Infrared sensors determine the temperature of an object without making physical contact with it. Here’s how the noncontact technology helped optimize the performance of gas-fired ovens used in flour and corn food production operations.



It is no surprise flour and corn food products, including tortilla chips, are so popular with consumers -- they’re fresh and flavorful, whether served at a favorite restaurant or purchased from a local market. In fact, in the growing U.S. snack-food market, tortilla chips may soon overtake potato chips in total sales. According to the Snack Food Association in Alexandria, Va., Americans consume 1.54 billion pounds of potato chips annually with tortilla chips close behind at 1.43 billion pounds.

The Spanish first coined the term tortilla (from torta, or “cake”) in Mexico, where it was used it to describe flat corn and flour cakes. All tortillas were originally made from the pulp of ground corn. Later, when wheat was brought to the New World, white flour tortillas became prevalent. After tortilla products were first introduced in the southwestern United States, the popularity of the new food item spread rapidly. Tortilla chips can come in many different sizes and shapes, such as triangles, rounds, and rectangular strips.

The high quality of today’s tortilla products is made possible by modern production machinery and processing techniques. This equipment enables manufacturers to improve the taste, appearance and consistency of corn and flour tortillas, tortilla chips, flat breads, pizzas and other similar products.

The basic method of tortilla and tortilla chip production has changed little since ancient times. Traditional tortilla preparation involves cooking and steeping (soaking) the corn, pouring off the cooking liquor, and washing the nixtamal (the end product of the cooking, steeping and washing/draining process). The nixtamal then is ground into masa, a dried and ground corn flour.

In automated tortilla and tortilla chip factories, gas-fired ovens are used to bake the formed masa. Tortilla chips are baked at temperatures ranging from 500 to 554oF (260 to 290oC) with the baking time varying from 35 to 50 sec. Baking enhances the chips’ alkaline flavor and reduces moisture and oil absorption during frying. The tortilla chips are cooled to produce a more uniform consistency and reduce blistering.

Next, the chips are fried in oil ranging from 338 to 374oF (170 to 190oC). Salt and seasonings are applied immediately after frying while the chips are still hot. The chips are conveyed into an inclined rotating cylinder, where a liquid seasoning mix is sprayed on them. Upon cooling, the oil crystallizes, forming the seasoning coat.

Production Challenges. The quality-control aspects of tortilla and tortilla chip production are essential. Among the parameters controlled during the production process are:
  • Cooking, quenching, steeping, baking and frying times and temperatures.
  • The moisture content of the corn, nixtamal, masa and the end product.
  • The operating condition of the equipment (cooker, oven, fryer, cooling rack, etc.).
For Casa Herrera Inc., Pomona, Calif., a supplier of production machinery for flour and corn food products, temperature control is key to ensuring a consistent, high-quality manufacturing operation. Casa Herrera produces many gas-fired oven designs used for tortilla production. The company’s product line ranges from individual production machines to entire automated assembly lines, including ovens, flour presses, “sheeters” and other specialty equipment.

“Tortilla chip factories utilizing gas-fired ovens are concerned about ‘toast points’ -- small brown burn spots on the tips of the tortilla chip -- during the production process,” said Wes Lowery, technical services manager at Casa Herrera Inc. “Maintaining the desired toast point hinges on proper measurement and control of temperature in oven heating zones.”

Figure 1. In a typical gas-fired oven, three different conveyer belts are used to carry tortilla chips during baking.

In a typical gas-fired oven, a series of conveyer belts are used to carry tortilla chips during baking. Variable-rate burners are situated between the belts to sear the chips on each side. The product bakes on the top belt and then is flipped over onto the middle belt, which travels in the opposite direction. The chips are flipped one more time onto the bottom belt to complete the baking process. The chips exit the oven in the same direction they originally entered it (figure 1).

During the preparation of corn tortilla chips, a fast heating of both product surfaces is essential for retaining a suitable moisture level inside the chips.

“The baking process depends on precise control of belt temperature inside the oven,” Lowery said. “The tortilla chips must be seared in a way that bakes in flavor without leaving visible burn marks.”

In the past, most tortilla equipment manufacturers utilized thermocouples to indicate the ambient air temperature inside their ovens. Thermocouples consist of two strips or wires of different metals joined at one end. Changes in temperature at their junction induce a change in electromotive force (EMF) measurable across the leads. As temperature goes up, this thermocouple EMF rises. Sometimes an array of thermocouples (aptly called a thermopile) is used.

“Thermocouples are one of the cheapest and easier to use types of contact temperature sensors,” notes Frank Schneider, worldwide product manager for point sensor products at Raytek Corp., Santa Cruz, Calif. “Nevertheless, thermocouples are incapable of measuring direct surface temperatures, so their readings may not accurately reflect oven conditions during process heating. This drawback limits the ability of tortilla and tortilla chip factories to optimize baking cycles during production runs.”

