COVID-19 has brought an interesting term to our attention that most of us probably never heard before the advent of the vaccines being hailed as modern miracles. That term is lipids. Lipids are why the Pfizer and Moderna vaccines require special refrigeration and a key part of how they work. This is all well and good but, you are probably wonder how this has anything to do with making better hard cider. As I am often saying, good hard cider contains both art and science. The more you develop your understanding of both, the better chance you have of making exceptional cider. Lipids are part of the science behind how yeast work and researchers in a vast array of scientific fields have used eukaryotes to help them with numerous scientific discoveries, like vaccines. Eukaryotes are single cell organisms with nuclei and organelles. One of the most studied eukaryotes used in this research is what we call yeast and, not just any yeast but, Saccharomyces cerevisiae. Saccharomyces cerevisiae is the most commonly used yeast for commercial applications. It is used for bread, wine, beer, biofuels, and often cider (though I’m trying to change that).
So, what are lipids and why are they important to my cider making process? Let’s explore these two questions in more detail and give you some insights into the science behind how lipids decide how good your cider will be.
What are lipids?
Lipids are often generically called fats but the reality is that they are much more complex. As defined by Mbuyane and associates, lipids are hydrophobic molecules that are soluble in organic solvents and found in microorganisms, plants and animals with functions related to cellular structure, energy storage, enzyme activation and signaling(1). In other words, they are fats. Hydrophobic means they don’t mix well with water so to be dissolved, you need some type of solvent, just like oils and fats. The more interesting part is the later half of the definition. They provide key cellular functions that enable life. We will dig deeper into that in a minute but, let’s first explore the types of lipids in more detail. There are 8 categories of lipids found in Saccharomyces cerevisiae yeast.
| Lipid Category | Definitions |
|---|---|
| Fatty acids | long chain hydrocarbon attached to a carboxylic group |
| Glycolipids | three fatty acid chains attached to glycerol via an ester link |
| Glycerophospholipids | three fatty acid chains attached to glycerol via an ester link with a phosphoryl group |
| Sphingolipids | long chain fatty acids with a long chain amino alcohol and polar head group |
| Sterols | three six membered and a five membered ring fused together |
| Prenols | like sterols but synthesized from isopentenyl diphosphate and dimethylallyl diphosphate produced mainly via the mevalonic acid pathway |
| Saccharolipids | fatty acids linked to a sugar backbone |
| Polyketides | synthesized by polymerization of acetyl and propionyl subunits |
Some of these categories should sound familiar, like fatty acids and maybe sterols but, if you are like me, the first time you’ve heard of some of the others was when you read the above table. That’s okay because the goal isn’t to teach you every category so you can spout them off for a test or pop quiz. The main goal is to realize that saccharomyces yeasts as well as other yeasts and bacteria (and even our human bodies) use lipids to live. Yeast use lipids for structure. For example you will find lipids in the plasma membrane. Lipids are used for energy because synthesizing them can release ATP, the key energy source for yeasts processes. Lipids also signal or regulate reactions by blocking or allowing the transfer of nutrients and metabolites into and out of the yeast cell. Without lipids, yeast cells couldn’t function and this functioning is what makes lipids critical to the cider making process.
Why are lipids important to cider making?
The cell wall of yeast is fairly permeable to small atomic level particles – think nutrients or sugar. This doesn’t make it a very good regulator for the needs of the yeast cell. Yeast can process sugar into ethanol but too much sugar would make it incapable of functioning. It’s needs a way to regulate what compound pass into the cytoplasm for processing in its various organelles. That is where the plasma membrane comes into play. Located inside the cell wall, the membrane contains the nucleus, organelles, mitochondria, and other elements found in the cytoplasm. More importantly, the membrane is formed from a sea of lipids and proteins and those lipids control what goes into and out of the cell.
Let’s look at ethanol as an example. Yeast make ethanol and carbon dioxide (CO2) as part of the fermentation process. However, ethanol is toxic to yeast. As ethanol increases and juice becomes cider, the cider becomes a toxic environment for the yeast. Some yeast are better are surviving in this environment than others. Why are they better? Yeast that have higher ergosterol and monounsaturated fatty acid content possess a higher tolerance for ethanol. This would be many Saccharomyces cerevisiae yeasts while yeasts with lower levels, like T. delbrueckii, are more sensitive to ethanol(1). The plasma membrane is known to have three micro-domains known as MCC, MCT, and MCP. Again, the main point isn’t to memorize the domains but to realize that these domains each contain specific lipids that interact and react to select lipids and proteins(1). This is how lipids regulate what goes into or out of the cell. There are several ways by which lipids help regulate these compounds. One is from direct absorption. Lipids can absorb compounds from outside the cell and then deposit them into the cytoplasm. Another method identified is by working with proteins. An example is the Fat1p protein where the protein uptakes fatty acids and deposits it into the cytoplasm. The other method is from sterol uptake, which is oxygen dependent. When oxygen is present, the sterols transporters are open and allow sterols to flow but close this path in anaerobic conditions. This is similar to various genes, like SUT1, found in the yeast that regulate sterols uptake. Yeast posses a very complex structure and processing ability that even after sequencing the genome for Saccharomyces cerevisiae is only partially understood. Every year we learn more about the inner workings of yeast. As noted by Mbuyane and others, Saccharomyces cerevisuae yeast utilize lipids and other compounds differently from non-Saccharomyces strains like T. delbrueckii and L. thermotolarens. For me, this highlights the need to question preconceived notions about how cider should be fermented as well as make me interested in learning even more about how yeast work and what the best yeast really is for making great hard cider.
Hopefully, you also have a better appreciation for lipids and maybe want to do your own research on the subject. If not, don’t worry. I will continue to explore and write about them as much as I can.
(1) L.L. Mbuyane and associates, The metabolism of lipids in yeasts and applications in oenology, Food Research International 141, 2021
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