Sourdough Bread Video

Step-by-Step Video on how to make Sourdough Bread

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What are “finings” and what is their role in beer maturation?

Finings are electrically charged and attract yeast residence (Vaughan 1999).  Finings are used for the clarification of cloudy beer (Andrews 2008) and most beers use finings to remove the yeast sediments from a beer (Vaughan 1999). Prior to fermentation, brewing yeast is naturally flocculent at this stage; it also carries a negative charge, other smaller particles mostly protein-polyphenol complexes, precipitates at the lower temperature and these may exhibit either a positive or negative charge at beer pH, the sedimentation of the particles exhibiting a negative charge is often enhanced by use of “isinglass” or “white finings” (Freeman et, al. 2003).  Finings performance is importance for a number of reasons, the clarification time may have an effect on plant throughput by reducing the storage time needed in cold tanks, also the final clarity attained is important, since it affects the run length achieved by the beer filters and in the case of cash ales and it directly affects product quality, it is also very important that the sediment formed is as compact as possible (Freeman et al. 2003).  Over the years many substances have been employed to clarify beer, including egg-white albumen and gelatin, but most efficient has proved to be isinglass (Hornsey 1999).  Isinglass is the dried swin-bladder from certain species of fish (Hornsey 1999).   Brewers are particular about the quality of isinglass because one type is more effective than others (Andrews 2008).  The main source of isinglass are from fish caught in water 10°C either side of the equator such species as catfish, drumfish and threadfins (Hornsey 1999).  To prepare finings fit for brewery use the isinglass is removed from the fish and dries naturally and  if dried too rapidly the product will lose much of its clarification potential (Hornsey 1999). The finings action of isinglass is attributable to the protein collagen which is present in large quantities (Hornsey 1999). Collagen fining may be used in clarification or precipitation process for example for clarifying portable liquors such as beer and wine (Taylor 1997).  On acid hydrolysis, isinglass releases individual tripe polypeptide helices of collagen, each containing numerous positively-charged sites (Hornsey 1999).  The mechanism of fining is thought to be associated with electrochemical interactions between these sites on the collagen molecules and the net negative charge on the yeast all surface, thus promoting the sedimentation of large number of cells (Hornsey 1999).  Although the principle component of isinglass finings is collagen, small amounts of gelatin are also present.  Gelatin is a degradation product of collagen and in some parts of the world, notably the USA it represents the main means of fining beer (Hornsey 1999).  The sources of gelatin are normally animal bones and hides.  Gelatin has a clarification power approximately one-twentieth of that of collagen (Hornsey 1999).

The thermal stability of collagen from different exhibits great variability and this has been attributed to the amount of cross-linking between the individual helices, the higher the degree of cross-linking the greater the thermal stability however collagen from certain sources can even denature about 15°C rendering it totally impractical for brewery use (Hornsey 1999).  All isinglass finings lose their integrity rapidly at temperature above 25°C and storage in the brewery in liquid from should be between 4 and 10°C (Hornsey 1999).  Isinglass will also react with lipids and any negatively charge proteins within the bear and these will precipitate out with the yeast (Hornsey 1999). The reactions between finings and brewer’s yeast are seriously retarded by cations of salts, those of higher valency inhibiting more effectively the reactions.  The best pH level for fining action is 4.4, but at 4.0 the reaction is almost as good. (Briggs 1999).  Copper or kettle fining results in dramatic improvements in cold wort clarity compared to unfined worts and confers several significant process benefits (Leather and Ward 1995).

However some beers do not clarify satisfactory with isinglass finings, this may be due to using to little or too much or the yeast may have a relative small negative charge or concentration of yeast maybe too high,  and the poor finings action may be caused by an excess in positively charged colloidal material in beer, therefore auxiliary finings are used, these can be derived from alginates, carrageenin, or silicic acid and having a negative charge, are added to the beer carefully before normal finings in order to precipitate the positively charged collodials (Briggs 1999)

References:

Freeman, Gourrieree, Patel, Dawson, Powell-Evens, Shipper, Evans, Boulton, Grimmentt (2003), Improving the effectiveness of isinglass finings for beer clarification by optimization of the mixing process Part 1 laboratory scale experiments, 109(4) 309-317.

Hornaey Spenser Ian, (1999), The Royal Society of Chemistry, Cambridge, UK.

Briggs, Hough, Stevens, Young (1999), Malting and Brewing Science, Vol 2, hopped wort and beer, second ed., Aspen Publishers, New York.

