Berry development starts after a successful pollination and fertilization of the flowers at the flowering stage of the grapevine. When the berry grows, it is firm and fragile, and after approximately 4-6 weeks, it reaches its final size. The size of the fruit is strongly influenced by the seed development. Fruit contains from 1-4 seeds, while a smaller number of developing seeds generally result in smaller berry size.

Three general stages of berry growth have been determined: initial rapid growth, followed by a lag stage or slow growth, and finishing with another rapid growth and maturation of berries.

Berry growth stages

Stage I: Berry formation to lag phase (lasts approximately 40-60 days)
The first phase is related to berry formation, and starts immediately after the bloom. This is a phase of rapid berry growth, which lasts for approximately 60 days. Berry size increases due to cell division and cell enlargement. In this stage, berries are hard, and colored green. At that time, sugar content of the berry is low, while organic acids, such as tartaric and malic acids, accumulate. While tartaric acid has the highest accumulation in the skin, malic acid has the highest concentration in the flesh. Tannins also accumulate during the first growth phase, and are present in the skin and the seeds.

Stage II: A lag phase (lasts for approximately 7-40 days)
The second stage of berry growth stage is called the “lag phase”. In this period, berry growth slows down, or completely pauses. At the same time, seed embryos grow rapidly and few days before veraison seeds reach their final size. The lag phase lasts for approximately 1 to 4 weeks, depending on the location, climatic conditions, and grapevine variety. Berries continue to accumulate acids and tannins at this phase, and reach their highest level at the start of veraison. However, the boundary between the second and the third stage is often unclear.

Stage III: Veraison to berry maturation (lasts for approximately 35-55 days)
The third stage of berry growth starts with veraison (berry ripening process). Berries start to soften and color, in red varieties the pigments begin to accumulate in the skin. During this stage, sugars (soluble solids) start to accumulate, as well as varietal aroma and flavors starts to accumulate. At the same time, the concentration of organic acids declines. Sugar flows to the berry as sucrose, and there it is quickly converted into glucose and fructose. Most of the sugars and acids accumulate in the flesh (pulp) of the berry, only a small amount of sugar accumulates in the skin. During this period, the rapid growth of berries occurs due to cell enlargement, and lasts for approximately 6 to 8 weeks.

Grape berry growth

Photo (Practical Winery&Vineyard Journal): Grape berry growth

There are two nutrients necessary for berry development: the phloem, and xylem, which are supplied from the vascular system to the berry through the berry stem. At the berry formation stage (from the early stage until veraison) xylem represents the primary source of nutrients for the berry. It’s responsible for transporting water, minerals, growth regulators, and nutrients from the root system. While in the berry ripening stage (after the veraison) phloem becomes the primary source of nutrition for the berry development. At that stage, rapid sugar accumulation occurs in the berries, and phloem is the one responsible for carbohydrate transport (sucrose) from the leaf canopy to the vine, and thus to the berries.

Technological maturity of grapes

The technological maturity of the grapes refers to the moment when berries achieve the desired concentration of compounds i.e. reach optimal “sugar/acid” ratio for the production of individual wine style. Ripeness factors that normally determine the harvest time are sugar and acid content, pH, color, and flavor. We have already written about how to know when the grapes are ripe for harvest here. Depending on the variety and areas of growth, the concentrations of the compounds that determine the ripe berries, differs. Therefore, grapes are harvested at different times, neither premature, nor too late, especially in varieties that have a low acid content. Harvest date also differs on the intended purpose of the harvest. For example, Pinot noir grapes for sparkling wine production are harvested earlier (lower sugar and higher acid content), than Pinot noir grapes for non-sparkling wine.

Ripening is a continuous process and is strongly influenced by the weather, therefore, there is only a short period of time during which the berries remain within the desired ripeness parameters and need to be harvested. If the harvest is delayed, and berries developed beyond the desired range of ripeness, they become overripe.

Another factor to consider when scheduling the harvest time is rotting. Namely, berries of grape varieties with gentle skin tend to rot. Strong fertilization with nitrogen, and wet autumn, can accelerate rotting. Rotting in red varieties is more dangerous than in white, because the fungus can destroy the red color of the berry.

 

Sources:
Kennedy, James A.2002. Understanding grape berry development, Practical Winery and Vineyard, July/August 2002. [online]
Nick K. Dokoozlian. Grape Berry Growth and Development. ucdavis.edu
Edward W. Hellman. Grapevine Structure and Function. GENCO Winemakers
Eric Stafne. 2012. Stages of Grape Berry Development. eXtension.org [online]
Mary Retallack. 2012. Grapevine Biology. Retallack Viticulture
Different manuals for winegrowing