Research Publications
Books Co-authored
Zoecklein, B.W., K.C. Fugelsang, Gump, B.H. 2001. Análisis y Producción de Vino. Acribia, S.A. Zaragoza, España.
Zoecklein, B.W., Fugelsang, K.C., Gump, B.H. 1999. Wine Analysis and Production. Chapman & Hall, New York, NY. 621p.
Zoecklein, B.W., Fugelsang, K.C., Gump, B.H., Nury, S.F. 1990. Production Wine Analysis. VonNostrand and Reinhold, New York, NY. 475p.
Book Chapters
Zoecklein, B.W., K.C.Fugelsang and B.H. Gump. 2005. In Press. Analytical Techniques in Wine and Distillates. Handbook of Enology, Haworth Press, Inc. N.Y.
Gump, B.H., Zoecklein, B.W., Fugelsang, K.C. 2000. Prediction of prefermentation nutritional status of grape juice--The formol method. In: Methods in Biotechnology, Vol. 14: Food Microbiology Protocols. J.F.T. Spencer , A. L. Ragout de Spencer (eds.) Humana Press, Inc. Totowa NJ. pp. 283-296.
Zoecklein, B.W., Gump, B.H., Fugelsang, K.C. 2000. Nutritional status of grape juice. In: methods in Biotechnology, Vol. 14: Food Microbiology Protocols. J.F.T. Spencer, A. L. Ragout de Spencer (eds.) Humana Press, Inc. Totowa NJ. pp. 413-423.
Titles and Abstracts
Discrimination of Wines Produced from Cabernet Sauvignon Grapes Treated with Aqueous Ethanol Post-Bloom Using an Electronic Nose
Amanda Martin1, Kumar Mallikarjuan2, Bruce Zoecklein3*
1, 3Enology-Grape Chemistry Group, Virginia Tech, Blacksburg, Virginia, USA, 24060-0418
2Associate Professor, Biological Systems Engineering, Virginia Tech, Blacksburg, VA 24061
*Corresponding Author [Fax: 540-231-9293; Email: ]
Abstract: This study evaluated two electronic nose systems (conducting polymer and surface acoustic wave-based) to differentiate volatiles of grapes and wines treated with an aqueous ethanol spray (5% v/v) at véraison. Ethanol spray induced fruit ethylene production immediately post-treatment, which then declined progressively. Electronic nose evaluations of grape volatiles were compared with Cabernet franc and Merlot physicochemistry and wine aroma sensory data. Canonical discriminant and principal component analysis found that both electronic nose systems and the physicochemical measures (Brix, TA, pH, color intensity, hue, total phenols, glycosides, and berry weight) were able to discriminate between ethanol-treated and untreated grapes and wines for both cultivars. Grape physicochemical treatment differences were due mainly to variations in hue, phenolic-free glycosides, and total phenols. Aroma sensory evaluations differentiated between ethanol treatments and control for Merlot, but not Cabernet franc wines.
Electronic Nose Analysis of Cabernet Sauvignon (Vitis vinifera L.) Grape and Wine Volatile Differences during Cold Soak and Post-Fermentation
Denise M. Gardner1, Bruce W. Zoecklein2*, and Kumar Mallikarjunan3
1Graduate Student and 2Professor, Enology-Grape Chemistry Group
3Associate Professor, Biological Systems Engineering, Virginia Tech, Blacksburg, VA 24061
*Corresponding Author [Fax: 540-231-9293; Email: ]
In press: Am. J. Enol. Vitic. 62:1. 2011
Abstract: Cold soak is a pre-fermentation maceration process at cold temperatures, traditionally used to enhance red wine color. This study monitored changes in Vitis vinifera L. cv. Cabernet Sauvignon volatiles using a commercial conducting polymer electronic nose (ENose) during a five day cold soak and post-fermentation. PCA distribution of juice volatiles detected by the ENose during cold soak showed PC1 accounted for 95.7% of the variation. Various volatile associations were made with specific ENose sensors. In comparison, PCA distribution of must chemistries had 52.4% of the variation accounted by PC1. The PCA distribution of wine volatiles detected by GC-MS showed PC1 accounted for 97.1% of the variation between control and cold soak treatments where control wine volatiles were associated with several ethyl esters, while cold soak wine volatiles were associated with diethyl succinate, isovaleric acid, benzyl alcohol, 3-methyl butanol, cis-3-hexenol, gamma-nonalactone, benzaldehyde, 2-methyl propanol, phenethyl acetate, 1-octanol, beta-damascenone, terpinene-4-ol, gamma-butyrolactone, ethyl acetate, hexanoic acid, citronellol, phenethyl alcohol, and n-butanol. Comparatively, PC1 accounted for 100% of the total variance when using the ENose to measure volatile composition. Sensory evaluation did not demonstrate significant differences in aroma between control and cold soak wines. This study demonstrates differences in volatile chemistry between control and cold soak wines, as well as the ability to use a conducting polymer as a simple tool for analysis of volatiles.
Electronic Nose Evaluation of the Effects of Canopy Side on Cabernet franc (Vitis vinifera L.) Grape and Wine Volatiles
Yamuna S. Devarajan1, Bruce W. Zoecklein2*, and Kumar Mallikarjunan3
1Graduate Student and 2Professor, Enology-Grape Chemistry Group
3Associate Professor, Biological Systems Engineering, Virginia Tech, Blacksburg, VA 24061
*Corresponding Author [Fax: 540-231-9293; Email: ]
In press: Am. J. Enol. Vitic. 62:1. 2011
Abstract: The effect of grapevine canopy side (north versus south, and east versus west) on grape and wine volatiles of Cabernet franc was evaluated using two electronic nose systems (conducting polymer-based and surface acoustic wave-based) during two growing seasons. Data from three sampling dates for each season from both electronic noses were compared with physicochemistry and wine aroma sensory evaluations. Univariate and multivariate statistical analyses generally indicated grape physicochemistry indices were not able to differentiate (p>0.05) between canopy sides consistently across growing seasons and sampling dates. Both of the electronic nose systems corroborated 100% discrimination of canopy sides for grapes and wine using canonical discriminant analysis. On average, the surface acoustic wave-based electronic nose was able to explain <50% of variation for grapes and <60% for wine using the first principal component, compared to >80% for the conducting polymer-based electronic nose. Wine aroma sensory evaluation was able to differentiate canopy sides in three of four evaluations.
