During liquefaction, the α-1,4-linkages are hydrolyzed at random. This reduces the viscosity of the gelatinized starch and increases the dextrose equivalent (DE), a measure of the degree of the hydrolysis of the starch. The liquefaction is carried out in such a way as to give the required DE for the subsequent process. For saccharification to dextrose, a DE of 8-12 is commonly used. Higher DE values are often necessary for maltodextrin production. The maximum DE obtainable is about 40.
Industrially important α-amylases are those made from Bacillus licheniformis, Bacillus subtilis, and Aspergillus oryzae. In Table 2.3 some application conditions and a few comparative characteristics are shown for various enzymes available for starch processing. The four amylases from Novo Nordisk are described here.
|Application temperature range (oC)
|Application pH range
|Minimum Ca+ dosage (ppm)
|Bacterial thermophilic α-amylase
Bacterial thermophilic α-amylases
Termamyl is an endo-amylase that hydrolyses the α-1,4-linkages in starch (amylose and amylopectin) almost at random. The breakdown products formed are mainly soluble dextrins and oligosaccharides. In a concentrated solution of starch, the hydrolysis results in a rapid viscosity reduction. In consequence Termamyl is often referred to as a ‘liquefying amylase’. Termamyl is available in liquid form with a standardized activity of 60 or 120 KNU g-1. One KNU (Kilo Novo α-amylase Unit) is the amount of enzyme that hydrolyses 5.26 g of (standard) starch per h at 37°C, pH = 5.6 under standard conditions.
Fungamyl® is a fungal exo-amylase, which hydrolyses the α-1,4-linkages in liquefied starch (amylose and amylopectin); a prolonged reaction results in the formation of large amounts of maltose. Fungamyl is used for production of high maltose syrups or high conversion syrups (see Saccharification of liquefied starch). Fungamyl is available in liquid form with a standardized activity of 800 FAU g-1. One FAU (Fungal alpha-amylase unit) is the amount of enzyme that hydrolyses 5.36 g of (standard) starch per h at 37°C, pH = 4.7 under standard conditions.
AMG® is a glucoamylase (exo-amylase), which hydrolyses α-1,4-linkages as well as α-1,6-linkages in liquefied starch (amylose and amylopectin). The hydrolysis proceeds in a stepwise manner. The breakdown product formed is glucose, which has been split off from the non-reducing end of the substrate molecule. Maltotriose and in particular maltose are hydrolysed at a lower rate than higher saccharides, and 1,6-linkages are broken down more slowly than 1,4- linkages. Eventually, a practically complete conversion of starch into glucose is obtained. In consequence AMG is referred to as a ‘saccharifying amylase’. AMG is available in liquid form with a standardized activity of 200, 300 or 400 AGU g-1. One AGU (Amyloglucosidase Unit) is the amount of enzyme that hydrolyses 1 μmol of maltose per min at 25°C under standard conditions.
Promozyme® is a pullulanase, which hydrolyses the α-1,6-glycosidic bonds in starch which has been partly hydrolysed by α-amylase, provided that there are at least two glucose units in the side chain. Promozyme is therefore well suited for debranching starch after liquefaction. Promozyme is available in liquid form with a standardized activity of 200 PUN g-1. One PUN (Pullulanase unit Novo) is the amount of enzyme that hydrolyses pullulan liberating reducing carbohydrate with a reducing power equivalent to 1 μmol glucose per min, using a substrate concentration of 0.2% pullulan at 40°C, pH = 5.0 under standard conditions.