In the process of obtaining tailor-made glucose syrups, hydrolysing starch (mainly from wheat, maize, tapioca, and potatoes) has to be undertaken. This process cleaves the bonds linking the dextrose in the starch chain. The method and the extent of hydrolysis (conversion) affect the final carbohydrate composition and therefore many of the functional properties of starch syrups. The degree of hydrolysis is commonly defined as the dextrose equivalent, which is the following;
Dextrose equivalent (DE): Glucose (also called dextrose) is a reducing sugar. Whenever an amylase hydrolyses a glucose-glucose bond in starch, two new glucose end-groups are exposed. At least one of these can act as a reducing sugar. Therefore the degree of hydrolysis can be measured as an increase in reducing sugars. The value obtained is compared to a standard curve based on pure glucose- hence the term dextrose equivalent.
Originally, acid conversion was used to produce glucose syrups. Today, because of their specificity, enzymes are frequently used to predetermine exactly how the hydrolysis will take place. In this way, tailor-made glucose syrups with well-defined sugar spectra are manufactured.
The sugar spectra are analysed using different techniques, two being high-performance liquid chromatography (HPLC) and gel permeation chromatography (GPC). HPLC and GPC data provide information about the molecular weight distribution and the overall carbohydrate composition of the glucose syrups. These are used to define and characterise the type of product, e.g. high maltose syrup. Although these techniques help to optimise the production of glucose syrups with the required sugar spectra for specific applications, indirect methods such as viscosity measurements are also used in producing tailor-made products.
In processing and enzymology, the modern enzyme technology is used extensively in the maize wet-milling sector. Current research focuses on refining the basic enzymatic conversion processes in order to improve process yields and efficiency. The three enzymatic steps are explained as follows;
The first step is liquefaction. Maize starch is the most widespread raw material used, followed by wheat, tapioca and potatoes. As native starch is only slowly degraded using alpha-amylases, a suspension containing 30-40% dry matter needs to be first gelatinised and liquefied to make the starch susceptible to further enzymatic breakdown. This is achieved by adding a temperature-stable alpha-amylase to the starch suspension. The mechanical part of the liquefaction process involves the use of stirred tank reactors, continuous stirred tank reactors or jet cookers.
The alpha-amylase Termamyl is added to the starch slurry after pH adjustment, and the slurry is pumped through a jet-cooker. Live steam is injected here to raise the temperature to 105C, and the slurry's subsequent passage through a series of holding tubes provides the five-minute residence time necessary to gelatinise the starch fully. The temperature of the partially liquefied starch is then reduced to 90-100C by flashing, and the enzyme is allowed to react further at this temperature for 1-2 hours until the required dextrose equivalent is obtained. The enzyme hydrolyses the alpha-1,4-glucosidic bonds in pregelatinised starch, whereby the viscosity of the gel rapidly decreases and maltodextrins are produced. The process may be terminated at this point, the solution purified and dried. Maltodextrins (DE 15-25) are commercially valuable for their rheological properties. They are used as bland-fasting functional ingredients in the food industry as fillers, stabilisers, thickeners, pastes and glues in dry soup mixes, infant foods, sauces, gravy mixes, etc.
The next step is saccharification, when maltodextrins are saccharified by further hydrolysis using glucoamylase or fungal alpha-amylase, a variety of sweeteners can be produced. These have dextrose equivalents in the ranges 40-50 (maltose), 50-55 (high maltose), 55-70 (high conversion syrup). By applying a series of enzymes including beta-amylase, glucoamylase and pullulanase as debranching enzymes, intermediate-level conversion syrups with maltose contents of nearly 80% can be produced.
A high yield of 95-97% glucose may be produced from most starch raw materials (maize, wheat, potatoes, tapioca, barley and rice).
