Distillation
Distillation is a means of separating liquids through differences in their vapor pressures.
Known since antiquity, the concentration of alcohol by the application of heat to a fermented liquid mixture is perhaps the oldest form of distillation (see distilled beverages). However, the technique is now widely used for a variety of liquids in the chemical industry and in the production of petroleum products, among other fields, despite the fact it is energy-consuming.
The liquid mixture evaporates, such that the vapor has a composition determined by the chemical properties of the mixture. Distillation of a given component is possible, if the vapor has a higher proportion of the given component than the mixture. This is caused by the given component having a higher vapor pressure — and thus a lower boiling point — than the other components.
However, chemical bonding between the components of the mixture creates properties unique to the mixture. The important one is an azeotrope. This is a composition, where both components have the same vapor pressure in the mixture, even if they have different ones when pure or in other compositions. At an azeotrope, the mixture contains the given component in the same proportion as the vapor, so that evaporation does not change the purity, and distillation does not effect separation. For example, ethyl alcohol and water form an azeotrope of 95% at 78.2°C.
By the nature of the process, it is theoretically impossible to completely purify the components using distillation, as distillation only tends to purity, never reaching it. This is comparable to dilution, which never reaches purity. If ultra-pure products are the goal, then further chemical separation must be used.
The minimum in distillation is flash distillation, where either the temperature is rapidly increased or pressure reduced, and vapor and liquid fractions are thus obtained, which may be processed as such. The device used in distillation is referred to as a still and consists at a minimum of a reboiler (pot) in which the source material is heated, a condenser in which the heated vapor is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid is collected.
The equipment may affect separation by one of two main methods. Firstly the vapours given off by the heated mixture may consist of two liquids with significantly different boiling points. Thus, the vapour that is given off is in the vast majority of one or the other liquid, which after condensation and collection effects the separation.
The second method (fractional distillation) is more effective at separating liquids with similar boiling points. This method relies upon a gradient of temperatures existing in the condenser stage of the equipment. Often in this technique, a vertical condenser, or column, is used. By extracting products that are liquid at different heights up the column, it is possible to extract liquids that have different boiling points. The greater the distance over which the temperature gradient in the condenser is applied leads to easier and more complete separation.
It is also possible to separate fractions by cooling, using differences in their freezing points. In the American vernacular, this was known as 'jacking'. The most popular drink produced by this process during Colonial times was applejack, which was fermented apple cider that was then frozen in the winter months, and for which the liquid (containing the most alcohol) would be poured off to make applejack, with the result of approximately doubling the alcohol content of the resulting beverage.
Many countries tax distilled alcohol, and preserve government income by legal restrictions on the use of a still.
Distillation was developed into its modern form with the invention of the alembic by Arab-Yemeni (Iranian-born) alchemist Jabir ibn Hayyan c. 800; he is also credited with the invention of numerous other chemical apparatus and processes that are still in use today. Chemists often use distillation in their work as a means of separating compounds or components. A distillation apparatus sometimes used by chemists is a rotary evaporator to distill (or evaporate) away solvent from a mixture.
Infections in brewing and distilling
Introduction
Microbial contamination of alcohol production systems usually brings out formation of compounds such as organic acids that inhibit yeast function or metabolites that impart undesirable properties to the finished product (BACK, W., 2003; CONNOLLY, C., 2000). Scavenging of essential nutrients is also a problem because most micro-organisms of concern are well adapted to the nutrients rich environment of mash or wort and represent very potent competitors to yeast. Bacterial and fungal contamination can lead to reduced yields of ethanol, lower yeast crops, reduced carbohydrate utilisation and a dramatic drop in pH value and therefore distort the efficiency of the process no matter if brewing beer or producing fuel ethanol (MAKANJUOLA, D.B. et al., 1992).
Microbial contamination can arise from various points in an alcohol production process. Raw materials, air, brewing liquor, additives and even pitching yeast can act as a constant supply of contaminants. Residues remaining in tanks, pipelines, valves and heat exchangers harbour bacteria that represent a potential source of recontamination of the process. Of most concern in this context are synergistically acting societies of bacteria and fungi that form bio-films in various stages of the production and therein maintain their own ideal microclimate (BACK, W., 2003). Modern clean-in-place (CIP) systems have evolved to be extraordinarily effective but still can not guarantee 100% cleanliness. Therefore the purpose of sanitary control and process observation in the brewhouse or distillery is to maintain conditions that will control spoilage organisms in the operations to manageable levels, i.e. levels that will not impair production efficiencies or product quality (BACK, W. et al., 1988, WRIGHT, S.A., 2000, BAMFORTH, C., 2002).
