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From: Food Quality & Safety magazine, February/March 2008

Pathogen Can't Hide From Biocides

Biocides can affect food safety from harvest through production

by Charles Giambrone, MS

Over the past 10 to 15 years, industry and the government have sought intervention strategies to reduce general microbial numbers and, specifically, to reduce or try to eliminate all produce pathogens. The most notable recent produce pathogen outbreak, which involved bagged baby spinach from California, was caused by E. coli 0157:H7. This occurrence resulted in a multistate outbreak, leading the U.S. Food and Drug Administration (FDA) to draft the “Guide to Minimize Microbial Food Safety Hazards of Fresh-cut Fruits and Vegetables.”

One method for dealing with microbial food hazards is the introduction of biocides into produce processing waters. A critical component for most produce manufacturers, it can enhance or jeopardize food safety from the harvesting step through the production processes.

“Guide to Minimize Microbial Food Safety Hazards for Fresh Fruits and Vegetables,” an FDA Center for Food Safety and Applied Nutrition (CFSAN) guidance document published in October 1998, devotes a whole section to water as a microbial hazard, control of potential hazards, agricultural water, and processing water.

Processing water systems are the main focus of your direct intervention food safety approach and are more critical for fresh produce markets than any other food market segment. If the direct intervention and sanitation program at a produce processor ignores processing water treatment, it is not a complete program and pathogen issues will arise.

Water used for food product and food contact surfaces is in the good manufacturing practices (GMP) regulations in the Code of Federal Regulations (CFR); the specific sections are CFR Title 21 Part 110.37 and 110.80. Facilities that deal solely with raw produce commodities are exempt from some segments (see 21 CFR 110.19). Market pressures inherent in these produce markets encourage most raw produce processors to adhere to GMPs regarding water quality.

A Race Against Time

From the time produce products are picked until they are processed and/or consumed, the natural or artificial ripening and spoilage attributes of a specific product create a race against time. This push begins the moment a product is picked in the fields and continues through the ripening process.

Hydrocooling is one way to slow a product’s respiration and enhance shelf life, but not all produce commodities are hydrocooled. Those that are quite sensitive to wetting are usually air cooled. Water does, however, remove heat from a product roughly 15 times faster than air. All hydrocoolers utilize a water pump to put the warm product into contact with the chilled water. Some hydrocoolers use a refrigeration system, while others use block or crushed iced to cool the water supply.

Produce products with a large volume versus surface area—like sweet corn, apples, cantaloupes, and peaches—do well with hydrocooling. There are four types of hydrocoolers: conventional, batch, immersion, and truck.

In conventional hydrocoolers, the produce is placed onto conveyors in cartons/bulk bins that go into a chilled water shower. Cooling rates are based upon a belt speed of roughly one foot per minute, so the cooling capacity of a unit is largely dependent upon the conveyor belt length of the cooling zone. Water volume usages are fairly high, as much as 20 gallons per minute per square foot of active cooling area. Large production units like cooperatives use these types of hydrocoolers.

Batch hydrocoolers have no conveyors; instead, they create a cooling zone using palletized cartons or bulk bins. Depending on their size, these batch units will chill one to eight pallets at once. Some units use a hybrid system that employs air chilling as well as water chilling.

Immersion hydrocoolers are large, shallow, rectangular tanks that use recirculated, chilled water. Crates of warm produce are placed into the tank with a submerged conveyor. Either a refrigeration system or crushed ice keeps the water cold using a recirculation pump. An example of immersion hydrocooling is the bulk fluming of products like string beans prior to grading.

Truck hydrocooling, used with field sweet corn, is the fourth method. The truck holds are designed with portable tanks containing crushed ice and cold water. Crates of produce are loaded, and portable perforated pipes above the product load shower it with cold water. The water is then collected, re-cooled, and recycled.

