Washington Winegrowers Convention Kennewick, WA, February 6-8, 2018 Growing Grapes for White Wine Production: Do s and Don ts in the Vineyard Markus Keller
Aroma, flavor: Volatiles for white wine Norisoprenoids (β-damascenone, β-ionone) Mainly glycosides in skin (must be hydrolyzed to smell) Accumulate during ripening (carotenoid breakdown) Monoterpenes (geraniol, linalool, ) Mainly glycosides in skin, less in pulp Accumulate mostly during ripening Methoxypyrazines Volatiles in skin and seed, released upon crushing Accumulate before veraison, then decline (mostly early) Volatile thiols (S-containing mercaptans) Bound to glutathione or cysteine in skin, less in pulp Accumulate during ripening (fatty acid breakdown) C6 compounds (hexenal, hexanal) Green-leaf volatiles Accumulate in crushed fruit/must (fatty acid breakdown)
Mouthfeel: Phenolics for white wine Hydroxycinnamates Volatile phenols Tartrate esters in pulp and skin Accumulate early, then often decline Astringent (tartrate esters) or bitter (ethanol esters) Early and rapid oxidation (juice/must browning) Flavonols Glycosides in skin (myricetin branch missing) Accumulate throughout berry development Bitter and/or astringent Flavan-3-ols Tannins Monomers in skin and seed Accumulate through veraison Bitter, late and slow oxidation Tannins Oligo- and polymers in skin, seed, stem Accumulate through veraison May continue to polymerize during ripening Astringent (bitter oligomers mostly in seeds) Bogs et al. (2007)
Harvest berry weight (g) Water deficit impacts berry development 2.2 2.0 1.8 1.6 1.4 r = 0.84*** 50-100% ET 100-100% ET 1.2 2010 1.0 2011 2012 0.8 0.4 0.5 0.6 0.7 0.8 0.9 Lag-phase berry weight (g) Berry weight loss (%) Water deficit before veraison Small berries -1.2-1.0-0.8-0.6 Post-veraison Ψ stem (MPa) Berry size is determined early It is difficult to manipulate after veraison Water deficit after veraison Less sugar, berry shrinkage 50 40 30 20 10 0-10 -20-30 Merlot (r=-0.45*) Syrah (r=-0.41*) Chardonnay (ns) Keller (2015)
Water deficit: It s not just about berry size 122 116 F Temperature ( F) 104 86 68 50 32 Ambient 89 F Fruit exposure 25% ET Exterior Interior 100% ET Exterior Interior 245 246 247 248 249 250 Day of year Water deficit Small berries, low vigor Open canopy, restricted shoot growth High cluster sun-exposure High light and high temperature Exposed berries are warm berries 68 F 50 F Monoterpene window
Sun exposure: Light or heat? Light (>50% full sun) strongly increases flavonols, increases monoterpenes, hydroxycinnamates, and slightly skin tannins; reduces methoxypyrazines (early) Temperature below 68 F increases flavonols, above 95 F inhibits them (tissue temperature!) Temperature optimum for monoterpenes is 50-68 F, volatilization increases above 68 F High preveraison temperatures increase malate, tannins; reduce methoxypyrazines High postveraison temperatures reduce malate, methoxypyrazines; increase K + ph! +25 F Kolb et al. (2003) Spayd et al. (2002)
Deficit irrigation for white wines Irrigation per se has little effect on white fruit composition, but water deficit leads to smaller, more open canopy, and smaller, warmer berries Sun-exposed white grapes have up to 8-fold higher flavonols and up to 4-fold higher flavan-3-ols (monomers, oligomers, polymers) than shaded berries More bitter and astringent phenolics Wine quality? Mild water deficit increases β-damascenone, monoterpenes and volatile thiol precursors, reduces methoxypyrazines Moderate deficit reduces aroma potential Do: Impose mild water deficit Don t: Impose severe water deficit
Sun exposure: How much is too much? Low nitrogen It depends! Morning sun High nitrogen Afternoon sun Do: Remove leaves early! Be careful on west/south side! Prebloom Reduces cluster compactness, overcropping 2-4 weeks after fruit set Enhances sun exposure Don t: Too much, too late (veraison or later) Sunburn
Nitrogen (N): Moderation is a virtue 20 28 Soluble solids ( Brix) 19 18 17 16 15 no N 100 kg N/ha 14 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Yield (t/ha) r = -0.65*** Soluble solids ( Brix) 27 26 25 24 23 22 Hot Warm Ambient Cool r = -0.47*** 0 5 10 15 20 25 Growing tips/shoot at veraison More N Higher yield, more lateral growth, denser canopy Growing shoot tips compete with fruit Delayed ripening N increases malate and K + ph tends to increase N suppresses secondary metabolism Lower phenolics N (and S) enhance aroma precursors (volatile thiols) Keller et al. (2001, 2010)
High N increases disease susceptibility Berries per cluster 100 Powdery mildew severity (lesions/leaf) 50 0 8 6 4 2 Botrytized Healthy 0 0.2 1 2 Soil N (g/vine) Soil N 0 g/vine 3 g/vine 1.5 1.0 0.5 0.0 Berry weight (g) 0 Chardonnay Cabernet S. Do: Apply moderate rates of N (<60 lbs/acre) Don t: Allow N deficiency or excess
Potassium (K + ) and the ph conundrum K + may counter influence of organic acids (TA) by substituting for protons (H + ) in grapes Juice ph Both TA and K + drive variation in juice ph Late harvest TA (or if berries shrink), K + ph Crop load K + (phloem import) ph Juice ph is not very responsive to soil K + ( Malate ) Ca 2+ and K + compete for root uptake: Soil ph, juice ph Titratable acidity (g L -1 ) 22 20 18 16 14 12 10 8 Soluble solids (ºBrix) Year 1 Year 2 Year 3 6 4 2 14 16 18 20 22 24 26 28 Juice ph 3.9 3.8 3.7 3.6 3.5 3.4 3.3 Merlot Syrah Chardonnay 3.2 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Juice K + (g/l)
Harvest time: Decisions, decisions Harvest date impacts wine style Physiological maturity: Seeds able to germinate Water increases then decreases Concentration effect Natural sugar maximum at 23-25 Brix Tannins polymerize but may bind to cell walls Malate, methoxypyrazines decline early, not much late Terpenes continue to accumulate late Aroma potential but so do some volatile phenols Brett fodder? Some fruity ester precursors decline Fruit character? Danger: High ph Microbial stability? Do: Ripen grapes to winery specifications (communication!)
Machine harvest: Bad for wine quality? Sensory differences to hand harvest are rare Quality effect depends on presence of MOG and time to processing, not harvest method Pie and peas (leafy, gamey, meaty flavors) from mix of green and overripe berries, leaves, shoots, sometimes insects (e.g. Asian ladybeetle) Machine (not hand) harvest + long-distance transport: Continued metabolism Flavor loss Physical damage, maceration Protein extraction Juice protein concentration increases Heat instability (haze formation) increases Fining (bentonite) requirement increases Worse in warm temperatures
Summary: Vineyard practices for whites Do s: Apply mild water deficit (preveraison RDI) Remove leaves early, preferentially on east/north side Apply moderate rates of nitrogen Ripen (harvest) grapes according to winery specifications Don ts: Apply severe (or even moderate) water deficit Overexpose fruit, especially on west/south side Remove leaves late (veraison or later) Undercrop vines Apply very low or high rates of nitrogen Machine harvest and delay processing in warm weather