Unit 3: Sugar and Starch Industries: Extraction, Refining & Bioproducts

Industrial sucrose ($\text{C}_{12}\text{H}_{22}\text{O}_{11}$, $\alpha\text{-D-glucopyranosyl-(1}\to\text{2)-}\beta\text{-D-fructofuranoside}$, $M = 342.30\text{ g/mol}$) is extracted from two major commercial agricultural crops:

1. Sugarcane (*Saccharum officinarum*): Tropical C4 perennial grass. Mature cane stalks contain $11 - 16\text{ wt}\%$ sucrose, $0.5 - 1.5\%$ reducing sugars (glucose and fructose), $11 - 16\%$ fiber (cellulose, hemicellulose, lignin), and $68 - 75\%$ water.

2. Sugar Beet (*Beta vulgaris*): Temperate biennial root crop. Contains $15 - 20\text{ wt}\%$ sucrose, negligible reducing sugars ($< 0.1\%$), $4 - 5\%$ pulp, and $75 - 80\%$ water.

Sugar Inversion and Optical Rotation

Sucrose is a non-reducing disaccharide without an anomeric free hemiacetal group. In acidic aqueous media or via yeast $\beta$-fructofuranosidase (invertase), sucrose undergoes hydrolysis into equimolar amounts of D-glucose and D-fructose:

This hydrolysis is termed inversion because of the reversal of optical polarization:

• Pure sucrose is dextrorotatory: $[\alpha]_D^{20} = +66.5^\circ$
• D-Glucose is dextrorotatory: $[\alpha]_D^{20} = +52.7^\circ$
• D-Fructose is strongly levorotatory: $[\alpha]_D^{20} = -92.4^\circ$

The equimolar equiconcentration mixture (invert sugar) has a net levorotatory optical rotation:

Because invert sugars are hygroscopic and hinder sucrose crystallization, industrial extraction operations must maintain strictly alkaline to neutral pH conditions ($\text{pH } 7.2 - 8.2$) to minimize sucrose inversion losses.

Juice extraction from sugarcane stalks involves physical cell rupturing followed by mechanical squeezing or countercurrent liquid diffusion.

1. Cane Preparation

Raw cane stalks arriving at the factory are dumped onto feeding tables and pass through:

• Cane Knives: High-speed rotating blades ($500 - 600\text{ rpm}$) that level and chop stalks into small billets.
• Heavy-Duty Shredders: Hammer-mill rotors with pivoting hammers that shatter rind cells without expressing juice, achieving an Open Cell Index ($\text{OCI}$) exceeding $88 - 92\%$.

2. Multi-Mill Tandems & Compound Imbibition

The prepared cane passes through a tandem of 4 to 6 three-roller mills. Each mill comprises a Top roller, Feed roller, and Discharge roller arranged in a triangular configuration with a central trash plate:

• Compound Imbibition: To extract residual sugar from the fiber matrix without excessive dilution, fresh imbibition water ($65 - 75^\circ\text{C}$, $20 - 30\text{ wt}\%$ on cane) is applied exclusively to the bagasse entering the final mill.
• The expressed thin juice from the final mill is recycled backward to spray bagasse entering the penultimate mill, moving countercurrently against the advancing fiber mat.
• Modern milling tandems achieve total sucrose extraction efficiencies ($\text{Pol Extraction}$) of $95.5 - 97.0\%$, discharging moist bagasse ($48 - 50\text{ wt}\%$ moisture, $1.5 - 2.5\%\text{ Pol}$) directly to factory steam boilers.

Multiple-Effect Evaporator Heat Transfer & Boiling Point Elevation Table

In sugarcane and beet sugar refining, thin juice is concentrated from $14 - 16^\circ\text{Brix}$ to $65 - 70^\circ\text{Brix}$ syrup across a quintuple-effect evaporator train:

