Topics on “Libra” electronic nose for fruit quality

Abstract

Quartz crystal microbalances (QCM) are piezoelectric devices. To turn a quartz crystal into a chemical sensor it is necessary to coat it with a layer of a material capable of capturing molecules from the environment. When a mass is absorbed or placed onto the quartz crystal surface, the oscillation frequency changes decreasing in proportion to the amount of mass (Di Natale et al. 1997). The ability to control a QCM´s selectivity by applying different coatings is an important feature, and makes this sensor type extremely versatile. However, the coating of QCM is, ironically, their greatest drawback. Batch-to-batch variability in the manufacturing leads to inadequate reproducibility(Sarig 2000). Indeed, the response of sensors depends on numerous factors that may be difficult to control, such as the temperature and the humidity of the carrier gas. All these factors cause changes in the selectivity of sensors affecting the reproducibility of measurements. The detection threshold of the human nose is typically between 1000 ppm (10-6)and <1ppt (10-12). Therefore, the absolute detection threshold of a gas sensor should be very low. For a QCM e-nose, only a few molecules are required to react with the sensitive elements leading to sensitivities close to the ppm or tenth of a ppm range as measured in the vapour phase (Mielle 1996) . Despite previous studies using electronic nose, no QCM calibration statement has been established in relation to the sensitivity needed in the sensors for organoleptic quality measurement in pear or in other fruit. Present work will show the possibilities of using this equipment as a non destructive method to evaluate fruit quality. The work will be developed in different steps or topics to get a better understanding