Americans are eating a healthier diet due, in no small part, to the explosive growth of prepackaged salads and vegetables. A recent report shows that 59 percent of consumers say they eat salads as meals for dinner or lunch at least once a week.

This trend helps explain the 15 percent market share enjoyed by packaged salad mixes in the $45 billion fresh vegetable category. The fact that packaged salad continues to grow at much higher rates than other fresh vegetables clearly demonstrates that Americans will eat salads if they are washed and ready to go.

This trend also underlies the creation and growth of an entirely new manufacturing technology to support the production and distribution of packaged salads. As might be expected for a product intended to go directly from the package to the consumer’s table, washing and sanitizing the produce are absolutely critical elements of the process.

The New York Times described the process this way: “Inside, the packing plant is cold and wet, and loud as a jackhammer, as enormous production lines ferry the tiny greens from bin to bag. First, they are upended onto conveyors, passing a row of inspectors and sweeping down a flume into the world’s largest salad spinners.”

It is in the flume water that the produce is both cleaned and sanitized, which makes the supply, filtration and treatment of that water a major concern — and a major expense — for the plant. The recent experience of a Tennessee processing company demonstrates just how sophisticated the technology has become.

Flume water in the plant is cleaned and recycled into a closed loop. At the Tennessee food processor, the original system used chlorine as a disinfectant, but the water quickly became discolored and contaminated with suspended solids and chlorine residue, which required it to be replaced every two or three hours.

To improve product quality and conserve water resources, the owner of the company and his team of consultants spent nearly five years developing an ozonated wash system that applied the known benefits of injecting ozone into the flume water as a means of sanitizing it. His goals were to:

•  Obtain a longer shelf life for the product.

•  Provide a fresher taste and lower bacterial counts for the product.

•  Reduce the plant’s water consumption.

In the original system prototype, coarse suspended solids were removed from the flume water by a rotary screen with a 0.079" (2 mm) gap. It became apparent during initial testing, however, that fine suspended solids also needed to be removed before the ozone was injected in order to improve its dissolution in the flume water.

After further design and testing, a new water cleaning and ozonation system was installed, which included mechanically cleaned filters equipped with 50-µm wedgewire screens. The filters were placed immediately after the coarse rotary screen to capture the fine lettuce particles and prepare the water for ozonation injection.

The mechanically cleaned filter is specifically designed to remove suspended solids from viscous fluids. The filter assembly includes a proprietary cleaning disc that moves up and down the filtering screen, scraping debris from the screen and collecting it in a chamber at the bottom of the filter. Debris is purged periodically from the collection chamber by a discharge valve in a process that takes less than seven-tenths of a second without interrupting production. Because the screen is cleaned without affecting production, a consistently high flow rate is maintained.

The filter assembly’s continuous filtering capability means the chilled 50 psi (3.4 bar) flume water is cleaned and recycled in a 211 gal/min loop without interruption. Fine filtering improves the dissolution of ozone in the flume water, resulting in cleaner flume water, longer runtimes between water changes, and reduced water consumption.

Now, flume water replacement is required only once a day instead of every two to three hours. The company owner is also pleased with the system’s impact on product quality. His goals of longer product shelf life, fresher taste, lower bacterial counts and decreased water consumption have been realized.

In addition, the filters have improved the performance of the water chiller in the sanitizing system. Immediately after filtering, the flume water is pumped to a chiller, where it is cooled to 41°F (5°C) by spraying the water onto heat exchanger plates. The high-quality filtering eliminates the risk of suspended solids clogging the sprayer nozzles, keeping the water chiller at optimum performance.

 The mechanically cleaned filter’s continuous filtration makes it a suitable solution for produce-washing applications.  

Web Exclusive: How Mechanically Cleaned Filters Work