Stuck Ferments
Every vintage and every region has its own particular issues. So there is always something to keep us interested in the lab. Often the issues are minor or sometimes they are region-wide and a bit of a pain. One problem that appears to some degree, in some place, every year, is the issue of stuck and sluggish ferments.
This year we had one region with a large number of stuck whites; other regions had problems with reds. The reasons for sluggish ferments are now well known and are typically to do with unkind or cruel treatment of the yeast by winemakers: poor nutrition, temperature swings or other bugs getting into the ferment and creating competition.
We supply some products that usually sort the problem quite rapidly. Yeast hulls (such as Extraferm) can and do really help; a complex nutrient (Maxaferm) and a strong rescue yeast (like the very popular Fermivin Champion) can be indispensable. Most winemakers nowadays are quick to recognise the slow progress of a ferment as an issue and do also recognise the value of a couple of good products and the right protocol, in resolving the situation.
There has been a slow but steady increase in the number of YAN tests we do throughout vintage. This is a very good test to determine the nutrient status of the must and as a useful guide to adding yeast nutrient supplements. Ensuring adequate nutrition is a good step to eliminating or reducing sluggish or stuck ferments.
No export due to incorrect rounding of alcohol content for labels – again!
Just prior to vintage a lot of wineries are finishing off wines and getting them into bottle. One critical and very important analytical parameter to get right is alcohol. This is usually the only test result that is on the label and due to regulations both here and overseas it is vital that the result is accurate and is used in the correct format.
One quite technical but important aspect of this result is that, for those wineries exporting to Europe, it must be represented to the nearest 0.5 or whole number. And the alcohol statement must also be within a range of 0.8% volume/volume (v/v) of what is stated on a VI1 Export Certificate of Analysis.
So as an example of rounding, if a test result is 12.7% volume/volume (v/v), this is best rounded to 12.5 % v/v, which is to the nearest 0.5% or 1%. It could also be rounded to 13.0%. But the other option of rounding to 12.0% is not without risk.
We recently had an unhappy client who got a result from us of 13.2% prior to bottling and rounded it to 12.5% for the label. When we were asked to complete a VI1 Certificate of Analysis (COA) by the customer many months later on the bottled wine, the result was 13.4%. Consequently, the wine was rejected for export as it was outside the specified tolerance. That is, the difference between the label and the VI1 COA was 0.9%.
There was a difference between the two test results from the lab of 0.2%. The tests were done some months apart, one on the tank sample, and one on the bottled wine. We were questioned on this difference and there are a couple of factors to consider. First the repeatability of the test method is 0.2%. That is, the statistical variation between two tests on the same sample by the same lab and same operator at the same time is 0.2%. This is well recognised internationally and was within our NATA accreditation criteria. The second factor to consider is that the wine may have had a slight variation in composition from tank to bottle.
So in this instance, to not round a figure to the nearest 0.5% increment has caused grief for the winery because they cannot export this wine to Europe with these labels. They rounded by 0.7%, when it would have been much safer to round the number by 0.2%. The key issue to keep in mind is the tolerance that has to be abided by, with Europe having the tolerance of 0.8%.
We have seen this numerous times over our twenty plus years of doing this same work. Hopefully this article will remind exporters of this tricky little issue.
A Case study
What is the cause of a cloudy white wine? Well, there are a few possibilities and it can take a bit of detective work to figure out the correct one. A 2017 bottled but hazy white wine was recently submitted for such a problem. There is a sequence of tests that are done initially to try to determine the underlying cause.
Samples of the wine were taken aseptically and centrifuged to obtain a deposit from the wine. This deposit was then subjected to microscopic analysis. There were no obvious microbial or crystalline materials that were the likely cause.
The deposit was then dissolved and analysis by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES, or just commonly called ICP) was performed. This is a widely used technique to determine trace metals in various matrices; in wine it is typically used to determine whether copper or iron are present above certain levels. In this case the deposit had a high copper level, indicating that copper casse may be the cause of the problem. Next we did a heat stability test to see what the protein levels in the wine were. Unfortunately the protein stability test also failed.
It was thus quite obvious that the haze that was in this wine was an example of copper casse. This is a well-known but seldom-seen problem in wine, in our experience. Nowadays it is easy to measure copper and most winemakers are aware of the problems that levels above 0.5 parts per million (ppm) can cause. This client now understands it even better than before.
The main sources of copper in wine come from vineyard sprays and from the copper sulfate used in fining to remove sulfides. Very careful additions are needed to ensure that a copper casse doesn’t occur – and they rarely do.
