Everyone know the statistics: a case of salmonella in Mexican cucumbers in 2015 affected 341 people, the well-publicized 2011 case of Listeria in cantaloupe sickened 147 people, and E. coli 0157:H7 in romaine lettuce in 2011 affected 58 people.
Lee Stivers, Penn State Extension educator, said although there are no strict regulations stating fresh produce must be washed before it’s sold, many customers require it. And from a marketing perspective, it just makes sense — especially when it comes to leafy greens.
Stivers said the level of soil on produce is directly associated with how close to the ground it grows. “We are trying to minimize the contamination of fruits and vegetables that people eat raw,” she said. “Anything growing closer to the ground is more likely to become contaminated with dirt and pathogens that we want to keep out of fresh produce. Those pathogens could be coming from fresh manure, feces of domestic or wild animals, runoff or rain.”
Growers can’t change the height of plants, but there are some practices which keep plants from coming in contact with soil. Cultural practices such as using clean straw or plastic mulch, or growing crops like tomatoes on trellises help minimize contact with soil.
Most produce will inevitably harbor a degree of soil after harvest, and that soil can be a source of numerous microorganisms. The potential for a problem comes when fresh produce is contaminated with microorganisms through the soil, then that produce is eaten fresh without cooking.
Although pathogens which are harmful to humans are not usually present in soil in levels high enough to cause illness, pathogen contamination of produce can occur and cause outbreaks if exposed to incompletely composted animal manure in the field, wild or domestic animal manure or contaminated irrigation water. In some cases, human workers are the source of contamination.
But does washing leafy greens actually improve food safety? “Unless you wash the produce correctly and safely, there’s a chance you’ll make it less safe to eat,” said Stivers. “The more we learn about food safety, the more we start to look at things with food safety glasses on.”
Stivers explained proper washing can minimize risk of contamination by harmful pathogens. First, remove heavy soil deposits, including muddy outer leaves. Discard and leave muddy leaves in the field, and avoid harvesting in muddy conditions to keep as much soil out of the wash line as possible.
Next, wash with water. “You’re now handling a product that’s very close to the final consumer,” said Stivers. “It isn’t going to be cooked, so you need to be very careful about the safety of that product. Anyone who is washing leaves needs to use potable water.” It should be obvious pond water is unsuitable for washing leafy greens, but even untested well water can pose a risk.
The third step is to dry the produce to remove as much surface moisture as possible. “Surface moisture decreases shelf life and sets up conditions for any bacteria that are still on the surface to grow and multiply,” said Stivers. “One good way to dry leafy greens is to use a centrifuge.”
Stivers describes several types of wash systems which are suitable for leafy greens and other vegetables. In a spray wash system, water comes from above and washes produce below. The water passes through once, with no immersion, then goes down the drain. “It’s a single-pass system, and you should still be using potable, clean water,” said Stivers. “Because water is passing over and there isn’t a big chance of cross contamination, using a sanitizer is optional.”
Because spray wash systems use a lot of water, some are designed to recirculate water, which means there’s a chance of cross-contamination. “In a multi-pass, recirculating system,” said Stivers, “you should use a sanitizer to minimize cross-contamination.”
An immersion wash system involves placing the product into a tank or buckets for swirling or washing. This system may be non-recirculating, where the produce is dumped in and pulled back out, or include a recirculation system which washes produce more thoroughly. “The risk of cross-contamination can very high, whether it’s a circulating or non-circulating system,” said Stivers. “You need to use a sanitizer with this system.” Avoid galvanized containers in favor of plastic, which can be cleaned better.
A triple wash system involves washing produce in three sinks. Packaged salad greens widely available at grocery stores are usually advertised as ‘triple washed’. “By the time the product is out of the third sink, it’s very clean,” said Stivers. “Most of the contaminants are removed in the first sink, then the second sink and third clean it more. Sanitizer should be used in the second and third sink, but it can be used at a lower concentration.”
Stivers cautions with any of the immersion systems, the user must keep the bins or tanks clean. “If you’re using them over and over and running water with organic material, biofilm builds up,” she said. “Biofilm is a nice place for bacteria to hang out and contaminate the next load. The recommended practice is to drain the tank at the end of the day, thoroughly clean it with a brush and good detergent and allow the bins or tank to air dry.”
Removing surface moisture is critical when it comes to cleaning both produce and washing equipment because pathogenic bacteria grows and leads to spoilage. Stivers said some growers have retrofitted old washing machines to serve as a centrifuge, but those must be cleaned and air-dried after use.
When sanitizers are used in washing produce, it’s important to use them correctly. Stivers said sanitizers are regulated by the EPA and can only be used in certain formulations and concentrations for fruits and vegetables. As is the case with pesticides, the label is the law.
One commonly used wash water sanitizer is chlorine. “Chlorine is cheap and we use it a lot, but it has some problems,” said Stivers. “One is that it is very pH sensitive. It only works well at around six to eight pH. It doesn’t work well as the pH goes higher. But if the pH of the wash water is low, like under five, you get toxic chlorine gas formation. Also, chlorine becomes denatured, or ineffective, by dirt and organic matter. So if you’re washing something and the water becomes dirty, the chlorine is tied up by the organic matter and is no longer killing microbes in the wash water. It is also corrosive and irritating, and forms potentially toxic byproducts.”
Peracetic acid products are also widely used, and are less sensitive to pH changes or the presence or organic matter. They’re essentially non-toxic, but more costly than chlorine products.
“It’s important to follow the directions on how much to use,” said Stivers, describing the use of any sanitizer. “With chlorine, ideally, you want two to seven ppm of free chlorine in a solution of a pH between six and 7.5. But the instructions will often tell you to add more up front, and that’s because of the issues with organic matter.”
How do you know if the product you’re using is registered for use for washing fresh fruits and vegetables? The label will include an EPA number and instructions for proper use. The National Pesticide Information Retrieval System (NPIRS) includes a search function which allows growers to find out who supplies appropriate sanitizers.
“All sanitizers are effective if they’re used properly,” said Stivers. “Make a choice based on what you need for your system.”