Herbal spices are important sources of antimicrobials, and the use of spices, their essential oils or active ingredients for controlling microbial growth in food materials constitutes an alternative approach to chemical additives.

Some of the spice essential oils (individual or combinations) are highly inhibitory to selected pathogenic and spoilage micro-organisms. The fractionation of essential oils and further application help to improve the level of activity in some cases. The optical isomers of carvone from Mentha spicata and Anethum sowa (Indian dill) were more active against a wide spectrum of human pathogenic fungi and bacteria than the essential oils as such.

Mixing compounds such as carvacrol and thymol at different proportions may exert total inhibition of Pseudomonas aeruginosa and Staphylococcus aureus. The inhibition is due to damage in membrane integrity, which further affects pH homeostasis and equilibrium of inorganic ions. Such knowledge on the mode of action helps spice extracts/ingredients to be applied successfully in foods. Also, application of active ingredients instead of essential oil will not change the food’s flavour very much.

Plant extracts or seed diffusates could be used for the control of seed-borne pathogens and can be a substitute for costly chemicals for seed treatment. Plant extracts of pimento can be used for controlling fungal growth during storage of wheat grains. Likewise, the seed diffusates of  Anthem graveolens and  Coriandrum sativum gave a high level of growth inhibition against seed-borne fungi such as Alternaria alternata  and Fusarium solani.

Of the various herbal spices, oregano and thyme show the highest antimicrobial activity. Carvacrol, present in the essential oils of oregano and thyme, has been proved to be the most important fungitoxic compound.