Precision at Every Stage: Analysis in Winemaking & Bottling

Dr Carly Gamble – Technical Manager/WA State manager
Dr Alana Seabrook – Operations manager
Dr Victoria Hughes – R&D Manager/VIC State manager

There are a number of factors that can influence the overall winemaking process.  These factors can stem from the vineyard, the grapes, juice chemistry and the particular wine processing methods being undertaken.  Understanding the type of analysis that is available can assist at each stage of this process.  Having access to accurate and reliable data can be vital to guide decision making and help protect the quality of the final product.  This technical article will discuss the critical points in the winemaking process, rather than growing process, including chemical analysis at harvest, fermentation and bottling that promote and protect wine quality.  Virus and plant testing other than petiole analysis will not be discussed.

Grapevine Flowering

Petiole analysis is often overlooked but can provide critical information regarding vine nutrition pre-harvest.  Like all plants, vines need certain nutrients to ensure good growth and to produce healthy fruit. To make good wine from the grapes, the range and quantities of nutrients must be optimum, including the correct amount of all essential nutrients. (Note: in this article the term ‘nutrients’ will be used in place of the less accurate but commonly used term ‘minerals’). Nutrients for vine health are primarily extracted from the soil in which the vine grows. Healthy soil, which contains good levels of nutrients, is necessary for healthy plant growth. Petiole testing identifies deficiencies in the amounts of nutrients; these deficiencies can be ameliorated using supplementary nutrient application by soil fertilizers and/or foliar sprays.

The optimal time to consider petiole analysis is at the full flowering stage (80% cap-fall).  Petiole analysis can provide a snapshot of the nutrient status of the vine, as results obtained can be compared to published standards (Table 1) which relate to vegetative and reproductive performance.  Obtaining quantitative information on nutrient levels can assist in determining the effectiveness of fertiliser applications and also help identify the cause of any specific vine problems.

The main macro nutrients are: N, K, Ca, Mg, P; the main micro nutrients include Cl, B, Fe, Mn, Zn, Cu. This range of nutrients, along with nitrate and sodium, are used by most grape growers as there is an accepted standard of levels for them, as shown below in Table 1. 

Table 1. Petiole Nutrient Guidelines (Adapted from Iland et al., 2011).

Berry Ripening

Maturity analysis
As harvest time approaches, it is important to monitor the ripening and quality of berries by tracking parameters such as soluble solids, pH and titratable acidity (TA) to help guide picking decisions.  Baume – a measure of density - is commonly measured in Australia and provides an indication of the sugar content of the berries and the potential alcohol content of the wine.  pH and TA analysis provides the hydrogen ion concentration (pH) and the acidity of the grapes which is vital information in regards to berry ripening and harvest.  This information provides a baseline to make picking decisions and subsequent winemaking additions.  During berry ripening, it is also possible to perform basic analysis in the field, refractometers and hydrometers​​ are straightforward to use to monitor the sugar composition of the grapes.  

Colour analysis

Analysis of berry colour and tannins can also provide useful information at this stage.  Colour analysis can indicate the development of anthocyanins during ripening and provide information on grape quality.  Tannin measurement can allow the management of tannins from the berry stage to the final product.

Smoke taint

​In the unfortunate event of bushfires, smoke taint analysis can assist in determining whether berries have been impacted, and if so, by what extent.  These results can allow the implementation of management techniques to reduce the impact of smoke taint aromas.  In extreme cases, results can also be fundamental in deciding whether to harvest at all, potentially saving the costly exercise of harvesting grapes that will produce a tainted and commercially unviable product.  There are a several key cited methods of testing for smoke taint in grapes and wine which will provide very different results, it is important to work consistently with your local laboratory (Favell et al 2022).

Botrytis cinerea

Botrytis cinerea is a mould which can develop on grape bunches in the vineyard. The growth of Botrytis cinerea on grapes can lead to major issues during the production of wine. Laccase is an oxidative enzyme that is produced when Botrytis affects grapes. Its presence in both red and white musts and wine can result in rapid browning and loss of aroma.  Laccase analysis can be used to assist in treatment and processing decisions of particular batches.  An alternative indicator of the presence of Botrytis cinerea is gluconic acid which can also be measured analytically (Zoecklein et al., 1995).  Small, portable analysis units like Absorbance ONE and Vintessential test kits also make it possible to measure analytes such as gluconic acid in the vineyard with minimal capital outlay.

Table 2. Pre-harvest Analysis Options (excludes soil testing)