Wine analysis using spectroscopy
Within quality assurance and process monitoring practices, spectroscopy has been widely used for many years1. Increasingly spectroscopic techniques and advanced data processing are being adopted for authenticity testing.2
Such verification is particularly important in the wine industry, which provides a premium-priced commodity. The retail value of wine is determined by a host of factors, including the origin, the varietal, and the year of production. The price of a bottle can exceed $13K in the fine wine market.
Spectroscopic technologies can be used to validate:
- a wine's vintage
- country of origin
- the grape variety
Marsala wine is an example of a fortified wine marketed by virtue of the origin of its grapes. There is a whole industry dedicated to monitoring the age and characterization of Marsala wines to provide quality control and ensure fair practice for both wine growers and purchasers.
Analyzing Marsala wine
Marsala is one of the most important dessert wines and was one of the first Italian wines to receive the DOC recognition. It is thus a product of great economic importance.
It is exclusively produced in the province of Trapani in Sicily, Italy, from autochthonous grapes. Marsala is marketed in twenty-nine different categories, which are differentiated according to the particular grape variety and technology used in its production and the duration of the aging process3.
Both single-variety and blended Marsala wines are available. Grillo, Inzolia, Catarratto and Damaschino white grape varieties are used in the production of Golden (Oro) and Amber (Ambra) Marsala wines, whereas the red grapes, Nero d’Avola, Perricone and Nerello Mascalese, are mixed with white grape (up to 30%) to produce Marsala Rubin (Rubino). Each Marsala type is further classified as dry, semi-dry or sweet according to the amount of sugar present, which can range from100 g/L.
The aging time in barrels is an important consideration in the classification and pricing of Marsala. Fine Marsala has been aged for a minimum of 1 year; Superior Marsala for a minimum of 2 years; Superior Reserve Marsala for a minimum of 4 years; Virgin Marsala for a minimum of 5 years; and Virgin Reserve Marsala has had a minimum of 10 years of aging.
The aging of a Marsala wine, together with the grape features, is fundamental in determining its quality. Oak and/or cherry barrels are used to store the Marsala during the aging period and these too influence the characteristics of the wine.
The ability to determine the aging history of a particular bottle of Marsala wine is thus of critical importance for confirming its quality, and consequently its value.
Determining the quality of Marsala wine using analysis
Such analyses, however, are not suitable for routine monitoring since they are expensive and time consuming, requiring the use of complex instrumentation operated by specialized technicians, such as high-performance liquid chromatography (HPLC) with mass spectroscopy detection. Furthermore, the sample preparation needed for such analyses necessitates the use of organic solvents.
Food authenticity testing must be conducted regularly to be effective, and so cost-effective, easy-to-use analysis techniques that provide robust data rapidly are essential. A variety of spectroscopic techniques have proved to meet this brief and are becoming increasingly popular for the determination of the origin and authenticity of food and drink products.
The development of automated spectroscopic instrumentation at reasonable cost has made these powerful analytical tools widely accessible. These techniques have the additional advantages that no sample preparation is needed and that they are not detrimental to the product being tested.
Fourier transform infrared (FTIR) spectroscopy
Although chemometrics, such as Fourier transform, are required for the interpretation of IR spectroscopic data, software that performs this automatically is now readily available. Since the data are captured electronically, they can be easily searched to match the results of an analysis with spectral libraries facilitating rapid identification/comparison.
Fourier transform IR spectroscopy (FTIR) provides versatility as either near-, mid-, or far-IR regions can be utilized and is widely used across a variety of applications5. The technique has recently been further enhanced by the development of sampling accessories, such as attenuated total reflectance (ATR) cells, which has promoted FTIR as a tool for routine analysis by simplifying sample handling and improving accuracy.
Mid-infrared technology has already been applied to the differentiation, classification and authentication of wines and brandies6-8. The technique has now also been applied to the monitoring of Marsala wine9.
Fourier transform infrared spectroscopy for Marsala wine monitoring
The wines of different aging time, color and sugar level were analyzed using a Shimadzu IRAffinity™-1S FTIR spectrophotometer with a Specac Quest™ ATR accessory.
Using principal component analysis of the obtained spectral data, the Fine, Superior and Superior reserve samples were readily distinguished from the most valuable Virgin samples. In addition, linear discriminant analysis identified Marsala wines of different aging times with a coefficient of variance exceeding 20%9.
The results showed a complete discrimination of 100%. The confusion matrix of cross validation was equal to 87.76%, indicating a high percentage of correct classification.
It is hoped that the technique can provide objective data that will be useful to verify the conformity of the Marsala wines to the declaration labeled and also to monitor the wine during the aging process in the winery.
Use of the ATR accessory further simplified sample handling and improved the accuracy of the analysis. Specac produce a range of high-quality ATR accessories to meet all FTIR spectroscopy needs10.
The Quest™ ATR Accessory is a performance single-reflection ATR accessory designed for laboratory spectroscopic sample analysis in the mid- and far-infrared range. The innovative optical design and durable monolithic diamond ATR crystal ensure the highest quality in performance. A range of interchangeable ATR crystal pucks are also available to allow the Quest ATR accessory to be tailored to meet a specific research need.
The Quest ATR accessory contains around all reflective gold-coated optics, and features Specac's Synopti™-Focal Array technology with precision-moulded aspheric mirrors. These features allow for high throughput with an extended wavelength capability suitable for both mid- and far-IR spectroscopy.