If I say « Botrytis cinerea », does this word sound familiar to you ? Probably not, at least not if you aren’t a professional winemaker!

Botrytis cinerea is a fungus that plays an important role in winemaking. It is responsible for fruit rot in many fruit plants. Believe or not, it affects the wine grapes significantly and can give a very different ending to the wine you thought you were going to have. During specific climatic conditions, the grapes infected by this fungus develop something called “noble rot”, which is responsible for the production of some sweet dessert wines.

Botrytis cinerea observed in microscope

There are actually two different types of rot, B. cinerea can cause. The first one (mentionned before)  is called « noble rot ». Both temperature and humidity are the main factors influencing the development of this noble rot.

After the grapes have been infected, B. cinerea releases hydrolytic enzymes, which makes the grape’s cell wall more permeable. Under these conditions, the fruit starts to dehydrate. This loss in moisture and drying of the fruit help concentrate the sugar and the others constituents in the fruit. Finally, as the osmotic pressure increases in the berry, the activity of the fungus decreases.

Example of grapes having been infected with Botrytis cinerea

Example of grapes having been infected with Botrytis cinerea

The second rot that this fungus can cause is called « vulgar rot » or « sour bunch rot ». If the drying of the berry does not occur and if the humidity is not decreasing, the fungus continues to grow and can change the fruit’s properties. The berry starts to swell and will finally split apart. This makes the spoilage by other organisms, such as molds and acetic acid bacteria, much easier.

B. cinerea changes the fruit’s composition in different ways. The most important changes that happen when noble rot is present are the following :

As mentionned before, the sugar concentration increases, as the berry gets dehydrated. Apart from that, the fungus uses 35-45% of the sugar present in the fruit.

Moreover, the fungus metabolizes some of the organic acids, such tartaric and malic acids, making the amount of organic acid drop and the pH increase.

Furthermore, complex polysaccharides can form and lead to the inhibition of the alcoholic fermentation or to filtration problems later on.

Some aroma compounds such as terpenes are destroyed, but are then replaced by other characteristic B. cinerea aroma.

So, roble rot basically does two things : it helps to increase the sweetness level AND it gives other types of flavor to a wine. However, as the grapes dehydrate, more grapes are needed to make the same amount of juice. Moreover, it is to be noted that not all grapes get noble rot at the same time. Thus, the grapes have to be selected through the whole vineyard and at the right time. 

If you happen to taste such a sweet wine, you will certainly hear the winemaker or the sommelier say words such « honey » or « ginger » to describe these types of wine!