The Manufacture Of Glucose Syrup

Glucose syrup is manufactured from starches usually derived from maize (corn) or rice. There are seven process steps which include:-

• mixing process,
• liquefaction,
• deproteinization,
• saccharification,
• decolorization,
• decarburization,
• ion exchange process
• evaporation process.

Most glucose syrups rely on an acid hydrolysis or one using acid with enzyme hydrolysis.

Dextrose Equivalent

The degree of hydrolysis is measured using the dextrose equivalent (D.E.), which is the reducing power calculated as dextrose and expressed on a dry weight basis of the final product. The higher the D.E. the higher the level of hydrolysis of the starch. Starch has a zero D.E. whilst a 100% glucose syrup would be 100 D.E. The higher the DE too the greater the extent of the conversion process.

The Dextrose Equivalent is measured using the following from the  Association of Official Analytical Chemists (AOAC, 14th edition, 1984) method 31.035 (Lane–Eynon method). 

A 0.5-ml sample aliquot is transferred to a 100-ml volumetric flask and made up to volume by the addition of deionized water. Five milliliters of Fehling A solution and 5 ml of Fehling B
solution are added to a 250 ml flask and 3 drops of 1% (w/v) methylene blue solution is added.

The resulting solution is heated and maintained at boiling for 2 min. This solution is then rapidly titrated in less than a minute with the sample solution until the end point is reached which is a colourless solution.  

Total dextrose calculated as the reducing sugar is obtained from the AOAC conversion table and dextrose equivalent is calculated using the following equation (Pontoh and Low 1995):

{[Weight of glucose rom the conversion table]/[Weight of dry syrup sample]}  x 100

An alternative method uses a cryoscope or freezing point osmometer.

The amount of dextrose left in a syrup is determined using HPLC (high-pressure liquid chromatography).

Definitions

Fructose syrups can also be called glucose syrups is the source is starch. Fructose is also a reducing sugar and like dextrose has a D.E. value.

Processing Methods

Acid hydrolysis of starch follows a well defined route. The D.E. is related to the quantity and type of sugars present (Palmer, 1970).

Acid Hydrolysis

The most common method for producing a 35 or 43 DE syrup.

A starch slurry of 35 to 40% solids is acidified to pH 1.65 using hydrochloric acid. using indirect steam heating, the starch is hydrolysed in a conversion process. The converted syrup is neutralised to pH 4.5 with the addition of sodium carbonate. The slightly acidic syrup is either centrifuged or filtered to remove fats, proteins, oils, fibre etc. The most common filter material is spent carbon fines.

Colour is removed using further activated carbon treatment.

The syrup is treated in an ion-exchange processes which removes further colour but also salts and then this is repeated as a polishing step.

The syrup is concentrated by evaporation under vacuum (24-26 inches Hg) to about 70% solids. Vapour is condense and returned to the process.

Enzyme-Acid Hydrolysis Process used in the Manufacture of Glucose Syrup

When an enzyme hydrolysis process for manufacture is used then no such strict relationship is found between the D.E.. Using specific enzyme selection a glucose syrup rich in one particular sugar may be produced as in the production of high maltose syrups for example. Maltose syrups are purified and concentrated starch hydrolysates containing from 40% to 90% (dry basis) maltose (Gaouar et al. 1998).

α-Amylase

Typical enzymes include α-Amylase (AA) for liquefaction (EC3.2.1.1). The activity of the enzyme is defined as 1 unit to liberate 1.0 mg maltose from starch in 3 min at pH 6.9 at 20 °C in phosphate buffer.

The enzyme catalyses hydrolysis of glucose 1-4 links in gelled starch to produce dextrose and maltose. It cannot hydrolyse 1-6 linkages because of steric interference. Any carbohydrates left in a glucose syrup will often have units bound through 1-6 linkages. These will be key components of the higher sugars in glucose syrups.

Most alpha-amylases have a pH optimum between 5 and 7 and are best at a temperature of 65 to 70°C. For manufacturing purposes, high temperature/heat stable amylases are used. These can withstand temperatures up to 120ºC for 20 minutes, pH 6.  

β-amylase

The other enzymes for processing are  β-amylase (BA) for saccharification (EC 3.2.1.2). The activity of this enzyme is defined as 1 unit to liberate 1.0 mg maltose from starch in 3 min at pH 4.8 at 20°C in acetate buffer.

There are two industrial sources of this enzyme. One is from diastatic malt that comes from malted barley and is important in its own right for brewing. Diastatic malt contains small amounts of alpha-amylase. This is an especially useful enzyme mix for handling poor quality starch sources.

