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Investigation of the variation in phosphate content of wines
Elizabeth Lindley1,2,6, John Agar4, Mike Bosomworth3, Seán Costelloe3, Julienne Freeman3, David Keane1,2,6, Tony Shadbolt5, Douglas Thompson3and Tracey Ying4
1Renal Medicine, 2Medical Physics and 3Blood Sciences, Leeds Teaching Hospitals NHS Trust,4Renal Medicine, Geelong Hospital, Australia,5SHORT SHEEP Micro-Winery, Mudgee, Australia and 6NIHR Devices for Dignity Healthcare Technology Co-operative
INTRODUCTION: For many patients with chronic kidney disease, restriction of dietary phosphate intake is an essential component of the management of hyperphosphataemia. Beverages usually contain phosphate in a form that is readily absorbed, especially when drinks are takenbetween meals, without a binder. The phosphate content of famous brands of cola (due to the addition of phosphoric acid or A338)is published, but it is often impossible to obtain this information for less well-known soft drinks and other beverages, such as beer and wine.
We recently showed that the standard ‘phosphomolybdate’ assay used by most laboratories to measure phosphate could reliably be usedwith a range of beverages. While validating the test, we observed a very wide range in phosphate content of wines. A better understanding of this variation could allow renal patients, and their dietitians, to identify products with lower levels.
LITERATURE SEARCH: We began by searching the internet and found that plants store phosphate in their seeds as the indigestible molecule, phytic acid. Fermentation releases this phosphate. This would explain why red wines, in which the grape juice begins fermenting with the pips and skins, tended to have higher phosphate levels than the whites. But several of the white wines in our study also had high levels. The most likely explanation for this was di-ammonium phosphate (DAP), a yeast nutrient that is widely used to supplement the nitrogen content of the crushed grapes. To learn about the relative contribution of intrinsic and added phosphate, we tested wines from an Australian micro-winerycontaining little or no DAP.
METHOD:After checking the agreement between the UK and Australian laboratory analysers using classic Coca-Cola (which is strictly regulated), 10 samples of wines from 5 grape varieties were tested using the standard assay for inorganic phosphate. Four reds, two white and two rosés were DAP-free. One red and one white had been fermented with addition of 100 and 150 mg/L DAP respectively. One roséwas ‘straight-pressed’, the other was ‘cold-soaked’ (where the juice is macerated for 24-48 hours with pips and skins, compared to up to 14 days for reds).
RESULTS:The phosphate content of Coca-Cola measured in the two laboratories was very similar (4.8 mmol/L in Australia compared to 5.0 mmol/L in the UK), allowing our data to be pooled. The DAP-free white wines contained 2.1 and 2.7 mmol/L phosphate, the reds 4.5 to 6.6 mmol/L, the straight-pressed rosé 3.1 mmol/L and the cold-soaked rosé 6.0 mmol/L. The increase in phosphate in the micro-winery wines with added DAP (0.7 and 1.2 mmol/L) was in good agreement with the documented addition. Phosphate levels in Australian wines from UK supermarkets were substantially higher 4.5 to 7.4 mmol/L (whites) and 6.5 to 9.9 mmol/L (red).
DISCUSSION:These measurements show that maceration, where colour and flavour is extracted into the juice from pips and skins, inevitably increases the phosphate content of wines. The high level in the cold-soaked rosé suggests that phytic acid, from whichphosphate is released, is extracted quite early in the process. Addition of much larger quantities of DAP (up to about 600 mg/L) would explain the higher phosphate levels in the mass-produced wines.
CONCLUSION:This investigation showed that white, or straight pressed rosé wines made with little or no DAP are relatively low in phosphate, with a 175 mL glass having less than a third of the phosphate content of a 330 mL can of Coca-Cola. Patients who have problems with hyper-phosphataemia need to be aware of the high phosphate levels in red wine. The renal community should lobby for more informative labelling which might encourage wine makers to use less DAP. The phosphate content of the wines and other beverages we have measured is available on-line.