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Beverages and Foods from Fungi: Wine and Beer Yeasts are single-celled fungi that belong to the sac fungus subdivision, Ascomycotina; in the absence of oxygen, yeasts will undergo fermentation as their form of respiration to obtain energy In the process of fermentation, one molecule of sugar (glucose or fructose) is converted into two molecules of ethyl alcohol and two molecules of carbon dioxide Most of the yeasts used for fermentation are strains of Saccharomyces cerevisiae (see fig. 24.1) Making Wine The practice of growing grapes for wine is an ancient one; whatever the exact origins, wine making was practices by many ancient cultures The ancient Egyptians developed a sophisticated system of cultivating wine grapes that included building trellises for training grape vines, developing precise pruning methods, and constructing irrigation canals to the vineyards (see fig. 24.2) The Romans introduced many innovations to wine making such as classifying grape varieties and adding spices and herbs to flavor wine, and they brought the wine grape and wine making to what is now France, Germany, and England The skill of wine making was preserved and passed on in the monasteries of Europe during the Middle Ages and the Renaissance In the mid-19th century, innovations introduced by Louis Pasteur transformed the ancient art of wine making into the science of enology In North America, the wine grape was introduced into California in 1769 by the Franciscan missionary Father Junipero Serra, but wine making did not become established until the 1800s The wine industry in California was almost destroyed by prohibition in the early 20th century, but renewed growth in the 1960s and 1970s established California as one of the major wine centers in the world The wine grape Wine begins with the wine grape, Vitis vinifera, native to the area around the Caspian Sea between Europe and Asia North America has many species of native grape, but only Vitis labrusca of the eastern U.S. has been used to any great extent for making wine (see fig. 24.3) The wine grape is basically of two types, red or white, referring to the color of the grapes; thousands of varieties of these two types exist (see table 24.1) Since the beginning of the 20th century, shoots of the wine grape have been routinely grafted onto rootstocks of North American vines North American vines are resistant to an insect pest (the root-feeding aphid Phylloxera) that devastated vineyards throughout France, Germany, Austria, and even California (if they were planted with European wine grapes) (see fig. 24.4) The cultivation of grapes is called viticulture (see fig. 24.5); it usually takes 3 years for new vines to produce enough grapes to harvest; fruit production improves annually until a plateau is reached when the vines are 6 to 8 years old; vines can be productive for 40 to 50 years Grapevines are subject to several fungal diseases, powdery mildew of grape being the most devastating Harvest Approximately 20% of the juice of ripe grapes is a mixture of glucose and fructose, and approximately 1% is a mixture of organic acids In the fall, grapes are harvested when their sugar content reaches a certain critical level; too sugary a grape produces wine that is flat and more susceptible to bacterial spoilage; too acidic, the wine tastes green or hard Red or white Grapes must be crushed, pressed, and fermented to become wine (see fig. 24.6) Crushing breaks the skins to start the flow of juice; the resulting mix of skins, seeds, pulp, and juice is called must Pressing squeezes the skins to express the last bit of liquid in such a way as to free the juice of skins, pulp, and seeds Leftover skins and seeds make up a cake, or pomace, usually returned to the vineyard as fertilizer; the expressed juice is transferred to tanks or barrels in preparation for fermenting The juice of all wine grapes, whether red or white varieties, is initially white; wines develop a red color when the juice is left in contact with broken red skins, which contain anthocyanin pigments and tannins that impart the color For white wine from white or red grapes, the skins are separated from the juice in the press before fermentation – the grapes are crushed, pressed, and then fermented; for red wine, red skins are not removed from the juice until after fermentation – the grapes are crushed, fermented, and then pressed; pressing to remove red skins in the middle of fermentation produces rosé, which has a pink or pale red shade Fermentation Sulfur dioxide is added to the juice to inhibit the growth of any unwanted yeasts and bacteria brought in with the grapes; it also removes oxygen from the fermentation vat Specially developed wine yeast strains are then added to the juice, and the yeast ferments the grape sugars to ethanol and carbon dioxide; waste heat, which is produced during the process, can negatively affect the wine and must be monitored carefully If fermentation temperature is too low, white wine develops an overly yeasty taste and red wine becomes too fruity and thin-bodied; if temperature is too high, wine loses its fruitiness and tastes “cooked” (and the yeast may be killed, stopping fermentation completely) Red fermentation is very active because pressing takes place after fermentation; a bubbling surface cap of skins and seeds forms on the surface of the