N60—What It Is and What It Is Not
By Dean A. Danilson, Ph.D.
N60 is a term used to describe a sampling system for beef trimmings for Escherichia coli O157:H7. N60-Trim was initially implemented in 2003; by 2007, many of the larger beef companies had widely adopted it. Today, N60-Trim sampling is recognized as the gold standard sampling procedure for beef trimmings destined for raw ground use across the beef industry.
In 2007, the U.S. Department of Agriculture (USDA) Food Safety and Inspection Service (FSIS) adopted an N60 sampling procedure for regulatory applications to verify that E. coli O157:H7-preventing interventions at plants are working as intended, and to emphasize to plants that FSIS is monitoring them. The FSIS N60 sampling program also provides valuable surveillance data to the agency and the beef industry.
What Is N60-Trim Sampling?
As used by the beef industry, N60-Trim is composed of several key factors:
• 100 percent of trim from the production period is subjected to sequential sampling
• Minimum of 60 sample units/lot
• Thin surface slices, maximized surface area to internal tissue
• Sample units (n=60) targeted for combined total of 375 g
• Targeted external carcass surface, when available
• Maximum five combos or 10,000 pounds per lot
• Enrichment sample preparation in which surface slices are massaged, not ground
• Enrichment method that should be capable of growing targeted organisms to minimum detection level of the rapid test method in time allocated for enrichment growth
• Rapid method specificity and sensitivity that minimally achieves the USDA-defined specificity and sensitivity targets and can detect a minimum of 15 CFU/375 g, 100 percent of the time
• Scientific validation of lab methods, with periodic verification through check-samples and external audits
• Event-window management, which involves intensified investigations on multiple positives coupled with expanded product disposition judgments on “negative-but-associated” trim and/or “untested” primals produced within an event-window situation; potentially higher-risk product should be further evaluated and/or diverted from raw, nonintact end use
N60-Trim is more than just the collection of 60 pieces from a batch of meat. When each of these elements above are in place, N60-Trim serves as ongoing verification of an establishment’s food safety systems and processes employed for the control of E. coli O157:H7.
Data from an N60-Trim program is critical in assessing multiple-hurdle intervention capabilities and individual plant performance, as well as in providing valuable historical baseline data for trend analysis and continuous plant improvement. In its totality, N60-Trim is a robust and effective food safety system management program for short-term prevention and long-term improvement of E. coli O157:H7-related illness attributed to raw beef products.
In their simplest binomial form, n=60 samples provide 95 percent probability of finding a contaminant in a lot when the population incident is 5 percent or greater, the contaminant is uniformly distributed and samples are randomly acquired. In this context, n=60 is the basis of the ICMSF Case 15 sampling plan.
In conditions of 5 percent or greater population incident, n=60 is an effective “accept/reject” sampling plan. In conditions of lower population incident, n=60 sampling alone has lower probability of detection and thus becomes less reliable for accept/reject decisions for a stand-alone product lot.
However, when n=60 lot testing is coupled with systematic sequential sampling across “all” trim in a production period, the power of an N60-Trim system to demonstrate process control and identify high-risk incident windows is significantly magnified. With all of the above elements in place, this constitutes a robust sampling program.
For example, consider a plant that conducts 100 lot tests/day over two shifts. During that production period, 6,000 pieces are tested, representing a very comprehensive surveillance on the trim production output for that day. Multiple positives found in a production period, or in windows within the period, signal that an event may have occurred and that additional product may need to be diverted from raw ground and raw nonintact end use.
Multiple positives trigger event-window management procedures. Test results are received approximately 12 to 18 hours after the sampling is actually conducted. When an “event alert” signal occurs, related products are immediately brought under control pending intensified investigation for root cause determination and/or unique line-time-product relationships that define the scope of a contamination event and the level of other actions that may be needed.
Definitive investigative outcome allows for more targeted and specific product scope and actions, so the more information and data, the better the decision making on necessary product actions. In a multiple positive event, a specific root cause, a process or system failure or a specific line-time-product relationship may or may not be identified. Each case is different and requires understanding of the respective processes and plants along with experiences from previous incidents to direct expanded product actions to minimize risk and provide safe raw product to the consumer.
Are All N60 Programs the Same?
Yes and no. For plants that employ a robust N60-Trim program, the basic elements of N60-Trim detailed above are essentially the same. Variations do exist, things like five-combo versus single-combo lots, more than n=60 samples per lot, different sampling tools or methods, different rapid methods, etc. These variations, if appropriately validated, do deliver to the robust expectation of the standard N60-Trim program.
More importantly, an N60-Trim program not designed appropriately or executed effectively by a plant is of much greater concern than these validated variations. This concern applies to all elements of the N60-Trim program described above, especially the critically important event-window management procedures. For a purchaser of raw beef trim from different plants, it is prudent to be knowledgeable of the N60-Trim program used by an establishment and watchful of the execution of the program day-in and day-out.
What N60 Sampling Is Not
When used as a stand-alone test on a population of very low incidence of E. coli O157:H7, N60 in and of itself is not a high-probability accept/reject sampling tool.
However, an N60-Trim program executed consistently with discipline is a very powerful and effective food safety system management process that has been a key contributor in the reduction of illnesses and recalls attributed to consumption of ground beef. As Dennis Johnson of Olsson Frank Weeda Terman Bode Matz PC stated in the National Provisioner, “Of the outbreaks I have worked from 2007 to 2010, in all but one, no packer that conducted an aggressive N-60 excision sample even had product in the implicated blend.”
This testimonial, along with the beef industry’s N60-Trim track record over the past several years, speaks volumes about the value of N-60 sampling.
Dean A. Danilson, Ph.D., is the vice president, food safety and quality assurance at Tyson Foods. He received his Ph.D. in animal science from Virginia Polytechnic Institute & State University.
1. ICMSF. 2002. Microbiological testing in food safety management. New York: Kluwer Academic Plenum Publishers.