Tea processing - Wikipedia, the free encyclopedia - Sent Using Google Toolbar

Tea processing - Wikipedia, the free encyclopedia

Tea processing

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Worker picking tea flushes in Tanzania.
Worker picking tea flushes in Tanzania.

Tea processing is the method in which the leaves and flushes from Camellia sinensis are transformed into the dried leaves for brewing tea. The types of tea are distinguished by the processing they undergo. In its most general form, tea processing involves oxidising the leaves, stopping the oxidation, forming the tea and drying it. Of these steps, the degree of oxidation plays a significant role of determining the final flavour of the tea, with curing and leaf breakage contributing to a lesser amount flavour.

[edit] General

Although each type of tea has different taste, smell, and visual appearance, tea processing for all tea types consists of a very similar set of methods with only minor variations:

  1. Picking: Tea leaves and flushes, which includes a terminal bud and 2 young leaves, are plucked from Camellia sinensis bushes twice a year during early spring and early summer or late spring. Autumn or winter pickings of tea flushes are much less common, though they occur when climate permits. Picking is done by hand when a higher quality tea is needed, or where labour costs are not prohibitive. Hand-picking is done by pulling the flush with a snap of the wrist and does not involve twisting or pinching the flush, since doing the latter reduces the quality of the leaves. Tea flushes and leaves can also be picked by machine, though there will be more broken leaves and partial flushes. It is also more difficult to harvest by machine on mountain slopes where tea is often grown.
  2. Wilting: The tea leaves will begin to wilt soon after picking, with a gradual onset of enzymatic oxidation. Wilting is used to remove excess water from the leaves and allows a very light amount of oxidation. The leaves can be either put under the sun or left in a cool breezy room to pull moisture out from the leaves. The leaves sometimes lose more than a quarter of their weight in water during wilting.
  3. Bruising: In order to promote and quicken oxidation, the leaves may be bruised by tumbling in baskets or by being kneaded or rolled-over by heavy wheels. This also releases some of the leaf juices, which may aid in oxidation and change the taste profile of the tea.
  4. Oxidation: For teas that require oxidation, the leaves are left on their own in a closed room where they turn progressively darker. In this process the chlorophyll in the leaves is enzymatically broken down, and its tannins are released or transformed. This process is referred to as fermentation in the tea industry, although no true fermentation happens since the process is not driven by microorganisms. The tea producer may choose when the oxidation should be stopped. For light oolong teas this may be anywhere from 5-40% oxidation, in darker oolong teas 60-70%, and in black teas 100% oxidation.
  5. Kill-green: Kill-green or shāqīng (�青) is done to stop the tea leaf oxidation at a desired level. This process is accomplished by moderately heating tea leaves, thus deactivating their oxidative enzymes, without destroying the flavour of the tea. Traditionally, the tea leaves are panned in a wok or steamed, but with advancements in technology, kill-green is sometimes done by baking or "panning" in a rolling drum. In CTC black teas, kill-green is done simultaneously with drying.
  6. Yellowing: Unique to yellow teas, warm and damp tea leaves from after kill-green are allowed to be lightly heated in a closed container, which causes the previously green leaves to yellow.
  7. Shaping:The damp tea leaves are then rolled to be formed into wrinkle strips. This is typically done by placing the damp leaves in large cloth bags, which are then kneaded by hand or machine to form the strips. This rolling action also causes some of the sap and juices inside the leaves to ooze out, which further enhances the taste of the tea. The strips of tea can then be formed into other shapes, such as being rolled into spirals, kneaded and rolled into pellets, or tied into balls and other elaborate shapes.
  8. Drying: Drying is done to "finish" the tea for sale. This can be done in a myriad of ways including panning, sunning, air drying, or baking. However, baking is usually the most common. Great care must be taken to not over-cook the leaves.
  9. Curing: While not always required, some teas required additional aging, secondary-fermentation, or baking to reach their drinking potential. As well, flavored teas are manufactured by spraying with aromas and flavors or by storing them with their flavorants.

A flow-chart for tea processing

Without careful moisture and temperature control during its manufacture and life thereafter, fungi will grow on tea. This form of fungus causes real fermentation that will contaminate the tea with toxic and sometimes carcinogenic substances and off-flavours, rendering the tea unfit.

[edit] Type specific processing

Tea is traditionally classified based on the degree or period of "fermentation" the leaves have undergone:[1]

