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Yeast nutrition and successful fermentations

Introduction

Successful primary fermentations of grape juice depend upon a number of factors: nutritional aspects are of critical interest as more becomes known about the nutritional needs of yeast. The following article discusses some of the relevant issues in regard to yeast nutrition in fermentations.

Nutrients

A number of different nutrient types are necessary for yeast to perform a successful fermentation. The main nutrient groups to be discussed are sugars, vitamins and nitrogen compounds.

Sugars

Of the nutrients needed by yeast, sugars are by far the most abundant in grape juice, typically being in the range 180 – 250 g/L. The sugars present are glucose and fructose. In this range the sugars are well in excess of the yeast’s needs. It is unlikely that sugars are ever a limiting factor in yeast fermentations in grape juice.

Vitamins

Yeasts do need a number of vitamins to be able to produce a healthy fermentation. These include biotin, folic acid, inositol, niacin, pantothenate, pyridoxine, riboflavin and thiamin.

Grape juice is not usually deficient in vitamins unless the vitamin content is depleted by winemaking practices such as cold settling, filtration, fining, pasteurisation, ion exchange, sulfiting and uncontrolled growth of other organisms such as non-Saccharomyces yeasts and lactic acid bacteria during fermentation.

Thiamin is one vitamin that has been shown to reduce fermentation problems in some juices.

Several vitamin supplements for yeast are available commercially.

Nitrogen compounds

Nitrogen compounds are the second most important macronutrients for yeast, after sugars. Nitrogen is a limiting factor in most grape juice fermentations and normally needs to be supplemented if a clean and uninterrupted fermentation is desired. A deficiency of nitrogen can lead to slow or stuck fermentations.

Fermentation rate and total fermentation time are related to initial must nitrogen content. The strain of yeast used and the fermentation conditions determine the nitrogen content needed.

Nitrogen is accumulated by yeast more easily in the early part of the fermentation, with the bulk of the nitrogen being taken up during the yeast growth phase. During fermentation the yeast membrane becomes affected by alcohol and becomes more and more permeable or “leaky” as the alcohol content increases. This makes it more difficult for the yeast to take up and retain nitrogen as the fermentation progresses. Making the yeast membrane stronger at the start of fermentation can reduce the permeability problem. This can be improved by the presence of oxygen, which enables the yeast to make more survival factors such as sterols and long chain fatty acids that protect the membrane.

Nitrogen content can be inadvertently reduced by some winemaking processes, particularly those practices that permit the growth of unwanted yeasts or bacteria. Low sulfur dioxide (SO2) contents, high pH levels and cold settling can contribute to this problem.

Yeast Assimilable Nitrogen (YAN)

Only a portion of the total nitrogen compounds in juice is available for yeast to use; this portion is best known as Yeast Assimilable Nitrogen (YAN). The two sources of nitrogen compounds that contribute to YAN are ammonium ions and alpha amino acids.

YAN can be measured in the laboratory as a combination of the ammonium nitrogen content and the amino acid nitrogen content. Both of these are measured separately in units of mg/L of nitrogen, these two parameters are then added to give YAN in mg/L.

Current knowledge is that approximately 150 mg/L of YAN is necessary for a fermentation to proceed. However a larger amount (approximately 400 – 500 mg/L of YAN) is generally quoted as necessary for yeast to reach its maximum fermentation rate and for no issues to arise during fermentation.

The growth rate of yeast does appear to be limited by the ability of the yeast cell to accumulate amino acid nitrogen. In juice a number of amino acids are present naturally - arginine and proline being the main ones. However, proline cannot be utilised by yeast under normal anaerobic conditions.

To complicate things further, yeast cells metabolise ammonium in preference to amino acid nitrogen. The commonly used nitrogen additive diammonium hydrogen phosphate (DAP) is a source of ammonium ions only; it does not contain any amino acids. So when DAP is added the ammonium content increases but the amino acid content does not. The yeast will use the ammonium in preference to the amino acids present in the juice and so may miss out on taking up enough amino acids and essential nutrients needed for a successful fermentation.

Measuring YAN

This parameter is relatively easy to measure. It does require the availability of a visible light spectrophotometer. A lot of wineries already have this equipment for measuring other analytes such as malic acid and glucose/fructose in juice and wine using enzymatic analysis kits.

Two kits are required for measuring YAN: one is the enzymatic kit for Ammonia, the second kit for Alpha Amino Acids does not employ enzymes but is based on a colourimetric reaction known as the NOPA reaction. The same equipment is used for both determinations.

Nitrogen Supplements

As previously mentioned, DAP is a commonly used nitrogen supplement as it is a cheap nitrogen source. It does, however, only provide a source of ammonium ions and not any amino acids or other nutrients apart from the phosphate ion.

The supplements used for amino acids are products derived from yeast. These yeast products are known by several names such as hydrolysed yeast, autolysed yeast or yeast extract. The products can be composed of the entire yeast at the time of drying the yeast cells, or sometimes only the soluble part. They can contain proteins, amino acids, carbohydrates lipids, vitamins and minerals. It is hard to be specific on composition because the production methods vary and so the product compositions can be quite different.

These products are different to yeast hulls, also known as yeast ghosts. Yeast hulls are made up of the insoluble portion of the yeast and contain the cell wall, membrane and insoluble cytoplasmic material.

Yeast hulls are also useful in fermentations and can alleviate some fermentation problems. They can provide growth factors, adsorb toxic components in the must and act as nucleation sites for the fermenting yeast cells.

Several blended nutrient products are available, for example DSM Maxaferm. These all vary but usually contain DAP, hydrolysed yeast, yeast hulls and vitamins and/or minerals.

Adding Nutrients

The timing and addition rate of nutrition additives is important. It is desirable that the yeast inoculum has the opportunity to take up the existing nutrients before supplements, but also important that nutrients are not in excess at the end of fermentation so allowing spoilage organisms greater opportunity to grow.

It is better to determine the YAN of the juice and calculate the amount of nitrogen required than just add a fixed amount of DAP to every ferment.

Also DAP on its own is not the best answer for optimising the nutrition of yeast. From our own experience many of the problems of stuck and sluggish fermentations can be eliminated by paying greater attention to yeast nutrition.

Recommendations

The following recommendations are not exhaustive. New research work is being done all the time on yeast nutrition and with new discoveries, further guidelines will emerge. The following is a basic set of recommendations to help reduce the incidences of fermentation problems from nutritional causes:

1. Measure the YAN of each batch to determine the most appropriate level of nitrogen addition.

2. Consider using other supplements as well as just DAP.

3. Add the nitrogen supplement just after the yeast has an opportunity to absorb existing nutrients from the juice, typically after a 1-2 Brix drop from the start of fermentation.

4. Do not add the DAP in one step but divide it into two or three steps during the first half of the fermentation.

5. Monitor the generation of hydrogen sulphide and use this as a guide to the time of nitrogen addition.

 

Article from the  Australian and New Zealand Grapegrower and Winemaker, Issue 573, 2011
Page Number(s): 101-102

Author: Greg Howell

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