WINETECH Technical Yearbook 2019

Tartaric acid is not only one of the major fixed acids in grapes, but also plays a role in the composition and taste of wine. Due to its different forms in wine it also influences the pH and cold stability of wine. The Australian Wine Research Institute (AWRI) recorded the change in the pH and total acid concentration since the 1960s and the various interesting conclusions raised from it. When the Australian wine spectrum shifted from fortified wines to dry red wines from the 1960s to the 1970s, the high pHs of the wines was a common problem. This led to different microbiological instabilities, which prompted different adjustments of winemaking practices. It included amongst others the application of cooling, acid additions and discerning sulphur dioxide management. The median pH of Australian wines was for example 3,45 in 1988, but increased to 3,62 in 2014. During this period the total acid concentration (TA) remained approximately constant between 6,0 and 6,4 g/L. The TA concentration consists of approxi­ mately 40% tartaric acid, 30% malic or lactic acid and 20% succinic acid. During cool years the malic acid concentration can be twice as much. Although a pH increase of 0,2 units may appear negligible, it can almost halve the acidity or quantity of hydrogen ions in a wine. It is important to be aware that higher pH grape juice is more prone to bacterial spoilage and the potential growth of Brettanomyces.

The potas s i um (K) concent rat i on of Australian Shiraz juice varies from 1 800 to 3 600 mg/L. The skins of red grapes contain higher concentrations (up to 9 000 mg/L) than the pulp. The K concentration of young red wines varies between 900 to 1 100 mg/L and is usually approximately 20% lower than the original juice. Much higher K concentrations were observed in especially the warmer wine regions during the 1970s. As result of the present trend to harvest grapes earlier, less time is available for the accumulation of potassium in the berries. During the 2018 harvest the K concentration of many red wines was approximately 1 800 mg/L, which is almost twice the above- mentioned concentrations. During the alcoholic fermentation, changes in the K and TA concentrations of the fermenting juice occur. During the first three days of alcoholic fermentation, the K concentration increases by 50-120% as result of the K extraction from the skins. Thereafter the concentration deceases as result of the precipitation of K-bitartate (cream of tartar), which removes an H-ion from the solution. This causes an increase in the pH of the fermenting juice. The tartaric acid concentration at the end of alcoholic fermentation is consequently nearly half the original tartaric acid concentration. A possibility exists that the formed alcohol may extract more K from the skins. At that stage a pH decrease can also occur when malic acid

THE ROLE OF TARTARIC ACID IN WINEMAKING PHOTO. K-bitartrate (cream of tartar) is precipitated in big volumes during winemaking.

CHARL THERON: Private consultant KEYWORDS: Tartaric acid, winemaking. AUGUST 2019