It is well know that edible oils deteriorate during cooking, particularly when they are repeatedly heated to high temperatures. These oils are typically heated to temperatures of the order of 180° C. to fry food. A multitude of chemical reactions occur at these temperatures, such as polymerisation, thermo-oxidation, etc., which significantly alter the quality of the oil. The quantity of some products of these reactions must not exceed a threshold imposed by legislation, since the oil is deemed unfit for consumption beyond the threshold. It is thus important to be able to detect the threshold in a reliable manner, so that the oil is replaced as soon as it becomes necessary. For a long time, it was left to cooks to judge, after a visual and/or olfactory inspection, whether the oil was still fit for consumption. Of course, that method is entirely subjective and is consequently unreliable.
EP Patent No, 1 588 158, which corresponds to U.S. Patent Application Publication No. US 2006/0288877 A1, discloses a device for the capacitive measurement of the quality and/or deterioration of a cooking oil to overcome these drawbacks. The content of EP Patent No, 1 588 158 and corresponding U.S. Patent Application Publication No. US 2006/0288877 A1 is incorporated herein by reference. In this device, the capacitive sensor is directly arranged in the vat of the cooking apparatus, with the sensor encapsulated in a perforated protective case, secured in a submerged area of the vat.
Although the device disclosed in that Patent Application operates satisfactorily, performing a capacitance measurement inside a deep fat fryer remains a highly delicate operation. Indeed, a variation of a few picofarads between the new oil and used oil, greatly influenced by temperature, water and impurities, is not easy to detect. Added to this is the fact that the device has to operate in a very harsh environment in which the capacitive sensor, and, possibly, the temperature sensor associated therewith, are subjected to temperatures higher than 200° C., and to shocks when careless operators strike the sensor with the baskets holding food for frying.
Protection of the capacitive sensor, and possibly, the temperature sensor associated therewith, thus constitutes an extremely important problem that needs to be addressed to avoid deterioration in measurement accuracy and/or reducing the life time of the measuring device, since the measurements are directly dependent thereon.