Table of Contents · Web viewThe main pathway for the fermentation of glucose is the...

BIOL 443 Fermentation Biotechnology Nadine Kallweit (20288655)“Biology of brewing” November 16 th , 2007

Table of Contents

Introduction......................................................................................................................................2Yeast.................................................................................................................................................4Malting.............................................................................................................................................5Mashing............................................................................................................................................5Wort boiling......................................................................................................................................6Hops..................................................................................................................................................6Fermentation.....................................................................................................................................7Traditional Batch Systems................................................................................................................8

Top-fermentation skimming systems and their successors..........................................................8Yorkshire stone squares................................................................................................................8Burton Unions..............................................................................................................................8Lager fermentation.......................................................................................................................8

Continuous Fermentations................................................................................................................9A Specific Example: Cylindrical Fermenting Tower with Baffles to Inhibit Backflow..............9

Beer Treatment...............................................................................................................................11Haze Prevention..........................................................................................................................11Yeast Removal............................................................................................................................11Preservation against microbiological spoilage...........................................................................12Color and Clarity........................................................................................................................12Foam...........................................................................................................................................13Flavor..........................................................................................................................................13

Conclusion and Perspectives..........................................................................................................14References......................................................................................................................................15

1


Introduction

Beer was produced without any knowledge of micro-organisms or enzymes thousand of years

ago. Therefore, brewing is often mentioned as a typical example of traditional or old

biotechnology. Whereas, with today’s new developments, there are more possibilities to improve

the beer brewing process. Examples for this are, transgenic barleys, proper starter cultures in

malting, or continuous bioreactors with immobilized yeast cells. [1]

What is beer? First, in the Pocket Oxford Dictionary beer was defined as “an alcoholic beverage

made from fermented malt, etc. flavored with hops, etc.” or it is generously described as “barley

or other grains prepared by steeping and germination or otherwise, for brewing and distilling”.

Finally, beer is any liquid which is a fermented hop-flavored cereal product that contains four

important ingredients, such as, water, malt, hops and yeast. Especially, the German purity law of

1516 allows only water, barley malt and hops to avoid adding cheaper and poorer ingredients.

The newer Beer Tax Law permits also yeast, and for top-fermenting beers other cereals malts. [2],

[3]

In Figure 1 the conventional brewing process is illustrated. The whole process consists of four

steps: malting, word production, fermentation and down-stream processing. [1]

Fig. 1: Simplified Scheme for brewing [1]

2


Beer Brewing has a very long history. Before 6000 BC, beer was made from barley in Sumeria

and Babylonia. 2400 BC in Egyptain, barley was crushed, mixed with water, and dried into

cakes. The basis techniques of brewing came to Europe from the Middle East. Hops were in use

in Germany in the 11th century. During the 15th century, they were introduced into Britain from

Holland. In 1420, beer was firstly made in Germany by a bottom-fermentation process. [5]

In 1837, T. Schwann recognized the fungal nature of yeast and he called the organism

´Zuckerpilz´, which is translated into Saccharomyces. [7]

In the 1860s, Louis Pasteur established many of the microbiological practices. Additional, Emile

Hansen, a Danish botanist, devised methods for growing yeasts in culture. He discovered the pure

culture technology, which was a meaningful step to this time. And now, in the 21st century,

brewing is a large-scale industry, with stainless-steal equipment, computer-controlled and

automated operations. [5]

The main aim of brewers and researches is to understand the chemistry, biochemistry and

microbiology of the raw materials as well as the processes of malting, mashing and fermentation. [4]

3


Yeast

There is no satisfactory definition for yeasts. They are generally accepted as fungi. Three of the

four groups include yeasts: Ascomycetes, Basidiomycetes and Deuteromycetes.

Saccharomyces cerevisiae is mainly used in brewing processes; therefore, the vegetative growth

cycle and sporulation are very good investigated. Wild sporulation strains are normally diploid.

Different species are named according to their fermentation of sugars, industrial properties or cell

morphology. [8]

Yeast grows in simple media which contain fermentable carbohydrates, such as, glucose,

fructose, sucrose, maltose and maltotriose, ample minerals and accessory nutrients like

potassium, iron, magnesium, manganese, calcium, copper and zinc. A nitrogen source is met by

amino acids. Notably, for a starch-based alcoholic fermentation is the addition of exogenous

enzymes such as α- and β-amylases, β-glucanases and peptidases required. [9], [10]

The main pathway for the fermentation of glucose is the Embden-Meyerhof-Parnas (EMP) route.

