THE BASICS OF COOLING FOOD

1999, by O. Peter Snyder, Jr., Ph.D.
Hospitality Institute of Technology and Management
 

Introduction
While there has been a lot of talk about food cooling, there are, in fact, virtually no scientifically correct studies that have been performed on food cooling in the kitchen to identify the hazards and critical limits. This paper reviews some government standards for cooling and then, presents a review of the correct methodology for cooling analysis and graphing.

The cooling problem stems from the survival of the three spores, Clostridium perfringens, Clostridium botulinum, and Bacillus cereus, in pasteurized foods. When meat, fish, poultry, vegetables, starches, cereal products, and dairy products are pasteurized, for example, at 150ºF for 1.21 minutes, the vegetative cells are reduced to a safe level, but the spores are activated. If the food is controlled with pH below 4.6, these three pathogens will not germinate and multiply. Food does not need to be refrigerated for safety. This does not mean that the foods cannot spoil, however, because there are some spores that grow at refrigerator temperatures or low pH that can spoil food.

There are thousands of foods and sauces on sale in food markets that illustrate pH control. Many types of acidified gravies, tomato sauces, chilis, sauces, dressings, etc. are simply pasteurized, pH controlled below 4.6, and then, packaged. Traditionally, mold and yeast control is by small amounts of sodium benzoate and potassium sorbate added to the food.

In retail food operations today, we are wasting a lot of effort cooling volumes of sauces, gravies, etc. that are sufficiently acidified so as to not need refrigeration. The simple way to identify these foods is to measure the pH with a pH meter. Unfortunately, neither most regulatory personnel nor operators have meters or are trained to measure pH in food. Hence, operators are being required to cool and refrigerate many foods that do not require spore hazard control. Actually, one does not need to get to pH 4.6 to have a cooling benefit. If a gravy, soup, or sauce has a pH of about 5.2, the multiplication times for bacteria are at least twice as long, and cooling times could be twice as long. Currently, this cannot be predicted; it must be measured.

Food Cooling Regulations
When the pH is greater than 4.6 (e.g., 6.0), we must control the spore pathogen problem. The 1976 model FDA food code identified, for the first time, food cooling as a major hazard control and specified 4-hour cooling from hot to 45ºF in order to assure the safety of the food. The FDA gave no reference for these critical limits. In 1993, the FDA included as the reference for cooling the paper by Longrée and White (1955). The FDA also changed the standard from 4-hour cooling, hot to 45ºF, to 6-hour cooling. The food is supposed to cool from 140 to 70ºF in 2 hours and from 70 to 41ºF in 4 hours. FDA officials thought that cooling was a broken curve when, in fact, they specified a continuous cool process, when the logarithmic nature of cooling is considered.

Actually, the Longrée and White study does not provide any data that support the FDA's 4-hour or 6-hour cooling recommendation. If you doubt me, read the paper. In this study, the researchers cooked white sauce, cooled it to approximately 115ºF, and put live E. coli cultures into the white sauce. Then, they divided the white sauce into various sized containers in order to study different cooling rates. The cooling was conducted in a brine-wall-cooled walk-in refrigerator on a cart at an unspecified airflow rate over the containers. The containers varied in depth and shape, to include 2-1/2-gallon, 4-gallon, and 8-gallon containers. The refrigerator temperature was reported as being 47ºF, although my analysis indicates that it was closer to 49ºF around the experimental shapes. The study temperatures were very accurately taken with thermocouples, but the study only cooled from approximately 115 to 60ºF and stopped at 60ºF. Cooling times over this temperature range varied from 8 hours to 21 hours. Remember, under normal conditions of cooling from 140 to 41ºF, the time to 60ºF is about 1/2 of the cooling time that it takes to reach 41ºF.

Not surprisingly, in every one of the experiments in this study, the E. coli multiplied--from a few bacteria to millions. There was no "safe" cooling identified in this study. This study simply showed that rapidly growing E. coli, if it were put into pasteurized food at 115ºF, would multiply under the typical cooling procedures used in 1955 in institutional food operations.

