Thomas Henick-Kling

Dynamics of indigenous and inoculated yeast populations and their effect on the sensory character of Riesling and Chardonnay wines

To study the impact of yeast populations on wine flavour and to better understand yeast growth dynamics, wines were produced by the (i) indigenous microflora, (ii) vigorous yeast starter EC1118 and (iii) slowly fermenting yeast Assmannshausen. Sensory analysis revealed that wines differed depending on the fermentation type. However, these yeast‐related differences did not exceed the varietal character. Both added starter cultures clearly dominated the Saccharomyces population from the middle of fermentation onwards. The starter cultures differed in their repression of indigenous non‐Saccharomyces yeast. EC1118 limited growth of non‐Saccharomyces yeasts more strongly than Assmannshausen. Sulphite addition further repressed growth of non‐Saccharomyces yeasts. On completion, more than one Saccharomyces strain was present in each fermentation, with the largest variety in the non‐inoculated and the smallest in the EC1118‐inoculated fermentation. Results from the two genetic assays, karyotyping, and PCR using δ‐primers were not fully equivalent, limiting the usefulness of δ‐PCR in studies of native Saccharomyces yeasts.

Effect of wine type on the detection threshold for diacetyl

Abstract Diacetyl, an important wine flavorant synthesized during alcoholic and malolactic fermentation, has been reported to have a sensory threshold of 2–3 mg/l. A comparative study of threshold for diacetyl in wines was undertaken to determine the effect of wine type on that value. Sensory threshold was determined according to the forced-choice ascending concentration series of limits method described by ASTM (E 679-79), using trained panelists. Panel detection thresholds and standard deviation from the geometric mean were found to be 0.2 mg/l and 0.32 in Chardonnay, 0.9 mg/l and 0.21 in Pinot noir, and 2.8 mg/l and 0.38 in Cabernet Sauvignon. These results demonstrate the important effect of wine type on diacetyl threshold, invalidating the use of a single threshold value for all wines.

Effects of Diffuse Colonization of Grape Berries by Uncinula necator on Bunch Rots, Berry Microflora, and Juice and Wine Quality

ABSTRACT Production of grape (principally cultivars of Vitis vinifera) for high-quality wines requires a high level of suppression of powdery mildew (Uncinula necator syn. Erysiphe necator). Severe infection of either fruit or foliage has well-documented and deleterious effects upon crop and wine quality. We found that berries nearly immune to infection by U. necator due to the development of ontogenic resistance may still support diffuse and inconspicuous mildew colonies when inoculated approximately 3 weeks post-bloom. Fruit with diffuse mildew colonies appear to be healthy and free of powdery mildew in late-season vineyard assessments with the naked eye. Nonetheless, presence of these colonies on berries was associated with (i) elevated populations of spoilage microorganisms; (ii) increased evolution of volatile ethyl acetate, acetic acid, and ethanol; (iii) increased infestation by insects known to be attracted to the aforementioned volatiles; (iv) increased rotting by Botrytis cinerea; and (v) increased frequency of perceived defects in wines prepared from fruit supporting diffuse powdery mildew colonies. Prevention of diffuse infection requires extending fungicidal protection until fruit are fully resistant to infection. Despite a perceived lack of improvement in disease control due to the insidious nature of diffuse powdery mildew, potential deleterious effects upon crop and wine quality thereby would be avoided.

A simple and accurate GC/MS method for the quantitative analysis of diacetyl in beer and wine

A GC/MS method for the quantification of diacetyl is described. Diacetyl is derivatized with 4,5-dichloro-1,2-diaminobenzene to form 6,7-dichloro-2,3-dimethylquinoxaline (DCDMQ). The derivative is extracted in benzene and quantified by GC/MS. Formation of DCDMQ is linearly correlated with diacetyl concentration. The method is rapid, sensitive (determination limit 0.0005 μg/mL) precise (standard error < 2%), and accurate (recovery of diacetyl 91.5% + 1.5%).