| Evaporator Effect | Operating Pressure ($\text{bar abs}$) | Vapor Temp ($^\circ\text{C}$) | Juice Brix ($^\circ\text{Bx}$) | BPE ($\text{K}$) | Boiling Temp ($^\circ\text{C}$) | Overall $U$ ($\text{W/(m}^2\cdot\text{K)}$) | |---|---|---|---|---|---|---| | Effect 1 | $2.05\text{ bar}$ | $121.0^\circ\text{C}$ | $18.5^\circ\text{Bx}$ | $0.6\text{ K}$ | $121.6^\circ\text{C}$ | $2,600\text{ W/(m}^2\cdot\text{K)}$ | | Effect 2 | $1.45\text{ bar}$ | $110.3^\circ\text{C}$ | $24.0^\circ\text{Bx}$ | $1.0\text{ K}$ | $111.3^\circ\text{C}$ | $2,100\text{ W/(m}^2\cdot\text{K)}$ | | Effect 3 | $0.98\text{ bar}$ | $99.1^\circ\text{C}$ | $32.5^\circ\text{Bx}$ | $1.8\text{ K}$ | $100.9^\circ\text{C}$ | $1,600\text{ W/(m}^2\cdot\text{K)}$ | | Effect 4 | $0.55\text{ bar}$ | $83.7^\circ\text{C}$ | $45.0^\circ\text{Bx}$ | $3.5\text{ K}$ | $87.2^\circ\text{C}$ | $1,100\text{ W/(m}^2\cdot\text{K)}$ | | Effect 5 | $0.20\text{ bar}$ | $60.1^\circ\text{C}$ | $68.0^\circ\text{Bx}$ | $8.4\text{ K}$ | $68.5^\circ\text{C}$ | $650\text{ W/(m}^2\cdot\text{K)}$ |

Boiling Point Elevation ($\text{BPE}$) of sugar solutions follows the empirical Dühring line relationship:

where $B$ is sucrose concentration in $^\circ\text{Brix}$ and $T_{\text{sat}}$ is saturation temperature of pure water in $^\circ\text{C}$.

Raw sugarcane juice is an opaque, turbid, acidic liquid ($\text{pH } 5.0 - 5.5$) containing suspended bagasse particles, colloidal proteins, polysaccharides, organic acids, polyphenols, and coloring matter.

