1. Raw materials
1.6. Enzymes
Introduction
First of all it should be noted that enzymes are proteins and that they are substrate specific. This means that a given enzyme only will work on a certain substrate and only do a very particular job. Secondly it should be remembered that, although they take part in a chemical (enzymatic) reaction, they do not change during that reaction. They are what we call biological catalysts that accelerate or facilitate chemical reactions.
Because they are proteins, they are heat sensitive and all enzymes have an optimum temperature and pH for activity. Within that range, activity increases with temperature until the denaturation point is reached. At that point the enzyme will lose its functionality. Apart from temperature and pH, enzymes are also dependent upon the availability of water, amount of enzyme used, the availability of the substrate and the time allowed for the reaction.
Enzymes are biological compounds, usually proteins, which expedite the conversion of one substance into another. Their presence accelerates the rate of a chemical reaction and they are often specific and act upon only one substrate, or catalyze only one kind of reaction, in different, but related, substrates.
The activity of enzymes is dependent upon temperature. Enzymes used in baking are usually stable at room temperature and the rate of enzyme activity doubles with each 10°C increase up to the temperature of denaturation, at which the enzyme is inactivated. Most enzymes are inactivated above 60°C. An exception would be bacterial alpha-amylase, which retains its activity up to 85°C.
Enzymatic activity is pH dependent; there will be a pH optimum where maximum activity is achieved. Enzymes are usually stable at pH values between 4 and 9. Most doughs have pH values between 5 and 6. There is seldom an issue of enzyme denaturation due to pH. However, the acidity does effect the ionization of groups at the active site of the enzyme, rendering the enzyme more or less effective, depending upon the pH of the dough.
Enzymatic activity is dependent upon the concentration of the enzyme and the substrate. A higher concentration of enzyme will increase the reaction rate although not in direct proportion to substrate availability. The amount of time the enzyme and substrate are together directly affects the extent of reaction.
Many enzymes require the presence of a non-protein group, or co-enzyme, to be active. There are also compounds that act as inhibitors of enzyme activity by binding reversibly or irreversibly to the enzyme and/or substrate or in some way inhibit enzyme action. Oxidizing agents, such as bromates and iodates, and some heavy metal ions, have this effect.
There are 4 main groups of enzymes which are commonly encountered in baking : amylases, proteases pentosanases and lipoxygenases. Amylases are divided into α-amylase and β-amylase.
Two amylases are common to the baking industry, alpha-amylase and beta-amylase also known as alpha-1,4-glucan glucanohydrolase and alpha-1,4-glucan maltohydrolase.
Amylases convert starch into sugar : the α-amylase will cleave the starch randomly (the so called 1-4 bonds in the starch) while the β-amylase can only chop off two sugar units at the time at the end of the starch chain. Normally there is enough β-amylase present in the flour but sometimes addition of α-amylase is needed. The α-amylase will cut the starch into smaller units called dextrins and the more α-amylase activity there is, the better for the β-amylase because there are more extremities available. So the substrate for the β-amylase is either starch or dextrins and the product is maltose.
Alpha-amylase is an endoenzyme that attacks linkages within the molecular structure. It randomly cleaves starch chains at interior a-1,4-glycosidic linkages producing short chains of glucose molecules or dextrins. Beta-amylase is an exoenzyme and cleaves maltose units from the non-reducing end of the starch molecule. In order for these enzymes to function, the starch granule must be ruptured so that the individual starch molecules are available for enzymatic action.
Depending upon their origin, alpha- and beta-amylases show differences in pH and temperature optima, thermostability, and other chemical stability. They do not require co-enzymes for activity, although alpha-amylase activity is enhanced by the presence of calcium.
The pH optimum for alpha-amylase is 4.5 and it is inactivated at a pH of 3.3 to 4.0. This pH dependence decreases the efficacy of this enzyme in sour doughs. Beta-amylase is active across a much broader pH range, 4.5-9.2, with a pH optimum of 5.3. Alpha-amylase is relatively thermostable up to 70°C, whereas beta-amylase loses about half of its activity at this temperature. Fungal amylase is the least temperature stable, followed by cereal amylase, while bacterial amylase is stable at higher temperatures. New intermediate stability enzymes have been developed that are active above the gelatinization temperature of starch (60°C), but are totally inactivated at the later stages of baking (80-90°C). The objective is to maximize the anti-staling effect without creating a gummy, sticky product.