It starts with α-D-glucose, which is phosphorylated to hexose monosaccharide glucose. The

major step for fermentation is decarboxylation of pyruvic acid to acetaldehyde and carbon

dioxide using the enzyme pyruvate decarboxylase. With the help of alcohol dehydrogenase and

NADH2 as cofactor the produced acetaldehyde is reduced to ethanol. [9]

Saccharomyces cerevisiae is a glucose-sensitive yeast type. That means, respiration is repressed

in the presence of small concentration of free glucose in the medium. Brewers´ wort typically

contains approximately 1% glucose. [9]

Many strains of Saccharomyces cerevisiae can attain ethanol concentrations of 12-14%. Thus, the

development of alcohol tolerant yeast strains, for increasing the plant productivity and decreasing

distillation costs, is becoming increasingly necessary. The knowledge about temperature- and pH-

optimum to achieve high cell growth and fermentation activity is not neglectable as well. [10]

A ´good´ brewing yeast should have: rapid fermentation rate, an efficient utilization of maltose

and maltotriose, an ability to withstand the stresses, an ideal flocculation character and good

´handling´ characteristics. [12]

4


Malting

The malting process consists of four steps. First, the freshly harvested barley goes through a

period of rest and ´sweating´ from about 30 to 60 days. Within the second step the barley kernels

are immersed in a tank of water at a temperature from about 50°F to 65°F. During the steeping

process, respiration of the kernels becomes noticeable. In the third step, the germination, the

respiration and growing process is accelerated. The barley is removed from the steeping tank to

appropriate conditions of temperature, moisture and oxygen. During this step the barley kernel

begins to ´chit´ at the base of the kernel by showing a white tip. The third step has three

important functions. With the aid of various hydrolytic enzymes including α- and β-amylases, β-

glucanases and peptidases, starches are converted to dextrins and sugars, insoluble proteins to

soluble proteins and a source of desirable taste and aroma is provided. In the fourth step, the

batch of green malt is conveyed to the drying or kiln compartments. Finally, the dried malt is

cleaned and stored. [6], [10], [11], [15]

The malting process is very important because the starch of the endosperm becomes mellow,

enzymes are developed, and the flavor and color ingredients are formed. [6]

Mashing

During the mashing process, the raw materials are extracted and carbohydrates and other

nutrients are solubilized and enzymatically hydrolyzed. The mash is started at a relatively low

temperature, e.g. 122°F, and held to allow sufficient time for the proteinase and β-glucanase to

act. After that, the temperature is raised up to 149°F level which is optimum for amylolysis.

β-Glucanase is an endo-enzyme, which acts on the cell walls of the endosperm and it keeps the

viscosity down. The proteinase is needed to release a range of nitrogenous compounds.

Aminoacids are needed for the yeast growth. Large polypeptides are the most important surface-

active compounds in beer foam. At higher temperatures α-amylase solubilises both the amylose

and the amylopectin compounds of starch. β-Amylase renders the straight-chain carbohydrates to

maltose and the branched-chain compounds to mixtures of maltose and small dextrins. It is

essential that all the added enzymes are inactivated during boiling. It is common, that the grist

contains a small proportion of unmalted grain, say 5-20%. In this case, some brewer may use

industrial enzymes as aids in the mashing process. [10], [11]

There are three systems of mashing: first, infusion mashing, evolved in Britain, where beer wort

is sterilized by boiling before the yeast is added, secondly, decoction mashing, the typical lager

5


mashing system of continental Europe, which works at several temperatures to enable enzymatic

or bacterial action and thirdly, American development of combination or cooker mash, using

steam and the boiling adjunct to accomplish these temperatures. In particular, the higher the

temperature of conversion the faster the reaction, but the lower the quantity of sugar formed. [2], [6],

[10]

In the sparging process, the sweet wort is drawn into a vessel called an underback, and washed

with water at a temperature of about 167°F. The effect is to dissolute all residual soluble

materials and to inactivate any remaining enzymes. Silicates and tannins are concentrated in the

poorly buffered final sparge and phospholipids can be removed. The average gravity value for

final spargings from British mash tuns is 1.0001, for continental Europe 1.004 and for North