Monitoring the effect of ethanol spray on Cabernet franc and Merlot grapes and wine volatiles using electronic nose systems
Bruce W. Zoecklein1*, Yamuna S. Devarajan2, Kumar Mallikarjunan3, and Denise M. Gardner2
1 Professor, Head, Enology-Grape Chemistry Group,
2Graduate Student and 3Associate Professor, Biological Systems Engineering, Virginia Tech, Blacksburg, VA 24061
*Corresponding Author [Fax: 540-231-9293; Email: ]
International Journal of Food Engineering Vol 4: 2 Article 14. 2008
Abstract: Wine discrimination and analysis is typically done through chemical analysis and sensory evaluation by a trained panel. Both of these methods are proven to be successful in wine discrimination, but require extensive preparation, time and money. The electronic nose is an objective, rapid-analysis tool that has been used in the food industry for a number of applications. The purpose of this study was to determine if an electronic nose can accurately discriminate between Cabernet Sauvignon (Vitis vinifera L.) wines made from grapes that have received different pre-harvest but post-bloom spray treatments to enhance growth.
Aqueous ethanol, which has been shown to impact fruit maturity, was sprayed on the grape clusters at 13 weeks post bloom in different concentrations (control, 5% and 10% v/v). Chemical analysis was able to accurately discriminate between the wines produced from these grapes. Triangle difference testing by a consumer panel was not able to differentiate between the different treatments. The electronic nose data was able to accurately identify the control group and the 5% EtOH treatment 90% of the time. Placement of the 10% EtOH group was only 13% correct. The results show the promising potential for an electronic nose to discriminate between control and treated wine samples.
Electronic Nose Evaluation of Cabernet Sauvignon Fruit Maturity
Ahmad I. Athamneh,1 Bruce W. Zoecklein,2* and Kumar Mallikarjunan3
1 Graduate Student, and 3 Associate Professor, Department of Biological Systems Engineering;
2 Professor, Enology-Grape Chemistry Group, Department of Food Science and Technology, Virginia Tech, Blacksburg, VA 24061, USA.
* Corresponding author [ ; fax (540) 231-9293]
Abstract: The ability of an electronic nose to classify cabernet sauvignon (Vitis vinifera L.) fruit based on maturity levels was investigated over two seasons. Maturity of samples collected 18, 19, and 20 weeks post-bloom was evaluated by measuring berry weight, pH, Brix, titratable acidity, total phenols, color intensity, hue, total anthocyanins, and total and phenol-free glycosides. Results were compared, using discriminant and canonical discriminant analysis, with analysis of headspace volatiles via a hand-held electronic nose. The electronic nose was able to determine differences among the three sample groups in both seasons. Additionally, in one season electronic nose measurements were compared to chemical analyses of samples collected from east and west sides of north-south oriented vineyard rows. Results demonstrated the ability of the electronic nose to distinguish fruit from vine canopy sides. Field measurements demonstrated the potential for the electronic nose as a rapid, non-destructive tool for evaluating grape maturity.
Key words: grape maturity, cabernet sauvignon, electronic nose, grape volatiles, grape aroma
Effect of Délestage with Partial Seed Deportation on Merlot and Cabernet Sauvignon Wines
Bruce W. Zoecklein1*, Lisa M. Pélanne, and Sandy S. Birkenmaier
1Professor and Head, Wine/Enology-Grape Chemistry Group, Department of Food Science and Technology (0418), Virginia Tech, Blacksburg, VA 24061.
*Corresponding author [Fax: 540-231-9293; email ]
Abstract: Délestage (rack and return) involving partial seed deportation was compared with merlot produced by manual cap punching for three seasons, and cabernet sauvignon produced by mechanical punch-down (pigeage) systems for one season. Fermentation reduced monomeric pigment color and increased polymeric pigment color for all treatments. Délestage wines generally had more large polymeric pigment color than cap-punched or pigeage wines. Total glycosides increased during cold soak and fermentation, and were in greater concentration in cap-punched merlot, and similar among cabernet sauvignon treatments. Discrimination testing (triangle difference analysis) demonstrated merlot wines generally differed in aroma and/or flavor. Cabernet sauvignon wines differed in both aroma and flavor.
Key words: délestage, rack and return, polymeric pigments, glycosides, seed removal, volatile compounds
Effect of Vertical Shoot-Positioned, Smart-Dyson, and Geneva Double Curtain training systems on Viognier grape and wine composition
Bruce W. Zoecklein1*, Tony K. Wolf2, Lisa Pélanne1, M. Kay Miller2 and Sandy S. Birkenmaier1
1 Enology-Grape Chemistry Group, Department of Food Science and Technology, Virginia Tech, Blacksburg, VA 24061 USA
2 AHS Jr. Agricultural Research and Extension Center, Virginia Tech, Winchester, VA 22602 USA
*Corresponding author
Abstract: Viognier grapes grown in northern Virginia, USA, and resultant wines were evaluated as a function of training system. Treatments included Vertical shoot-positioned (VSP), Smart-Dyson (SD), and Geneva double curtain (GDC), with vines of all treatments spaced 2.4 m apart in 3.0 m-wide rows. In addition to increased cluster numbers and crop yield, GDC training generally increased fruit zone sunlight interception and fruit exposure, while it decreased cane pruning weights per m of cordon, compared to SD and VSP. Crop adjustments were made between bloom and véraison in six seasons, to result in average yields of 10.5 kg/vine (GDC), 9.9 kg/vine (SD) and 6.0 kg/vine (VSP), with the lower SD canopy bearing 30 to 40% less crop than the corresponding upper SD canopy. Crop loads (yield/cane pruning weight) were generally between 4 and 12; GDC crop load approached 20 in three seasons, while SD approached 14 in one. Leaf area per crop ratio was determined one season and exceeded 1.8 cm2/kg of fruit for all systems. Fruit was harvested at similar Brix values, with differences in berry weight, pH, titratable acidity, malic and tartaric acids among treatments generally not significant. Volatile compounds were analyzed using headspace solid-phase microextraction GC-MS. Fruit showed consistent differences in linalool, α-terpineol, β-damascenone and n-hexanol concentrations among training systems. SD had the highest concentration of most free volatiles quantified in both juice and wines, while GDC frequently produced wines that had the highest concentration of phenol-free glycosides. Triangle difference sensory testing demonstrated identified differences between GDC and SD in wine aroma and flavor, and between VSP and SD in flavor. GDC wines generally had higher fruity and floral aromas compared to the other systems.