Amylase supplementation can occur at the flour mill or at the bakery in doughs and sponges. Malted barley flour has been used as an ingredient in bread for more than a century. Today, malted barley flour, malted wheat flour, fungal and bacterial alpha-amylases are used. The use of amylases provides a source of sugar for yeast fermentation. The increased sugars also improve flavour and enhance crust colour. Through starch modification, amylases improve moisture retention, have a crumb-softening effect and decrease staling.
Flour tends to lack α-amylase and the miller will supplement the flour with α-amylase. The diastatic activity of the flour is expressed by the falling number or the Hagberg number. A good flour has a falling number between 200 and 250 seconds. The α-amylase the miller will add can come from three different sources : cereal source (malted barley), fungal source (Aspergillius oryzae) or bacterial source (Bacillus subtilis). Bacterial amylase do denature at relatively high temperatures and some will remain in the bread after baking. The enzyme will continue to chop up starch in the baked bread. As starch is one of the main players in the staling mechanism, bacterial enzymes are used as crumb softeners because they will continue to work while the bread sits on the shelf of the supermarket. However there is a danger to it : the bread becomes softer and softer, it becomes more and more gummy and it is not uncommon that it will flatten, collapse while it sits on the shelf in case there is too much bacterial amylase left in the bread.
Fungal amylases work in exactly the same way bacterial and cereal amylases work. They however are denaturated at lower temperatures.
Protease will react with proteins and weaken them. As a result mixing time will be reduced, machinability will be improved as well as pan flow (the dough will fill more easily the shape of the pan). These effects are accomplished by breaking the long protein chains, cutting peptide bonds, into smaller units.
Proteins are made up of long chains of amino acids. Proteolytic enzymes, or proteases, include proteinases and peptidases. Proteinases split proteins at the CO-NH linkages, creating polypeptides, peptides and peptones. These are then further hydrolyzed into amino acids by peptidases.
In addition to supplying amylases, malted barley flour was also once relied upon for proteolytic enzymes, but these have been replaced by proteases from plant and fungal sources. Addition of proteases enables high speed bread production by decreasing the mixing time needed to achieve pliable dough. A protease acts to decrease the size and binding ability of the gluten molecules. Their action has much the same effect as reducing agents, except their effect is permanent; it cannot be reversed by the addition of an oxidizing agent.
Pentosans are polysaccharides comprised predominantly of the five-carbon sugars xylose and arabinose. While they are present in wheat flour in very small quantities, about 2 to 3 percent, they account for as much as one-quarter of the water absorption of dough made from wheat flour. This increases the viscosity of the dough and negatively affects loaf volume. Pentosanases cleave the polysaccharide chains thereby decreasing viscosity and improving loaf volume. Pentosanases have only been available commercially in recent years.
Finally also lipoxygenase is used in the bakery which is found in soy flour. This enzyme will bleach the flour. Flour contains a yellowish pigment that will be broken down by lipoxygenase. As a result one will obtain a whiter crumb.
Understanding the function of enzymes in baked goods
Many food product designers consider enzyme use new and innovative. While this is true for many categories, the baking industry actually has a long history of enzyme study and application. In fact, some references to the use of added enzymes in bakery foods are over 100 years old.
Even without this track record, enzymes are appealing functional ingredients for a variety of reasons. Enzymes are, for example, naturally occurring components of many bakery ingredients. If an enzyme is added, it often is destroyed by the heat of the baking process. In both cases, designers can obtain the functional benefits of the enzyme while maintaining a "clean label" image for the finished product. Enzymes also are specific to a particular function, eliminating concerns about undesired effects.
Nevertheless, getting the most out of enzymes in bakery products requires some planning on the part of the designer and a better understanding of what enzymes can do.
What are enzymes?
Because they consist of long amino acid chains, enzymes are classified as proteins and share many protein-like qualities. An enzyme molecule, however, is folded into a specific three-dimensional globular structure. Within these contorted folds are cavities that match the external features of a substrate molecule - a fat, protein or starch, for example - much like a key fits into a lock.
