FermentationWine Wine is an alcoholic drink. The word wine is usually used to talk about drinks made from the juice of grapes, although people sometimes call alcoholic drinks made from the juice of other fruits (such as plums or blackberries) "wine". This article only deals with wine made from grapes. Wine is made by the fermentation of the sugar in grapes. There are two main types of wine, red wine and white wine. Red wine is made from red grapes, and white wine is made from white grapes. Rosé wine is made by leaving red grapes in skin red and white wine
Transcript
FermentationWineWine is an alcoholic drink. The word wine is usually used to talk about drinks made from the juice of grapes, although people sometimes call alcoholic drinks made from the juice of other fruits (such as plums or blackberries) "wine". This article only deals with wine made from grapes.
Wine is made by the fermentation of the sugar in grapes. There are two main types of wine, red wine and white wine. Red wine is made from red grapes, and white wine is made from white grapes. Rosé wine is made by leaving red grapes in skin contact for a very short time. The colour comes only from the skin, so if you have short skin contact, the wine will not turn red but only "pink" (rose wine). Wine sometimes has bubbles in it; this wine is called sparkling wine. The most popular sparkling wine is champagne, which comes from France.
People have been making wine for about 5000 years.
Fermentation is the process in which cells (mostly yeast) convert glucose (or sugar) into either alcohol or vinegar along with some other products. Fermentation is an anaerobic reaction.
Lactic acid fermentation produces lactic acid. It happens in muscles of animals when they need lots of energy fast.
The chemical logic behind... Fermentation and Respiration
Enzymes relay the electrons released by substrate oxidation to special molecules we call electron acceptors. Electron acceptors may be organic or inorganic, and the most common examples thereof are NAD+ and FAD. Each of these molecules ca accept two electrons, yielding NADH+H+ and FADH2, respectively. Since cellular amounts of NAD+ and FAD are very small, special mechanisms are needed in order to convert NADH+H+ and FADH2
back into NAD+ and FAD. This is performed through electron transfer from NADH+H+ and FADH2 to other molecules, which may occur through either fermentation or respiration. Contrary to general belief, the distinction between these two processes does not lie on a requirement for O2!
Fermentation
In fermentation, NADH (or FADH2) donates its electrons to a molecule produced by the same metabolic pathway that produced the electrons carried by NADH (or FADH2). For instance, during
intense physical exercise by muscles, NADH generated through glycolysis transfers its electrons to pyruvate (an organic molecule produced by glycolysis), yielding lactate.
(The relationship between the pH drop in muscles during lactate production and the occurrence of cramps is discussed in detail in these two papers). This process is called lactic fermentation . Many other kinds of fermentation have been found in microorganisms, and the most well-known among these is alcoholic fermentation:
Respiration
In respiration, the final acceptor of NADH (or FADH2) electrons is not a product of the metabolic pathway that released the electrons carried by NADH
as final electron acceptors. Mammals use O2, and their respiration is therefore called aerobic respiration. Aerobic respiration happens in the inner mitochondrial membrane, which contains the relevant electron-transfering protein complexes. each of these complexes accepts electrons from a molecule and transfers them to a different compound, and the full assembly is therefore termed theelectron transport chain:
NADH dehydrogenase or complex I. In mammals, this complex contains more than twenty polypeptide chains, many of which with no known function. This complex accepts two electrons from NADH+H+ and transfer them, through Fe-S clusters, to a lipophylic molecule, ubiquinone (Q), which gets converted to ubiquinol (QH2). In this protein complex, electron transfer releases enough energy to transfer protons (H+) from the mitochondrial matrix to the intermembrane space, decreasing its pH relative to the matrix.(More details, including a three-dimensional structure, are available here)
sucinate dehydrogenase or complex II. This is the sole membrane-bound enzyme in the citric
acid cycle. It oxidizes succinate to fumarate and transfers both released electrons to FAD, yielding FADH2. As in complex I, these electrons ultimately get transfered to ubiquinone.(More details, including a three-dimensional structure, are available here)
cythocrome bc1 or complex III. It accepts electrons from ubiquinol (generated by complexes I e II), and transfers them to cythocrome c, a small solule protein present in the intermenbrane space. (More details, including a three-dimensional structure, are available here).
cythocrome c oxidase or complex IV. It receives four electrons from cythocrome c and transfers
them to O2,thereby reducing it to two water molecules. (More details, including a three-dimensional structure, are available here)
In complexes I, III and IV , electron transfer releases enough energy to transfer H+ from the mitochondrial matrix to the intermembrane space. This causes
an increase of H+ concentration (and electric potential) in the intermembrane s+ace, i.e. a larger chemical potential of H+ in the intermembrane space relative to the matrix. However, when we have two solutions with different concentrations on both sides of a membrane, solute tend to diffuse from the regions of higher chemical potential to areas of lower potential (for a neutral species, this is equivalent to moving from areas of higher concentration to areas of lower concentration).
