Microorganisms can cause food-borne illnesses when they are alive and present in significant quantities capable of overcoming the body’s defenses. Food safety can be achieved by killing any microorganisms that might be present. One technique used is to lower the pH of the medium by adding acid to the point where microorganisms cannot grow. This technique is used in making pickles.
Heat is a particularly effective means of killing microorganisms. Canning is a preservation technique that works because the very high temperatures used are sufficient to kill all of the microorganisms that might be present. Heat and acidity interact in the canning of fruits. The comparatively high acidity (low pH) of fruits makes it
possible to can them safely by boiling the cans or jars for a prescribed, acceptably brief time without added pressure. However, vegetables are not very acidic, so they need to be processed at a temperature hotter than 212°F (100°C). By using a pressure canner, the necessary temperature can be reached and the vegetables are preserved safely.
Jams and jellies are a means of preserving fruit by adding a large amount of sugar. The concentration of sugar outside the cell walls is far greater than that inside the cell walls of microorganisms. The result is that osmotic pressure develops and draws out the water in the cells of the microorganism, effectively killing any that might be present.
staphylococcal poisoning Food poisoning due to ingestion of enterotoxin produced by S. aureus; violent disturbances of gastrointestinal tract for one or two days and occurring usually within eight hours of ingestion.
vibrio cholerae Bacterium sometimes found in foods and water with fecal conta mination; causes cholera.
botulism Food poisoning caused by eating the toxin produced by C. botulinum; human infection is most commonly associated with types A, B, or E.
clostridium botulinum Type of bacteria producing a toxin that is highly poisonous and frequently fatal to humans when consumed.
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section one | foundation for food studyhours after ingestion of the food containing the toxin. If a patient survives until the ninth day, the prognosis for recovery is good, although the recuperative process may be quite slow.
Occasional instances of botulism result from home-canned foods that have been processed inadequately, with the result that some spores of C. botulinum survive and the toxin is generated within the can. Although similar problems can occur in commercial canning, standards of quality control generally are far in excess to ensure avoiding this potential risk. Even so, an occasional scare does develop. Such episodes are publicized extensively in the media, and potentially hazardous items (cans from the problem lot) are recalled and removed from all markets.
To avoid problems, home-canned vegetables, meat, poultry, and fish should be boiled actively for 15 minutes prior to even tasting them. This is not necessary with fruits, because their acid generally is sufficient to discourage C. botulinum growth.
An important requisite for home canning any vegetables, fish, poultry, or meat is a pres- sure canner, for this is the only way that the food can be brought to a temperature high enough to ensure inactivation of the spores. In a pressure canner, the temperature will reach 240°F (116°C) if the food is processed at a pressure of 15 psi, which allows adequate heat processing for safety within a reasonable period of time.
People processing these low-acid foods must maintain pressure at the appropriate 15 psi throughout the required processing time. The safe processing period ordinarily is at least 25 minutes at 15 psi, but the number and size of containers being processed will influence the recommendation. Since the time–temperature relationships are so critical to achieving safety in canned vegetables and meats, careful timing is essential for the number of minutes indicated at the correct pressure in a pressure canner.
listeriosis Listeria monocytogenes can cause humans to develop listeriosis in 12 hours to 6 weeks after eating food contaminated with this type of bacteria. The symptoms are described in Table 3.1 on page 33. Actually, L. monocytogenes can be found widely in farm animals and rural environs, which means that careful sanitation is important in preventing the bacteria from entering the food supply.
Prevention or avoidance of contamination is especially important because L. monocyto- genes can survive and multiply at refrigerator temperatures; heat, acid, and salt also are of limited effectiveness in preventing multiplication and/or in killing the bacteria. Contamination needs to be avoided by
1. using only pasteurized milk and dairy products;
2. keeping raw foods separate from cooked foods until very close to serving time;
3. keeping all cutting boards and other items that contact food meticulously clean;
4. washing all raw foods before eating;
5. maintaining foods below 40°F (5°C) or above 140°F (60°C); and
6. cooking poultry to 180°F (82°C) and other meats and fish to 160°F (71°C).
campylobacter infection Campylobacter bacteria can be found in the intestinal tract of meat animals and poultry, which can mean that they might be found in some meats. Campylobacter jejuni contamination in water, unpasteurized milk, and raw and undercooked meats has caused illness and some deaths. However, heating all meats and poultry to at least 160°F (71°C) will destroy any Campylobacter that might be present. Untreated water and unpasteurized milk are other potential sources of contamination, but pasteurization or boiling will assure safety from Campylobacter.
escherichia coli Depending upon the specific type of Escherichia coli ingested, infections caused by E. coli (Figure 3.6) may be classified as enteroinvasive, enterotoxigenic, or enterohemorrhagic. The most common type is enterotoxigenic, often called traveler’s diarrhea, which usually strikes between 8 and 44 hours after eating E. coli and lasts about 24–30 hours. There usually is an outpouring of fluid in the form of diarrhea and possible vomiting, which leads to dehydration, but does not invade the epithelial tissue of the small intestine.
Figure 3.5
Culture of Clostridium
botulinum type B. Its
toxin can be found in canned meats and vegetables if they are not processed to a high enough temperature for a sufficient time. Courtesy of U.S. Department of Agriculture.
listeria monocytogenes
Type of bacteria that can cause listeriosis; sometimes found in unpasteurized milk.
listeriosis Potentially very serious food-borne illness caused by ingesting viable
L. monocytogenes.
campylobacter jejuni Type of bacteria sometimes found in poultry and meats.
escherichia coli (e. coli)
Group of bacteria often found as the cause of food-borne illnesses.
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In contrast, E. coli O157:H7 is classified as enterohem-orrhagic; it is manifested by bloody diarrhea and may lead to renal failure due to this organism’s ability to produce virulent toxins in the host. The cause of E. coli O157:H7 infections is linked to eating undercooked meat, raw milk, unpasteurized juices, and contaminated leafy greens. This type of infection has received great media attention since the first outbreak in 1993, which was traced to a fast-food operation and that had tragically fatal consequences.
shigella Fecal contamination is usually the source of
Shigella boydii, a type of bacteria that can cause shigellosis and bacillary dysentery. By using chlorinated water and practicing good sanitation standards when handling food, food workers can prevent this food-borne illness.