Jeffrey A. Trogolo, Ph.D.

Summary
Members of the water industry work hard to produce high quality, clean, delicious water. Unfortunately, the wetted surfaces of water systems are working against you by serving as substrates for microbial colonization and biofilm formation. While not all microbes are dangerous to our health, after all the effort to produce quality water and beverages, microbes on wetted surfaces can cause discoloration, taste, odor, and slow flow in filtered systems. New silver antimicrobial smart materials now available to the water industry for production, distribution and dispensing applications allow manufacturers to fight back and maintain the superior water quality through to the consumer.

Introduction
The bottled water and beverage equipment industry has a strong focus on hygiene in processing, distribution and dispensing environments. Food processing, including beverages, has been under growing scrutiny by government, media and consumers. Much of the attention focuses on proper handling at the bottled water or beverage producer; however, more recently the distribution market has had more opportunity to contribute to product quality.

The surfaces in bottled water and beverage dispensing systems are constantly wet or damp, which is ideal for colonization by microorganisms. Often such beverage systems are in public areas with high traffic and low control over contact with the dispensing end of the system, where contamination can begin and ultimately form hard-to-clean biofilms. Bacterial biofilms can impart taste and odor; in addition, portions can slough off and colonize surfaces, clog filters, or end up as unsightly particles floating in a drinking glass.

Advanced silver materials containing antimicrobials are a new tool for the water and beverage distribution and dispensing industry. These additives to plastics and coatings combat bacterial colonization and biofilm formation on the wetted internal surfaces of products such as water coolers, filters, hydration bags, ice machines, beverage tubing and dispensing equipment. The objective is not to treat the water or beverages themselves, but to maintain a neutral surface and avoid biofilms that can influence quality by adding or changing flavor.

Microorganisms are not necessarily harmful. Most heterotrophic plate count (HPC) organisms found in drinking water are not a health concern. However, taste, odor and appearance can be affected by their presence. Hygiene surrounding beverage dispensers and water coolers is important for aesthetic reasons, but are more critical in public environments where regulatory requirements are involved. The European Bottled Water Association (EBWA) has recently produced guidelines for water cooler sanitation and disinfection in response to publicly voiced concerns about hygiene. The guidelines published by the EBWA, arguably the highest quality standards in the bottled water industry worldwide, establish sanitization and disinfection protocols, cleaning frequency and metrics for measuring the hygienic condition of a water cooler.

Existing solutions
The existing approaches to preventing an increase in the microbial population within a water distribution and dispensing system involve periodic cleaning or significant modifications of standard cooler or beverage dispensing system designs.

While cleaning is an obvious course of action to combat microbial colonization, it has drawbacks. Most of the components in a cooler or dispensing system are hard to access; chemical treatments are rarely successful at removing established biofilms and the cleaning agents themselves raise environmental and safety concerns. The only sure way to completely remove biofilms from a surface is to mechanically remove them, which requires careful cleaning that is not always practical. Even if the biofilm is physically scrubbed away, the mechanical action of cleaning roughens the surface, making fine scratches from which biofilms are even harder to remove.

Another approach to water and beverage distribution and dispensing hygiene is to incorporate active systems such as UV or ozone into the system to control microorganisms. These systems can be effective; however, they require maintenance and are costly. In addition, they require expensive redesign of existing models to accommodate the new technology, and then only customers willing to upgrade to the new designs will benefit from the improved technology.

The smart materials solution
In beverage and water applications an ideal approach to controlling surface colonization and biofilms would have the following characteristics:

  • passive, no maintenance required
  • no product redesign necessary
  • proven long-term efficacy in water applications
  • meets all regulatory requirements
  • economical in existing product designs
  • designed to control surface colonization and biofilm formation

A new commercial technology able to meet these criteria is one of the new silver based smart antimicrobial materials. The term smart material refers to a materials system that senses environmental conditions and appropriately responds to perform a useful function. In water and beverage systems, smart antimicrobial materials can provide long-term performance by “turning off” when not needed and then activating again when conditions are right for bacterial colonization.

Inorganic antimicrobial additives
Inorganic antimicrobial additives are fine powders that are easily blended into plastics and coatings using conventional methods. Of the three primary silver-based systems available, zeolite and zirconium phosphate (ZrPO3) utilize ion exchange for controlled release of silver, while phosphate glass depends upon dissolution to for release.