The inner mitochondrial membrane is not permeable to H+. Under normal conditions, the only way for protons to flow back to the matrix is through a special protein: ATP synthetase. This complex protein contains two major portions: an intermembrane proton channel (F0) coupled to a catalytic protein complex (F1) facing the mitochondrial matrix. The F1 portion contains several subunits with different functions, and converts the energy released by the return of protons to the matrix into chemical energy used to synthesize ATP from ADP and Pi.
NADH is unable to cross the mitochondrial membrane. There are therefore mechanisms to transfer electrons from NADH molecules produced in
the cytoplasm during glycolisis to the electron transport chain. These are:
the malate-aspartate shuttle (which also works in gluconeogenesis): NADH transfers its electrons to oxaloacetate, converting it to malate. Malate can enter the mitochondrion, where it is dehidrogenated to oxaloacetate and transfer is electrons to NAD+. This NADH then transfers its electrons to teh electron transport chain through complex I. Through this shuttle, approxiamtely 3 ATP are produced from each cytoplasmic NADH.
the glycerol-3-P shuttle. In this shuttle, which is very active in brown adipose tissue, cytoplasmic NADH transfer its electrons to the glycolytic intermediate DHAP (dihydroxyacetone phosphate). DHAP is converted to glycerol-3-P, which donates its electrons to ubiquinone through a FAD-linked
glycerol-3-P dehydrogenase located in the outer face of the inner mitochondrial membrane. Through this shuttle, approxiamtely 2 ATP are produced from each cytoplasmic NADH.
The amount of ATP produced by ATP synthase is therefore related to the difference in H+ concentration across the membrane. Since NADH oxidation causes prroton efflux from the matrix in three protein complexes (I, III e IV), whereas FADH2 oxidation to FAD is only accompanied by such an efflux in two complexes (III e IV), more ATP can be produced from NADH than from FADH2. Oxidation of a
NADH molecule produces almost 3 ATP, and FADH2 oxidation yields almost 2 ATP.
Mitochondrial respiration may occur without ATP production, as long as the released protons are able to return to the matrix without passing through the ATP synthetase. This can happen e.g. if ionophores (lipid-soluble molecules with the ability to transport ions) are added to the mitochondria. In brown adipose tissue, a special protein (thermogenin) forms a proton channel in the mitochondion inner membrane. The flow of protons back into the matrix through this protein instead of ATP synthetase is responsible for the heat generation characteristic of this tissue. Primary Fermentation
Primary fermentation is when the wort finally becomes beer through the conversion of sugars into alcohol and carbon dioxide. This conversion is done by the yeast which "eat" the sugars; you just need to provide the right conditions for the yeast to do its job.
Primary Fermentation for the Beginning Homebrewer
As a beginner, you'll probably be using the plastic bucket fermenter that came in your equipment kit. Assuming you have a bucket fermenter and a
fermentation lock, the most important thing you can do is find a location with a stable, appropriate temperature for your yeast to work.
The Fermentation Environment
Once the wort is aerated and the yeast has been pitched, the yeast should begin to reproduce, and eventually ferment the beer converting the fermentable sugars into alcohol and CO2.
Signs of fermentation include bubbles (burps) in the airlock and a layer of foam called krausen on top of the beer. The amount of time between pitching the yeast and the first signs of fermentation is referred to as "lag time". The goal of the attentive brewer is to keep the lag time within a reasonable limit. Lag times typically vary from as short as a few hours, to as long as 72 hours! Using a yeast starter and aeration methods, as well as keeping the fermentation temperatures within the accepted range, can help to reduce lag time.
Most ale fermentations should be mostly complete within 10 days although fermentation times can vary anywhere from 4 days to 3 weeks. Lagers may take longer to finish fermenting, typically 2 to 3 weeks, and require at least a month of lagering.
A critical component to the fermentation environment is the temperature of the wort at pitching and during fermentation.
A Carboy fermenting beer with a Three Piece Airlock on top.
Fermentation Temperature
The appropriate fermentation temperature varies from one yeast strain to the next. In general, ales ferment anywhere between 60 and 70 degrees F while lagers ferment between 50 and 60 degrees F. Fermenting at too low a temperature can result in a slow or "stuck" fermentation in which the yeast become dormant before all the sugars have been converted. Fermenting at too high a temperature can lead to off-flavors due to the production of esthers. There are different steps brewers can take to control the temperature throughout the fermentation process.
There are three commonly used vessels for primary fermentation: plastic food grade buckets, glass carboys, and stainless steel conical fermenters. Which one you choose is largely a function of cost, ease of use, and technique.
Plastic Buckets
6.5 gallon plastic food grade buckets are the most common fermentation vessel for the new brewer; they are often sold with beginner equipment kits.
