What is the sour or herbaceous period?

The intricate dance of nature and science in the vineyard is beautifully exemplified during the sour or herbaceous period, a critical phase in the life cycle of grapevines. This period, often overlooked in casual discussions of viticulture, plays a pivotal role in the development of the grapes that eventually yield some of the world's finest wines.

The sour period, also known as the herbaceous period, is initiated post-fertilization, marking the beginning of the transformation of flowers into future grape clusters. This metamorphosis starts when temperatures consistently stay above 15°C, usually at the cusp of summer following spring. During this time, the phenomenon of flowering, or "cierna," occurs, where flowers evolve into grape clusters. Temperature plays a crucial role; temperatures below 15°C or above 35°C can delay fertilization in vines, leading to subsequent uneven maturation.

Most grapevines are hermaphroditic, self-pollinating from one flower to another through anemophily. Pollen grains fall onto the sticky surface of the stigma, germinate due to the sugary liquid it contains, and travel down the pistil's neck to the ovarian cavity, where they fertilize the ovules.

The fertilization process itself is brief, lasting about 2 to 3 days. However, for a complete vineyard, this might take a week or more. It's during this stage that the sour or herbaceous period occurs. Lasting between 45 to 65 days, this period's duration varies based on the grape variety and environmental conditions.

During the sour period, the grape berries increase in size due to the cellular multiplication of the ovary's tissues, yet maintaining the same number of cell layers. This growth occurs tangentially between the cells of each layer.

In the first phase, the pericarp, or the outer layer of the grape, develops significantly, while the embryos show almost no evolution. Conversely, in the second phase, the ovules develop rapidly, and the growth of the grape berry slows.

Throughout this period, the grape berries act as another green organ of the plant, performing photosynthesis thanks to the chlorophyll in the epicarp, and respiration through 25 to 40 stomata distributed on each berry's surface. Until the next phase of cluster development, veraison, all cells in the skin and pulp contain chloroplasts, although these are distributed differently within the berry. They are more abundant in the periphery of the skin, while absent in the deeper cells of the pulp. Chloroplasts are also present in the superficial tissues of the seeds.

Interestingly, berries richest in chloroplasts are those most exposed to sunlight and located at the top of the cluster, having a thicker skin with more cell layers.

At the end of the sour period, just before veraison, the grape contains about 20 grams of sugars per kilogram of pulp and nearly the same amount of acidity. Contrary to what might be assumed, during this stage, the berries have high metabolic activity, characterized by elevated respiratory intensity and rapid accumulation of acids.

The sour period, thus, is a time of significant transformation and growth in grapevines, setting the stage for the subsequent stages of grape development and ultimately influencing the quality and character of the wine produced.