Zeolites
The unique crystalline structure of zeolites results in interconnected internal pore structure with pore diameters on the order of a nanometer or less. The ion exchange release mechanism is primarily responsible for the controlled release capability of these materials. Ion exchange requires charge neutrality at the surface of the zeolite; therefore, no silver ions can be released unless a cation is present for exchange, providing a crucial limit on release kinetics. In addition, the low equilibrium concentration limits total release. Therefore when a surface is wet, the zeolite-containing surface becomes active, releases silver to an antimicrobial concentration, then “turns off”, reserving the silver reservoir.

In this way the zeolite creates a truly smart material surface that senses conditions in which microorganisms can grow. For example, Figure 2 is from a study published by Dr. Charles Gerba at the University of Arizona in which water containing Legionella pneumophelia was applied to an epoxy coating containing silver zeolite. The coating will not be active in dry conditions but water containing viable organisms triggers silver release. Therefore, the conditions that activate silver release are precisely those required for bacterial colonization and multiplication.

In addition to excellent controlled release characteristics, zeolites meet the safety requirements required for use in a drinking water system. The U.S. requires that the antimicrobial agent be registered with the EPA and approved by the FDA for use in materials contacting food and water. Certain grades of silver zeolite have undergone extensive safety testing and have been registered and approved for food and drinking water contact. If considering a particular product, confirm that drinking water applications are specifically listed on the EPA label.

Zirconium phosphate
Zirconium phosphate is a clay-like mineral that, unlike the three-dimensional pore structure of the zeolite, possesses a layered structure with silver in the layers. The silver is released from between the layers by ion exchange with ambient ions. However, one ion exchange can only occur at the edges of the layers, which can impede release kinetics when embedded in a plastic or coating. Since silver does not permeate most polymers, only the ZrPO3 particles oriented at the surface with an edge exposed will release silver. The ZrPO3 powder particle size is under one micron so particle exposure at the surface is hindered by the surface energy of the polymer; therefore, silver surface availability is relatively low. These characteristics may be detrimental to ZrPO3 in challenging water applications where the release rate from a surface must be high enough to inhibit colonization, and may explain why products containing ZrPO3 have not produced the type of long-term quantitative test results published by products using silver zeolites.

Phosphate Glass
Certain formulations of phosphate glass can be designed to dissolve gradually in water. Silver has been incorporated into these glasses, which are powdered and blended into plastics and coatings much like the zeolite and ZrPO3 powders. When the glass dissolves, silver ions are released to provide an antimicrobial effect. One characteristic of silver phosphate glass is that it is transparent, which means that it has only a minor effect on transparency of thick sections. Dissolution as a release mechanism, however, is challenging because the kinetics of aqueous dissolution are very sensitive to water chemistry, temperature and, in particular, flow rate. Therefore, in a water system the antimicrobial glass product lifetime may vary widely depending on local conditions, type of input water, and system use. Few water applications have utilized silver phosphate glass and presently no quantitative long-term water test data has been provided by manufacturers using the technology.

Commercial applications of silver antimicrobials
Filters and Filtration Systems
Water filtration systems are a natural application of silver-based antimicrobials. Colonization of filter media results in increased pressure drop and reduced absorptive performance of activated carbon. Silverized carbon has long been used to combat microbial growth in filters; however, it does not benefit from the controlled release mechanism employed by silver-based additives. The data in Figure 3 demonstrates the controlled release capability of silver zeolite in an activated carbon filter. Note the “burst effect” observed with silverized carbon, which can exceed regulatory limits and reduce end-of-life performance, since that burst of silver is not available later.

The controlled release behavior of silver zeolite is beneficial to filter manufacturers using it in their filter units. One of the largest filter manufacturers in the United States has been using silver zeolite for several years and has produced in excess of 4 million filters containing silver zeolite. In addition to the filter media, filtration equipment manufacturers can utilize silver antimicrobials in the housings and fittings to control bacterial colonization and biofilms on all surfaces in their products.

Water coolers
As mentioned above, the European Bottled Water Association (EBWA) has recently produced guidelines for water coolers that require sanitization and disinfection of the wetted surfaces of the water cooler every six months and recommend it every three months. The bottled water supplier, who provides the coolers and water supply to the customer, is responsible for the added cost of these hygienic procedures.

