"Increasingly, all types of consumers are demanding minimally-processed foods that are high in quality, nutritionally superior, and easy to prepare," he says. "Food processors have met this demand by developing refrigerated foods with extended shelf life, [such as] complete heat-and-eat meals, fresh pasta, and [pre-washed or deli-style] salads."
However, contrary to past conventional wisdom, scientists now know that several pathogens, such as Escherichia coli O157:H7, Listeria species, and certain strains of Clostridium botulinum, can grow at refrigeration temperatures, which means that manufacturers must stringently apply control measures to refrigerated foods with extended shelf life.
According to Marth, good manufacturing practices (GMPs), sanitation, and hygiene are key to microbiological control. These include using high-quality raw materials with low levels of microorganisms, selecting food processing equipment that is easy to clean and does not harbor contaminants, sanitizing equipment regularly to prevent build-up of bacteria, checking equipment for cleaning adequacy with microbiological tests, filtering the air of food processing areas to reduce airborne contaminants, and training personnel to use hygienic food handling practices. Expanding upon GMPs, establishing a Hazard Analysis Critical Control Point system is also important to fully identify and control food safety hazards. Beyond food production, acceptable product storage temperature and time must be established and carefully monitored.
The heat treatments used for refrigerated products with extended freshness are lower than those used to sterilize food, thus, they may effectively inactivate bacteria, but not microbial spores. These spores can germinate and grow under conditions caused by product abuse, Marth notes. Proper handling after heating is critical to avoid introducing microbial hazards.
Other control measures that may enhance food safety and are commercially applied include irradiation (approved for meat and poultry, but not seafood), the bacteriocin nisin (an anti-microbial protein produced by certain bacteria), and high hydrostatic pressure.
Spoilage microorganisms, such as yeasts, molds, and Lactobacillus bacteria, are also a concern with refrigerated foods because they may alter product appearance, taste, texture, and odor, Marth says. For example, some species of Lactobacillus produce acetic and formic acid, ethanol, and carbon dioxide, which can spoil a variety of foods, including milk, meats, vegetables, fruit juices, sugary products, alcoholic beverages, and products containing vinegar.
Modified atmosphere and aseptic packaging are useful in extending the freshness of refrigerated products by reducing oxygen and/or increasing gases like carbon dioxide in the food environment, which inhibits bacterial growth. As with all food processing steps, however, such packaging must be used with control measures to be effective, Marth notes.
* This summary, to be published in the February 1998 issue of Food Technology, was produced by IFT's Expert Panel on Food Safety and Nutrition. Principal author Elmer H. Marth, Ph.D., is an emeritus professor in the Dept. of Food Science at the University of Wisconsin. Founded in 1939, IFT is a non-profit scientific society with 28,000 members working in food science, technology and related professions in industry, academia and government. As the society for food science and technology, IFT brings sound science to the public discussion of food issues.