Clarification Chemical Processes

``` SUGAR JUICE CLARIFICATION PATHS │ ┌──────────────────────────────┼──────────────────────────────┐ ▼ ▼ ▼ SIMPLE DEFECATION SULPHITATION PROCESS CARBONATATION PROCESS Milk of Lime [Ca(OH)₂] Milk of Lime + SO₂ gas Excess Ca(OH)₂ + CO₂ gas Target pH: 7.2 - 7.6 Target pH: 6.8 - 7.0 Double Carbonatation (pH 10.5 ──► 8.5) Calcium Phosphate Floc Calcium Sulfite [CaSO₃] Floc Bulk CaCO₃ Crystal Surface Adsorption │ │ │ ▼ ▼ ▼ Raw Sugar Production Direct Plantation White Sugar Refined White Sugar (> 99.8% Pol) ```

1. Simple Defecation (Raw Sugar Manufacture)

Raw juice is heated to $70 - 75^\circ\text{C}$ and treated with milk of lime ($\text{Ca(OH)}_2$, $0.05 - 0.10\text{ wt}\%\text{ CaO}$ on juice) to neutralize organic acids and raise pH to $7.4 - 7.8$. Soluble natural phosphate ($\text{PO}_4^{3-}$, native concentration adjusted to $> 300\text{ ppm}$) reacts with calcium ions to form a flocculent tricalcium phosphate precipitate:

The juice is heated to boiling ($103 - 105^\circ\text{C}$) to flash off entrained air bubbles and fed to continuous multideck clarifiers (Dorr-Oliver clarifiers) with synthetic polyacrylamide flocculants. Clear juice overflows the top, while settled muds are filtered on rotary vacuum drum filters.

2. Sulphitation (Plantation White Sugar)

Simultaneous addition of milk of lime and sulfur dioxide gas ($\text{SO}_2$, generated by burning sulfur in air) precipitates insoluble calcium sulfite:

The $\text{CaSO}_3$ precipitate adsorbs gums, waxes, and colored impurities. Simultaneously, the reducing sulfite ion ($\text{SO}_3^{2-}$) bleaches melanoidin and polyphenolic pigments by chemical reduction.

3. Continuous Carbonatation (Refined Sugar Standard)

Juice is treated with a large excess of lime ($1.5 - 2.5\text{ wt}\%\text{ CaO}$) followed by sparging with flue gas carbon dioxide ($28 - 32\%\text{ CO}_2$ from lime kilns):

The massive precipitation of microcrystalline calcite ($\text{CaCO}_3$) creates an extensive surface area that occludes colloidal impurities, delivering clarified liquor suitable for white refined sugar.

Clarified juice enters the evaporator station at $13 - 16^\circ\text{Bx}$ (percent dissolved solids) and must be concentrated to thick syrup at $60 - 68^\circ\text{Bx}$, requiring the evaporation of approximately $75 - 80\text{ wt}\%$ of the initial water content.

The Physics of Multiple-Effect Evaporation (Norbert Rillieux, 1843)

In a multiple-effect evaporator, live boiler exhaust steam ($1.8 - 2.4\text{ bar}$, $115 - 125^\circ\text{C}$) is supplied exclusively to the heating calandria of the First Effect. The water vapor boiled off from the first effect is used as the heating medium in the calandria of the Second Effect, which is maintained at a lower pressure and boiling point. This cascading sequence continues through 4 or 5 effects in series, ending in a barometric condenser operating under deep vacuum ($0.15 - 0.25\text{ bar}$, boiling point $55 - 65^\circ\text{C}$).

Steam Economy Principle

For an ideal $N$-effect evaporator with negligible heat loss:

For a standard Quadruple-Effect station ($N = 4$), $1\text{ kg}$ of live exhaust steam evaporates $3.2 - 3.5\text{ kg}$ of water.

Boiling Point Elevation (BPE)

As juice concentrates across the train, the boiling point of the solution exceeds the boiling point of pure water at the same pressure, governed by the Dühring rule and Raoult's law:

In the 4th effect ($65^\circ\text{Bx}$ syrup), BPE reaches $5.5 - 7.5^\circ\text{C}$, reducing the effective temperature driving force ($\Delta T_{\text{eff}} = T_{\text{steam}} - T_{\text{boil}} - \text{BPE}$) available for heat transfer.

Syrup from the evaporators ($65^\circ\text{Bx}$) is converted into crystalline sugar within single-effect vacuum pans operating under controlled vacuum ($0.15 - 0.20\text{ bar}$, boiling at $60 - 68^\circ\text{C}$ to avoid carmelization and thermal color development).

Supersaturation and Crystallization Regimes

The crystallization driving force is governed by the supersaturation coefficient ($SS$):

where $(S/W)$ is the mass ratio of dissolved sucrose to water:

1. Metastable Zone ($1.00 < SS < 1.20$): Existing crystal seed faces grow by mass-transfer diffusion without spontaneous nucleation.

2. Intermediate Zone ($1.20 < SS < 1.40$): Existing crystals grow, and false grain (uncontrolled secondary nucleation) occurs if agitated.