Other Cleaning Methods

Flumes in produce processing plants are often used to remove dirt and transport fruit and vegetables around the processing plant. This system, though highly efficient from a processing perspective, can cause a number of bacteriological problems due to the potential spread of contamination across a batch. A flume system will typically transport product from the initial sorting equipment to washing equipment and then, in some facilities, to a blancher. After the product is cooled and cut or chopped, depending on the facility, it may be transported via flume to de-watering equipment, like a basket centrifuge.

Due to organic loading, flume systems are prone to biofilm formation. Biocides like chlorine, peracetic acid (PAA), and chlorine dioxide are used for microbial control and process water reuse/recycling.

Spray washers are designed to remove the majority of the soil and attached debris from produce products. Equipment units like the one available from FMC FoodTech (Chicago) employ approved food-grade cleaners that apply a thin film prior to using chlorinated water and/or water treated with another biocide like chlorine, PAA, chlorine dioxide, or ozone.

There are typically four types of commercial washing equipment: flatbed brush washers, U-bed brush washers, rotary washers, and pressure washers. From a sanitation perspective, washers with brushes pose additional challenges, including getting proper cleaning while minimizing detergent retention and maintaining brush integrity and equipment life.

Whatever the cleaning method, all utilize biocides to reduce and control microbial levels on the produce and in the process water. The point of application determines whether the biocide label is controlled by the Environmental Protection Agency (EPA) or the FDA. If it involves raw agricultural commodities, as is the case with field hydrocoolers, the biocide must have an EPA label. Congress has specified that the EPA controls raw agricultural biocide usage in the field, and the EPA believes the intent is to treat the process water, not the product itself.

If the biocide used to treat the process water is employed in any further processing, including fresh cut, then the label is used for microbial control and must be consistent with FDA regulations. In either case, we are treating the process water for a dual purpose:

  • To reduce contamination and bioburden in the process water in order to minimize microbial contamination and biofilm formation in the water stream and the equipment; and
  • To treat the actual produce product to reduce/eliminate spoilage or pathogenic microbes on the consumed product.

Common Biocides

A wide variety of biocides and chemistries are used to treat processing water in the produce markets, including chlorine, PAA, chlorine dioxide, and ozone. While some processors still use actual chlorine gas cylinders to generate hypochlorous acid in solution, most don’t because of safety issues. Chlorine is the oldest, cheapest, and easiest of these four biocides to dose and control. However, it also needs to be in a fairly narrow pH range to be effective and safe. The truly safe range for biocidal hypochlorous acid from hypochlorite is from pH 6.5, where we achieve a 90% safety range, to pH 8.0, where we achieve a 20% safety range.

Chlorine can easily be dosed and controlled using either in-line or portable oxidation-reduction potential (ORP) probes. ORP needs to be kept in a functional range, from an optimum of 750 mV to a low of 350 mV. Time, temperature, and pH—along with exposure to direct sunlight—all factor into the ORP reading and the free hypochlorous acid you will have in your process water stream.

Hypochlorous acid is effective against vegetative bacteria, but it has two huge drawbacks: disinfectant byproducts (reaction with organics) and corrosion. The off flavors created by chlorine, its reactions with organic molecules, and its impact on wastewater effluent treatment are strong negatives for many produce customers.

The reduced life span for produce processing equipment due to hypo-chlorite corrosion is also a major factor. In addition, hypochlorous acid, even within the optimal pH and temperature parameters, is not as effective a biocide against fungal and bacterial spores and protozoal cysts as are chlorine dioxide and ozone. Hypochlorite is still employed in many hydrocooler applications and in processing spray equipment, however, so it needs to be considered as a functional chemistry.