White tea
Young leaves (new growth buds) that have undergone no oxidation; the buds may be shielded from sunlight to prevent formation of chlorophyll. White tea is produced in lesser quantities than most other styles, and can be correspondingly more expensive than tea from the same plant processed by other methods. It is less well known in countries outside of China, though this is changing with increased western interest in organic or premium teas.
Green tea
The oxidation process is stopped after a minimal amount of oxidation by application of heat, either with steam, or by dry cooking in hot pans, the traditional Chinese method. Tea leaves may be left to dry as separate leaves or they may be rolled into small pellets to make Gunpowder tea. This process is time consuming and is typically done with pekoes of higher quality. The tea is processed within one to two days of harvesting.
Oolong (Wulong)
Oxidation is stopped somewhere between the standards for green tea and black tea. The oxidation process takes two to three days. In Chinese, semi-oxidized teas are collectively grouped as blue tea (青茶, literally: blue-green tea), while the term "oolong" is used specifically as a name for certain semi-oxidized teas.[2]
Black tea/Red tea
The tea leaves are allowed to completely oxidize. Black tea is the most common form of tea in southern Asia (Sri Lanka, India, Pakistan, Bangladesh, etc.) and in the last century many African countries including Kenya, Burundi, Rwanda, Malawi and Zimbabwe. The literal translation of the Chinese word is red tea, which is used by some tea lovers. The Chinese call it red tea because the actual tea liquid is red. Westerners call it black tea because the tea leaves used to brew it are usually black. However, red tea may also refer to rooibos, an increasingly popular South African tisane. The oxidation process will take between two weeks and one month. Black tea is further classified as either orthodox or as CTC (Crush, Tear, Curl, a production method developed about 1932). Unblended black teas are also identified by the estate they come from, their year and the flush (first, second or autumn). Orthodox processed black teas are further graded according to the post-production leaf quality by the Orange Pekoe system, while CTC teas use a different grading system.
Post-fermented tea
Teas that undergo a second oxidation, such as Pu-erh, Liu'an, and Liubao, are collectively referred to as secondary or post-fermentation teas in English. In Chinese they are categorized as Dark tea or black tea. This is not to be confused with the English term Black tea, known in Chinese as red tea. Pu-erh, also known as Póu léi (Polee) in Cantonese is the most common type of post-fermetation tea in the market.
Yellow tea
Either used as a name of special tea processed similarly to green tea, or high-quality tea served at the Imperial court.
Also called winter tea, kukicha is made from twigs and old leaves pruned from the tea plant during its dormant season and dry-roasted over a fire. It is popular as a health food in Japan and in macrobiotic diets.

Green Pu-erh tuo cha, a type of compressed raw pu-erh

[edit] References

  1. ^ StarChefs (2006). "THE RAINBOW OF TEA". Retrieved on 2006-12-21.
  2. ^ The Best Tea House Co. Ltd. (2005). "茶�分���作". Retrieved on 2006-12-21.
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Brewing - Wikipedia, the free encyclopedia - Sent Using Google Toolbar

Brewing - Wikipedia, the free encyclopedia


From Wikipedia, the free encyclopedia

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This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (January 2008)
This article is about beer. For brewing of wine, see Winemaking.
Brewing can also refer to steeping, as in the preparation of tea.
A 16th century brewery
A 16th century brewery

Brewing is the production of alcoholic beverages and alcohol fuel through fermentation. This is the method used in beer production, although the term is also used to describe the fermentation process used to create wine and mead. It can also refer to the process of producing sake and soy sauce. The term is also sometimes used to refer to any chemical mixing process.

Brewing has a very long history, and archeological evidence suggests that this technique was used in ancient Egypt. Descriptions of various beer recipes can be found in Sumerian writings, some of the oldest known writing of any sort.

The brewing industry is part of most western economies.



[edit] Brewing beer

Brewing Real Ale
Hot Water Tank

All beers are brewed using a process based on a simple formula. Key to the process is malted grain--depending on the region, traditionally barley, wheat or sometimes rye. (When malting rye, due care must be taken to prevent ergot poisoning (ergotism), as rye is particularly prone to developing this toxic fungus during the malting process.)

Malt is made by allowing a grain to germinate, after which it is then dried in a kiln and sometimes roasted. The germination process creates a number of enzymes, notably α-amylase and β-amylase, which will be used to convert the starch in the grain into sugar. Depending on the amount of roasting, the malt will take on a dark colour and strongly influence the colour and flavour of the beer.

The malt is crushed to break apart the grain kernels, increase their surface area, and separate the smaller pieces from the husks. The resulting grist is mixed with heated water in a vat called a "mash tun" for a process known as "mashing". During this process, natural enzymes within the malt break down much of the starch into sugars which play a vital part in the fermentation process. Mashing usually takes 1 to 2 hours, and during this time various temperature rests (waiting periods) activate different enzymes depending upon the type of malt being used, its modification level, and the desires of the brewmaster. The activity of these enzymes convert the starches of the grains to dextrins and then to fermentable sugars such as maltose. The mash tun generally contains a slotted "false bottom" or other form of manifold which acts as a strainer allowing for the separation of the liquid from the grain.

A mash rest from 49-55°C (120-130°F) activates various proteinases, which break down proteins that might otherwise cause the beer to be hazy. But care is of the essence since the head on beer is also composed primarily of proteins, so too aggressive a protein rest can result in a beer that cannot hold a head. This rest is generally used only with undermodified (i.e. undermalted) malts which are decreasingly popular in Germany and the Czech Republic, or non-malted grains such as corn and rice, which are widely used in North American beers. A mash rest at 60°C (140°F) activates beta-glucanase, which breaks down gummy beta-glucans in the mash, making the sugars flow out more freely later in the process. In the modern mashing process commercial fungal based beta-glucanase may be added as a supplement. Finally, a mash rest temperature of 65-71°C (149-160°F) is used to convert the starches in the malt to sugar, which is then usable by the yeast later in the brewing process. Doing the latter rest at the lower end of the range produces more low-order sugars which are more fermentable by the yeast. This in turn creates a beer lower in body and higher in alcohol. A rest closer to the higher end of the range creates more higher-order sugars which are less fermentable by the yeast, so a fuller-bodied beer with less alcohol is the result.

