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Dem Thoi Gian | Mon Mar 14, 2011 4:59 am by ๑۩۞۩๑ (¯`•admin•´¯) ๑۩۞۩๑ | Công sức đó hen !!!!
Bạn hãy …
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Chay Nguoc | Sun Mar 13, 2011 10:53 pm by ๑۩۞۩๑ (¯`•admin•´¯) ๑۩۞۩๑ | VisualBasic Chay Nguoc thoi gian
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Tổng Phần Tử..1+1/2.+..+1/n | Thu Mar 10, 2011 2:18 pm by ๑۩۞۩๑ (¯`•admin•´¯) ๑۩۞۩๑ | Dim i,tong As Integer
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| 9.10 BEER The popularity of products resulting from the conversion of sugars into ethanol by yeasts is almost universal and there is hardly a culture without its own indigenous alcoholic beverage. All that is required is a material that will furnish sufficient fermentable carbohydrate; a condition fulfilled by honey, cereals, root crops, palm saps and many fruits, pre-eminently grapes, but also, apples, pears, plums and others. The ethanol concen- tration achieved by fermentation is limited by the sugar content of the raw material and also by the ethanol tolerance of the yeast which is normally around 14%v/v. Sake, Section 9.12.2 below, is something of an exception. Potency can be increased by distillation of a fermented wash to produce spirits such as whisky, vodka, brandy, calvados and arrack, and ethanol partially purified by distillation can also be added back to a fermented product to give fortified wines such as port, sherry and madeira. Here, we will concentrate on a single product which has spread throughout the world and is now produced more widely than any other alcoholic drink: European-style beer. Brewing is thought to have originated inMesopotamia where it is said that as much as 40% of total cereal production was used for this purpose. Because of the relative complexity of the process, it is likely that beer was a later discovery than wine. The Romans were disinterested | |
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| and after tasting British ale in the 4th century the Emperor Julian was compelled to pen a little poem: Who made you and from what By the true Bacchus I know you not He smells of nectar But you smell of goat. Clearly the unhopped ale of the time was not to his taste, but even today beer enjoys an inferior reputation to that of wine. Barley is the principal cereal used in the production of beer, although other cereals are occasionally used and wheat beers such as Berliner Weisse and the Gueuze–Lambic beers of Belgium are notable exceptions. Africa has a number of traditional beers produced from local cereals such as sorghum or millet and some of these are produced on a substantial industrial scale. These however are the result of a mixed lactic/ethanolic fermentation and bear little resemblance to European- style beers. One reason for barleys pre-eminence is that the grain retains the husk which affords protection during storage and transport and also acts as an aid to filtration during wort separation. The gelatinization temperature of malted barley starch is also low relative to that of other cereals (52–59 1C) and this enables the starch to be gelatinized (solubili- zed), prior to enzymic digestion, at temperatures which will not inac- tivate the starch degrading enzyme a-amylase. A further advantage is the presence in barley of substantial quantities of a second enzyme, b-amylase, which is essential for the rapid conversion of starch and dextrins to maltose. Since the brewing yeast, Saccharomyces cerevisiae, is unable to fer- ment starch, the first stage in the production of any alcoholic beverage from starchy materials is conversion of the starch into fermentable sugars. Human ingenuity has come up with a number of ways of doing this. In the Oriental Technique, exemplified by products such as sake, mould enzyme preparations like koji are used, whereas the prevalent Western technique uses endogenous starch-degrading enzymes produced in the grain through the process of malting. A third technique used in some native cultures in South America is to use salivary amylase by chewing the substrate so that it becomes coated with saliva and then spitting it out to saccharify and ferment. This approach is not amenable to industrialization and is not, as far as we are aware, the basis of any large-scale commercial production of alcoholic beverages. In malting, the grain is moistened by steeping in water and is then spread on to a malting floor and allowed to germinate. During germi- nation, hydrolytic enzymes, produced in the aleurone layer surrounding the grain endosperm, attack the endosperm, mobilizing the nutrient and | |
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| energy reserves it contains for the growing barley plant. To encouraged this, maltsters sometimes add gibberellins, plant growth hormones which are the natural regulators of this process. The development from a seed to a plant is arrested by kilning which reduces the moisture content of the malt to 3–5%. During kilning, some non-enzymic browning reactions occur between amino acids and sugars in the malt and these contribute to the final beer colour. Darker beers tend to include malts that have been kilned at higher temperatures to promote browning reactions. Nowadays malts are usually bought in by brewers as one of their raw materials and the brewing process proper starts with its conversion into a liquid medium (wort) capable of supporting yeast growth: a step known as mashing (Figure 9.11). The malt is ground to reduce the particle size and increase the rate of enzymic digestion and is then mixed with hot water. Water, known in brewers parlance as liquor, is an important ingredient in brewing and the quality of the local water was one of the reasons for the development of traditional UK brewing centres such as Burton on Trent, London and