Bacterial Contaminants
Gram positive Bacteria
Gram positive bacteria are generally regarded as the most threatening contaminants in alcohol production. Most hazardous to the alcohol industry are those belonging to the genera Lactobacillus and Pediococcus. They are virtually present in every alcohol plant and can cause a variety of problems (CONNOLLY, C., 2000; BACK, W., 2003). Because of their rapid growth rate and tolerance to high temperatures and low pH conditions they are the most troublesome contaminants found in breweries, distilleries and fuel alcohol plants. These bacteria spoil beer by producing haze or rope and cause unpleasant flavour changes such as sourness and atypical odour (JACOBSEN, M. and JESPERSEN, L., 1996, SAKAMONTO, K and KONINGS, W.N., 2002). Gram positive bacteria produce a variety of metabolic by-products that affect flavour and quality distilled spirits and decrease ethanol yield in fuel alcohol production processes.
Lactic acid is their major end product of bacterial carbohydrate metabolism and the group of bacteria involved can be roughly divided into three classes: obligately homofermentative, facultatively heterofermentative and obligately heterofermentative. Lactic and acetic acids are toxic to yeasts at 1 – 4 % (w/v) and 0.05 – 0.9 % (w/v) respectively (CONNOLLY, C., 2000). Lactic and acetic acid act synergistically to inhibit yeast growth. According to CHIN, P.M. and INGLEDEW, W.M. (1994) they do not necessarily affect the yeast’s ability to ferment sugars but can cause substantial losses in yeast viability. This would present serious problems in continuous fermentation systems or if yeast were to be harvested for reuse.
Lactobacilli
Lactobacilli are anaerobic, microaerophilic or aerotollerant, catalase negative rod-shaped cells. Like all lactic acid bacteria they are very acid tollerant and have complex nutritional requirements (KANDLER, O. and WEISS, N. 1986). The genus Lactobacillus is the largest genus of lactic acid bacteria and includes numerous species, amongst these L. brevis, L. lindneri, L. curvatus, L. casei, L. buchneri, L. coryneformis, L. plantarum, l. brevisimilis, L. malefarentans and L. parabuchneri are the most predominant in alcohol production systems (JACOBSEN, M. and JESPERSEN, 1996). Lacobacillus ssp. are hop tolerant and may produce silky turbidity and make beer sour. Some strains produce diacetyl (SPEDING, G. and LYONS, T.P, 2001).
The heterofermentative L. brevis appears to be the most important beer spoiling Lactobacillus species and is detected at high frequency in beer and breweries. It is reported to cause superattenuation due to its ability to ferment dextrins and starch. (LAWRENCE, D.R., 1988) More than half of the bacterial incidents in brewhouses around the world are caused by this species. (BACK, W. et al., 1988, BACK, W. et al. 1996).
Pediococci
Pediococci are homofermentative, round (coccus-shaped), catalase negative bacteria which grow in pairs and tetrads. Beer spoilage caused by these cocci is characterized by acid formation and buttery aroma of diacetyl. Pediococcus spp. produces rope and extensive amounts of diacetyl like Lactobacillus casei. (SAKAMONTO, K and KONINGS, W.N., 2002). They are found at many stages in the brewing process from wort to finished beer. Several Pediococcus spp. are found in breweries among them P. damnosus and P. inopiantus represent the most common beer spoilers. P. acidilactici and P. pentosaceus can grow during early stages of the mashing process but are destroyed during boiling or by hop bitterness (SIMPSON and TAGUCHI, 1995, JACOBSEN, M. and JESPERSEN, L., 1996, SPEDING, G. and LYONS, T.P, 2001; BACK, W., 2003).
P. damnosus is described to produce biogenic amines mainly in contaminated bottled beer but also during sweet wort production (KALAC, P. and KRIZEK, M., 2003). Biogenic amines are a group of undesired substances in foodstuffs and beverages. They are reported to cause hypertensive crisis, headaches and are probably capable of triggering allergic or allergic like responses.
Micrococci
Micrococci are only represented by one species relevant to breweries. BACK and co-workers (1998, 2003) report a fruity aroma when Micrococcus kristinae is present in wort even in low numbers.
