Chelating agents such as phytic acid extracted from rice bran could be added to enhance the activity of the natural preservative. There are a number of suppliers for this material. Another option is to use a naturally produced material such as sodium gluconate. Sodium gluconate is the sodium salt of gluconic acid, produced by the fermentation of glucose. It is a white crystalline powder, very soluble in water. Non-corrosive, nontoxic and readily biodegradable (98% after two days), sodium gluconate is an effective chelating agent especially in alkaline and concentrated alkaline solutions.

It forms stable chelates with calcium, iron, copper, aluminum and other heavy metals. It is as effective as other chelating agents, such as ethylenediaminetetraacetic acid (EDTA) and related salts. Aqueous solutions of sodium gluconate are resistant to oxidation and reduction, even at high temperatures. However, it is easily degraded biologically (98% after two days) and thus presents no wastewater problem. It is used in the food industry.

Chelating agents interfere with the cellular membranes that surround all organisms and weaken them by depriving them of the trace elements that they need for cellular function. Extremely high levels of chelating agent have been used as preservatives on their own.

Source: Dweck, Anthony. Handbook of Formulating Natural Cosmetics (Dweck Books 1) (pp. 83-84).