Vaughan A., (1999), Bitter Harvest, Better Beer, The impact of beer production and consumption on people and the environment, Sustain, London.

Andrews (2008), Food and Beverage Management, Tata McGraw-Hill Ltd, New Delhi.

Leather and Ward (1995), The Effect of Wort pH on Longer Fining Performance, Vol. IO, pp. 187-190, May-June.

Taylor (1997), Collagen Finings and preparation thereof, Dec.  

Question 2.What changes to the malting parameters are required to dark and pale crystal malt and what is its function in beer?-Eoghan Fulham

Brewers can use various techniques such as the standard reference method (SRM) for example, when evaluating the colour of grains and product. Most techniques used by professional brewers incorporate spectrophotometers to judge the colour of their products while amateurs can use less complex colour scales such as Degrees Lovibond to determine their products colour.  In the case of SRM, pale lagers typically are given a value of 2 and the scale can range as high as 700 for dark malt products.

Barley is the classic ingredient used in brewing. Malted barley (Barley grain that has undergone controlled germination) has significantly positive enzymatic properties vs. ingeminated barley depending on the temperature which it is fermented. For example, malted barley has increased amylase activity, which is desirable in fermentation as it provides yeast with a starch energy source. Controlling the fermentation process and selection of the grain can dramatically change the colour of the overall beer product.

Dark Crystal Malts

Dark malts use dark malt grains that have undergone extensive modifications. The color of the malt, in this case dark may also indicate the flavors and bitterness that will be present in the finished beer product. For example bitterness decreases as malt content increases.  Dark malts are steeped at a degree of 44-47 and are dried at a temperature of 22°-22°C. They are then stored in a moist warm environment. The grains are then subjected to extensive enzyme activity. The grains are kilned at high temperatures, as high as 200°C where the approach a point of carbonising. After the grain has been exposed to temperatures of 105-110°C, the mailliard reaction becomes apparent. This  gives the grain and product a dark appearance.  Due to the high Killen temperatures which dark malts endure, enzyme activity ceases after the kilen processing. These high temperatures also give dark malt products signature colors and flavors. Depending on the quantities used, dark malts can be used to darken the color of beers and other products that may need darkening. In terms of flavor, the mailliard reaction can produce slightly sweet tastes in beer products. Dark malts can also provide a finished product with a slightly charred caramel taste. Dark malts also yield higher protein contents (~12%) then lighter beers which may also contribute to taste

Pale Crystal Malt

Pale malts, as the name suggests use a malt grain pale in colour. The key objective when formulating pale malts is to retain adequate brewing enzyme concentration. This is achieved by steeping at a degree of 40-44 and drying the grain quickly at relatively low temperatures (17°-18°C) the most common pale malts used, are kilned at 90°-110°C. These temperatures do not destroy the grains brewing enzymes. Due to pale malts popularity, it is often used as base malt; this means that it composes a large portion of the grist in many beers. The pale malting process is classified as limited modification process that inactivates enzymes and gives the final product a protein content of ~9%.

References:

Bamforth, C. W., & Barclay, A. H. P. (1993). Malting technology and the uses of malt. In A. W. MacGregor & R. S. Bhatty (Eds.), Barley chemistry and technology, AACC Press.

Coghe, S., Gheeraert, B., Michiels, A., & Delvaux, F. R. (2006). Development of Maillard reaction related characteristics during malt roasting. Journal of the Institute of Brewing, 112, 148–156.

Forster, C., Narziss, L., & Back, W. (1998). Investigations of flavour and flavour stability of dark beers brewed with different kind of special malts. Tech. Q. Master Brewers Association of the Americas, 35

T.H Shellhammer (2009). Beer-A Quality Perspective. USA: Academic Press. p213-227

Question 9: Both Bottle -filling and canning of beer can affect the head of stout and the carbonation level of lager. Discuss this statement.

Beer foam is a colloidal system being built form a gas (CO2) and a liquid (beer). It is formed by CO2 release when bubbles rising in the beer are loaded with surface active substances form the beer, which form more or less stable foam when reaching the surface of the liquid according to Hedreul C. and Frens G. (2001). Foaming properties are often characterised by foaming capacity and foam stability. Foaming capacity or formability is defined as the capacity of the continuous phase to entrap air or gas and foam stability as the ability to retain the gas for a certain period of time according to Indrawati L et al (2008). During the brewing process the largest loss of head retention is during the fermentation process this is due to the loss of foam stabilizing material into the foam head such as alcohols Aldehydes, yeast crop and other negative foam factors. Other Fermentation by- products such as compounds such as Sulphur compounds, Organic acids and Diacetyl can also have a negative impact on head retention as stated by Amalia (2012). Finished beer should be handled gently and finally the maximum head retention will be observed in the beer that is cooled and poured fairly vigorously into scrupulously clean tall narrow glasses- quaternary ammonium detergents remaining on the glassware can reduce head retention as stated by Hough S. J et al (1999).

