Food Safety Focus (130th Issue, May 2017) – Food Safety Platform
Bacteria in Raw Meat vs Cooked Meat
Reported by Dr. Fiona FONG, Research Officer,
Risk Assessment Section,
Centre for Food Safety
Recently, bacterial contamination of meat has attracted public attention. According to the World Health Organization, contamination of food by microbiological agents is a worldwide public health concern; and most countries have documented significant increases over the past few decades in the incidence of diseases caused by microorganisms in food. In this article, we will introduce some factors that determine the growth of bacteria in food, and discuss the different food safety considerations for bacteria in raw meat and cooked meat and the measures to reduce risks of food poisoning caused by bacteria.
Factors Determining the Growth of Bacteria in Food
Bacteria grow best when intrinsic and extrinsic properties are optimal for their growth. Intrinsic properties are the properties that are inherent parts of the food, such as pH and water activity, while extrinsic properties are the properties of the environment in which the food is stored, such as temperature. Water activity is not the same as the moisture content of the food but is a measure, ranging from 0 to 1, of the availability of water in food which determines the growth and survival of bacteria. By controlling these factors (e.g. controlling the storage temperature of the food), bacterial overgrowth can be prevented.
Bacteria in Raw Meat
Fresh meat is a highly nutritious substrate with water activity of about 0.99, meaning that it is suitable for the growth of most microorganisms. Raw meat in general contains bacteria, including pathogenic and spoilage ones. As warm-blooded animals naturally carry bacteria such as Salmonella spp. in their intestines, raw meat may be contaminated with bacteria during the slaughtering process such as evisceration and dressing procedures. In addition, the equipment and tools used in the processes, the hands and clothing of personnel as well as the environment may also contaminate the meat with bacteria.
Raw meat should be cooked thoroughly before consumption.
Meat has potential to carry foodborne pathogens that can cause illness and lead to food safety problems. These pathogenic bacteria are able to invade our bodies or produce toxins to cause illness. They cannot be seen or smelled on the meat, but can generally be killed by normal cooking conditions (i.e. cooking to a core temperature of at least 75°C instantaneously or other effective time/ temperature combinations).
Pathogenic bacteria may need to compete with other bacterial flora (e.g. spoilage bacteria) for growth on the meat. Certain pathogenic bacteria such as Staphylococcus aureus are relatively poor competitors and may be outgrown by other flora. Spoilage bacteria will cause food to deteriorate or lose quality by developing a bad odour or feeling sticky on the outside of the meat, signs that consumers would normally notice. These spoilage bacteria are normally not harmful, however, when consumed in very large numbers, they can cause gastrointestinal disturbance. Consumers should throw away the meat that shows any signs of food spoilage.
Bacteria in Cooked Meat
As mentioned above, thorough cooking can generally destroy most bacteria on raw meat, including pathogenic ones. Nevertheless, if there are subsequent lapses in food safety practices, food poisoning may still occur. To start with, raw meat may be contaminated with spores of certain pathogenic bacteria (e.g. Clostridium perfringens) and spores are not readily destroyed by normal cooking temperature. Heat of cooking can rather activate the spores to germinate and develop into vegetative cells which can multiply rapidly in foods that are placed at ambient temperature for a long period. Consuming foods that contain high levels of Clostridium perfringens vegetative cells may lead to foodborne illness.
In addition, pathogenic bacteria may be introduced into the ready-to-eat cooked meat through cross-contamination and multiply to larger amount as a result of time and temperature abuse of the food, causing foodborne illness in consumers.
Measures to Reduce Risks of Food Poisoning Caused by Bacteria
To prevent food poisoning, raw meat should be cooked thoroughly before consumption. The ready-to-eat cooked meat should be discarded if it has been held at room temperature for more than 4 hours. If the cooked meat is held at room temperature for less than 2 hours, it can be refrigerated for final use later or used before the 4 hours limit is up.
Moreover, good hygienic practices should be observed. Hands, cutting boards, knives, and other utensils should be washed thoroughly after touching raw meat. Measures should be adopted to the prevention of cross contamination between raw meat and ready-to-eat foods including cooked meat, e.g. using one cutting board for ready-to-eat foods and a separate one for raw meat.