Scott Labs Yeast Nutrient Choosing Guide
Last Updated: 7/2021
Applies to: Winemakers looking for assistance in selecting and comparing Scott Labs yeast nutrients and yeast derivatives. This guide contains a downloadable pdf choosing chart that characterize our yeast nutrients and derivatives by specifications and sensory impact.
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Supplementing Yeast Nutrients for Success
What nutrients do yeast need?
Nitrogen. Nitrogen controls cell number, fermentation rate, and the production of some aroma compounds. Yeast assimilable nitrogen (YAN) consists of most amino acids, ammonia, and some types of peptides. The amount of YAN in the juice/must will vary based on geographic location, grape variety, maturity at harvest, and processing decisions.
Survival factors. Survival factors (sterols and unsaturated fatty acids) are essential for healthy plasma membranes which help yeast withstand increasing ethanol concentrations. When yeast have sufficient survival factors, sugar uptake can continue throughout fermentation and the toxic effects of ethanol can be minimized.
Vitamins and minerals. Vitamins and minerals are cofactors for growth and metabolism and yeast cannot survive without them. Interestingly, there is also a link between vitamins and aroma production. When vitamins are present in an assimilable form, fruitiness is increased and negative sulfur off-odors are decreased. Additionally, the higher the YAN, the greater the cell number, which means more vitamins and minerals are required.
WHAT NUTRIENTS ARE PRESENT IN GRAPES?
While grapes contain nitrogen, vitamins, minerals, and survival factors, they often do not contain them in levels that will support healthy fermentation. YAN supplementation is often necessary.
YAN in grapes exists in two main forms: ammonia and amino acids. Yeast use each of these nitrogen sources differently. Although yeast prefer ammonia, it is used quickly and does not give yeast the staying power to complete fermentation, nor does it support the production of positive aromas. In general, amino acids are taken up more slowly. This form of nitrogen lasts longer and can give yeast the staying power to complete fermentation. Importantly, amino acids also support yeast aroma production.
How Much Nutrient Should be Added?
See our "Fermentation Nutrition Planner" for step by step advice on creating a yeast nutrition plan. The amount of supplementation required for a healthy fermentation depends on multiple factors:
Initial juice chemistry. Prior to fermentation, sugar content and YAN should both be measured. Higher sugar and lower YAN fermentations will both require higher levels of YAN supplementation. It is crucial to measure YAN and sugar immediately prior to fermentation. Wineries may conduct pre-fermentation processes like clarification or cold-soaking that take a few days. During this time, native microflora will consume YAN as well as vitamins and minerals, even in healthy fruit and juice/ musts. Measuring YAN before these processes may not accurately represent the YAN at inoculation.
Yeast strain nutrient needs. Different yeast strains have different nitrogen demands and are classified as low, medium or high nitrogen need according to the following:
- Low nitrogen-demand: 7.5 ppm YAN per 1 °Brix
- Medium nitrogen-demand: 9 ppm YAN per 1 °Brix
- High nitrogen-demand: 12.5 ppm YAN per 1 °Brix
Turbidity. When juice is over-clarified (<50 NTU), many nutritional factors for yeast are removed, making it necessary to supplement with complete and balanced nutrients.
Fruit quality. The presence of molds and rot will impact grape juice/must chemistry. Studies have shown that grapes impacted by Botrytis cinerea and other molds are highly deficient in YAN and other essential nutrients.
Fermentation temperature. Higher fermentation temperatures stimulate fermentation rate and yeast growth, thereby requiring more nitrogen than cooler fermentations.
How to Calculate YAN
YAN is calculated using both forms of assimilable nitrogen in grapes (ammonia and amino acids):
YAN = (0.8225 x [NH3]) + [PAN]
Ammonia (NH3): Typical methods for measuring ammonia (NH3) report total ammonia concentration but only 82.25% of ammonia is nitrogen and this must be accounted for when calculating YAN.
Amino acids (PAN): Amino acids are reported as PAN (primary amino nitrogen), AAN (assimilable amino nitrogen), or FAN (free amino nitrogen), which are interchangeable. Typical amino acid analysis measures only the nitrogen content of assimilable amino acids, so that number is used directly when calculating YAN. It is important to note that yeast cannot assimilate the amino acid proline, so this measurement typically excludes proline.
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