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Infrared light as an indicator of wine quality

Guaranteeing the quality of wines is becoming increasingly important so that they are able to compete in the international market. However, the quality control methods currently used are often rather time-consuming and costly. In addition, it is difficult to detect microbial grape contaminations that might jeopardise the quality of the wines. Researchers at the National Training and Research Institute for Viticulture and Horticulture in Weinsberg, Germany, are now working with researchers from the University of Hohenheim on the development of a new method to overcome these difficulties and to make the assessment of wine quality more reliable.

Dr. Martin Pour Nikfardjam from the National Training and Research Institute for Viticulture and Horticulture (LVWO) in Weinsberg  and his team are working together with a research team led by Prof. Dr. Dr.  Reinhold Carle at the University of Hohenheim. Their plan is to develop a new method for the rapid and efficient assessment of the quality of grape mash based on near-infrared spectroscopy.

Current wine quality assessment methods are inadequate

The ripeness and health of grapes is currently assessed mainly on the basis of visual inspection and measurement of the must weight which is based on the sugar content and is measured in Oechsle (Oe) degrees. However, the Oechsle scale only gives a rough indication of the future quality of the wine. While it allows the potential alcohol content after complete fermentation to be calculated, the quality of the wine depends on many other factors. 

Dr. Martin Pour Nikfardjam, head of the Department of Analytics at the National Training and Research Institute for Viticulture and Horticulture in Weinsberg. © private

“Various fungi are present in the vineyards and have a greater or lesser impact on the quality of the individual wines,” said Nikfardjam. “We therefore need an objective tool for analysing the grapes. At present, grape rot is only assessed by visual inspection, which provides a rather subjective assessment of the grapes’ condition,” Nikfardjam explained.

“In autumn, during the grape harvest, the producers come with big containers of grapes to the cooperative wineries where the grapes are inspected. The inspectors only see the grapes on the surface of the grape heap. The most perfect looking grapes might be lying at the top while grapes of bad quality might be hidden away at the bottom and escape the inspectors’ attention.

The amount paid to the wine growers is determined by the quantity of grapes, Oechsle degree and visual appearance. This is another reason why we need a new method that allows the objective and comprehensive assessment of the quality of the grapes at the time of delivery. The method also needs to be fast. Many wine growers deliver tons of grapes to the cooperatives during the harvesting period in autumn and these need to be processed quickly in order to prevent them from rotting. 

First improvements through the introduction of spectroscopic methods

For quite some time, a method based on the spectroscopic measurement in the mid-infrared range has been used to measure the quality of grapes. This method allows a comprehensive analysis of the mash ingredients and potential microbial contaminations. However, the method has the disadvantage that the mash needs to be processed in a particular way prior to measurement: a homogeneous mash sample is withdrawn, centrifuged and filtered in order to remove coarse particles. The method only works with a clear liquid. The preparation of such a sample takes valuable time, which is not available during the hectic harvesting period when the grapes are delivered. In addition, this small sample is only a small ‘snapshot’, yet determines the fate of the entire harvest delivered.

“Although measurements in the mid-infrared range improved the situation, we were nevertheless far from our goal of automating the process as well as integrating the method into mash processing. We wanted to improve the situation and had the following scenario in mind: the wine growers deliver the grapes, these glide past a measurement device which carries out constant measurements of the entire load without requiring a specific sample to be prepared,” Nikfardjam said. 

Irradiation with infrared light provides information about grape composition

Diode array spectrometer of NIR-Online GmbH used for the analysis of grape mash using near-infrared spectroscopy as the grapes are processed © NIR-Online GmbH

The cooperative research project of the LVWO and the University of Hohenheim has the potential to solve the aforementioned problems. The project started in 2010 and is supported by the FEI (Research Association of the German Food Industry) with funds provided by the BMWi – Federal Ministry of Economics and Technology. The project also involved the Cooperative Winery Heilbronn-Erlenbach-Weinsberg, the Weinbiet Vintners’ Cooperative based in Neustadt/Weinstraße and the company NIR-Online based in Walldorf as an industrial partner. 

