Technical Reports
The Real Truth About Cooking

Introduction

It has been said that understanding the process of meat cookery is a problem of characterizing unsteady, simultaneous heat and mass transfer in a continuously changing, complex porous structure. This description of cooking makes it seem as if cooking processes are hopelessly complicated and would be next to impossible to understand. In reality, however, even the most complicated processes follow a few simple cooking principles, and can be readily understood once those basic principles are explained.

The purpose of this paper is to give you an overview of the basic principles for cooking of meat products. This paper will also explain the underlying technical aspects of cooking processes, and show you how to apply these principles to real-life processes.

Cooking Equipment

The most common types of heating media used in meat cooking equipment are hot water (e.g. steam kettles), steam (e.g. steam cook cabinets), and forced-air (e.g. smokehouses). This paper will focus on the use of forced-air convection ovens (smokehouses) in the preparation of smoked and unsmoked meat products.

Many different types of meat processing systems are currently in operation in meat processing plants around the world. These systems include small batch ovens capable of producing 450 lb. per load, large batch ovens capable of cooking more than 55,000 lb. per load, and continuous processing systems capable of production rates in excess of 10,000 lb./hr of product. The majority of these systems are some variation of the standard forced-air convection oven commonly known as a smokehouse.

Oven variables

Regardless of whether an oven is large or small, old or new, or batch or continuous, most forced-air meat processing ovens are designed to control the following four variables:

  • cooking time
  • dry-bulb temperature
  • wet-bulb temperature
  • air velocity

Accurate measurement and control of these four variables is essential to maintain proper control of the cooking process.

Cooking time. The cooking time in a batch process is typically controlled by using an established cooking schedule that includes several preset, timed cooking and smoking steps. The total cooking time is usually determined by the time required to achieve a specific product core temperature.

Instead of cooking the product using fixed step times, some smokehouse control systems can be programmed to change steps in a cooking schedule according to preset target core temperatures. As such, the control system would change steps when the product core reached a preset target temperature during a step, rather than simply running the step for a preset length of time before changing to the next step.

Another method of controlling the cooking time is the use of a process known as Delta-T cooking. In a Delta-T process, the oven control system is programmed to maintain a constant temperature difference between the product core temperature and the oven dry-bulb temperature. The control system may also be programmed to control the oven wet-bulb temperature by maintaining a constant temperature difference between the dry- and wet-bulb temperatures, or by maintaining a constant relative humidity.

For example, if the Delta-T (temperature difference) for a process was programmed to be 60F and the product core temperature at the beginning of the process was 50F, then the control system would set the initial dry-bulb temperature at 110F (60+50= 110F). As the product core temperature gradually increased during cooking, the oven control system would gradually increase the dry-bulb temperature to maintain a constant temperature difference (Delta-T) of 60F between the product core temperature and the dry-bulb temperature.

Dry-bulb temperature. The dry-bulb temperature is the temperature of the oven air when measured with a clean, dry temperature sensor. The heat source used to control the dry-bulb temperature can be a gas burner (natural gas or propane), steam coils, electric heat, or live steam.

For most cooking schedules used in production cooking processes, the highest dry-bulb temperature setpoint would be less than 210F, even though a standard-design batch oven would be capable of operating at temperatures of up to 240F. Ovens that are designed for high temperature operation may be run using dry-bulb temperatures as high as 400F. These high temperature ovens are used for roasting and browning of products.

Wet-bulb temperature. The wet-bulb temperature in an oven is measured by fitting a wet, moisture-wicking cloth over an ordinary dry-bulb sensor and placing it in the oven air stream. If the air is not saturated with moisture (i.e. the relative humidity is less than 100%), evaporation of moisture from the sock will cool the sensor to a temperature that is lower than the dry-bulb temperature.

The difference between the dry- and wet-bulb temperatures is used to determine the relative humidity of the oven air. The relative humidity of the air is a measure of the actual amount of moisture in the air compared to the amount of moisture in saturated air at the same dry-bulb temperature.

The wet-bulb temperature is the temperature at which moisture is evaporating in the oven. Drier, less humid air will create more evaporative cooling, resulting in a lower wet-bulb temperature. On the other hand, a saturated atmosphere, such as steam or 100% relative humidity air, will not create any evaporative cooling, and therefore the dry- and wet-bulb temperatures will be the same.