fermentation tank, and the cap must be broken up several times a day Fermentation goes on for days or weeks, and the rate is usually monitored by measuring decreasing sugar concentration; when there are no more fermentable sugars in the wine, fermentation is complete In the production of dry wine, fermentation is complete when there is no residual sweetness For sweet wine, fermentation is interrupted while there are still sugars present by lowering the temperature, filtering the wine of yeast, or adding sulfur dioxide to inhibit the yeast The alcohol concentration of table wines averages 12%; yeast cells die at alcoholic concentrations much above this Another way to produce sweet wine is to interrupt fermentation while there are still sugars present by adding alcohol to a concentration that will kill the yeast; sweet wines produced this way usually have alcohol contents of about 15-21% Most of the carbon dioxide produced during fermentation is allowed to escape, producing a still wine, a wine that is not highly carbonated Clarification During clarification, wine loses its murky appearance and becomes crystal clear Clarification begins with racking (see fig. 24.6); sediments collect on the bottom of the settling vat, clarifiers are added to remove suspended particles, and the wine is filtered Filtering through millipore filters, which have very tiny pores, excludes unwanted microbes; pasteurization – heating the wine to 81º C (180º F) – or adding a preservative are other ways to inhibit microbes Aging and bottling White wines, if aged at all, are generally not aged as long as red wines The aroma and flavor important to red wines (and some whites) are obtained by aging the wine in oak barrels (usually white oak from France) or small casks for several months to a year of more (see. Fig. 24.6) A second fermentation may be induced in the barrel during aging to convert malic acid to lactic acid; lactic acid is a weaker acid, and this reduces overall acidity of the wine During bottling, the wine is placed in sterile bottles, corked, sealed, and stored bottoms up; if Acetobacter bacteria and too much oxygen get into the bottle, ethanol can be converted to acetic acid (vinegar) The label provides a great deal of information about the wine, such as the grape(s) used to produce it, where the grapes were produced, the year of production, where the wine was produced, and who produced the wine (see fig. 24.7) “Drinking Stars” Sparkling wines result when excess carbon dioxide from a secondary fermentation remains trapped within the bottle True champagne is sparkling wine produced from grapes grown in the Champagne region of France The art of champagne making was greatly influenced by a 17th-century Benedictine monk from the Champagne region, Dom Perignon; he described the sensation of champagne as “drinking stars” Dom Perignon learned how to deliberately induce the bubbles; he also perfected the art of blending different wines to come up with the base for the champagne, and he developed the method of pressing red grapes before fermentation to get white wine The base wines (cuveé) are made separately from three principal grape varieties: the white Chardonnay grape and the red Pinot Noir and Pinot Meunier grapes The three young wines are then blended according to the champagne producer’s specifications to achieve a characteristic taste A mass method of production involves placing the cuveé in large pressurized tanks to which sugar and yeast have been added; in cheaper productions, the wine is artificially carbonated In the traditional method (Méthode Champenoise), the blended wine is bottled and undergoes a second fermentation in the bottle (enabled by the addition of sugar and yeast to the bottled wine) The champagne bottles are made of thicker glass than typical wine bottles so that they can withstand the pressure from the carbonation; also, the bottles have been sealed with metal crown caps After the secondary fermentation in Méthode Champenoise, the wine remains in contact with the lees (sediment composed mainly of spent yeast cells) for several months to a year to develop aroma and flavor The next step is riddling, the process of collecting sediments from the secondary fermentation (see fig. 24.8); upended bottles are slowly turned each day until the sediments have collected in the neck in a tiny plastic bucket that was inserted during bottling just inside the crown cap During the disgorgement step, the sediments are removed from the bottle; the neck of the inverted bottle is immersed in a subzero solution to freeze the sediments into a plug, the crown cap is removed, and carbonation pressure blows out the plug During the dosage step, wine or brandy is added to replace liquid lost during disgorgement, then the bottle is corked; sugar may also be added during the dosage step if a sweeter champagne is desired; brut champagne is the driest, and the designation doux indicates the sweetness Pink champagne can be made by blending a red wine in the cuveé or by permitting some skin contact with the red grapes before pressing Fortified and dessert wines If an alcoholic content above 14% is desired (concentrations above 14% kill the yeast), then alcohol, usually in the form of brandy, must be added and the wine is said to be fortified (fortified wines usually have an alcoholic content between 15% and 21%) In the production of a dessert wine, natural