Celia Barril

Ascorbic Acid: A Review of its Chemistry and Reactivity in Relation to a Wine Environment

Extensive reviews of research are available on the use of ascorbic acid, and its consequent degradation pathways, in physiological conditions or food matrices. However, very little information can be found for wine-related systems. This review highlights the relevant chemistry and reactivity of ascorbic acid with a focus on its behavior and potential behavior in a wine environment. The review describes the use of ascorbic acid as a complementary antioxidant preservative to sulfur dioxide along with the metal-catalyzed and radical-dependent manner by which it achieves this role. The relevant degradation products of ascorbic acid in aerobic and anaerobic conditions are presented as well as the interaction of these degradation products with sulfur dioxide and other wine-relevant sulfur compounds. Limitations in existing knowledge, especially regarding the crossover between the antioxidant and pro-oxidant roles of ascorbic acid, are identified.

Chemistry of ascorbic acid and sulfur dioxide as an antioxidant system relevant to white wine

The impact of the combined ascorbic acid and sulfur dioxide antioxidants on white wine oxidation processes was investigated using a range of analytical techniques, including flow injection analysis for free and total sulfur dioxide and two chromatographic methods for ascorbic acid, its oxidative degradation products and phenolic compounds. The combination of different analytical techniques provided a fast and simultaneous means for the monitoring of oxidation processes in a model wine system. In addition, the initial mole ratio of sulfur dioxide to ascorbic acid was varied and the model wine complexity was increased by the inclusion of metal ions (copper(II) and iron(II)). Sulfur dioxide was found not to be a significant binder of ascorbic acid oxidative degradation products and could not prevent the formation of certain phenolic pigment precursors. The results provide a detailed insight into the ascorbic acid/sulfur dioxide antioxidant system in wine conditions.

The Influence of Stereochemistry of Antioxidants and Flavanols on Oxidation Processes in a Model Wine System: Ascorbic Acid, Erythorbic Acid, (+) -Catechin and ())-Epicatechin

The stereochemical influence of antioxidant and flavanol compounds on oxidation processes in a model wine system was studied. The diastereoisomers, ascorbic acid and erythorbic acid, were used as antioxidants in a model wine system containing either (+)-catechin or (-)-epicatechin as the oxidizable flavanol compound. Samples were stored at 45 degrees C for a period of 14 days and analyzed by UV/visible spectrometry, CIELab, UPLC-PDA, and LC-MS. The results showed that less brown oxidative coloration occurred for samples with erythorbic acid for a given flavanol compound, while (+)-catechin provided less yellow coloration for a given antioxidant. Although erythorbic acid was degraded faster than ascorbic acid, it was associated with less decay in the accompanying flavanol compound. Xanthylium cation pigments were identified as the major contributor to color development. Furthermore, the production of pigment precursors, previously identified as furanone-substituted flavanols, was confirmed in all cases and their corresponding xanthylium cation pigments were lower in the presence of erythorbic acid than ascorbic acid. The results demonstrate that erythorbic acid is more efficient at minimizing oxidative color development than ascorbic acid in the model wine system.

Measurement of antioxidant activity with the thiobarbituric acid reactive substances assay

The thiobarbituric acid reactive substances (TBARS) assay is widely used to measure lipid oxidation and antioxidant activity in food and physiological systems. However, there has been no review (to our knowledge) that focuses exclusively on this test. This review presents an overview of the current use of the TBARS test in food and physiological systems, before looking at the various ways in which the assay is used in studies on antioxidant activity. As an antioxidant assay, the TBARS test may lack acceptable reproducibility, and long reaction times may preclude its adoption as a rapid screening method. Despite these potential limitations, there are features of the TBARS test that make it useful as a complement to popular screening tests such as Trolox equivalent antioxidant capacity. This review concludes with proposals for development of the TBARS test so that it can be used as a rapid and robust antioxidant assay.

