A research team at the DVO consisting of Dr Jose Luis Aleixandre-Tudo, Dr Hélène Nieuwoudt and Professor Wessel du Toit is one step closer to simplification of grape phenolic measurements. The importance of phenolic compounds is due to their crucial role played on the colour and mouthfeel properties of red wines. In a feasibility study recently published, the phenolic content of grapes on a moving conveyor belt was measured using a contactless NIR (Near Infrared) spectroscopy instrument. This inline-online approach was compared against static measurements using the same instrument. Intact as well as crushed berries were measured following the reasoning that phenolic content would be better quantified if grapes are crushed (e.g. the phenolic information of the seed phenolics will be better exposed to the NIR light). The results were compared against two chemical reference methods. The spectral measurements of the crushed berries and the homogenate extraction protocol provided the most accurate prediction models for the inline-online system going forward. The grapes were obtained in collaboration with a number of cellars including Boschendal, Bellingham, Franschhoek Cellar, Rickety Bridge, Remhoogte, Glenwood and Alto wines.
The use of NIR spectroscopy to measure grape phenolics is not new and various studies have reported on it. However this is the first time where a contactless (no sample preparation) system was tested on a moving conveyor belt in real winery conditions, with very promising results. Such a system can be applied to red grape grading in addition to basic chemical parameters and sanitary status. It can also serve as a decision making tool as to which winemaking practices are needed during the fermentation phase, i.e. amount of pump overs per day, when to press off the skins, enzyme dosage, whether or not to use fermentation tannins, etc. The technology can also potentially be transferred to vineyards to attempt the monitoring of phenolic ripeness during the ripening season.
This project falls into a broader strategy of the research being conducted at the DVO in which efforts are currently being made towards the development of integrated systems to optimise the winemaking process. With the use of spectroscopy (light) applications a number of tools are in the process of being developed, covering from phenolic ripeness and content in grapes to fermentation and ageing monitoring in wines. An example of this can be seen in the South African Phenolics Laboratory (SAPhlab) web based application that will be launched in 2019. Making use of ultraviolet-visible light and optimised prediction models this application will provide phenolic information in grapes, fermenting samples and wines with minimal analytical time. Other capabilities of the application include the possibility of comparing the generated data against an extensive database of phenolic levels, which can be very valuable for wine producers.
Jose Luis Aleixandre-Tudo, Helene Nieuwoudt, Wessel du Toit (2019). Towards on-line monitoring of phenolic content in red wine grapes: A feasibility study. Food Chemistry: 270, 322-331.