Keywords: Viognier, training system, volatile compounds, glycosides, wines
Effected of Vertical Shoot-Positioned, Smart-Dyson and Geneva Double Curtain Training on Cabernet Franc Grape and Wine Composition
B.W. Zoecklein1, T.K. Wolf2, L. Pélanne1, M.K. Miller2 and S.S. Birkenmaier1
1 Enology-Grape Chemistry Group, Department of Food Science and Technology, Virginia Tech, Blacksburg, VA 24061 USA
2 AHS Jr. Agricultural Research and Extension Center, Virginia Tech, Winchester, VA 22602 USA
Abstract:Cabernet franc grapes grown in Virginia, USA, for six seasons, and resultant wines were evaluated as a function of training system, which included Vertical Shoot Positioned (VSP), Smart-Dyson (SD), and Geneva Double Curtain (GDC). Vines were spaced 2.4 m apart in 3.0 m-wide rows. Crop adjustments were made between bloom and véraison each year to result in average yields of 11.3 kg/vine (GDC), 12.2 kg/vine (SD) and 8.3 kg/vine (VSP), The lower SD canopy bore about 50% less crop than the corresponding upper SD canopy. Leaf area per crop ratios, determined in one season, averaged 11.2 cm2/g (VSP), 11.8 cm2/g (combined SD canopies), and 23.4 cm2/g for the GDC vines. GDC training generally increased fruit zone sunlight interception and decreased cane pruning weight per meter of cordon, compared to the other two systems. Leaf layers in the fruit zones were measured near veraison in two seasons and ranged from about 1.5 to 2.0, with no appreciable differences between training systems. Percent exposed fruit, measured at the same times, did not vary significantly among training systems.
Keywords: Cabernet franc, training system, volatile compounds, glycosides, wines
Effects of Capture and Return on Chardonnay (Vitis vinifera L.) Fermentation Volatiles.
Hodson, Emily and B.W. Zoecklein. 2004.
M.S. Thesis
Effectiveness of a capture and return system for the partial retention of fermentation volatiles, as a means of improving white wine quality, was evaluated. Twenty-three aroma-active volatiles including ethyl esters, acetate esters, fusel alcohols, and fatty acids, were quantified using head-space solid phase microextraction with GC/MS. Volatile analysis of fermentations maintained at 15°C demonstrated a trend of increased concentrations of fusel alcohol acetates, fatty acids and higher alcohols in treatment wines. When fermentation temperature was maintained at 30°C there was increased concentration and retention of fusel alcohols, fatty acids and higher alcohols compared to 15°C. Sensory analysis of wines fermented at 15°C, using triangle difference testing, indicated variable differences in aroma among treatments.
Effect of Training Systems on Viognier (Vitis vinifera L.) Musts and Wines
L. Millard*, B. W. Zoecklein, and T. Wolf
*M.S. Candidate
Vitis vinifera L. cv. Viognier grapes were grown in Northern Virginia for three seasons under three different training and trellising systems: Vertical Shoot Positioned (VSP), Smart Dyson (SD), and Geneva Double Curtain (GDC), and evaluated for the effects on grape and wine aroma and flavor compounds. Treatments were set up in a randomized block, split-plot design. Fruit was harvested at the same Brix, and no difference in pH or berry weight was observed. VSP-trained vines had the lowest crop load and lowest light levels in the fruit zone. Seventeen compounds were analyzed using headspace solid-phase microextraction, GC-MS. Fruit showed differences in linalool, a-terpineol, ß-damascenone, and n-hexanol concentrations as a function of training system. Wines showed differences in both grape-derived and fermentation-derived volatiles. Wines from VSP-trained vines had lower phenol-free glycosides and lower free volatiles than either GDC or SD. SD typically had the greatest concentration of glycosides and free volatiles. Triangle difference sensory testing demonstrated differences between GDC and SD in wine aroma and flavor, and differences between VSP and SD in flavor for two of the three seasons.
Effect of Ethanol Sprays on Cabernet Sauvignon (Vitis vinifera L.) Grape Maturity and Wine Quality
Kerem Baki* and Bruce W. Zoecklein
*M.S. Candidate
Aqueous ethanol (5% or 10% v/v) was sprayed on Cabernet Sauvignon (Vitis vinifera L.) clusters at 11 and/or 13 weeks post-bloom to evaluate the impact on fruit maturity. Fruit ethylene concentration increased as a result of sprays. The week-13 spray fruit at 17 and 21 weeks post-bloom had higher color intensity, total anthocyanins, and total and phenol-free glycosides than controls, while differences in berry weights, Brix, TA, and pH were not observed. Wine total and copigmented anthocyanins, polymeric pigments, total phenols, color intensity, and total and phenol-free glycosides were higher in week-13 ethanol spray treatments than control wines. Differences in volatiles were noted in musts and wines of treatments, compared to controls. Triangle difference discrimination testing indicated differences in wine aroma between control and 5% week-13 ethanol spray, and in flavor between 5% week-13 and 10% week-13 ethanol spray treatment.
Effects of Extended Cold Soaking on the Overall Wine Quality of Vitis vinifera L. cv. Cabernet Sauvignon
Baki*, K. and B. W. Zoecklein. 2004.
*M.S. Candidate
The overall wine quality of four-day vs. twenty-day cold soaking of Vitis vinifera L. cv. Cabernet Sauvignon must was evaluated. The twenty-day cold soak wines exhibited lower TA, malic acid, and tartaric acid, with higher pH and ethanol (p < 0.05). Also, the twenty-day cold soak wines had higher total anthocyanins, total phenols, flavone cofactor and greater color hue, however lower color intensity (p < 0.05). Polymeric pigments, total and phenol-free glycosides were lower in the twenty-day cold soak wines (p < 0.05). There was no difference in copigmented anthocyanins between treatments (p < 0.05). Benzyl alcohol and n-hexenol levels were greater in the twenty-day cold soak wines, whereas citronellol, acetic acid, ethyl octanoate, and ethyl dodecanoate levels were lower in the twenty-day cold soak wines (p < 0.05). Sensory evaluation, via triangle test, of wine indicated differences between the treatments concerning aroma (a = 0.001, ß = 0.05, Pd = 30%), and flavor (a = 0.01, ß = 0.05, Pd = 30%).
Effects of Prohexadione-calcium on Grape Yield Components and Fruit and Wine Composition
D. Lo Giudice, T. Wolf, and B.W. Zoecklein. Am. J. Enol. Vitic. 51: 73-83. 2004.
Prohexadione-calcium (prohexadione-Ca) was applied to field-grown Cabernet franc, Cabernet Sauvignon, Chardonnay and Seyval to evaluate rates and timing effects on fruit yield components and on fruit and wine composition. Application (250 mg/L) to single clusters of Cabernet Sauvignon and Chardonnay at modified Eichhorn and Lorenz (E-L) stages 21 and 23 decreased fruit set, whereas applications at E-L stages 26, 27, and 29 reduced berry weight, with no impact on fruit set. Berry weight reduction correlated to increased color intensity (420 nm + 520 nm), total anthocyanins, total phenols and phenol-free glycosyl-glucose (PFGG) in Cabernet Sauvignon. Prohexadione-Ca increased Cabernet franc must color intensity, total anthocyanins and total phenols, despite having minimal effects on berry weight or crop yield.