Understanding the contribution of ascorbic acid to the pigment development in model white wine systems using liquid chromatography with diode array and mass spectrometry detection techniques

The present study investigated the contribution of ascorbic acid to the formation of coloured species in model white wine systems containing (+)-catechin as the oxidisable phenolic substrate. Reactions were carried out in the presence or absence of ascorbic acid in model wine systems buffered with either tartaric acid or formic acid. High performance liquid chromatography with diode array detector (HPLC-DAD) or mass spectrometry (HPLC-MS) analyses demonstrated that glyoxylic acid-derived xanthylium pigments were the main coloured species produced in all samples except those containing just (+)-catechin and formic acid. Higher concentrations of these pigments were detected in the tartaric acid based model system containing both (+)-catechin and ascorbic acid than in the corresponding formic acid model system. The inability of formic acid to form an aldehyde, unlike the known oxidative formation of aldehydes from tartaric acid, contributes to the lower colour development in the formic acid model system. Significantly, these observations imply that ascorbic acid must break down to provide an aldehyde, or ketone, capable of reacting with (+)-catechin to generate the glyoxylic acid-derived xanthylium cations.

Measurement of labile copper in wine by medium exchange stripping potentiometry utilising screen printed carbon electrodes

The presence of copper in wine is known to impact the reductive, oxidative and colloidal stability of wine, and techniques enabling measurement of different forms of copper in wine are of particular interest in understanding these spoilage processes. Electrochemical stripping techniques developed to date require significant pretreatment of wine, potentially disturbing the copper binding equilibria. A thin mercury film on a screen printed carbon electrode was utilised in a flow system for the direct analysis of labile copper in red and white wine by constant current stripping potentiometry with medium exchange. Under the optimised conditions, including an enrichment time of 500s and constant current of 1.0μA, the response range was linear from 0.015 to 0.200mg/L. The analysis of 52 red and white wines showed that this technique generally provided lower labile copper concentrations than reported for batch measurement by related techniques. Studies in a model system and in finished wines showed that the copper sulfide was not measured as labile copper, and that loss of hydrogen sulfide via volatilisation induced an increase in labile copper within the model wine system.

Light-induced changes in bottled white wine and underlying photochemical mechanisms

ABSTRACT Bottled white wine may be exposed to UV-visible light for considerable periods of time before it is consumed. Light exposure may induce an off-flavor known as “sunlight” flavor, bleach the color of the wine, and/or increase browning and deplete sulfur dioxide. The changes that occur in bottled white wine exposed to light depend on the wine composition, the irradiation conditions, and the light exposure time. The light-induced changes in the aroma, volatile composition, color, and concentrations of oxygen and sulfur dioxide in bottled white wine are reviewed. In addition, the photochemical reactions thought to have a role in these changes are described. These include the riboflavin-sensitized oxidation of methionine, resulting in the formation of methanethiol and dimethyl disulfide, and the photodegradation of iron(III) tartrate, which gives rise to glyoxylic acid, an aldehyde known to react with flavan-3-ols to form yellow xanthylium cation pigments.

The decay of ascorbic acid in a model wine system at low oxygen concentration

The present study investigated the impact of temperature on the degradation of ascorbic acid in low oxygen conditions in a model white wine. The concentrations of ascorbic acid, furfural, sulfur dioxide and phenolic-type products were monitored in a model white wine stored under non-oxidative conditions at 45.0, 36.5 and 24.0 °C for up to 693 days. The concentrations of both ascorbic acid and sulfur dioxide decreased over the analysis period while furfural and other colourless phenolic products increased in concentration, despite the presence of residual sulfur dioxide. The decay of ascorbic acid in the low oxygen conditions followed first-order kinetics and the rate constants were determined to be (3.5±0.2)×10(-8), (1.02±0.07)×10(-8), and (0.184±0.009)×10(-8) s(-1) for 45.0, 36.5 and 24.0 °C (n=5, standard error), respectively, and the activation energy was 110±3 kJ/mol (n=3, standard error). Importantly, these data allow more accurate prediction of the temperature-induced loss of ascorbic acid in low oxygen conditions during transport or storage of wine.

Implications of the Presence of Maturing Fruit on Carbohydrate and Nitrogen Distribution in Grapevines under Postveraison Water Constraints

