Distillation

1. Distillation Philip Meaden • Part 1: Malting, Mashing and Fermentation • Part 2: Distillation of Malt Spirit • Part 3: Distillation of Grain Spirit • Part 4: Distilled Beverages other than Scotch Whisky • Part 5: Flavour and Maturation • Part 6: Cooperage • Part 7: Blending and Packaging

2. Sources of Flavour Compounds • Raw materials • cereals (especially malt), peat • Yeast and bacteria • metabolic products, cell biomass • Distillation • reaction products formed during distillation • Maturation • wood extracts, products of reactions with wood extracts

3. phenol guaiacol o-cresol m-cresol p-cresol p-ethylphenol Phenolic Compounds From Peat

4. Volatile Compounds From Malt • During malting and kilning, amino acids and reducing sugars undergo Maillard reactions to produce a wide range of volatile compounds • heterocyclic nitrogen-containing compounds such as pyrazines (nutty aroma) and pyrroles (corn/bread aroma) • heterocyclic sulfur-containing compounds (thiazoles) • other heterocyclic compounds such as furans and furfural (caramel/vanilla aroma), and maltol (toffee aroma) • Dimethyl sulfide (DMS) and polysulfides (cabbage aroma)

5. furans furfural maltol pyrazines pyridines pyrroles thiazoles Volatile Compounds From Malt

6. Volatile Compounds From Distillation • Maillard reaction products (see previous slide for examples) • Esters • Acetal (imparts “delicate” fragrance) • Phenolic compounds derived from coumaric and ferulic acids (from barley) (impart spicy/medicinal aroma) • Acrolein

7. Acetal Formation During Distillation acetaldehyde ethanol + acetal water

8. Formation of 4-Vinyl Compounds During Distillation cinnamic acid derivatives (p-coumaric [ferulic] acid) 4-vinyl derivatives 4-vinyl phenol [guaiacol]

9. Formation of Acrolein During Fermentation and Distillation glycerol (from yeast) lactic acid bacteria -hydroxypropionaldehyde distillation acrolein

10. Maturation • Maturation is required to improve the sensory quality of spirits • Maturation is responsible for the non-volatile components of spirits • For Scotch whisky, the minimum maturation period is three years; for others it may be less (e.g., two years for Bourbon) • Maturation uses oak casks, which may be new or used, charred or uncharred (depending on the type of product)

11. The Chemistry of Maturation • Many hundreds of compounds are present in matured spirits • Correlation between specific flavours and aromas, and individual compounds, is difficult to demonstrate • Many compounds that are likely to be contributing to flavour and aroma cannot be reliably detected • Flavour and aroma properties of purified compounds may be modified in matured spirits (“matrix effects”)

12. Factors Affecting the Rate of Maturation • Cask size and type (new or used) • Cask age and history • Temperature and humidity • Charring • Ethanol concentration of the spirit (usually 62 to 68% v/v ethanol)

13. Charring • Produces a layer of active carbon for removing undesirable flavour components • Increases the yield of lactones and phenolic extracts from the wood • Provides the major source of colour • Destroys resinous wood flavours

14. Chemical Changes During Maturation • Additive reactions • release of components of the cask wood into the spirit • Subtractive reactions • evaporation of volatile compounds from the spirit • adsorption of compounds onto the surface of the cask • chemical reactions that reduce volatile compounds • Interactive/productive reactions • chemical reactions between components of the spirit, or between components of the spirit and wood

15. Additive Reactions Examples of compounds released from the wood are: • Lactones • Sugars • Tannins • Phenolics

16. Components of Oak Wood

17. Cell Wall Components of Oak Wood • Lignin (about 30%) • polymer constructed from the monolignols p-coumaryl, coniferyl and sinapyl alcohol; the last two predominate in hardwoods including oak • Hemicellulose (about 20%) • highly branched polymer containing xylose, arabinose, glucose and other sugars • Cellulose (about 50%) • polymer of glucose

18. p-coumarylalcohol coniferyl alcohol sinapyl alcohol The Structural Components of Lignin