Aroma and flavor triangle difference tests did not distinguish treatment differences with young Cabernet franc wines. This first report of prohexadione-Ca effect on grape reproductive development illustrates fruit thinning and berry size reductions. The impact on must quality was generally positive while the full impact on wine quality remains equivocal, but worthy of further evaluation.
Effect of Délestage with Partial Seed Deportation on Merlot and Cabernet Sauvignon Wines
Bruce W. Zoecklein, Brad McCarthy, and Karen Vaillant.
Tannin management and understanding of quantitative and qualitative influences of grape phenols on wine quality are important in premium wine production. This study compared délestage (rack and return) with partial seed deportation, with Merlot produced by manual cap punching and Cabernet Sauvignon produced by a mechanical pigeage system. Wine pigment profiles were impacted by treatment. Fermentation reduced monomeric and increased polymeric pigment. Délestage wines generally had a higher percentage of large polymeric pigments than control wines. Total glycosides increased during cold soak and fermentation, and were in greater concentration in the control Merlot wines and identical in the Cabernet Sauvignon treatments. Merlot phenol-free glycosides were in greater concentration in the délestage-produced wines at dejuicing. Discrimination testing (triangle difference analysis) demonstrated Merlot wines differed in aroma and/or flavor in two of three vintages. Cabernet Sauvignon wines were perceived to differ in both aroma and flavor. Headspace solid-phase microextraction GC/MS analyses generally demonstrated higher concentrations of 3-methylpropanol, 3-methylbutanol, ethylhexanoate, and n-hexanol in control wines for both cultivars.
Effect of Fermentation, Postfermentation, and Postbottling Heat Treatment on Cabernet Sauvignon Glycoconjugates
Anna K. Mansfield and Bruce W. Zoecklein
American Journal of Enology and Viticulture. 54(2):99-104. 2003.
In Cabernet Sauvignon must, total and phenol-free glycosides (expressed as glycosyl-glucose) rose during fermentation while skin concentrations dropped. Wines were heated postfermentation, prior to dejuicing (rising 2 to 3°C per day from 23 to 42°C, and held for one day at 42°C), or after bottling (at 42°C for 21 days) to determine the effect on total glycosides, glycosidic fractions, and anthocyanin complexing. Pre-dejuicing thermal vinification resulted in higher total (12%) and phenol-free (18%) glycosides. Large polymeric pigments rose 208% and small polymeric pigments rose 41%. Skins had lower total glycosides (-16%), and no significant difference in phenol-free glycosides. Postbottling heat treatment resulted in lower total (-15%) and phenol-free (-16%) glycosides, and increased hue (25%). Large polymeric pigments increased 62% compared to control wines.
Quantification of Glycosidase Activity in Selected Strains of Brettanomyces bruxellensis and Oenococcus oeni
A. K. Mansfield, B. W. Zoecklein and R. S. Whiton
American Journal of Enology and Viticulture. 53(4): 303-307. 2003.
Brettanomyces bruxellensis and lactic acid bacteria are common micoroorganisms capable of modifying wine aroma and flavor. The activities of ß-glucosidase against p-nitrophenyl-ß-D-glucopyranoside was determined in a model system for 14 strains of Brettanomyces bruxellensis yeast and 9 strains of lactic acid bacteria (Oenococcus oeni). All Brettanomyces strains and 7 Oenococcus strains exhibited enymatic activity against this substrate. B. bruxellensis ß-glucosidase activity was primarily intracellular; O. oeni showed some extracellular activity. Strains showing activity greater than 1000 nmole mL-1 g dry cell mass-1 24 hr-1 for Brettanomyces, or 100 nmole-1 mL-1 g dry cell mass-1 24 hr-1 for Oenococcus, were evaluated for their effect on Viognier grape glycosides. Neither genus were active on Viognier grape glycosides.
Population Dynamics and Effects of Brettanomyces bruxellensis Strains on Pinot noir (Vitis vinifera L.) Wines
B. W. Zoecklein and K. C. Fugelsang.
American Journal of Enology and Viticulture. 54(4):294-300. 2003.
Replicated, sterile-filtered Pinot noir (Vitis vinifera L.) wines were individually inoculated with one of eight strains of Brettanomyces bruxellensis (Ave, M, 213, 216, Vin 1, Vin 3, Vin 4, and Vin 5) at initial viable cell numbers 50 cfu/mL. In two separate studies, population changes were monitored for 23 months or until cell densities declined to 30 cfu/mL. Significant variation was noted in both growth rate and peak population densities among strains. The concentration of select volatile components was monitored using solid phase micro-extraction, GC/MS. Significant increases in the concentration of 4-ethyl phenol occurred after accumulated cell populations reached 25 x104 cfu /mL. Brettanomyces - inoculated wines were found to have detectable concentrations of ethyl-2-methylbutyrate, isoamyl alcohol, ethyldecanoate, isovaleric acid, guaiacol, 2-phenylethanol, 4-ethylguaiacol, and 4-ethylphenol, with significant differences in their concentrations among strains. Duo-trio testing demonstrated sensory differences between the control and all inoculated wines, and between wines inoculated with strains Ave and Vin 5, M and 216, and M and Vin 4. Quantitative descriptive analysis suggested that all Brettanomyces wines had a lower perception of fruity descriptors than did uninoculated control wines.
Comparison of Analytical Methods for Prediction of Prefermentation Nutritional Status of Grape Juice
B. H. GUMP, B. W. ZOECKLEIN, K. C. FUGELSANG, and R. S. WHITON
American Journal of Enology and Viticuture. 53(4): 325-329. 2002.
Five methods for evaluating nitrogen status were compared using 70 Cabernet Sauvignon juice samples: nitrogen by o-phthalaldehyde (NOPA), arginine NOPA, enzymatic ammonia, Formol, and high-performance liquid chromatography (HPLC). Parallel recovery studies using model solutions of various amino acids and ammonia, presented singly and in combination, were also conducted. The results from two fruit-processing methods were compared using immature and mature berries. NOPA measurements were significantly higher in mature, pressed whole berry-derived samples, compared with homogenized juice. Adjustment of formaldehyde pH prior to analysis was found to be critical to consistency of the Formol method. Average amino acid recoveries for the Formol titration ranged from 82 to 99%. Average recovery for proline was 16.9 ± 0.4%. Ammonium nitrogen was also recovered (84 ± 3%) in the Formol procedure. Formol results trended significantly with NOPA. The correlation coefficient between Formol and NOPA plus NH4+ was 0.87, with the Formol values being higher. The average deviation between the Formol and HPLC plus NH4+ was 7.3%, while that between the NOPA plus NH4+ and HPLC plus NH4+ was -7.3%.
