high temp in food

17-High-Temp-2012-Pt1-1up.pdf

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This is module 17, Food protection using high temperatures. This module is split into two parts. Here is the first.

High temperatures preserve foods based on their destructive effects on cells or spores.

At the cellular level the first main damage begins to occur in the cytoplasmic membrane. Here lipids become more soluble and start to leak. As the heat increases ribosomes and RNA begin to degrade. This is followed by breaks in DNA and eventual DNA degradation. Finally proteins and enzymes will reach the point of denaturation and inactivation.

Bacterial spores are far more resistant to heat than cells. Cortex properties are very important to heat resistance of spores. Once heat reaches the inner membrane and the protoplast or core, the damage occurs. Even in a very dry state proteins and DNA can be damaged preventing the spore from ever germinating.

Some bacteria are capable of producing spores. A spore is a tough protein coat surrounding a dormant bacterial cell. Spores are very important because they allow some pathogen bacteria to survive high cooking temperatures, acid foods, dry foods, and other conditions that normally kill bacteria. There are three bacteria that cause food borne illness and produce spores: Clostridium botulinum, Clostridium perfringens, and Bacillus cereus. There are many other sporeformers of the same genera and some related genera that can spoil foods.

We discussed the effects of temperature on microorganisms as a extrinsic property. Basically, psychrophiles, yeasts and molds are the most sensitive to heat. The gram negative mesophiles are next most sensitive followed by the gram positives. As we get more heat resistant the next group is the thermodurics like lactobacillus and finally the thermophilic vegetative microorganisms. The last two groups are very heat resistant. These are the mesophilic sporeformers and the strict thermophilic sporeformers.

Here is a table listing some of the common sporeformers and their relative heat resistance. The time in the right column is the time at 121°C to kill 1 log of spores. Note that Clostridium botulinum is the most heat resistant pathogen, but not the most heat resistant sporeformer.

There are four general categories of heat treatments applied to foods. The first is blanching. Blanching is a short application of heat to foods. It can be a hot water dip or steam. Blanching is predominantly used for quality purposes to destroy natural enzymes that can reduce shelf life. However, blanching will reduce the microbial load on any food its applied to. Cooking is another method of heat treatment. The goal of cooking is to affect a change in the food. Pasteurization is similar to a cooking process, but the objective is usually to not affect any change in the food product. For both cooking and pasteurization, heat is applied to reach a level that destroys all hazardous vegetative pathogens in a food sample. Since different vegetative pathogens may be hazardous, there are no set rules on the time or temperature standards. Lastly, sterilization is considered heating foods to a level that destroy both vegetative and sporeformer pathogens and spoilage organisms. We will look at the last two methods in detail, starting with commercial canning. Part 2 of this module will look at pasteurization.

A discussed in the history of food microbiology, the concept of “canning” was invented in 1810 by Nicolas Appert in France. His technology consisted of placing foods in bottles and heating them in boiling water. Throughout the 1800’s it was understood that this method was imperfect. Some bottles still managed to spoil. It wasn’t until 1895 that research at the Massachusetts Institute of Technology discovered that the bulged and spoiled cans of seafood were due to heat resistant bacteria. And that these bacteria were forming a spore. When the spores were studied, it took heating at 250ºF to kill them. A major milestone in canning had occurred.

MIT deduced that canned seafood spoilage was due to spores and that 250ºF or 121ºC was needed to destroy them. The dilemma was exactly how do you heat a food to 250°F? The answer is found in water physics. If we change the pressure water is under we change the temperature at which it boils and hence its maximum temperature. The chart shows that water boils at 100oC when its at 1 atmosphere or sea level. Logan, Utah for example is approximately 4500 feet above sea level and water boils at 204oF. As altitude goes up the atmospheric pressure goes down. As atmospheric pressure goes down, the temperature of water boiling goes down. The opposite is also true. If the atmospheric pressure is increased to 2 ATM, then water will boil at 121oC or 250oF. This is the reference temperature for commercial canning.

To understand and work with commercial canning processes three important numbers must be determined to ensure enough heat is applied to kill all sporeforming target organisms. This includes D, z and F.

Now let’s look at the microbial population in a different manner. Let’s look at death, rather than growth. Since the goal of heat preservation is to kill off all of the targeted microorganisms in a particular food sample, we start off with a large population. In the graph, this is 10 +6th. To keep things simple, this is a graph using a constant temperature. Let’s say 60ºC. Then say at 5 minute intervals the survivors are plated to see their numbers. These are graphed on semi-log paper versus time. The result is a linear function.

