Info

Source: Refs. 2, 22, and Manufacturer's Product Data Sheets.

Notes: DP is the degree of polymerization (DP-2 represents disaccharides, DP-3, trisaccharides, etc). Values are typical for the syrup types shown but will vary depending on the process conditions used during manufacture. Values are expressed as weight percent dry basis.

Source: Refs. 2, 22, and Manufacturer's Product Data Sheets.

Notes: DP is the degree of polymerization (DP-2 represents disaccharides, DP-3, trisaccharides, etc). Values are typical for the syrup types shown but will vary depending on the process conditions used during manufacture. Values are expressed as weight percent dry basis.

crystallizer. The common form of crystallizer is a horizontal cylindrical tank fitted with a slowly turning agitator and a means of cooling the contents by indirect heat transfer. The initial temperature of the mixture is kept below 50°C (122°F) to ensure that crystals grow in the monohydrate form. The mass is slowly cooled, usually over a period of days, until about 50 to 60 wt % of the dry substance has crystallized out as monohydrate crystals of a form suitable for separation and washing.

The values for concentration, initial and final temperature and rate of cooling vary depending primarily on the purity of the supply material. Monohydrate dextrose crystals can be successfully grown in liquors of purity ranging from pure dextrose to syrups containing less than 60% db dextrose. Dextrose crystallizes to form monohydrate adextrose although the existence of a hydrated /(-dextrose has been reported (26).

The resulting magma in the crystallizers is passed into perforated screen centrifuge baskets where it is spun to remove as much of the mother liquor as practical. Then a spray of water is applied to the rapidly rotating cake to wash off residual mother liquor. After spinning an additional period to remove as much liquid as practical, the cake is plowed out of the basket and fed to a dryer to reduce the moisture content to about 8.5 to 9.0%, slightly below the theoretical monohydrate moisture content.

Monohydrate crystallization can be done in several sequential steps starting with high purity hydrolysate to maximize the production of higher value crystalline product and to minimize the amount of mother liquor (usually sold as a lower value process co-product called Hydrol) that contains the polysaccharides that result from incomplete starch hydrolysis. Or, a portion of the mother liquor can be recycled and mixed with the incoming hydrolysate to provide an equivalent yield of crystals, usually about 75 to 80% db of the incoming hydrolysate. Such processes still result in 20 to 25 wt % of the hydrolysate supplied to the crystallization process being sold as Hydrol. Enzymatic reconversion of the mother liquor has been reported to allow nearly complete elimination of the production of lower value Hydrol (27).

The anhydrous form of dextrose is more fastidious than is the monohydrate and does not form easily separated crystals when crystallized from liquors below about 88 to 90% db dextrose content. Although it can be produced by batch crystallization, the predominant method uses vacuum evaporative crystallizers operated either continuously or in a batch mode. And because the starting material requires a minimum purity of about 95% db to produce purgable crystals, the dominant production process begins with a supply produced by remelting monohydrate crystals, resulting in the production of a very high purity product that meets the requirements of the United States Pharmacopeia. When producing USP-grade dextrose, the supply liquor can be passed through an ultrafilter membrane filter to reject pyrogens and make the final product acceptable for injection into the human body. Anhydrous a-dextrose is produced when the crystallization temperature is between about 55 and 100°C (about 131 and 212°F), preferably about 65°C (149°F) to minimize color development during the 6 to 8 h crystallization cycle required to produce a magma containing about 50% of the dry substance on the crystalline form. Crystals are separated, washed, and dried much like the monohydrate variety, although centrifuge cycles are usually shorter. Anhydrous [i-dextrose crystallizes at temperatures above about 100°C (212°F), although the /?-form will continue to crystallize at lower temperatures if all a-form crystals have been excluded from the crystallizer. The jS-form is the most soluble and rapidly dissolves in water.

Clarified, refined high dextrose content hydrolysate can be evaporated to about 71% ds and sold as a liquid product. Or, it can be solidified to either a monohydrate or anhydrous total sugar product (containing all the polysaccharides that result from incomplete starch hydrolysis) using a variety of processes (28). If a higher purity product is desired, chromatographic separation technology can be used to separate the high dextrose content syrup into a very pure (>99.5% db dextrose) product and a raffinate stream that contains nearly all the polysaccharides that result from incomplete starch hydrolysis. The raffinate stream can either be concentrated and sold as a lower value (Hydrol-type) product or be enzymatically reconverted. The very pure dextrose syrup can be sold as a liquid product or converted to a very pure total sugar. Liquid dextrose can also be produced by remelting crystalline dextrose.

Dry dextrose products can be shipped in vapor-resistant bags usually containing 50 or 100 pounds, or the approximately equivalent metric sizes of 20, 25, and 40 kg. Reusable containers holding about 1 metric ton of material are used for customers who do not require bulk delivery but who do not want the bother and expense of bag opening and disposal. Bulk transport via truck or railcar (jumbo railcars can hold as much as 82 metric tons or 180,000 lb) is also used. Solid products should be stored below 40°C (104°F), and rapid temperature changes and extremes should be avoided to minimize product caking.

Although most liquid products are shipped in bulk trucks and railcars (the largest railcars holding as much as 90 metric tons), some products are sold in 200-L (55 gallon) drums and 1000-L (250-gallon) disposable containers. Most small customers are supplied the dry equivalent (where available) of the desired syrup until their needs grow large enough to justify the installation of facilities for bulk truck delivery.

Liquid product storage facilities must be carefully designed to minimize product oxidation and microbial problems, especially in the headspace of storage tanks. Typical storage conditions of common liquid starch sweeteners are shown in Table 5. Storage temperatures selected are a compromise between a low temperature that would minimize product color development with time and the higher temperatures needed to either reduce the viscosity of the lower DE products to allow pumping or to prevent formation of crystals in the higher DE syrups and HFCS.

Economic Aspects

The price of starch is a major factor in the cost of production of starch sugars such as crystalline dextrose. Equipment investment, notably for crystallizers, is another im-

Table 5. Typical Storage Conditions of Liquid Starch Sweeteners

Syrup type

Solids content (wt %)

Temp. (°C)

Factor-controlling storage conditions

42 DE acid syrup

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