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Find information on animal health topics, written for the veterinary professional.

Basics of Food Safety

By Donald L. Noah, DVM, MPH, DACVPM, Midwestern University ; Stephanie R. Ostrowski, DVM, MPVM, DACVPM, Department of Pathobiology, College of Veterinary Medicine, Auburn University

The USA has the safest and most plentiful food supply in the world. These attributes are frequently and mistakenly taken for granted. In truth, however, they are maintained by motivations such as increases in production efficiency, reductions in market uncertainty, and the goal of higher profits.

Foodborne Pathogen Requirements

Nearly all foods contain pathogens, which originate from either the product itself or contamination during processing. As an example, CDC reports that 24% of sampled raw chicken parts were contaminated with Salmonella organisms. Regardless of their source, pathogens have common environmental/nutritional requirements, each of which presents various control opportunities. The acronym FAT TOM can aid in learning these common requirements.

Food (some source of energy): Controlling this requirement is perhaps the most difficult because, as the nutrient content of the food is reduced, the quality and usefulness is similarly reduced. However, certain methods of processing and preserving serve to limit microbial access to nutrient sources. Nearly all foods are washed, rinsed, sifted, sorted, or trimmed during processing, which serves to limit the initial level of contamination. Additionally, simple packaging increases the shelf life of many foods and prevents contamination from the environment and other food items.

Acidity (measured by pH): The optimal pH range for microbial growth is 4.6–7.5. Therefore, for items such as bottled sauces/condiments and pickled foods, a common and effective preservation technique is to lower the pH (acidify) of the item below 4.6. Conversely, some food products (eg, fish, olives, eggs) can be processed with sodium hydroxide (lye) to raise the pH above this range.

Temperature: The optimal temperature range for many foodborne pathogens is 40°F–140°F. For this reason, prepared foods are held in this range for as little time as possible. As a general rule, 4 hr is considered the maximal time in this danger zone to limit microbial growth.

Time: Bacteria require time to propagate to the point of being infective and virulent. In general, 4 hr is considered the maximal time period in the temperature danger zone in retail food establishments.

Oxygen: Most foodborne pathogens are aerobic in nature, ie, they require oxygen for multiplication. Oxygen also facilitates the spoilage process. Techniques that limit available oxygen include drying, canning, bottling, vacuum packaging, adding antioxidants, and modifying the storage atmosphere by replacing oxygen with an inert gas such as nitrogen or carbon dioxide. An added benefit to a modified/controlled atmosphere is that it slows the ripening process and can extend the shelf life of many fruits and vegetables by months.

Moisture: Foods lower in water activity are more resistant to the growth of many foodborne pathogens. By definition, the water activity (aw) of water is 1. Foods with an aw <0.85 are considered generally safer than foods with a higher aw. Techniques that lower the water activity of foods include drying, dehydrating, freezing, salt/sugar curing, and pickling.

Food Processing and Preservation

Virtually all foods are processed to some degree. Examples of minimal processing techniques include washing, peeling, slicing, juicing, freezing, drying, fermenting, and pasteurizing. More extensive processing techniques include baking, frying, smoking, toasting, puffing, shredding, flavoring, coloring, and fortifying. Regardless of the degree, foods are processed for the purposes of preservation, safety, variety, convenience, nutritional enhancement, and increased marketability. Although these processing techniques have resulted in a safer and more plentiful supply, health concerns about food processing have been raised because of certain attributes of processed foods, such as added sugar, sodium, saturated/trans fats, refined grains, low fiber content, and their conducive nature toward unhealthy behaviors. Moreover, processing techniques such as grinding increase the surface area of foods, as in the case of hamburger, which renders them more capable of supporting the growth of pathogenic microorganisms.

In the 1960s, the National Aeronautics and Space Administration (NASA) asked the U.S. Army and the Pillsbury Company to develop safe foods for manned space flights. In a novel approach, Pillsbury required contractors to identify “critical failure areas” and eliminate them from the production system. This systematic approach became known as the Hazard Analysis Critical Control Point (HACCP) process. The underlying principle of HACCP, which focuses on health safety issues rather than quality issues, is to prevent the hazards during processing rather than merely inspecting the end product. Since that time, HACCP has been mandated by FDA and USDA for many food production sectors and also has been implemented in the cosmetics and pharmaceutical industries.

