In this blog we discuss why berry sugar monitoring is important beyond sugar concentration.
Changes in sugar accumulation reflect other chemical changes
As berries accumulate sugars, the vine reaches the ultimate stages of its season cycle. Once a maximal amount of sugar per berry has been accumulated, complex chemical changes occurs in the berry. Changes involve variations in aromas profile, cell wall degradation, modification of tannins structure, etc…The “volatility” of these changes makes the timing of harvest window critical. A later harvest date may emphasize the aromatic expression of dried fig, prune aromas. An earlier harvest date may happen before the maximum amount of color has been reached.
Sugar time profile variations reflect season influence on the timing berry chemical changes
The amount of sugars accumulated per berry (which is different from berry “sugar concentration expressed in Brix or grams per liter) describes how fruit internal clock is ticking during ripening. From a winemaker point of view, an “early season” means the ultimate phase of the seasonal cycle is reached early according to the civil calendar and compared to historical trends. Whether the season is “early” or “late” it typically reflects the effects of environmental parameters on ripening dynamics. Interestingly, extreme conditions have similar effects on ripening profile: they tend to slow down sugar accumulation per berry. Very high temperatures and very high water deficit can reduce photosynthesis and consequently the sugars flow to the berry. Very low temperatures and very high water supply can also slow down sugar accumulation.
For those reasons, the start, duration, end of the processes of berry sugar loading directly reveal how season and practices affect maturation dynamics. As such, beyond indicating how the season shapes the maturation profiles, the timing of berry sugar accumulation also reflects deeper changes about other berry metabolic changes. What are those?
Sugar is a signal for color accumulation
Sugars induces anthocyanins accumulation as reported by a team of chinese researchers led by Zheng in 2009. In practical terms, sugars accumulation on a per berry basis can be seen as a signal for other biochemical changes affecting fruit enological composition (i.e. color, aromas).
The next question is: How do environmental parameter affect the relationship between sugars and color? To address this question, a team of australian researchers led by Sadras investigated the relationship between sugars and anthocyanins in Cabernet Sauvignon in a warm environment. Different treatments modulating irrigation and cluster thinning were applied (Figure 1).
Figure 1: relationship between amount of sugars and amount of anthocyanins during linear phase of accumulation (5 treatments, 3 seasons) – from Sadras et al, 2007
Figure 1 shows a linear relationship between color and sugars (when expressed in a log-log scale). In practical terms this means that the relationship between anthocyanins and sugars per berry is such that anthocyanin accumulation happens at a much greater rate than sugars. Furthermore authors showed that there was an effect of water regime on the relationship. Water deficit substantially increased the relative rate of anthocyanin accumulation in relation to sugars. In conclusion, the time variations in sugar accumulation and water deficit will directly impact color accumulation.
Practical Take home from this study
- The rate of berry sugar accumulation can be used as a proxy for berry color accumulation. Color accumulation starts after sugar accumulation starts. Color accumulation rate (in mg per berry) will be faster than sugar accumulation rate (in g per berry).
- Restricted water supply increases the rate of color accumulation in relation to sugars accumulation
At this point, we understand how sugars and color accumulation rates are related; but why would that be important for a winemaker? In a next blog we will discuss the relationship between fruit color accumulation and wine composition