After the mashing, the resulting liquid is strained from the grains in a process known as lautering. Prior to lautering, the mash temperature may be raised to about 75 °C (165-170 °F) (known as a mashout) to deactivate enzymes. Additional water may be sprinkled on the grains to extract additional sugars (a process known as sparging).

At this point the liquid is known as wort. The wort is moved into a large tank known as a "copper" or kettle where it is boiled with hops and sometimes other ingredients such as herbs or sugars. The boiling process serves to terminate enzymatic processes, precipitate proteins, isomerize hop resins, concentrate and sterilize the wort. Hops add flavour, aroma and bitterness to the beer. At the end of the boil, the hopped wort settles to clarify it in a vessel called a "whirl-pool" and the clarified wort is then cooled.

The wort is then moved into a "fermentation vessel" where yeast is added or "pitched" with it. The yeast converts the sugars from the malt into alcohol, carbon dioxide and other components through a process called Glycolysis. After a week to three weeks, the fresh (or "green") beer is run off into conditioning tanks. After conditioning for a week to several months, the beer is often filtered to remove yeast and particulates. The "bright beer" is then ready for serving or packaging.

There are four main families of beer styles determined by the variety of yeast used in their brewing.

Main article: Beer styles

[edit] Ale (top-fermenting yeasts)

Main article: Ale

Ale yeasts ferment at warmer temperatures between 15-20°C (60-68°F), and occasionally as high as 24°C (75°F). Pure ale yeasts form a foam on the surface of the fermenting beer, because of this they are often referred to as top-fermenting yeast—though there are some British ale yeast strains that settle at the bottom. Ales are generally ready to drink within three weeks after the beginning of fermentation, however, some styles benefit from additional aging for several months or years. Ales range in colour from very pale to black opaque. England is best known for its variety of ales. Ale yeasts can be harvested from the primary fermenter, and stored in the refrigerator.

[edit] Lager (bottom-fermenting yeasts)

Main article: Lager

While the nature of yeast was not fully understood until Emil Hansen of the Carlsberg brewery in Denmark isolated a single yeast cell in the 1800s, brewers in Bavaria had for centuries been selecting these cold-fermenting lager yeasts by storing (lagern) their beers in cold alpine caves. The process of natural selection meant that the wild yeasts that were most cold tolerant would be the ones that would remain actively fermenting in the beer that was stored in the caves. Some of these Bavarian yeasts were stolen and brought back to the Carlsberg brewery around the time that Hansen did his famous work.

Traditionally, ales and lagers have been differentiated as being either a top fermentor or bottom fermentor, respectively. But, as the years go by homebrewers and microbrewers alike keep pushing the envelope of the craft these clean cut definitions are starting to grey. The main difference between the two is lager yeast's ability to process raffinose. Raffinose is a trisaccharide composed of galactose, fructose, and glucose.

Lager yeast tends to collect at the bottom of the fermenter and is often referred to as bottom-fermenting yeast. Lager is fermented at much lower temperatures, around 10°C (50°F), compared to typical ale fermentation temperatures of 18°C (65°F). It is then stored for 30 days or longer close to freezing point. During storage, the beer mellows and flavours become smoother. Sulfur components developed during fermentation dissipate. The popularity of lager was a major factor that led to the rapid introduction of refrigeration in the early 1900s.

Today, lagers represent the vast majority of beers produced, the most famous being a light lager called Pilsner which originated in Pilsen, Czech Republic (Plzeň in Czech). It is a common misconception that all lagers are light in color—lagers can range from very light to deep black, just like ales.

[edit] Beers of Spontaneous Fermentation (wild yeasts)

Main article: Lambic

These beers are nowadays primarily only brewed around Brussels, Belgium. They are fermented by means of wild yeast strains that live in a part of the Zenne river which flows through Brussels. These beers are also called Lambic beers. However with the advent of yeast banks and the National Collection of Yeast Cultures,[1] brewing these beers, although not through spontaneous fermentation, is possible anywhere.

[edit] Beers of mixed origin

These beers are blends of spontaneous fermentation beers and ales or lagers or they are ales/lagers which are also fermented by wild yeasts.

[edit] The brewing process

At present, there are two types of brewing; notably the brewing done in monasteries and large (commercial) firms (eg Bud, Boston Beer Company, Miller, Anheuser Bush, D. G. Yuengling & Son, InBev, ...), and the brewing done at home.

[edit] Industrial brewing

Work in the brewery is typically divided into 7 steps: mashing, lautering, boiling, fermenting, conditioning, filtering, and filling.