Gushing, wild over foaming beer is an undesirable quality in packaged beer. A beer is said to gush when on releasing the over pressure innumerable minute bubbles appear throughout the volume of the beer rapidly expand and displace the contents of the bottles. This can be caused by rough handling during filling and storage, Hough S. J et al (1999). Bubble nucleation is an incident that occurs in many processes in the presence of either a supersaturated or superheated liquid. This is commonly seen in the Beverage Industry. It is particularly important to understand bubble nucleation during beer dispense in the case of bubble haze production. Bubble haze is a characteristic of beer which can either be desirable or undesirable to the consumer. Bubble haze occurs when large numbers of micro bubbles are produced, which circulate within the beer rather than rise to the top. Bubble nucleation occurs in the flowing supersaturated liquid during beer dispense. The rate of nucleation during the dispense stage of beer into a glass is expected to be affected by several critical factors which include the effects of gas bubble entrainment and the presence of surfactant on the bubble surface according to Hepworth J. N. Et al (2003). In the study carried out by Hepworth J. N.. Et al (2003) it was found that the rate of bubbles produced increased with increasing flow rates and decreasing nitrogen content in the gas headspace. According to Amalia (2012) the stability of foam is improved by the use of N₂ and CO₂ gas for keg dispenser, using small and narrow bottles, addition of Iron Salts and the addition of gums.

A normal beer keeps its CO2 content until it is in the bottle. Sometimes the CO2 content can decreases a result of a process treatment at lower pressure or increased temperatures and must be renewed before filling. This process is known as Carbonation. The CO2 is injected in bubbles which are as small as possible so that it can dissolve in the beer. During filling it is important to avoid loss of CO2 from beer. It is essential to the quality of the beer to prevent access of Oxygen into the beer during filling which could cause oxidation as stated by Kunze W. (1999).Counter pressure fillers are always used for Beer, as it is critical that high pressures are maintained for the quality of the beer, when the bottle comes out of the filler, foam slowly rises in the bottle neck. The surrounding air has a very great tendency to dissolve in the foam and thus into the beer. The filling of beer cans is based on the same principals as bottle filling. Beer produces a much more stable foam with Nitrogen gas than with CO2 according to Kunze W. (1999). This is incorporated in a different approach in can filling. A plastic/Aluminium insert referred to as a widget is placed inside the can. They are filled with Nitrogen gas. The pressure is greater inside the widget therefore upon opening the can a pressure gradient is formed which causes the Nitrogen to escape out of the widget. The foam produced with Nitrogen consists of very fine bubbles which collapse slowly. It is also more resistant to fatty substances as stated by Kunze W. (1999).

It is seen that both bottle Filling and canning can affect the carbonation and Head retention of beer.

References:

Hedreul C. and Frens G. (2001): Foam Stability. Colloids and surfaces: A Physiochemical and Engineering Aspects. 186(1-2):73-82.

Hepworh J. N., Boyd R. W.J., Hammond M.R.J., and Varley J.(2003): Modelling the effect of liquid motion on bubble nucleation during beer dispense. Chemical Engineering Science. 58; 4071-4084.

Hough S.J., Briggs D. E., Stevens R and Young W. T. (1999): Malting and Brewing Science, Volume 2: Hopped Wort and been. 2^nd Edition. Aspen Publication.

Indrawati  L., Wang Z., Narsimhan G. And Gonzalez J:( 2008) Effect of processing parameters on foam formation using a continuous  system with mechanical whipper. Journal of food Engineering volume.88( 1) 65-74.

Kunze W. (1999): Technology Brewing and Malting. 2^nd Edition. VLB Berlin.

Scannell, A. (2012) Fermented Foods.UCD.

Question 1: How is Munich Malt produced?

   Munich malt is high-kilned malt. High-kilned malt is malt that is kilned at a higher temperature. So it undergoes the same production as normal malt but differs in the conditions used for kilning the malt. Munich malt is associated with dark coloured and high flavour ales.

