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Yeast Nutrients
by Thomas Henick-Kling
UPTAKE OF SOLUTES (nutrients)
UPTAKE OF SUGARS
in Saccharomyces crevisiae glucose and fructose are taken up via facilitated diffusion, a non-concentrative process several carriers have specificity for different sugars
NITROGEN UTILIZATION DURING WINE ALCOHOLIC FERMENTATION
- Amino acids: 800-1500 mg/L
- Ammonia: 60-140 mg/L
- Proteins: some during later phase of fermentation
CAUSES FOR H2S ACCUMULATION DURING ALCOHOLIC FERMENTATION
- Lack of assimilable nitrogen, vitamin deficiency (pantothenic acid)
- Presence of elemental sulfur from sprays to control fungal infections
- High must pH leading to high fermentation rate
- High fermentation temperature leading to high fermentation rate
- Tall tanks inhibiting removal of excess H2S by CO2
- Addition of SO2
AVOIDING REDUCED SULFUR OFF-ODORS IN WINE FERMENTATIONS
- Use yeast starter culture
- Correct nitrogen deficiency
- Use moderate fermentation rate (temperature control)
- Avoid residual sulfur in must, strongly clarify juice (racking, centrifugation)
- At end of alcoholic fermentation, rack with aeration, add SO2
A lack of nitrogen sources causes an imbalance in the sulfur metabolism and carbon skeletons which can result in excretion of H2S, sulfur containing amino acids, and ketoacids, higher alcohols, esters.
STARTER CULTURE - INOCULATION
inoculation rate = inoculation level = number of cells added with starter culture (cell density in the inoculated must or wine after inoculation)
high inoculation rate: addition of large amount of yeast or bacterial cells resulting in high cell density in the inoculated wine.
High inoculation rate (5x106 to 1x107 cells/mL) leads to rapid onset of fermentation because very little growth of the culture is required and alcoholic fermentation or malolactic fermentation can begin immediately.
CORRECTING NITROGEN DEFICIENCY IN GRAPE MUST
- Addition of DAP (diammonium phosphate dibasic)
- Addition of yeast extract
- Addition of proprietary blends of DAP, yeast extract, and vitamins
Survey of available nitrogen for yeast growth
in New York grape musts
HENICK-KLING,T., EDINGER, W.D., Larsson-Kovach, I.-M.
Summary
In New York State hydrogen sulfide (H2S.) off-odors and sometimes stuck fermentations have frequently been associated with particular grape varieties, especially Seyval Blanc, Chardonnay, Riesling, and Pinot Noir. During our studies on the origin of ethyl carbamate in wines we noticed that grape must from many New York grown grape varieties contained very low amounts of free amino acids (<1 g/L) and free ammonia (<40 mg/L). This was found in cultivars from Vitis vinifera, V. labrusca, and interspecific hybrids. Initial trials in our experimental winemaking program showed that with the addition of diammonium phosphate the incidence of H2S. formation was dramatically reduced in Pinot Noir and Seyval Blanc and extended, slow fermentation avoided. During the 1993 and 1994 vintage we started a survey of the available nitrogen in major wine grape cultivars in the four wine growing regions of NY. This survey has confirmed the generally low concentrations of available nitrogen in NY grape musts. Consultation with many winemakers and subsequent experimentation in wineries also showed that additions of nitrogen supplements are in most cases beneficial. Further fermentation trials are under way to determine what type and amounts of nitrogen supplements are most effective to improve fermentation rates and avoid formation of H2S.
Nitrogen requirement by yeast
The rate of sugar utilization by yeast during fermentation depends on yeast specific catabolic capacity, the physical environment, and on the total number of viable cells. During the fermentation of grape juice, the yeast must grow under the adverse growth conditions of high sugar concentration, low temperature, and in the presence of SO2. Such conditions may lead to inadequate yeast growth and slow or stuck fermentation. Under wine fermentation conditions, a lack of available nitrogen has also been shown to limit yeast growth and fermentation. Current studies have found that a minimum of 130 mg/L of available nitrogen (FAN) is required for fermentation and that at least 400-500 mg/L is recommended to avoid stuck fermentation and off-odors (Agenbach 1978, Bell et al. 1979, Jiranek et al. 1995).
Available nitrogen and the production of hydrogen sulfide
Often, hydrogen sulfide (H2S) is formed in wine by the yeast through the reduction of elemental sulfur used for the dusting of grapes. Yeast may also produce H2S in response to nitrogen and vitamin deficiency (pantothenate) in the juice (Monk 1982, Dittrich 1987). The addition of a nitrogen source such as diammonium hydrogen phosphate (DAP) to a fermenting must can eliminate the H2S, but DAP must be added while the yeast is still growing. If it is added at the end of fermentation, it will have no effect on the H2S produced. During the past vintage we added DAP to several wines within 24 hours after an H2S off-odor was noticed. In all cases the off-odor disappeared and did not reappear. If a variety frequently produces H2S during fermentation due to nitrogen limitation, an addition of DAP and/or other yeast foods to the juice can help avoiding this problem. Lack of available nitrogen has also been linked to production of large amounts of higher alcohols giving the wine untypical fruity aromas (Dittrich 1987). Nitrogen supplementation therefore may also help to produce wines with cleaner, more characteristic aromas.
Survey of results and conclusions
In a two-year survey of New York juices it was observed that in 1993 most samples of all cultivars surveyed were near or below the required amount of 130 mg/L free amino nitrogen (FAN) and all were below the optimum 400-500 mg/L for satisfactory rate of fermentation and for avoiding hydrogen sulfide off-odors. In 1994 the amounts of available nitrogen were generally higher yet again all samples contained less than the optimum concentration of 400-500 mg/L. In both years samples were found for all cultivars which contained less than the minimum of 130 mg/L FAN. The mean FAN content for Pinot Noir and Riesling was below the minimum of 130 mg/L in both years. More samples are necessary to make specific recommendations. However already this limited overview shows that in most years the available nitrogen in New York musts is critically low and likely causes reduced sulfur off-odors and incomplete fermentations which could be avoided with additions of nitrogen nutrients to the must. To all musts 1 g/L of DAP (or equivalent) could be added. This would even in the musts with the highest FAN content not exceed 600 mg/L FAN. In some juices 2 g/L DAP should be added to reach a satisfactory amount of available nitrogen.
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