Brewing is also still practiced in certain (mostly catholic) monasteries. The beer made in Belgian monasteries is labeled as Trappist beer. Many beers produced in German monasteries are labelled Kloster Bier.

[edit] Homebrewing

Today, many simplified brewing systems exist which can be used at home or in restaurants. These homebrewing systems are often employed for ease of use, although some people still prefer to do the entire brewing process themselves (by also making the wort, ...)

[edit] Mashing

Main article: Mashing

Mashing is the process of combining a mix of milled grain, known as wort (typically malted barley with supplementary grains as corn, sorghum, rye or wheat; in a ratio of 90-10 up to 50-50), with water, and heating this mixture up which rests at certain temperatures (notably 45°C, 62°C and 73°C [2][3]) to allow enzymes in the malt to break down the starch in the grain into sugars, typically maltose.

Boilers at the Samuel Adams brewery
Boilers at the Samuel Adams brewery

[edit] Lautering

Main article: Lautering

Lautering is the separation of the extracts wort during mashing from the spent grain. It is achieved in either a lauter tun, a wide vessel with a false bottom, or a mash filter, a plate-and-frame filter designed for this kind of separation. Lautering has two stages: first wort run-off, during which the extract is separated in an undiluted state from the spent grains, and sparging, in which extract which remains with the grains is rinsed off with hot water.

[edit] Lauter tun
Main article: Lauter tun

A lauter tun is a special container used in all-grain brewing for separating the sweet wort from the spent grains (malted barley etc.). In essence it is simply a large strainer. It can be as simple as a plastic bucket with holes in the bottom or as complex as a stainless steel kettle with a specialized straining device attached to a spigot welded into the side of the kettle.

Sparging is the rinsing of the grains with water and is most often conducted in a lauter tun.

[edit] Mash filter

A mash filter is a plate-and-frame filter. The empty frames contain the mash, including the spent grains, and have a capacity of around one hectoliter. The plates contain a support structure for the filter cloth. The plates, frames, and filter cloths are arranged in a carrier frame like so: frame, cloth, plate, cloth, with plates at each end of the structure. Newer mash filters have bladders that can press the liquid out of the grains between spargings. The grain does not act like a filtration medium in a mash filter.

[edit] Boiling

Boiling the malt extracts, called wort, ensures its sterility, and thus prevents a lot of infections. During the boil hops are added, which contribute bitterness, flavour, and aroma compounds to the beer, and, along with the heat of the boil, causes proteins in the wort to coagulate and the pH of the wort to fall. Finally, the vapours produced during the boil volatilise off flavours, including dimethyl sulfide precursors.

The boil must be conducted so that is it even and intense. The boil lasts between 50 and 120 minutes, depending on its intensity, the hop addition schedule, and volume of wort the brewer expects to evaporate.

[edit] Boiling equipment
Brew kettles at Coors Brewing Company.
Brew kettles at Coors Brewing Company.

The simplest boil kettles are direct-fired, with a burner underneath. These can produce a vigorous and favourable boil, but are also apt to scorch the wort where the flame touches the kettle, causing caramelization and making clean up difficult.

Most breweries use a steam-fired kettle, which uses steam jackets in the kettle to boil the wort. The steam is delivered under pressure by an external boiler.

State-of-the-art breweries today use many interesting boiling methods, all of which achieve a more intense boiling and a more complete realisation of the goals of boiling.

Many breweries have a boiling unit outside of the kettle, sometimes called a calandria, through which wort is pumped. The unit is usually a tall, thin cylinder, with many tubes upwards through it. These tubes provide an enormous surface area on which vapor bubbles can nucleate, and thus provides for excellent volitization. The total volume of wort is circulated seven to twelve times an hour through this external boiler, ensuring that the wort is evenly boiled by the end of the boil. The wort is then boiled in the kettle at atmospheric pressure, and through careful control the inlets and outlets on the external boiler, an overpressure can be achieved in the external boiler, raising the boiling point by a few Celsius degrees. Upon return to the boil kettle, a vigorous vaporization occurs. The higher temperature due to increased vaporization can reduce boil times up to 30%. External boilers were originally designed to improve performance of kettles which did not provide adequate boiling effect, but have since been adopted by the industry as a sole means of boiling wort.

Modern brewhouses can also be equipped with internal calandria, which requires no pump. It works on basically the same principle as external units, but relies on convection to move wort through the boiler. This can prevent overboiling, as a deflector above the boiler reduces foaming, and also reduces evaporation. Internal calandria are generally difficult to clean.

[edit] Energy recovery

Boiling wort takes a lot of energy, and it is wasteful to let this energy escape into the atmosphere. The simplest way to recover this energy is with a kettle vapor condenser (German: Pfaduko, from the much longer Pfannendunstkondensator). A kettle vapor condenser is often nothing more than a plate heat exchanger.

[edit] Whirlpooling

At the end of the boil, the wort is set into a whirlpool. The so-called teacup effect forces the more dense solids (coagulated proteins, vegetable matter from hops) into a cone in the center of the whirlpool tank.