USDA Cooling Standards
Perhaps about 30 years ago, the USDA established cooling standards for chunked and formed roast beef that we see sold as sliced roast beef in delis. These standards were not based on any scientific data. They were apparently based on the times and temperatures then used by manufacturers to cook and cool chunked and formed deli roast beef and turkey. It was obviously safe, because there were no outbreaks of illness caused by the food produced according to the standards. The standard was that cooling would begin 1 1/2 hours after the food had come out of cooking. The food center temperature was to go from 120 to 55ºF in 6 hours. When the food reached 55ºF center temperature, cooling was to continue, and the food was not to be packed and stored until the center reached 40ºF.

Later, the USDA General Cooling Rule was written, which required that general meat and poultry products would be cooled from 130 to 80ºF in 1 1/2 hours and 80 to 40ºF in 5 hours. The reason for the two-part cooling was the belief that the first part of cooling had to be fast to prevent the outgrowth of pathogenic spores. Because the USDA was not using correct mathematics to analyze cooling, it did not realize that what it had specified is actually almost a straight line cooling requirement. Again, cooling is a logarithmic process, because heat removal is exponential. Today, USDA allows processors to use either cooling standard or to do their own if they do not like the USDA's standards.

The Correct Science for Cooling
Let's discuss the correct scientific basis for cooling. Pflug and Blaisdell (1963) and Pflug et al. (1965) have provided the correct mathematical treatment for cooling.

First, cooling is Newtonian and is based on the relationship

d (T - T1) / dt = -K (T - T1) This can be resolved into

(T - T1) = (To - T1)e -K1 t

or

log (T - T1) = -Kt + log (To - T1)

where T is the temperature of the center of the food at some time, t; T1 is the temperature of the coolant; and To is the initial center temperature of the food. K is the rate of cooling and also the slope of the cooling line. Incorporated in it are all of the variables that effect cooling such as air velocity, thermal diffusivity of the food, thickness of the food, etc.

A plot of log (T - T1) vs. time is linear, and the resulting straight line can be described by the slope K. In the Newtonian case, it is assumed that no lag exists in the start of heat transfer from the center to the surface. The presence of a time lag between a change in temperature at the surface and at the center of a product requires a modification of the basic equation. Normally in retail food cooling, there is enough time between putting a sample into a cooling system and beginning to record data (e.g., 15 minutes) that equilibrium of heat transfer is established, and one does not see the lag.

True asymptotic solution. Ball (1923) used the following equation to describe the heating and cooling of cans of food in terms of the straight line asymptote to the true heating or cooling curve when conduction heating data are plotted on semi-logarithmic paper,

log (T - T1) = (-t / f) + log j (To - T1).

j is the possible lag factor at the start of cooling and is equated as follows:

j = (Ta - T1) / (To - T1), where Ta is the apparent initial temperature.

f is the time in minutes for the asymptote of the cooling curve to cross 1 log cycle, that is, the time required for a 90% reduction of the center temperature of the food on the linear portion of the cooling curve (Ball, 1923).

Figure 1 is a correct graphical presentation of government cooling standards.

In using the method of Ball, the time-temperature data from conduction heat transfer processes can be plotted as either log (T - T1) vs. time (t), as shown on the left axis of Figure 1, or directly on a scale log (T - T1) + T1 vs. time, as shown on the right axis of Figure 1. The asymptote cooling line can be represented and constructed directly from the two parameters, f and j, assuming initial and final temperature data are known.

Actually, a cook does not need to use this complicated mathematics, although all regulatory officials should be trained to understand it. The analysis of a cooling study can be reduced to a very simple procedure without knowing any formulas. All that one needs to know is how to plot cooling on a semi-logarithmic graph, as shown in Figure 1. The relationship is that the difference between food center temperature and ambient or driving force temperature (i.e., the temperature around the food, such as of ice water or air) is the important relationship. If one plots the difference in temperature between food center temperature and driving force temperature on a semi-log plot, one gets a straight line relationship for cooling. (Actually, this mathematical relationship is precisely the same in cooking, except turned upside down.)

The beauty of this semi-log plotting is that this is predictive cooling. One can take a few temperatures and connect the points with a line, and this cooling line can predict the endpoint temperature. Of course, the measurements need to be done accurately. This means that cooling does not need to be monitored for 15 or 20 hours to do a cooling study, provided the data are handled in the mathematically correct way presented in this paper. It can be done in 2 hours for a 15-hour cooling process.

Let's now examine the various government-prescribed cooling rules, which are not based on science, but rather on professional judgment in the past.

