
after irradiation by membrane filtration method. Challenging tests with Escherichia coli were
performed in order to evaluate the disinfection efficiency of gamma radiation treatment.
Characterization of M. officinalis and L. citriadora microbiota indicated an average bioburden
value of 102 CFU/g. The inactivation studies of the bacterial mesophilic population of both dried
plants pointed out to a one log reduction of microbial load after irradiation at 5 kGy. Regarding
the fungal population, the initial load of 30 CFU/g was only reduced by 0.5 log by an irradiation
dose of 5 kGy. The dynamics with radiation doses of plants microbial population’s phenotypes
indicated the prevalence of gram-positive rods for M. officinalis before and after irradiation, and
the increase of the frequency of gram-negative rods with irradiation for L. citriadora. Among
fungal population of both plants, Mucor, Neoscytalidium, Aspergillus and Alternaria were the
most isolated genera. The results obtained in the challenging tests with E. coli on plants pointed
out to an inactivation efficiency of 99.5% and 99.9% to a dose of 2 kGy, for M.officinalis and L.
citriadora, respectively.
The gamma radiation treatment can be a significant tool for the microbial control in medicinal
plants.The authors are grateful to project PRODER nº 53515, AROMAP,
for financial support of the work. The authors thank to "MaisErvas - Aromáticas e Medicinais"
company for supplying the samples. The authors are grateful to FCT (RECI/AAGTEC/
0400/2012) for financial support