In most large breweries, there is a separate tank for whirlpooling. These tanks have a large diameter to encourage settling, a flat bottom, a tangential inlet near the bottom of the whirlpool, and an outlet on the bottom near the outer edge of the whirlpool. A whirlpool should have no internal protrusions that might slow down the rotation of the liquid. The bottom of the whirlpool is often slightly sloped towards the outlet. Newer whirlpools often have "Denk rings" suspended in the middle of the whirlpool. These rings are aligned horizontally and have about 75% of the diameter of the whirlpool. The Denk rings prevent the formation of secondary eddies in the whirlpool, encouraging the formation of a cohesive trub cone in the middle of the whirlpool. Smaller breweries often use the brewkettle as a whirlpool. In the United Kingdom, it is common practice to use a device known as a hopback to clear the green wort (green wort is wort to which yeast has not yet been added). This device has the same effect as, but operates in a completely different manner than, a whirlpool. The two devices are often confused but are in function, quite different. While a whirlpool functions through the use of centrifugal forces, a hopback uses a layer of fresh hop flowers in a confined space to act as a filter bed to remove trub (pronounced tr-oo-b, tr-uh-b in the UK). Furthermore, while a whirlpool is only useful for the removal of pelleted hops (as flowers don't tend to separate as easily), hopbacks are generally used only for the removal of whole flower hops (as the particles left by pellets tend to make it through the hopback.)

In homebrewing, where a brewer has the power to lift the entire stock and manipulate it by hand; the process of trub removal (the process addressed by the whirlpool and hopback) is generally accomplished by simply allowing the trub to settle to the bottom of the brew kettle and slowly decanting the green wort from the top so as not to disturb the thin layer of trub. Siphoning may also be employed but this is rare.

[edit] Wort cooling

After the whirlpool, the wort must be brought down to fermentation temperatures (20-26°Celsius [2]) before yeast is added. In modern breweries this is achieved through a plate heat exchanger. A plate heat exchanger has many ridged plates, which form two separate paths. The wort is pumped into the heat exchanger, and goes through every other gap between the plates. The cooling medium, usually water, goes through the other gaps. The ridges in the plates ensure turbulent flow. A good heat exchanger can drop 95 °C wort to 20 °C while warming the cooling medium from about 10 °C to 80 °C. The last few plates often use a cooling medium which can be cooled to below the freezing point, which allows a finer control over the wort-out temperature, and also enables cooling to around 10 °C. After cooling, oxygen is often dissolved into the wort to revitalize the yeast and aid its reproduction.

[edit] Fermenting

Modern fermenting tanks
Modern fermenting tanks

Fermentation, as a step in the brewing process, starts as soon as yeast is added to the cooled ,and afterwards, aerated wort. Aeration of usually done with sterile air. When these stadia have been completed, the product can be called beer for the first time. It is during this stage that sugars won from the malt are metabolized into alcohol and carbon dioxide. Fermentation tanks come in all sorts of forms, from enormous tanks which can look like storage silos, to five gallon glass carboys in a homebrewer's closet.

Most breweries today use cylindro-conical vessels, or CCVs, have a conical bottom and a cylindrical top. The cone's aperture is typically around 60°, an angle that will allow the yeast to flow towards the cones apex, but is not so steep as to take up too much vertical space. CCVs can handle both fermenting and conditioning in the same tank. At the end of fermentation, the yeast and other solids which have fallen to the cones apex can be simply flushed out a port at the apex.

Kraeusen in an English brewery's fermentation tank
Kraeusen in an English brewery's fermentation tank

Open fermentation vessels are also used, often for show in brewpubs, and in Europe in wheat beer fermentation. These vessels have no tops, which makes harvesting top fermenting yeasts very easy. The open tops of the vessels make the risk of infection greater, but with proper cleaning procedures and careful protocol about who enters fermentation chambers when, the risk can be well controlled.

Fermentation tanks are typically made of stainless steel. If they are simple cylindrical tanks with beveled ends, they are arranged vertically, as opposed to conditioning tanks which are usually laid out horizontally. Only a very few breweries still use wooden vats for fermentation as wood is difficult to keep clean and infection-free and must be repitched more or less yearly.

After high kraeusen a bung device (German: Spundapparat) is often put on the tanks to allow the CO2 produced by the yeast to naturally carbonate the beer. This bung device can be set to a given pressure to match the type of beer being produced. The more pressure the bung holds back, the more carbonated the beer becomes.

[edit] Conditioning

When the sugars in the fermenting beer have been almost completely digested, the fermentation slows down and the yeast starts to settle to the bottom of the tank. At this stage, the beer is cooled to around freezing, which encourages settling of the yeast, and causes proteins to coagulate and settle out with the yeast. Unpleasant flavors such as phenolic compounds become insoluble in the cold beer, and the beer's flavor becomes smoother. During this time pressure is maintained on the tanks to prevent the beer from going flat.