America 1.008. The specific gravity shows how many times heavier than water the liquid of

interest is. For instance, 1.050 SG wort is 1.05 times heavier than equal volume of water. [2]

Wort boiling

The medium which is fermented to produce beer or ale is called wort. [6]

The objectives of wort boiling are: inactivation of any enzymes, sterilization of the wort,

completion of ionic interactions, concentration of the wort, denaturation and precipitation of

proteins, dissolution of any additional sugars, isomerization of hop α-acids and volatilization and

removal of unwanted flavor components. [2], [10]

A small portion of malt is first prepared by boiling in a deep closed vessel (cocker). While the

main portion is mixed with water in a large vessel and heated. The cooker mash is then added, the

temperature increases and the conversion of starch to carbohydrates of lower molecular weight

takes place. In the usual European practice, the cooker mash may be omitted. [15], [6]

Hops

Important compounds present in the hope cone are the resins, the tannins and the essential oils.

There are although some other ingredients like pectins, hop waxes and water. The essential oils

are responsible for the beer flavor, known as hop character. The tannins combine with protein

during normal kettle boiling. Not precipitated tannins contribute to haze formation in the finished

beer. There are two classes of hop resins, the hard and the more important soft resins. The other

compounds like pectins, waxes and water have an improving effect on beer stability, the choice

of extraction and isomerization conditions and the foam retention of the beverage. [6], [12], [15]

6


Fermentation

After adjustment to the desired specific gravity, the clarified wort is cooled to about 50°F.

Then it is often aerated with sterile air and injected with yeast for the fermentation stage of the

process. This process is called ‘pitching’. For a period of one or two weeks, the yeast ferments or

breaks down the fermentable sugars of the wort into alcohol and carbon dioxide. The

fermentation process is performed either batch or continuous. [15]

After the fermentation, when more than 95% of the fermentable sugars in a wort are converted,

the yeast cells tend to flocculate and precipitate to form a sediment layer along the bottom of the

tank (Saccharomyces karlsbergensis) or rise to the surface (Saccharomyces cerevisiae). This

process is usually enhanced by cooling the liquid in the fermenting vessels down to about 40°F.

The fermentation of the sugars in the wort is finished after five or nine days. Three or six days are

additionally required for cooling and further yeast sedimentation. The freshly fermented beer is

then removed from the fermentation vessel. After filtration, pasteurization and other treatments,

the beer is ready for dispatch. [9], [15]

According to the system of fermentation is employed, beers may be classified in three different

groups. First, there are bottom-fermented beer types, as the German Lager Beers (Pilsner,

Wiener, Muenchener), the American Lager Beers and the American Steam Beers. On the other

hand, the British beer types like Ale, Porter and Stout and the German Weiss Beer belong to the

top-fermented beer types. The last group comprises the spontanously-fermented beer types, for

instance, the Belgian beers, Lambic and Faro. Basically, the fermentation process consists of two

main parts. During the ‘main fermentation’, maltose is splitted at relative high temperatures. In

the further step, called the ‘after fermentation’, the residual malto-dextrin is digested at lower

temperatures. [14]

There are seven main factors, which influence the fermentation performance and beer quality. 1.

choice of yeast strain, 2. condition of the yeast at the time of pitching, 3. amount of yeast added

to the wort, 4. distribution of the yeast in the fermentation wort throughout the fermentation, and

the size and geometry of the fermentation vessel, 5. Aeration, 6. Wort composition and pH and 7.

Fermentation temperature and pressure. [7]

7


Traditional Batch Systems

Top-fermentation skimming systems and their successors

This is the traditional ale fermentation in a modern enclosed vessel, usually constructed of

stainless steel, with wall attemperation, collection points for carbon dioxide and facilities for in-

place cleaning. The skimming process is difficult, very expansive but very fast. [2]

Yorkshire stone squares

There is a deck or platform slightly below the level of the wort which represent the skimming

system. The yeast collects on the deck and leaves the beer. Therefore is highly flocculent yeast

required. [2]

Burton Unions

Fermenting wort is transferred after about 36 hours to a row of casks each provide with a swan-

neck tube which opens into a common inclined through above the set of casks. [2]

Lager fermentation

Lager Fermentation and ale fermentation differ in the species of yeast used and in the

fermentation temperature, which is not allowed to exceed 50°F. [2]

The starting yeast count is 107 cells/ per ml, the initial carbohydrate concentration is 12°P, the

specific gravity is 1.050 and the initial temperature is about 50°F. During the lager fermentation

process the temperature is rising and cooling is required. The formation of many acids, such as,

acetic acid, drops the pH about 1 unit from an initial value of approximately 5.2. The whole

fermentation takes about 10 days. 70-80% of the wort carbohydrates are converted to ethanol.

