FSCN 4112: Food Chemistry and Functional Foods - Carbohydrates 4

Carbohydrates 4: Oligosaccharides, Polysaccharides, Starch, and Dietary Fiber

Oligosaccharides

• Definition: Produced by the reaction of a lactol group with a hydroxyl group of another sugar.

• Composition: Made up of $3$ to $10$ monosaccharide units glycosidically linked to each other.

The "Raffinose Family"

• Raffinose:

  • Structure: $\alpha$-D-Galp-($1\rightarrow6$)-$\alpha$-D-Glcp-($1\rightarrow2$)-$\beta$-D-Fruf.

  • Presence: Up to $0.05\%$ in sugar beets; generally found in legumes (beans, lentils, soybean), e.g., $0.4\%$ (on dry matter base) in garden beans.

  • Cleavage: Cleaved by invertase into fructose and melibiose, and by galactosidase into sucrose and galactose.

• Stachyose:

  • Structure: $\alpha$-D-Galp-($1\rightarrow6$)-$\alpha$-D-Galp-($1\rightarrow6$)-$\alpha$-D-Glcp-($1\rightarrow2$)-$\beta$-D-Fruf.

• Verbascose:

  • Structure: $\alpha$-D-Galp-($1\rightarrow6$)-$\alpha$-D-Galp-($1\rightarrow6$)-$\alpha$-D-Galp-($1\rightarrow6$)-$\alpha$-D-Glcp-($1\rightarrow2$)-$\beta$-D-Fruf.

• Predominance in Legumes: Both stachyose and verbascose are the predominant oligosaccharides in legumes.

  • Example: Garden beans contain $2.6\%$ stachyose; lentils contain $1.2\%$ verbascose (weight-$$ on the dry matter).

Problems with the "Raffinose Family"

• Production of Beet Sugar:

  • Higher raffinose concentration reduces the rate of sucrose crystallization.

  • Produces needle-shaped sucrose crystals.

• Digestive Issues: May cause flatulence.

  • Intestinal micro-organisms hydrolyze them into monosaccharides.

  • These monosaccharides are further degraded, producing CO$\text{<em>2}$, CH$\text{</em>4}$, and H\text{_2}.

Polysaccharides (Glycans)

• Composition: Consist of monosaccharides linked via glycosidic linkages.

• Hydrolysis: Acidic or enzymatic hydrolysis yields monosaccharides.

• Classification:

  • Homoglycans: Homogeneous in monomer units, may be linear or branched (e.g., cellulose, amylose, amylopectin).

  • Heteroglycans: Made up of two or more different monosaccharides (e.g., galactomannans, arabinoxylans).

• Structure: Monosaccharides may be joined in a linear or branched fashion.

  • Figure examples of branching patterns:

    • A) Unbranched molecule.

    • B-D) Molecules with short branches of mono-, di-, or trisaccharide units that are evenly spaced (B), randomly spaced (C), or clustered (D) along the backbone chain.

    • E) A slightly branched but effectively linear molecule, such as amylose.

    • F) The cluster type of branching found in amylopectin.

    • G) A branch-on-branch bush-like structure such as that of gum arabic.

• Most Important Polysaccharides: Starch, inulin, glycogen, cellulose, hemicelluloses, pectins, gums.

Nomenclature of Polysaccharides

• General Term: Denoted by the general term "glycans."

• Naming Convention: Add the suffix "-an" to the name(s) of the principal sugar(s) in the structure (the one forming the majority of the backbone).

  • Example: For a polysaccharide made up of $[\beta$-($1\rightarrow4$)-linked] D-glucopyranose units: glucan.

  • Example: For a polysaccharide made up of $[\beta$-($1\rightarrow4$)-linked] D-xylopyranose units as backbone and $\alpha$-L-arabinofuranose side-chains: arabinoxylan.

Starch

• Function: Storage carbohydrate.

• Importance: Most important carbohydrate source in human nutrition.

• Composition: Made up of D-glucose units.

  • Common sources: Rye, barley, rice, oats, corn.

• Components: Starch consists of two main polymers: amylose and amylopectin.

  • Amylose:

    • Structure: Linear polymer of $\alpha$-D-glucose residues linked by ($1\rightarrow4$)-linkages (repeating unit is maltose).

    • Conformation: Linear chains form a helix.

  • Amylopectin:

    • Structure: Contains additional side chains attached at the $6$-position of glucose residues, i.e., $\alpha$-($1\rightarrow6$)-linkages.