Often, the beer is then racked (siphoned) into another container, usually a carboy, for aging or secondary fermentation. Fermentation is virtually complete, so the term secondary fermentation actually refers to conditioning. Use of a hydrometer is recommended to be absolutely sure all fermentation is finished; this is especially important as a precaution when the beer is to be bottled. Racking is done to separate the beer from the trub so that the remaining active yeasts do not consume it, as this can give the beer an off-flavor. Racking also helps separate the beer from sediment, making it less likely to find its way into the finished product. During secondary fermentation some chemical by-products from the primary fermentation are digested, which considerably improves the taste. Secondary fermentation can take from 2 to 4 weeks, sometimes longer, depending on the type of beer. Additionally lagers, at this point, are aged at near freezing temperatures for 1-6 months depending on style. This cold aging serves to reduce sulfur compounds produced by the bottom-fermenting yeast and to produce a cleaner tasting final product with fewer esters.

If the fermentation tanks have cooling jackets on them, as opposed to the whole fermentation cellar being cooled, conditioning can take place in the same tank as fermentation. Otherwise separate tanks (in a separate cellar) must be employed. This is where aging occurs.

[edit] Filtering

A mixture of diatomaceous earth and yeast after filtering.
A mixture of diatomaceous earth and yeast after filtering.
Main article: Filtered beer

Filtering the beer stabilizes the flavour, and gives beer its polished shine and brilliance. Not all beer is filtered. When tax determination is required by local laws, it is typically done at this stage in a calibrated tank.

Filters come in many types. Many use pre-made filtration media such as sheets or candles, while others use a fine powder made of, for example, diatomaceous earth, also called kieselguhr, which is introduced into the beer and recirculated past screens to form a filtration bed.

Filters range from rough filters that remove much of the yeast and any solids (e.g. hops, grain particles) left in the beer, to filters tight enough to strain color and body from the beer. Normally used filtration ratings are divided into rough, fine and sterile. Rough filtration leaves some cloudiness in the beer, but it is noticeably clearer than unfiltered beer. Fine filtration gives a glass of beer that you could read a newspaper through, with no noticeable cloudiness. Finally, as its name implies, sterile filtration is fine enough that almost all microorganisms in the beer are removed during the filtration process.

[edit] Sheet (pad) filters

These filters use pre-made media and are relatively straightforward. The sheets are manufactured to allow only particles smaller than a given size through, and the brewer is free to choose how finely to filter the beer. The sheets are placed into the filtering frame, sterilized (with hot water, for example) and then used to filter the beer. The sheets can be flushed if the filter becomes blocked, and usually the sheets are disposable and are replaced between filtration sessions. Often the sheets contain powdered filtration media to aid in filtration.

It should be kept in mind that pre-made filters have two sides. One with loose holes, and the other with tight holes. Flow goes from the side with loose holes to the side with the tight holes, with the intent that large particles get stuck in the large holes while leaving enough room around the particles and filter medium for smaller particles to go through and get stuck in tighter holes.

Sheets are sold in nominal ratings, and typically 90% of particles larger than the nominal rating are caught by the sheet.

[edit] Kieselguhr filters

Filters that use a powder medium are considerably more complicated to operate, but can filter much more beer before needing to be regenerated. Common media include diatomaceous earth, or kieselguhr, and perlite.

[edit] Packaging

Packaging is putting the beer into the containers in which it will leave the brewery. Typically this means in bottles, aluminium cans and kegs, but it might include bulk tanks for high-volume customers.

[edit] Secondary fermentation

Secondary fermentation is an additional fermentation after the first or primary fermentation. For the secondary fermentation, the beer is transferred to a second fermenter, so that it is no longer exposed to the dead yeast and other debris (also known as "trub") that have settled to the bottom of the primary fermenter. This prevents the formation of unwanted flavors and harmful compounds such as acetylaldehydes, which are commonly blamed for hangovers.

Among homebrewers, secondary fermentation is a common source of discussion and debate. Some believe that the majority of homebrewed beers can simply be fermented in a single fermenter for approximately two weeks and then bottled, making secondary fermentation unnecessary. However, secondary fermentation is a necessary step when brewing beers with long fermentation times, such as lagers. Many homebrewers use secondary fermentation as a way of Conditioning, to enhance both the flavor and appearance of the beer.[4]

During secondary fermentation, most of the remaining yeast will settle to the bottom of the second fermenter, yielding a less hazy product. Some beers may have three fermentations, the third being the bottle fermentation.

Bottle fermentation

See Bottle conditioning.

Most homebrewed beers undergo a fermentation in the bottle, giving natural carbonation. This may be a second or third fermentation. They are bottled with a viable yeast population in suspension. If there is no residual fermentable sugar left, sugar may be added. The resulting fermentation generates CO2 which is trapped in the bottle, remaining in solution and providing natural carbonation.

Cask conditioning

See Cask ale.

Beer in casks are managed carefully to allow some of the carbonation to escape.

[edit] See also

Wikibooks has a book on the topic of

[edit] References

Wikisource has the text of the 1911 Encyclopædia Britannica article Brewing.

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Fermentation (wine) - Wikipedia, the free encyclopedia

Fermentation (wine)

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Fermenting must.
Fermenting must.