When an enzyme's active site meets with a corresponding substrate molecule, they temporarily bind to form an enzyme-substrate complex. By forming the complex, the enzyme lowers the energy required for certain reactions to take place. These reactions may either break up the substrate molecule or join it with another molecule. In addition, the complex will limit the reaction to specific bonds on the substrate molecule.
Enzyme activity is highly specific. Depending on its three-dimensional structure, a particular enzyme may hydrolyze or synthesize only one type of molecule. Others are less specific to a given type of molecule, but promote a certain chemical reaction on entire classes of compounds sharing common structural elements. Whatever the reaction, the enzyme itself will remain unchanged and this is why enzymes are considered catalysts.
Enzymes are named by adding the suffix "ase" to the end of the substrate. For simplicity, the substrate's name often is abbreviated. In baking applications, the general types of enzymes most commonly used are carbohydrases, proteases and lipoxygenases.
Starch segregation
Carbohydrases hydrolyze glycoside linkages in carbohydrates. This linkage specifically refers to the bond between one sugar molecule's reducing functional group and the -OH (hydroxyl) group of another molecule -- usually a sugar molecule, as well. Amylases are the carbohydrases that offer the greatest number of potential functions in bakery foods. These hydrolyze amylose and amylopectin in starch, as well as starch derivatives such as dextrins and oligosaccharides.
The alpha-amylase enzyme hydrolyses starch into soluble dextrins. These dextrins may subsequently be hydrolyzed by beta-amylase to yield maltose, and/or amyloglucosidase to yield glucose. Because starch exists as a tightly packed granule, amylases must act upon starch granules that are damaged (as many are during flour milling) or on granules that have been gelatinized by moisture and heat (such as when a dough is mixed and baked).
The sugars resulting from amylase activity act as food for yeast in yeast-raised products. As a result, the presence of these enzymes in the proper proportions is critical to carbon dioxide generation. Most flour naturally contains both alpha- and beta-amylase. The beta-amylase is, however, the only one naturally present in sufficient quantities. Thus, controlling the gassing power of the dough requires added alpha-amylase.
Amylases also can affect the consistency of a dough. Damaged starch granules absorb more water than intact granules. This ability is reduced when the damaged granules are acted upon by amylases. With their ability to immobilize water reduced, the damaged granules release free water which softens the dough and makes it more mobile.
A third function of amylases is their ability to retard staling. Over time, the crumb of baked products firms due to a complex set of changes that includes recrystallisation (or retrogradation) of amylopectin in the starch. By hydrolyzing the amylopectin into smaller units, bacterial alpha-amylase can maintain softness and extend shelf life.
One theory behind this suggests that amylopectin still crystallizes at the same rate with added enzymes, but that the shortened chain length maintains greater flexibility and softness when crystallized. Another theory is that the shortened amylopectin chains have a lesser tendency to retrograde. Either way, the enzyme must continue to hydrolyze starch after baking is completed. The fact that bacterial alpha-amylase is more thermally stable than other alpha-amylase sources is the reason it is used.
Because the enzyme is active in the finished baked product, it is possible for the enzyme activity to go too far. Rather than maintaining softness, the crumb can actually become gummy. The starting enzyme dosage is critical to preventing this. For even greater assurance against overdosing, amyloglucosidase or pullulanase may be added along with the alpha-amylase. These enzymes don't contribute to anti-staling when used alone, but help prevent gumminess when combined with the amylase.
A final use for amylases in bakery products is for replacing potassium bromate, an oxidizing agent that strengthens gluten strands. Strengthened gluten produces a dough with improved gas retention and, consequently, higher volume in the finished product. Based on various health studies, bromate use is on a sharp decline. Other oxidants -- such as ascorbic acid - can promote comparable volume, but they don't provide a direct match for bromate. To compensate, alpha-amylase can be added with ascorbic acid to improve the volume and increase the quality of the crumb. Bakeries may either add alpha-amylase and ascorbic acid separately or select a custom blend featuring an optimized mixture of the two components.
Amylases are not the only carbohydrases useful in bakery products. Pentosanases also can be added to improve quality. Both wheat and rye flour contain pentosans. These non-starch polysaccharides are highly hydrophilic and contribute significantly to the water absorption properties of a dough. In wheat flour-based products, pentosans also interfere with volume development.
