Wine quality is intrinsically tied to the quality of fruit provided for processing (Jackson and Lombard, 1993). Quality parameters typically measured in wine grapes are concentration of soluble solids, which determines final sugar and alcohol levels in wine; TA, which influences perception of acidity to the taster; and wine pH which can influence aging potential (Jackson, 2008). Additional factors that can influence final fruit “quality” are color (anthocyanins), tannins and phenolics, and aromatic volatiles (Jackson and Lombard, 1993).
The rate of ripening in grape berries is controlled dominantly by the interaction of time and temperature during the growing season (Keller, 2010), so much so, that classification systems for site-appropriateness for different grape varieties tend to rely heavily on heat accumulation during the growing season (Amerine and Winkler, 1944; Jackson and Cherry, 1988; Yau et al., 2014). The accumulation of anthocyanins is also related to heat. Higher temperatures, independent of sun exposure during véraison result in reduced color accumulation (Spayd et al., 2002). Similar results are seen with TA. An increase in juice pH is also associated with an increase in heat exposure (Spayd et al., 2002).
The inland Pacific northwestern United States, particularly eastern Washington, is a fast-growing region for wine and wine grape production; it is second in the United States only to California (U.S. Department of Agriculture, 2013). The region is typified by the risk for cold damage during the dormant season (Ferguson et al., 2011), dry summers with average annual rainfall of 7.15 inches (AgWeatherNet, 2014), warm summers with average monthly high temperatures from June to August of 77.5 to 88.3 °F (AgWeatherNet, 2014; Yau et al., 2014), and average daylengths of 14 to 15.75 h between June and August. This semiarid climate provides sufficient heat units to adequately ripen the varieties being grown there. However, sugar accumulation during the ripening period may be delayed by large diurnal temperature changes of upwards of 30 °F (AgWeatherNet, 2014). This delay can sometimes make it challenging to meet contract soluble solid specifications, which are typically between 24% and 28% for red wine grape varieties, before the first killing frost of the fall. Since natural maximum sugar accumulation is ≈25% (Keller, 2010), higher soluble solids concentration is generally due to dehydration; thus, extended hang time, preferably during warm, dry conditions, is necessary.
A machine patented in 2003, which applies heated air to plants, is currently being marketed to wine grape growers with the intention of improving fruit set, wine quality, and controlling diseases and pests (Agrothermal Systems, 2013). The relevant questions to pose are whether increasing the fruit’s exposure to short periods of heat during the growing season and through véraison could 1) override these potential limiters on ripening, 2) advance vine phenology and potential harvest date, 3) increase the potential soluble solids in fruit, or 4) enhance the fruit set and increase yield.
This field trial was designed to evaluate if this commercially marketed machine was able to increase fruit set, advance vine phenology, and improve final basic fruit quality metrics (soluble solids, pH, and TA) through the use of transient, applied heat in commercially grown red wine grape varieties in irrigated eastern Washington.
Agrothermal Systems2013Heat treatment offers wine growers better production. 6 May 2014. <http://agrothermalsystems.com>
AgWeatherNet2014Historic climate summary—WSU HQ weather station. 6 May 2014. <http://weather.wsu.edu>
BergqvistJ.DokoozlianN.EbisudaN.2001Sunlight exposure and temperature effects on berry growth and composition of Cabernet Sauvignon and Grenache in the central San Joaquin Valley of CaliforniaAmer. J. Enol. Viticult.5217
BurdenR.L.FairesJ.D.2000Numerical analysis. 7th ed. Brooks-Cole Belmont CA
FergusonJ.C.TararaJ.M.MillsL.J.GroveG.G.KellerM.2011Dynamic thermal time model of cold hardiness for dormant grapevine budsAnn. Bot. (Lond.)107389396
GuS.JacobsS.McCarthyB.GohilH.L.2012Forcing vine regrowth and shifting fruit ripening in a warm region to enhance fruit quality in ‘Cabernet Sauvignon’ grapevines (Vitis vinifera L.)J. Hort. Sci. Biotechnol.87287292
IlandP.EwartA.SittersJ.MarkidesA.BruerN.2000Techniques for accurate chemical analysis and quality monitoring during winemaking. Wine Promotions Campbell Town Australia
JacksonD.I.CherryN.J.1988Prediction of a district’s grape ripening capacity using latitude-temperature index (LTI)Amer. J. Enol. Viticult.391928
JacksonD.I.LombardP.B.1993Environmental and management practices affecting grape composition and wine quality: A reviewAmer. J. Enol. Viticult.44409430
JacksonR.S.2008Wine science: Principles and applications. Elsevier-Academic Press Burlington MA
KellerM.2010The science of grapevines: Anatomy and physiology. Elsevier-Academic Press Burlington MA
KliewerW.M.1977Influence of temperature, solar radiation, and nitrogen on coloration and composition of Emperor grapesAmer. J. Enol. Viticult.2896103
MeierU.2001Growth stages of mono- and dicotyledonous plants. BBCH Monogr. Blackwell Wissenschafts-Verlag Berlin Germany
SmartR.1985Principles of grapevine canopy microclimate manipulation with implications for yield and quality. A reviewAmer. J. Enol. Viticult.36230239
SpaydS.E.TararaJ.M.MeeD.L.FergusonJ.C.2002Separation of sunlight and temperature effects on the composition of Vitis vinifera cv. Merlot berriesAmer. J. Enol. Viticult.53171182
TararaJ.M.JungminL.SpaydS.E.ScagelC.F.2008Berry temperature and solar radiation alter acylation, proportion, and concentration of anthocyanin in Merlot grapesAmer. J. Enol. Viticult.59235247
U.S. Department of Agriculture2013Non-citrus fruits and nuts: 2012 preliminary summary. 1 May 2014. <http://usda.mannlib.cornell.edu/usda/current/NoncFruiNu/NoncFruiNu-01-25-2013.pdf>
YauI.-H.DavenportJ.R.MoyerM.M.2014Developing a wine grape site evaluation decision support system for the inland Pacific NorthwestHortTechnology248898