The process of fermentation in wine is the catalyst function that turns grape juice into an alcoholic beverage. During fermentation yeast interact with sugars in the juice to create ethanol, commonly known as ethyl alcohol, and carbon dioxide (as a by-product). In winemaking the temperature and speed of fermentation is an important consideration as well as the levels of oxygen present in the must at the start of the fermentation. The risk of stuck fermentation and the development of several wine faults can also occur during this stage which can last anywhere from 5 to 14 days for primary fermentation and potentially another 5 to 10 days for a secondary fermentation. Fermentation may be done in stainless steel tanks, which is common with many white wines like Riesling, in an open wooden vat, inside a wine barrel and inside the wine bottle itself like in the production of many sparkling wines.[1][2]



[edit] History

See also: History of wine

While there may never be evidence to pinpoint the moment when fermentation was first observed-the natural occurrence of fermentation, with little need for human interaction, makes it likely that fermentation was observed quite early in human history.[3] The earliest uses of the word "Fermentation" in relation to winemaking was in reference to the appearance "boiling" within the must that came from the anaerobic reaction of the yeast to the sugars in the grape juice and the release of carbon dioxide. The Latin fervere means, literally, to boil. In the mid-19th century, Louis Pasteur noted the connection between yeast and the process of the fermentation in which the yeast act as catalyst and mediator through a series of a reaction that convert sugar into alcohol. The discovery of the Embden–Meyerhof–Parnas pathway by Gustav Embden, Otto Fritz Meyerhof and Jakub Karol Parnas in the early 20th century contributed more to the understanding of the complex chemical processes involved the conversion of sugar to alcohol.[4]

[edit] Process

See also: Ethanol fermentation and Glycolysis
"Bloom", visible as a dusting on the berries, contains waxes and yeasts.
"Bloom", visible as a dusting on the berries, contains waxes and yeasts.

In winemaking there are distinctions made between ambient yeasts which are naturally present in wine cellars, vineyards and on the grapes themselves (sometimes known as a grape's "bloom" or "blush") and cultured yeast which are specifically isolated and inoculated for use in winemaking. The most common genus of wild yeasts found in winemaking include Candida, Klöckera/Hanseniaspora, Metschnikowiaceae, Pichia and Zygosaccharomyces. Wild yeasts can produce high-quality, unique-flavored wines; however, they are often unpredictable and may introduce less desirable traits to the wine, and can even contribute to spoilage. Traditional wine makers, particularly in Europe, advocate use of ambient yeast as a characteristic of the region's terroir; nevertheless, many winemakers prefer to control fermentation with predictable cultured yeast. The cultured yeasts most commonly used in winemaking belong to the Saccharomyces cerevisiae (also known as "sugar yeast") species. Within this species are several hundred different strains of yeast that be used during fermentation to affect the heat or vigor of the process and enhance or suppress certain flavor characteristics of the varietal. The use of different strains of yeasts are a major contributor to the diversity of wine, even among the same grape variety.[5]

The addition of cultured yeast normally occurs with the yeast first in a dried or "inactive" state and is reactivated in warm water or diluted grape juice prior to being added to the must. To thrive and be active in fermentation, the yeast needs access to a continuous supply of carbon, nitrogen, sulfur, phosphorus as well as access to various vitamins and minerals. These components are naturally present in the grape must but their amount may be corrected by adding nutrient packets to the wine, in order to foster a more encouraging environment for the yeast. Oxygen is needed as well but in wine making the risk of oxidation and the lack of alcohol production from oxygenated yeast requires the exposure of oxygen to be kept at a minimum.[6]

Dry winemaking yeast (left) and yeast nutrients used in the rehydration process to stimulate yeast cells.
Dry winemaking yeast (left) and yeast nutrients used in the rehydration process to stimulate yeast cells.

Upon the introduction of active yeasts to the grape must, phosphates are attached to the sugar and the six-carbon sugar molecules begin to be split into three-carbon pieces and go through a series of rearrangement reactions. During this process the carboxylic carbon atom is released in the form of carbon dioxide with the remaining components becoming acetaldehyde. The absence of oxygen in this anaerobic process allows the acetaldehyde to be eventually converted, by reduction, to ethanol. During the conversion of acetaldehyde a small amount is converted, by oxidation, to acetic acid which, in excess, can contribute to the wine fault known as volatile acidity (vinegar taint). After the yeast has exhausted its life cycle they fall to the bottom of the fermentation tank as sediment known as lees.[7]

[edit] Other compounds involved

Brettanomyces, also known as "Brett", is a yeast strain commonly found in red Burgundy wine.
Brettanomyces, also known as "Brett", is a yeast strain commonly found in red Burgundy wine.

