FEATURE ARTICLE
Safer Salads
Contaminated fruits and vegetables are more common than ever. Why? And what can consumers do to protect themselves?
Jorge M. Fonseca, Sadhana Ravishankar
Which Sanitizer?
Although washing doesn't necessarily remove attached bacteria, growers can get rid of surface pathogens and inhibit decay by using a sanitizing treatment at the time of harvest. The spectrum of treatments includes chemical, physical and nuclear processes, all of which have unique pros and cons.
The most common sanitizers are chlorine based, including chlorine gas (Cl2), sodium hypochlorite (NaOCl, also known as household bleach) and calcium hypochlorite (CaClO2). The last is cheapest and used most often. Although effective, their antimicrobial activity depends on the amount of free chlorine in solution, which in turn depends on pH, temperature and the amount of organic matter in the water. A fourth form, chlorine dioxide gas (ClO2), is relatively unknown, but recent reports suggest that it works particularly well.
As a sanitizing agent, ozone gas (O3) can also be very effective, but this depends on concentration, exposure time, relative humidity, temperature, microbial load and the type of fruit or vegetable. Although it is well suited to certain applications, ozone is the most expensive sanitizer approved by regulatory agencies and probably produces more corrosion than any other.
Acetic acid, hydrogen peroxide and peroxyacetic acid are three other disinfectants that show promise for specific uses. Unfortunately, the chemical concentrations needed to lower bacterial counts also make the product look less appealing. Several groups of food scientists are currently working on ways to overcome this drawback.
Calcinated calcium is a new and very promising agent to control pathogenic microorganisms in fresh produce. In one study, this substance (which is made from furnace-blasted bones, whey, shells or coral) was more than 10,000 times as effective as chlorine at reducing the levels of Listeria monocytogenes on tomatoes.
Another recently introduced product is called electrolyzed oxidizing water, which is produced when an electric current passes through dilute saline. This process generates an acidic liquid that has high oxidation-reduction potential and reactive chlorine compounds. Several studies show it to be effective for eliminating food pathogens in vitro and on kitchen surfaces. Unfortunately, the preparation of electrolyzed oxidizing water requires specialized equipment, and the technique has not gained widespread use.
A good alternative to chemical purification techniques is the use of ultraviolet light at a wavelength of 200-280 nanometers. This so-called ultraviolet-C (UVC) light offers several advantages over other treatments: It leaves no residue, it doesn't require a drying step after treatment, and it doesn't need complex safety equipment. However, UVC treatments do have some disadvantages, including the inability to penetrate tissue and negative effects on quality at high doses.
Among sanitizing treatments, gamma irradiation may be the most effective at eliminating bacteria from intact and fresh-cut produce. However, the dose needed to get rid of pathogens can have an unwelcome effect on pectic substances in the cell walls of the plant, causing the tissue to soften. For this reason, irradiation is not suited for widespread use. Extensive mechanical requirements and public apprehension about the safety of irradiated foods also present obstacles to the application of this technique.
One surprisingly ineffective treatment is plain detergent and water. (Soap, because it contains many trace chemicals, isn't desirable either.) One study showed no significant differences in the levels of Salmonella and Shigella between produce washed with plain water or with water containing two detergents called Tween 80 and sodium lauryl sulfate. As a result, most consumer groups simply recommend a thorough rinse with lukewarm tapwater.
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