The pathways for production of the various sweeteners share many common steps.
Maltodextrins
Maltodextrins are a type of saccharide polymer made up of D-glucosyl units linked primarily with alpha-1,4 bonds and have a DE less than 20. They are considered non-sweet and nutritive. The GRAS affirmation in 21 CFR, Section 184.1444 has been modified to include maltodextrins derived from potato starch as GRAS. Maltodextrins are usually manufactured through an acid or acid-enzyme process. Acid-converted maltodextrins contain a high percentage of linear fragments, which can cause haze in certain applications due to retrogradation. This can be overcome by using alpha-amylases. Enzyme-catalyzed conversion is another process used to produce maltodextrins. The process involves pasting the starch slurry, treating it with a bacterial alpha-amylase for liquefaction, and stabilizing with calcium ions. Multiple enzyme treatments may also be used. After conversion, the solution is filtered to remove protein and fats, refined, and decolorized before being evaporated or dried. Maltodextrin solutions are not evaporated to as low a solids level as glucose syrups due to their high viscosity. Commercial maltodextrins are used in applications where high viscosity and a neutral taste are desirable.
Glucose/corn Syrups
Corn syrups are considered safe for consumption and have specific standards outlined in the 21 CFR, Section 184.1865, as well as in Sections 168.120 and 168.121. In the US, over 7 billion pounds (3×10^9 kg) of corn syrups were produced from starch in 1999. The conventional process of manufacturing glucose/corn syrups involves either an acid or acid-enzyme process. While acid-catalyzed hydrolysis was traditionally used for corn syrup production, acid-enzyme processes are now preferred for producing sweeteners with a DE of 55 or higher, as acid-catalyzed hydrolysis can result in the formation of undesirable flavors in the syrup.
Acid-catalyzed Hydrolysis
Starch slurry is pumped into a pressure vessel called a ‘converter’ and acidified with dilute hydrochloric acid to a pH of about 2.0 at a temperature of 140-160 °C and a pressure of 80 psi. This process breaks down the starch into lower molecular weight products and produces syrups with a predictable carbohydrate profile. The reaction is stopped by raising the pH with sodium carbonate, and the liquor is then filtered to remove suspended fats and impurities. It is then passed through activated carbon beds for clarification and bleaching. Some syrups undergo ion exchange to improve color and stability, reduce ash levels, and enhance flavor. The pH is adjusted, and the liquor is evaporated using multiple-effect, falling-film evaporators under precise conditions. The syrup is then stored and analyzed before shipment.
Acid – Enzyme Processes
Starch is hydrolyzed in a converter to the desired starting DE, and then enzymes are added to reach the final DE or carbohydrate profile. The enzymes are added to the acid-converted slurry and allowed to react in an enzyme tank. The use of multiple enzymes may be necessary to achieve the desired carbohydrate profile.
Alpha-amylases are bacterial or fungal enzymes that break down alpha-1,4 linkages in amylose and amylopectin, producing dextrose and maltose. The reaction is initially fast and then slows down.
Beta-amylases from barley and yeast act on the non-reducing ends of starch molecules and produce maltose in the beta form. They are used to produce high-maltose syrups. The enzyme cannot cleave branch points, so the yield of maltose from amylopectin is only 55%.
Glucoamylases are fungal enzymes that break down maltose to produce glucose. They hydrolyze alpha-1,3, alpha-1,6, and beta-1,6 linkages. Their primary reaction is on the 1,4-linked α-D-glucopyranosyl units of non-reducing ends, releasing β-D-glucopyranose. When branch points are reached, the enzyme will cleave the 1,6 bonds, but at a much slower rate.
Pullulanase and isoamylase are debranching enzymes that break down 1,6 linkages in starch to produce high maltose syrups. Table 21.3 lists commercial enzymes used in corn refining, along with typical operating requirements.
Maintaining proper pH and temperature is important during the 48-hour batch enzyme conversion process, which involves filling enzyme tanks sequentially from the converter and monitoring the reaction’s progress by checking DE or carbohydrate profile.
After enzyme conversion, the tanks are emptied, and the liquor is processed through filtration and carbon bleaching, as previously described. Enzyme-converted syrups make traditional measurement methods such as DE less relevant since two syrups with the same DE can have different carbohydrate profiles and performance characteristics.
High-fructose Syrups
Immobilized enzyme technology can be used to make high-fructose syrups with varying levels of fructose, including 42%, 55%, and 90%. In 1999, over 24 billion pounds of high-fructose syrup were shipped in the US.