The metabolism of amino acids and breakdown of sugars by yeasts has the affect of creating other biochemical compounds that can contribute to the flavor and aroma of wine. These compounds can be considered "volatile" like aldehydes, ethyl acetate, ester, fatty acids, fusel oils, hydrogen sulfide, ketones and mercaptans) or "non-volatile" like glycerol, acetic acid and succinic acid. Yeast also has the effect during fermentation of releasing glycoside hydrolase which can hydrolyse the flavor precursors of aliphatics (a flavor component that reacts with oak), benzene derivities, monoterpenes (responsible for floral aromas from grapes like Muscat and Traminer), norisoprenoids (responsible for some of the spice notes in Chardonnay), and phenols. Some strains of yeasts can generate volatile thiols which contribute to the fruity aromas in many wines such as the gooseberry scent commonly associates with Sauvignon blanc. Brettanomyces yeasts are responsible for the "barnyard aroma" characteristic in some red wines like Burgundy Pinot noir.[8]

[edit] Winemaking considerations

Carbon dioxide is visible during the fermentation process in the form of bubbles in the must.
Carbon dioxide is visible during the fermentation process in the form of bubbles in the must.

During fermentation there are several factors that winemakers take into consideration. The most notable is that of the internal temperature of the must. The biochemical process of fermentation itself creates a lot of residual heat which can take the must out of the ideal temperature range for the wine. Typically white wine is fermented between 64-68 °F (18-20 °C) though a wine maker may choose to use a higher temperature to bring out some of the complexity of the wine. Red wine is typically fermented at higher temperatures up to 85 °F (29 °C). Fermentation at higher temperatures may have adverse effect on the wine in stunning the yeast to inactivity and even "boiling off" some of the flavors of the wines. Some winemakers may ferment their red wines at cooler temperatures more typical of white wines in order to bring out more fruit flavors.[7]

To control the heat generated during fermentation the winemaker has to choose a suitable vessel size or to use cooling devices of various sorts from the ancient Bordeaux traditions of placing the fermentation vat on top of blocks of ice to today's modern use of sophisticated fermentation tanks with built in cooling rings.[9]

A risk factor involved with fermentation is the development of chemical residue and spoilage which can be corrected with the addition of sulfur dioxide (SO2), although excess SO2 can lead to a wine fault. A winemaker who wishes to make a wine with high levels of residual sugar (like a dessert wine) may stop fermentation early either by dropping the temperature of the must to stun the yeast or by adding a high level of alcohol (like brandy) to the must to kill off the yeast and create a fortified wine.[7]

[edit] Other types of fermentation

In winemaking there are different processes that fall under the title of "Fermentation" but might not follow the same procedure commonly associated with wine fermentation.

[edit] Bottle fermentation

Bottle fermentation is a method of sparkling wine production originating in the Champagne region where after the cuvee has gone through a primary yeast fermentation the wine is then bottled and goes through a secondary fermentation where sugar and additional yeast known as liqueur de tirage is added to the wine. This secondary fermentation is what creates the carbon dioxide bubbles that sparkling wine is known for.[10]

[edit] Carbonic maceration

The process of carbonic maceration is also known as whole grape fermentation where instead of yeast being added to grape must fermentation is encouraged to take place inside the individual grape berries. This method is common in the creation of Beaujolais wine and involves whole clusters of grapes being stored in a closed container with the oxygen in the container being replaced with carbon dioxide.[11] Unlike normal fermentation where yeast converts sugar into alcohol, carbonic maceration works by enzymes within the grape breaking down the cellular matter to form ethanol and other chemical properties. The resulting wines are typically soft and fruity.[12]

[edit] Malolactic fermentation

Instead of yeast, bacteria plays a fundamental role in malolactic fermentation which is essentially the conversion of malic acid into lactic acid. This has the benefit of reducing some of the tartness and making the resulting wine taste softer. Depending on the style of wine that the winemaker is trying to produce, malolactic fermentation may take place at the same time as the yeast fermentation.[13]

[edit] References

  1. ^ J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 267-269 Oxford University Press 2006 ISBN 0198609906
  2. ^ J. Robinson Jancis Robinson's Wine Course Third Edition pg 74-84 Abbeville Press 2003 ISBN 0789208830
  3. ^ H. Johnson Vintage: The Story of Wine pg 16 Simon and Schuster 1989 ISBN 0671687026
  4. ^ J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 267 Oxford University Press 2006 ISBN 0198609906
  5. ^ J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 778-779 Oxford University Press 2006 ISBN 0198609906
  6. ^ J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 779 Oxford University Press 2006 ISBN 0198609906
  7. ^ a b c J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 268 Oxford University Press 2006 ISBN 0198609906
  8. ^ J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 780 Oxford University Press 2006 ISBN 0198609906
  9. ^ J. Robinson Jancis Robinson's Wine Course Third Edition pg 82 Abbeville Press 2003 ISBN 0789208830
  10. ^ K. MacNeil The Wine Bible pg 168-169 Workman Publishing 2001 ISBN 1563054345
  11. ^ K. MacNeil The Wine Bible pg 33-34 Workman Publishing 2001 ISBN 1563054345
  12. ^ D. Bird "Understanding Wine Technology" pg 89-92 DBQA Publishing 2005 ISBN 1891267914
  13. ^ K. MacNeil The Wine Bible pg 35 Workman Publishing 2001 ISBN 1563054345
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