# Competitive Detection of Volatile Compounds from Food Degradation by a Zinc Oxide Sensor
## 1 Introduction  
Food quality evaluation, shelf-life determination, and raw product control are important fields in which conductometric gas sensors have been applied in recent years. In these sensors, the gas detection mechanism is based on the conductance variation of a semiconducting metal oxide (MOX) layer caused by the chemical/physical adsorption of the gas molecules on the MOX surface [1]. MOX sensors have been intensively studied and used for their low production cost, reduced size, and high sensitivity to a broad range of chemicals. Compared to typical chemical analytical techniques used in food industry, the lower accuracy of MOX sensors is compensated by a faster response, higher portability for field usage, and less expansive instrumentation. A possible application of MOX sensors in the food industry for meat quality control is the detection of hexanal, a typical decomposition product of meat formed during lipid oxidation in stored and packed meat products [2, 3, 4]. Hexanal, indeed, is considered an important indicator of the freshness status [5, 6] of raw meat products. In a preliminary work, we compared three different MOX sensors for hexanal detection and we found that zinc oxide (ZnO), used as the detecting layer for resistive sensors, showed the best compromise between the sensing temperature (T = 523 K) and response amplitude [7, 8]. Indeed, the ZnO resistive gas sensor was tested with a broad range of hexanal concentrations in different working conditions, especially interms of the working temperatures and relative humidity. However, although the suitability of the ZnO sensor for hexanal detection was proved, the influence of other factors needs to be considered. In particular, in real food monitoring conditions, the decomposition process leads to the formation of numerous volatiles even during the early stage and the sensor must have the ability to detect the target volatile with minimal interference from others. In other words, the sensor must show sensitivity and selectivity towards hexanal. In general, as known, selectivity is one of the major issues in conductometric sensors made of semiconductor oxides because the sensing mechanism based on the exchange of electrons on the sensor surface during the \"oxidation\" of the volatile molecules is not a selective process and similar compounds can oxidize at the same time [9]. Several alternatives to improve the selectivity and sensitivity of MOX sensors are under continuous development, for example, the synthesis of particular architectures, such as nanowires, nanotubes, and nanosheets [10], exploiting the effect of nanostructured materials with respect to the bulk form [11], or the preparation of conductometric sensors based on p/n-type heterojunctions [12]. Similarly, the development of electronic noses requires the implementation of different sensor arrays to improve the ability to discriminate between similar volatile compounds [9; 13; 14; 15]. However, electron noses, which ensure the widest applicability, are complex and voluminous equipment that require trained specialists for their use. A single sensor or at most a combination of two makes the detection system easy to use even for unskilled operators and easily transportable, even if it is not usable for generic analyses but suitably developed for a specific application. From this point of view, a complete characterization of the sensor's behavior in the environment of interest is a fundamental step for the realization of the measurement system. Therefore, in this work, we studied the effect of the coexistence of the main volatiles formed during meat degradation on the response of a ZnO sensor to hexanal, which, to the best of our knowledge, has not been reported yet. In the first stage of the degradation process, 1-pentanol and 1-octen-3-ol are the two main compounds released together with hexanal that can unexpectedly and unwantedly interfere during the detection of the MOX sensor [4; 6; 16].