FDA 6-hour Cooling
The FDA says that we will cool food from 140 to 70ºF in 2 hours and then, 70 to 41ºF in 4 hours. It is obvious from Figure 1 that this is virtually a straight-line cooling relationship from 140 to 41ºF. The calculated time to 70ºF is 1.97 hours for a straight line vs. the FDA's 2 hours--a trivial difference.

USDA General Product Cooling
The next data plotted in Figure 1 are of the USDA general product cooling. Again, it is obvious from the line that it is virtually straight from 130 to 40ºF. The calculated time to 80ºF is 1.33 hours, which is a trivial difference from 1.5 hours.

Even more interesting is the USDA cooling requirement for chunked and formed roast beef. Typically, the raw roast has an average count of approximately 500,000 bacteria per gram in the center. USDA 9 CFR 318.17(h)(10) says that this food is to be cooled from 120 to 55ºF within 6 hours. This meat has been prepared and sold as deli roast beef for more than 25 years, and the process has been shown to be absolutely safe. After the meat gets to 55ºF, cooling will be continued until the meat gets to 40ºF before the food is packaged in cartons and stored. If this cooling line is extended to 40ºF, it can be seen from Figure 1 that the total cooling time will be about 15 hours. This assumes that the same refrigeration heat transfer coefficient continues from 120 to 40ºF. Remember, this has been used for the safe manufacture of many millions of pounds of C. botulinum-C. perfringens-B. cereus-contaminated, pasteurized meat and poultry. Actually, the regulation would allow virtually any amount of time to get from 55 to 40ºF, because the time from 55 to 40ºF is unspecified. Note, C. botulinum multiplies down to 50ºF, and B. cereus to 40ºF.

Juneja et al. Study
In 1992, this author received an agreement from Ray Beaulieu and Jeffery Rhodehamel at the FDA that there was indeed no scientific basis for the FDA retail food cooling regimes, and that it was appropriate to do a study. With the help of Dr. Vijay K. Juneja, USDA ARS ERRC, a study was conducted using hamburger as the food item and C. perfringens as the target organism (Juneja, 1994). Clostridium perfringens was selected, because, of the three spores, C. perfringens has the shortest lag and fastest generation time. Hamburger was selected as the media, because C. perfringens is found in hamburger, and hamburger has often been involved in C. perfringens outbreaks. Various cooling times were evaluated in order to determine the safe cooling time. One cooling time chosen arbitrarily was 15 hours to go from 130 to 45ºF, with a 38ºF temperature of coolant, in this case, air in the refrigerator. The 15-hour cooling time showed about 3 multiplications of C. perfringens. The USDA has accepted this cooling time as safe (Federal Register, January 6, 1999), because it now accepts cooling when there are 3 or less multiplications of C. perfringens.

How were the food parameters chosen in this study--start at 130ºF, cool to 45ºF, and cool with a 38ºF driving force? This author chose these numbers, based on the fact that the standard NSF refrigerator typically runs at 38ºF. Since C. perfringens does not multiply above 127.5ºF, 130ºF is a practical starting point for cooling. This means, food can be cooled from a high temperature to 130ºF anywhere in the environment. It does not have to be in a refrigerator prior to reaching 130ºF. The 45ºF was based on the fact that there must be driving force left at the end of the cooling process. Otherwise, it is not possible to achieve the end temperature.

For the FDA code to say that food needs to reach 41ºF in a 40ºF refrigerator is simply naïve and scientifically invalid. It is not possible to achieve 41ºF in a 40ºF NSF refrigerator, unless one waits many, many hours. As shown in Figure 1, if one extrapolates the Juneja et al. data from 45 to 40ºF, this cooling curve has a time of almost 23 hours to get to the USDA 40ºF critical limit. This has been proven to be absolutely safe.

