Sweetener Enzymes

The starch industry began using industrial enzymes at an early date. Special types of syrup that could not be produced using conventional chemical hydrolysis were the first compounds made entirely by enzymatic processes. Many valuable products are derived from starch. There has been heavy investment in enzyme research in this field, as well as intensive development work on application processes. Reaction efficiency, specific action, the ability to work under mild conditions and a high degree of purification and standardization all make enzymes ideal catalysts for the starch industry. The moderate temperatures and pH values used for the reactions mean that few by-products affecting flavour and colour are formed. Furthermore, enzyme reactions are easily controlled and can be stopped when the desired degree of starch conversion is reached.

The first enzyme preparation (glucoamylase) for the food industry in the early 1960s was the real turning point. This enzyme completely breaks down starch to glucose. Soon afterwards, almost all glucose production switched from acid hydrolysis to enzymatic hydrolysis because of the clear product benefits of greater yields, a higher degree of purity and easier crystallisation. However, the most significant event came in 1973 with the development of immobilised glucose isomerase, which made the industrial production of high fructose syrup feasible. AETL offers SEBStar HTL a thermostable bacterial amylase and SEBamyl GL - glucoamylase for grain processing.

Glucose syrups are obtained by hydrolysing starch (mainly from wheat, maize, tapioca/cassava, and potato). This process cleaves the bonds linking the dextrose units in the starch chain. The method and extent of hydrolysis (conversion) affect the final carbohydrate composition and, hence, many of the functional properties of starch syrups. The degree of hydrolysis is commonly defined as the dextrose equivalent.

Liquefaction

Maize starch is the most widespread raw material used, followed by wheat, tapioca and potato. As native starch is only slowly degraded using alpha-amylases, a suspension containing 30-40% dry matter needs first to be 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. In most plants for sweetener production, starch liquefaction takes place in a single-dose, jet-cooking process. The alpha-amylase SEBstar HTL or Starzyme HT 120Lis added to the starch slurry after pH adjustment, and the slurry is pumped through a jet cooker. Here, live steam is injected to raise the temperature to 105°C, and the slurry's subsequent passage through a series of holding tubes provides the five-minute residence time necessary to fully gelatinise the starch. The temperature of the partially liquefied starch is then reduced to 90-100°C by flashing, and the enzyme is allowed to further react at this temperature for one to two hours until the required DE is obtained. The enzyme hydrolyses the alpha-1,4-glycosidic bonds in pregelatinised starch, whereby the viscosity of the gel rapidly.

Saccharification:

When maltodextrins are saccharified by further hydrolysis using glucoamylase (SEBamyl GL) or fungal alpha-amylase, a variety of sweeteners can be produced. These have dextrose equivalents in the ranges 40-45 (maltose), 50-55 (high maltose), and 55-70 (high conversion syrup). By applying a series of enzymes including beta-amylase (SEBmalt BA), 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).

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 immobileised enzyme system is used. The optimum reaction parameters are a pH of about 7.5 or higher and a temperature of 55-60°C. 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 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 halflives 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. Thus, 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.