This linear function is called the thermal death curve. In this example we are looking at the thermal death of Clostridium botulinum spores in canned soup at 121°C. From this data we calculate the an important number called the thermal death value or “D value”. D=the time in minutes to kill one log of cells (or spores) at a given temperature. The value is only representative of the specific microorganism used and the specific food matrix it was heated in. On this curve we can calculate D by determining the time required to kill one log of spores from log 4 to log 3. In this case the time equals 5 minutes. Therefore D=5 min.

As mentioned the D value is expressed in minutes. Since it is related to temperature it is often expressed as D with the temperature in subscript followed by the minutes. The greater value of D means the more heat resistant a specific microorganism is. The D value is often used to create a standard. For example, the USDA requires a 6 D reduction in Listeria monocytogenes for cooked ready to eat meats. That means that if a manufacturer were to make a cooked ready-to-eat roast beef they would have to cook the meat for time at temperature equal to 6

times the D value. We determined D155 at 5 minutes in the previous slide. What is 6D? If D160=1 min, then what is the required minimum cooking time for roast beef at 160°F?

The thermal death time or “F” value is the total time at temperature a food product

must be subjected to obtain a targeted kill. First, what is the worst case scenario of target organism? For Clostridium botulinum it is 100 spores. So, if we need to kill three logs of spores, then can we set the thermal death time to D=3? The actual D121 value for CB is

0.2 min. That would mean a TDT of 0.6 min.

If we started with 100 spores and performed a 3D process, is that enough to kill all spores? Since there is only a fraction of a spore left, then that would appear to be the case.

Let’s look at the 3D process closer. The previous slide indicated that only 0.1 spore would be left after processing in each can of food. Look at that probability level as the number of cans in a batch goes up. Since one spore could lead to a botulism death, just one is unacceptable. Based on this data, the canning industry wants to be sure there will be no spores in a million can batch. That is a 12 D process. The 12 D Clostridium botulinum process is now the standard for commercial canning.

Which canned green beans do you think consumers want? To prevent overcooking foods, canning scientists carefully manage the thermal process to deliver just the right amount of heat to kill 12 D of Clostridium botulinum spores, while attempting to maintain the quality of the foods cooked. To do this a little food engineering and mathematics is required.

Remember the D value is the destruction of the target microorganism at one temperature. However, during a canning process there is both a come up heating time and a come down heating time. Any cumulative heat will add to the destruction of the target. How is this accounted for? Here is where the z value comes in. The z value is used to calculate the effect of other temperatures on the destruction of the target microorganisms. The z value is determined by plotting several D values derived at different temperatures on a semi log graph. The example above experimentally determined that the D value was 113 minutes at 220 ºF, 31 minutes at 230 ºF, 8 minutes at 240 ºF, 2.3 minutes at 250 ºF, and finally 0.65 minutes at 260 ºF. By subtracting any two points on the line that are 1 log apart determines the z value. In this case it is 17.5.

A calculated F value is used to determine a safe but not excessive heat sterilization process. Once the z value has been determined, the caluculated F value can be determined using the equation above. Basically it is the sum of all lethality (L) for the change in temperature. You will be happy to know we are not going any further with the complex mathematics and modeling used in canning processes.

Here is a chart of the standardized F sub zero values or F value at 121ºC. Note that these times are different for different foods and different for different types of containers. An additional factor in canning has not yet been discussed --heat penetration. In order for a complete heat lethality to take place, the heat must reach all parts of the food in all parts of the can, and in all cans in a run. More mathematics and physics are involved, but again you will be glad to know we are not going to get into this concept any further.

Let’s take a quick look at the actual canning process. Canning is accomplished using commercial sterilizers called retorts. Retorts can be set up horizontally or vertically. The process is quite simple. First racks are loaded with foods in containers. These can be cans, jars, or even soft heat stable packages. The door is opened and the rack is inserted. The door is closed and sealed. The chamber is then filled with steam and the air is exhausted out. As the steam builds up it can reach 2 atmospheres or 15 psi. A valve controls the pressure by releasing steam when the pressure exceeds 15 psi. Once the temperature of the retort reaches 121 ºC the process will be timed to achieve the stated F value. After that time, steam is vented until the pressure reaches 1 atmosphere. Then the product is unloaded and sent off to a separate cooling process.

Here is a photo of two horizontal retorts. Note that this operation has a rack loading system on tracks that can move to the different retorts.