Grapevine (Vitis vinifera) berries are sugar and nitrogen (N) sinks between veraison and fruit maturity. Limited photoassimilation, often caused by water constraints, induces reserve total nonstructural carbohydrate (TNC) remobilization, contributing to berry sugar accumulation, while fruit N accumulation can be affected by vine water supply. Although postveraison root carbohydrate remobilization toward the fruit has been identified through C tracing studies, it is still unclear when this remobilization occurs during the two phases of berry sugar accumulation (rapid and slow). Similarly, although postveraison N reserve mobilization toward the fruit has been reported, the impact of water constraints during berry N accumulation on its translocation from the different grapevine organs requires clarification. Potted grapevines were grown with or without fruit from the onset of veraison. Vines were irrigated to sustain water constraints, and fortnightly root, trunk, shoot, and leaf structural biomass, starch, soluble sugar, total N, and amino N concentrations were determined. The fruit sugar and N accumulation was also assessed. Root starch depletion coincided with root sucrose and hexose accumulation during peak berry sugar accumulation. Defruiting at veraison resulted in continuous root growth, earlier starch storage, and root hexose accumulation. Leaf N depletion coincided with fruit N accumulation, while the roots of defruited vines accumulated N reserves. Root growth, starch, and N reserve accumulation were affected by maturing fruit during water constraints. Root starch is an alternative source to support fruit sugar accumulation, resulting in reserve starch depletion during rapid fruit sugar accumulation, while root starch refills during slow berry sugar accumulation. On the other hand, leaf N is a source toward postveraison fruit N accumulation, and the fruit N accumulation prevents root N storage. Grapevine berries are sinks for the incorporation of both carbohydrates (Davies and Robinson, 1996) and N (RoubelakisAngelakis and Kliewer, 1992) between veraison and fruit maturity. Restricted TNC availability, induced by limited leaf photoassimilation, can cause starch redistribution from the roots during berry sugar accumulation (Candolfi-Vasconcelos et al., 1994), while N concentrations in the berries and roots are affected by abiotic conditions, such as vine water availability, during the growing season (Araujo et al., 1995). Furthermore, apart from also being affected by vine N supply, the N reserve accumulation in the roots is restricted by the presence of fruit before and after veraison (Rodriguez-Lovelle and Gaudillere, 2002). It is still unclear how the postveraison distribution of TNC and N reserves among the different organs, are affected by a combination of fruit presence and sustained water constraints. A further question remains on how this distribution contributes to, or inhibits, TNC and N reserve storage, or the contents of TNC and N in the fruit during berry maturation. In plant roots, TNC are mainly stored as starch, which can be hydrolyzed, yielding osmotic active soluble sugars (Regier et al., 2009). Apart from the possible remobilization of sugars via phloem sucrose transportation (Ruan et al., 2010) between Received for publication 23 Nov. 2016. Accepted for publication 9 Jan. 2017. This work was supported by the National Wine and Grape Industry Centre, and the Australian grapegrowers and winemakers through their investment body, Wine Australia, with matching funds from the Australian Government. Current address: Institut f€ur Allgemeinen und €okologischen Weinbau, Hochschule Geisenheim University, Geisenheim, 65366, Germany Current address: Montpellier SupAgro, Montpellier, 34060, France Corresponding author. E-mail: grossouw@csu.edu.au. J. AMER. SOC. HORT. SCI. 142(2):71–84. 2017. 71 the perennial vine parts (the roots and trunk) and the ripening berries, thereby contributing to berry sugar accumulation (Candolfi-Vasconcelos et al., 1994), sugars also accumulate in various tissues of water stressed plants, to aid in osmotic regulation (Regier et al., 2009). Therefore, upon water constraints during berry sugar accumulation, the accumulation of soluble sugars in different vine parts could theoretically contribute to various functions; e.g., to facilitate TNC remobilization, and to improve abiotic stress tolerance. As grapevines are perennial plants, the storage of starch reserves at the end of the season is essential for reserve TNC utilization the following season, required for vegetative and reproductive development from budburst (Holzapfel et al., 2010). Depleted root starch concentration can then lead to limited initiation and development of the inflorescence, and decreased fruit set and fruit yield the following season (Smith and Holzapfel, 2009). The N allocation to grapevine berries, and subsequent accumulation during berry maturation is, from a wine quality perspective, essential as it determines the juice yeast assimilable N content, influencing fermentation and wine composition. However, root N accumulation late in the growing season is important for its overwintering storage (Cheng et al., 2004). Similar to TNC, N reserves are used toward the initiation of early season vegetative growth, where their mobilization regulate spring growth and account for most of the N distribution until around flowering, as N soil uptake is usually still insufficient at this stage (Zapata et al., 2004). Nitrogen accumulation in the