Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
email: Office@practicalwinery.com
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SUMMER 2012
WINEMAKING
BY
Paul A. Henschke, Cristian Varela, Simon Schmidt, Tracey Siebert,
Radka Kalouchova, Chris D. Curtin and I. Leigh Francis
The Australian Wine Research Institute website
Diego Torrea and Carmen Ancin-Azpilicueta
Universidad Pública de Navarra, Spain
Mar Vilanova
Misión Biológica de Galicia, Spain email
Maurizio Ugliano
Nomacorc, France email
A wine’s style is largely defined by a combination of the varietal attributes contributed by the grape, with the vinous character provided by the yeast. In this context, viticultural decisions and fermentation choices enable winemakers to manage wine style, ranging, for example, from complex and savoury to fruity, and reductive to oxidative
This article provides insights into how managing fermentation nitrogen affects not only the wine’s aroma profile but also its perceived style. The scope for modulating aroma profile and style can be considerable when making wine from musts having initially low assimilable nitrogen content.
P/L; this being equivalent to 1700 mg/L DAP, assuming that the must previously contained no phosphates. Since must phosphate levels are highly variable,9 phosphate analysis should be made before considering large additions of DAP. The wine constituents showing greatest variation in response to nitrogen concentration are glycerol; malic, succinic and acetic acids; pH; titratable acidity, and sulfur dioxide. The general effects of DAP addition on these compounds are summarized in Figure 1.
Ethanol and glycerol – DAP supplementation of model and authentic grape juice produces variable but small reductions in ethanol suggesting that DAP supplementation is not an effective strategy for reducing ethanol content of wine.
In laboratory studies, ethanol yield, which is calculated as the proportion of sugar that is converted into ethanol, is reduced by up to 10% when fermentation is performed in a synthetic medium containing ammonium nitrogen rather than amino acids as sole nitrogen source.1 This reduction in ethanol yield results from the need to synthesize each amino acid from simple sugars and inorganic nitrogen. However, in studies with grape juice medium, in which amino acids are an importance source of YAN, DAP addition has produced variable results, ranging from no change to 0.7% (v/v) reduction, 4,8,13 further studies are required to understand this source of variability in ethanol yield.
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Y
east possessess a remarkable ability to sense its environment, which triggers metabolic responses to enable acclimatization to hardships or opportunities. In addition to physical and chemical stresses, such as temperature and acidity, yeast also senses nutrient status.
In fact, yeast responds to the presence and amounts of nutrients in its immediate environment by tailoring its metabolism accordingly. Consequently, the profile of fermentation-derived aroma active metabolites (esters, alcohols, carbonyls, volatile fatty acids, and volatile sulfur compounds) reflects a yeast’s genetically determined metabolic response to its immediate environment (grape must).
Grape must provides all of the nutrient requirements for yeast, usually in sufficient concentrations, to enable complete fermentation. This paper focuses on nitrogen sources, which, in grape must, are predominantly organic (amino acids, peptides) unless inorganic (ammonium salts) nitrogen has been used as a supplement.
In many viticultural regions around the world, limited nitrogen availability is an important constraint on vine development, which also restricts accumulation of amino nitrogen in the berry. This leads to nitrogen-deficient must, which can cause suboptimal (slow or sometimes “stuck”) fermentations and hydrogen sulfide production. Consequently, ammonium salts have become widely used as a
fermentation stimulant, especially when grape must is deficient in yeast assimilable nitrogen (YAN). Despite its wide application, the impact that inorganic nitrogen has on wine flavor, especially sensory profile, is poorly understood.3 We have initiated an investigation into this question for Albariño, Chardonnay, and Shiraz wines, however only white wines will be discussed here.
Fermentation nitrogen – general effects
The effects of nitrogen on the basic composition of wine have been determined mainly by studying the effects of adding diammonium phosphate (DAP) to model and grape juice/must media.3 A wide range of assimilable nitrogen values have been studied, approximately 50 to 500 mg N/L, which reflects YAN values that are likely to be encountered in most viticultural regions. This results in a wide range of fermentation kinetics, from slow or stuck through moderate to fast,3,4,8,10 that should be taken into account when considering DAP addition.
For low YAN juices, the addition of DAP necessary to achieve this range is large and may not be permitted under wine regulations in some countries. For example, U.S. wine regulations restrict nitrogen addition to 960 mg/L as (NH4)2HPO4, which increases YAN by 203 mg N/L (DAP contains approximately 21% N).12 Australian wine regulations restrict DAP nitrogen addition indirectly by limiting total soluble phosphates in wine to 400 mg