Agriculture Reference
In-Depth Information
throughout development. Minerals act as catalysts in biochemical reactions and
they are also components of structural molecules such as proteins and nucleic acids.
Cell walls, for instance, contain calcium and potassium is an osmotically active
cation involved in sugar transport. Potassium has a strong influence on berry pH
because it can substitute for protons. High juice calcium and potassium levels can
result in undesirable precipitation of tartrate in wines while high manganese can
promote acetaldehyde production (Cacho et al.
1995
). The micronutrient, iron, can
alter the tint of a wine towards the blue spectrum while copper can prevent oxida-
tion of the phenolics.
Acid Depletion
Berry titratable acidity is dependent on variety and climate. Both tartrate and malate
accumulate early in berry development and decrease during ripening. Tartrate con-
centrations are higher near the skin while malate concentrations are higher towards
the interior of the berry. Other organic acids, such as citrate, are present in the berry
but these are in much lower concentrations. In warm to hot regions berry titratable
acidity is often less than desired and this is due to the rapid degradation of malic
acid (Kliewer
1971
). As a result, tartrate is often added in the winery to adjust the
pH of the juice and wine. Malic acid can undergo transformation to sugars under
conditions when berry metabolism is slow but the proportion relative to sugar im-
port is very small.
Nitrogenous Compounds
The amino acids serve as precursors of aroma molecules and are an important
source of nitrogen for yeast during the fermentation process. Nitrogen enters the
berry through the xylem and phloem mostly as glutamine and concentrations are
dependent on variety, nitrogen fertilisation, weather and soil type (Huang and Ough
1991
). Most of the nitrogen that enters the berry is transferred to the seeds and
skin with approximately 20 % allocated to the flesh. While proteins take up only a
small portion of the nitrogen fraction within the berry they are important for stress
management and defence against pathogens. The amino acids proline and arginine
account for approx. 70 % of the amino acids in the berry. Proline concentrations
increase before harvest while in some cultivars arginine remains relatively stable
after veraison (Kliewer
1968
). Therefore the proline to arginine ratio can be a good
indicator of berry maturity. Shading can, however, decrease the proline to arginine
ratio (Pereira et al.
2006
).