3. Labile Zone ($SS > 1.40$): Spontaneous homogeneous nucleation occurs, generating irregular fine crystals.

The Three-Boiling Scheme (A, B, C Massecuites)

To maximize sucrose recovery from mother liquor, sugar factories employ a multi-strike boiling sequence:

• A-Strike (High Purity, $\sim 85 - 90\%$ Purity): Boiled from virgin syrup and seeded with fine slurry. Discharged as A-Massecuite into centrifugals; yields commercial raw sugar and A-Molasses.
• B-Strike ($\sim 75 - 78\%$ Purity): Boiled from A-Molasses and syrup; yields B-Sugar and B-Molasses.
• C-Strike ($\sim 58 - 62\%$ Purity, Low Grade): Boiled from B-Molasses; cooled slowly in continuous water-cooled crystallizers over $24 - 48\text{ hours}$ to exhaust mother liquor down to $SS \to 1.0$. Centrifugation separates low-grade C-Sugar (recycled as seed magma) from Final Blackstrap Molasses ($30 - 35\%$ sucrose, $15 - 20\%$ reducing sugars, purity $< 35\%$), from which no further crystallization is economically viable.

Sucrose Hydrolysis (Inversion) Kinetics & Optical Mutarotation

During heating at acidic $\text{pH}$, sucrose hydrolyzes into equimolar D-glucose and D-fructose:

• Optical Inversion Phenomenon:
• Pure sucrose is dextrorotatory: $[\alpha]_D^{20} = +66.5^\circ$.
• Inverted hydrolysate contains D-glucose ($[\alpha]_D^{20} = +52.7^\circ$) and strongly levorotatory D-fructose ($[\alpha]_D^{20} = -92.4^\circ$).
• Net specific optical rotation shifts from positive to negative:

This reversal of polarimetric sign from $+66.5^\circ \to -19.85^\circ$ designates the reaction as sugar inversion.

Raw sugar crystals ($97.0 - 98.5\%$ sucrose) are covered with a thin film of dark, sticky mother liquor containing ash, invert sugars, and colorants. Refining removes this molasses film and purifies sucrose to $> 99.9\%$ purity.

Industrial Refining Sequence

1. Affination: Raw sugar crystals are mixed with hot, concentrated affination syrup ($70 - 75^\circ\text{Bx}$, $65 - 75^\circ\text{C}$) to soften the outer molasses film without dissolving the crystalline sucrose core. The magma is spun in high-speed basket centrifugals, washing off $85\%$ of surface impurities to produce Affined Sugar ($99.0 - 99.3\%\text{ Pol}$).

2. Melting & Clarification: Affined sugar is dissolved in hot demineralized water to produce raw liquor ($60 - 65^\circ\text{Bx}$) and clarified by phosphatation or carbonatation.

3. Decolorization: Clarified liquor passes through deep beds of granular activated carbon (GAC), bone char, or macroporous styrenic/acrylic strong-base anion exchange resins:

• Polymeric colorants (melanoidins from Maillard reactions, caramels, and alkaline degradation products of fructose) carry negative charges and bind to quaternary ammonium resin sites ($\text{Resin-N}^+\text{Me}_3\cdots\text{Color}^-$).
• Effluent liquor reaches $< 20\text{ ICUMSA}$ color units (sparkling water-white).

4. Refined Sugar Boiling & Conditioned Granulation: White liquor is boiled in vacuum pans, centrifuged, and dried in rotary drum granulators using dehumidified air to moisture $< 0.03\text{ wt}\%$.

Starch is an endosperm energy-storage polysaccharide comprising two $\alpha$-glucan polymers:

• Amylose ($20 - 30\text{ wt}\%$): Linear chain of $\alpha\text{-(1}\to\text{4)-linked}$ D-glucopyranose units ($M \approx 10^5 - 10^6\text{ g/mol}$) forming helical coils.
• Amylopectin ($70 - 80\text{ wt}\%$): Branched polymer containing $\alpha\text{-(1}\to\text{4)}$ linear links with $\alpha\text{-(1}\to\text{6)}$ branch points every $24 - 30$ glucose residues ($M \approx 10^7 - 10^8\text{ g/mol}$).

1. Corn Wet Milling Extraction

Shelled corn kernels ($70\%$ starch, $10\%$ protein, $4.5\%$ oil, $2\%$ fiber) undergo a 4-step wet separation:

1. Steeping: Kernels soak in warm water ($50 - 52^\circ\text{C}$) containing $0.10 - 0.20\text{ wt}\%\text{ SO}_2$ and lactic acid for $24 - 48\text{ hours}$. $\text{SO}_2$ reduces disulfide crosslinks in the glutelin protein matrix, freeing starch granules.

2. Germ Separation: Coarsely ground corn passes through hydrocyclones; low-density germ ($50\%$ corn oil) floats and is recovered for corn oil pressing.

3. Fiber Washing: Finely milled slurry passes through curved DSM screens to remove coarse fiber (bran/pericarp).