The principle advantages of using plastic buckets for fermentation are that they are easy to use and clean and are the least expensive option. Since the top is wide open, the wort can be poured in quickly without the use of a funnel or siphon; the wide opening also makes cleaning much simpler.
The main disadvantages are that they scratch easily making them difficult to sanitize, it is not possible to view what is occurring during fermentation without opening the lid thus exposing the beer to possible infection, and the plastic is oxygen permeable which limits the amount of time the beer should spend in the bucket. For these reasons many people switch to glass carboys after their first few brews.
The majority of home brewers use 6.5 gallon glass carboys as their primary fermentation vessel. While not as inexpensive as plastic buckets, carboys eliminate the disadvantages associated with buckets and are only moderately more difficult to work with.
The small opening at the top of the carboy necessitates the use of a funnel or siphon when adding the cooled wort to the fermenter. In addition, the small opening makes carboys more difficult to clean; typically a carboy brush and bottle washer are required.
Stainless Steel Conical
Conicals are significantly more expensive than either buckets or carboys running anywhere from $500 to several thousand depending on the size and accessories. Some home brewers use conicals because it allows them to remove the trub from the cone in the bottom without racking and disturbing the fermenting beer. This allows both primary and secondary fermentation to occur in the same vessel. Many conicals are also fitted with a spigot allowing the brewer to bottle or keg their beer more easily.
Various grades of food safe plastic are used to manufacture what are essentially "plastic carboys" or "water cooler" bottles. Many brewers swear by these bottles, while others argue that they may be semi-permeable, and eventually allow oxygen into the fermenter. Either way the fact that they do not break when dropped makes them in at least one way superior to glass fermenters. Some of these bottles hold exactly 5 gallons, and so are not suitable for fermenting a 5 gallon batch which would produce another half gallon or so of krausen. They are good for smaller primary fermenters and for conditioning or "clearing" tanks, although conventional wisdom says not to bulk condition in them for more than a month or so.
Airlocks and Blow-Offs
The majority of modern brewing is done in a closed environment through the use of airlocks which allow carbon dioxide produced during fermentation to escape while preventing unwanted microbes from getting into the beer.
A "Blow Off" is a modification of an air lock. It is typically a hose attatched to the primary fermentation vessel where the air lock usually goes. The other end of this hose is run into a container of water or sanitizer and submerged. The resulting C02 can
"bubble" out of the hose and the container like a normal air lock. The purpose of the blow off hose is to allow krausen to spill out through the hose and into the container if it grows too large for the space allowed by the fermentation vessel.
Fermentation Temperature Control
Given the importance of temperature during primary fermentation, many brewers have devised methods of controlling the temperature ranging anywhere from placing the fermenter in a water bath to utilizing chest freezers and a digital temperature controller.
Inexpensive temperature control methods include:
Placing the fermenter in a water bath. This increases the thermal mass of the fermentation system, and can stabilize temperatures in changing ambient temperatures. If heating is required, an aquarium heater is employed by some homebrewers to a) increase the temperature of the system and b) ensure a stable temperature. A heater with a temperature control is recommended, but be wary that most do not go below 67F / 19C.
Wrapping a wet towel or T-shirt around the fermenter. This works best if the bottom of the towel/T-shirt is left in the water and a fan is
aimed at the fermenter. Brew Strong suggests that this works better in dry climates.[1]
Modifying a large cooler (60 quarts or more) to fit the fermenter along with several bottles of ice.
Moving the fermenter to a cooler location. Specifically one that may be thermally independent and/or stable (e.g. a closet).
Advanced temperature control typically involves the purchase of a refrigerator or chest freezer and a digital temperature controller. Many home brew supply shops carry these controllers for home use.
Another popular temperature control method for home brewers who do not have the space or budget for a refrigerator or freezer is the Son of Fermentation Chiller, a fun and easy Do-It-Yourself project for those who want to build their own fermentation chamber. While more involved than simply purchasing a cooler, this fermentation chamber tends to hold temperatures more easily and constantly than the modified cooler method discussed above.
When is Primary Fermentation Complete?
Primary fermentation typically takes anywhere from 7-14 days and is considered finished when the fermentation process is complete. So-called
"secondary" fermentation is for conditioning and clearing the beer rather than additional fermentation.
Beer should be left in the primary fermentation vessel for at least 7 days, even if fermentation appears to be complete.
The only way to determine whether or not fermentation has finished is by taking gravity readings on consecutive days; if this reading remains constant, fermentation is complete. Some home brewers use the lack of airlock activity or the fallen krausen as an indication that fermentation is finished but these methods are inaccurate and can be misleading. If you think your fermentation is done, use your hydrometer to make sure.
What do I do next?
Once primary fermentation is complete, you may want to condition the beer through a secondary fermentation stage or lager it if appropriate. If not you can skip straight to the final step in the beer brewing process: Packaging and Carbonation.
