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 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, as well as its U.S. equivalent, namely, U.S. Pat. No. 7,504,836 B2, are 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.
During his research, the Applicant discovered that the position or orientation of the sensor inside the vat is crucial for obtaining a reliable capacitive measurement. Indeed, both water and impurities in the oil and temperature gradients present in the vat have a significant effect on the measurement and the accuracy thereof.
Water, mainly present in new oil and in food for frying, finds its place at the bottom of the vat when the oil temperature is less than 100° C. When the temperature of the oil rises, it creates a “hot” area above the heating element (approx. 180° C.) and possibly a “cold” area (approx. 80° C.) below the heating element if the deep fat fryer is provided for this purpose. The water generated by cooking may remain fractionated with the oil when its temperature is less than 100° C.
When food is being cooked, water is discharged because of the cooking chemistry. Part of the water passes from the liquid state to the steam state, which will produce natural mixing, which will also cause the water at the bottom of the vat to rise. The operator often performs mechanical stirring to cook the food better.
It is when drops of water or steam become blocked in the sensor or at the surface thereof, that the measured value becomes unusable.
Moreover, impurities, mainly formed of breadcrumbs and bits of cooked food, dirty the sensor, mainly on the sensitive surface thereof and over any closed part of the encapsulating case. These impurities thus also tend to damage the measurement quality.
The capacitive measurement is also greatly influenced by the temperature gradients present in the vat. When a deep fat fryer is switched on, the temperature above the heating element reaches approximately 180° C. Underneath the heating element, the temperature increases slowly if there is no mixing caused by the insertion of products for frying, which pushes the hot oil from the top to the bottom. Mixing the oil causes temperature differences on the sensitive surface of the sensor. This thermal gradient may be very marked and the temperature variations may be very rapid when products for frying are dipped into the vat.
The temperature greatly influences the dielectric constant of the oil, and thus its capacitance, which, in this case, decreases. Temperature measurement, and mainly the measuring point thereof, is thus also an essential element for the accuracy of the system.