The new method is based on near-infrared spectroscopy and is already successfully used in quality analyses and process control in breweries, corn mills and the pharmaceutical industry. It can be integrated directly into the grape mash processing step as the samples do not need to be specifically prepared for the measurement. 

Near-infrared spectroscopy is a spectroscopic method that uses the near-infrared region of the electromagnetic spectrum. Upon irradiation, every molecule type in the sample reveals a specific vibrational signature which provides information about the ingredients of the sample and their concentration. 

No routine application without calibration

Axel Mattes, member of Dr. Nikfardjam’s team analysed over one thousand mash samples as part of his doctoral thesis. © private

The method is an indirect one, which means that the NIR device needs to be calibrated with reference samples before it can be used for the routine analysis of unknown samples. 

The calibration of the device using reference samples was one of the major priorities of the research project. Axel Mattes of the Nikfardjam team analysed more than 1000 mash samples in order to obtain information on composition and component concentration. Mattes first applied NIR spectroscopy and then reference analytical methods. The results of the latter were matched with the spectroscopic ones for calibration and determination of the concentration of the individual components. It is quite a lengthy process to compare a large number of samples and adapt the calibration to each vintage and wine growing area.

Ergosterol as an indicator of mould infestation

Numerous parameters need to be included in the comprehensive assessment of grape quality, degree of ripeness and health. Nikfardjam and his fellow researchers based their analysis on the grapes’ glucose and fructose content as well as on the concentration of tartaric and malic acid. 

Other important quality parameters were what are known as decay markers such as acetic acid and glycerine which are present as the result of microbial activity. In addition to these standard parameters, Professor Carle and his team included a decay marker that can be detected using NIRS: ergosterol. 

Ergosterol is a component of fungal cell membranes and can therefore be used as an indicator of mould infestation. Ergosterol is already used for assessing the decay of other food products, including tomatoes, apple juice as well as of animal feed. “There is a linear correlation between decay and ergosterol content. The amount of ergosterol increases with the increase of decay,” Nikfardjam explained.

"We have now reached an important stage“

Grapes being turned into mash. © LVWO

The results presented in the final report are promising. “We are now at a very important stage and have created a basis on which we can build,” said Nikfardjam highlighting that the project partners will now test whether the NIR device provides reliable results. This seems likely as the device has been thoroughly tested for a period of three years during which it was fed with huge amounts of data. They will also optimise the calibration models and focus on improving the predictive quality of the method. 

“Our goal is to optimise the system so that it will eventually work without significant human intervention. With this I mean that the system will record the near-infrared spectrum of the entire grape mash and analyse the individual quality parameters. Ideally, the mash will also be separated into different quality fractions by way of valves and further processed,” said Nikfardjam highlighting his expectations. 

The integration of the NIR method into grape processing would provide wine inspectors with a quick and cheap way of objectively assessing grape quality and health. This knowledge leads to better wine quality and increased consumer safety. In addition, Nikfardjam expects that the system can be used to develop a fair and quality-based payment system, though it will still take some time before the product can be launched commercially.

Further information about LVWO:

Dr. Martin Pour Nikfardjam is the head of the Department of Analytics at the National Training and Research Institute for Viticulture and Horticulture in Weinsberg, which is specifically focussed on chemical and microbiological analyses of a broad range of samples, including samples of fruit, juices, wines and other alcoholic beverages, distillates as well as vinegar. Nikfardjam’s department mainly services the LVWO, but also carries out analyses for others, including local wine growers. 

 

Further information:
Dr. Martin Pour Nikfardjam
National Training and Research Institute
for Viticulture and Horticulture (LVWO)
Traubenplatz 5
74189 Weinsberg
Tel.: +49 (0)7134 / 504 170
Fax: +49 (0)7134 / 504 174
E-mail: Martin.PourN(at)lvwo.bwl.de

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/infrared-light-as-an-indicator-of-wine-quality