19. Lignin Degradation Products • Lignin degradation products (such as vanillin, syringealdehyde, coniferaldehyde) have been found in all oak-matured spirits • Lignin degradation products provide floral, spicy, smooth, mellow attributes to flavour and aroma • Sources of lignin-degradation products are • degradation of lignin by toasting or charring of the cask • ethanol extraction of monomers from lignin • ethanolysis of lignin

20. CH O 3 Structures of Some Lignin Degradation Products OH OCH 3 CH=CHCHO coniferaldehyde vanillin vanillic acid OH OH OCH CH O OCH 3 3 3 CH=CHCHO CHO syringaldehyde sinapaldehyde

21. Lignin Degradation Products From Oak Wood Chips Oak wood chips were stored in 60% (v/v) ethanol for six months. Amounts are ppm.

22. Effect of Toasting or Charring on Production of Lignin Degradation Products from Oak Chips Oak wood chips were extracted with 60% (v/v) ethanol. Amounts are ppm.

23. Content of Lignin Degradation Products in Oak-Matured Spirit Amounts are g per 100 litres of 50% alcohol

24. Release of Sugars From Oak Wood Amounts are g per 100 litres of 50% alcohol

25. Tannins • Water-soluble polyphenols commonly found in plants • Hydrolysable tannins • gallotannins, ellagitannins, which hydrolyse to form gallic and ellagic acids • contribution to flavour remains questionable, but may contribute to bitterness and astringency • Condensed tannins • flavanoids such as quercitin (can be a source of haze)

26. gallic acid ellagic acid quercitin Structures of Tannins

27. Release of Tannins From Oak Chips(over five months) *Determined as gallic acid

28. trans-3-methyl-4-octanolide cis-3-methyl-4-octanolide Structure of Oak Lactones Two isomers occur naturally in oak wood:

29. Lactones in Oak-Matured Spirits • Important in providing “oak” characteristic to oak-matured spirits • Amount of each isomer varies with the type of oak (e.g., cis isomer is more abundant than trans isomer in American oak) • cis isomer has an odour threshold 2.5 to 20 times lower than the trans isomer • Synthesized oak lactones contain all four possible isomers: adulteration of oak-matured spirits with synthetic mixture can therefore be detected

30. Sensory Perception of cis-Isomer of Oak Lactone

31. Subtractive Reactions Examples include: • Evaporation of volatile compounds such as ethanol, acetaldehyde and dimethylsulfide • Oxidations: ethanol to acetaldehyde, dimethylsulfide to dimethyl sulfoxide • Ethanolysis of acrolein to 1,1,3-triethoxypropane

32. Ethanol Loss During Maturation • In Scotland (damp, cool conditions), ethanol is lost more rapidly than water • Allowances are made by the regulatory authorities (Customs & Excise) for this loss • 2% per annum plus a further 2% for casks of 500 litres • 2% per annum plus a further 3% for casks of 250-300 litres • For American whiskies (matured in warm, dry climates), water evaporates more rapidly than ethanol; ethanol concentration rises by about 2% per annum

33. Changes in Levels of Sulfur Compounds During Maturation Dimethyl sulfide disappears within about one year

34. Ethanolysis of Acrolein During Maturation acrolein 1,1,3-triethoxypropane

35. Interactive Reactions • Oxidation, such as acetaldehyde to acetic acid • Esterification, such as acetic acid to ethyl acetate • Acetal formation from acetaldehyde and ethanol • Oxidation and esterification of lignin degradation products • Water-ethanol interactions

36. Acetaldehyde, Acetic Acid and Acetal Formation ethanol acetaldehyde acetic acid ethanol + acetal water

37. Oxidation and Esterification of Lignin Degradation Products vanillin vanillic acid ethyl vanillate OH OCH CH O 3 3 COOC H 2 5 syringaldehyde syringic acid ethyl syringate

38. Changes in the Structural Properties of Water-Ethanol Mixtures • Water and ethanol interact through hydrogen bonding • Maturation leads to the formation of stable clusters of water and ethanol, and an increase in viscosity • Stable clusters of water and ethanol are thought to contribute to the “mellowness” of the spirit • Non-volatile components (lignin, tannins and inorganic salts) contribute to the formation of stable clusters of ethanol and water

39. Summary of Changes During Maturation of Malt Spirit Units (except for ethanol) are ppm