Gram-Negative Bacteria
Important Gram-negative contaminants in the context of brewing and distilling are acetic acid bacteria, Pectinatus ssp., Megashera ssp., Zymomonas ssp. and various Enterobacteriacae. Several members of this group not only distort the fermentation process or produce undesired by-products but also have been reported to survive the fermentation process and to transfer into the finished product. As obligate beer spoilage organisms they may cause severe damage to product stability and flavour (JACOBSEN, M. and JESPERSEN, L., 1996, WRIGHT, S.A., 2000).
Acetic Acid Bacteria
The relevant acetic acid bacteria are rod shaped aerobic bacteria from the genera Acetobacter and Glucanobacter. Both are capable of converting ethanol into acetic acid. Both are very small catalase positive and oxidase negative cells (BACK, W., 2003; SPEDING, G. and LYONS, T.P, 2001). Acetobacter is responsible for ropiness of beer whereas Glucanobacter causes a cider-like aroma and prefers high gravity environments. The most likely places to find acetic acid bacteria are the yeast propagation tank and the beer well. Conditions that promote yeast growth are also favourable for the propagation of acetic acid bacteria. Through ethanol scavenging, significant acetic acid production and the competition for nutrients both types of bacteria can cause losses of ethanol yield (CONNOLLY, C., 2000).
Pectinatus
Pectinatus cervesiiphilus and P. frisingens are reported as obligate beer spoilage organisms. They are strictly anaerobic rod shaped and produce a range of off flavours through acids and organic sulphur compounds (BACK, W., 2003; SPEDING, G. and LYONS, T.P, 2001; ALLEN, F, 1994). They can survive in low alcoholic beer and have become more important with the increasing production of non-pasteurised or flash pasteurised beer. Improved bottling technology which results in significantly reduced oxygen content in packed beers may produce conditions in favour of Pectinatus ssp. (JACOBSEN, M. and JESPERSEN, L., 1996). P. cervesiiphilus has proved to be very tolerant towards hop bitterness and is capable of causing beer spoilage by production of butyric acid, hydrogen sulphide and the development of turbidity (HAIKARA, A and LOUNATMAA, K., 1987).
Zymomonas ssp.
Zymomonas ssp. form clumps or rosettes of plump rod shaped cells. They are anaerobic, survive under low pH conditions and are ubiquitous contaminants of water, soil and plants. They are responsible for off-flavours like actetaldehyde and rotten apple (ALLEN, F, 1994; SPEDING, G. and LYONS, T.P, 2001). Off importance is the species Z. mobilis which is aerotolerant and stable at ethanol concentrations below 10% (w/v). It can ferment glucose and fructose and occurs typically in ale and distillery mashes (JACOBSEN, M. and JESPERSEN, L., 1996). Z. mobilis is commonly found where molasses is used as a feedstock such as in rum production.
Enterobacteriaceae
Enterobacteriaceae are a family of catalase positive and oxidase negative rod shaped cells that are often found in water and culture yeast. Only some genera have the potential to cause problems in alcohol production systems. Enterobacteriaceae produce a variety of endproducts from glucose such as lactic, acetic and succinic acids, acetoin, 2,3 - butanediol and ethanol. Herewith they have a significant effect on production efficiency and quality of the final product (CONNOLLY, C., 2000). Commonly found are Hafnia protea, H. alvei, Klebsiella pneumonieae, Enterobacter cloacae, E. aerogenes, E. agglomerans (Rahnella aquatilis) which represent wort spoilers but do not survive into beer (VAN VUUREN, H.J., 1996)
Fungal Contaminants
Moulds
Aureobasidium pullulans, Moniliella, Geotrichum candidum are the most predominant moulds that are brought into the process through air, raw materials and return bottles. Most moulds are relatively easy to control and not of concern by directly threatening the process of alcohol production but through their potential to initialise the agglomeration of other harmful moulds such as Aspergillus ssp. or Penecillium ssp. and spoilage bacteria (BACK, W., 2003).
Yeasts
Yeasts occurring as contaminants in the context of alcohol production are defined as "wild yeast" and in practise they are distinguished from cultures of brewing yeast by their ability to grow on a number of well-defined selective substrates. Brewing contaminants are traditionally divided into Saccharomyces and the non-Saccharomyces yeast genera Bretannomyces (Dekkera), Kloeckera, Pichia, Candida, Rhodotorula, Cryptococcus and Hansenula (VAN DER AA KÜHLE, A. and JESPERSEN, L.; 1998; JACOBSEN, M. and JESPERSEN, L., 1996, SPEDING, G. and LYONS, T.P, 2001; BACK, W., 2003).
