Aroma chemistry

Question 1

Varietal- What is Methoxypyrazines?

Answer 1

• cyclic nitrogen-containing compounds found in red and whites
• 3-isobutyl (IBMP), 3-isopropyl (PMP), 2-sec-butyl (SBMP)
• Herbaceous characters (green bell pepper)
• Olfactory perception thresholds sit in low ng/L
• concentration in wine can be influenced by:
  clone, climate, canopy management, multi-coloured Asian lady beetle, skin contact, pressing.

Question 2

Varietal- What is Polyfunctional Thiols?

Answer 2

• 3- mercaptohexanol (3MH), 3-mercaptohexyl acetate (3MHA).
• found in NZ sauvignon blanc
• Low perception threshold in wine
• positive aromas of tropical fruits
• Marlborough Sauvignon Blanc contains on average 7080ng/L 3MH and 516ng/L 3MHA. South African wines on average have 1526 ng/L of 3MH and 38ng/L 3MHA.

Question 3

Varietal- What is Terpenes?

Answer 3

• Floral aromatic varieties in Riesling, Muscat and Gewurztraminer
• free, free volatile terpenes and bound, potentially volatile terpenes
• Bound terpenes undergo acid or enzymatic hydrolysis
• influenced by harvest date, canopy management, skin contact and pressing condition.

Question 4

Varietal- What is C13 Norisoprenoids?

Answer 4

• Fruity and flowery aromas, balsamic tones
• arise from the oxidative breakdown of carotenoids in grapes.
• Lutein and Beta-carotene
• Level of these carotenoids decreases through the progression of grape maturity.
• bound forms and located in the flesh of the grape berry
• Bound forms increase with grape maturity and can undergo hydrolysis during winemaking practices, eg. ageing to contribute to the odour active free form in wines

Question 5

Pre-fermentative- what are C6 compounds?

Answer 5

• herbaceous, green grassy aroma
• formed by polyunsaturated fatty acids in grapes
• alpha linoleic and alpha-linolenic during grape tissue damage
• Position 13 of the polyunsaturated fatty acid is oxidised to form respective 13-hydroperoxide, which then cleaved into C6 aldehydes by grape 13-hydroperoxide lyase
• C6 aldehydes in the juice are hexanal and 2-hexanal.

Question 6

Fermentative- What are alcohols and aldehydes?

Answer 6

• produced by yeast during fermentation
• pungent and unpleasant odours
• not exceeding 300mg/L add complexity to a wine aroma
• exceeding 400mg/L negative effect.

Question 7

Fermentative- What are esters?

Answer 7

• formed by condensation of a hydroxyl group and an acid
• the reversible reaction at wine pH.
• longer carbon chain in ester result more waxy or floral aroma
• Acetate ester formed by the condensation of acetic acids and higher alcohol

Question 8

Fermentative- what are Fatty acids?

Answer 8

• Hexanoic, octanoic, decanoic
• Fatty acids are produced by yeasts during fermentation.
• less than 10mg/L complexity wine
• more than 20mg/L unpleasant

Question 9

Post fermentation- what is cork taint?

Answer 9

• Woody plant cork oak
• 1-Octen-3-one, mushroom, metallic
• 1-Octen-3-ol, Mushroom, metallic
• 2-methylisoborneol, earthy, mouldy, dirty
• 2,4,6- Trichloroanisole (TCA), mouldy, damp carboards
• Geosmine, earthy, mouldy
• Guaiacol, smoky, medicinal.
  TCA can be caused by excessive bleaching of corks after punching out, absorption of chloride