Times at Specified Temperatures for Salmonella Inactivation (Kill)
Food pasteurization standard. In some countries, Enterococcus faecalis is used as the pasteurization standard. Since it is doubtful that E. faecalis is actually a pathogen, it is more appropriate to use destruction of the known pathogenic species, salmonellae, for the standard. Salmonella spp. cause 30% of documented foodborne illnesses and deaths in the United States (14). Some species of salmonellae are rather difficult to inactivate. The resistance of salmonellae to inactivation also varies with water activity of the food in which it is found. The D-value [time at a given temperature needed to destroy 90% (1 log) of the organism] of salmonellae in food containing a high amount of sugar or in a food with a lower water activity is much larger than in food in which the water activity is sufficient for the growth of these organisms (6, 152). If the D-values and z-values described by the USDA for the destruction of Salmonella in beef (70) are used as the basis for pasteurization control, there will be adequate safety from other vegetative pathogens such as E. coli O157:H7 (99, 100).
5D and 7D inactivation. The 5D and 7D inactivation times are shown in the Figure 5-2. The D-values are derived from standards for cooking roast beef (29, 70).
Thermometers. Currently throughout the world, the bimetallic coil thermometer is commonly used to measure food temperature. Figure 5-3 illustrates the construction of the typical bimetallic coil thermometer.
The bimetallic temperature-sensing
coil extends up the stem of the thermometer from the tip approximately
3 inches (7.62 cm) and averages the temperature over this distance.
This device is not tip sensitive. Hence, there is no way that this
instrument can be used to find the coldest spot in hamburgers, chicken
breasts, or any other thin, small-volume food and accurately validate that
the food has been adequately pasteurized.
To ensure correct food temperature measurement (essential for correct pasteurization of the contaminated food that comes from the wholesale system), all cooks must use tip-sensitive thermocouple thermometers. These thermocouple thermometers should have a tip diameter of 0.040 inch (1.0 mm) or less and an accuracy of + or -1.1ºF (+ or -0.56ºC) over the range of 0 to 400°F (-17.8 to 204.4°C), to verify food pasteurization. An accurate tip-sensitive device is the Atkins 33040 (Atkins Technical Inc., Gainesville, Florida) thermocouple with 0.040 hemi-tipped probe. If the food is an inch or more in thickness, and a time of 20 seconds is allowed for an accurate temperature reading, a thermistor meter such as the UEI PDT 300 (Universal Enterprises, Inc., Beaverton, Oregon) can be used.
Food Cooling Calculations
Every foodservice operator has the responsibility to make sure that refrigerators have the capacity to cool foods safely. The spores of C. botulinum, B. cereus and C. perfringens survive pasteurization. The 1999 FDA Food Code (204) states that food should be cooled from 140 to 70°F (60 to 21.1°C) in 2 hours and from 70 to 41°F (21.1 to 5.0°C) in 4 hours. This guideline implies that cooling food is a two-step process. It is not actually a two-segment cooling, but really is a straight-line cooling process from 140 to 41°F (60 to 5°C) in 6 hours.
Juneja et al. (98) have shown that the critical cooling time to control the multiplication of surviving C. perfringens spores is 15 hours, if food is continuously cooled from 130 to 45°F (54.4 to 7.2°C) in a 38°F (3.3°C) cooling environment. This is the normal time needed to cool 2 inches of hot food in a 2 1/2-inch steam table pan, from 130 to 45°F (54.4 to 7.2°C) in a 50-fpm [feet per minute (0.25 meter per second)] air flow, which is characteristic of commercial walk-in and reach-in refrigerators (169). This means that normal storage refrigerators with 1/4-horsepower compressors can cool food overnight if the mass is less than 25 pounds and the heat load starts at 150°F (65.6°C).
Since foodservice refrigerators are designed only to hold (store food at cool temperatures) and not designed to cool food (136), additional horsepower (Btu per minute) of refrigeration capacity or a lower chilling temperature of 35 to 25°F (1.7 to -3.9°C) must be requested when blast-cooling units are purchased. A temperature below 25°F (-3.9°C) should not be used, because food begins to freeze, and cooling is slowed. Alternatively, operators can purchase a freezer with a thermostat that allows it to operate at 25 to 35°F (-3.9 to 1.7°C), and the problem is solved. Specifying this type of cold storage unit means that there will be an electric or hot gas defrost evaporator coil in the refrigerator, which prevents normal refrigerator coil freeze-up below 35° (1.7°C). Blast-chilling units that are used to cool foods to 41° (5.0°C) in 6 hours or less must have a rapid flow of air at 1,000 fpm (5.08 meters per second) blowing directly across the container of food. An important part of the cooling rate is turbulence of the air around the food container. Air at 1,000 fpm (5.1 meters per second) cools three times faster than typical refrigeration air flows of 50 fpm (0.25 meter per second) (147). If cooling is done in turbulent water, then the heat transfer at the surface of the food is improved above high-velocity air by a factor of about five. The graph on the following page shows a typical food cooling process. Note, the process follows a semilog relationship. When the difference in food center temperature minus air temperature is plotted on the Y axis and the cooling time on the X axis, the cooling curve is a straight line (142, 143).
Since 75% of the heat is removed through the bottom of the pan of food because of the direct contact of food with the bottom surface, containers of food must rest on an open or wire rack where there is no blockage of air across the bottom of the pan. Research has shown that the maximum food thickness that can be cooled to 41°F (5.0°C) in 6 hours in a 30°F (-1.1°C) high-velocity air stream is about 2 inches (5 cm). The use of either stainless steel pans or plastic containers has little effect on the cooling rate.
The food should be covered to prevent mold contamination of the food surface from the air circulating through the evaporator coil, which is never cleaned. If the food is acid, and the pan is covered with aluminum foil in contact with the acid food, the acid will attack and make holes in the aluminum foil. On the other hand, if plastic wrap is used, it may blow off. The best solution is to put a layer of silicone paper or plastic wrap over the food first, and then cover with aluminum foil.
In ordinary retail food operations that are not preparing large volumes of food ahead of time that must be cooled, the high-capacity fan is not required. Justification for this statement is based on the research by Juneja et al. (98), which has shown that 15-hour continuous cooling to 45°F (7.2°C) is safe, and that food in an ordinary refrigerator will cool in this time if the food is 2 inches (5 cm) deep or less, in a covered pan (170).
Figure 5-4. Typical Food System
FDA Food Code 6-hour cooling (204) vs. Juneja et al., 1994 - 15-hour cooling (98)
(Food center temperature is shown in °F beside plot points.)
Validation of cooling.
The ability of refrigerators and cold-holding units in food operations
to cool food can be validated in a simple manner. A mixture of "soupy"
instant mashed potatoes can be made in a food pan by adding 7% by weight
potato flakes to water, until the potato-water mixture in the pan is "gelled."
For cooling tests, the temperature of this potato-water mixture should
be 140 to 150°F (60 to 65.6°C). A thermocouple or thermistor
thermometer is mounted on the pan to measure the temperature in the geometric
middle of the mixture. The temperature of the potato-water mixture
is measured and recorded at the time it is placed in the refrigeration
unit and at 30-minute intervals. The temperature data can then be
plotted as shown in Figure 5-4. When the temperature data are plotted,
there will be a linear cooling relationship.
Holding temperatures in salad bars can be checked in the same manner, except that the potato-water mixture is prepared with cold water and the mixture should be 39 to 40°F (3.9 to 4.4°C). The mixture should hold a temperature of less than 41°F (5.0°C) [but above 32°F (0°C) to prevent freezing] throughout the container as long as it is in a salad bar, cold prep table, or cold display.
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