Peracetic Acid

PAA is effective as a direct food contact biocide because it breaks down into acetic acid and peroxide, substances generally recognized as safe. PAA has a number of advantages and disadvantages (See Table 1, left). As for regulatory approvals for PAA as a direct produce biocide, 40 CFR 180.1196 states that this biocide is exempt from the requirement of residue data when use solution is less than 100 parts per million (ppm). PAA has a wide variety of applications:

  • Packing house sanitation at 85 ppm;
  • Sanitizer for processing equipment such as peelers, slicers, and saws at 0.3-0.4 ounces/5 gallons water (85-123 ppm);
  • Field equipment at 85 to 125 ppm; and
  • Processing water treatment to control spoilage at 1 ounce/16 gallons of water (85 ppm).

Another biocide to consider is chlorine dioxide, which, after ozone, is the most powerful anti-microbe biocide available. Like other biocides, chlorine dioxide has several advantages and disadvantages (See Table 2, below). The most typical method of activation in food processing applications is the two-part system requiring acidification. There are a number of acids used to generate chlorine dioxide from its chlorite precursor.

Hydrochloric (HCl) acid is the most efficient acid activator. It is typically used in water treatment applications in which the activated product is diluted to a very low concentration. At higher use concentration levels, the excess chloride ion from the hydrochloric acid can create corrosion issues. Food-grade phosphoric acid is commonly used for many two-part activation or generation units. While not as efficient as HCl, it is more efficient than any other acid used. The only drawback occurs if you are using high concentration levels for certain approved applications: the excess phosphate could pose a problem to wastewater effluent.

A number of organic acids, including lactic and acetic, can be utilized to activate the chlorine dioxide precursor. Citric acid may be preferred in situations where you desire a low reaction level and where the citric acid helps in the appearance of sensitive produce products. They are all inefficient chemical reactions, however.

Chlorine dioxide precursors are various concentrations of tech grade sodium chlorite ranging from 7.5% up to 25% solutions. These solutions have an alkaline pH. Stabilized chlorine dioxide precursors are available, like the one manufactured by Bio-Cide International, Inc. (BCI; Norman, Okla.) These are buffered solutions of chlorite; the solution pH is far less alkaline than technical grade sodium chlorite.

These stabilized chlorine dioxide solutions are distinctly different in pH, especially when they are acidified to produce active, free chlorine dioxide solutions. There are a variety of safe, efficient generators to choose from. Two user-friendly units from BCI are a batch system for smaller applications called the Automatic Activation Non-Electric system and the continuous On-Line Activation System.

Safety Considerations

When using PAA and chlorine dioxide, you should remember that both products are strong oxidants; pay careful attention to the product data sheets and other information. Concentrated PAA can cause serious burns and inhalation discomfort. Use appropriate personal protective equipment when tubes are taken up. Manual dilution of the product is strongly discouraged; use a pump from Dosatron (Clearwater, Fla.) for auto dosing and dilution. Your best bet is to order a D116/ Di150 PVDF-H pump model that is oxidant compatible.

PAA has no federal limits for air inhalation simply because there is no reliable way to measure it in air. If respiratory issues arise, you need to test air limits for either peroxide or acetic acid. Also, remember that any chlorine dioxide precursor concentrates that spill can dry and result in dried sodium chlorite powder, which is flammable and explosive. Any chlorite solution spills of tech grade or even stabilized chlorine dioxide need to be thoroughly diluted with water.

Activated chlorine dioxide solutions contain free chlorine dioxide gas—the active biocide—dissolved in solution. The higher the concentration, pressure, or solution temperatures are, the greater the amount of dissolved gas that will leave the solution. Unlike PAA, you can measure free chlorine dioxide in the air using a variety of air monitoring devices, including portable Drager units, specific detection tubes, and a number of more accurate and more expensive types of air monitoring equipment.

Direct intervention treatment methods for produce products are simply another key component in the multipronged food safety control strategies required by today’s ready-to-eat food markets. These direct interventions are useless, however, without strictly enforced, validated GMPs and hazard analysis and critical control point systems.

Giambrone is senior technical support manager at Rochester Midland Corp. Reach him at cgiambrone@ or (800) 762-4448, ext. 7263.



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