   Woonton, B. et al explain that malt is produced by the controlled germination of barley grains which is initiated by steeping barley grains in water, followed by germination and kilning periods.

   Briggs, D. 1998, defines the malting process as the limited germination of cereal grains such as barley. The stages of malting are steeping, germination and kilning.

Scannell, A. 2012, defines the objectives of malting to include:

• Initiate germination and embryonic growth
• Production and activation of barley enzymes
•  Enzymatic breakdown of proteins and cell wall carbohydrates

Steeping -

                  Steeping initiates the germination of the barley grain. Mayolle, J. et al state how steeping results in water uptake by the grain, increasing the grain water content up to 43-46%. Water and oxygen are supplied to the interior of the barley kernel, allowing for enzymes to become active and start germination. The rate of water intake influences enzyme formation and growth, also the warmer the water the faster the uptake. Darker malts, such as munich malt have a higher degree of steeping (44-47%) than pale malt (42-44%). The water content of steeped barley is also known as the degree of steeping. Barley is steeped for 36-52 hours to obtain the high degree of steeping found in munich malt. (Scannell, A. 2012)

Germination –

                  The germination process provides water to the barley ensuring that the moistuer content is maintained above 40% in the grain. Controlling the water content along with controlling the temperature during germination is very important. For the production of darker malts such as Munich malt the temperature is kept between 23-25°C. The duration of germination is 8-11 days for dark malts (Gallagher, E. 2009, p 183). During Germination the barley is agitated or turned to ensure even germination of grains. (Scannell, A. 2012)

Kilning –

                  The aim of the kilning process is to dry the malt to obtain a longer shelf life and to stop all biochemical reactions that occur during germination. Another aim of kilning drying is the formation of typical malt flavours from the barley. Kilning is a two step drying process consisting of initial drying and kilning process itself. In initail drying the water content of the malt is reduced to about 10% at temperatures of 45-65°C. After 10-12hours, the end of this initial drying is marked by an increasing outlet of air temperature. In the second phase, the drying air is heatd to 80-85°C, this is done to decrease the moisture content of the malt to 3.5-4%. (Gallagher, E. 2009, p 183)

Vandecan, S. et al; 2011, discuss how important features of dark speciality malts (e.g. Munich Malts), such as colour, flavour and reducing power, are mainly formed due to nonenzymatic browning reactions during the kilning and roasting process. The development of caramel-like flavour components is highly influenced by the roasting temperature and moisture content of the malt kernels.

References:

Briggs, D (1998). Malt and Malting. London: Blackie academic and professional. 713.

Gallagher, E (2009). Gluten-free food science and technology. Uk: Wiley Blackwell. 183.

Mayolle, J.; Lullien-Pellerin, V.; Corbineau, F.; Boivin, P.; Guillard, V. (2012). Water diffusion and enzyme activities during malting of barley grains: A relationship assessment. Journal of Food Engineering. 109, 358-365.

Woonton, B.; Jacobsen, J.; Sherkat, F.; Stuart, I. (2005). Changes in Germination and Malting Quality During Storage of Barley. Journal of the Institute of Brewing.

Vandecan, S.; Daems, N.; Schouppe, N.; Saison, D.; Delvaux, F.R. (2011). Formation of flavour, color and reducing power during the production process of dark speciality malts. Journal of the American Society of Brewing Chemists. 69 (3), 150-157.

Question 3. Explain the objective of "mashing" in beer production and compare and contrast the different mashing processes available to a developing brewer- Roisin Mac Donald

Mashing is an extraction process by which milled barley or malt (usually called grist) is combined with hot water and adjuncts (other cereal ingredients) to produce a fermentable substrate on heating, which provides all the necessary nutrients and precursors for yeast in the fermentation stage (Tse et.al 2003).

The main objective of the mashing process is to maximise the production of fermentable matter by:

·         The enzymatic hydrolysis of the insoluble polysaccharide starch into fermentable carbohydrates (sugars).

·         Breakdown of non-starch polysaccharides into smaller carbohydrate units eg beta-glucans and arabinoxylans (Durand et.al 2009).

·         Degradation of storage proteins into amino acids and peptides, by the action of proteinases  (Jones & Marinac 2002), which can contribute to a haze or foam in the beer (Scannell 2012).

·         Increasing nitrogen compounds available for yeast to use during fermentation (Preedy 2009).