Saccharomyces spp. in general are considered to be the most hazardous. The majority of the Saccharomyces brewing contaminants detected belong to S. cerevisiae but also other Saccharomyces spp. have been reported. The taxonomy of yeasts as brewing contaminants as for yeasts in general is not very clear, especially as the taxonomy within the genus Saccharomyces has been subject to several changes within recent years. Moreover, the genetic diversity among these contaminants as well as their relationships to culture yeasts have never been thoroughly investigated (PETERSEN, K.M. et al., 2000; JESPERSEN, L. 2000).
Detection
Considerable efforts have been made by many microbiologists to understand and characterise contaminants of the alcohol production process. The most commonly used method today for detecting bacteria and fungi is still traditional incubation on culture media with various staining procedures and the use of a microscope (HOLZAPFEL, W. H., 1992). A number of selective media (Table 1) were developed since PASTEUR, L (1876) but the development of rapid detection methods is increasingly important. In modern alcohol production only the technology of ATP bioluminescence has gained widespread acceptance as a method of routine rapid sanitary inspection. Testing is very fast, accurate and inexpensive. Portable testing devices enable production staff to take responsibility for the effectiveness of cleaning programmes. SAKAKIBARA T. et al. (2003) even published methods without cultivation.
The most troublesome spoilage organisms Lactobacillus ssp. and Pedicoccus ssp. are commonly isolated and enumerated by a variety of commercially available microbiological nutrient media such as tomato juice agar (TJA), Raka Ray No.3 agar or VLB-S7 agar. These are made selective by the addition of compounds that inhibit growth of yeasts or Gram-negative genera like the antibiotic cycloheximide and 2-phentylethanol, respectively (ALLEN, F, 1994; STEWART, G.G. and RUSSELL, I., 1998, WRIGHT, S.A., 2000, CONNOLLY, C., 2000, SPEDING, G. and LYONS, T.P, 2001). Several media incorporate beer from the actual brewery to detect the contaminant’s spoilage potential in the individual facility or brew (JACOBSEN, M. and JESPERSEN, L., 1996; BACK, W., 2003). According to the nature of individual lactic acid producing bacteria the samples are incubated anaerobically or microaerobically for further differentiation and to suppress growth of strict anaerobes.
Comments
Microbial contamination of alcoholic beverages could also be regarded as a matter of concern but beer has been recognized for hundreds of years as a safe food. It is hard to spoil and has a remarkable microbiological stability. The reason is that beer is an unfavorable medium for many microorganisms due to the presence of ethanol (0.5–10% w/w), hop bitter compounds (approximately 17–55 ppm of iso-α-acids), the high content of carbondioxide (approximately 0.5% w/w), the low pH (3.8–4.7), the extremely reduced content of oxygen (<0.1 ppm) and the presence of only traces of nutritive substances such as glucose, maltose and maltotriose. These latter carbon sources have been substrates for brewing yeast during fermentation. As a result, pathogens such as Salmonella typhimurium and Staphylococcus aureus do not grow or survive in beer (SAKAMONTO, K and KONINGS, W.N., 2002 BACK, W., 1994;).
No single medium is capable of detecting all strains of Gram-positive bacteria and various media and incubation procedures must be used in combination. The selection of methods is often based on the individual experience of each laboratory and adapted to the individual distillery or brewhouse. Various very exact methods of identification and enumeration are available and latest focus on genetic fingerprinting. Most authors agree that very few genetic methods seem attractive as a means of routine quality assurance in practical production.
Automated CIP programmes alter basic and acid solutions in combination with the physical force of turbulent rinses not only to reduce occurring spoilage organisms but to maintain an environment that appears unattractive to potential contaminants. In this respect numerous publications emphasise that batch fermenting systems have a great advantage over continuous systems. Modern process engineering and bottling systems have improved the anaerobic conditions throughout the process and reduce aerophillic species but might pose a new threat through the promotion of obligate anaerobes (BACK, W., 2003; SPEDING, G. and LYONS, T.P, 2001). The widespread use of antimicrobials in fuel ethanol production (CONNOLLY, C., 2000) do not represent an option for the beverage industry or the upcoming fuel ethanol industry in Europe which intends to market distillery by-products as animal feeds (SPANDERN, M. 2003). Good manufacturing practice and cleanliness are key to reducing the risk of contamination.
See also
External links
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