WINETECH Technical Yearbook 2019

It is known that the cultivar (the genetic make-up) plays a big role in the YAN level and composition. In the second scenario, a model built on data obtained from major cultivars (Sauvignon blanc, Chenin blanc, Chardonnay, Shiraz, Merlot and Cabernet Sauvignon) was tested on minor cultivars (Marsanne, Roussanne, Pinot gris, Verdelho, etcetera). Practically, when analysing samples, certain cultivars are well represented in the data set, while other, more ‘exotic’, will appear only seldom. It can thus happen that a calibration can be set up to include only major cultivars. In this case, again the WineScan and the MPA performed best, but the Alpha-P also gave

accurate results. The other two instruments considered, the MicroNIR and the FieldSpec 4, produced unreliable results. TAKE-HOME MESSAGE Even t hough t he pe r fo rmance va r y depending on the IR region and mode i n wh i c h t he i n s t r umen t doe s t he measurements, the study showed that it is possible to calibrate IR spectroscopic instruments for the accurate measurement of YAN, FAN and ammonia concentrations using various data sets and scenarios. In all cases, transmission FT-IR spectroscopy (FOSS) and FT-NIR spectroscopy (MPA) produced models capable of good to

excellent quantification. Both of these instruments showed sufficient robustness against samples originating from different varieties, growing conditions and vintages, addressing the concerns of applying this technology to the wine industry. Given the accuracy, robustness, high throughput and cost-effective nature, the models produced by both FT-IR and FT-NIR spectroscopy provide winemakers with the opportunity to make timelier and informed nutrient supplementation decisions, facilitating the achievement of their desired wine style and quality. As a result, these calibrations have been added to the existing IR method portfolio in the CA Lab of the DVO.

SUMMARY The use of infrared (IR) spectroscopy, combined with chemometrics, is gaining t rac t i on due to i t s advantages over traditional methods. Therefore, various IR spectral instruments measuring in different modes and ranges of the IR spectrum (FT-IR, FT-NIR and ATR-MIR), were compared and evaluated for their accuracy to measure total YAN, as well as its components, FAN and ammonia, separately. Furthermore, the robustness of the calibrations against the inherent variability of the grape juice matrix was assessed to ascertain the feasibility of the integration of this technology in an industrial context.

– For more information, contact Astrid Buica at abuica@sun.ac.za.