The different mashing processes available to a developing brewer are: 

(a) Infusion mashing system

The infusion mashing system is normally used by traditional British ale brewers. This system is very simple compared to other methods, in terms of number of vessels used and takes place in a single vessel only, called mash tuns (Hough1991), where the conversion and separation of the sweet wort from spent grains take occurs. The texture of infusion mash differs from others as a thick, viscous mash is produced at around 63-67°C by mashing coarsely ground grist (Briggs et.al 2004), containing a large quantity of well-modified malt with a maximum of 10% adjuncts (Hough1991). The liquid wort is removed from the mash, after a standing for 30 mins to 2 and half hours. This wort is re-circulated as it is cloudy and then then runoff becomes clear due to the filtering action through the grist particles. This clear wort is then collected in a holding vessel or transported to a copper where it is boiled with hops. The residual extract, which is carried along in the wet grains, is washed out by spraying hot liquor, at 75-80ºC over the goods (Briggs et.al 2004) Protein rests are not normally used because temperature steps are not employed and some types of beers such as wheat beer usually need a protein rest at lower temperatures, so they are more difficult to brew with this system (Colicchio et.al 2012). The maximum number of brews set in one vessel each day is normally five (Hough1991).

(b)  Temperature-programmed infusion mashing system

Temperature-programmed infusion mashing is normally used by brewers in the UK and mainland Europe and has now started to replace older mashing systems. Finely ground grist and a thin mash are created to permit stirring. Mashing of the grist, containing poorly modified malt or sometimes well modified malt (Briggs et.al 2004) with 30-50% adjuncts (Hough1991), takes place in 1-3 stirred heated mash-mixing vessels, unlike single temperature infusion mash, which are externally heated to 35°C. Stands heat the mash at 50°C, 65°C and 75°C to give proteolysis and starch breakdown (Hough1991). Then a lauter tun or mash filter collects the sweet wort (Briggs et.al 2004). This system uses protein rest, unlike the infusion and double mash systems. The maximum number of brews set in one vessel each day is normally is 12-14, significantly larger than infusion mashing (Hough1991).  

 (c)  Double mash system

The double-mashing system commonly used by North American brewers, which uses very well defined malts that are enzyme-rich and contain nitrogen and also large amounts of maize or rice grits (Briggs et.al 2004). This takes place in three vessels only. High levels of adjuncts are normally used for this process (30-50%) to make use of enzymes and dilute excess nitrogen compounds. This process involves mixing grits with a small amount of malt in a cereal cooker and heated to 65°C, where the viscosity of the mixture decreases due to the action of the malt enzymes, then is boiled unlike the other processes. Throughout this system, the starting temperature of the main malt mash is 45°C to promote starch breakdown and proteolysis. The content of the cereal cooker are transferred to the main mash and the temperature is increase to 67°C causing an increased breakdown of the malt and adjunct starch. To reduce viscosity the mash is heated to 72°C before been pumped into the lauter or mash filler in order to separate the wort. Like infusion mash systems, this system does not use a protein rest. The maximum number of brews set in one vessel each day is normally is 12-14, identical to temperature-programmed infusion mashing (Hough1991).

(d) Decoction system

Decoction mashing is a traditional system used by larger brewers in mainland Europe. Finely ground grist’s are used made from less modified malts.  These mashes are very thin like temperature programmed systems, however they need to be moved by pumping and stirred. This system use 3-4 vessels, which include a stirred mash mixing vessel, a stirred decoction vessel and a device for separating the wort. An initial temperature of 35°C is achieved by mashing the grist. Decoction occurs after standing, where a third of the mash is pumped to a cooker and is heated to boiling point (Briggs et.al 2004). This is the only system that in which the boiling of the malt occurs, and is very important particularly for a less modified malt in order to breakdown the cell walls making the starches more readily accessible for the malt enzymes (Colicchio et.al 2012). The temperature rises to 50°C, when the boiling mash is pumped back into the mash mixing vessel and mixed with its contents. Decoction is carried out after a final stand, where the temperature is increased to 65°C then to 76°C for the final decoction. The mash is then moved to a lauter tun or a mash filter (device for separating the wort) where the sweet wort and spargings are collected, and boiled with hops. Like temperature-programmed infusion mashing, this system uses a protein rest. The maximum number of brews set in one vessel each day is normally is 8, making it the second largest amount of brews set after temperature-programmed infusion and double-mashing systems (Hough1991).