The third enzymatic step is known as isomerisation. Glucose can be isomerised to fructose in a reversible reaction. Under industrial conditions, the equilibrium point is reached when the level of fructose is 50%. The reaction also produces small amounts of heat that must be removed continuously. To avoid a lengthy reaction time, the conversion is normally stopped at a yield of about 45% fructose. The isomerisation reaction in the reactor column is rapid, efficient and economical if an immobilised enzyme system is used. The optimal reaction parameters are a pH of about 7.5 or higher and a temperature of 55-60C. These parameters ensure high enzyme activity, high fructose yields and high enzyme stability. However, under these conditions glucose and fructose are rather unstable and decompose easily to organic acids and coloured by-products. This problem is countered by minimising the reaction time in the column by using an immobilised isomerase in a column through which the glucose flows continuously. The enzyme granulates are packed into the column but they are rigid enough to prevent compaction.
The immobilised enzyme loses activity over time. Typically, one reactor load of glucose isomerase is replaced when the enzyme activity has dropped to 10-15% of the initial value. The most stable commercial glucose isomerases have half-lives of around 200 days when used on an industrial scale.
To maintain a constant fructose concentration in the syrup produced, the flow rate of the glucose syrup fed into the column is adjusted according to the actual activity of the enzyme. So towards the end of the lifetime of the enzyme, the flow rate is much slower. With only one isomerisation reactor in operation, there would be great variation in the rate of syrup production over a period of several months. To avoid this, a series of reactors are operated together and some or all of the enzymes in the columns are renewed at different times.
Starch is an extremely large industry, especially when dealing with the processes and the uses of enzymes within it. The sugar industry is equally as large, putting to use many similar enzymes and processes. The sugar industry is literally part of the starch industry, as sugar is processed from starch.
Starch is a natural component of sugar cane. When the cane is crushed, some of the starch is transferred into the cane juice where it remains throughout subsequent processing steps. Part of the starch is degraded by natural enzymes already present in the cane juice, but if the concentration of starch is too high, starch may be present in the crystallised sugar (raw sugar). If this is to be further processed to refined sugar, starch concentrations beyond a certain level are unacceptable because the filtration of the sugar solution will be too difficult. In order to speed up the degradation of the starch, it is general practice to add enzymes during the evaporation of the cane juice. Due to its extreme thermal stability, Termamyl may be added at an earlier stage of the multistep evaporation process than conventional enzymes. Termamyl is therefore the preferred product for starch degradation.
Another polysaccharide, dextran, is not a natural component of sugar cane, but it is sometimes formed in the sugar cane due to bacterial growth. This happens in particular when the cane is stored under adverse conditions (high temperatures and high humidity). Dextran has several effects on sugar processing: clarification of the raw juice becomes less efficient; filtration becomes difficult; heating surfaces become 'gummed up', which affects heat transfer; and, finally, crystallisation is impeded, resulting in lower sugar yields.
These problems may be overcome by adding a dextran splitting enzyme at a suitable stage of the process. Companies supply a fungal enzyme called dextranase for this application. It should be added that dextran problems may also be encountered in the processing of sugar beet, although the cause of the dextran is different. In this case, dextran is usually a problem when the beets have been damaged by frost. The cure, however, is the same: treatment with dextranase.
All of the enzymes that are used in the starch and sugar industries are wide ranging and are used for various different applications within the processes. Dextranase is used for the breaking down of dextran in raw sugar juice, as mentioned above. Finizym is used to improve the filterability of wheat starch hydrolysates. Fructozyme is applied in the hydrolysis of inulin to fructose. Both fungamyl and maltogenase are fungal alpha-amylases used for making high maltose and special glucose syrups. There is also an enzyme called promozyme, a pullulanase for debranching starch after liquefaction and reducing the oligosaccharide content of glucose syrups. You can see where this particular enzyme would be used, in the liquefaction stage as mentioned earlier. Shearzyme is an enzyme used improved wheat gluten/starch separation. Sweetzyme is a glucose isomerase for converting glucose into fructose. Termamyl has already been mentioned earlier on, as it is used in the liquefaction of starch to dextrin, incidentally there is more recent enzyme called termamyl type LS, this is an improved version of termamyl. Another enzyme put to use in these industries is toruzyme, a heat-stable cyclomaltodextrin glucanotransferase (CGTase) for cyclodextrin production.
As you can see there are many various enzymes in the modern day starch and sugar industries, which result in improved yields of products. They are incredibly important for today's growing demand, and without them we would find things to be a fair bit different.
Duncan Beard.