The process for making a 42% high-fructose syrup involves mixing a starch solution with an alpha-amylase enzyme and heating it for several minutes before cooling and adding more enzyme. Glucoamylase is then added and the mixture is left to react for 24-90 hours, producing a liquor containing 94% dextrose. The liquor is filtered and purified before being isomerized.
To convert the liquor to high-fructose syrup, it is reacted with immobilized glucose isomerase in a reactor. Nonenzymic processes for isomerization can also be used but they produce undesirable by-products.
Enzyme technology can convert glucose to fructose on a commercial scale. Current production of high-fructose syrups typically use immobilized enzymes from various species.
Fixed bed systems with immobilized enzymes can process large quantities of product through small reactors quickly, reducing the development of undesirable compounds.
The mechanism for isomerization of glucose to fructose is well-documented and follows Michalis-Menten characteristics. Proper demineralization of the liquor prior to isomerization is essential, and optimal operating conditions include a pH range of 6.5 to 8.5 and a temperature of 40-80°C.
Enzyme decay is exponential, so typical systems contain several reactors containing enzymes in varying stages of output. Fructose levels can be controlled by varying reaction time, temperature, and pH.
Once conversion is complete, the liquor is pumped through beds of activated carbon and then evaporated to the proper solids level, generally 71% or 80% dry solids.
Enzyme technology was developed in the 1950s to convert glucose to fructose on a commercial scale. The production of high-fructose syrups currently uses immobilized enzymes from different sources. The isomerization of glucose to fructose has been studied extensively, and the reaction requires divalent cations and operates best at a pH range of 6.5 to 8.5 and a temperature of 40-80°C.
The 42% high-fructose syrup produced by isomerization is not sweet enough for some applications, so enriched fructose syrups are made by separation technology. The technology uses the differences in affinity of dextrose and fructose for strong acid ion exchange resins to separate the two carbohydrates into enriched streams. Through automatic valves, enriched dextrose and fructose streams are drawn off, and the dextrose fraction is returned to the front of the system to be reisomerized. The enriched fructose fraction is blended with a 42% fructose stream to produce 55% high-fructose syrup.
The 42% fructose syrup from the isomerization column is first demineralized and then pumped into the separator at 36-60% solids. The system uses a simulated moving bed to selectively absorb fructose relative to dextrose, resulting in the separation of the two carbohydrates. The separated streams of dextrose and fructose have a typical purity of 85% to 90%. The purity of the separated streams affects the evaporation cost. The enriched fructose stream can be used to make 90% high-fructose syrup or crystalline fructose.
Crystalline Fructose
If there is enough fructose compared to non-fructose materials, it can be turned into crystals. To do this, the fructose must be at least 90% pure. The process involves adding dry fructose crystals to a liquid mixture and cooling it until the pure fructose crystallizes. The crystals are then washed, separated, and dried to create a material that is 99.5% pure. Adding ethanol or methanol can help improve the crystallization process. Care must be taken with dihydrate fructose crystals as they can dissolve in their own water. Crystalline fructose is very sensitive to moisture and should be stored in conditions below 50% relative humidity.
Crystalline Dextrose and Dextrose Syrups
In the past, separating and crystallizing dextrose was time consuming and expensive. Nowadays, commercial processes start with a solution made by liquefying starch in a jet cooker. This solution is turned into a 94% dextrose liquor through saccharification. To make dextrose syrup, the fats and proteins are removed, and the syrup is carbon bleached, demineralized, and evaporated. The liquor can also be further refined to 99% dextrose through adsorption-separation chromatography. Anhydrous dextrose or dextrose monohydrate can be obtained through crystallization. Monohydrate crystallizers use batch tanks or continuous systems. During crystallization, the syrup is cooled below 50°C in a controlled manner. The crystallized dextrose is washed and centrifuged to remove the mother liquor. The remaining crystals are dried and packaged.
Anhydrous dextrose is made by dissolving the monohydrate in hot water and refining it. Temperature control is important during crystallization to ensure nuclei formation. These nuclei are grown under controlled conditions, separated, washed, and screened as before. The mixture is then evaporated under reduced pressure with heat and agitation and dried to a moisture level of 0.1%. This results in a free-flowing powder that is separated and packaged as in the case of the monohydrate.
Oligosaccharide Syrups
In the 1990s, people started paying more attention to sweeteners made from sucrose, soy flour, or corn starch. These sweeteners are called oligosaccharides and are made up of short chains of sugar molecules. They can be made by using certain enzymes, like transglucosidase. The process involves passing a liquid with a lot of maltose through a column to separate out the maltose. The remaining liquid can be turned into a syrup containing mostly maltotriose. Maltotriose syrups can also be made using enzymes. These syrups can be further processed to contain high levels of isomalto-oligosaccharides.