The HACCP process consists of the following seven principles: 1) Conduct a hazard analysis, ie, determine the hazards and preventive measures that could be applied to prevent them. 2) Identify critical control points, ie, any point, step, or procedure in a food manufacturing process at which controls can be applied. 3) Establish critical limits for each critical control point, ie, the maximum or minimum value to which a physical, biologic, or chemical hazard must be controlled at a critical control point. 4) Establish critical control point monitoring requirements, which are necessary to ensure that the process is under control at that point. 5) Establish corrective actions, or actions that must be taken when monitoring indicates a deviation from an established critical limit. 6) Establish procedures to ensure the HACCP system is working as intended, ie, validate the specific HACCP plan. 7) Establish record keeping procedures; each plant must maintain records of the entire process, as well as records to verify the plan.

The primary purposes of food preservation are to inhibit the growth of pathogenic microorganisms and to retard organic degradation such as oxidative rancidity. Many processes involve the use of multiple techniques. In addition to extending the shelf life of food products, the quality and acceptability of many food products, such as cheese, yogurt, and pickled onions, are actually enhanced by the preservation process. These purposes are attained through one or more actions of the preservation process. They include reducing the existing pathogen load, altering the pH and temperature, lowering the oxygen content, reducing the water available for microbial growth, and providing a physical barrier to contamination.

Traditional food preservation techniques, aimed toward one or more of the above purposes, include the following: drying (reduces water necessary for microbial growth), refrigeration (slows microbial growth and enzymatic activity), freezing (preserves food for longer periods), salt or sugar curing (reduces water [especially in meats and fruit]), smoking (coats foods with natural antimicrobials), pickling and brining (reduces both aw and pH), canning and bottling (reduces oxygen and provides physical barrier to contamination), jellying (cooking in a medium that cools to form a gel [eg, aspic]; reduces aw), jugging (meats stewed in earthenware jugs or casseroles), burying (reduces aw, oxygen, light, temperature, and pH), and fermenting (adds beneficial microbes that successfully compete with pathogens; also lowers pH and adds alcohol).

More modern food preservation techniques are listed below.

1) Pasteurization is used mainly for dairy products and other liquid foods, and it can reduce microorganisms by 99.999%. In a “high temperature, short time” process, the product is held at 161°F for 15 sec. In an “ultra-high temperature” process, pasteurization is achieved after reaching 275°F for 2 sec. In a less conventional technique, home pasteurization involves holding the product at 145°F for 30 min. Finally, an “extended shelf life” process combines heating with filtration.

2) Vacuum packaging is usually in air-tight bags or bottles.

3) Additives typically involve the addition of antimicrobials to limit the growth of microorganisms or antioxidants to inhibit spoilage. Common antimicrobials include calcium propionate, sodium nitrate, sodium nitrite, disodium EDTA, and various sulfites. Common antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), ascorbic acid (vitamin A), and tocopherol (vitamin E).

4) Irradiation, which is commonly termed “cold pasteurization,” involves exposing the product to low-dose ionizing gamma rays from a radioactive source such as cesium (Cs-137) or cobalt (Co-60). The process does not render the food radioactive but results in the killing of nearly all surface pathogens. Commonly used for products such as spices and fruits, food irradiation is endorsed by the World Health Organization and approved by the FDA. Since 1986, all food treated with irradiation must display the radura.

5) Pulsed electric field electroporation consists of brief pulses of a strong electric field that enlarges cell membrane pores, which kills microorganisms. It is commonly used for fruit juices.

6) Modified atmosphere replaces oxygen with an inert gas such as nitrogen or carbon dioxide. It is commonly used for salads, grains, apples, bananas, and fish.

7) Nonthermal plasma treats food surfaces with “flame” of helium or nitrogen.

8) High pressurization reduces microorganisms while retaining freshness. It is commonly used for deli meats and guacamole.

9) Biopreservation is the addition of beneficial microbes such as Lactobacillus to compete with pathogens.

10) “Hurdle” technology combines multiple techniques to produce additive benefits. An example of placing multiple hurdles in such a combination might involve high temperature and salt curing during processing, addition of antioxidants before packaging, and low temperature during storage and transportation.

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