The application of microwave sensing to the measurement of cheese curd moisture

The application of microwave sensing to the measurement of cheese curd moisture

There is a need in the dairy industry for instrumentation capable of providing on-line information about the moisture content of cheese during manufacture. Present measurement techniques are usually performed off-line and can be susceptible to human error. It is demonstrated that microwave-based moisture sensing techniques offer a number of potential advantages over conventional methods due to the strong interaction of microwaves with water. The permittivity of cream cheese curd and low-fat cheddar cheese curd has been measured over a range of frequencies and moisture contents in order to establish the relationship between these variables. A vector reflection coefficient measurement engine based on a six-port reflectometer has been built and tested. A suitable sensing head has been fabricated from a short length of microstrip transmission line. Two sensor characterisation models have been developed and compared with measured data. A novel algorithm has been developed to resolve the ambiguity inherent in many permittivity measurement techniques. It has been discovered that surface waves can propagate on a grounded dielectric slab covered by a material with a higher dielectric constant, provided the loss factor of the covering medium is greater than zero. It has also been found that the dominant mode of microstrip can radiate when the line is covered by a high-permittivity material, although this can be suppressed if the covering material is sufficiently lossy. There are three principal conclusions to draw from the investigation in this thesis. Firstly, changes in the moisture content of cheese curd during manufacture produce measurable variations in permittivity. Secondly, these changes can be measured accurately and cheaply using off-the-shelf microwave hardware. Finally, considerable attention must be paid to the characterisation of the sensing head if the instrument is to achieve its full potential. Promising results have been obtained in this area, however certain issues pertaining to the propagation of multiple dominant modes and higher order modes have not been fully resolved and would repay further theoretical analysis.