One innovative water cooler manufacturer in Italy, Cosmetal, has lead the industry by enhancing their flagship Avant cooler with a silver zeolite product specifically designed for long-term water applications. To quantify the ability of antimicrobial surfaces to control microorganisms and biofilms in the cooler, Cosmetal tested coolers with and without silver zeolite, measuring the bacteria content of water from the cooler as an indirect measure of colonization, since the incoming bottled water contained relatively few microorganisms.

The results of the study, provided in Figure 4, indicate that the cooler with antimicrobial surfaces significantly inhibits microbial colonization for over 12 months. In addition to testing the water from the cooler, the internal wetted surfaces of the antimicrobial cooler were swab tested after 13 months using EBWA protocols, demonstrating that even after a year of use, silver zeolite protected the cooler surface, keeping surface colonization to less than 50% of the EBWA limit. Conventional cooler designs can only meet that standard if the bottled water supplier performs two to four sanitization and disinfection procedures per year.

Therefore, the silver zeolite antimicrobial has provided Cosmetal with the market advantage of offering cost savings to the water distributor, as well as top quality water to the consumer. “Cosmetal recognized a market need for antimicrobial surfaces and biofilm control in our products and found the AgION silver zeolite to perform the best.” states Giampiero Galizia, Director of Technical Operations at Cosmetal, “Silver zeolite is the only antimicrobial to produce long-term data showing excellent results. This new Self-Cleaning Technology positions our products at the forefront of innovation in the water cooler industry.”

Tubing, fittings, and dispensing
Water and beverage distribution and dispensing systems have a potential to form biofilms that can affect the taste and odor, and are difficult to clean. Tubing manufacturers, however, are now taking advantage of the easy processability and marketability of inorganic silver antimicrobials. Eldon James Corporation, the manufacturers of Flexelene Silver tubing and fittings, which utilize the silver zeolite technology, is pursuing the water and beverage markets.

To evaluate the long-term performance of the tubing, high volume performance studies including antimicrobial efficacy tests were performed. The results, presented in Figure 5, demonstrate the antimicrobial efficacy of the tubing through 40,000 liters.

The market is responding positively. Brewer Adolph Coors Company has recently approved Flexelene Silver tubing, allowing distributors to specify it for their draft beer accounts. Currently Coors requires that lines over 25 feet long be cleaned weekly and shorter lines be cleaned every two weeks. Use of the antimicrobial tubing may allow extended time between cleanings, which translates into significant cost savings. Mike Smith, national draft manager for Coors, told the Northern Colorado Business Report, “I think it’s going to benefit Coors and the industry. It will allow us to fight a lot of the bacteria that harms beer.” Smith added that the antimicrobial tubing will help distributors reduce cost, save time, and cut down on liability, a potential issue anytime a cleaning solution is introduced into a beer line.

Conclusion
The overall mission of the water and beverage industry is to produce a high quality product using practical, economical and effective methods. Distribution and dispensing systems have the potential to reduce quality by affecting taste, odor and appearance. Safe, effective silver antimicrobials that have proven long-term performance are a sensible part of total hygiene.

Increasing attention from government, industry organizations and consumers is being placed on the quality of food and water, particularly with respect to microorganisms. Actions taken by the EBWA are likely only the beginning of a trend toward more strict hygienic standards in drinking water and beverage systems.

Commercial products containing silver zeolite have emerged as industry leaders with proven long-term performance and have demonstrated value to manufacturers and users. It is expected that other technologies will also be evaluated now that silver zeolite has provided an industry performance benchmark.

Acknowledgements/References
We would like to thank Dr. Charles Gerba, University of Arizona, for his contributions.

1“Rapid Reduction of Legionella pneumophila on stainless steel with zeolite coatings containing silver and zinc ions” P. Rusin, K. Bright and C. Gerba Letters in Applied Microbiology 2003, 36, p69.

About the author
Dr. Jeffrey A. Trogolo is Chief Technology Officer at AgION Technologies, Inc., developer of controlled release antimicrobial solutions for consumer, industrial and medical applications. He holds a doctorate in materials engineering from Rensselaer Polytechnic Institute. Dr. Trogolo has over 10 years experience in surface engineering processes, including controlled antimicrobial release and surface modification of biomaterials in the medical device industry.

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