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Oxidative Pinking in White Wines

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Oxidative Pinking in White Wines Vitis 16, 286-294 (1977) The Australlan Wine Research Institute, Gien Osmond, Australia Oxidative pinking in white wines by R. F. SIMPSON Oxydative Rosafärbung in Weißwein Z u s a m m e n f a s s u n g . - Es wird eine spektrophotometrische Methode be­ schrieben, die es ermöglicht, die Intensität der Rosafärbung in Weißwein zu messen. Bei Weinen, die zu Rosafärbung neigen, wird diese durch Zugabe von Wasserstoff­ peroxid herbeigeführt. Weine, die mit 75 mg Wasserstoffperoxid/! behandelt und 24 Stun­ den lang bei 25 oc gehalten wurden, entwickelten eine rosa Verfärbung, die Rückschlüsse auf den Präcursorgehalt im Wein erlaubt. Zugabe von 15 mg Wasserstoffperoxid/! führ• te zu einer wenigE,r starken Verfärbung, die sich in Abhängigkeit vom S02-Gehalt des Weines entwickelte.; diese Reaktion ermöglicht es nachzuweisen, in welchen Weinen während der Lagefung eine rosa Verfärbung entstehen könnte, so daß gegebenenfalls eine PVPP-Behandlung vorgenommen werden kann. Der Einfluß des Wein-pH, der Lagerungstemperatur, des S02-Gehaltes und der PVPP-Behandlung auf die Entstehung der durch Zusatz von Wasserstoffperoxid ausge­ lösten rosa Weinverfärbung wurde geprüft. Die durch Zugabe von S02 und .Änderungen im pH verursachten Reaktionen zeigen, daß die rosa Substanzen keine einfachen (mono­ meren) Anthocyane sind. Die spektralen und chemischen Eigenschaften dieser rosa Sub­ stanzen ähneln jenen, die von Anthocyanen mit höherem Molekulargewicht und Aryl­ Substitution in Stellung 4 zu erwarten sind. Solche Anthocyane könnten von Proantho­ cyanen herstammen, deren Vorkopimen in den Trauben bekannt ist und die wahrschein• lieh auch im Wein vorhanden sind. lntroduction With white table wines the term "pinking" is used to describe the troublesome discolouration which sometimes develops during the later stages of production or storage. Pinking is most likely to occur following contact with air and in these in­ stances the colour development is fairly rapid (i.e. over several days). Even where the extent of pinking is slight the colour of the wine is unfavourably affected; where pinking is more intense the wine may be commercially unacceptable. Wines made from the main white grape varieties (vine sp. Vitis vinifera) grown in Australia (Muscat Gordo Blanco, Sultana, Palomino, Riesling and Crouchen) have been found susceptible to pinking with, however, considerable regional variation in such susceptibilities. The reported incidence of pinking has increased in recent vintages and this has coincided with an incre-ase in the use of colder fermentation and inert gas to protect against oxidation. Wines produced under these conditions are often those exhibiting higher susceptibility to pinking; this is in agreement with observations made by S1NGLETON (1972). Pinking can be removed or prevented by treating the wine with nylon, casein or PVPP (polyvinylpolypyrrolidone). As it is desirable to keep the number of wine­ making operations to a minimum and because of increased production costs, treat­ ments to protect against pinking should only be considered where the development Oxidative pinking in whjte wines 287 of pinking is likely. A better understanding of the causes and nature of pinking in white table wines would assist in overcoming this problem. In the present studies, a spectrophotometric method of quantitatively measur­ ing pinking has been developed and an assay formulated to determine the potential of a wine to develop pink colour. Materials and methods Spe c t r a l m e a s ur e m e n t s : Visible spectra and optical densities were obtained using a Varian Techtron model 635D u.v.-visible spectrophotometer and Linear model 252A chart recorder. The optical densities (E) using 10 mm sample cells were corrected by subtracting the values obtained at the same wavelength (l) with distilled water. Wine samples were filtered through a membrane filter with 1.2 pm pore size prior to measurement. D e t e r m in a t i o n o f p i n k i n g : From an examination of the visible spectra of wines having a range of pink colour it was evident that the wines with more pinking were those with spectra exhibiting higher optical densitil$ at about 500 nm and which deviated by a greater extent from a smooth curve neaJ_'this wave­ length. The visible spectra of two wines, one having no pinking and the :other ap­ preciable pinking are shown in Figs. 1 and 2, respectively. In these studies the pink­ ing values were determined using statistical means. A