Adding pentosanase to a wheat flour-based product can improve product volume by hydrolyzing the pentosans present. At the same time, though, hydrolyzed pentosan will release water, making the dough very slack. When using pentosanase, the water absorption of the dough must be adjusted to compensate. If the dough is too slack, not only will it be difficult to machine, but the volume-building benefits of the pentosanase will not occur.
In rye bread, the pentosans in the rye flour are critical to building structure since rye flour's gluten content isn't sufficient. If pentosan content is too high, though, it will compete for water with the starch and prevent it from swelling and gelatinizing properly. Pentosanase will help control the pentosan content so there is enough to build structure, but not so much as to interfere with the starch functionality. Pentosanases that hydrolyze cellulose also are available. These may be added to high-fibre bakery products to help improve their eating qualities by breaking up the long cellulose chains that contribute to gritty mouthfeel.
Protein pruning
Proteases hydrolyze the peptide bond between the amino group of one amino acid and the carboxyl group of the next amino acid in a protein. In dough, this serves to weaken the gluten chains. This can affect the dough in two ways, depending on when the protease is added. If the protease is allowed to hydrolyze a portion of a dough early in the process -- added to the sponge of white pan bread, for example -- it will reduce the mixing time necessary to develop the dough. Early addition of a protease to a complete dough, however, will cause the gluten to become too weak to build structure properly, resulting in a course, uneven crumb.
Nevertheless, protease could be added to an entire dough later, at the mixing stage. This won't reduce the mixing time because the enzyme will not have had enough time to hydrolyze much gluten. Still, as hydrolysis occurs through shaping, floor time and proofing, the protease will help improve the flow of the dough. This procedure might be used to eliminate short pan fills in a straight (non-sponge) dough system or to help the pan flow of buns and English muffins.
Another application for proteases is in replacing sodium sulfites in cracker doughs. Cracker doughs contain low levels of fat and water, making them rather stiff. This stiffness makes it difficult to laminate the dough into layers and to sheet it to cracker thinness. Sodium sulfites hydrolyze the disulfide bridges on the gluten molecule, reducing its resistance to extension and making the resulting dough more plastic.
Sulfites have undesirable side effects, however. They break down vitamin B2, inhibit browning reactions that are desirable in baked products, and are a marketing no-no because some consumers exhibit allergic reactions to the substance. In fact, many countries have banned or are considering banning sulfite's use in bakery products. Adding a protease to the formula and allowing sufficient time for the enzyme to act (sulfites, by comparison, react more rapidly) can achieve the desired workability in the dough without the negative side effects.
Bond-building catalysts
While proteases help make dough more slack, lipoxygenases can help do the opposite. Lipoxygenases catalyze the addition of an oxygen molecule to polyunsaturated fatty acids to form peroxides such as hydroperoxy-linoleic acid. These then will interact with a gluten side chain, making the gluten more hydrophobic and, subsequently, stronger. With stronger gluten, the dough will have better gas-retention properties and increased tolerance to mixing.
In a way, lipoxygenases offer results similar to those obtained with dough strengtheners such as sodium stearoyl-2-lactylate, but they also offer additional benefits. Although the exact mechanism behind it is not fully understood, lipoxygenase can bleach fat-soluble flour pigments to produce a whiter crumb in finished bread and rolls.
Understanding and selecting enzymes
Understanding what different types of enzymes do to bakery products is the first step in enzyme selection. Considering how specific enzyme action is, once the desired results are determined, the enzyme to use will be a straightforward decision. Other factors in enzyme selection and use aren't so easy. These include the enzyme source and form, the strength of the enzyme activity and how much to use, and the conditions under which the enzyme will be used and handled. Amylases used in bakery foods come from three primary sources.
Like amylases, proteases for bakery applications can be extracted from both fungi and bacteria - most often with the same species used for alpha-amylase production. Different types of protease have different catalytic mechanisms. The different mechanisms primarily control how the enzyme responds to different pH conditions.
Lipoxygenases aren't available in concentrated forms like proteases and amylases. They are added as a natural constituent of full-fat and defatted soy flour. These flours often are offered with other functional ingredients such as calcium peroxide for additional oxidation, dicalcium phosphate for dough conditioning, and corn flour to improve absorption and mix tolerance.