• Typical Amylose/Amylopectin Ratios:

  • Normal starches: $20-30\%$ amylose, $70-80\%$ amylopectin.

  • Waxy starches: $0-8\%$ amylose, $92-100\%$ amylopectin.

  • High-amylose starches: >$40\%$ amylose, <$60\%$ amylopectin.

Starch Granule

• Location: Found in special compartments in plants called starch granules.

• Structure: Radial layers of crystalline and amorphous regions.

• Birefringence: Native starch granules are birefringent, showing a characteristic Maltese cross pattern under a polarized microscope (sensitive reflective light in a particular direction).

  • Birefringence implies a high degree of molecular orientation.

Interaction with other Chemicals

• Iodine Affinity:

  • Amylose has an affinity for iodine; when enclosed by starch, iodine exhibits strong light absorption (intense blue color).

  • Shorter chains (e.g., in amylopectins) form purple-pink colors.

• Clathrates:

  • Other compounds also form clathrates with amylose.

  • Amylose helix is internally hydrophobic, so the "enclosed" guest molecule must be lipophilic (e.g., monoglycerides, fatty acids).

  • Clathrates: Chemical substances that trap molecules within a lattice structure, from Latin clathratus meaning "with bars, latticed."

Starch Gelatinization & Retrogradation

• Gelatinization/Pasting:

  • Process: Irreversible changes occur when a starch suspension is heated, starting around $50-70$ $^{\circ}\text{C}$ (gelatinization temperature, depends on starch type).

  • Changes: Starch granules swell and adsorb $20-40$ g of water/g of starch, leading to a rise in suspension viscosity.

  • Amylose Diffusion: Amylose diffuses out of the granule, and granules eventually burst.

  • Birefringence Loss: Process is accompanied by a loss of birefringence.

  • Cooling Effects:

    • Rapid cooling with mixing: viscosity of a starch paste generally increases.

    • Rapid cooling without mixing: a starch gel is formed.

  • Influence: Amylose/amylopectin ratios influence the pasting profile.

  • Viscosity Profile: \begin{itemize}
    \item Temperature $\approx 60-70^{\circ}\text{C}$: Viscosity begins to rise.
    \item Temperature $\approx 95^{\circ}\text{C}$: Peak viscosity reached.
    \item Cool down (RT): Viscosity increases again, forming a gel.
    \end{itemize}

• Retrogradation:

  • Process: Largely irreversible transition from a solubilized or highly swollen state to an insoluble, shrunken, microcrystalline state.

  • Dependencies:

    • Temperature: Maximum around $0^{\circ}\text{C}$.

    • pH: Maximum around pH $7$.

    • Concentration: Higher concentrations cause higher retrogradation tendencies.

Modified Starches

• Purpose: Starch properties (and those of amylose and amylopectin) can be improved by physical and chemical methods.

• Examples: Pre-gelatinized starch, starch ethers, starch esters, cross-linked starches.

  • Pre-gelatinized/cold-water swelling starch: Used in instant puddings. Ingredients include sugar, modified food starch, etc.

  • Cross-linked starches:

    • Mechanism: Strengthening of the granule by introducing covalent linkages (most common chemical starch modification in foods).

    • Example Linkage: Phosphorylation links OH groups from two different sugars.

    • Benefits: Pastes from cross-linked starches are less likely to break down with extended cooking times, increased acidity, or severe shear.

    • Improvements: Improved viscosity and textural properties.

    • Degree of Cross-linking: Only a low degree of cross-linking is required for desired effect (often < $0.1\%$).

    • Food Examples: Used in cherry pies, cream of mushroom soup to improve texture and stability.

Starch Digestion

• Saliva ($\alpha$-amylase):

  • Partial cleavage of ($1,4-$)$\alpha$-glucosidic linkages.

  • Reduction of chyme viscosity.

• Gastric pH: At pH $2$, further starch hydrolysis by saliva amylase occurs during meal.

• Luminal Digestion (Pancreas $\alpha$-amylase):

  • Results in oligosaccharides.

• Membrane-Located Digestion (Maltase, Isomaltase):

  • Breakdown of oligosaccharides into monosaccharides.

Resistant Starch (RS)

• Definition: Not all starch is digested; resistant starch gets fermented in the large intestine and acts as dietary fiber.

• Classification in Nutrition: Rapidly digestible, slowly digestible, resistant.

• Further Classification of Resistant Starch (RS):

  • RS1: Physically inaccessible due to entrapment of starch within a protein matrix or a plant cell wall (e.g., in uncooked food or food containing uncooked starch).

