RISKS OF FRESH PRODUCE CONTAMINATION
Why is it important to improve sanitization of fresh produce?
In recent years, fresh-produce-related outbreaks, mainly due to Norovirus, Salmonella, and E.coli O157:H7 contamination, as well as other pathogens such as Cyclospora, are on the rise. The fast, efficient distribution chain that has arisen to accommodate the short shelf life of fresh produce has provided the consumer the advantage of year-round access to affordable fresh produce. However, in the case of an outbreak, this advantage creates increased risk due to wide distribution and potential rapid spread of illness. The recent trend of increased consumption of fresh, ready-to-eat produce also makes outbreaks more likely.
Shouldn't we focus on identifying the source of contaminated produce and taking it off the shelves?
Currently, identifying an outbreak may take in the range of 2 weeks, by which time the affected product may have been distributed widely. Therefore, it is important that efforts by processors, distributors, retailers, government, and consumers be directed towards prevention of foodborne illness outbreaks. Producers must find a way to sanitize produce and deactivate microbes before they reach the consumer.
Why can't we prevent contamination in the field instead?
It is impossible to completely control contamination of product growing in open fields – some contamination is inevitable in systems that interact with the natural world. At the end of the day, it’s always possible that animals, birds, or insects will come into a field. Many of these are harmless, but those in disease state are depositing pathogenic microorganisms in the field. To deal with that small fraction of problem microbes, processors must rely on post-harvest decontamination.
Why start with leafy greens?
Leafy greens can be difficult to clean because of crevices and pores where bacteria can hide and may not be accessible by conventional sanitizers. The physical structure of the leaf makes this inevitable. Fresh produce can also be easily bruised, creating areas that are more susceptible to bacterial growth. According to a report released by the Center for Science in the Public Interest (CSPI) in October 2009, leafy greens occupy the first place in the top ten riskiest foods regulated by the FDA based on the reported cases of illnesses between 1990 and 2006. As a rough example, a bag of leafy greens that has been handled perfectly, under the present methods, might have 1 million cells per gram of microbes, ranging up to 10 million.
LIQUID SANITIZING VS. GASEOUS SANITIZING
Why are gaseous methods more effective than liquid methods of sanitizing?
The speed at which molecules of sanitizer can penetrate crevices on the leaf depends on the innate diffusivity of different substances. Liquid sanitizers such as chlorine have a diffusivity 1000 times slower than gaseous sanitizers. That means it takes them much, much longer to penetrate crevices, giving them less chance to reach microbes and deactivate them.
What about new methods of disinfecting vegetables in water, such as the P3 Tsunami (peracetic acid, acetic acid, and hydrogen peroxide)?
Several new formulations of liquid sanitizers have bee shown to be effective against E. Coli and some, such as P3 Tsunami, show improvement over the traditional chlorine solution. However, any sanitizing method that relies upon creating an acid solution still faces the general challenges of liquid sanitizing, such as low diffusivity (requiring long time exposure), high water usage and the need to frequently replace water, cross-contamination, and the difficulty of accessing pores and crevices in the leaf. In addition, microbes like E. coli have a long history of adapting to thrive in acidic conditions, and over time it is likely that these solutions will reduce in efficacy due to selective pressure on the population of pathogens. (Think of lactic acid bacteria, which have evolved to thrive in lactic acid.) Gaseous sanitizing operates on a more basic aspect of microbe physiology, making it less likely that microbes will evolve resistance.
How can liquid sanitizing be made more effective?
Applying a liquid sanitizer alongside a surfactant (such as ProSan) can help the sanitizer reach the leaf surface and penetrate its waxy cuticle. This increases microbial kill.
What gaseous sanitizers have been tested for use with produce?
Ozone gas and chlorine dioxide have both demonstrated effectiveness against microbes on fresh produce, including leafy greens. Ozone gas is somewhat safer to work with, as it breaks down into harmless oxygen.
OZONE GAS
Are there adverse health effects from exposure to the gaseous sanitizer ozone?
Ozone breaks down into oxygen, and produce that has been treated with gaseous ozone retains no chemical contamination. Most bottled water you drink is ozonated, for example. Ozone-sanitized produce is completely safe for consumers to eat, just as water purified with ozone is safe to drink. Although breathing ozone as a component of smog does pose a risk, and people should avoid direct exposure to ozone, these risks do not pertain to ozone sanitization of produce.
Will microbes become resistant to ozone?
Ozone has no single target in the cell – it reacts with any compound with a double wall, including enzyme components and lipids in membranes. The cell therefore has no way to overcome it or adapt to it. The mildest form of ozone death is oxidative stress – ozone is such a strong oxidizer that microbes have no chance to develop resistance.
Won't ozone bleach leafy greens?
It is true that uncontrolled application of ozone to produce, without controlling the dosage, will bleach lettuce and spinach. However, when applied in a controlled way at sufficiently low concentrations, ozone can reduce bacterial concentrations by 10x to 100x without bleaching leaves. In addition, since this treatment decreases spoilage organisms as well as pathogens, the produce remains fresh longer.
Is ozone sanitizing FDA approved?
The gaseous ozone sanitizing process has been approved by the FDA since 2001.
Is ozone-sanitized produce organic?
Since ozone breaks down cleanly into oxygen, there are no chemical residues from ozone sanitization, unlike from chlorine liquid sanitizing. Many health food consumers are troubled by chlorine residues and would likely be attracted to ozone sanitization for this reason.
Is it still necessary to wash produce?
Washing produce to remove large particles, clods of dirt, etc., will still be necessary, but sanitizing with ozone either before or after the washing process will make the overall sanitization much more effective.
Where in the supply chain can we introduce gaseous sanitizing?
Gaseous sanitization can fit in to various times in the supply chain, depending on the specifics of the situation. After produce has been vacuum treated is a great time to treat it, since it is dry and cool. One idea that shows great promise is installing an ozone generator onto a shipping container and treating your produce during transit in a refrigerated shipment. An effective way to treat produce with ozone is to dose with a low concentration of ozone over a very long period of time, ideal for produce undergoing 3 or 4 days of shipment. The key point is that there are many opportunities in a supply chain when produce is sitting (in transit, in storage). Those times are ideal for gaseous treatment.
How much does this cost?
Sanitizing with gaseous ozone is not cost-prohibitive. In most cases, existing processes and practices can be adapted to include ozone. Ozone can be added as part of an existing vacuum cooling step by adding on an ozone generator on top of the existing system. (It is necessary to modify some materials to avoid damage by ozone, but these refitting are minimal.) A rough estimate would be $20,000 to add an ozone generator to a vacuum unit, plus $10,000 of adaptations. Another good solution is to add an ozone generator on top of a shipping container and sanitize with low doses of ozone during shipment.