Fermentation Primer - Lesson V

                     Fermentation Primer - Lesson V

In last month’s lesson we discussed how yeast cells process pyruvate under anaerobic conditions. We explored both the Alcohol Fermentation Pathway and the Lactic Acid Fermentation Pathway. In this month’s issue we will look at the nutritional/chemical needs of the yeast.

Q: Does yeast need nutrients in order to carry out fermentation?

The short answer is “Yes”. The follow-up question usually is: what typeof nutrients and in which quantities does it need? Answering this latter question is more complex: as you are aware by now, there are many different varieties of yeasts (the Saccharomyces cerevisiae species alone has over 1,000 identified varieties!), there are also many different sources of fermentable sugars, each with its own chemical and biological composition.

In this lesson we will focus on the general nutritional needs of yeast, with the understanding that actual needs may differ for a specific combination of yeast species and fermentation media.

Nitrogen

The most important yeast nutrient is neither a vitamin nor a mineral, but rather a gas: Nitrogen. Nitrogen (chemical element symbol “N”) is essential to life on earth: it is a component of all proteins and it can be found in all living systems. Nitrogen compounds are present in organic materials, foods, fertilizers, explosives and poisons. Named after the Greek word nitron, for “native soda,” and genes for “forming,” nitrogen is the fifth most abundant element in the universe. Nitrogen is metabolized by yeast to synthesize proteins and also stimulates yeast multiplication, keeps yeast metabolism active and prevents H2S and mercaptan formation. Yeast Assimilable Nitrogen (YAN) is composed of ammonium ions and amino acids (except proline).

Vitamins

The word “vitamin” was coined in 1911 by the Warsaw-born biochemist Casimir Funk (1884-1967). At the Lister institute in London, Funk isolated a substance that prevented nerve inflammation (neuritis) in chickens raised on a diet deficient in that substance. He named the substance vitamine because he believed it was necessary to life and it was a chemical amine. The “E” at the end was later removed when it was recognized that vitamins need not be amines. The letters in the names (A, B, C and so on) were assigned to the vitamins in the order of their discovery. The one exception was vitamin K which was assigned its “K” from “Koagulation” by the Danish researcher Henrik Dam.

The vitamins most likely to be essential for the satisfactory growth of any particular strain of saccharomyces cerevisiae are (in alphabetical order): Biotin, Calcium Pantothenate, Inositol, Niacin, Pyrdoxine, Riboflavin and Thiamin.

Minerals

The metal ions most likely to be needed for the efficient metabolic functions of saccharomyces cerevisiae are (in alphabetical order): Calcium, Copper, Iron, Magnesium, Manganese, Potassium and Zinc.

Q: What happens when the yeast lacks the nutrients it needs?

Absence or reduced presence of nutrients can lead to yeast stress, resulting in lower ethanol yields, longer fermentation times, production of off-flavor congeners or undesirable aromas and possibly even death of the yeast population. Again, there is no general answer that applies to all yeast species or varieties. each fermentation must be analyzed individually, based on: 

  • the fermenting material and its composition (cane juice, High Test Molasses, other molasses grades, raw sugar, etc.)
  • the environmental conditions (temperature, pH, osmotic pressure)
  • properties of the water being employed (minerals present in the water)
  • species and variety of yeast being used
  • yeast recycling/propagation operations at the distillery (versus the consistent use of active dry yeast, for example)

Join us again next month, as we continue our deep-dive into this fascinating world!

Source: Creative Commons (creativecommons.org) and Khan Academy (khanacademy.org)