Determination of Ethyl Carbamate in Wine by Solid-Phase Microextraction and Gas Chromatography/Mass Spectrometry
R. S. Whiton and B. W. Zoecklein
American Journal of Enology and Viticulture. 53: 60-63. 2002.
A method for the rapid determination of ethyl carbamate in wine by headspace solid-phase microextraction with detection by gas chromatography/mass spectrometry has been developed. The analysis parameters of fiber coating, extraction time, and sample temperature have been optimized. The optimized method of headspace sampling with a Carbowas/divinylbenzene fiber for 30 minutes has been found to be reproducible and linear from 10 to 80 µg/L. The method is simple, requires little operator effort, and can be automated.
Evaluation of Glycosyl-Glucose Analytical Methods for Various Glucosides
R. S. Whiton and B.W. Zoecklein.
American Journal of Enology and Viticulture. 53(4): 315-317. 2002.
The accuracy of two methods for determination of glycosyl-glucose (GG) in grapes and wine has been tested using several glycoside standards containing alkyl, benzenoid, and phenolic aglycones. The total GG method using C-18 sorbent material was found to exhibit poor recoveries of benzenoid glycosides. The recoveries for the phenol-free GG method were satisfactory for alkyl and benzenoid glycosides. In addition, an adaption of the phenol-free GG method to 96-well microplate format has been demonstrated.
Measurement of 3-Alkyl-2-Methoxypyrazine by Headspace Solid-Phase Microextraction in Spiked Model Wines
P. J. Hartmann, H. M. McNair, and B. W. Zoecklein
American Journal of Enology and Viticulture. 53(4): 285-288. 2002.
The effect of wine matrix ingredients and conditions on the headspace sampling of 3-alkyl-2-methoxypyrazines was investigated with solid-phase microextraction (SPME) and capillary gas chromatography, using a nitrogen phosphorus detector. Changes in the recovery of 3-ethyl-, isopropyl-, sec-butyl-, and isobutyl-2-methoxypyrazines from the static headspace of synthetic wine matrices spiked with 5 µg/L of each analyte were investigated and reported as a function of SPME fiber type, extraction time, and temperature. The influence of pH, ethanol, phenolics, and oak was studied. Divinylbenzene/carboxen/polydimethylsiloxane (PDMS) SPME fibers at an extraction temperature of 50°C for 30 minutes with 30% (w/v) added sodium chloride resulted in the highest analyte recoveries. Although PDMS (100 µm) SPME fibers at an extraction temperature of 35°C for 30 minutes with 30% (w/v) added sodium chloride resulted in lower analyte recoveries, the fiber remained functional for 50 to 75 analyses after other coatings deteriorated. Changing the sample ethanol concentration from 0 to 20% (v/v) resulted in an exponential decrease in the recovered analytes. Below pH 2, there was extensive loss of the analytes in the headspace. No measurable impact on alkyl-methoxypyrazine headspace concentrations was observed with exposures to selected phenolics and to oak.
Effect of Sour Rot on the Composition of White Riesling (Vitis vinifera L.) Grapes
B.W. Zoecklein, J.M. Williams and S.E. Duncan.
Small Fruits Review. 1(1): 63-77. 2000.
Sour rot on White Riesling grapes increased Brix, glucose to fructose ratio, titratable acidity, tartaric acid, glycerol and gluconic acid while reducing berry weight. Sour rot did not influence pH, acetic acid, ethanol or laccase activity. Two classes of grape aroma components, potentially volatile terpenes (PVT) and free volatile terpenes (FVT), were not influenced by sour rot. However, quantitative differences in selected free aroma compounds between "clean" and rot degradated fruit were observed. Rot reduced free geraniol, nerol, and linalool concentrations, while increasing trans-furan linalool oxide, benzyl alcohol, 2-phenylethanol, 2-methyl-1-propanol, and 3-methyl-1-butanol.
Evaluation of the Phenol-Free Glycosyl-Glucose Determination
B. W. Zoecklein, L. S. Douglas and Y. W. Jasinski
American Journal of Enology and Viticulture, 51 (4): 420-423. 2000.
Grape glycosides are, in part, an important source of varietal wine aroma and flavor. Aglycones may be aliphatic residues, monoterpenes, sesquiterpenes, norisoprenoids or phenolic compounds. Studies were undertaken to evaluate and compare methods of isolating non-phenolic or phenol-free glycosides. Phenol- free glycoside fractions were obtained by three isolation methods: the addition of polyvinylpolypyrrolidone (PVPP), isolation at pH 10.0 using C18 solid phase extraction cartridges, or isolation at pH 13.0 using OasisTM hydrophilic-lipophilic balance (HLB) extraction cartridges. Glycoside separation using the Oasis cartridges reduced the phenol content of the phenol-free glycoside fractions in Chardonnay juice and wines by an average of 49.7 % compared to 33.6 % using C18. The differences in the phenol concentration in the phenol-free glycoside fractions with these two methods were greater with red juices and wines. For the Cabernet Sauvignon juice and wines the phenol concentration was reduced by an average 103% and 73% using the Oasis and C18 cartridges, respectively.
Optimization of Headspace Solid-Phase Microextraction for Analysis of Wine Aroma Compounds
R. S. Whiton and B. W. Zoecklein
American Journal of Enology and Viticulture, 51(4): 379-382. 2000.
Headspace solid-phase microextraction (SPME) is a fast and simple sampling method for analysis of volatile compounds, but quantitation can be affected by sample matrix and sampling conditions. Sampling time and temperature are particularly important in controlling analyte response, and the effects vary for different compound classes and volatilities. Tests with model solutions containing a range of typical wine volatiles show that increasing temperature and sampling time can increase sensitivity for higher boiling polar compounds but decrease sensitivity for very volatile compounds. Sample matrix elements such as ethanol concentration can also have different effects on the responses of different compounds.
Supercritical Fluid Extraction of Grape Glycosides
M. Palma, L.T. Taylor, B.W. Zoecklein, and L.S. Douglas.
Journal of Agricultural and Food Chemistry. 48(3):775-779. 2000.
Supercritical fluid extraction with methanol-modified CO2 was used to extract glycosides from grapes. An optimization design involving 12 extraction variables was applied to achieve quantitative recoveries. The most important factor was the amount of organic modifier, a consequence of the high degree of glycoside polarity. By the proposed method, the total time of analysis can be decreased relative to that required for more conventional extractions. The full method can also be automated.
