When Microbial, Processing & Packaging Issues Collide

Microorganisms in food can be good, bad or ugly. In some cases, they can be very beneficial; for example, pickles, sauerkraut, and fermented sausages have specific species of lactic acid-producing microbes that produce antimicrobial agents against other microorganisms but do not harm people. On the other end of the spectrum are pathogens which tend to get a lot of attention. In the middle are problematic non-pathogenic microbes that pose a unique challenge for the food industry: spoilage microorganisms.

Spoilage microorganisms can be opportunistic pathogens, but the major challenges they present relate to food quality, not safety. These include bacteria, yeasts, and molds that are found in foods from natural environments and manufacturing facilities. Several types of degeneration can result from these. Understanding the nature of raw materials and processing and storage conditions can help prevent these microbes from impacting product quality. Although there are non-microbial defects that can impact quality, this column focuses on microbial causes.

When spoilage has a microbial cause, it is generally associated with microorganisms not being killed or controlled during the food manufacturing process or by penetrating what appears to be a sealed package. It is critical to understand that packaging can play a role, so when the GMA lab receives a sample for spoilage analyses, the microbial analysis is paired with an analysis of the package.

Microbial spoilage for packaged goods is usually defined by the following five types of spoilage (See Table 1, below):

1. Thermophilic spoilage (caused by bacteria that grow at temperatures above 130°F). Spore-forming bacteria are able to withstand harsh conditions that would otherwise kill vegetative cells. Spores can endure heat, acid, and other adverse conditions. When favorable conditions exist, germination occurs and cells start replicating and growing in food. Because most food processes are established to kill vegetative cells, thermophilic spores will sometimes remain in food products. This doesn’t usually result in spoilage because after being processed, products are not generally stored or held at thermophilic temperatures. In addition to the ability to grow at temperatures above 130°F, some thermophilic spore-forming microbes produce gas during growth resulting in a hard swell of the package. If thermophilic microbes do not form gas they can darken the food product or significantly lower the pH.
    
2. Flat-sour spoilage. When thermophilic spores germinate, grow, and do not produce gas, the pH of the product will drop; this is called flat-sour spoilage. This also can be caused by mesophilic (spoilage caused by bacteria that grow at ambient temperatures of 86°F to 98.6°F) spore-forming microorganisms. If flat-sour spoilage occurs in a finished product, the only clues that the product is spoiled are an off odor or appearance, or a low pH found when the package is opened, because the package remains flat. This spoilage can occur in low-acid canned foods and acidified foods, but it may be harder to detect with acidified foods since their pH is already low. Other types of spoilage organisms related to these foods are those that can evade the mild heat treatment required. As explained by Moir et al. in “Spoilage of Processed Foods: Causes and Diagnosis,” heat-resistant molds such as Bysscochlamys fulva or fungi such as Talaromyces flavus can survive the processes established by process authorities for acidified foods.
    
3. Under-processing. When the appropriate kill step is not properly applied, vegetative cells may be present after food processing to cause this type of spoilage. Gross under-processing will reveal a mixed flora of yeasts, cocci, coccobacilli, and bacilli. Under-processing is described by Weddig et al. in “Canned Foods: Principles of Thermal Process Control, Acidification and Container Closure Evaluation” and by Moir et al. as occurring when processing times or retort operations are not correct, where food products aren’t completely thawed prior to manufacturing, when errors occur in filling weights, and with excessive microbial contamination. In low-acid shelf-stable products, gross under-processing is a serious concern because any type of microorganism from the environment or food prior to processing can be present, including pathogenic Clostridium botulinum which an anaerobic (i.e., canned) environment may allow to grow and produce toxin. The food industry works hard to control this hazard. Most of the reported cases of foodborne botulism caused by C. botulinum are associated with home-prepared foods; the last canned food outbreak occurred in 2007.
    
4. Post-process contamination. Packaging failures or microbial contamination after processing during conventional canning can lead to another type of spoilage: post-process contamination. This occurs when an event after the final kill step causes spoilage, such as microbial contamination from false seams or a double seam that is out of specification allowing for microorganisms to pass the barrier. If the packaging has no defects allowing contamination and the food is properly processed, the only real opportunity for contamination is during seaming. This is most likely to occur when the seaming is setting and containers are exposed to the cooling water after thermal processing. Bulging, stressed cans are susceptible to microorganisms that may be present in the cooling water. Therefore, properly chlorinating the cooling water, or using an effective sanitizer, can help lower spoilage contamination risks after processing. Additionally, Weddig et al. notes that post-process contamination in an aseptic system can result from contaminated valves, an unsterile air environment/air filter, or other issues because the food and packages are sterilized separately before coming together in the final product. This means that, in addition to testing for microbial recovery, a review of packaging records is essential, because packaging defects may cause spoilage events. Post-processing packaging defects include food reactions to exposed metal in cans (hydrogen swells, pinhole formations, and sensory differences).
    
5. Incipient spoilage. In some instances, one may see many microorganisms under the microscope that cannot be recovered in the laboratory. In this particular case, heavy contamination of a product with nonviable organisms is called incipient spoilage. This type of spoilage occurs when manufacturing lines of packaged food are open to the environment giving microorganisms the chance to contaminate the food product or allow microorganisms present in food to multiply and reach high populations prior to processing. The thermal process may be effective in killing the high levels of organisms present, but differences in odor, flavor, or even gas formation may persist, resulting from the initial growth of microorganisms before processing.
    

Summary.

Although a focus on pathogens in food products is justified, spoilage issues continue to be an ongoing concern and have the ability to damage a company’s brand in the eyes of the consumer. Therefore, completing an analysis of the types of microorganisms recovered in spoiled shelf-stable products, as well as an analysis of processing factors, packaging considerations, and other environmental conditions, can help identify the problem. Lessons learned from the spoilage event then can be used to prevent future problems.