The residual 20-30% consists mainly of higher dextrins or limit dextrins which are not

susceptible to attack by malt amylases. With the help of microbial amyloglucosidase, dextrins can

be hydrolyzed. [10]

8


Continuous Fermentations

A Specific Example: Cylindrical Fermenting Tower with Baffles to Inhibit Backflow

During the cylindrical fermenting tower process, is used by M.G. Royston (U.S. Patent, 1967),

the hopped wort moves continuously into and through a fermentation vessel. The vessel

construction inhibits the backflow of the wort and prevents the intermixing of the progressively

fermented wort with the new one. [15]

The high overall rate of fermentation leads to lower costs and low floor space requirements. For

instance, the vessel has a measured value of 1 foot diameter by 20 feet high and the whole

fermentation is completed after 3 to 5 hours. [15]

Fermentation vessel comprises a cylindrical tower (1). At the lower end of the tower (1), inlet

means (2) are provided for introducing the hopped wort, so that the inflow of wort to the tower is

distributed over the cross-selection of the tower. A perforated baffle (3) is may be provided.

For the introduction of air or oxygen at the base of the tower (1), a sparging ring (4), is may be

provided with connection to a source of air or oxygen. The spaced baffle (5) serves to inhibit

backflow of the ascending liquid and assists in maintaining a forward flow of the liquid. When

the apparatus is in full operation, the cell concentration of the yeast will be high and will form a

yeast plug above the level of the perforated wort-distributing baffle (3). The yeast growth

increases in direct proportion to the sugar conversion taking place as the process proceeds. [15]

Fig.2: Cylindrical Fermenting Tower [15]

9


1: cylindrical tower

2: wort inlet

3: perforated baffles

4: sparging ring

5: spaced baffles

6: separating zone

7: beer outlet

8+9: water jacket

The yeast culture is isolated on a wort agar plate and finally transferred in liquid wort in a 2-liter

Pasteur flask. There is a sterile connection between the Pasteur flask and a sample point on the

tower. The flow introduction is continued and aerated with sterile air. The fermentation

temperature is held constant by 60°F. [15]

10


Beer Treatment

After the primary fermentation beer is said to be ´green´. It must undergo a number of changes,

including the elimination of certain volatile fermentation products, super-saturation which carbon

dioxide, separation of yeast cells, and removal of some of the polyphenolic and other materials. [2], [7]

The maturation process is carried out in close containers. Beer treatment involves six processes:

1. Carbonation, 2. Flavor/aroma changes, 3. Additions of coloring and flavoring materials, 4.

Stabilization against non-biological haze and flavor change, 5. Clarification, 6. Biological

stabilization. [2]

Haze Prevention

Haze consists of polyphenolic compounds and polypeptides of high molecular weight, together

with trace quantities of metallic ions. There are two types of protein-tannin haze; chill haze, when

beer is cooled to about 37,4°F and it redissolves on warming, and the permanent haze, which is

formed at temperatures of und does not redissolve in beer. [2]

A widely used method of delaying the onset of non-biological haze is to add an endoprotease to

the beer in the final stages of preparation. The most used enzyme is papain which acts through

cleavage of the polypeptide chains present in the beer. A critical concentration is necessary

before the polymerising polyphenol/polypeptide system becomes visible as a haze. To achieve

the best results, the enzyme should be kept in a reducing environment free from metal ions and be

added to the beer as late as possible. [11]

Yeast Removal

Centrifugation and filtration are common procedures to remove residual yeast. Care must be

taken to avoid any rise in temperature lest the material deposited previously redissolve. Filtration

occurs through pulp, kieselguhr, composite filter sheets or more rarely, through cellulose ester

membranes. [2]

11


Preservation against microbiological spoilage

Wort has a natural resistance to many infectious microorganisms because of its low and acidic pH

between 5.0 and 5.4. [5] Nevertheless, the microbiological spoilage of beer is a big problem. [11]

Acetic-acid bacteria (Acetobacter spp. and Acetomonas spp.), the lactic acid-forming

