Wine Chemistry and Biochemistry

668 M. Dubernet

obtained is thus is two light beams of the same wavelengths with a phase inter-

ference ofp. According to the phase difference recombination of the interference

signals is constructive or destructive. In other words, the intensity of the interference

signal will vary according to the value ofp. The plot of the variation in intensity as

a function of the phase is called an interferogram and its mathematical model is an

integral. Fourier transform is a mathematical procedure which converts the inter-

ferogram into a signal the intensity of which varies according to wavelength and

which in turn enables the infra-red spectrum to be constructed. This is an intricate

calculation which can only be carried out using powerful computing techniques.

The spectrum thus obtained enables the technique to be used as a classical

method of spectrophotometric analysis, in which specific absorption wavelengths of

organic compounds are employed to determine concentrations of the target analytes.

12.5.3 Installation and Acquisition of Infra-Red Spectra

The sample to be analysed does not require any particular sample pre-treatment.

However, in the case of musts or cloudy wines, a preliminary clarification by cen-

trifugation or filtration is recommended in order to prevent system blockage. Carbon
dioxide levels exceeding 750mg/L should be reduced or eliminated before analysis

to avoid degassing of the sample in the analytical circuit.

The circuit (Fig. 12.2) starts with a sampling needle. A peristaltic pump transfers

the sample into a heating chamber where it is heated to 40◦C. After passing through

a filter, the sample is transferred into the measurement cuvette which is constructed

of fluorosilicate as glass or quartz absorb strongly in the mid infra-red. After analy-

sis, the sample is ejected to waste.

As the complete cycle lasts only 30s, with automation, the analysis rate is of the

order of 120 samples per hour.

12.5.4 Chemometrics

Generally the mid-IR spectrum of a wine or must contains information of analytical

interest the extraction of which in the majority of cases requires highly complex

sampling

needle

Pump

Heating unit

Filter Cuvette

waste

Fig. 12.2Simplified schematic diagram of the analytical circuit