Let's take a brief look back at the grape berry and its maturation to better understand where the constituents of wine come from. Several components of wine are produced by the vine itself and therefore will be greatly influenced by terroir, grape variety and climatic conditions. This is particularly the case for aromatic molecules, polyphenols, metals and sugars. The type of vinification will also influence greatly wine composition. Maceration of skins in presence of juice will promote the extraction of tannins and anthocyanins. In this article, we will focus on the oenological maturity grapes. This includes technological maturity, phenolic maturity and aromatic maturity. This article will review grape morphology, its components and its maturity indices.
Berries have an envelope named skin. The pulp and seeds are found inside the skin. Berry support is provided by the stalk which contains many tannins responsible for astringency which are also found in seeds. Grape nutrition is provided by peripheral and central vascular networks that flow across the pedicel.
Figure 1. Grape morphology
"Grape berry diagram en" by Original: LadyofHatsDerived: Peter coxhead is licensed under CC BY-SA 4.0.
Following pollination or flower fecundation, berries will grow rapidly due to growth hormones secreted by the vine. At this stage, cellular activity will increase dramatically in the berry and will be dependant on available levels of nitrogen as well as water accessibility. It is the herbaceous growth stage where significant amounts of tartaric and malic acids are accumulated within the pulp and tannins increase in seeds. In the next phase, “veraison“, berry growth is significantly reduced. However, many changes are induced by abcisic acid at this stage. Sugars start to accumulate in the pulp and benzoic acids, cinnamic acids, flavonols, tannins and anthocyans (red berries) cause variations in skin colour. After veraison comes berry maturation where a second growth spur occurs leading to sugar and potassium increase in the berry.
Sugar Ripening or "Technological Maturity"
During maturation, sucrose produced by photosynthesis in leaves is transported actively to the pulp against osmotic pressure. Through the action of several enzymes, the berry will accumulate glucose and fructose in vacuoles present within the pulp. The ripening speed depends on overall climatic conditions and the level of acidity in the berry. This stage can be defined as “technological maturity” because the amount of sugars that has accumulated in the berry can be measured by refractometry.
Tartaric and malic acids are produced in leaves as well as in berries. These acids are major components in grapes and levels vary greatly from one type of grape to the next. During the herbaceous stage, total acidity increases as tartaric and malic acids levels increase. During maturation, tartaric acid levels will remain relatively constant while malic acid is used as a substrate to provide energy in the pulp. Therefore, the level of malic acids will decrease rapidly leading to lower levels of total acidity. Water will start to accumulate in the berry causing an increase in the pH value and subsequent dilution of tartaric acids.
Of all the minerals present in the vine, potassium will undergo the most significant changes. While levels of calcium, magnesium, copper and zinc will slighly decrease during maturation, potassium will accumulate in high levels in the pulp mirorring changes observed with glucose and fructose. Potassium is directly involved in sugar increase in the berries. Low levels of potassium will impact negatively the accumulation of sugars while to high levels of potassium will cause berries to mature quickly which can lead to loss of aroma.
Nitrogen compounds have a fundamental role in yeast fermentation of sugars. Ammonia (NH4+) and amino nitrogen levels increase in berries during maturation. The total sum of ammonia and amino nitrogen refers to Total Nitrogen. Before veraison, nitrogen as ammonia accounts for 80% of total nitrogen. Ammonia levels will decrease during maturation down to 5 or 10% of total nitrogen levels.
Other than increasing levels of sugars in berries, maturation also involves the accumulation of phenolics and aromatic compounds. A large subset of molecules can be referred to as polyphenols. They are classified in two groups: flavonoids and non-flavonoids. Flavonoids refer to molecules such as anthocyans, flavanols, flavonols and condensed or hydrolysed tannins. Non-flavonoids compounds include phenolic acids (hydrobenzoic and hydrocinnamic acids) and stilbens. These compounds contribute to organoleptic properties of wine such as flavor, astringency, bitterness, taste and appearance. They also influence its stability and oxydation resistance. Polyphenols contribute significantly to wine quality. Physiological maturity is the point where phenolic compounds such as anthocyans are at their highest levels in the berry and tannins become less astringent. Physiological maturity is not necessarily correlated to technological maturity. Under very warm climate, high sugar levels occur before phenolic and aromatic maturity. Aromatic compounds such as terpenes, thiols and aroma precursors contribute to grape typicity. These molecules can be found in free form in berries or linked to other compounds like sugars. Because of the diversity of polyphenols, a dedicated article will follow shortly.
Why Maturity Monitoring is Important
Grape ripeness is a complex process. In a context of climate change, producers must deal with variable environmental conditions that have a considerable influence on oenological maturity. Sugar ripeness, physiological and aromatic maturity are not necessarily synchronous. Maturity control analyzes make it possible to establish a starting point for the development of the wine and allows the winemaker develop the best winemaking strategy. Partnership with an oenologist and a wine testing laboratory will help determine the optimal oenological maturity.
Understanding the way grape compounds change during maturation will help the winemaker produce quality wines with complex flavors. Climate change introduces challenges in managing oenological maturity. Must analysis is an efficient tool to address these challenges and determine the optimal time for harvest.