Standard methods and procedures for the quality

Standard methods and procedures for the quality assessment of alcoholic beverages are based on gas and liquid chromatography, often coupled with mass spectrometry for the structural identification of the individual components [7]. For the enrichment of the aroma substances, several methods have been used such as liquid–liquid extraction, column chromatography, solid phase extraction, and solid phase microextraction. The presence of a large variety of compounds that contribute to taste and flavour means that the analysis of the comprehensive spectral data on some wines is not sufficient for assessing their quality. Therefore, pattern recognition approaches have also been applied to the classification of “multi-component” wines and are useful for quality control evaluations [2,8]. In fact, pattern recognition methods for classification and identification are used increasingly in fields such as food chemistry, process monitoring, medical sciences, pharmaceutical chemistry, and social and economic sciences. Classification is one of the fundamental methodologies in chemometrics and basically consists of finding a mathematical model capable of recognising the membership of each object to its proper class. Once a classification model has been obtained, the membership of new objects to one of the defined igf 1 antagonist can be predicted [8]. Sensory evaluation by trained experts is also employed, but it does not always provide an objective analysis of the consumer products and is an expensive procedure. Recently, electronic noses and tongues, combined with statistical approaches, and chemometrics have been used intensively for food and beverage identification and ageing and quality assessments [8–12]. Typically, electronic noses and tongues mimic the human olfactory system and are based on an array of nonspecific sensors (or sensors with different selectivities) combined with pattern recognition software. Most electronic nose systems are based on metal oxide sensors or mass sensitive transducers that are inherently highly sensitive to water, making the detection of flavour chemicals present at low concentrations in alcoholic beverages extremely difficult. In this regard, fibre optic sensors provide an alternative, universally adaptable sensing platform because of the inherent stability of the optical fibres, their high sensitivity, and the ability to multiplex sensors in series [13]. In particular, the combination of optical fibre devices and nanomaterials offers the prospect for the development of measurement techniques using an optical waveguide with a nanostructured coating layer that exhibits changes in its optical properties upon exposure to targeted chemical species. Recently, we have demonstrated a novel chemical sensor based on a layer-by-layer (LbL)-deposited mesoporous film composed of silica nanospheres (SiO2 NPs) on an optical fibre long period grating (LPG) for the detection of organic compounds [14] and ammonia [15] and for the measurement of the refractive index (RI) of substances [16]. An LPG consists of periodic modulation of the refractive index of the core of an optical fibre with the period lying typically within the range from 100μm to 1mm [17]. It couples light from the forward propagating mode of the core of the fibre with a discrete set of co-propagating cladding modes at wavelengths governed by the phase matching condition, as shown in Eq. (1):where λ(x) represents the wavelength at which the coupling occurs for the linear polarised (LP0x) mode, ncore is the effective refractive index of the mode propagating in the core of the fibre, nclad(x) is the effective index of the LP0x cladding mode, and Λ is the period of the grating. The presence of a coating layer with a sub-μm thicknesses, deposited via LbL film deposition techniques such as Langmuir Blodgett (LB), electrostatic self-assembly (ESA), and dip coating, modulates the transmission spectrum (TS) of the LPG and makes it highly sensitive to the external refractive index. Based on this principle, sensors for pH, humidity, and chemical and biological applications have been demonstrated [17].