Sugar processing biotech

Sugar processing is a crystallisation operation: dissolve sucrose out of cane or beet, purify the juice, and crystallise. Its biotechnology is mostly defensive — the microbiology of the process is a source of loss, and the enzymes used are there to undo damage.

Dextran: the industry’s characteristic infection

Intact cane is protected. Once it is cut or damaged, sucrose-rich juice is exposed, and Leuconostoc mesenteroides colonises it rapidly. This organism secretes dextransucrase, an extracellular enzyme that acts directly on sucrose: it cleaves the glycosidic bond, uses the released energy to add the glucose unit to a growing α-1,6-linked glucan chain, and frees the fructose.

The result is dextran — a high-molecular-weight polymer — and its effects are disproportionate to the amount formed.

It destroys sucrose. Every glucose polymerised is sucrose that cannot be recovered.

It raises viscosity enormously, because a small mass of very long polymer thickens juice far more than the same mass of sugar. Evaporation, filtration and pumping all slow.

It poisons crystallisation. Dextran adsorbs onto growing sucrose crystal faces and blocks them selectively, so crystals grow elongated and needle-like instead of the regular form. Elongated crystals are difficult to separate in centrifuges, retain more molasses, and lower both yield and quality. Dextran also causes false readings in polarimetric sugar analysis, because it is strongly dextrorotatory — hence the name.

This is why cut-to-crush time is a controlled variable in cane logistics: the whole problem is a function of delay between harvest and processing, and rain, mechanical harvesting and frozen or damaged cane all make it worse.

Dextranase — an endo-hydrolase cleaving α-1,6 bonds — is dosed into the juice to cut the polymer into fragments too short to affect viscosity or crystal habit. It cannot recover the lost sucrose; it limits the downstream damage. It is a rare example of an enzyme deployed against a specific microbial product rather than to transform a substrate.

Inversion is the other loss

Sucrose hydrolyses into glucose and fructose — inversion — under heat and acid, and by microbial invertase. Inverted sugar does not crystallise, so it goes to molasses. Juice pH is therefore held slightly alkaline through liming and clarification precisely to suppress acid hydrolysis, and the balance is delicate because excessive alkalinity degrades reducing sugars into coloured compounds instead.

Colour formation runs alongside: Maillard reactions between reducing sugars and amino acids, and caramelisation, generate colorants that must be removed by carbonatation or ion exchange, since colour carries into the crystal.

Beet has its own molecule

Sugar beet contains raffinose, a trisaccharide of galactose, glucose and fructose. Like dextran, it interferes with sucrose crystal growth and accumulates in recycled molasses streams, and it is not digestible by human enzymes. α-Galactosidase cleaves the galactose, converting raffinose to sucrose plus galactose and removing the crystallisation inhibitor while recovering some sugar.

Beet processing is also where fructans and betaine end up in molasses, which is one reason beet molasses is used as a fermentation substrate rather than a feed the way cane molasses is.