least squares fit to the em­ pirical formula 2 lnE = a0 + a 1ln(l - 350) + a 2ln(Ä. - 350) using E400, Euo, E420, Eooo, E625 and E050 determined the values of the coefficients a0, a 1 and a 2 for each curve, i.e. spectrum (for example, see WILLIAMS 1959, 41--49). For wines showing no pinking a consistently good fit was obtained in the range 420-600 nm using the derived formulae. Estimation of pinking for a wine is based on the difference between E500 (observed) and the unpinked (calculated) value at 500 nm. For convenience, the pinking values were multiplied by a factor of 10a. A s s a y o f p in k i n g s u s c e p t i b i 1 i t y : Pink colour could be produced in susceptible wines (i.e. wines of which some lots had pinked during storage) by the addition of hydrogen peroxide. The extent of pinking present after treatment with hydrogen peroxide was determined by the method outlined above. As such the value obtained is the sum of the pinking initially present in the wine (generally a low value) and the pink colour produced by the hydrogen peroxide treatment. Wine samples (10 ml lots) were placed in screw-capped test tubes (20 ml). Hy­ drogen peroxide was added and the samples were stored for 24 h at 25 °c in the dark, unless otherwise stated. The samples were mixed thoroughly (i) after the ad­ dition of hydrogen peroxide and (ii) before measuring optical densities. Effect of hydrogen peroxide concentration on pinking pro du c e d in w in e : 10 ml lots of 5 wines showing a range of ability to pink were treated with 0.05 to 0.40 ml of freshly prepared 0.3 % (w/v) hydrogen peroxide solution (1 ml of 30 % hydrogen peroxide made up to 100 ml with glass-distilled water) to give a concentration of 15 to 120 mg hydrogen peroxide per 1. The com­ mercial hydrogen peroxide solution was stored at 4 °c. Analyses by the method of KoLTHOFF and SANDELL (1952) showed a concentration of 29.9 % (w/v). Effects of reaction time and temperature on pinking p rod u c e d i n w i n e : The pinking produced in 5 wines with the addition of 288 R. F. Su.tPSON O· 25 0·25 2 0·20 0·20 > > 1- ~ 0· 15 ;;; 0·15 "'z z w w C C ... 0 · 10 ... 0·10 ,t ,t -u u ..1- ..1- 0 0·05 0 0·05 40 0 50 0 60 0 70 0 400 500 600 70 0 WAVELENGTH ( nm) WAVELENGTH ( nm) Fig. 1: Spectrum of wine showing no pinking. Pinking value, 0 (absorbance units X 103). Fig. 2: Spectrum of wine showing pinking. Pinking value, 27 (absorbance units X 103). Abb. 1: Spektrum eines Weines ohne Rosafärbung. Farbintensität = 0 (Absorptionsein- heiten X 103). Abb. 2: Spektrum eines Weines mit Rosafärbung. Farbintensität = 27 (Absorptionsein­ heiten X 103), 75 mg hydrogen peroxide per l and storage at 25 °c was determined after 3, 6 and 12 h, and over 14 days. The pinking produced in the same wines stored at 3 °C for 1 and 2 days was similarly obtained. Effect of S0 2 content on pinking present and produced in w in e o n t h e ad d i t i o n o f hydrogen per o xi de : After acidifica­ tion with fü1 HCl (to give a pH <1) wine samples were sparged with nitrogen to remove free S02• 5M NaOH was added until the pH of each wine was returned to the original value. Potassium metabisulphite was added to the 5 wines to give a range of samples having up to 60 mg free S02 per 1. After allowing the wines to equilibrate (2 days at room temperature) free and bound S02 levels were deter­ mined by the method of RANKINE and PococK (1970). Pinking values were obtained after the addition of 15 and 75 mg hydrogen peroxide per 1. PVPP treatment of wine and the effect on pinking: 100 ml lots of 5 wines were placed in stoppered measuring cylinders with 12 to 100 mg polyvinylpolypyrrolidone (PVPP) and briefly shaken at 10 min intervals over 1 h. The wines were filtered using a 1.2 ,um membrane filter. Pinking values were obtained prior to and after the addition of 75 mg hydrogen peroxide per 1. Effects of acidification and pH change on pinking: Additions of 5M HCl and 5M NaOH to 50 ml lots of 5 wines having appreciable pink colour were made to provide a range in pH (for each wine) from 2.75 to 4.00. Pink­ ing values were obtained (after 4 h) prior to the addition of hydrogen peroxide, after the addition of 75 mg hydrogen peroxide per 1 and after acidifying the treated samples with 5M HCl (to give a pH <1). Oxidative pinking in white wines 289 Analysis of errors (in assays using hydrogen peroxide) : Triplicate determinations were made and the pinking values shown (see Tables) are means of these determinations. Examination of the data collectecl for each table showed that the standard error was proportional to the pinking value.
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