  • RS2: Native starch granules, typically uncooked (e.g., uncooked potato starch, high-amylose corn starch).

  • RS3: Retrograded, non-granular starch, formed by heat-moisture treatment of starch (e.g., cooked and cooled potatoes).

  • RS4: Chemically modified starches, rendered not digestible by modification (e.g., some modified food starches).

  • RS5: Self-assembled starch-lipid complexes.

Food Applications for Resistant Starch

• Manufacturing Processes: Generally utilize the tendency of high-amylose starch to retrograde or highly crystalline areas resistant to enzymatic hydrolysis.

• Challenges: Incorporation of dietary fiber (DF) into foods can be challenging.

• Functional Properties: RS gives food products unique functional properties.

  • Improves crispness (e.g., in crackers).

  • Provides good expansion properties to low-moisture products.

  • Offers good handling in processing.

• Sensory Impact: Some products with RS even rated better by a sensory panel (e.g., waffles).

• Taste: RS does not negatively affect taste.

• Incorporation Amount: Amount of RS incorporation depends on the product.

• Processing Stability: Some commercial RS have good processing stability.

• "Invisible DF": Can be an "invisible DF" in products where "regular" starch is typically found (e.g., bread with resistant corn starch).

Dietary Fiber

• FDA Definition:

  • Non-digestible carbohydrates (degree of polymerization >3) and lignin that are intrinsic and intact* in plants.

    • Intact: "having no relevant component removed or destroyed."

    • Intrinsic: "originating and included wholly within a food."

    • Intact and intrinsic fibers: Naturally present such that they are integrated within the plant matrix and contain other nutrients naturally present in proportions that exist in the plant cell.

  • Added (isolated or synthetic) non-digestible carbohydrates (>3 monomeric units) that have been determined by FDA to have a physiological benefit.

    • Isolated: Non-digestible carbohydrates isolated from plant sources, no longer intrinsic or intact.

    • Synthetic: Non-digestible carbohydrates chemically synthesized, not isolated from plant sources.

• Currently Approved (FDA): $\beta$-glucan soluble fiber, cellulose, guar gum, hydroxypropylmethylcellulose, locust bean gum, pectin, psyllium husk, inulin.

• Codex Alimentarius Definition:

  • The Codex Alimentarius is a collection of standards, guidelines, and codes of practice adopted by the Codex Alimentarius Commission (CAC).

  • Protects consumer health and promotes fair practices in food trade.

• Properties/Possible Effects of Dietary Fiber:

  • Decrease intestinal transit time and increase stool bulk.

  • Fermentable by colonic microflora (prebiotic effect).

  • Reduce blood total and/or LDL cholesterol levels.

  • Reduce post-prandial blood glucose and/or insulin levels.

  • Increases satiety, aiding in weight management.

  • Effects depend on the type of dietary fiber.

Dietary Fiber Recommendations and Intake

• DRV (Daily Reference Value) for Nutrition Label: $28$ g/day (other agencies recommend different amounts).

• Actual Intake (Adults): $13-15$ g/day.

• Dietary Fiber Contents of Selected Foods (g/100g):

  • Wheat bread (whole grain): $7.5$ g

  • Wheat bread (refined flour): $3.5$ g

  • Oats flakes: $5.6$ g

  • Potatoes: $2.5$ g

  • Tomatoes: $1.8$ g

  • Lettuce: $1.5$ g

  • Apple: $2.3$ g

Soluble vs. Insoluble Dietary Fiber

• Distinction: Name implies solubility in water (buffer).

• Properties: These two fiber types can have rather different properties, including caloric content.

  • Soluble fiber: $2$ kcal/g.

  • Insoluble fiber: $0$ kcal/g.

Plant Cell Wall Polysaccharides (Covered in this lecture)

• Cellulose

• Arabinoxylans

• Mixed-linked $\beta$-glucans

• Pectins

• Additional non-starch polysaccharides widely used in food (though not all will be covered).

Cellulose

• Structure: Homoglycan consisting of $\beta$-D-glucopyranosyl units joined via ($1,4$)-linkages.

• Conformation: Straight linear.

• Properties: Intermolecular hydrogen bonding between glucose residues leads to the formation of microfibrils, making cellulose durable and practically insoluble.

• Modified Cellulose: Has some OH groups replaced to enhance solubility (e.g., carboxymethylcellulose in ice cream).

Hydroxypropylmethylcellulose (HPMC)

• Structure: Cellulose OH partially derivatized with $\text{-CH}<em>2\text{-CH}(\text{OH})\text{-CH}</em>3$ and $\text{-CH}<em>3$ groups (R = H, CH$\text{</em>3}$, -CH$\text{<em>2}$CH(OH)-CH$\text{</em>3}$).

