Status of yeast assimilable nitrogen in Italian grape musts and effects of variety, ripening and vintage

The content of promptly assimilable nitrogen by yeast (PAN) was analysed in 586 juices from technologically ripe grapes collected in several Italian regions. A spectrophotometric method within the reach of each wine laboratory was used, adapting a previous method of the American Society of Brewing Chemists used for wort to grape juices. 58.3 % of the samples were below the classic deficiency threshold of 140 mg·l-1. Among varieties and vintages there were significant differences. An overall trend of PAN to slightly decrease with ripening was observed. The variability of the PAN content of numerous samples harvested in a vineyard to check ripeness seems to be larger than that for sugars and total acidity

3-hexen-1-ol isomers in Müller-Thurgau wines: A ''varietal'' characteristic affected by must sulfiting time

Research NoteMust sulfiting time can be responsible for differences in the (3E)/(3Z)-hexen-1-ol ratio of wines produced on a semi-industrial and industrial scale. However, as the (3E) isomer is normally higher than the (32) isomer, the ratio could be used, with some caution, as a contributing parameter to verifying the varietal origin of some wines, in particular of Müller-Thurgau. After hexanol, (3E)-hexen-1-ol seems to be the main Cb-alcohol in Müller-Thurgau wines from Trentino (Italy)

Selective use of wine yeast strains having different volatile phenols production

Among Saccharomyces cerevisiae wine yeasts, we found a high frequency of strains having the ability to decarboxylate 4-hydroxycinnamic acid and 3-methoxy-4-hydroxy-cinnamic acid. From Gewurztraminer juices fermented by S. cerevisiae wine strains with and without such character, we obtained wines with considerably different levels of volatile phenols and some interesting evidences of the likely precursors of 4-vinylguaiacol and 4-vinylphenol. The identification of yeast strains by electrophoretic karyotyping gave us the possibility of evaluating the effective contribution of the yeast in the organoleptic characteristic of Traminer wines associated with the concentration of such volatile phenols

Aroma components of Galician Albariño, Loureira and Godello wines

Wines of the three most interesting Galician white grape varieties have been deeply analyzed for three aroma categories: the volatile compounds, except for monoterpenols, the monoterpenols and the bound forms. Many compounds showed significant differences between the wine groups, as proved by Tukey's test, e.g. for methanol, trans and cis 3-hexen-1-ols and relevant ratio, benzaldehyde and 4-vinylguaiacol in the first category, and for the most part of compounds of the other two categories. Loureira and most Albariño wines contain linalool and ho-trienol at a level of sensorial contribution. The relevant average terpene profiles show many similarities, especially for Albariño and particularly if considering the contents and relevant ratios of furan and pyran linalool oxide isomers, of ho-diendiols I and II and of geraniol. On the other hand differences could be stated for some compounds, mostly under the bound forms. Godello wines, with the poorest content of monoterpenols under both forms, are principally characterized by a marked level of bound benzaldehyde, with contemporary presence of a considerable average level of the free form. PCA data treatments on both monoterpenols and aglycons from the bound forms, showed a good separation among the groups as well a good homogeneity and varietal correspondence of the wines

Structural identification and presence of some p-menth-1-enediols in grape products

The present work aims at clearing the structure of some p-menthenediols, which are already known to be present in musts and wines as glycosides, referred to as isomers 1, 2 and 3. Isomers 2 and 3 correspond to cis-p-menth-1-ene-6,8-diol and p-menth-1-ene-7,8-diol; isomer 1, which is only found in traces, is probably identical with trans- p-menth-1-ene-6,8-diol. Their concentration is reported for both free and bound forms in musts from some varieties and compared with the alpha-terpineol and 2-exo-hydroxy-1,8-cineole concentrations. In general, the free forms were scarcely present; among the bound forms p-menth-1-ene-7,8-diol first and then cis-p-menth-1-ene-6,8-diol prevailed. Besides improvement of varietal classification, the compounds identified give evidence of allylic hydroxylase activity in grape berries

Flavour compounds of clones from different grape varieties

The selection of highly productive and characteristic clones is essential for the maintenance and improvement of the viticultural and commercially valuable qualities of certain grape varieties. In the future, this very difficult and time consuming selection of adapted clones will be supplemented with more precise analytical techniques. In this regard, GC analysis will be most valuable. Analytically, German white wines can be divided into three groups by the content of only a few monoterpene compounds ('terpene profiles'): 'Riesling', 'Muscat' and 'Sylvaner' or 'Weissburgunder' types. Furthermore, the use of linear discriminant analysis on some components allows the differentiation within types (e. g. for 'Riesling'-tpye: Riesling, Müller-Thurgau, Bacchus, Kerner, Ehrenfelser, Scheurebe). Till now no significant differentiation could be made from flavour compounds of Riesling clones, although clear differences can be perceived between wines from Traminer clones. Wines with a low sensory appraisal typically have a low content of monoterpene compounds. Differences can also be recognized in the aroma profiles of Chardonnay clones, allowing an analytical differentiation between aromatic and non-aromatic types. Certain clones, however, show differing compositions of flavour compounds when grown in different areas (clones with an 'unstable' aromatic character). Also during ripening of the berries, differences occur within individual Chardonnay clones, resulting in a few clones having recognizable ripening curves for monoterpene compounds

Methylanthranilate (''foxy taint'') concentrations of hybrid and Vitis vinifera wines

Research NoteMethylanthranilate, contributing to the typical hybrid/foxy taint of wines from American hybrids and wild vines, was also detected in white wines from Vitis vinifera varieties in concentrations up to 0.3 µg/l. In wines from newly bred fungus resistant cultivars methylanthranilate contents were found to be at the same low level, not exceeding the perception threshold of ca. 300 µg/l and therefore not negatively affecting the wine quality

A new 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN) precursor isolated from Riesling grape products: Partial structure elucidation and possible reaction mechanism

A heteroside, which produces 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN) by acid hydrolysis, was isolated from Riesling grapes by retention on Amberlite XAD-2 resin, followed by preparative TLC and HPLC techniques. It was partially identified by NMR spectroscopic procedures. The presence of a megastigm-4-en-9-one structure with an enol-ether function in the C4 position and a OH/OR function in the C6 position was ascertained. The sugar part should be constituted of two or three glucose moieties with the same NMR characteristics. The linkage of these moieties to the megastigmane structure in the C4 position and possibly also in the C6 position remains to be determined. The isolated conjugated form produced only a TDN-d4 isomer when reacted at 50 degrees C in D2O at different acid pH values. A possible reaction mechanism was proposed, considering the kinetics of TDN-d(4) formation during the hydrolysis of the raw glycosidic fraction from two differently aged Riesling wines at pH 2, and comparing it with the kinetics of TDN formation as well. The latter may correspond to the mechanism proposed by WINTERHALTER (1991). Thus, the presence of at least two different TDN precursors in grape products at different concentrations was proved

Methyl trans geranate and farnesoate as markers for Gewürztraminer grape skins and related distillates

Methyl ester of trans geranic and farnesoic acids, farnesol and two alpha-farnesene isomers are remarkable compounds in skins of mature grapes as well as in marc distillates of Traminer variety. Considerations about their level in both products of other floral varieties, like Yellow and Rose Muscats and Müller-Thurgau, as well as about their relationships with the main skin monoterpenols and other compounds, including two unidentified stereoisomeric sesquiterpenes present only in distillates, were discussed. Finally, results of PCA data treatments as for the distillates are show