Activity problems
Because they are a naturally occurring component of soy flour, lipoxygenase activity is not as standardized as it is with the tablets, powders, etc. available with amylases and proteases. Though these forms sport standardized activity levels, product designers still may be confused by the different methods of activity measurement. The amylose activity of malt extracts and syrups, for example, is typically expressed as degrees Lintner, while concentrated amylase sources are often expressed in Sandstedt-Kneen-Blish (SKB) units.
On top of the tremendous number of standardized tests, individual enzyme suppliers often have a custom method of determining enzyme activity. This presents a challenge to product designers trying to compare activities in order to predict usage levels and cost impact.
The goal of most methods of measuring enzyme activity is to determine how quickly the enzymes convert substrate molecules to product molecules. Because of this, the activity measurements often have little to do with the enzyme's activity in actual use, particularly in baked products. Designers will probably wish to create their own assay by testing enzymes at different levels in actual doughs. The observed effects can then be related to the amount of enzyme added.
By using the level of activity per gram of enzyme as the measuring unit, product designers will have a common basis for comparing enzymes. In addition, the activity measurement will include a weight that can be directly related to the price of the ingredient in order to determine the cost of a given degree of effectiveness.
Factors influencing the performance of enzymes
When creating a test for comparing enzyme activity and when preparing to put enzymes to work in the formula, remember that conditions the dough encounters through the process will greatly affect enzyme activity.
Time is critical for successful application of many enzymes. Put simply, the chemical reaction must have enough time to proceed. An enzyme's catalytic reaction can, of course, be sped up by increasing the enzyme level to increase the amount of available catalyst. However, this can be expensive and, in the case of bacterial amylases for shelf life extension, be impossible due to detrimental effects in the finished product.
Keep in mind also that amylases can only act on damaged or gelatinized starch granules. A certain amount of mixing and/or dough development will be required before these enzymes begin to work. A protease will start to act as soon as a dough is wetted. On top of that their performance depends on the fat and sugar content of the product. An amylase that performs well in bread will not give as good results in cake for instance.
Temperature influences enzyme activity in both a positive and negative way. Every 18 degree F increase in dough temperature increases the enzyme activity up to two-fold. On the down side, the same temperature increase also will accelerate the rate of enzyme denaturation by a factor anywhere from 10- to 30-fold. At a high enough temperature, the rate of denaturation catches up with the reaction rate, slows it and eventually stops it. Just as the time and enzyme amount must be optimally balanced, so must the time and temperature. A longer reaction time can actually increase the efficiency of the enzyme conversion at a lower temperature.
Acidity, or pH, affects enzyme activity. Different enzymes, and even enzymes from different sources, have optimum pH ranges under which they are most active. This was previously discussed for different proteases, but also is true for amylases.
When formulating, designers must not only be aware of the starting pH of the formula, but how it changes over time. For example, as chemical leaveners are consumed, the overall dough pH may be altered out of the optimum range for the enzyme. The same is true for yeast-leavened products, as the pH can change dramatically as fermentation proceeds in products such as crackers and bread.
Care also is necessary when adjusting the formula pH. Cocoa powder and other chocolate-flavoured ingredients require an alkaline system for optimum flavour. Adjusting such a system to be more acidic for the enzyme can adversely affect the flavour. Colour development is strongly related to pH, and any alterations will affect a product's crust colour.
Salt level can affect the enzyme's activity because salt can help stabilize certain enzymes. The opposite is true, however, for proteases, which are inhibited by high salt concentrations. This could be the result of salt making gluten less available to the action of the enzymes.
If salt levels can't be adjusted, the order of addition can overcome this limitation. In a sponge-and-dough bread, for example, enzymes can be added to the sponge. Because salt won't be added until the dough stage, the enzymes will have more time to react uninhibited. Salt also can be added to later steps in multiple-stage mixing procedures for other products, but the time between stages isn't nearly as significant as it is between a sponge and a dough.
Certain enzymes may require ionic co-factors to be active. Many carbohydrases will not function without calcium ions. Zinc is necessary for neutral fungal proteases.
Enzymes are indeed a highly specific, useful collection of ingredients for bakery products. Enzyme activity itself is useful, and many enzyme applications offer clean-label advantages. Although the number of different enzymes and the cacophony of different activity measurement methods may seem intimidating, product designers can sort through this to determine the best enzyme and the conditions that enzyme requires in the formula and during processing for maximum effectiveness. All it takes is a new understanding of these old ingredients.
Noël Haegens
