Beer: Quality, Safety and Nutritional Aspectsby Paul S Hughes
For centuries, beer has been a favourite drink throughout the world. The art of brewing has more recently evolved into the science it is today as a result of the increased knowledge of both the ingredients and the process. Considerations such as appearance, taste and the nutritional value of beer are important topics for consumers and brewing scientists alike. This book looks at the chemistry behind those aspects of beer that are of particular interest to beer drinkers, namely flavour and nutritional aspects, in combination with a discussion of maintenance of quality and safety, the areas more relevant to the brewing scientist. Beer: Quality, Safety and Nutritional Aspects brings the reader right up to date with current thinking, and will be valued by both interested consumers and those employed in industries related to the brewing industry.
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Beer: Quality, Safety and Nutritional Aspects
By E. Denise Baxter, Paul S. Hughes
The Royal Society of ChemistryCopyright © 2001 The Royal Society of Chemistry
All rights reserved.
An Overview of the Malting and Brewing Processes
The story of beer starts with ripe barley grain, plump and sound, with a moderate (for a cereal) protein content of 10–12%. The barley kernel is roughly ovoid in shape, surrounded by protective layers of husk, with a small embryo at one end. This embryo is the part that will grow into the new plant, given the chance. The remaining part of the kernel is the endosperm, which is basically just a store of food for the young plant.
Most of the endosperm consists of large dead cells with thick cell walls consisting mainly of β-glucan (a polymer of glucose molecules linked by β-glycosidic bonds) together with some pentosan (an arabinoxylan polymer) and a little protein.
These cells are stuffed with starch granules, which come in two sizes; large (about 15–20 µm diameter) and small (about 2 µm diameter). There are very many more small granules than large granules but they account for less than 5% of the weight of the starch. These starch granules are embedded in a matrix of hordein. This is an insoluble protein which provides a store of peptides and amino acids for the new plant. The whole of the starchy endosperm is surrounded by the aleurone, which is a triple layer of living cells.
The whole aim of the malting process is to get rid of as much as possible of the the β-glucan cell walls and some of the insoluble protein which would otherwise restrict access of enzymes to the starch granules. At the same time enzymes are developed which will, in the brewhouse, convert the starch into soluble sugars.
In the maltings the barley is steeped to raise the water content from 12% to around 45%. This process takes about 48 hours and consists of two or three periods when the grain is totally immersed in water, interspersed with 'air rests' when the water is drained off and fresh humidified air is blown through the grain bed to provide oxygen. The increased water content stimulates respiration in the embryo and hydrates the stores of starch in the endosperm. As the embryo activity increases, gibberellins are produced. These are natural plant hormones that diffuse into the aleurone, where they stimulate the production of hydrolytic enzymes during germination.
The moist grain is then allowed to germinate for a few days. During this time cool humidified air is again blown through the grain bed to keep the temperature down to around 16 °C and to stop the grain drying out. As gibberellins diffuse into the endosperm from the embryo they stimulate the aleurone cells to produce hydrolytic enzymes. These include amylolytic enzymes, which break down starch, proteolytic enzymes, which attack the protein, and cellulytic enzymes, which break down cell walls. Proteolytic enzymes include carboxypeptidases, which release one amino acid at a time starting from the carboxyl end of an amino acid chain, and endopeptidases, which can break peptide bonds in the centre of long amino acid chains. They can therefore very rapidly reduce the size of a protein or polypeptide. Next β-glucanases are produced. These break down the endosperm cell walls, making it easier for the other enzymes to diffuse out into the starchy endosperm. Last, but not by any means least, amylolytic enzymes are produced. The two most important are α-amylase and β-amylase, both of which can break down α-1,4 bonds. A debranching enzyme, which can attack the 1,6 bonds, is also produced, but this enzyme is quite sensitive to heat and so is normally inactivated during malt kilning.