Figure 2. For its current oven designs, Casa Herrera uses an infrared temperature sensor from Raytek Corp. that can be mounted outside the oven’s heating zone.

Sensor Solution

In situations such as this, where accurate product temperature measurements are critical to product quality, manufacturers of process heating equipment are choosing infrared temperature measurement technology over traditional thermocouples. Infrared thermometers are useful for measuring temperature under circumstances where thermocouples or other probe-type sensors cannot be utilized or do not produce accurate data. A unique characteristic of infrared thermometers is their ability to determine the temperature of an object without making physical contact with it.

The principle of infrared and its noncontact nature offers many advantages for process ovens. Infrared temperature monitoring eliminates the risk of contaminating the product, which is important in the food processing industry. Furthermore, infrared sensors save time and money in situations where another type of contact sensor might require machines to be shut down.

Measuring Tortilla Temperature. For its current oven designs, Casa Herrera employs the Thermalert TX infrared temperature sensor from Raytek Corp. This device combines noncontact temperature measurement with industry standard two-wire technology. The sensor provides digital communications as well as 4 to 20 mA output, allowing remote configuration and monitoring. If needed, multiple sensors can be installed on a single multi-drop network (figure 2).

The smart TX sensor used in the Casa Herrera application has a temperature measurement range of 0 to 1,000oF (-18 to 500oC) with an optical resolution of 33:1. Among the other features, the sensors “provide remotely adjustable temperature and output subranges, adjustable emissivity, ambient temperature check, and a user-defined alarm output,” according to Schneider. “Averaging and advanced peak/valley hold algorithms enable accurate measurement and control of process heating applications.”

Using the infrared system software, oven operators can set temperature and output ranges, emissivity and alarm points, and then monitor temperature data from multiple sensors. Production data can be archived or exported to other applications for analysis and process documentation.

Figure 3. Casa Herrera’s Odyssey oven employs three infrared thermometers -- one for each product conveyor belt -- so that it can monitor tortilla temperature throughout the baking process.

Casa Herrera’s Odyssey, an oven that includes a fuel-efficient combustion system for flour and corn tortillas and tortilla chips, employs a total of three infrared thermometers, one for each product conveyor belt. As noncontact devices with extended optical resolution, the infrared thermometers from Raytek can be mounted outside the oven’s heating zone (figure 3).

“The infrared devices provide a precise measurement of belt surface temperature as tortilla chips go through the oven. Data is fed to a temperature controller, which, in turn, governs an actuator in open or closed position and maintains a constant temperature on the belts under all load conditions,” Lowery says. He noted that this technique improves control of temperature setpoint limits in the baking process, and ultimately ensures a better final product.

In summary, due to infrared temperature measurement technology, factories producing flour and corn food products can monitor temperature levels in their production processes with increased efficiency. This results in higher quality products of greater consistency.

Improved methods for temperature detection and control also enable tortilla and tortilla chip manufacturers to reduce scrap and thus increase product yields.

“Prior to the use of infrared thermometers, ovens had to remain idle during reheating after a production shutdown. Otherwise, a certain amount of product on the conveyor belts would burn during the restart period,” Lowery said.

Now, with precise, direct conveyor-belt temperature measurements supplied by infrared sensors, process-heating cycles can restart immediately without any loss of product. Manufacturers see the benefits of this technology improvement at their bottom line. 

An infrared thermometer can be compared to the human eye. The lens of the eye represents the optics through which the radiation (flow of photons) from the object reaches the photosensitive layer (retina) via the atmosphere.

Sidebar
Infrared Temperature Measurement: A Primer

Infrared instruments measure temperature according to Planck’s Law of black body radiation, which states every object emits radiant energy, and the intensity of this radiation is a function of the object’s temperature. The sensor simply measures the intensity of radiation, thereby measuring an object’s temperature.

An infrared thermometer can be compared to the human eye. The lens of the eye represents the optics through which the radiation (flow of photons) from the object reaches the photosensitive layer (retina) via the atmosphere. This is converted into a signal which is sent to the brain after being compensated for ambient temperature variation (see figure).

Every form of matter with a temperature above absolute zero emits infrared radiation according to its temperature. This phenomenon, know as “characteristic radiation,” is caused by the internal mechanical movement of molecules. The intensity of this movement depends on the temperature of the object. Since the molecule movement represents charge displacement, electromagnetic radiation (photon particles) is emitted. These photons move at the speed of light and behave according to the known optical principles. They can be deflected, focused with a lens, or reflected from reflective surfaces.


For more information about infrared temperature-measuring devices from Raytek Corp., Santa Cruz, Calif., call (800) 227-8074; e-mail support@raytek.com; or visit www.raytek.com. For more information about tortilla production equipment such as ovens, flour presses and other specialty equipment from Casa Herrera Inc., Pomona, Calif., call (800) 624-3916; e-mail wesl@casaherrera.com; or visit www.casaherrera.com.

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