perennial vine parts usually initiates before berry maturity, and the reserves continue to increase until leaf fall (RoubelakisAngelakis and Kliewer, 1992). The presence of fruit reduces N assimilation in grapevine roots (Morinaga et al., 2003). Nitrogen is mostly stored in the roots, and these reserves consists of a range of amino acids and proteins (Zapata et al., 2004). Amino acids in plants are involved in the regulation of N metabolism, and play essential roles in N transport and storage (Roubelakis-Angelakis and Kliewer, 1992). The metabolic pathway related to the a-ketoglutarate family of amino acids, yields glutamic acid, glutamine, arginine, and proline. These amino acids are abundant in plants, and have distinct roles in N metabolism (Verma et al., 1999). Glutamic acid is the intermediate product of nitrate and ammonium assimilation, and a precursor for the synthesis of glutamine, arginine, and proline (Berg et al., 2002). Arginine is considered the main N-storage amino acid in grapevines (Xia and Cheng, 2004), while glutamine is an essential N transporter (Coruzzi and Last, 2000). Proline accumulation is linked to osmotic adjustment following abiotic plant stresses (Hare and Cress, 1997). The metabolism of the a-ketoglutarate-derived amino acids is therefore essential to regulate plant N partitioning and distribution, particularly during abiotic constraints. The aim of this experiment was to determine the effect of fruit presence during sustained postveraison water constraints, on the TNC and N distribution within grapevines. The first goal was to investigate the response in the structural development of the leaves, shoots, trunk, and roots, based on the presence of fruit, during sustained water constraints. The second goal was to determine how fruit presence affects TNC accumulation in the different organs, during the sustained water constraints, and to assess which individual sugars accumulate in the grapevine roots during the two phases of berry maturation (rapid and slow sugar accumulation). The final goal was to determine how the presence of maturing fruit affects the allocation of N between the grapevine organs, and to identify potential contributions of amino N, and especially the amino acids yielded from a-ketoglutarate metabolism, toward N storage or translocation. Materials and Methods EXPERIMENTAL DESIGN. Own-rooted ‘Shiraz’ grapevines, grown in 50-L pots containing commercial potting mix, were used for this experiment in the 2014–15 growing season. The grapevines were grown in an outside bird-proof cage in the warm to very warm climate Riverina region of New South Wales, Australia. The 3-year-old grapevines were winter pruned to 10 spurs with two buds each, and arranged in four rows of nine vines each. From just after budburst, the grapevines were fertilized every 3 weeks with 250 mL of 50:1 diluted complete liquid fertilizer (MEGAMIX Plus; Rutec, Tamworth, Australia). In total, 2.6 g N was applied to each vine through fertilization, and the fertilization events were ceased 1 month before the start of the experiment, aiming to limit soil N uptake during the experiment. The grapevines were well watered between budburst and veraison, when irrigation was supplied three times a day to the point of visual free drainage from the pots. Vines were shoot thinned so as to leave 17 shoots per vine from fruit set, and at the onset of veraison, 2 d after the first sign of berry softening was observed, the treatments were initiated. Four randomly selected vines, one from each row, were destructively harvested on the day when then the treatments were initiated, to represent the population of grapevines before the implementation of the treatments. After removal of the four initial vines, the eight remaining vines per row were evenly spaced out in the row. All bunches were removed on half the vines, to have 16 vines with, and 16 vines without fruit. Two vines in each row (one with fruit and one without fruit) were used as a visual reference to control irrigation scheduling, and received double the irrigation volume than the other vines. Irrigation was scheduled three times per day (0800, 1400, and 1800 HR) for all vines, and the vines receiving double the irrigation, were rewatered each day just to the point of visual free drainage from the pots during the 1400 HR irrigation event, through two irrigation emitters per pot. The remaining 24 vines were irrigated throu

Photoproduction of glyoxylic acid in model wine: Impact of sulfur dioxide, caffeic acid, pH and temperature.

Glyoxylic acid is a tartaric acid degradation product formed in model wine solutions containing iron and its production is greatly increased by exposure to UV-visible light. In this study, the combined effect of sulfur dioxide, caffeic acid, pH and temperature on the light-induced (⩾300nm) production of glyoxylic acid in model wine containing tartaric acid and iron was investigated using a Box-Behnken experimental design and response surface methodology (RSM). Glyoxylic acid produced in the irradiated model wine was present in free and hydrogen sulfite adduct forms and the measured total, free and percentage free glyoxylic acid values were modeled using RSM. Sulfur dioxide significantly decreased the total amount of glyoxylic acid produced, but could not prevent its production, while caffeic acid showed no significant impact. The interaction between pH and temperature was significant, with low pH values and low temperatures giving rise to higher levels of total glyoxylic acid.