Current Sanitation Practices for Leafy Green Vegetables For Processors, Retailers and Consumers

More consumers are making an effort to increase their fruit and vegetable intake because of their heightened awareness of their value for a healthy diet. However, fresh produce, especially leafy green vegetables, are the first among the top ten riskiest foods in the United States. Since 1998, the Centers for Disease Control and Prevention (CDC) Foodborne Disease Outbreak Surveillance System reported thousands of outbreaks, illnesses, hospitalizations and deaths due to current sanitation and handling practices. Of the 9.6 million cases of total foodborne incidents associated with fruits, nuts and vegetables, leafy vegetables account for 22 percent of the illnesses, 14 percent of the hospitalizations, and 6 percent of the deaths each year. In addition to health benefits, fresh cut produce also provides convenience to consumers. Green leafy vegetables such as iceberg and romaine lettuce, and spinach are typically more challenging to sanitize than other vegetables because of their relatively large surface area, large number of crevices and pores, their shape and fragility.  

The FDA Food Safety Modernization Act (FSMA) was signed into law on January 4, 2011, with the goal of preventing contamination. FSMA’s Final Produce Safety rule was made available to the public on November 13, 2015, to inform farmers, packers and regulatory personnel about the standards for growing, harvesting, packing and holding produce ⁽⁵⁾. This ruling is also important for retailers and consumers to understand, because the fresh produce shelf-life period includes storage by retailers and consumers. 

This fact sheet will explain the operations used in current practices of fresh produce processing. The goal for understanding this information is to identify potential improvements on sanitation processes, while at the same time, reduce the number of outbreaks and foodborne illnesses in the United States.

Why Have Leafy Vegetables Become a Point of Heightened Concern?

Although food sanitation techniques have improved with the development of new processes, so has the detection time of outbreaks. Improved surveillance, reporting and communication have helped the CDC to determine the outbreak source and announce recalls to the public more rapidly. However, fresh produce recalls still have limited success due to the short shelf life of fresh leafy products (15 to 19 days). By the time an outbreak has been identified and traced back to the source, it is usually too late to warn the consumer ⁽⁶⁾. The increased number of reported outbreaks related to green leafy vegetables is partly due to improved capability of identifying the source more accurately.

What is Involved in the Processing of Green Leafy Vegetables?

For development of effective detection techniques, it is important to determine the processing steps involving the highest safety risk. Understanding the individual unit operations in fresh produce processing will lead to the design of better alternative unit operations. Figure 1 shows the processing steps and the duration of each step from harvest to consumption.

Figure 1: Current sanitation processing steps for fresh green leafy produce and the approximate time elapsed at each unit operation.

The purpose and method for each major step are provided below. 

Crop Harvest: Produce is mechanically removed from the ground and placed in chilled water to begin removing field heat. Each hour of delay in cooling of produce reduces the shelf life by approximately one day. Bacteria from the ground and processing equipment may be present at this point. Extra care must be taken to avoid excessive damage to the product during harvesting. Equipment used to remove the produce from the soil must be sharp, and handling must be done with care to avoid additional bruising. Locations on the produce that have sustained damage are more likely susceptible to bacteria and will allow the bacteria to infiltrate the subsurface structures, which will reduce effectiveness of sanitation on damaged areas ⁽¹⁴⁾.

Chlorinated water application: Chlorinated water is used to remove and potentially to reduce bacteria contracted from the soil, harvesting equipment or human contact. If the surface of the product is covered in dirt, it must be washed prior to treatment in the submersion tank. Chlorinated water is applied by either submerging the product, or spraying the product at a total chlorine concentration of 20 to 200 ppm. Submersion for 1 minute commonly achieves a microbial reduction of approximately 1.3 to 1.7 log unit ⁽²⁾. After submersion, the product is rinsed with chlorinated water at a concentration of 2 to 20 ppm prior to loading into crates for transportation ⁽⁸⁾.

Transportation: Transportation needs for leafy vegetable have to be considered separately each time the product is loaded and unloaded to analyze the true contamination risk. Transportation trucks must be refrigerated to avoid product spoilage (1 to 3 degrees Celsius). Trailers must be cleaned often, and the products must be placed within palletized crates and should not directly contact the trailers floor ⁽¹²⁾. The first transportation will be from the field to the cooler. Once the product is cooled to approximately 1 to 3 degrees Celsius, it is transported to the processing plant. Since 80 percent of lettuce grown in the United States is grown in Salinas, California, shipping times to major processing plants in Illinois, Ohio and Iowa can be nearly three days. Once processed and packaged for sale, the product is shipped to retailers within the region.  