• Regulatory Status: Approved as a food additive by FDA and in EU (E$464$).

• Properties: Viscous, soluble fiber, but not (or only poorly) fermentable.

• Health Benefits: Has a cholesterol-lowering effect.

• Thermal Gelling: Forms reversible gels when heated (becomes liquid upon cooling).

  • Derivatization allows for hydrophobic interactions during heating.

• Food Applications: Used in a variety of foods (e.g., sauces).

  • Special property: Reduction of oil uptake during frying due to thermal gelation.

  • Prevents moisture loss.

  • Example: Used in fries to make them crispy but not oily outside, moist inside.

  • Example: Used in sausages as stabilizers.

Arabinoxylans (AX)

• Backbone: Made up of xylose units.

• Side-chains: On OH $2$ and/or $3$ positions, can be single arabinose units or more complex.

• Sources: Aside from grains, psyllium husks contain AX.

• Enzymatic Cleavage: Some links between xyloses can be cleaved with enzymes to obtain oligosaccharides (AXOS), used in cookies.

• Function: Contributes to viscosity.

Mixed-Linked $\beta$-Glucans

• Sources: Mostly found in grasses, especially oats & barley.

• Structure: Formed by $\approx 70\%$ of ($1\rightarrow4$)-linked $\beta$-D-glucopyranosyl units, interrupted by ($1\rightarrow3$)-linked $\beta$-D-glucopyranosyl units ($30\%$).

• Properties: Insertion of ($1\rightarrow3$)-linkages gives the polysaccharide chain an irregular shape, enhancing its solubility compared to cellulose.

• Function: Can form very viscous solutions.

Fructans

• Sources: Found in many types of edible plants (e.g., garlic, onion, chicory, grains).

• Inulin: Usually obtained from extraction of chicory root; linkages are predominantly $\beta$-($2\rightarrow1$).

  • Structure: $\beta$-D-((2$\rightarrow1$\text{)fructosyl)}_{\text{n}}-fructose$, where$n \le 60$.</p></li><li><p>Possible branching via$\beta$-($2-6$) linkages (e.g., in fructans from Agave tequilana).</p></li><li><p>Oligofructose is the hydrolysis product of inulin.</p></li></ul></li><li><p><strong>Properties</strong>: Prebiotic soluble fiber with low caloric value.</p></li><li><p><strong>Applications</strong>: Can be used to (partially) replace sucrose or fat.</p></li><li><p><strong>Digestive Issues</strong>: Fructan fermentation proceeds rapidly and may cause gastrointestinal discomfort, especially for people with irritable bowel syndrome (IBS).</p><ul><li><p>In clinical trials, IBS symptoms (abdominal pain, bloating, flatulence, etc.) could be reduced for the majority of IBS patients who followed a diet low in FODMAPs (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols).</p></li><li><p>Example: Chicory root fiber used in nonfat yogurt.</p></li></ul></li></ul><h5 id="85b6e625-9606-4c21-9e78-de80a3e3b079" data-toc-id="85b6e625-9606-4c21-9e78-de80a3e3b079" collapsed="false" seolevelmigrated="true">Pectins</h5><ul><li><p><strong>Sources</strong>: Often the major fiber constituents in dicotyledonous fruits and vegetables (contrary to cereals where arabinoxylans and mixed-linked$\beta$-glucans dominate).</p></li><li><p><strong>Industrial Production</strong>: Industrially produced from citrus peels and apple pomace.</p></li><li><p><strong>Components</strong>:</p><ul><li><p><strong>Homogalacturonan (smooth region)</strong>:$\alpha$-D-galacturonic acid units joined by ($1,4)-linkages.

    • Galacturonic acid carboxyl groups are esterified to a variable extent with methanol.

  • Rhamnogalacturonan I & II (hairy regions): More complex structures, often containing arabinan and arabinogalactans, branching from rhamnose residues.

Pectin Gelling Properties

• High-ester pectins: Require an increasing amount of sugar with rising esterification degree to form gels.

• Low-ester pectins: Require very low pH and/or Ca\text{^{2+}}$ions; gelatinize in the presence of low sugar contents.</p></li><li><p><strong>Gel Formation Conditions</strong>:</p><ul><li><p>At a pH of about$3$and in the presence of Ca$\text{^{2+}}.

• At higher pHs, pectins also form thermally reversible gels.