A Survey of Virginia Apple Cider Producers' Practices
J. R. Wright, S. S. Sumner, C. R. Hackney, M. D. Pierson, and B. W. Zoecklein.
Dairy Food Environ. Sanitation. 20:190-195. 2000.
During the summer of 1998, questionnaires were mail to 90 Virginia cider producers in an effort to gain a better understanding of current production practices. The questionnaire covered production levels and sales location, orchard management, facilities, processing, preservation measures, and additional safety measures. Most operations are seasonal, produce less than 5,000 gallons annually, and confine sales to Virginia. Although few producers fertilize orchards with manure, animals often graze nearby, and producers sometimes use drop and damaged apples. Most producers process in separate, enclosed areas and test water for bacteria. All indicated that equipment and faciltities are cleaned and sanitized daily. The majority of producers sort apples before washing, use refrigerated storage, prevent contamination during storage, and wash and brush apples before pressing. However, most use only water for washing, and only 37% use a chemical sanitizer following the wash step. Few pasteurize cider or add preservatives; however, almost all store cider refrigerated or frozen. Although only 25% have an operating HACCP program, most have considered implementing HACCP. Few producers conduct microbiological tests on cider or include code numbers or expiration dates on labels. Finally, most producers expressed an interest in the use of alternative processing technologies to help assure the safety of their cider.
Reduction of Escherichia coli O157:H7 on Apples Using Wash and Chemical Sanitizer Treatments.
J. R. Wright, S. S. Sumner, C. R. Hackney, M. D. Pierson, and B. W. Zoecklein
Dairy Food Environ. Sanitation 20:120-126. 2000.
Unpasteurized apple cider has been implicated in outbreaks involving Escherichia coli O157:H7. Apples used for cider production may become contaminated by contact with animal feces. The objective of this study was to determine if wash and sanitizer treatments can reduce or eliminated E. coli O157:H7 on apples. Apples were subjected to six wash or sanitizing treatments: 200 ppm hypochlorite, a commercial phosphoric acid fruit wash, 5% acetic acid, 5% acetic acid followed by 3% hydrogen peroxide, a commercial peroxyacetic acid sanitizer, and distilled water. Apples that had been inoculated with a five-strain mixture (~2 x 103 CFU E. coli O157:H7 per cm2) were immersed in treatments for two minutes. The water wash, which caused reductions of only 1.1 logs when cells were enumerated on Sorbitol MacConkey agar (SMAC) and 0.6 logs when Tryptone Soy agar with 1% pyruvic acid (TSAP) was used, and was the only treatment that did not differ significantly from the no-wash control. Hypochlorite caused reductions of 2.1 logs on both media but differed significantly from the most effective treatment, 5% acetic acid. Phosphoric acid resulted in a reduction of 2.9 logs when cells were enumerated on SMAC buto only 2.3 logs when TSAP was the recovery medium, indicating that the treatment caused some sublethal injury. For the acetic acid/hydrogen peroxide treatment, reduction was 2.5 log with SMAC and 2.4 logs with TSAP. The 5% acetic acid and peroxyacetic acid solutions were the most effective, causing reductions of 3.1 logs and 2.6 logs respectively, without apparent sublethal injury.
Efficacy of Ultraviolet Light for Reducing Escherichia coli O157:H7 in Unpasteurized Aple Cider
J. R. Wright, S. S. Sumner, C. R. Hackney, M. D. Pierson, and B. W. Zoecklein
Journal of Food Protection. 63:563-567. 2000.
This study examined the efficacy of UV light for reducing Escherichia coli O157:H7 in unpasteurized cider. Cider containing a mixture of acid-resistant E. coli O157:H7 (6.3 log CFU/ml) was treated using a thin-film UV disinfection unit at 254 nm. Dosages ranged from 9,402 to 61,005 µW-s/cm2. Treatment significantly reduced E. coli O157:H7 (P<=0.0001). Mean reduction for all treated samples was 3.81 log CFU/ml. Reduction was also affected by the level of background microflora in cider. Results indicate that UV light is effective for reducing this pathogen in cider. However, with the dosages used in this experiment, additional reduction measures are necessary to achieve the required 5-log reduction.
The Effects of Prefermentation Maceration Temperature and Percent Alcohol (v/v) at Press on the Concentration of Cabernet Sauvignon Grape Glycosides and Glycoside Fractions.
H.M. McMahon, B.W. Zoecklein, and Y.W. Jasinski.
American Journal of Enology and Viticulture, 1999. 50(4): 385-390
Grape-derived aroma and flavor precursors exist partially as nonvolatile, sugar-bound glycosides. Hydrolysis of these compounds may modify sensory attributes and potentially enhance wine quality. Prefermentation skin contact at two temperatures and alcohol content at press were monitored to determine effects on Cabernet Sauvignon glycosides. Total, red-free, and phenol-free glycoside concentratiions were estimated by the quantification of glycosyl-glucose. After three days, cold soak (10 degrees C) increased total glycosides by 103%, red-free glycosides by 81%, and phenol-free glycosides by 88%. During the same period, ambient soak (20 degrees C) increased total glycosides by 177%, red-free glycosides by 144%, and phenol-free glycosides by 106%. A reduction in the ratio of red-free glycosides to the total and the ratio of phenol-free glycosides to the total occurred during fermentation. At fermentation completion, red-free glycosides comprised 30% of the total, and phenol-free glycosides were 23% of the total glycosides. Wines produced by early pressing (10% sugar) had 25% more total and red-free glycosides than those pressed at dryness.
Quantification of Glycosidase Activities in Selected Yeasts and Lactic Acid Bacteria.
H.M. McMahon, B.W. Zoecklein, K. Fugelsang, and Y. Jasinski.
Journal of Industrial Microbiology and Biotechnology, 23:198-203. 1999.
Using a model system, the activities of alpha-L-arabionfuranosidase, beta-glucosidase, and alpha-L-rhamnopyranosidase were determined in 32 strains of yeasts belonging to the following genera: Aureobasidium, Candida, Cryptococcus, Hanseniaspora, Hansenula, Kloeckera, Metschnikowia, Pichia, Saccharomyces, Torulaspora and Brettanomyces (10 strains); and seven strains of bacterium Leuconostoc oenos. Only one Saccharomyces strain exhibited beta-glucosidase activity, but several non-Saccharomyces yeasts species showed activity of this enzyme. Aureobasium pullulans hydrolyzed alpha-L-arabinofuranoside, beta-glucoside, and alpha-L-rhamnopyranoside. Eight Brettanomyces strains had beta-glucosidase activity. Location of enzyme activity was determined for those species with enzymatic activity. The majority of beta-glucosidase activity was located in the whole cell fraction, with smaller amounts found in permeablilized cells and released into the growth medium. Aureobasidium pullulans hydrolyzed glycosides found in grapes.