The second source is from bacteria and is purer. It is more expensive than diastatic malt.

Beta-amylase also hydrolyses 1-4 linkages producing mainly maltose, small amounts of dextrose. It cannot hydrolyse 1-6 linkages and these too will form part of the higher carbohydrate range. The operating pH is between 4.5 and 5.0. The optimum temperature is 55 to 60°C.

Glucoamylase

Glucoamylase, also known as amyloglucosidase (AMG) or saccharifying enzyme is critical  for catalysing the hydrolysis of 1-4 and 1-6 links. Its operating pH is 4.5 to 5.0 and an optimum or operating temperature of 55 to 60°C. 

Pullulanase

The classic debranching enzyme because it hydrolyses 1-6 linkages. In conjunction with alpha-amylase will produce glucose syrups with the highest DE possible. Maltose syrups are produced this way using pullulanase and beta-amylase to yield high levels of maltose. The operating parameters are similar to those of beta-amylase which is ideal from an industrial perspective.

Glucose Isomerase

A key enzyme in the conversion of dextrose and glucose syrups to make glucose-fructose syrups. All the previous enzymes are relatively cheap and are not recovered from the process. The isomerase is usually immobilised in or on beads so that it is retained in an immobilised enzyme reactor. Continuous operation reduces the cost of manufacture.

The enzyme operates at pH 7.5 to 8.0 with a temperature optimum of 60 to 65°C. Each enzyme has different performance characteristics depending on the source.

Green Saccharification

A two-step process using β-amylase and pullulanase is claimed to be one of the best methods for green saccharification. It supersedes the one-step process because this is usually an inefficient constant temperature method (Li et al., 2020). The manipulation of the pullulanase addition time and adjusting its temperature of processing has the biggest impact on saccharification. 

The reduction in starch as a substrate is also monitored using changes in paste viscosity and observing any acceleration during amylolysis.

and pullulanase (PN) for saccharification (EC 3.2.1.41, 1 unit to liberate 1.0 mmol of maltotriose from pullulan/min at pH 5.0 at 25 °C in acetate buffer).

In enzyme liquefaction, partial swelling of a normal maize (corn) starch at 70 °C helps with hydrolysis when using a granule starch hydrolyze enzyme (GSHE). Using this method, 95% of the maize starch was converted to glucose. The preswelling of the starch granule would allow the enzyme greater access to the amylose (Tong et al., 2019).

Concentrated Starch Liquefaction Processing

A highly concentrated starch liquefaction process of 45% w/w is viewed as a green process to improve the productivity of starch syrup and related fermentation products. Generally highly concentrated corn starch slurry is the feedstock of choice but the production efficiency and product performance cannot perfectly match the conventional liquefaction process (30 %, w/w). 

Potato and tapioca starch deliver the best substrates for liquefaction and there is much more rapid degradation of the starch molecule (Kong et al., 2021). Both these starches have long external and internal chains which implies a higher  proportion of consecutive α-1,4 linkages. This makes the starch molecule more susceptible to enzymatic attack under highly concentrated substrate condition and a better substrate for sugar production.

Normal corn starch and waxy corn starch, which contain relatively shorter linear fragments, are less accessible to α-amylase. The liquefaction process is slower and more of the larger starch fragments survive.

References

Gaouar, O., Zakhia, N., Aymard, C., & Rios, G. M. (1998). Production of maltose syrup by bioconversion of cassava starch in an ultrafiltration reactor. Industrial Crops and Products, 7, pp. 159–167

Kong, H., Yu, L., Gu, Z., Li, Z., Ban, X., Cheng, L., … & Li, C. (2021). Fine structure impacts highly concentrated starch liquefaction process and product performance. Industrial Crops and Products, 164, 113347.

Li, C., Kong, H., Yang, Q., Gu, Z., Ban, X., Cheng, L., … & Li, Z. (2021). A temperature‐mediated two‐step saccharification process enhances maltose yield from high‐concentration maltodextrin solutions. Journal of the Science of Food and Agriculture. (Article)

Palmer, T. J. (1970) In: Glucose Syrups and Related Carbohydrates (Ed. G.G. Birch, L.F. Green and C.B. Coulson). Elsevier, London.

Pontoh, J., & Low, H. N. (1995). Glucose syrup production from Indonesian palm and cassava starch. Food Research International, 28, pp.379-385

Tong, Z., Tong, Y., & Shi, Y. C. (2019). Partial swelling of granules enables high conversion of normal maize starch to glucose catalyzed by granular starch hydrolyzing enzyme. Industrial Crops and Products, 140, 111626 (Article).