The Concept of Berry Sugar Loading

We heard from a few of our friends who farm grapes that in this past year, 2016, they saw their sugar accumulation plateau at a low sugar level when it neared harvest time. I was rereading some of my previous blogposts in an effort to do a better job of categorizing all the previous posts and came across a blog that I had written in September 2013 called: Predicting Harvest Date.
In that post, I wrote about the concept of berry sugar loading which occurs after véraison, the onset of fruit maturation.
Berry sugar loading leads to berry softening as sucrose is being transported to the berry and hydrolyzed into it's components, glucose and fructose. This allows glucose and fructose to be accumulated in the berry, creating a concentration gradient.
According to the article written by Alain Deloire of Charles Sturt University, Bathurst, it is possible to distinguish three principal sugar loading profiles:
     1. Continual and rapid loading is considered to be beneficial for the production of rosé, fresh fruit red wines, or pleasant aromatic white wines.
     2. Slow sugar loading – inhibition of ripening is considered to be a form of “blocked” ripening and this could be indicative of major physiological problems, such as mineral deficiencies, viral diseases etc., as well as excessive water deficit or to an excessive crop load in relation to the exposed leaf surface.
     3. Sugar loading presenting a plateau phase vines showing this tendency present a phase of active sugar loading in the berry (ripening), followed by a plateau representing a cessation of sugar loading corresponding to maturity.
The concept can be visualized in the following figure.

The take home message for me from reading this article is that a continual and rapid loading of sugar to achieve the desired level of Brix can be disrupted by disease states which include excessive water deficit, which some farmers in our area experienced last harvest.
Reference:
1. Alain Deloire, The concept of berry sugar loading, January 6, 2016.