Cooling: Green leafy vegetables need to be rapidly cooled down to remove field heat, to reduce respiration, to reduce water loss and to limit microorganism growth. After harvest, produce is cooled from field temperature to 0 to 1 degrees Celsius rapidly. Green leafy vegetables should be held at this temperature throughout the shelf life. The three most common cooling techniques for leafy vegetables are ice-packing, hydro-cooling and hydrovac cooling ⁽¹²⁾.  

• Ice-packing involves filling the spaces between the produce with crushed ice in an ice to product ratio of approximately 1-3 ratio. This is the least expensive method, but it is slow to remove heat from the product.  
• Hydro-cooling involves placing the produce on a conveyor belt that is doused with chilled water, cooling the produce in approximately 20 minutes.  
• Hydrovac cooling involves cooling vegetables under a vacuum. Vacuum cooling is extremely effective in preserving food quality because of rapid cooling rates ⁽⁷⁾.  

Post-harvest processing: Cutting, washing and packaging are done in a processing plant (1). The product is cut specific to the final use and centrifuged or air dried to remove most of the moisture, before weighing for packaging. Modified atmosphere packaging (MAP) may be used to reduce oxygen by using carbon dioxide and nitrogen within the packaging to extend shelf life of the product ⁽⁹⁾. For fresh fruit and vegetables, the recommended gas mixture is 5 percent O_², 5 percent CO₂ and 90 percent N₂ which can increase the shelf life of the product to eight days from four or five days under regular air conditions ⁽³⁾.

Post-processing product storage: Maintenance of required product storage conditions during distribution, by retailer and by consumer are essential for the shelf life of green leafy vegetables. Once the produce arrives at the store, the time it takes to get from the refrigerated truck into the coolers must be minimized. Industrial refrigeration systems should keep the fresh produce cooled at a temperature of about 1 degrees Celsius until the product is purchased by the consumer ⁽¹¹⁾. Following purchase, the product is exposed to ambient temperature until it is placed in a consumer’s refrigerator causing temperature fluctuations. The temperature changes may accelerate moisture loss, further reducing shelf life of fresh produce. Additionally, once opened, the product is exposed to air and the benefits of MAP are not maintained. Given that the shelf life of fresh leafy green vegetables is short, emphasis should be placed on proper storage conditions throughout the shelf life of the product.

What are the Major Limitations in the Current Process?

Several potential practices affect the sanitation of fresh produce:

1. Stomata (microscopic pores for gas exchange) in leafy vegetables allow bacteria to penetrate deeper into the produce.
2. Leafy vegetation has crevices which makes it difficult for liquid sanitizers to penetrate and to inactivate bacteria residing therein.
3. Chlorinated water works better at lower pH values, but to assure product quality, equipment safety and to avoid producing chlorine gas, the solution is held between 6.5 and 7.5 pH.

What Can Be the Result of a Failed Sanitation Procedure?

Annual cost of medical treatment, productivity loss and illness-related deaths was estimated to be more than $50 billion without the inclusion of cost to the food industry due to recalls and to public health agencies ⁽¹³⁾. If current sanitation standards are not met, a variety of negative scenarios can arise.  

• Bacteria can cause product spoilage.
• Bacteria can cause foodborne illness.
• Contaminated products have the potential to contaminate additional products they come in contact with.
• Costly recalls can occur, reducing the food supply and damaging company’s reputation.

Although chlorinated water has been prominent in the industry for a long time, alternative sanitation methods need to be explored to improve the safety and shelf life of fresh produce. 

What Alternative Sanitation Practices are Being Investigated?

Research is being conducted on alternative sanitation technologies to improve the safety of fresh produce. Alternative sanitation strategies may include:

• Use of other liquid sanitizers—Hydrogen peroxide, calcium-based solutions, organic acids, peroxyacetic acid, ozonated water, chlorine dioxide solution and electrolyzed water have shown positive results in a number of research publications.
• Use of gas sanitizers—Gases, such as chlorine dioxide and ozone, offer promising results because of their ability to penetrate deeper and faster than liquid sanitizers to bacteria located in crevices.  
• Designing and implementing equipment modifications—Modifications to current equipment to combine the application of multiple sanitation approaches can produce more effective results. For example; injection of chlorine dioxide or ozone gas into a hydrovac cooling unit would provide an opportunity to combine a sanitizing agent with cooling operation to improve the efficiency and effectiveness of fresh produce sanitation thereby providing consumers safe products.

References

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