References

Briggs D.E, Boulton, C.A, Brookes, P.A and Stevens R (2004). Brewing Science and practice. Woodhead Publishing Limited, Cambridge, England.

Colicchio,T.,  Bamforth, C.,  Philliskirk, G., Villa, K., Stempfl, W & Hayes, P (2012). The Oxford Companion to Beer. Oxford University Press Inc, New York.

Durand, G.A., Corazza, M.L., Blanco, A.M., Corazza F.C. (2009). Dynamic optimization of the mashing process. Food Control, 20: 1127–1140.

Hornsey, I.S (1999). Brewing. Royal Society of Chemistry (Great Britain), RSC Paperback, Cambridge, UK.

Hough, J.S (1991).The biotechnology of malting and brewing, Cambridge University Press, New York.

Jones, L.B & Marinac, L (2002). The Effect of Mashing on Malt Endoproteolytic Activities. J. Agric. Food Chem.  5: 858-864.

Preedy,V.R (2009).Beer in health and disease prevention. Elsevier Inc, USA.

Scannell, Amalia (2012) Lecture material, file 5 – Yeast.

Tse, K. L., Boswell, C. D.,  Nienow, A. W.  & Fryer, P. J (2003). Assessment of the Effects of Agitation on Mashing for Beer Production in a Small Scale Vessel. Trans IChemE, 81, Part C, 1-12.

Q6. Compare and contrast the maturation process undergone in ale and lagar production- Sinead Mannion

Beer can be classified into two main groups, ale and lager. Ales and lagers differ slightly, mainly due to the fermentation process. Ales are fermented with top fermenting yeasts such as Saccharomyces Cervesiae, where as lagers are fermented with the addition of a top fermenting yeast such as Saccharomyces pastorianus, Hutkins, (2006). 

There are two main methods of fermentation, consisting of primary fermentation, and secondary fermentation (otherwise known as lagering or maturation), (Linko et al., 1998). The primary fermentations of ales are for a short period of 3-6 days at a warm temperature of (15-23°C), where lager takes a slightly longer fermentation period of 1-3 weeks at a colder temperature (7-12°C), (Hutkins, 2006). At the end of the primary stage of fermentation, rancid flavours are released such as diacetyl and acetylaldehyde, (Scannell, 2012). However, according to a study conducted on production of volatile sulphur compunds by ale and lager brewing strains of Saccharomyces Cervesiae, the flavour compounds, hydrogen sulphide, methanethiol and methyl thioacetate appear to be stronger in lager than when the than ale when Saccharomyces Cervesiae is added to both beers during fermentation, Walker and Simpson, (1993).

The fermentation process of lager is long, taking up to many weeks, even months. According to Steward, it can take up to nine months for the maturation of beer in Europe, (2002). The maturation stage is also known as lagering, and is generally the longest step during the entire process. Lager is stored at approximately 0-4°C for a few weeks or months, depending on the type of lager produced, (Saito, 2003). During this period of storing the lager in cool temperatures, the lager begins to turn into a somewhat clear colour, and the natural production of esters and other by products is inhibited, which in turn can give the beer a “crispy” taste. 

The storage conditions of ale and lager differ significantly during maturation. Ales are stored in casks, where the addition of priming sugars, hops, isinglass finings and potassium or sodium metabisulphite are added as preservatives in the beer to prolong the shelf life of the product by eliminating pathogenic organisms that may be present, (Scannell, 2012). Casks are suitable to store ale, as beer stored in casks tend to be low in carbon dioxide, rich in flavour, and the cost of production is low, which is a major benefit to companies producing the product, (Scannell, 2012). However lagers are generally stored in tanks, where the presence of yeast assists the improvement of the flavour present (diacetyl) and the lager begins to form a clear colour.

This step can be accelerated with the addition of the enzyme a- acetolactate decarboxylase, which changes directly to acetoin, (Rostgaard- Jensen et al., 1987). This method can be commonly used in the food industry where demands are high for swift beer production. Conditioning agents are also sometimes used during the maturation stage. Sugar known as “priming sugar” is sometimes added to ales to provide a sweeter taste and colouring such as caramel can also be added for improvement to the color of the ale or lager. During the maturation stage, carbonation also occurs. This step sometimes involves the injection of carbon dioxide into the beer. Ales require less carbon dioxide than lager; therefore the injection of carbon dioxide is more common in lager products than in ales, (Lea et al., 2003).

References:

Hutkins, Robert, W. (2006). Microbiology and Technology of Fermented Foods. Oxford: Blackwell Publishing. p320.