Edinburgh. In particular, calcium content has a significant impact on the brewing process because calcium ions precipitate out as calcium phos- phate during mashing. This decreases the wort pH from 6.0 to 5.4, nearer the optimum for a number of malt enzymes, and thus increases the yield of fermentable extract. Starchy adjuncts may be added during mashing to boost the fermentable sugar content of the wort. There are two traditional systems of mashing: the British technique of infusion mashing where the mash is held in a single vessel at a constant temperature of around 65 1C, and the continental decoction system where the mash is heated through a range of temperatures by removing a portion, heating it, then adding it back. Nowadays a number of variations on these techniques are used so that the differences are less distinct. In mashing, a number of enzymic activities contribute to the produc- tion of the clear liquid medium known as sweet wort. For instance, it requires two enzymes operating in concert to break down starch into maltose, a disaccharide of glucose fermentable by the brewing yeast. Barley starch is composed of two fractions: amylose (20–25%), a linear polymer of a-1,4-linked glucose units, and amylopectin (75–80%), a branched polymer containing linear chains of a-1,4-linked glucose units with branches introduced by occasional a-1,6-linkages. Alpha amylase hydrolyses a-1,4-linkages to produce a mixture of lower molecular weight dextrins while the exoenzyme, b-amylase, attacks dextrins at their non-reducing end, snipping off maltose units. Limit dextrins containing the a-1,6-linkages are left in the wort largely untouched unless the non- malt enzyme amyloglucosidase is added to the mash. | |
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| Proteinases solubilize malt proteins and supply yeast nutrients so that about 35–40% of malt protein is solubilized during mashing compared with 90–95% of the malt starch. Phosphatases release inorganic phos- phate, which is important for yeast nutrition and for contributing to the buffering capacity of the wort. The activity of b-glucanases can also be useful in breaking down b-glucans which can cause subsequent handling problems with the beer. After mashing, sweet wort is boiled. This stops the degradative proc- esses by inactivating the malt enzymes. It also pasteurizes the wort, completes ionic interactions such as calcium phosphate precipitation, denatures and precipitates proteins and tannins which separate as a material known as hot break or trub and helps dissolve any sugars which may be added at this stage as an adjunct. Hops are also added during boiling. These are the cones or strobili of the plant Humulus lupulus whose principal purpose is the bittering of the wort. The hop resin contains a-acids such as humulone and cohumulone which are only partially soluble in wort. During boiling they isomerize to isohumulones which are more soluble and more bitter than a-acids (Figure 9.12). Although hop resins have some antibacterial action, they play little part in assuring the bacteriological stability of beer as spoilage bacteria such as lactobacilli rapidly acquire a tolerance to them. Wort boiling lasts for 1–2 h during which 5–15% of the volume is evaporated. Hop residues are then strained off, hot trub is removed in a whirlpool separator, and the hopped wort cooled to the fermentation temperature. The yeasts used to brew ales and lagers are strains of Saccharomyces cerevisiae, known as S. cerevisiae var. cerevisiae and S. cerevisiae var. carlsbergensis (uvarum) respectively. The distinctions between the yeasts used in ale and lager brewing are slight. Traditionally, ale yeasts were regarded as top fermenters which formed a frothy yeast head on the surface of brewing beer and was skimmed off to provide yeasts for pitching (inoculating) subsequent batches, while lager yeasts were bot- tom fermenters which formed little surface head and were recovered from the bottom of the fermenter. Nowadays this is a less useful distinction as many ales are brewed by bottom fermentation. The cardinal temperatures of the two organisms differ and this is reflected in the different temperatures used for lager fermentations (8– 12 1C) and for ale fermentations (12–18 1C). They can also be distin- guished by the ability of S. cerevisiae var. carlsbergensis to ferment the disaccharide melibiose, although this is of no practical import since the sugar does not occur in wort. During fermentation the yeast converts fermentable carbohydrate to ethanol via the EMP pathway. Although this is an anaerobic process, a vigorous fermentation is often helped by aeration of the wort before pitching with yeast. This supplies oxygen, necessary for the synthesis of unsaturated fatty acid and sterol components of the yeast cell membrane, and may sometimes be repeated later in the fermentation. A time course of a typical ale fermentation is shown in Figure 9.13. After an initial vigorous phase during which there is active yeast growth, ethanol production and a drop in pH as nitrogen is removed from the wort, there is a second phase of slower ethanol production in the absence of further yeast | |
