Lesson 4 - Yeast Biochemistry
This lesson describes the biochemical pathways that explain the activities of yeast in brewing. It begins by looking at what metabolism is and how we control it. It then looks at how nutrients, including sugar, are taken up by brewing yeast. Next, it explores metabolism in more detail, focusing on sugar metabolism and looking at two particular modes of metabolism, respiratory and fermentative. Finally, it considers the need for redox control and takes a brief look at gluconeogenesis and the storage of carbohydrates in yeast.
Storage Carbohydrates
Within the brewing process, yeast cells are starved at the end of fermentation and during storage before re-pitching. They use storage carbohydrates to get them through these lean periods.
Yeast cells have stores of two carbohydrates. These are glycogen and trehalose.
Glycogen is a polymer made up of multiple units of glucose. It is similar in structure and function to starch in plant cells.
Trehalose is a much simpler molecule, a disaccharide made from two glucose units.
Glycogen is a true storage carbohydrate. It is formed when cells have a lot of sugar but are starved of another essential nutrient such as nitrogen. In brewing yeast it can be up to 40% of the cell’s dry weight. We can test this when we do a starch test during mashing; we get some pitching yeast and add a drop of iodine. If it is packed with glycogen, as it should be, it will go dark brown.
Glycogen is formed in times of plenty and broken down to generate glucose for energy when cells cannot grow and so it is particularly important that cells have adequate glycogen stores to ensure they survive the period between cropping and re-pitching. Glycogen is also rapidly broken down and utilised immediately after pitching and it has been suggested that in addition to wort sugars, it provides some energy and carbon for the syntheiss of lipids that occurs in the initial aerobic phase of fermentation. Depending on the physiological state of the pitching yesast, the cells may not able to use wort sugars very efficiently because their membranes do not function properly until they have used the oxygen to synthesise sterols and unsaturated fatty acids. A big trigger here is oxygen and that is why it is best to store pitching yeast under an inert gas such as CO2 otherwise glycogen is more quickly broken down, before pitching.
Trehalose was thought for a long time to be another carbon store but this is not true. It is part of the package of systems used by yeast cells (and other organisms) to deal with stress. It can bind to membranes and protect them from damage.
Did you know, there is a plant (Selaginella lepidophylla) called the resurrection plant? It lives in very hot dry deserts and is famous because it can withstand droughts for many years. On the rare occasions that rain comes, it miraculously turns from a dried up thing into a green plant. This is because its cells contain a lot of trehalose which protects the delicate membranes and gets it through the hard times.
Trehalose is important in brewing yeast when conditions become very stressful, for example during very high gravity brewing or when active dried brewing yeast is put through the dehydration process. Levels are typically less than 5% of the cell’s dry weight but can reach 20% when conditions are very stressful.