Lea, A. and Piggott, J.R., (2003). Fermented Beverage Production. 2nd ed. New York: Plenum Publishers. p146-150.

Linko, M., Haikara,A., Ritala,A., Penttila, M. (1998). Recent advances in the malting and brewing industry. Journal of Biotechnology. Vol. 65, p85-88.

Saito, J. and Takamoto, Y. (2003). Thermal Convection in Cylindro-Conical Tanks During the Early Cooling Process. Journal of the Institute of Brewing. Vol. 109 (1), p80-83.

Stewart, G. (2002), `Fermentation of high gravity worts its influence on yeast metabolism and morphology', Proc. 28th EBC Cong., Budapest, 36.

Walker, M. D. and Simpson W. J. (1993). Production of volatile sulphur compounds by ale and lager brewing strains of Saccharomyces cerevisiae. Letters in Applied Microbiology. Vol. 16 (1), p40-43.

Question 7 - Laura Cooley

Compare and contrast the fermentation process of top and bottom fermented beer

In 1883, Emil Hansen used serial dilutions to separate yeast cells based on morphology and show that top and bottom fermenting strains produce unique fermentations (Rank et al., 1988).

In ale beers, the strain Saccharomyces cerevisiae is used and is known as a ‘top’ fermenting yeast. On the other end of the beer scale, lager fermentation requires the use of the yeast strain Saccharomyces pastorianus, (formerly known as Saccharomyces carlsbergenisis), which is a ‘bottom’ fermenting yeast strain. Although both strains belong to the same species, they are distinguishable by their biochemical and physiological properties (Leskošek and Stojanović, 2002).

 The names ‘top’ and ‘bottom’ fermenters do not refer to the area of the vessel where fermentation takes place it is, in fact, referring to the type of yeast used and how the yeast grows during the fermentation process. A ‘top’ fermenting yeast produces low density clumps as it grows in the wort. These clumps trap carbon dioxide and rise to the surface of the fermentation vessel. In contrast to this, ‘bottom’ fermenting yeasts flocculate which then settles on the bottom of the fermenter vessel (Hutkins, 2006).

‘Top’ and ‘bottom’ fermenters were originally categorised due to their flocculation behaviour. The behaviour of each type is so distinct that the two main classes of beer, ale and lager, are based on the yeast types (Lodolo, et al., 2008). The temperatures at which these yeasts ferment are different. Top fermenting yeasts, used in ale production, ferment at fairly high temperatures of 18˚C to 27˚C. While, in contrast, lager production uses yeast strains that are capable of fermentation at temperatures below 15˚C. In ale production, fermentation is followed by a short aging period or even no aging period, however lager production, after fermentation is subjected to a long aging period, which can last up to a few weeks, also known as ‘lagering’. Within these categories there can be a range of differences including alcohol content, colour, flavour etc (Kodama, et al., 2006).

Top fermentation is the oldest method of beer production, and until the middle of the 19^th century was the only method used. Beers produced from top fermenting yeast differs from bottom fermenting yeast in their ingredients and by their aroma, which is primarily induced by the strain used, S. cerevisiae (Michael Eblinger, 2009).

Top and bottom fermenters have many differences. For example, top fermenting yeasts are chains of budded cells, only ferment one third of raffinose, have a greater yield crop after fermentation and have a high enzyme content while in contrast, bottom fermenting yeasts are single cells or pairs of cells, ferment raffinose completely, produce a lower yield crop after fermentation and have a low enzyme content. (Scannell, 2012).

References

Hutkins, Robert, W. (2006). Microbiology and Technology of Fermented Foods. Oxford: Blackwell Publishing. p320.

Kodama, Y., Kielland-Brandt, Morten C.,Hansen, J.. (2006). Lager Brewing Yeasts. In: Per Sunnerhagen, Jure Piskur Comparative Genomics Using Fungi as Models. Berlin: Springer Berlin. p145 - 164.

Leskošek, I.,Stojanović, M. . (2002). A possible application of ale brewery strains of Saccharomyces cerevisiae in lager beer production. World Journal Of Microbiology And Biotechnology. 9 (1), p70-72.

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Michael Eblinger, Hans (2009). Handbook of Brewing. Germany: Wiley-VCH. p222

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Scannell, Amalia (2012) Lecture material, file 5 – Yeast.

