The Australian Wine Research Institute

Grape and colour analysis

* Response time may vary depending on sample numbers submitted and the current workload in the laboratory, and refers to days after receipt of samples.

Colour of red grapes

The red grape berry colour assay uses spectrophotometry to provide an independent measure of red grape colour for comparison to the measurements made by near infrared spectroscopy (NIRS). The analysis measures the concentration of extractable anthocyanins which are the pigments found in red grapes. The assay can also determine the total phenolic compounds in the grapes. The total extractable anthocyanin value has been shown to correlate well with the potential wine quality and is the measure used to calibrate NIRS instruments currently in use and check for any instrumental bias.

What can I use this colour assay for?

You will be able to use the assay to:

• observe differences among vineyards at harvest;
• observe differences among grapes harvested from the same location in different vintages
• compare with the colour measurements as determined by NIRS

Which samples are best?

Berry samples are likely to provide the most useful colour result for assessment of the ripeness of the fruit and can be used in conjunction with brix or baumé and pH to decide when to pick the fruit. Results will be expressed in mg/g. Commercial Services can analyse pH, titratable acidity, brix and tannins on your grape samples.

MCP tannin assay – red grapes

The AWRI has developed a new method allowing tannin quantification in grape and juice samples. The method, called the MCP (Methyl Cellulose Precipitable) tannin assay, is based on precipitation of tannin with methyl cellulose and a UV–Vis measurement at Abs 280 nm (for total phenolics). The MCP tannin assay is a robust selective method to quantify the pool of tannins and provide the winemaker with information which can assist in improved understanding and management of the tannins in the process from berry to bottle.

In general terms, tannins refer to polyphenolic compounds which can be divided into two classes; hydrolysable tannins (oak derived) and condensed tannins (primarily grape derived). In most wines the grape derived tannins originating from skin and seeds will contribute the majority (> 90%) of the total pool of tannins. Studies using the MCP tannin assay at the AWRI showed the following ranges of tannin concentration (expressed as g/L epicatechin eq.) within red grape varieties: Cabernet Sauvignon (0.83 –2.47 g/L) , Shiraz (0.59–2.3 g/L) and Merlot (0.84–2.0 g/L) (Smith, P. personal communication).

Which samples are best?

The MCP assay can be performed on fresh or frozen red grape berry samples (min. 200 berries) or homogenates. Tannin results are expressed as g/L epicatechin equivalents.
Note: Tannins are polymers composed of monomer subunits such as catechin and epicathechin.

Grape maturity

Commercial Services can also determine pH, titratable acidity and brix on your grape or juice samples.

Titratable acidity and pH provide measures of acidity of the grapes and their hydrogen ion concentration and are measured on free run juice from fresh grapes. They cannot be measured from frozen samples due to the insolubility of tartrates on freezing.

Brix is the measure of the sugar or soluble solids concentration of the grapes and can be measured on juice. Brix can be easily converted to baumé by dividing the brix result by 1.8.

How can I take a 'representative' grape berry sample?

It is extremely difficult to take a single representative sample from a truck or crusher pit. If this is your only course of action we suggest that you take more than one sample and average your results.

There are several prescribed methods to take berry samples from a vineyard, but the preferred method for this assay is to take a large random sample of berries from all over the vineyard (approximately 1,000 berries), mix them all together in a tray and take a sub-sample of approximately 200 berries and submit in a container. Submitting multiple samples from different vineyards is recommended due to the natural variability between vineyards. Correlating your results with your own observations and from vintage to vintage will help to develop this useful tool for assessment of grape quality.

If you are comparing results with samples analysed by a winery using NIRS or spectrophotometry it is recommended that you arrange for your sample to be taken at the winery when the winery takes its sample from a truck or bin. Samples taken in different ways or at different times will most likely be different, meaning comparison of the results will not provide a means of confirming the validity of either result.

How do I send my samples in?

Berries:
Send in approximately 200 berries in a container. It is preferable to freeze the berries prior to dispatch and to pack them so that they will remain frozen during the journey. Freezing is not suitable if pH or titratable acidity is required. Do not add powdered sodium metabisulfite to the grapes as it will cause bleaching, resulting in the colour measurement being erroneous (low).

Juice:
Juice must be submitted in plastic containers. You will need to freeze the juice prior to dispatch and it is advisable to add a protective amount of SO₂ (~ 200 mg/L) to prevent fermentation. 50 mL of juice is sufficient. Freezing is not suitable if pH or titratable acidity is required.

How do I send my grape sample to you without breaching the quarantine regulations?

Quarantine requirements only apply to samples sent from outside of South Australia. A brief explanation of the quarantine requirements can be found by contacting Randell Taylor, Trace Analysis Laboratory Manager. Further information can be found in the December 2010 edition of the Commercial Services Investigations Bulletin.

For more information, contact Customer Service at The Australian Wine Research Institute. Email Commercial Services or telephone 08 8313 6600, facsimile 08 8313 6621.