Although salinity or sodicity may be localised to certain production areas, the study aimed to provide insight into the potential impact of cations and anions in wine and its origins, from a positive, as well as potentially negative perspective.
Soil salinity and sodicity occurs mostly in arid and semi-arid environments. Saline soils have high concentrations of soluble salts (e.g. NaCl) in the soil solum/regolith with electrical conductivity values of over 4 dS/m and resistance of less than 300 Ohm, an exchangeable sodium percentage of lower than 15 and pH lower than 8,5. Sodic soils in turn have a high concentration of sodium ions compared to other cations, with electrical conductivity values of lower than 4 dS/m, sodium adsorption ratio of over 13, an exchangeable sodium percentage of over 15 and pH between 8,5 and 10. Grapevines should grow normally at values below 2.5 dS/m, resistance over 300 Ohm, and at soil pH levels between 5 and 7.5. Grapevines are known to be moderately sensitive to salinity, but salinity and sodicity can have an adverse effect on plant growth, whether directly or indirectly. These conditions also affect the grapevine’s physiological responses, causing yield reduction, decrease in shoot growth and increase in cation and anion concentrations in the fruit and final wine. It may also affect the biochemical pathways, leading to toxicities, deficiencies and mineral imbalances in the plant.
The most important cations associated with salinity are Na+, Ca2+ and Mg2+, whereas the most important anions are Cl–, SO42- and HCO3. They may occur naturally in the soil, but more commonly are added to the soil through irrigation, also exacerbated through persistent droughts. Cation and anion analysis in the leaves, petioles and grape components is essential for the prevention of the negative effects it may have on grapevine physiology, the grape juice and also the wines made from it.
The OIV resolution (Oeno 6/91) with regard to sodium states that: “When wine contains excess sodium (excess sodium is equal to the content of sodium ions less the content of chloride ions expressed as sodium), it is generally less than 60 mg/L, a limit which may be exceeded in exceptional cases…”. As a result of these restrictions, some wines may even be rejected from the export market. High concentrations NaCl also has an effect on the sensorial quality of wine, and may as a result be described as flat, dull, soap, seawater-like and saline.
Some local wine tank samples were found to exceed the stated OIV limit, as well as the local beverage limit, of 100 mg/L significantly in some cases (see Table 1). Australia already in 1997 published an article (Leske et al., 1997) to create awareness of sodium levels in Australian wines. In Australia, for example, the sodium content may not exceed 1 000 mg/L in wine, as the country has a high occurrence of saline and sodic soils. It seems that some regulators do not even consider chloride levels as recommended in the OIV legislation, which makes a huge difference in interpretation.
Objectives and rationale
The main questions were related to the primary origin of the salinity/sodicity, cation and anion concentrations occurring in the grapevine, the cation and anion content of the grape juice and wine, the different cation and anion measurement techniques locally and overseas, as well as the practices implemented for the management of salinity/sodicity. In addition to this, anecdotal evidence suggested that some wines with higher mineral contents may also have positive style attributes. The potential sensory impact of salts in wine therefore had to be considered.
Materials and methods
Areas with soil salinity and sodicity were selected in Chenin blanc and Pinotage vineyards from two farms in an arid area of the Western Cape with hot and dry summers. The plots, of which one was rain-fed and one irrigated, were divided into ‘high’ and ‘low vigour’ according to salinity and sodicity levels and aerial imagery. Soil analysis was conducted at three depths to confirm the presence of high cation and anion concentrations. Meso-climate loggers were installed on both farms in order to analyse the climatic effects on the grapevine. Vegetative and reproductive measurements were conducted including trunk circumference measurements, shoot measurements, destructive leaf area measurements, berry sampling and harvest measurements. The cation and anion concentrations in the soil, different grapevine parts (leaves, petioles and canes), grape berry parts (juice, homogenised grapes, skins and the sediment after juice settling), and in the subsequent wines were also assessed and the sensory profile of the wines were determined.
Results and discussion
Soil samples confirmed the presence of salinity/sodicity in the plots, which affected the growth (as measured through shoot growth, trunk circumference and leaf area), as well as yield per vine. Shoot, petiole and leaf analysis showed high levels of sodium, reaching values greater than 1 500 mg/L. The juice cation and anion analysis showed high levels of sodium for some plots, however, chloride levels were found to be below harmful limits. There were differences between juice, sediment, skin and homogenised sample analysis, confirming that the sediment contained the highest cation and anion content. Some marked differences were reported between analyses of the different laboratories. Descriptive sensory analysis showed no significant differences in terms of saltiness, however, some wines exhibited significant differences between aroma and taste descriptors. The high salt content in the wine may also have had a positive effect on the taste of the wine. At low salt concentrations wines may appear to be sweeter, or less bitter.
The study confirmed ion transfer from soil to vine to grape and into the wine, and also showed how high concentrations of certain cations can exist in the juice sediment. This could affect wines that undergo skin/lees contact for long periods of time. The wines made from the high cation content juice surprisingly had lower levels of cations and anions, which needs further investigation. Sensory analysis has indicated that, at certain concentrations, sensory factors could be affected positively or negatively, however, this was dependent on the concentrations of the cation and anions. Considering that we found different interpretations in accepted limits, as well as considerable differences in analysis results between laboratories on the same samples, we recommend clear guidelines to be set for accepted limits, as well as analysis methods.
Wines with high cation and anion concentrations can be found in many wine grape-growing regions across the world, and this have also been reported for some South African wines, originating from grapes grown on soils with high levels of salinity or sodicity. Questions arose concerning the origin of the salinity/sodicity, cation and anion concentrations occurring in the grapevine, as well as the grape juice and wine, different measurement techniques, as well as the practices implemented for the management of salt content in wines. The sensory impact was also investigated. The study confirmed ion transfer from soil to vine to grape and into the wine, and showed high concentrations of certain cations in the juice sediment, stressing the importance of judicious skin contact. Wines made from high cation content juice did not always show high levels of cations and anions, warranting further investigation. Sensory analysis showed that at certain concentrations sensory factors could be affected positively or negatively by high salt content in wine. Analysis methods differed between laboratories, and there is a need for standardisation of methods and calibration between the labs.
Leske, P.A., Sas, A.N., Coulter, A.D., Stockley, C.S. & Lee, T.H., 1997. The composition of Australian grape juice: Chloride, sodium and sulphate ions. Australian Journal of Grape and Wine Research 3, 26 – 30.
Muller, K., 2017. Grapevine cation and anion transfer: A perspective from the soil to wine chemical and sensory properties. MSc(Agric) Thesis, Stellenbosch University, March 2017.
– For more details on this project, please refer to Muller (2017) and for more information, contact Albert Strever at firstname.lastname@example.org.