Is Environmental Sampling Sufficient for Food Safety?

In October 2019, the Food Safety and Operating Committee of the Innovation Center for U.S. Dairy published its updated environmental Pathogen Control Guidance Document, a comprehensive document intended to help the U.S. dairy industry control pathogens in wet and dry dairy processing environments (available at usdairy.com/foodsafety). In its guidance document, the Innovation Center details five principles that should be followed to ensure effective pathogen control. These include:

1. Separate raw from ready-to-eat (RTE);
2. Follow Good Manufacturing Practices (GMPs);
3. Institute sanitary facility and equipment design;
4. Implement effective cleaning and sani­tation procedures and controls; and
5. Initiate environmental pathogen monitoring.

These principles are in keeping with a 2022 systematic literature review showing that 10 of the 12 (83%) foodborne illness outbreaks involving pasteurized dairy products from 2007 to 2021 were due to contamination with Listeria, an environmental contaminant (Can J Pub Health. 2022;113:569-578). A similar study that looked at reported outbreaks from 1998 to 2011 coming from both pasteurized and unpasteurized cheese showed that, in 44 outbreaks stemming from cheese made with pasteurized milk, 24% were attributed to Listeria and the remainder were a mix of Salmonella, Campylobacter, Bacillus, E. coli, and others, all considered environmental contaminants (Foodborne Pathog Dis. 2014;11:545-551). The importance of focusing on the five principles of pathogen control is very clear.

One Step Further

But within the realm of environmental monitoring, is the vitally important task of environmental sampling to control pathogens enough? Will a good pathogen environmental monitoring program (PEMP) sufficiently and consistently ensure product safety and a high level of product quality? According to Neil Bogart, a highly regarded expert in dairy safety and the president of Bogart Food Safety and Sanitation Associates, Inc., an Alabaster, Ala.-based food safety and sanitation advisory firm with a primary focus on dairy processing, the answer is, “Perhaps not.”

“While swabbing, [adenosine triphosphate] ATP surface monitoring, and other environmental sampling methods are crucial steps for controlling widespread pathogens,” says Bogart, “they do not provide the complete picture in wet milk processing. Thermoduric organisms, for instance, can carry over from the raw milk supply, or pockets of contamination can become established in processing equipment where swabbing is impractical. This underscores the necessity of a robust process monitoring program to fully validate sanitation procedures and pinpoint contamination hotspots that can significantly impact quality and safety.”

When considering a process monitoring program for cheese and dairy powder processing, for example, emphasis must be placed on spore-forming bacteria due to their ability to survive extreme processing conditions, their potential pathogenicity, and because they possess strong spoilage capacities, which could lead to proteolysis, lipolysis, gas formation, and other quality defects. These bacteria can originate from the soil, feces, bedding, feed, or milking equipment but can also enter the milk via contaminated teats, milking cups, bulk tanks, or transport tankers. Pockets of contamination can also develop within the processing plant due to failures in milk handling, sanitation, or preventive maintenance. Extended production run times exacerbate the problem. Endospores formed by these organisms may survive pasteurization and subsequently germinate into vegetative cells that may be psychrotolerant but prefer to grow in warm conditions, giving them an even greater chance to contaminate many dairy processing environments (Front Micro. 2017;8:1-15).

Sporeformers of primary concern to dairy processors are members of the genera Bacillus and Clostridium; however, except in certain cheese processing, concern over the anaerobic Clostridium is often less than that for their aerobic counterparts. While many sporeformers are not pathogenic and are seen primarily as indicators of hygiene during milk collection, transport, or processing, certain members of these genera are well-known pathogens and, therefore, worrying from a food safety standpoint.

The formation of homogeneous or heterogeneous bacterial biofilm communities on the internal surface of processing equipment is of particular concern to dairy processors because, when present, biofilms can lead to persistent problems of microbial contamination that are often intermittent and hard to pin down. Heat exchangers, pipelines, tanks, gaskets, seals, and other stainless steel processing equipment are primary sites for biofilm formation, especially once a conditioning layer of milk protein is laid down on the surface of the equipment during processing (Comp Rev Food Sci Food Safety. 2012;11:133-147). Biofilm formation is also a leading cause of fouling of reverse osmosis and microfiltration membranes and is a frequent concern in the continuous step of evaporation before spray drying, making these processes especially critical in controlling contaminant outgrowth (Food Res Int. 2021;150:110754; Comp Rev Food Sci Food Saf. 2014; 13:18-33).

