IntroductionThe measurement of orga

Introduction
The measurement of organic acids in wines is complementary to sensorial and microbiological quality assessment in fermentation control, for example, because the types and concentrations of the different acids influence the resulting color and taste of the wine. Winemakers must monitor the concentration of various organic acids to ensure the quality of their wines. Tartaric, citric, and malic acid are added to adjust the pH. Malic acid levels must be monitored closely as many wines undergo a malolactic bacterial fermentation which reduces the acidity of the wine, as the malic acid is converted to lactic acid. Acetic acid is the main product of oxidation and can also spoil a wine.
In this article, we describe two different applications, both of which benefit by using high quality organic acid standards:
Determination of organic acids in red and white wines
Separation of organic acids in fruit juices
Part 1: Organic acids in red and white wines
Our aim in this project was to use ion exclusion chromatography for the simultaneous determination of the following organic acids of primary importance in wine: phosphoric acid, citric acid, tartaric acid, galacturonic acid, malic acid, succinic acid, lactic acid and acetic acid. Additionally, we compared two different sample preparation techniques that are common in the food industry: off-line with SPE and on-line with dialysis.
The wine samples were from Switzerland and Germany:
White wine:
(1) Müller-Thurgau, Baden (D)
(2) RieslingxSylvaner, Graubünden (CH)

Red wine:
(3) Merlot, Ticino (CH)
(4) Zizerser Blauburgunder, Malans (CH)
The HPLC analyses were performed using ion exclusion chromatography with conductivity detection on a sulfonated resin in the H+ form (e.g. SUPELCOGEL C- 610H column or equivalent) by A. Rumi, Metrohm Ltd., Switzerland.
Sample preparation

1. Sample preparation using SPE
Wine samples spiked with organic standard solutions, non-spiked wine samples, a blank solution and standard solutions for calibration were extracted by SPE using the following protocol:
SPE protocol
Condition the Supelclean LC-4 cartridge (Supelco, Cat. No. 57089) with 2 mL methanol followed by 4 mL eluent
Add 1 mL sample solution with a micropipette having disposable tips
Elute with 4 x 2 mL eluent
Fill to the final volume with eluent

2. Sample preparation using dialysis
The wine samples and standard solutions were diluted with eluent to the final volume and dialysed (dialysis cell, Metrohm).
Examples of the separation of an SPE extract of organic acid standards and a white wine sample are shown in Figures 1 and 2, respectively. A linear response for each acid at five different concentrations was achieved. The concentrations of the standard solutions were chosen in such a way that the whole expected concentration range of each acid in the samples was covered. The precision was determined by consecutive injections of blank wine samples and spiked ones.
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