(Lactobacillus spp. and Pediococcus spp.) and Zymomonas anaerobia are the commonest beer

spoilage organisms. [2]

Pasteurization is very expensive. More important is the preservation with synthetic organic

chemical compounds because there is a reduced amount of bottle breakage. The beer contains

both yeast and bacteria, thus necessitating the use of a fungistatic chemical and a bacterostatic

chemical. As for instance, Imidazoline Hydrobromide, p-Hydrocybenzoic Acid Esters, 2,3

Epoxypropanol or Diethanolamides are effective chemicals. [15]

In the pasteurizing process the beverage is heated to the required temperature and is then filled

into sterilized transport containers or small containers in which the liquid is kept under a certain

pressure. The flash pasteurization, in which the entering beer flows countercurrent, first to hot

pasteurized beer and then to steam, until it reaches a holding area where it remains for 20sec at

158°F, is the commonest procedure for bulk beer. Tunnel pasteurizers are used with bottled or

canned beer. The Containers travel through an enclosed area in which they are sprayed with hot

water, left for an appropriate time, and then treated with a cold-water spray. This procedure

reduces the incidence of bottle breakage. There are also some other methods of pasteurization

such as the plate pasteuriser. [2, [15], [16]

Color and Clarity

Melanoidins from kilned malt introduce color to beer. Roasted barley is also used as a source of

color, which makes a characteristic contribution to the flavor and head retention of stout. The

control of beer clarity is now not difficult. Metal hazes, oxalate hazes, Silica hazes can occur. [2]

12


Foam

The foamy head on a cool glass is an honored characteristic. In the formation of the foam

aromatic constituents and hop flavoring materials are adsorbed. That is why the foam has a great

importance to the consumer to prolong the taste enjoyment of the beverage. The amount of foam

is directly related to its carbon dioxide content. [15], [2]

The foam of different beers collapses at different rates. This process is not completely understood

but the size of the gas bubbles is an important factor. The goal is to produce a beer with a long-

lasting head of foam. Detergents have a harmful effect on beer foam. [15]

Beside the foam stability, the foam adhesion, or ´cling´, is fundamental. This is the material

deposited on the wall of the drinking glass as beer foam collapses. The iso-α-acids are implicated

in the ability of beer foam to cling to the glass. To enhance the ´cling´ chemical compounds like

Polysaccharide, Zinc plus Chelating Agents or Propylene are added to the beer. [2],[15]

The condition of the glass containing the beer is highly important in relation to the head stability

and also the presence of traces of grease or dishwashing agent is a crucial factor. [2]

Flavor

The beer flavor is examined either within detailed chemical analysis or within some individual

attribute of its impact on the human sense organs. A lot of different groups of flavor compounds

have been reported. [2]

Beer flavor is never constant because during the aging process subtle changes occur. During the

first stage a papery, or cardboard-like flavor, causes by the formation of volatile, long chain,

unsaturated carbonyls, develops and the product decreased in fruity/estery and floral character.

During the next step the flavor becomes bready, sweeter and toffee-like. The sweetness of a beer

is, naturally, related to its sugar content. In the last step the beer attains a winey, sherry-like

character and the bitterness decreases. Generally, ales become a sweet with a molasses-like,

cloying character. However, darker, heavier beers have a better flavor stability. Additional,

oxidation is the principal cause of flavor instability, therefore brewers try to minimize the

introduction of oxygen into the product after the fermentation and during packaging.[2], [17], [18]

13


Conclusion and Perspectives

About Biotechnology is often told, that it was ´as old as civilization itself´. But now malting and

brewing is evolved into a multi-million dollar industry. [19]

As aforementioned there are new methods developed, so that the brewing process has been

reformed during the last years. Malting barley should give higher yields on the field, high extract

yield in the brewhouse, suitable activities of many enzymes, free as possible of harmful micro-

organisms etc. Therefore, transgenic barleys become more and more important. The main interest

has been to insert a thermostable β-glucanase gene into the genome. The produced heat-stable β-

glucanase reduces the viscosity of the wort in mashing experiments. [1]

Otherwise, a novel method is using lactic acid bacteria or Geotrichium Candidum as starter

cultures in malting to reduce the fungal contamination and to improve the malt quality. They also

restrict the growth of harmful Gram-negative and –positve bacteria. [1]

