Author(s):
Oliveira, Carla Maria 
; Ferreira, António César Silva ; Freitas, Victor De ; Silva, Artur M. S.
Date: 2011
Persistent ID: http://hdl.handle.net/10400.14/7632
Origin: Veritati - Repositório Institucional da Universidade Católica Portuguesa
Subject(s): Wine; Oxidation; Phenolic compounds; Antioxidant agents; Copper(I)/copper(II); ron(II)/iron(III)
Description
The present review aims to show the state of the art on the oxidation mechanisms occurring in wines, as well
as the methods to monitor, classify and diagnose wine oxidation. Wine oxidation can be divided in enzymatic
oxidation and non-enzymatic oxidation. Enzymatic oxidation almost entirely occurs in grape must and is
largely correlated with the content of hydroxycinnamates, such as caffeoyltartaric acid and paracoumaroyltartaric
acid, and flavan-3-ols. Non-enzymatic oxidation, also called chemical oxidation of wine,
prevails in fermented wine and begin by the oxidation of polyphenols containing a catechol or a galloyl group.
These phenolic reactions, both enzymatic and non-enzymatic, result in by-products named quinones.
However, in non-enzymatic oxidation, oxygen does not react directly with phenolic compounds. The
limitation on the reactivity of triplet oxygen is overcome by the stepwise addition of a single electron, which
can be provided by reduced transition metal ions, essentially iron(II) and copper(I). The sequential electron
transfer leads to the formation of hydroperoxide radical (HOO•), hydrogen peroxide (H2O2), and hydroxyl
radical (HO•). The later radical will oxidize almost any organic molecule found in wine and will react with the
first species it encounters, depending on their concentration. Sulfur dioxide (SO2) and ascorbic acid, when
added to wine, are able to reduce the quinones. Alternative options have been assessed for the prevention of
oxidation during wine storage; nevertheless, these are not fully understood or commonly accepted. During
aging, aldehydes are important intermediates in the chemical transformations occurring in wines, leading to
color and flavor changes. In the same way, a range of off-flavors can be formed from wine oxidation. At low
concentrations these flavors may add to the complexity of a wine, but as these increase they begin to detract
from wine quality. In addition to the major chemical browning involving wine phenols, the main oxidation
reactions occurring during grape juice heating or storage are caramelization and Maillard reaction, which are
temperature dependent. Different methods have been proposed in the literature, addressing the complexity
and multi-scale related with the oxidation process, to attempt the quantification of antioxidant activity in
wines. These methods can be broadly divided in: i) methods based on chemical reactions and ii) methods
based on the chemical-physical properties of antioxidants.
