What is pinking?
White wines that have been made using highly reducing conditions can sometimes develop a pink colouration on sudden exposure to air. The aroma and flavour often remain unaltered. The pink colour often gives the impression of oxidation; however, oxidation is a separate phenomenon. The material comprising pink colour is not a single monomeric anthocyanin such as that found in red wines, although it does appear to be formed from proanthocyanins.
It has been suggested that an addition of ascorbic acid might help to protect wine from oxidative pinking during bottling (Skouroumounis et al. 2005, click here to order). When only SO2 is present at bottling (and not ascorbic acid), it is assumed that the pinking precursors are competing with free SO2 for available oxidants, and that the rate of the pinking reactions is increased with increasing concentration of dissolved oxygen in the wine. In the scenario where both SO2 and ascorbic acid are present, it has been proposed that dissolved oxygen reacts almost exclusively with ascorbic acid, while SO2 combines rapidly with the oxidant produced. Hence oxygen will be consumed before it can react with pinking precursors, provided that the SO2 is readily available. Simpson et al. (1982) (click here to order) reported that commercial wines containing medium levels of ascorbic acid showed little potential for pink colour formation; however, there is a need to ensure that adequate concentrations of free SO2 are maintained in the wine.
Caution must be observed when using ascorbic acid. Ascorbic acid reacts rapidly with oxygen to produce dehydro-ascorbic acid, and then hydrogen peroxide (H202), which can oxidise wine in the absence of sufficient SO2. As a general rule, about 2.8 mg/L of ascorbic acid could potentially ‘use up’ 1 mg/L of SO2. Therefore, appropriate additions of SO2 should be made to the wine before any ascorbic acid addition.
Estimation of pinking potential (Replicated from Iland et al. 2004)
- Label a 100 mL clear glass screwcap bottle as ‘control’ and another as ‘test’
- Completely fill the ‘control’ bottle with wine
- Measure 40 mL of the same wine into the ‘test’ bottle and add 0.5 mL of 0.3% w/v hydrogen peroxide. Mix.
- Place the ‘test’ sample in a dark cupboard at approximately 25°C overnight.
- Observe the degree of pinking of the ‘test’ wine compared to that of the ‘control’. As well as this visual assessment, spectral measures of the ‘test’ and ‘control’ wine can be performed at 520 nm, which gives a quantitative comparison. In this case, the wines will need to be filtered through 0.45 µm filter for assessment.
Quantification of pink colour, pinking susceptibility and precursor content
Simpson (1977) (click here to order) details a method where oxidative pinking can be quantified using a UV-Vis spectrophotometer. This analysis is offered by AWRI Commercial Services and gives results for three measures: pink colour, pinking susceptibility and precursor content.
Pink colour is a quantitative measure of the degree of pinking in a wine.
Pinking susceptibility is a quantitative measure of the degree of pinking arising following the addition of a small amount of hydrogen peroxide (15 mg/L), which removes 28 mg/L of free sulfur dioxide. This is designed to model the loss of most of the free SO2 in a white wine and therefore predict how likely pinking is to occur as a wine ages. Closure trials conducted by the AWRI suggest that losses of free sulfur dioxide in Semillon sealed with a screw cap can be between 12 and 23 mg/L over 24 months in bottle.
Precursor content is a quantitative measure of the degree of pinking arising from a relatively high addition of hydrogen peroxide (75 mg/L). This larger addition rate is intended to remove all sulfur dioxide bound to pinking precursors, so the level of pinking seen after this addition is a measure of all pinking precursor content in a wine.
Wines exhibiting pinking will have increased absorbance at 500 nm. Both pinking susceptibility and precursor content methods compare the absorbance of a hydrogen peroxide-treated sample to that of an untreated sample over a range of wavelengths to measure the level of pinking that arises. For the interpretation of these results, the absorbance difference is multiplied by a factor of 1000.
Interpretation of results
Pink colour is generally recognisable in most white table wines if the ‘pink colour’ value is greater than 5.
Wines with a value greater than about 15 for ‘pinking susceptibility’ might be capable of developing pink colour during normal post-fermentation operations. Note that the pinking susceptibility is dependent on the concentration of free sulfur dioxide in the wine, as free sulfur dioxide will help to protect wine from oxidative pinking. Ascorbic acid also helps protect wine from oxidative pinking (in combination with sulfur dioxide) (Simpson et al. 1983, click here to order). Thus, wines containing a higher concentration of SO2, or free SO2 in combination with ascorbic acid, would be expected to have a lower pinking susceptibility and vice versa.
The ‘precursor content’ value gives an indication of the quantity of material present in the wine capable of forming pink colour. Simpson et al. (1982) reported a mean precursor content value of 35 for 62 commercial white wines. Values above about 50 would be considered high. The pinking precursor content measurement is less dependent on the concentration of free SO2 and other antioxidants such as ascorbic acid present in the wine than the measure of ‘pinking susceptibility’.
More details can be found in Simpson et al. (1982) (click here to order).
Treatment or removal
Removal of pink colour and/or pinking precursors from wine can be achieved by fining with PVPP. Laboratory trials, in conjunction with pinking analysis, should be performed to establish the addition rate of PVPP required to remove the pink colour from the wine and/or reduce the precursors to a level where the wine passes the pinking test. A fining procedure can be found here.
It is also possible that pink colour can be reversed by exposure to UV light; however, it is suggested that this be trialled in the first instance by placing a couple of bottles considered ‘pink’ on a window sill in direct sunlight.
Iland, P.G., Ewart, A., Sitters, J., Markides, A., Bruer, N. 2000. Techniques for chemical analysis and quality monitoring during winemaking. Adelaide, SA: Patrick Iland Wine Promotions.
Skouroumounis, G.K., Kwiatkowski, M.J., Francis, I.L., Oakey, H.,Capone, D.L., Peng, Z., Duncan, B., Sefton, M.A., Waters, E.J. 2005. The influence of ascorbic acid on the composition, colour and flavour properties of a Riesling and a wooded Chardonnay wine during five years’ storage. Aust. J. Grape Wine Res. 11(3) : 355-368. (click here to order)
Simpson, R.F. Oxidative pinking in white wines. 1977. Vitis 16: 286-294. (click here to order)
Simpson, R.F., Miller, G.C., Orr, G.L. 1982. Oxidative pinking of white wines: recent observations. Food Technol. Aust. 34(1): 44-47. (click here to order).
Simpson, R.F., Bennett, S.B., Miller, G.C. 1983. Oxidative pinking of white wines: a note on the influence of sulphur dioxide and ascorbic acid. Food Technol. Aust. 35(1): 34-37. (click here to order)