Wine is a highly complex mixture of compounds which largely define its appearance, aroma, flavour and mouth-feel properties. The compounds responsible for those attributes have been derived in turn from three major sources, viz. grapes, microbes and, when used, wood (most commonly, oak). The grape-derived compounds provide varietal distinction in addition to giving wine its basic structure. Thus, the floral monoterpenes largely define Muscat-related wines and the fruity volatile thiols define Sauvignon-related wines; the grape acids and tannins, together with alcohol, contribute the palate and mouth-feel properties. Yeast fermentation of sugars not only produces ethanol and carbon dioxide but a range of minor but sensorially important volatile metabolites which gives wine its vinous character. These volatile metabolites, which comprise esters, higher alcohols, carbonyls, volatile fatty acids and sulfur compounds, are derived from sugar and amino acid metabolism. The malolactic fermentation, when needed, not only provides deacidification, but can enhance the flavour profile.

The aroma and flavour profile of wine is the result of an almost infinite number of variations in production, whether in the vineyard or the winery. In addition to the obvious, such as the grapes selected, the winemaker employs a variety of techniques and tools to produce wines with specific flavour profiles. One of these tools is the choice of microorganism to conduct fermentation. During alcoholic fermentation, the wine yeast Saccharomyces cerevisiae brings forth the major changes between grape must and wine: modifying aroma, flavour, mouth-feel, colour and chemical complexity. The wine bacterium Oenococcus oeni adds its contribution to wines that undergo malolactic fermentation. Thus flavour-active yeasts and bacterial strains can produce desirable sensory results by helping to extract compounds from the solids in grape must, by modifying grape-derived molecules and by producing flavour-active metabolites. This article reviews some of the most important flavour compounds found in wine, and their microbiological origin.

Yeast and bacterial modulation of wine aroma and flavour

This review discusses the impacts of nitrogen addition in the vineyard and winery, and establishes the effects that nitrogen has on grape berry and wine composition and the sensory attributes of wine. Nitrogen is the most abundant soil-derived macronutrient in a grapevine, and plays a major role in many of the biological functions and processes of both grapevine and fermentative microorganisms. Manipulation of grapevine nitrogen nutrition has the potential to influence quality components in the grape and, ultimately, the wine. In addition, fermentation kinetics and formation of flavour-active metabolites are also affected by the nitrogen status of the must, which can be further manipulated by addition of nitrogen in the winery. The only consistent effect of nitrogen application in the vineyard on grape berry quality components is an increase in the concentration of the major nitrogenous compounds, such as total nitrogen, total amino acids, arginine, proline and ammonium, and consequently yeast-assimilable nitrogen (YAN). Both the form and amount of YAN have significant implications for wine quality. Low must YAN leads to low yeast populations and poor fermentation vigour, increased risk of sluggish/stuck/slow fermentations, increased production of undesirable thiols (e.g. hydrogen sulfide) and higher alcohols, and low production of esters and long chain volatile fatty acids. High must YAN leads to increased biomass and higher maximum heat output due to greater fermentation vigour, and increased formation of ethyl acetate, acetic acid and volatile acidity. Increased concentrations of haze-causing proteins, urea and ethyl carbamate and biogenic amines are also associated with high YAN musts. The risk of microbial instability, potential taint from Botrytis-infected fruit and possibly atypical ageing character is also increased. Intermediate must YAN favours the best balance between desirable and undesirable chemical and sensory wine attributes. ‘Macro tuning’, of berry nitrogen status can be achieved in the vineyard, given genetic constraints, but the final ‘micro tuning’ can be more readily achieved in the winery by the use of nitrogen supplements, such as diammonium phosphate (DAP) and the choice of fermentation conditions. This point highlights the need to monitor nitrogen not only in the vineyard but also in the must immediately before fermentation, so that appropriate additions can be made when required. Overall, optimisation of vineyard and fermentation nitrogen can contribute to quality factors in wine and hence affect its value. However, a better understanding of the effect of nitrogen on grape secondary metabolites and different types of nitrogen sources on yeast flavour metabolism and wine sensory properties is still required.

Implications of nitrogen nutrition for grapes, fermentation and wine

Publisher Summary Sulfiting agents have a long history of use as food ingredients. Sulfur dioxide and several forms of inorganic sulfites, which liberate sulfur dioxide under the conditions of use, are food additives, collectively known as sulfiting agents. In addition to their use as food additives, the sulfites can also occur naturally in foods. Foods contain a variety of sulfur-containing compounds, including the sulfur amino acids, sulfates, sulfites, and sulfides. The key to the understanding of sulfite toxicity may lie in elucidation of sulfite metabolism. Several researchers have proposed that defects in sulfite metabolism among certain segments of the human population may put them at greater risk to the possible toxic effects of sulfite ingestion. If the current generally recognized as safe (GRAS) review leads to some limitation on the continued use of sulfites, it will be necessary to consider alternatives. Enzymatic browning will be inhibited by any process that destroys or inactivates the enzyme. Blanching would obviously work but is impractical for using on fresh fruits and vegetables. Despite their long history of use as food additives, much remains to be learned about sulfites, which would be helpful to the present concerns about their safety.

Sulfites in foods: uses, analytical methods, residues, fate, exposure assessment, metabolism, toxicity, and hypersensitivity.

Saccharomyces cerevisiae-Oenococcus oeni interactions in wine: current knowledge and perspectives.

Saccharomyces cerevisiae wine-producing yeast cultures grown under model winemaking conditions could be induced to liberate hydrogen sulfide (H2S) by starvation for assimilable nitrogen. The amount of H2S produced was dependent on the yeast strain, the sulfur precursor compound, the culture growth rate, and the activity of the sulfite reductase enzyme (EC 1.8.1.2) immediately before nitrogen depletion. Increased H2S formation relative to its utilization by metabolism was not a consequence of a de novo synthesis of sulfite reductase. The greatest amount of H2S was produced when nitrogen became depleted during the exponential phase of growth or during growth on amino acids capable of supporting short doubling times. Both sulfate and sulfite were able to act as substrates for the generation of H2S in the absence of assimilable nitrogen; however, sulfate reduction was tightly regulated, leading to limited H2S liberation, whereas sulfite reduction appeared to be uncontrolled. In addition to ammonium, most amino acids were able to suppress the liberation of excess H2S when added as sole sources of nitrogen, particularly for one of the strains studied. Cysteine was the most notable exception, inducing the liberation of H2S at levels exceeding that of the nitrogen-depleted control. Threonine and proline also proved to be poor substitutes for ammonium. These data suggest that any compound that can efficiently generate sulfide-binding nitrogenous precursors of organic sulfur compounds will prevent the liberation of excess H2S.

Regulation of hydrogen sulfide liberation in wine-producing Saccharomyces cerevisiae strains by assimilable nitrogen.

Present knowledge of sulfite and sulfide formation by wine yeasts during the fermentation of grape juice is reviewed. Possible sources of sulfur, namely sulfate, sulfite, elemental sulfur and sulfur-containing amino acids, are discussed. Factors influencing sulfide as well as sulfite formation are emphasized. Low-sulfite and high-sulfite-forming yeast strains differ in their metabolism as well as in some aspects of regulation.

R. Eschenbruch

Papers

Sulfite and Sulfide Formation during Winemaking -- A Review