All of these enzymes must diffuse into the starchy endosperm and begin the process of breaking down the cellular structure (the cell walls) and the stores of protein, starch and lipid in order to provide nutrients for the new plant. This process is strictly controlled by the maltster, who curtails it after four or five days. By this time most of the cell walls should have been digested, since if these are allowed to remain they will cause processing difficulties at a later stage. Part of the high molecular weight, insoluble protein will also have been broken down into smaller fragments (peptides and amino acids) and sufficient amylolytic enzymes will have been syn-thesised. Most of the starch remains intact, except for the small granules, which are the first to be digested during malting. If these small granules persist in the malt they can cause filtration problems for the brewer during the later stages of beer production.
The damp 'green' malt is dried in a kiln to prevent further enzyme activity and to produce a stable material which can be safely stored until needed for brewing. The kilning process also removes the volatile components responsible for undesirable 'grainy' flavours, and encourages the development of more attractive malty, biscuity flavours. This flavour development depends very much upon temperature and thus can be controlled by the maltster in order to produce a wide range of malts. The majority of commercial malts are fairly lightly kilned (up to 85 °C) in order to produce lager malts. In the UK a substantial proportion is kilned to a higher temperature (usually 90–100 °C) to give somewhat darker and more flavoursome pale ale malts. Higher temperatures (up to 200 °C) are used to produce speciality malts with flavours ranging from the toffee, caramel flavours of crystal malts to the sharp astringent flavours of roasted malts. These different malts can then be used by the brewer to produce beers with a wide range of flavours and colours (see Chapter 3).
The brewing process converts the malt starch first to soluble sugars, then uses yeast to ferment these to alcohol. At the same time proteins are broken down into amino acids which can be used by the yeast as nutrients, coincidentally producing characteristic flavour compounds.
In the brewhouse the malt is crushed in a mill. Often a roller mill will be used – this keeps the husk largely intact so that it can serve as an aid to filtration later in processing. The crushed malt ('grist') is mixed with hot water in the mash tun and the whole mash is held at around 65 °C for about one hour. This temperature is chosen as it is the temperature at which malt (i.e. barley) starch will gelatinise – making it more susceptible to enzyme attack.
Sometimes other cereals ('adjuncts') may form part of the grist, in order to provide specific qualities in the beer. For example, small quantities of wheat are often used in ales to enhance the beer foam, while unmalted rice and maize grits may be used to improve the flavour stability of light-flavoured lagers. The more intensely kilned malts (crystal, amber, or brown malts) are used to provide colour and flavour in traditional British ales, while roasted malt and barley are used in the darker porters and stouts (see Chapter 3).
Like barley starch, wheat starch also gelatinises at 65 °C. Rice and maize starches gelatinise at higher temperatures, so if either of these cereals is used as an adjunct, it must be pre-cooked in a separate vessel (known as a cereal cooker) before being added to the mash.
In some mashing systems, particularly those used for lagers, where the malts may have been less completely modified during malting, the mash may initially be held at a lower temperature (around 45 °C) to allow the breakdown of cell walls and protein which commenced in malting to continue. After about 30 minutes the temperature is then raised to 70 °C. At this temperature the starch will gelatinise and it can then be broken down by the amylase enzymes in the mash.
During mashing the amylolytic enzymes in the malt break down the starch into fermentable sugars. Cereal starch consists of approximately 75% amylopectin and 25% amylose. Amylopectin is a very large, branched molecule (the molecular weight has been estimated at several million) made up of glucose units linked by α-1,4 bonds (which give linear chains) and α-1,6 bonds (which give branch points). On average, each branch is made up of around 25 glucose units. Amylose, on the other hand, is a linear molecule made up of up to 2000 glucose units linked by α-1,4 bonds only (Figure 1.4).
Both α-amylase and β-amylase can hydrolyse α-1,4 bonds, β-Amylase attacks from the outer reducing ends of the amylopectin and amylose molecules, releasing free maltose (two glucose units), but stopping when it reaches an α-1,6 bond. In contrast, a-amylase attacks lengths of α-1,4 chains between branch points, releasing smaller, branched dextrins with long straight side-chains. These provide more substrates for β-amylase action. α- and β-Amylase acting together reduce amylose to maltose, maltotriose and glucose, but amylopectin gives rise, in addition, to many small branched dextrins which cannot be further broken down during mashing.