• Standard conditions to form a stable gel in jam/jellies: pectin content < 1\%$, sucrose$58-75\%$, pH$2.8-3.5$.</p></li></ul></li></ul><h5 id="fe0c7f4c-9376-4aec-8ebe-737c84693910" data-toc-id="fe0c7f4c-9376-4aec-8ebe-737c84693910" collapsed="false" seolevelmigrated="true">Pectin Applications</h5><ul><li><p><strong>Stabilizer and Gelling Agent</strong>: Used in jams, jellies, drinks, milk products, ice cream.</p><ul><li><p>In drinks: Cloud-stabilizing.</p></li><li><p>In milk products (yogurt, milk drinks): Thickening effect creates a special mouth-feeling, prevents syneresis, prevents agglomeration of proteins.</p></li><li><p>In low sugar products, low-ester pectin is used with Ca$\text{^{2+}} ions.

Gums

• Definition: Heteropolysaccharides that provide thickening and stabilizing effects.

• Applications: Additives in a whole range of food products (ice cream, beverages, candies, salad dressings, desserts).

• Origins:

  • Seaweeds: Carrageenans, agar, alginates.

  • Plant Seeds: Locust bean gum, guar gum.

  • Microorganisms: Xanthan gum.

  • Plant Exudates: Gum arabic.

  • Examples shown illustrate stabilization, texture (smoothness), and gelation properties in various products (ice cream, salad dressing, gelatin desserts).

Gums in Ice Cream

• Function: Used as stabilizers; increase viscosity, hold water, effective at low concentrations (usually < 1\%$).</p></li><li><p><strong>Benefits</strong>: Especially useful in fat- and/or sugar-reduced ice cream.</p></li><li><p><strong>Examples</strong>:</p><ul><li><p><strong>Carob/locust bean gum</strong>: Creates uniform reproducibility, prevents heat shock.</p></li><li><p><strong>Carrageenan</strong>: Negatively charged polysaccharide due to OH groups replaced by sulfate groups. Often used as a secondary stabilizer to prevent phase separation. Stable in solution with milk proteins. Forms a weak gel that stabilizes casein micelles in solution.</p></li><li><p><strong>Cellulose gum (carboxymethylcellulose)</strong>: A cellulose derivative with bulky carboxymethyl side groups; high water-holding capacity leads to smaller ice crystals.</p></li><li><p><strong>Guar gum</strong>: Helps control ice crystal formation resulting from heat/thaw cycles during distribution.</p></li></ul></li><li><p><strong>Combinations</strong>: Gums are often used in combination (e.g., cellulose gum gels better this way) to achieve desired effects.</p></li></ul><h5 id="86e6da90-7ce2-4afa-b140-19ba67516f96" data-toc-id="86e6da90-7ce2-4afa-b140-19ba67516f96" collapsed="false" seolevelmigrated="true">Galactomannans: Guar Gum and Locust Bean (Carob) Gum</h5><ul><li><p><strong>Function</strong>: Both are galactomannans used as thickeners and stabilizers, FDA-approved as dietary fiber.</p></li><li><p><strong>Structure</strong>: Both have a backbone of$\beta$-($1\rightarrow4$)-linked mannoses, linked to$\alpha$-D-Galp on O$\text{_6}$.</p></li><li><p><strong>Substitution Extent</strong>: The extent of substitution varies.</p><ul><li><p><strong>Guar gum</strong>: More branched (up to$5$unsubstituted mannose units).</p></li><li><p><strong>Locust Bean Gum (LBG)</strong>: Less branched (up to$11 unsubstituted mannoses).

Xanthan Gum

• Production: Produced from Xanthomonas campestris.

• Backbone: (1-4$)-linked$\beta$-D-glucose.</p></li><li><p><strong>Side-chains</strong>: On every$2^{\text{nd}}$$ glucose unit, containing glucuronic acid (giving an overall negative charge).

• Conformation:

  • When dry: Side chains align with the backbone.

  • In solution: Side chains wrap around the backbone, protecting it.

• Viscosity: Exhibits non-Newtonian viscosity (shear-thinning behavior) at high and low shear rates.

• Stability: Stable over a wide range of pH and temperature.

• Usage: Perhaps the most widely used gum in foods.

  • Sauces and dressings: Stabilizes emulsions.

  • Dairy products: Improves stability, viscosity, ice crystal growth.

  • Baked goods: Crumb texture improver; high water-holding capacity can be advantageous in refrigerated dough to prevent dough syruping.

  • Example: Used in organic balsamic vinaigrette.