A key compound of maturation is diacetyl, which is formed as an unpleasant by-product during

the main fermentation. With the help of the enzym α-acetolactate decarboxylase, diacetyl is

reduced to acetoin and further to 2,3-butanediol. The taste threshold of acetoin is much higher

than that of diacetyl. But unfortunately brewer’s yeast lacks the enzyme α-acetolactate

decarboxylase. To solve this problem there are either exogenous enzymes during the process

added or the encoding gene for α-acetolactate decarboxylase is integrated into the yeast genome. [1]

Finally, big breweries produce 500 million liters of beer in a year. They must be efficient to

survive but the quality of the beer must be good and uniform. As well, they are interested in all

technical novelties making the processes cheaper, easier and safer. But also small breweries are

very popular although they are producing with higher costs and limited quantities. [1]

14

BIOL 443 Fermentation Biotechnology Nadine Kallweit (20288655)„Biology of brewing” November 16 th , 2007

References

[1] Matti Linko et al., 1998

Recent advances in the malting and brewing industry

Journal of Biotechnology 65, Finland: 85–98

[2] A. H. Rose, 1977

Economic Microbiology - Alcoholic Beverages

Volume 1, Academic Press, London/

M. Macleod

Beer

Department of Brewing and Biological Sciences, Edinburgh, Scotland: 43-137

[3] http://www.garshol.priv.no/blog/4.html

[4] Tor-Magnus Enari, 1999

From Beer to Molecular Biology - The Evolution of Industrial Biotechnology

Fachverl. Carl, Nürnberg: 11-31

[5] http://www.britannica.com/eb/article-9106004/beer

[6] L. A. Underkofler and R. J. Hickey, 1954,

Industrial Fermentations/

R. I. Tenney

The Brewing Industry

Volume 1, Chemical Publishing Co., INC., New York: 172-195

[7] J.S. Hough et al., 1982

Malting and Brewing Science - Volume 2 Hopped Wort and Beer

2nd Edition, University Press, Cambridge

15

http://www.britannica.com/eb/article-9106004/beer

http://www.garshol.priv.no/blog/4.html


[8] F.G. Priest and I. Campbell, 1996

Brewing Microbiology

2nd Edition, Chapman and Hall, London/

I. Campbell

Chapter 1: Systematics of yeasts: 1-11

[9] F.G. Priest and I. Campbell, 1996,



T.W. Young

Chapter 2: The biochemistry and physiology of yeast growth: 12-42

[10] Ward, O.P., 1989

Fermentation Biotechnology: Principles, Processes and Products

Pp. i-xii, 1-227, Prentice-Hall

[11] G. G. Birch and M.G. Lindley, 1985

Alcoholic Beverages

Elsevier Applied Science Publishers LTD, London/

J.C. Slaughter

2. Enzymes in the Brewing Industry

Department of Brewing and Biological Sciences, Edinburgh, UK: 15 – 28

[12] G. G. Birch and M.G. Lindley, 1985

Alcoholic Beverages

Elsevier Applied Science Publishers LTD, London/

A.L. Whitear and R. Sharpe

3. Hop Constituents and Beer Character

Whitbread and Company plc, Luton, UK: 29 - 50

16


[13] F.G. Priest and I. Campbell, 1996



J.R.M Hammond

Chapter 3: Yeast genetics: 43-82

[14] R.Wahl and M. Henius, 1902

American Handbook of the Brewing, Malting and Auxiliary Trades

2nd Edition, Chicago

[15] M.Gutscho, 1969

Alcoholic Malt Beverages

Noyes Development Corporation, United States

[16] G. B. Härnulv et al., 1995,

Beer Pasteurization – Manual of Good Practice

EPC Technology and Engineering Forum, Getränke - Fachverlag Hans Carl, Netherlands

[17] B. H. Gump and D. J. Pruett, 1992

Beer and Wine Production – Analysis, Characterization, and Technology Advances ACS

Symposium Series

San Francisco/

N. J. Huige

Chapter 5 - Progress in Beer Oxidation Control

Miller Brewing Company, Milwaukee

[18] B. Vanderhaegen et al., 2007

Aging characteristics of different beer types

Food Chemistry 103, Centre for Malting and Brewing Science, Belgium: 404-412

[19] D. Mc Elroy and J. Jacobensen, 1995

What is Brewing in Barley Biotechnology?

17


Biotechnology Vol 13, Nature Publishing Group, Australia: 245-249

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