Thus after the conversion stage (mashing) a sweet syrupy liquid known as 'wort' is produced. This liquid contains mainly maltose and glucose, which are fermentable, together with significant quantities of small branched dextrins, which are not fermentable. There may be traces of larger straight-chain dextrins, the amount of which depends upon the enzymatic activity of the malt and the mashing conditions, and thus can to some extent be manipulated by the brewer. No starch should survive the mashing stage. The wort will also contain soluble protein, polypeptides and amino acids.
In traditional British ale mashing, the wort is separated from the spent grist in the mash tun by being allowed to filter through the spent grain bed into the next vessel. Hot water (usually at least 70 °C) is sprayed onto the top of the grain bed in order to extract and wash out the soluble components. This is known as sparging. A more usual practice nowadays is for the whole mash to be transferred to a separate vessel, the lauter tun. This vessel has a perforated base plate which allows the wort to run through into the next vessel, the kettle or copper, leaving the insoluble remains of the malt, (the spent grains) behind in the lauter tun.
In the kettle, hops or hop extracts are added and the wort is boiled quite vigorously. This has three effects:
The wort is sterilised.
Much of the soluble protein is coagulated and can be separated off as the 'trub'.
The α-acids in the hops are extracted into the wort and isomerised into iso-α-acids, which provide the characteristic bitter taste of beer (see Figure 1.5).
In addition to α-acids, hops contain essential oils, which contribute to the hoppy, floral and spicy aromas in beer (see Chapter 3). Most of these compounds are volatile and can therefore be lost by evaporation during boiling. In order to effect a suitable compromise between sufficient boiling to coagulate protein and isomerise the hop acids, but still retain the desired quantity of aroma compounds, the brewer may add part of the hop recipe part-way through the boil.
Also during boiling, browning reactions take place between the reducing sugars and the primary amines (particularly amino acids) in the wort, resulting in an increase in wort colour and some loss of free amino nitrogen. Browning reactions are complex and still not completely characterised, but basically consist of condensation reactions between simple sugars, such as glucose, with primary amines (for example the amino acid glycine) to give aldosylamines. These are relatively unstable compounds and can undergo Amadori rearrangement to form ketosamines, which condense with another aldose molecule to form diketosamines. Reacting in the enol form, these diketosamines can undergo further condensation reactions with additional amines to form a mixture of reddish-brown pigments, most of which contain a furfural ring. A simplified reaction pathway is shown in Figure 1.6. To some extent the amino acids act as catalysts, and the increase in colour is much greater than the loss of amino acids.
After boiling, the coagulated protein or 'trub', together with the spent hops, must be removed. Traditionally this was achieved by filtering the wort through the bed of spent hop cones. In modern breweries most of the hops are in the form of pellets or extracts, with much less waste leafy material to form a filter bed, and a vessel known as a whirlpool is used instead. The hot bitter wort is pumped into the whirlpool tangentially. The resulting swirling motion causes the trub to collect at the centre of the vessel as a conical mound. The clear wort can be removed from an exit pipe, which is situated to the side of the vessel. The bitter wort is then cooled to fermentation temperature by passing it through a paraflow heat exchanger.
Fermentation takes place at 7–13 °C for lagers or 16–18 °C for ales. Yeast is mixed with the cooled wort and the mixture pumped into the fermenting vessel. During fermentation the yeast takes up amino acids and sugars from the wort. The sugars are metabolised, with carbon dioxide and ethyl alcohol being produced under the anaerobic conditions found in brewery fermentations (Figure 1.7):
C6H12O6[right arrow]2CO2 + 2C2H5OH (1.1)
Excerpted from Beer: Quality, Safety and Nutritional Aspects by E. Denise Baxter, Paul S. Hughes. Copyright © 2001 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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