The wet-bulb temperature in an oven can be controlled in two ways. The control system can either modulate the fresh air and exhaust dampers open and closed to control the amount of evaporated product moisture in the oven. Or it can regulate the wet-bulb temperature by injecting controlled amounts of steam or atomized water into the oven.

Air velocity. If air velocity control is required, an oven can be equipped with either a multiple- speed fan or a variable-speed fan capable of a range of air velocities. Many smokehouses are equipped with a single-speed fan, and therefore the air velocity in the oven is fixed at one speed.

Heat & Mass Transfer

To understand the effects of heating and evaporation on meat products during cooking, we must first understand the basic mechanisms of heat transfer and mass transfer.

Heat transfer

Heat is defined as energy that is transferred as the result of a temperature difference. The temperature difference is the driving force of heat flow. A greater temperature difference will create a faster rate of heat flow.

During cooking of meat products, the temperature difference (TD) between the product surface and the core determines the rate of heat flow to the product core. The greater the surface-to-core temperature difference, the faster the product will cook.

There are three basic modes of heat transfer:

  • conduction
  • convection
  • radiation

Conduction. In conduction heat transfer, heat is transferred by direct particle-to-particle contact. During cooking of meat products, conduction is the mechanism of heat transfer within the product interior, from the surface to the core.

Convection. Convection heat transfer is the result of bulk mixing of fluids (air or liquid) at different temperatures. Convection heat transfer can be either free or forced convection. Free convection is the natural movement of air or liquid due to density differences caused by temperature gradients within the system. Forced convection, on the other hand, moves the air or liquid by a mechanical means such as a fan or a pump.

In meat processing ovens, the air is moved by a main recirculation fan, which means that it is a forced convection system. During cooking, forced convection is the mechanism of heat transfer from the air to the product surface.

Radiation. Radiation is best visualized as a wave transmission of energy through space. An example of radioactive heating would be microwave heating. In forced-convection meat processing ovens, radiation is not a predominant mode of heat transfer.

Mass transfer

The term Amass transfer is a technical sounding term that might seem difficult to understand. But in reality, for a typical meat cooking process, mass transfer simply refers to moisture migration from the product interior to the surface, and moisture evaporation from the product surface to the air.

Moisture migration within the product interior is dependent on the product's temperature, composition, moisture concentration, and water-holding capacity. Moisture evaporation from the product surface to the oven air depends on the moisture concentration differences between the product surface and the air. During cooking, most of the cooking loss is due to evaporation of moisture from the product surface, while the moisture content of the product interior shows little change.

Simultaneous heat & mass transfer

Convection cooking is essentially a high temperature drying process. When meat products stuffed in moisture permeable casings or without casings are cooked in a forced-convection oven, heat is transferred from the air to the product, while at the same time, moisture evaporates from the product to the air. This process is known as simultaneous heat and mass transfer, and can be separated into four distinct mechanisms:

  • Heat transfer from the air to the product (heat convection)
  • Heat transport within the product (heat conduction)
  • Moisture transport within the product (moisture diffusion)
  • Moisture evaporation from the product to the air (moisture convection)

Product Heating Rates

The product-heating rate is determined by the temperature difference between the surface and the core of the product. The greater the temperature difference between the surface and the core, the faster the product will cook. Any changes in the cooking process that affect the product surface temperature will in turn affect the heating rate of the product core.

Effect of wet-bulb temperature on heating rates

Because moisture is evaporating from the product surface during cooking, the surface of the product acts much like a wet-bulb sock. Evaporative cooling of the product surface keeps the surface temperature at or near the oven wet-bulb temperature for much of the cooking process. Since the oven wet-bulb temperature has a strong influence on the product surface temperature, it also has a strong influence on the surface-to-core temperature difference that determines product heating rates and cooking times. For this reason, the oven wet-bulb temperature essentially controls the product heating rate for much of the cooking process, and is a critically important heat transfer variable.

The effect of the wet-bulb temperature and its importance in determining product heating rates and cooking times is further explained in the following paragraphs.

Heating rate of fine-cut bologna: 40% relative humidity cooking process

The figure titled Treatment 1-F diagrams the oven and product temperatures for 4.1" diameter fine-cut bologna stuffed in moisture permeable fibrous casings and cooked in a forced-convection batch oven. As shown on the figure, the temperature setpoints for Heat Treatment 1-F were 196F dry-bulb, 158F wet-bulb, and 40% relative humidity.