Real World Example

The importance of process monitoring was exemplified in a 2007 research study published in the International Journal of Dairy Technology (2007; 60:109-117.). In this study, a team of New Zealand researchers monitored a process stream during five whole milk powder manufacturing runs, each approximately 18 hours in length. The plant was operating at the rate of 40,000 liters per hour. A clean-in-place (CIP) cleaning occurred after every run, and after every five runs the evaporator and direct steam injection unit were manually cleaned to remove foulant build-up. Samples were collected every two hours during processing from 16 sampling locations, beginning with raw milk ahead of pasteurization, after pasteurization, following each of five evaporator passes, and through to the finished product. In addition to vegetative cells, samples were tested for the presence of endospores.

The study found low or no spore counts in samples taken from the end of raw milk treatment, although vegetative cells were found in low numbers. The researchers concluded that in this study, raw milk treatment had very little influence on the thermophile numbers of milk destined for powder manufacture.

Conversely, beginning with samples taken from between the plate heat exchanger and evaporator and carrying on through two stages of evaporation, there was a consistent increase in both vegetative cell growth and spore formation. Spores and vegetative cells were initially detected after about nine hours of production, and by 18 hours, counts exceeded 10,000 colony-forming units per milliliter (cfu/mL). Vegetative growth and sporulation did not increase during evaporator stages three through five. In some production runs, vegetative cell and spore levels decreased during processing after the second evaporation stage, but in other runs, the contamination levels remained relatively consistent.

The authors concluded that the study “confirms that spores were forming within the milk powder manufacturing process and were not a result of external contamination.” They further noted that low levels of contamination could come in from the raw milk, but the contamination found in later stages of production predominately arose from sporulation occurring within the plant, notably from bacteria trapped in foulant (from the evaporator or separator, for example) that remains in the equipment between CIP runs and may be only partially removed during manual cleaning. In this case, the heat exchanger, the preheat section of the evaporator, and the evaporator itself appeared to be the predominant sites of biofilm formation.

Every Situation is Different, but Some Things Remain the Same

Maintaining microbiological quality and safety in dairy processing presents a considerable challenge to dairy processors. In dairy operations where controlling thermoduric, thermophilic, and post-pasteurization contamination is requisite for ensuring consistent quality and safety, wet process monitoring is an essential adjunct to environmental surface monitoring. Microbiological sampling of wet process critical control points helps quality assurance professionals control contamination, validate cleaning and sanitation procedures, and identify sources of milk contamination coming from the raw milk supply, processing equipment, or the surrounding environment.

Every dairy processing operation is different, and processes determined to be “critical” will vary from process to process or plant to plant. However, some processes or plant operations require careful monitoring in every milk processing environment. These include the raw milk both at the time of receipt in the plant receiving bay and immediately before pasteurization; plate heat exchangers; microfiltration or reverse osmosis filtration equipment; any open vats or vessels, including cheese vats and blending or mixing vats; evaporators; scraped surface heat exchangers; filling equipment in wet milk filling operations; and other specialty equipment that may run for extended periods between cleaning cycles. In each case, biofilm formation is a threat, and it is critical to sample both upstream and downstream of the equipment to afford the ability to determine if biofilms are developing on internal surfaces.

Thermoduric and thermophilic vegetative organisms and their endospores are found frequently in dairy products, including milk powders. Single-species and multi-species biofilms formed on milk contact equipment surfaces are a primary contributor to pathogenic and spoilage organism bioburden. These biofilms are difficult to remove from milk processing environments and, if allowed to mature, can cause immeasurable damage to product safety, quality, and reputation, leading to disastrous economic consequences.

As Neil Bogart concludes, “From a practical viewpoint, a carefully conceived and well-implemented process monitoring program that allows managers to optimize and validate sanitation procedures and safely regulate plant operations is about the cheapest insurance money can buy.”

Johnson is a biotech innovator with a 25-year tenure founding and developing companies to advance health technology. A trailblazer in HACCP application in the dairy industry, his early career focused on enhancing dairy safety and quality assurance. He holds advanced degrees in microbiology and biochemistry and serves on the board of directors of QualiTru Sampling Systems.