Introduction
There are a number of different tests that are done on juice and must at the time of crushing and pressing. These tests are explained below with some new developments in tests and equipment noted.
Testing during crushing and pressing
The main analyses performed during the process of crushing and pressing grapes are well established. There are typically (in alphabetical order):
- pH
- Sulfur dioxide, free and total (measured in units of mg/L)
- Titratable acidity, TA (measured in units of g/L of tartaric acid)
- Total soluble solids, TSS (units in Brix or Baume)
- Yeast Assimilable Nitrogen, YAN (measured in units of mg/L of nitrogen)
Other tests may be done depending on the season and areas of particular interest to the winemaker. Some examples of other tests include Botrytis cinerea analysis (this has been very common in some regions the past few years), colour and malic acid.
Although the above tests are quite straight forward there are several different ways of doing them and the equipment used can vary widely as well. This will depend upon:
- The size of the winery facility
- The budget available
- The amount of grapes treated per day
- The number of tests done per day
Equipment available
There are always new developments in wine testing equipment; a number of new developments will be touched on below.
TSS (Brix and Baume)
There are now a number of portable electronic instruments available for this test. They are quite reasonably priced and, being portable, the testing can be done at the crusher or press. This can be a blessing or a curse as these devices are quite delicate and the normal hectic operation around a crusher or press can prove to be a none-too-delicate place to be.
From our experience these devices come in either Brix or Baumé measurement mode, but not both. Again it will depend upon the winery which unit of measurement they prefer.
Refractometers are also used extensively. Being a portable instrument (in fact they typically will fit in a pocket) they are very handy devices as well.
The traditional hydrometer is still widely used with the -2 to 10° and 10 -20º Baume the most popular of those that we sell.
pH and TA
The ever reliable pH meter is a very popular and versatile instrument for measuring both pH and TA. The meters of all brands that we have used or tested are very robust, particularly when used as a bench top instrument in the laboratory. Some portable units can be used on-site at the crusher or press but great care needs to be taken as if they are dropped, they will be most likely damaged.
The main developments in this technology have been in electrode design. There are several now on the market that are well designed for use in the protein rich grape juice environment. There are several descriptions for these – double junction, annular ring intermediate junction, double bridge etc. The key design feature is that the juice/wine does not enter the inner section of the electrode.
The traditional general purpose design that uses a ceramic frit is not recommended by us for use in wine as some components, particularly protein, can get into the inner section and cause problems such as clogging up the frit and causing inaccuracies. Although some manufacturers claim that this type of electrode can be cleaned and made good this has not been our experience. The price tag of ~$200 – $300 for a good double junction type electrode should be seen as a normal consumable expense. In our busy labs we use the best and typically get 6 months usage per electrode. The message: don’t be afraid to replace the electrode at least annually if you find performance is waning.
Sulfur dioxide
The traditional Rankine apparatus is widely used throughout Australia. Although this is a wet chemistry and manual method we still recommend it as the best alternative. Once set up there are very few running costs except for the chemical consumables, these being quite inexpensive. This is not a portable instrument and samples should be taken to the winery lab for Sulfur Dioxide testing.
YAN
This has become a more popular test over the last 10 years and is quite easy to perform, but does require a spectrophotometer and the use of test kits. The price of a basic spectrophotometer is now less than $2000 – this price having come down considerably over the past decade. Many smaller wineries are now utilising this equipment and are able to get YAN results for themselves instead of sending out to a consulting lab.
Using YAN results helps considerably in making decisions about the level of yeast nutrient needed per batch of fruit. This ability in turn should assist in reducing the number of problem ferments experienced in a winery.
Botrytis testing
The last 3 vintages have seen an enormous amount of testing done for Botrytis infection. This has not been across all regions but certainly the Eastern States have had quite an issue with this fungus.
The jury still seems to be out as to the most effective way to evaluate the level of Botrytis infection in grapes. In Australia the tests that have been used included:
- vineyard estimates of % Botrytis infection
- the white plate method
- laccase testing
- ELISA testing
There does not appear to be good correlation between these techniques and research is ongoing as to the best way forward.
One interesting method that is being used in Spain and now in the USA is to monitor Gluconic acid by utilising test kits and a spectrophotometer. Botrytis is the only organism of any interest that produces Gluconic acid. In some Spanish wineries limits are set on the acceptable levels of Gluconic acid with payments being tied to the levels of this acid in fruit at the weighbridge. This is a very simple test (similar to measuring malic acid, done now in lots of wineries) and one that provides an accurate result in real units, not on an arbitrary scale as laccase testing does. This is not new technology, having been done for many years in French wineries as well. Trials will be performed in our labs this coming vintage using Gluconic acid as a marker for Botrytis infection.
Conclusion
There are some new developments in testing of juice and must at the crushing and pressing stage of wine production. One area where more work need to be done to provide sound and usable results is in the checking for Botrytis. Gluconic acid testing may well be a more useful technique than those currently employed.
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