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| growth. Overall the yeast population increases about six-fold during fer- mentation.Thisyeastcanberecycled,usuallyafteranacidwashtocontrol bacterial contamination, but eventually its performance drops as viability declines and it is used in animal feed and the manufacture of yeast extract. Lager fermentations take longer due to the lower temperature. The name lager originates from the German word for store and describes the period of secondary fermentation (storage) at low temperature which these products undergo to improve yeast settling, clarification and CO2 dissolution. In the past, this could last for several months, but with modern techniques such as centrifugation and artificial carbonation it is less protracted and is now complete within one to two weeks. Depending on the product, beer from the fermenter can be subjected to a variety of downstream processes. It may be run to casks where priming sugars are added to stimulate the secondary fermentation neces- sary in cask-conditioned beers, or it may be filtered prior to pasteu- rization and kegging. Bottled and canned beers usually undergo a combination of filtration and centrifugation before packing and pasteurization. Brewing is quite a robust process microbiologically due to a combi- nation of factors such as low nutrient status, ethanol content and low pH and there is a limited range of micro-organisms of concern to the brewery microbiologist. Members of the Enterobacteriaceae such as Obesumbac- terium proteus, Klebsiella and Enterobacter species are sensitive to the low pH and ethanol of beer but can grow in wort producing off-odours like dimethyl sulfide which can persist through to the final product. They also contribute to the production of nitrosamines by reducing wort nitrate to nitrite. Obesumbacterium proteus is particularly associated with top-fermenting yeasts but can be controlled by acid washing. Acetic acid bacteria of the genera Acetobacter and Gluconobacter can be found throughout the brewery. As obligate aerobes they are partic- ularly associated with cask-conditioned beer where they cause spoilage as a result of turbidity, ropiness and the oxidation of ethanol to ethanoic (acetic) acid. Zymomonas mobilis is an anaerobic, Gram negative rod which can ferment sugars to ethanol. It causes more problems in ale brewing where it grows in the primed beer producing turbidity and off- flavours. Lactic acid bacteria of the genera Lactobacillus and Pediococcus can grow widely in the brewery environment and in beer where they produce acid, diacetyl, which gives beer a sweet butterscotch flavour, and Table 9.10 Possible taints in beer Taint Associated Flavours Possible Cause Acetaldehyde Apples, paint, grassy Bacterial spoilage: Acetic acid bacteria/Zymomonas Sulphur Bad eggs, drains Formed during fermentation, wild yeasts/Zymomonas Cloves Herbal phenolic Bacterial spoilage/wild yeasts Musty/fungal Mouldy, stale, hessian Mould or bacteria: Usually in water Fruity Estery, pineapples, solvent, bananas, pear drops Wild yeast/Brettanomyes: Wort bacteria/Enterobacter agglomerans Ethyl Acetate Solvent-like Wild yeast/Hansenula anomala Diacetyl Toffee, butterscotch, honey Formed during fermentation/ lactic acid bacterial spoilage: Lactobacillus/Pediococcus DMS Sweetcorn, jammy Bacterial spoilage: Hafnia/ Obesumbacterium. Wort bacteria TCP Acetic Acid Medicinal, antiseptic Sour, vinegar Bacterial spoilage: Wort bacteria Bacterial spoilage: Acetobacter/Gluconobacter Acidic Sourness and creaminess Sourness and apples Bacterial spoilage: Lactobacillus Acetobacter Courtesy L. Hargreaves | |
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| polymeric material known as rope. Yeasts that are not used for the fermentation can cause hazes and off-odours and are known as wild yeasts. Normally these are described as being Saccharomyces and non- Saccharomyces. Saccharomyces wild yeasts can be detected using a medium containing copper sulfate to inhibit the brewing yeast. Non- Saccharomyces yeasts such as Pichia, Hansenula, Brettanomyces and others can be detected with a medium containing lysine as the sole nitrogen source which Saccharomyces cannot utilize. Some of the possible taints in beer and their causes are presented in Table 9.10. 9.11 VINEGAR Vinegar is the product of a two-stage fermentation. In the first stage, yeasts convert sugars into ethanol anaerobically, while in the second ethanol is oxidized to acetic (ethanoic) acid aerobically by bacteria of the genera Acetobacter and Gluconobacter. This second process is a common mechanism of spoilage in alcoholic beverages and the discovery of vinegar was doubtless due to the observation that this product of spoilage could be put to some good use as a flavouring and preservative. The name vinegar is in fact derived from the French vin aigre for ‘sour wine and even today the most popular types of vinegar in a region usually reflect the local alcoholic beverage; for example, malt vinegar in the UK, wine vinegar in France, and rice vinegar in Japan. In vinegar brewing, the alcoholic substrate, known as vinegar stock, is produced using the same or very similar processes to those used in alcoholic beverage production.Where differences occur they stem largely from the vinegar brewers relative disinterest in the flavour of the intermediate and his concern to maximize conversion of sugar into ethanol. In the production of malt vinegar for example, hops are not used and the wort is not boiled so the activity of starch-degrading enzymes continues into the fermentation. Here we will concentrate on describing the second stage in the process, acetification. Acetification, the oxidation of ethanol to acetic acid is performed by members of the genera Acetobacter and Gluconobacter. These are Gram- negative, catalase-positive, oxidase-negative, strictly aerobic bacteria. Acetobacter spp. are the better acid producers and are more common in commercial vinegar production, but their ability to oxidize acetic acid to carbon dioxide and water, a property which distinguishes them from Gluconobacter, can cause problems in some circumstances when the vinegar brewer will see his key component disappearing into the air as CO2. Fortunately over-oxidation, as it is known, is repressed by ethanol and can be controlled by careful monitoring to ensure that ethanol is not completely exhausted during acetification.Most acetifications are run on | |
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