Why Monitor Food Temperature?

The two primary reasons for monitoring internal food temperature are taste and safety. Beef, for example is cooked at different temperatures for varying periods of time to provide very distinct taste and texture:

• Extra-rare: cooked between 115-120°F (46-49°C) to provide very red and cold meat
• Rare: cooked between 125-130°F (52-55°C) to provide soft meat with a cold red center
• Medium rare: cooked between 130-140°F (55-60°C) to provide firmer meat with a warm red center
• Medium: cooked between 140-150°F (60-65°C) to provide pink and firm meat
• Medium well: cooked between 150-155°F (65-69°C) to provide a small amount of pink in center
• Well done: cooked above 160°F (>71°C) to provide evenly spread gray-brown meat

Food safety is also a major reason to monitor the internal temperature of food. Food borne illnesses caused by pathogens like Clostridium perfringens, Salmonella and E coli can be easily exterminated by ensuring that food is well cooked and cooked at prescribed temperatures. The US Government sponsored Partnership for Food Safety Education, for example, recommends that poultry be cooked at 165°F as measured by a food thermometer. Likewise there are specific recommended temperatures at which other food products should be cooked. It is for this reason that oven users are strongly encouraged to use food thermometers and food probes.

Wireless Food Probes Using Surface Acoustic Wave Technology

Conventional, wired food probes greatly reduce the overall burden of monitoring food temperatures. By providing an embedded solution, wired food probes do away with the need to have external gadgets to monitor food temperature. However, the wire that links the probe to the oven has disadvantages associated with it:

• The wire can either be too long or too short.
• If too long, it has to be coiled up and neatly arranged within the oven cavity.
• If too short the probe can only be inserted at certain angles causing the user to potentially struggle with the placement of the food within the oven cavity.
• When hot, the wire can cause injury causing the user to be extra cautious to be careful while handling it.
• Cleaning the wired food probe can be difficult and unwieldy. This can greatly discourage use of the probe.

Today, the technology to make the food probe completely wireless exists. Surface Acoustic Wave (SAW) technology allows for passive (no batteries) sensors to be wirelessly interrogated to obtain temperature measurements. Using this technology, many of the disadvantages associated with a wired food probe are completely eliminated by wireless food probes:

• There is no need to worry about wire length as the probe is wirelessly linked to the oven.
• When hot, the form factor of the wireless food probe makes it very easy to remove and handle.
• The wireless food probe, just as any other utensil, can be easily cleaned.

Wireless Food Probes using SAW Temperature Sensors

The small size of SAW wireless temperature sensors (~3.8 mm²) make them ideal for insertion into the small confines of a food probe as shown in the figure below.

Figure: SAW Temperature Sensor and Antenna in Food Probe

The sensor is placed within the hollow cavity of the food probe, as close to the tip as possible. As shown in the figure, the SAW temperature sensor is connected to an antenna that is placed in the handle area of the probe. An oven interrogator antenna, connected to embedded oven interrogation electronics, is appropriately located within the oven cavity. An electromagnetic link is established between the probe and the interrogation electronics via the oven interrogation antenna. Changes in food temperature translate into changes in the frequency of the sensor response signal, which, after being processed through calibration algorithms, is converted into an accurate temperature measurement.

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