Effect of Fermentation, Aging and Thermal Storage on Total Glycosides, Phenol-free Glycosides and Volatile Compounds of White Riesling (Vitis vinifera L.) Wines.
B.W. Zoecklein, C.R. Hackney, S.E. Duncan, J.E. Marcy.
Journal of Industrial Microbiology and Biotechnology 22:100-107. 1999.
There is growing recognition of the significance of the products of glycoside hydrolysis to varietal wine aroma. White Riesling wines were produced from four strains of Saccharomyces cerevisiae. Wines underwent conventional aging or anaerobic thermal storage (20 days at 45 degrees C) either 2 or 40 months post-fermentation to quantify influences on total glycosides, phenol-free glycosides and selected volatiles. Glycoside and free volatile concentrations were estimated by analysis of glycosyl-glucose and gas chromatography/mass spectrometry, respectively. Thermal storage of wines 2 months post-fermentation reduced the total glycosides by an average of 33% for all yeasts and increased the concentration of free benzyl alcohol while decreasing the concentration of free linalool and geraniol. Conventional aging for 40 months reduced the total and phenol-free glycosides equally among yeasts by an average of 60%, with phenol-free glycosides averaging 80% of the total. Thermal storage of aged wines reduced the total glycoside concentration by an additional 29%. The effect of thermal storage on selected volatile phenols, higher alcohols, esters, acids, terpenes, carbonyl compounds, C-13 norisoprenoids and six-carbon alcohols was variable depending upon the component.
Effect of Fruit Zone Leaf Removal on Total Glycoconjugates and Conjugate Fraction Concentration of Riesling and Chardonnay (Vitis vinifera L.) Grapes.
B.W. Zoecklein, T.K. Wolf, S.E. Duncan, J.E. Marcy, and Y. Jasinski.
American Journal of Enology and Viticulture 49:259-265. 1998.
Grape glycosides are, in part, important aroma and flavor precursors. Their quantification may offer a means of determining the impact of viticultural practices, such as leaf removal, on potential wine quality. Fruit zone leaf removal of Riesling and Chardonnay grapevines grown at two sites for two seasons was evaluated for its influence on total and phenol-free grape glycosides at harvest. Vines were trained to a low bilateral cordon system at one vineyard and a high bilateral cordon at the other. The concentrations of total and phenol-free glycosides were higher in Riesling and Chardonnay fruit from leaf-pulled vs. control vines at three of four harvest dates. Phenol-free glycosides averaged 80% of the total in Riesling juice and 66% of the total in Chardonnay. Grapevine canopy microclimate plays an important role in determining fruit composition, although knowledge about the relationship between microclimate and aroma/flavor development is limited. This study illustrates the affect of microclimate manipulation on grape glycosides, important components of fruit quality.
Effect of Fruit Zone Leaf Thinning on Total Glycosides and Selected Aglycones Concentrations of Riesling (Vitis vinifera L.) Grapes
B.W. Zoecklein, T.K. Wolf, J.E. Marcy, and Y. Jasinski
American Journal of Enology and Viticulture 49:35-43. 1998.
Selective leaf removal from fruit zones of mature Riesling grapevines grown at two northern Virginia (approx. 39°N) sites was evaluated over two seasons for its influence on total grape glycosides. Two to four leaves per shoot were removed three weeks after bloom from around fruit clusters grown on high (1.8 m above ground) bilateral cordon at one vineyard and a low (1.2m) bilateral cordon at the other. Leaf removal increased the percentage of sunlight penetration into the canopy fruit zone in three of four measurements. The concentration of glycosides, estimated by the analysis of glycosyl-glucose, was mostly higher in the fruit from leaf-removed vines than control vines for both the low-trained vineyard and high-trained vineyard. The sum of the concentration of selected bound monoterpene alcohols (geraniol, nerol, and linalool), bound aromatic alcohols (benzyl alcohol and 2-phenolethanol) was also higher in fruit of leaf-removed canopies than controls at most sampling dates.
Effect of Fermentation, Aging and Aging sur lie on Total and Phenol-free Riesling (Vitis vinifera L.) Glycosides
B.W. Zoecklein, Y. Jasinski, and H. McMahon
Journal of Food Composition and Analysis 11:240-248. 1998.
Grape-derived glycosides are, in part, important aroma precursors. Quantification may offer a means of determining the impact of processing methods on potential wine quality. Several studies were conducted to evaluate the influence of four strains of Sacchromyces cerevisiae on total and phenol-free Riesling glycosides during fermentation and following aging for 40 months of 40 months sur lie (with yeast). Fermentation decreased the total and phenol-free glycoside concentration, as estimated by the analysis of glycosyl-glucose, although the maximum difference among yeasts was only 7.6%. Fermentation had little effect on the percentages of phenol-free to total glycosides which averaged 61.5%. Aging 40 months or 40 months sur lie reduced the total glycoside concentrations by an average of 59.6 and 73.4%, respectively, resulting in no differences among yeast strains. Phenolic-free glycosides averaged 80% of the total in the aged wines.
Effects of Fermentation, Storage Sur Lie or Post-fermentation Thermal Processing on White Riesling (Vitis vinifera L.) Glycoconjugates
B.W. Zoecklein, J.E. Marcy, and Y. Jasinski
Am. J. Enol. Vitic. 48:397-402 1997.
Four strains of Saccharomyces cerevisiae were evaluated for their influence on White Riesling glycoconjugates. Fermentation resulted in a decrease in total conjugated glycosides as estimated by the analysis of glycosyl glucose (GG). The greatest reduction occurred with the Fermiblanc strain although the maximum difference among yeasts was only 7%. The concentration of bound monoterpene alcohols, oxides, and aromatic alcohols was diminished due to fermentation, although generally similar among yeasts. Following fermentation, the concentration of free terpenes and aromatic alcohols differed slightly among yeasts, but differences were below the sensory thresholds reported for each compound. Post-fermentation thermal processing reduced the glycoconjugates by an average of 33% for all treatments, with the greatest reduction occurring with the VL1 strain. Storing wines for 45 days sur lie lessened the glycoside content uniformly among treatments by 52%.
Effect of Crop Level on Chardonnay and Cabernet Sauvignon (Vitis vinifera L.) Glycosides
B. Zoecklein, T.K. Wolf, C. Yoder, and Y. Jasinski
Proceedings for the 4th International Symposium on Cool Climate Viticulture and Enology. 1996.