In the early stages of the cooking process, the close association between the oven wet-bulb temperature and the product surface temperature was clearly evident. As shown on the figure, during oven come-up at the beginning of the process, the product surface temperature closely followed the oven wet-bulb temperature. As the wet-bulb temperature neared its setpoint of 158F, the product surface temperature broke sharply at the dewpoint (156F), just below the oven wet-bulb temperature.

The dewpoint temperature is the temperature at which moisture begins to condense out of the air. Since at the beginning of the process the product surface temperature was cooler than the dewpoint temperature of the air, moisture condensed on the product surface and formed a thin layer of moisture over the entire surface. The evaporation of that moisture from the surface and the resulting evaporative cooling caused the surface of the product to act much like a wet-bulb sock. As a result, the surface temperature was virtually the same as the wet-bulb temperature for the first 45 minutes of the cooking process ( Treatment 1-F).

During the wet-surface phase of a process, the product behaves as though it is being heated in a water bath that is at the wet-bulb temperature. Any change in the wet-bulb temperature will have an almost immediate impact on the product surface temperature. As long as the product surface is completely covered with a thin layer of moisture, the wet-bulb temperature essentially controls the product surface temperature and heating rate, while the dry-bulb temperature has little or no effect.

After the bologna had cooked for approximately 45 minutes using Heat Treatment 1-F, the surface began to dry off. Free moisture was no longer available over the entire product surface, and evaporative cooling at the surface was reduced. The drying of the product surface reduced the evaporative cooling, which allowed the surface temperature to increase above the wet-bulb temperature. The higher surface temperature increased the surface-to-core temperature difference, and as a result, the product-heating rate for this process was faster than it would have been had the surface stayed wet. If the product surface had stayed wet throughout the process, the constant rate of evaporative cooling would have caused the surface temperature to stay at the wet-bulb temperature for the entire process, resulting in a smaller surface-to-core temperature difference and a longer cooking time.

Of course, an easy way to increase the heating rate for this process would have been to increase the wet-bulb temperature. This would have resulted in an immediate increase in the surface temperature and the surface-to-core temperature difference, thus creating a faster heating rate. An increase in the dry-bulb temperature would have also eventually increased the surface temperature and the heating rate. But because it would have taken time to dry the product surface and allow the surface temperature to increase above the wet-bulb temperature, the effect would not have been as immediate as an increase in the wet-bulb temperature. Heating rate of fine-cut bologna: 80% relative humidity cooking process

The figure titled Treatment 3-F shows the oven and product temperatures for 4.1" diameter bologna cooked using a higher relative humidity than Heat Treatment 1-F. Heat Treatment 3-F used the same 158F wet-bulb temperature as Heat Treatment 1-F, but the dry-bulb temperature was reduced from 196F to 167F, thus increasing the relative humidity from 40% to 80%.

As shown on the Treatment 3-F figure, the product surface and core temperatures were dramatically different for this process than for Heat Treatment 1-F.

During the first 45 minutes of Heat Treatment 3-F, the close relationship between the surface temperature and the wet-bulb temperature was very similar to that for Treatment 1-F. The surface temperature closely followed the wet-bulb temperature during oven come-up, and then broke sharply when it reached the dewpoint temperature of the oven air. The product surface temperature then closely tracked the wet-bulb temperature through the first 45 minutes of the process. After the first 45 minutes, however, the surface temperature curve for Heat Treatment 3-F was distinctly different from that for Heat Treatment 1-F in that it stayed close to the wet-bulb temperature for the rest of the process.

After 45 minutes, the surface temperature curve for Heat Treatment 1-F began to increase above the wet-bulb temperature. For Heat Treatment 3-F, however, the surface temperature curve remained very close to the wet-bulb temperature throughout the process. Because of the high relative humidity for Heat Treatment 3-F (RH=80%), the product surface dried slowly. Since the surface stayed wet for most of the process, the evaporative cooling kept the surface temperature close to the wet-bulb temperature throughout the entire process. In contrast, for Heat Treatment 1-F (RH=40%), the surface began to dry after 45 minutes of cooking, reducing the evaporative cooling and allowing the surface temperature to increase above the wet-bulb temperature.

