Eric Stafne, Mississippi State University and Tim Martinson, Cornell University
A grape cluster is a complex flower, or inflorescence, that consists of a peduncle, cap stems (also called pedicels), a rachis, and berries that arise from individual flowers in the flower cluster. Each individual berry is made up of skin, flesh (or pulp), and seeds. On the outside of the skin is the bloom, or waxy layer, that helps to prevent water loss.
As the berry forms, necessary nutrients are supplied from the vascular system, namely the phloem and xylem. The xylem transports water, mineral nutrients, growth regulators, sugars, and other nutrients from the root system. The xylem plays an important part in berry development, primarily in the early stages up until veraison. The phloem is responsible for carbohydrate transport from the leaf canopy to the vine, and subsequently, to the berry. It has little function early on when the xylem is doing the primary work, but the phloem becomes the primary source of nutrition for the berry after the start of veraison. Berry size increases with an increase in sugar content after veraison, and this varies by cultivar.
Three stages of grape berry development have been identified (Keller, 2010; Winkler et al., 1974). During stage I, starting at fruit set, berries grow through cell division. Stage II, called the Lag Phase, starts with a pause in berry growth while seed embryos start to form and grow. Cell division stops, and further growth is through enlargement of cells. Stage III starts at veraison, when berries change color, soften, accumulate sugars, and metabolize acids. Up until veraison, berries represent the vine’s mechanism for dispersing mature seeds, through consumption by birds or other vertebrates. Acids and tannins that accumulate before veraison ensure that they remain unpalatable. After veraison, changes occur (color change, softening, sugar accumulation, and acid reduction), which increase attractiveness and palatability to avian and vertebrate animals that consume and disperse grape seeds.
The first phase is related to berry formation. This phase starts at bloom and goes for approximately 60 days. During this time, the berry is formed and rapid cell division occurs. The berry expands in volume and accumulates solutes such as tartaric and malic acids, but little sugar. Tartaric acid has the highest accumulation in the skin. It accumulates during the initial stages of berry development and provides acidity for winemaking, thus making it a critical component. Malic acid has the highest content in the flesh and is also important in the final wine making process. Other important acids also are in the berry at this time including hydroxycinnamic acid, which is in the flesh and skin. It is involved in browning reactions and is a precursor to volatile phenols, such as tannins. Tannins accumulate during the first growth phase of the berry and are present in the skin and seeds. They are responsible for bitterness and astringency, making it important especially for red wine quality characteristics including color, stability, and mouthfeel.
The lag phase is distinguished by a pause in berry growth, during which seed embryos start to grow rapidly. At the start of the lag phase, berries have reached at least half of their final size. Following the 5 to 10 day lag, cells expand and continue to accumulate acids and tannins, which reach their maximum levels at veraison. During this phase, seeds reach their final size by 10 to 15 days before veraison.
The third stage starts with veraison and includes the softening and coloring of the berry, accumulation of soluble solids (sugars), and reduction in acids. During this phase, the berry doubles in size and several changes occur. The malic acid content is reduced, although this is strongly correlated with climate. Warm region grapes typically have less malic acid, whereas cooler regions produce grapes with higher levels of malic acid. Seed tannins also decline during the second growth phase as a result of oxidation where they become fixed to the seed coat. Some aroma compounds also decline, but this can depend on berry exposure to sunlight. Some of the significant changes occurring after veraison are an increase in compounds like glucose and fructose from sucrose. Concentrations of these sugars are dependent on different factors such as hang time, crop load, canopy size, disease, and water. Secondary metabolites are also important and are major determinants of wine quality. In red grapes, anthocyanins are a secondary metabolite. In white grapes, volatile flavor compounds like terpenoids are secondary metabolites of importance.
The grape berry is an independent biochemical factory that contains water, sugar, amino acids, minerals, and micronutrients (Kennedy, 2002). The synthesis of other components like flavor and aroma compounds occurs in the berry as well. There can be a great deal of variability between berries within a cluster, between clusters within a vine, between vines within a cultivar (clone), and between vines within a vineyard. The determination of berry ripeness can be difficult due to this variability. The main objective of the grower is to achieve a uniformly ripe crop with great flavor and aroma compounds.
The berry itself has three main tissues that comprise it: flesh, skin, and seed. Each tissue contributes differently to overall wine composition. This composition can be manipulated by changing berry size, but other factors such as the number of seeds, environment, and nutrition can also affect final wine quality. However, every one of these components is tied initially to successful flower fertilization. For the grape vine, reproduction is the number one purpose of its existence. Humans manipulate this process for their benefit.
Keller, M. 2010. The Science of Grapevines: Anatomy and Physiology. Academic Press, Burlington, Massachusetts.
Kennedy, J. 2002. Understanding grape berry development. Practical Winery and Vineyard. July/August.
Winkler, A.J., J.A. Cook, W.M. Kliewer, and L.A. Lieder. 1974. General Viticulture. Univ. of California Press. Berkeley, California.
Cluster and Berry Variability during Ripening video, Oregon State University
Reviewed by Patty Skinkis, Oregon State University
and Tim Martinson, Cornell University