Question 8-What esters in beer produce undesirable flavours and where in the process can their production be controlled? Erin Fleming

Erin Fleming-11209224

Assignment 1-Task 2

Question 8

Due to their high volatility and low thresholds, esters contribute significantly to the unique flavours and aromas associated with specific varieties of beer. Over 100 different esters have been identified in beer (Eblinger 2009) but most brewing esters are formed by esterification of ethanol with fatty acids and acetyl coenzyme A (Yoshioka et al. 1981).  This reaction results in the formation of ethyl acetate, an ethyl ester which is the most common ester found in beer. Although these esters make up the largest percent of all esters in beer, those which are formed by the acetates of the higher alcohols, also called “banana esters” or “acetate esters” also play a role in the flavour and aromas (Briggs et al 2004).  At high levels and in certain varieties, acetate esters may be regarded as off-flavours.  Therefore, their formation must be controlled.

Controlling the formation of acetate esters in the brewing process can be done in a number of ways.  Manipulating the yeast strain, the wort specific gravity and sugar profile, wort oxygen and lipid content, genetic modification of the yeast, as well as the fermentation temperature will impact the level of acetate esters synthesized. Fewer acetate esters results in a beer with fewer off-flavours.

One of the most important factors affecting ester production is the yeast strain selection (Verstrepen et al. 2003).  The strain of yeast used will affect both the average ester production as well as the relative proportion of each individual ester produced (Verstrepen et al. 2003).  Equally, wild yeast strains such as Hansenula and Pichia produce large quantities of ethyl acetate by aerobic fermentation (Briggs et al. 2004). Therefore, if one wishes to minimize the acetate esters produced, the yeast population should be regulated to avoid wild strains.

Another important influence on esterification is the specific gravity and sugar profile of the wort.  High-gravity operations, which are very commonly used today, result in an unbalanced flavour profile due to the overproduction of acetate esters (Verstrepen et al. 2003).  This overproduction leads to beers which are “over-fruity” or “solvent-like.” The relative amounts of various sugars contained in the wort also affects the formation of acetate esters.  In general, worts which contain higher levels of glucose and fructose produce more esters than those containing more maltose (Younis et al. 2000).  Therefore, by adding a supplemental maltose syrup, one could reduce the acetate ester concentrations.

The formation of volatile esters is diminished with an increase in wort oxygen and lipid content.  Therefore, wort aeration or oxygenation is recommended as a possible means by which to decrease unwanted esters (Verstrepen et al. 2003).  Regarding lipids, unsaturated fatty acids in the wort also decrease the synthesis of unwanted esters.  This is most likely due to the repression of the ATF gene transcription (Meilgaard 2001).  Manipulation of the wort lipid content can be achieved through changes in the filtration process (Verstrepen et al. 2003).  

Generally, a decrease in temperature during fermentation also decreases ester synthesis (Eblinger 2009).  Likewise, increased fermentation temperatures ranging from 10-25°C results in an increase in ester production (Verstrepen et al. 2003).  The exact increases are strain-dependent, but by simply decreasing the temperature at which fermentation occurs, one can minimize esterification.

Acetate ester formation in brewer’s yeast can also be controlled with the genetic modification of specific genes.  According to Verstrepen et al., it is possible to “create new yeast strains with desirable ester production characteristics.”  Additionally, the manipulation of the ATF gene specifically is one of the most important control points affecting ester synthesis (Verstrepen et al. 2003).

References:

Briggs, D; Boulton, C; Brookes, P; Stevens, R (2004). Brewing Science and Practice. New York: Woodhead Publishing. 683-687.

Eblinger, H (2009). Handbook of Brewing Processes, Technology, Markets. Online: Wiley-VCH Verlag GmbH & Co. 134.

Meilgaard, M (2001). Effects on Flavour of Innovations in Brewery Equipment and Processing: a Review. The Journal of the Institute of Brewing and Distilling.107, 271-286.

Verstrepen, K; Derdelinckx, G; Dufour, J; Winderickx, J; Thevelein, J; Pretorius, I; Delvaux, F. (2003). Flavor-Active Esters: Adding Fruitiness to Beer. Journal of Bioscience and Bioengineering. 96 (2).

Yoshioka, K. Hashimoto, N. (1981). Ester Formation by Alcohol Acetyltransferase from Brewers' Yeast. Journal of Agricultural Biology and Chemistry.45

Younis, 0; Stewart, G (2000). The Effect of Wort Maltose Content on Volatile Production and Fermentation Performance in Brewing Yeast. Brewing Yeast Fermentation Performance.1(2).