Hey r/Homebrewing I hope I'm in the process of brewing my first gallon ever.

I will begin this chapter by reviewing the four stages of fermentation to see what the yeast is up to in each one. Then, with that background, I’ll move on to discuss the important steps that meadmakers can take to ensure a good fermentation and a successful mead. I’ll divide those steps into must preparation, yeast selection and preparation, and managing the fermentation. Lag period. During the lag period, or lag phase, the yeast acclimatizes to its new home. When yeast encounter a growth medium—a must with an acceptable amount of sugar and an appropriate pH—it first prepares itself to replicate. It takes on the necessary nutrients and compounds needed to support the massive changes involved in reproduction. After adjusting to the new temperature and sugar content of the environment, the yeast cells essentially begin to “beef-up” for the task at hand. They thicken their cell walls and set about hoarding the nutrients they will need to make healthy offspring. To understand the lag phase, it might help to know what the yeast cells are preparing for: reproduction on a massive scale. Let’s look at what’s coming. Respiratory phase (aerobic phase). To reach the ideal concentration of yeast in the must, the initial population that you add must multiply several times over. Each yeast will reproduce until it depletes the reserves required for replication. During reproduction, the yeast cell creates a bulge, known as a bud, in its cell wall. The cell replicates its nuclear chromosomal complement, infuses it into the bulge, and finally cleaves the bulged bud, creating an independent newyeast cell. This asexual process of budding requires the yeast cells to create large amounts of new cell wall tissue. During the lag phase, the cells take on oxygen and make precursors of amino acids and lipids, the components of the cell walls they’ll be creating. If conditions are right, the yeast cells create new cell walls that will be permeable by the types of sugars they will be metabolizing. Producing this new cellular membrane utilizes oxygen, as well as nitrogen and other nutrients, at a rapid pace. The terms “aerobic” and “respiratory” phase arise out of the yeast’s need for oxygen in this stage of the fermentation process. Different strains of yeast will require different levels of dissolved oxygen and nutrient in the new must, but all require these elements to some extent in order to replicate and thus perform well during the main fermentation. Providing these components will be a main focus of our discussion on managing fermentation later in this chapter. Fermentation phase. The transition from the aerobic (with oxygen) to the anaerobic (without oxygen) or fermentation phase occurs as the yeast depletes the molecular oxygen needed to create new cell wall tissue. At this time, the population of yeast cells shifts from a focus on reproduction to a focus on fermentation. At the height of the fermentation phase, the yeast population can rise to 1.5 x 108 cells/ml. This is not to say that reproduction ceases, or that fermentation will not occurat all in the presence of oxygen, but as the O2 level falls, the yeast cells begin absorbing the sugars in the wort and metabolizing them into alcohol and carbon dioxide. Yeast cells can utilize alternative energy sources to oxygen to continue reproduction, and reproduction generally continues on some small scale until the fermentable sugars are completely depleted or the yeast reaches its alcohol tolerance. Before the actual transformation of the dissolved sugars to alcohol begins, the yeast cells utilize transport compounds known as permeases to move the sugars across the cell wall. Through enzymatic activity of a-glucosidase, the maltose and other disaccharide sugars are reduced to glucose. The fermentation process begins in a series of metabolic steps that reduce the glucose to trioses. Through the glycolisis pathway, the trioses are then reduced to pyruvic acid, resulting in two molecules of ATP per molecule of glucose fermented. The pyruvic acid is then converted to acetaldehyde by the enzyme pyruvate decarboxylase; the acetaldehyde is in turn converted eventually to carbon dioxide and ethanol by the enzyme alcohol dehydrogenase. The cycle has many similarities to the metabolization of glucose in humans, including the use of ADP (adenosine diphosphate), ATP (adenosine triphosphate), and NAD/NADH (nicotinamide adenine dinucleotide), which alternately collects or contributes hydrogen molecules during the reduction process. The ability to sustain this reaction, and thus the amount of ethanol and carbon dioxide produced, varies from strain to strain. Depending on the strain of yeast you use, and the nutrient content of your must, the fermentation phase may begin as early as 12 to 24 hours after pitching, or as long as 72 hours. In most cases, if you have pitched enough yeast and added enough nutrient, 24 hours will be about right. During the fermentation, the number of yeast cells will begin dropping once the population hits its peak at about six days. It will fall in a bell-shaped curve. The yeast cells that remain will be selecting themselves for the conditions that prevail in their medium: higher alcohol and lower nutrient levels, and lower pH (see “Feeding a Fermentation” below). In a must with a healthy yeast culture, the fermentation should proceed strongly until the yeast has consumed all of the available sugar, or it reaches its alcohol tolerance. At this point, the cells become less active and enter the final stage of the fermentation process.

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