sweetness is maintained by adding brandy to bring the fermentation process to a halt while sugars are still present (e.g., port and sweet Madeira) Brandy can also be added to fully fermented and relatively dry wine (e.g., dry sherry) The taste and aroma of sherry are due to the oxidation of some of the ethanol to acetaldehydes; it is this oxidation that imparts the “nutty” flavor of a sherry After it is fortified with brandy, sherry has an alcoholic content of 17-20% The Brewing of Beer (see fig. 24.10) Records and artifacts indicate that ancient peoples such as the Sumerians, Egyptians, Hebrews, Incas, and Chinese all knew and practiced the art of brewing (see fig. 24.9) Barley malt Beer can be made from any starchy carbohydrate source, but barley, used by the ancients, is the basis of most modern beer Because beer making begins with starch rather than individual sugars, there are several preliminary steps in brewing before the yeast is introduced and fermentation can begin The first step is the preparation of the malt Barley grains are moistened with water, spread out on a malting floor, and allowed to germinate for a short time As barley grains germinate, they produce enzymes that catalyze the breakdown of starch The germinated grains are dried in a kiln (oven) and crushed to make malt powder The color of the finished beer is determined by how the malt was roasted; lighter colored beers are made with standard malts dried at lower temperatures, and darker beers are made with specialty malts that are roasted at higher temperatures for longer to caramelize the malt sugars Mash, hops, and wort The malt is added to the grain starch and water, and heated in a mash tun; this mixture is the mash, and during this step the malt enzymes break down the grain starch to fermentable sugars The starch of any cereal grain (e.g., wheat, corn, rice, barley) can be the start for the brewing process; beers made with barley malt and barley grain are known as malt liquors; beers in the U.S. typically use rice as the grain starch Eventually, the mash is strained, producing a clear, amber liquid called wort The wort is boiled with hops in a brew kettle; hops (Humulus lupulus, a member of the hemp family, Cannabaceae – see fig. 24.11) adds a source of desirable bitterness that counteracts the sweetness of the malt Much of the aroma and flavor of beer is attributable to the hops; hopping apparently has an antibacterial action, keeping the beer from spoiling, and hopped beers are said to retain the foamy head longer Fermentation and lagering The wort is strained and cooled and placed in the fermentation tank, and then the yeast is introduced to begin fermentation Two types of yeast are used; lager beers are fermented by yeast that settle to the bottom of the tank; other beers are fermented by yeast that rise to the top of the tank Ale is produced by a top-fermenting yeast that ferments at higher temperatures than the cold-fermenting lager yeasts; most beers in the U.S. are lager beers Fermentation continues for up to a week, at which time the wort is now a beer This new or green beer is transferred to lager tanks where it is aged for three weeks; the flavor is often developed by adding beechwood chips to the tank A second fermentation is started during the aging by adding freshly yeasted wort; this produces the natural carbonation of beer (note: most mass-produced American beers are carbonated at bottling) After lagering, the beer is clarified and packaged in bottles, cans, or kegs Light beer is produced by fermenting more of the carbohydrates in the mash; most nonalcoholic beer is produced by brewing normally and then reducing the alcoholic content to less than 1.1% by evaporation or distillation Sake, a rice “beer’ For sake, rice provides the grain starch that must be converted into sugars before yeast fermentation can begin To do this, steamed rice is mixed with the spores of the mold Aspergillus oryzae The inoculated rice is heated to 35º C (95º F) for about a week As the fungus grows on the rice, it produces enzymes that convert the starch into sugars After several days, the resultant koji is mixed with more steamed rice, and yeast is introduced to begin fermentation The resultant moto serves as a starter culture and is mixed with the remaining batch of steamed rice (the moromi), continuing fermentation for up to 3 weeks Sake has an alcoholic content of 20% Main Points of A Closer Look 24.1 – Disaster in the French Vineyards The mid- to late 19th century was an especially difficult time for the wine industry in France Starting in the 1860s, root-feeding aphids, Phylloxera, were attacking the grapevines In an attempt to solve the problem, rootstocks of North American grapes (resistant to damage by the aphids) were introduced into France and shoots of local French varieties were grafted onto the rootstocks Unfortunately, the North American plants introduced the fungus Plasmopora viticola, which causes downy mildew of grape By the late 1870s, the fungus threatened vineyards throughout France The turnaround came with the widespread use of a fungicide containing copper sulfate and lime, Bordeaux mixture This lecture outline was prepared mainly from Plants and Society, by Levetin and McMahon, 2003 (3rd edition), and may contain phrases or entire sentences taken verbatim from that source.
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