A miscellaneous post to clarify some aspects coming out sometimes when I teach WSET Level 3. Especially the sessions about grape growing and winemaking lend themselves to questions falling out of the scope of the program, questions that even the instructor is not sure about. They are very good chances to advance your own knowledge, although they should be treated outside the context of the lesson, to avoid overwhelming the students with concepts that they do not need for the exam.
Fermentation stopping in high sugar musts
Page 58 of the Level 3 manual on sweet winemaking tells us that sweet wine can be made by concentrating in various ways sugar in the must. The last paragraph specifies that in this kind of environment fermentation can stop even at 7%, leaving unfermented sugar, because yeasts struggle to survive in very sugary environment.
But why is that? Isn’t it good for the yeasts to have lot of sugar? One would understand yeast stopping to ferment when the alcoholis level reaches 15% or above, leaving maybe unfermented sugar behind, but why would this happen even at pretty low levels of alcohol?
The answer is osmotic pressure: inside the solution yeasts (being living creatures) are mainly composed by water (around 65%). They also have a semi-permeable membrane, which they use to eat and to expel what they do not need. The physical fact is that if the water concentration of the liquid outside the yeasts gets low, which happens in very sugary must, the yeast loses water, for reasons of osmotic pressure (two solutions separated by a permeable layer will tend to balance, in this case water will move from the element containing higher concentration – the yeast – to the element containing lower concentration of water – the very sugary must -).
In simple words, in very sugary must the yeasts dehydrate and shrink and they cannot keep functioning properly until they die, like any dehydrated living being.
Thus, at right conditions, fermentation may stop even when the alcoholic level is low.
How are barrels managed when you are not storing wine inside?
Although you do not normally think about it, this is a very interesting question.
You can store barrels empty or full. If empty, after careful washing and draining, the caretaker will burn sulfur dioxide inside at regular intervals to avoid bacteria colonising the wood. A barrel stored this way will have the wood dry and shrink over time, so you will need to swell it again when you decide to use them or it will leak.
Alternatively, you can store the barrels by filling them with sulfur-citric solution (citric acid + potassium metabisulfite). The solution will keep them clean and humid, avoiding drying and shrinkage, but it may not be ideal for new barrels, which will leak they aromatic extract to the liquid.
Sprinklers for frost protection
When water sprinkled on the vine freezes, it actually protects the vine against frost. This technique is called “aspersion”.
It is a bit counterintuitive, so as an instructor in the context of a Level 3 lesson it may be better to not linger too much on this concept and just go forward, especially since the session on natural and human factors in the vineyard is already very intense. However usually someone will ask how this is possible.
The answer is “latent heat”: when water (in this case) changes its state from liquid to solid, it releases heat. (1) The ice on the bud will also act a protection against the much lower temperature “outside”, like an igloo. Grape growers will keep doing this until the temperature rises above freezing, at which point they will stop and leave the ice to melt naturally.
It is a system that needs lot of water to operate, you need to continuously sprinkle the vines for 5-6 hours for many days until the frost period ends, and it may waterlog the vineyard. Furthermore, wind must be taken into account if you want to effectively cover all of your plot (in places like Burgundy and Chablis, where aspersion is regularly employed, you may end up sprinkling the vines of your neighbour). “Practical” problems may also happen (pipes bursting, sprinklers clogging etc). As such it is usually employed only on the best vineyards, where the price of the resulting wine will make it worth it.
Sources used for this post:
The Effects of Osmotic Pressure and Ethanol on Yeast Viability and Morphology, Patricia L. Pratt, James H. Bryce, Graham G. Stewart
Barrel Care Techniques, Winemaker Magazine
Viticulture, Stephen Skelton MW
The wines of Chablis and the Grand Auxerrois, George Rosemary
(1) Conversely, when solid water (ice) absorbs sufficient heat, it will go back to liquid. That’s the reason why you need heat to melt ice.