Copper toxicity in whisky by-products
Some traditional whisky distilleries use copper stills and pipework (rather than stainless steel ones), and their by-products tend to contain high levels of Cu. Pot ale syrup and dark grains contain about 80-120 mg/kg and 40-60 mg/kg DM of Cu, respectively. Draff is relatively low in Cu (12-18 mg/kg), since it is produced before distillation (Lewis, 2002). A trial compared Cu availability in sheep (estimated by measuring liver accumulation) of copper sulphate, pot ale syrup, wheat dark grains and barley dark grains. Copper availability in dark grains was similar to that of copper sulphate (5-6%) while copper from pot ale syrup was found to be much less bioavailable (1%) (Sutter et al., 1996). This may explain why, in spite of its high copper content, pot ale syrup has been fed to sheep in relatively large amounts without apparent problems of toxicity (Lewis, 2002; Crawshaw, 2004).
Generally, malt whisky by-products can be fed to cattle, though there has been a report of lethal Cu toxicity in calves that were fed pot ale syrup at a rate of 5 litres/head/day (Sargison et al., 1996). Malt whisky by-products may be used for non-susceptible breeds of sheep, but not for indoor-fed sheep or for prolonged periods. It is preferable to check the Cu levels of those products and to blend copper-rich materials in the diet with those low in Cu (Lewis, 2002; Crawshaw, 2004).
Copper is not an issue in industrial ethanol production.