Glycosides are, in part, important aroma and flavor precursors. Quantification may offer a means of determining the impact of cultural practices on potential wine quality. Crop level from two mature Virginia vineyards was evaluated for the influence on grape glycosides expressed as glycosyl glucose (GG) or red-free GG. Cabernet Sauvignon vines, trained to a mid-wire (0.9 cm) bilateral cordon system, were cluster thinned by hand 30 days post bloom. Crop levels averaged 3.2, 5.1 and 6.4 kg/vine, resulting in a leaf area to fruit weight (cm2g-1) of 34.4, 27.0 and 19.2 respectively. Chardonnay vines were trained to an open-lyre, divided canopy training system and cluster thinned by hand 45 days post-bloom. Crop levels averaged 6.5, 12.8 and 17.8 kg/vine providing a leaf area to fruit weight of 18.0, 9.5 and 7.6 cm2g-1, respectively. Crop level influenced the rate of fruit maturation (Brix) for both cultivars. Cabernet Sauvignon berry weight was unaffected, although pH, total phenols and total anthocyanins differed. Red-free GG per gram of fruit weight was affected by treatment at four of seven sampling dates, while GG concentration per gram of fruit weight was greatest in the low crop treatment at four of six sample dates.
Effect of Native Yeasts and Selected Strains of Saccharomyces cerevisiae on Glycosyl Glucose, Potential Volatile Terpenes, and Selected Aglycones of White Riesling (Vitis vinifera L.) Wines
B.W. Zoecklein, J.E. Marcy, J. M Williams, and Y. Jasinski
Journal of Food Composition and Analysis. 10:55-65. 1997.
Four strains of Saccharomyces cerevisiae and native yeasts were evaluated for their influence on White Riesling-conjugated aroma precursors, which were measured using three analytical procedures. Fermentation resulted in a decrease in glycosyl glucose with the greatest reduction occurring with native yeast fermentations. Potentially volatile terpene levels were similar among wines following fermentation, the exceptions being those fermented using the Fermiblanc strain and native yeasts which showed lower concentrations. Storing wines for 45 days on the yeast lees (sur lie) reduced the glycosyl glucose and potential volatile terpene content with slight differences among treatments. Gas chromatographic analysis of selected glycosidically bound monoterpene alcohols, oxides and aromatic alcohols showed a trend similar to the glycosyl-glucose and potentially volatile terpenes following fermentation. Fermentation had a variable effect on the free linalool concentration. Levels of nerol and geraniol decreased while free alpha-terpeniol, hotrienol, furanic and pyranic linalool oxides, benzyl alcohol, and 2-phenylethanol increased. The concentrations of free volatiles following fermentation differed among yeasts, but differences were below the sensory thresholds reported for each compound.
Effect of Fruit Zone Leaf Removal and Shoot Thinning on Vitis vinifera L. Grape Glycosides
B.W. Zoecklein, C.C. Yoder, T.K. Wolf, and Y. Jasinski
Proceedings for the 4th International Symposium on Cool Climate Viticulture and Enology. 1996.
The analysis of glycosides has been suggested as an objective index of grape quality. Two separate studies (leaf removal of Chardonnay and Cabernet Sauvignon on multiple training system; leaf removal and shoot thinning of Cabernet Sauvignon) were conducted in Virginia to evaluate the effect of vineyard management on these important secondary metabolites.
I. Selective leaf removal from the fruit zone of mature Chardonnay and Cabernet Sauvignon grapevines was evaluated for two seasons to determine the influence on grape glycosides, expressed as glycosyl glucose (GG). Two treatments were compared: 1) removal of two to four leaves per shoot from around the fruit zone three weeks post-bloom and 2) no leaf removal. Chardonnay vines were grown on a Hudson River Umbrella (HRU), Cabernet Sauvignon on a mid-wire, HRU and Lyre systems. Leaf removal (LR) increased fruit zone porosity as measured by the percentage of sunlight penetration and point quadrant analysis. Yield components were generally unaffected by treatment as were fruit soluble solids, pH and titratable acidity at harvest. LR increased Chardonnay GG for five of six sampling periods while having no influence on total phenols, flavonid and nonflavonoid phenols. Cabernet Sauvignon GG was greatest in fruit from leaf-pulled vines at four of six harvest dates. LR increased total phenols and anthocyanins, although not consistently.
II. In a separate study begun in 1995, shoot thinning, mechanical and hand fruit zone leaf removal of mature Cabernet Sauvignon grapevines were evaluated for their influence on red-free GG. Four shoot densities [14.0, 18.9, 23.3, and 25.4 shoots per meter (SPM)] were examined, each with no leaf removal (No LR), mechanical leaf removal (M LR) or mechanical plus hand leaf removal (M+H LR) imposed five weeks post-bloom. Shoot thinning (ST) reduced leaf area per vine while increasing fruit zone porosity as measured by the percentage of sunlight penetration and point quadrat analysis. LR increased fruit zone porosity. Fruit soluble solids, pH, and total phenols were generally increased with increased fruit exposure while titratable acidity and total anthocyanins were generally unaffected at harvest. Red-free GG concentration was greatest inthe most exposed treatment (14.4 SPM and M+H LR).
Effects of Native Yeasts and Selected Strains of Saccharomyces cerevisiae on Glycosyl Glucose, Potential Volatile Terpenes and Selected Aglycones of White Riesling (Vitis vinifera L.)
B.W. Zoecklein, Y. Jasinski, and S. Rigby
Proceedings of the Wine Spoilage Microbiology Conference. T. Tamiko and K.C. Fugelsang (Eds.). pp 65-74. California State University, Fresno. CATI Publication 960203, Fresno, CA.1996.
Native yeasts and four strains of Sacchromyces cerevisiae were evaluated for their influences on the fermentation rate, whine chemistry and conjugated aroma precursors of White Riesling grapes. Fermentation resulted in a decrease in glycosyl glucose (GG), with the greatest reduction occurring with the native fermentation. Potentially volatile terpenes (PVT) were similar among wines following fermentation, the exception being he Fermiblanc (FB) strain and fermentations with native yeasts. Storing wines for 45 days on the yeast lees (sur lies) reduced the GG and PVT content with slight differences in PVT among treatments. Gas chromatographic analysis of selected glycosidically bound monoterpene alcohols, oxides and aromatic alcohols showed a trend similar to the GG and PVT following fermentation. Fermentation had a variable effect on free linool, but reduced nerol and geranol, while increasing free -terpeniol, hotrienol, furanic and pyranic linalool oxides, benzyl alcohol and 2-phenylethanol. The concentrations of free volatiles following fermentation differed among yeasts, but differences were below the sensory thresholds reported for each compound.