4. Starch-Gluten Separation: The starch-gluten slurry passes to high-speed disk-nozzle centrifuges. Insoluble corn gluten protein (zein, density $\rho = 1.1\text{ g/cm}^3$) exits in the overflow, while dense starch granules ($\rho = 1.5\text{ g/cm}^3$) discharge in the underflow ($> 99.5\%$ pure starch).

2. Enzymatic Conversion into Sweeteners

Starch slurry ($30 - 35\text{ wt}\%$ solids) is converted into commercial syrups via two-stage enzymatic digestion:

1. Liquefaction: Slurry is adjusted to $\text{pH } 5.8 - 6.2$, stabilized with $\text{Ca}^{2+}$ ($50\text{ ppm}$), and injected with thermostable endo-$\alpha$-amylase (*Bacillus licheniformis*) in a steam jet cooker ($105^\circ\text{C}$ for $5\text{ min}$, then $95^\circ\text{C}$ for $90\text{ min}$):

The enzyme cleaves internal $\alpha\text{-(1}\to\text{4)}$ bonds, reducing viscosity.

2. Saccharification: Liquefied maltodextrin is cooled to $60^\circ\text{C}$, adjusted to $\text{pH } 4.2 - 4.5$, and incubated with fungal glucoamylase (*Aspergillus niger*):

Glucoamylase hydrolyzes both $\alpha\text{-(1}\to\text{4)}$ and $\alpha\text{-(1}\to\text{6)}$ linkages from non-reducing chain ends.

3. Isomerization to High Fructose Corn Syrup (HFCS-42): Purified dextrose liquor ($95\text{ DE}$) passes through immobilized glucose isomerase columns (*Streptomyces*), isomerizing glucose into an equilibrium mix of $42\%\text{ fructose}$ and $50 - 52\%\text{ glucose}$.

Starch processing extends beyond basic syrups into high-fructose corn syrups (HFCS) and renewable bioplastics that displace petroleum-derived polymers.

1. Simulated Moving Bed (SMB) Chromatographic Fructose Enrichment

Enzymatic glucose isomerase converts glucose into an equilibrium mixture containing at most $42\text{ wt}\%$ fructose (HFCS-42). To formulate beverage-grade HFCS-55 ($55\text{ wt}\%$ fructose, matching cane sucrose sweetness), the syrup must be chromatographically enriched:

• Separation Principle: Resins functionalized with calcium sulfonate ($\text{Ca}^{2+}$) form weak coordination complexes with fructose oxygen atoms (fructose contains a flexible furanose ring that coordinates strongly with divalent calcium), while glucose passes through unretarded.
• Simulated Moving Bed Technology: Continuous counter-current solid-liquid adsorption is achieved without physically moving the solid resin by cyclically shifting the fluid feed, eluent (pure water), extract (pure fructose), and raffinate (pure glucose) ports across $8 - 24$ interconnected packed columns:

The extracted stream yields $90\text{ wt}\%$ pure fructose, which is blended with HFCS-42 to formulate high-purity HFCS-55.

2. Starch Biopolymer Engineering: Polylactic Acid (PLA)

Starch is an industrial agricultural feedstock for biodegradable polyesters:

1. Lactic Acid Fermentation: Starch hydrolysate (pure glucose) is fermented anaerobically by *Lactobacillus delbrueckii* at $45 - 50^\circ\text{C}$ and $\text{pH } 5.5 - 6.5$:

2. Lactide Ring Formation: Lactic acid is pre-polymerized into low-molecular-weight oligomers, then catalytically depolymerized at $200^\circ\text{C}$ under vacuum with tin(II) octoate catalyst to yield cyclic dimer Lactide:

3. Ring-Opening Polymerization (ROP): Purified lactide undergoes coordinate-insertion ring-opening polymerization:

PLA exhibits tensile modulus comparable to polystyrene ($E \approx 3.5\text{ GPa}$) and degrades completely into water and $\text{CO}_2$ in industrial composting facilities.

University Honors Industrial Case Study: Dextran Contamination & Enzymatic Polysaccharide Mitigation

When sugarcane experiences post-harvest delays in tropical climates, epiphytic bacteria (Leuconostoc mesenteroides) metabolize sucrose into extracellular high-molecular-weight Dextran ($\alpha\text{-(1}\to\text{6)}$-linked D-glucan with $\alpha\text{-(1}\to\text{3)}$ branch points, $M_w > 10^6\text{ g/mol}$):

• Process Havoc: Dextran drastically elevates juice and syrup viscosity, distorting sucrose crystal growth into elongated needle-shaped "cigar" crystals that cannot be separated in centrifugal baskets.
• Biotechnological Remedy: Industrial cane mills inject thermo-tolerant fungal dextranase enzymes ($50 - 65^\circ\text{C}$) into mixed juice clarifiers, cleaving internal $\alpha\text{-(1}\to\text{6)}$ linkages into low-viscosity isomalto-oligosaccharides, restoring normal cubic crystal habit and sucrose crystallization velocity.