Assessment of relationships between grape chemical composition and grape allocation grade
Abstract
The aim of the project was to determine from existing and new objective chemical measures which ones can differentiate higher and lower value grape grades and whether any of these grape-derived compounds were also predictive of final wine style or grade for Australian Cabernet Sauvignon, Shiraz and Chardonnay grapes. The project identified objective chemical measures that can be used to define grape quality and are predictive of potential wine style in an Australian context. Targeted (individual compounds) and non-targeted (spectral fingerprints) analytical approaches were useful data types for grade prediction and specific chemical markers important for quality have been identified.
Summary
The first point in the wine value chain is the vineyard and the grapes, therefore decisions made at this point are critical to achieving the desired final wine style. Grape-derived compounds responsible for appearance, aroma, texture and taste in wine are the primary contributors to the degree of fitness for purpose of wine grapes. Many of these compounds are known and are measurable, have meaning to the final sensory characteristics of the wine and can be manipulated through viticultural and/or winemaking practices. However, the application of objective chemical measures in Australia tends to be at a rudimentary level, with only one or two measures implemented by some companies.
The value of the grapes is currently assessed in a range of ways that are mainly subjective. The subjectivity of the assessment can result in uncertainty as to whether the maximum value possible for those grapes is achieved. As such, many growers and winemakers want to support this decision-making process by using objective chemical measures that directly relate to attributes that confer value such as key compounds responsible for taste, aroma, texture and appearance. The grower can be paid based on a field-based assessment of, for example the condition of the vines, flavour of the fruit, presence of disease, some quantitative chemical measures such as colour, Brix, pH, TA or they can be paid based on the final value of the wine that is achieved using those grapes. For the winemaker, this assessment is critical to ensuring they have fruit that is appropriate to the value and style of the wine they intend to make as well as controlling raw material costs.
By measuring a range of chemical compounds in multiple grape batches, the project aimed to determine how variable these were across a wide range of fruit grades. The primary aim was to determine which of these chemical measurements in grapes, independently or in combination, were able to differentiate between grape grades. A further aim was to determine whether the fruit could be clustered (grouped) based on similarity of chemical composition. An expected output of the research was to assess the practical application of grape grading measurements and to support wine producers who intend to apply these measures in their systems. The pilot study was performed on a small scale in 2013 (CAS only), followed by a large-scale study in 2014 (CAS, SHZ and CHA).
An extension to the original 2013 and 2014 ‘fruit-focused’ project was a 2015 vintage trial that also included winemaking from different fruit grades for CHA and SHZ. After discussions with industry partners, it was decided that the most valuable objective to address in the next stage of the research was to establish objective measures in grapes that could be used to support decision-making about the most appropriate style of wine that could be created from an individual batch of fruit. It was also important to establish whether compounds that can differentiate between grape grades were also predictive of final wine grade.
The results of the 2013/2014 grape allocation grade study showed that targeted grape chemical analysis with data modelling gave a good indication that some traditionally measured compounds (pH, TA, Brix, colour) could be useful as objective measures, as could some new chemical measures that had not previously considered in the assessment of quality in the Australian context. Some of the new analytical measurements identified are already in use for quality assessment and grape streaming by international producers, and the results of this project support their observations, providing a strong demonstration of their value to the Australian wine industry. A limitation of the analytes identified is that complex analyses may not be viable for some producers, due to cost and time delay for the analyses – however many of these analyses are available through third-party laboratories. Alternatively, it was important to note that for all the varieties studied, ‘non-targeted’ spectral fingerprint measurements, in particular MIR, could be used to achieve a high degree of grape grade prediction accuracy (≥90%) using QDA. Furthermore, for CHA, 93% prediction accuracy was achieved using simple grape chemical analyses that would be readily accessible to a winery laboratory.
The 2015 ‘grape chemistry-wine style/grade’ prediction study demonstrated that grape composition can be used to support decisions about streaming fruit for particular wine styles or grades. Thus grape composition can support assessment of fruit potential for both wine quality grade (and style), with many of the objective measures identified in the 2014 ‘grape grade’ study also being associated with ‘wine style’ outcomes. The results give a strong demonstration that with further research, and greater sample numbers in stylistic categories, robust models could be developed to predict wine style or grade outcomes from objective grape-based analyses.
A number of benefits from the project are apparent in terms of economic returns to grapegrowers or winemakers, improvements in the quality of grapes or wine, environmental benefits or benefits to the broader community. Potential impacts of this project for the wine sector include the ability for grapegrowers to more efficiently produce grapes to defined specifications, and for winemakers to select fruit with greater confidence that it will be appropriate for a targeted wine style. In addition, it is significant to grapegrowers because it relates to confidence and transparency in the realisation of maximum economic value for their grapes. For both growers and winemakers, objective chemical measures can provide specifications that allow the most value to be achieved from grapes. Developing an understanding of the synergistic relationships of available objective measures to well-established subjective systems has the potential to significantly reduce production costs and increase value by ensuring that fruit is used in the most efficient production stream and that maximum value is returned from the end product. It also may lead to significant savings in the costs of monitoring crops through more effective application of resources and clearer understandings of geographical and climatic drivers.
The second (2015) phase of the work could additionally afford winemakers the opportunity to stream fruit based on objective chemical measures offer to maximise its value and minimise risk related to loss of value, for example by avoiding the batching of higher value fruit with lower value fruit, and allow them to choose fruit of the value that relates best to a desired wine style. Furthermore, the management of vineyards to supply grapes of defined composition fit for a particular style of wine is a process that involves a wide range of inputs with varying impacts on the environment, for example, chemicals for spraying, fuel for equipment, water and fertiliser. By better understanding the levels of chemical compounds in grapes required for each wine style, the resource inputs can be tailored and minimised, thus minimising the impact on the environment.
The maintenance of strong relationships along the value chain between key participants such as grapegrowers and winemakers is central to a sustainable Australian wine sector. Objective measures of quality may contribute significantly in some sections of the wine community to ensuring transparency, trust and the maximisation of value by providing an objective framework within which all parties understand what is expected to achieve the highest value and most effective use of available resources. Improving the profitability of vineyard enterprises and wine companies in rural Australia also provides social benefits for the regions.