For most of the process, the product surface temperature was lower for Heat Treatment 3-F than for Heat Treatment 1-F. This created a smaller surface-to-core temperature difference for Heat Treatment 3-F than for Heat Treatment 1-F, and so the product cooked more slowly. As shown on the figures, by the end of the cooking process, the surface-to-core temperature difference was 22F for Heat Treatment 1-F, but was only 5F for Heat Treatment 3-F. The smaller temperature difference resulted in a longer cooking time for Heat Treatment 3-F (177 minutes) than for Heat Treatment 1-F (141 minutes).

Production oven processes

The figure titled Boneless Ham Cooking Process diagrams the oven and product temperatures for an actual boneless ham production process that was run in a fully loaded 12-truck batch oven. The process shown was a four-step cooking and smoking process. The product was smoked during the second step using traditional smoke from a dry sawdust smoke generator.

As shown on the figure, the product surface temperature for this process exhibited drying patterns that were similar to those discussed previously for bologna. Even though this process was a multiple-step process instead of a single-step process, there was still a strong relationship between the oven wet-bulb temperature and the product surface temperature. As shown on the figure, every change in the wet-bulb temperature causes an almost immediate change in the product surface temperature.

Discussion

Wet-bulb temperature. The wet-bulb temperature is the processing variable that has the strongest effect on product cooking times. An increase in the oven wet-bulb temperature will cause an almost immediate increase in the product surface temperature. A higher product surface temperature will increase the surface-to-core temperature difference, resulting in faster heating rates and shorter cooking times.

In addition to its effect on heating rates, an increase in the wet-bulb temperature will also reduce the drying rate of the product surface, which will often result in increased cooking yields.

Since surface drying also helps to enhance surface color development, a reduction in surface drying caused by an increase in the wet-bulb temperature will often slow down surface color development in smoked and unsmoked product.

Dry-bulb temperature. A change in the dry-bulb temperature will also have an effect on product cooking times, but will not have as immediate an impact as that caused by a change in the wet-bulb temperature.

An increase in the dry-bulb temperature will dry the product surface more rapidly, eventually resulting in reduced evaporative cooling and a gradual increase in the surface temperature. This gradual increase in the surface temperature will increase the surface-to-core temperature difference, resulting in faster heating rates and shorter cooking times. As a general rule, when comparing several processes with the same wet-bulb temperature, the process with the highest dry-bulb temperature will have the fastest heating rate.

The increased drying caused by higher dry-bulb temperatures will usually result in lower cooking yields. However, since hot, dry conditions enhance the browning reactions associated with color development in both smoked and unsmoked products, the increased surface drying caused by higher dry-bulb temperatures will also promote surface color development.

Relative humidity. A common misunderstanding regarding cooking processes is that a high relative humidity process will create a more intense heat than a less humid process, and therefore cook faster. However, as I have shown, it is the surface-to-core temperature difference that determines product-heating rates, not the relative humidity. And since the product surface temperature is strongly influenced by the oven wet-bulb temperature, it is the wet-bulb temperature that plays the most critical role in determining product heating rates and cooking times, not the relative humidity.

Conclusions

Any number of different cooking processes can be used to successfully cook and smoke meat products. However, all oven-cooking processes have a few basic principles in common. Some of the most important of these principles are summarized as follows:

  1. The four heat transfer variables that can be controlled in a meat-processing oven are: (1) cooking time, (2) dry-bulb temperature, (3) wet-bulb temperature, and (4) air velocity.
     
  2. The heating rate of the product is determined by the temperature difference between the surface and the core. The larger the temperature difference, the faster the product will cook.
     
  3. The wet-bulb temperature has a strong effect on the product-heating rate. In an oven cooking process, the wet-bulb temperature controls the product surface temperature for much of the process. As a result, the wet-bulb temperature determines the surface-to-core temperature difference for much of the process, and therefore controls the product-heating rate. In general, the higher the wet-bulb temperature, the faster the product will cook.

     

  4. The dry-bulb temperature has a strong influence on the product surface temperature, but only after much of the surface moisture has dried off. If several cooking processes have the same wet-bulb temperature and different dry-bulb temperatures, the process with the highest dry-bulb temperature will usually have the fastest heating rate.