How Does Theory Match the "Real World?"
The 15-hour cooling matches the real world of food cooling in retail food operations when the food is covered 2 inches deep in a 2 1/2-inch pan. Snyder (1997) has shown that a typical pan of mock gravy (flour-thickened water) in an ordinary, commercial walk-in refrigerator with standard airflow, where the food is 2 inches deep, covered, in a 2 1/2-inch pan, will take approximately 13 hours to cool. All of the food that is placed 2 inches deep, covered, in 2 1/2-inch pans cools in the range of 13 hours. This assumes that the airflow is approximately 50 feet per minute across the bottom of the pan. If the pans are on a solid shelf, or in a corner where the airflow is low, or in a reach-in refrigerator, where the airflow is typically less than 50 fpm, the same pan of food can take 20 to 25 hours to cool to 45ºF. To predict cooling, one must not only know food thickness and driving force temperature, one must also know air velocity. I know of no sanitarian who uses an airflow meter to monitor or correct cooling.

In a recent conversation with a sanitarian from New York state, the sanitarian said that he commonly sees food in 4-inch pans and 5-gallon buckets of food in New York restaurants. It is common to see food 4 inches deep in pans in Minnesota's restaurants, even though the current cooling rules have existed for almost 25 years. Obviously, cooling in a 4-inch pan will take much longer than the 6 hours allowed in the FDA code. Since it is not causing illness, the 6-hour cooling is virtually never measured, unless it is a stirred pot of food.

Why are sanitarians not enforcing 6-hour cooling? The reason is simple. If 6-hour cooling were enforced for typically viscous food in a pan, roasts, etc., the only solution is for the restaurant operator to purchase a $15,000-to-$20,000 blast chiller. Sanitarians know that they cannot show, in a court of law, a need for rapid cooling. Hence, they do not press the point.

Summary
This paper has presented scientific facts regarding the safe cooling of food. Both the FDA and USDA have written cooling standards in past years that are not based on the correct mathematical science of cooling. Fortunately, in the case of the FDA, the 4-hour or 6-hour cooling standard has rarely been enforced. Hence, operators have not had a great financial burden placed on them. If, in 1976, the FDA had enforced its 4-hour standard, every restaurant in the US today would have an unnecessary blast chiller, at a cost of $15,000 to $20,000.

In USDA plants, the problem is not as significant, because processors typically want to pack and ship the food as fast as possible so that product does not collect in the production area. Processors invest in highly effective cooling systems, simply for throughput. Even if a processor were given 15 or 20 hours to cool food, most would probably not want it, because it would take up too much time and labor. It would be an advantage if the cooling system failed, because it would give them a lot more time to correct deviations before product must be destroyed.

If every restaurant, to include the numerous "mom-and-pop" restaurants in the US, were actually forced to comply with the current FDA 6-hour cooling for all of their food, millions of dollars of labor effort would be wasted, the industry would spend millions of dollars for unnecessary blast chillers and the electricity to run those blast chillers, and there would be no reduction of illness.

I invite retail food operators in America to contact their national food association or me, so that the industry can go to the Conference for Food Protection or write directly to the FDA / USDA, telling them to do the necessary research to change the regulations so that they only impose necessary safety standards on the industry.
 
 

References

  • Ball, C.O. 1932. Thermal process time for canned food. National Research Council Bull. 37.
  • FDA. 1976. Food Sanitation Manual. US Dept. of Health, Education, and Welfare, Public Health Service. HEW Pub. No. (FDA) 78-2081.
  • FDA. 1993. Food Cold. US Public Health Service, US. Dept. of Commerce, Technology Administration, National Technical Information Service. Springfield, VA. Pub. No. PB94-113941AS.
  • Federal Register. 1999 (January 6). Rules and regulations. 64(3):732-749.
  • Juneja, V.K., Snyder, O.P., and Cygnarowicz-Provost, M. 1994. Influence of cooling rate on outgrowth of Clostridium perfringens spores in cooked ground beef. J. Food Protect. 57(12):1063-1067.
  • Longrée, K. and White, J.C. 1955. Cooling rates and bacterial growth in food prepared and stored in quantity. J. Amer. Dietet. Assoc. 31:124-132.
  • Pflug, I.J. and Blaisdell, J.L. 1963. Methods of analysis of precooling data. ASHRAE J. 5(11):33-40, 49.
  • Pflug, I.J., Blaisdell, J.L., and Kopelman, J. 1965. Developing temperature-time curves for objects that can be approximated by a sphere, infinite plate, or infinite cylinder. ASHRAE Transactions Part I 71:238-248.
  • Snyder, O.P. 1997. Two-inch and four-inch food cooling in a commercial walk-in refrigerator. Dairy Food Environ. Sanit. 17(7):398-404.

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