A preservative is essentially a chemical agent that will destroy or inhibit micro-organisms in finished formulations. There is ample scope for microbial contamination at the end user level. It is therefore important that the product be protected by a suitable preservative to ensure safety in use and adequate shelf life.

Water activity and pH of the formulation, and the solubility and stability of the preservative in the base composition are the factors that determine preservative efficacy. Additives such as glycerol, butylene glycol, salt (sodium chloride), sugars, soluble starches, dextrin, xanthan gum and others absorb water, thereby lowering the water activity of the formula.

Analogs in the lists of natural actives include glucans, mannans and other polysaccharides extracted from commonly used culinary materials such as tamarind seed and fenugreek.

In the area of chemical preservatives that stop or inhibit microbial growth, common classes of chemicals used include acids, aromatic alcohols, N-methylol containing compounds, halogenated compounds, isothiazolinones, quaternary nitrogen compounds, and 1,2 diols.

A number of phytochemicals found in natural sources resemble these compounds in chemical structure and are natural preservatives. These classes of phytochemicals include essential oil constituents (such as eugenol, thymol, carvacrol, terpenoid compounds), flavonoids, phenolic compounds, tannins and alkaloids.

Some of these extracts when combined with ineffective antibiotics were found to be effective against antibiotic resistant strains.

Although no single natural extract has been found to be as effective as conventionally used preservatives such as parabens, combinations of naturals with synergistic activity have been identified.

Such extracts could offer protection against the invasive growth of bacteria and fungi. Parabens, for example, are poorly soluble in water and their action is strongest against gram positive bacteria and fungi and weakest against gram negative bacteria.

A combination of synthetic preservatives are often used in formulations, which can lead to safety and regulation issues, depending on where the product is marketed. (For example, the E.U. has considerably more strict regulations than does the U.S.)

It is here that blends of natural extracts may be useful. Natural blends also offer antioxidant action, with beneficial effects on product shelf life. One example is a proprietary natural extract of yellow curcuminoids from Curcuma longa (turmeric) roots and a colorless derivative Tetra-hydrocurcuminoids [INCI: tetrahydrodiferuloyl-methane, tetrahydrodemethoxydiferuloylmethane, tetra-hydrobisdemeth-oxydiferuloylmethane][1], that were found to offer more effective anti-oxidant protection than the conventionally used synthetic antioxidant butylated hydroxytoluene (BHT).

The skilled formulator has a plethora of natural options available for use as preservatives. Care should however be taken to ensure that these naturals are selected with physicochemical characteristics, global regulatory considerations and safety aspects in mind.

Reference

1. US Patent # 6,607,712, Sabinsa Corporation