FSC 401 Carbohydrates

Explain functions of CHO in foods, and be able to classify CHO

Functions:

• Sweeteners

• Texturing, thickening, and stabilizing agents – i.e. starch, gums

• Energy source – i.e. glucose, fructose, galactose, sucrose, maltose, lactose, starch

• Dietary fiber – i.e. cellulose, β-glucan, arabinoxylan, resistant starch

• Gelling agents – i.e. pectin

• Prebiotics – i.e. inulin, fructo-oligosaccharides, galacto-oligosaccharides

Monosaccharides:

• 3-9 carbons:

• Ex: glucose, galactose, mannose, and fructose

Explain the differences between and aldose and ketose sugar, give examples in foods

• Aldose: monosaccharide that contains one aldehyde group

  • glucose, mannose, and galactose

• Ketose: monosaccharide that contains a ketone group

  • fructose

Seliwanoff’s test:

• Add resorcinol and hydrochloric acid to sugar solution, then heat.

• Ketoses react faster (carbonyl stability?) → deep red color

• Aldoses react slowly → yellow/pink color

Define, recognize and give examples of a chiral carbon, isomer, epimer, enantiomer, mutarotation

• Chiral carbon - four different groups attached to it; asymmetric, can
  exist in two different spatial arrangements/configurations.

  • A chiral compound is one with a non-superimposable mirror image

• Isomer - Same molecular formula, but different chemical structures

• Structural or Conformational Isomers -

  • D-glucose vs. D-fructose

• Diastereomer - stereoisomer where two molecules have the same molecular formula and connectivity but are NOT mirror images of each other

  • D-glucose vs. D-galactose

• Epimer - Sugars that differ from each other at only one chiral center

  • D-mannose and D-galactose are C2 and C4 epimers of D-glucose

• Enantiomer - optical isomers - mirror images of each other (nonsuperimposable)

  • L-glucose vs. D-glucose

• Anomers - Hemiacteal or hemiketal that differ only in the configuration at the anomeric carbon

  • α- or β- forms

• Mutarotation - α- ←> β- forms

  • α-D-(+)-glucose (37%) ←> open chain D-(+)-glucose ←> β-D-(+)-glucose (63%)

Explain how sugars form ring structures, furanose vs pyranose

• Furanose – five member ring

• Pyranose – six member ring – most commonly seen in nature

• Anomeric carbon - a carbon atom in a sugar molecule that becomes a stereocenter when the sugar cyclizes

Identify and differentiate a hemiacetal vs hemiketal, and relevance to sugars

• Hemiacetal – Ring structure of an aldose sugar formed thru a reaction between aldehyde and -OH group

• Hemiketal – Ring structure of a ketose sugar formed thru a reaction between a ketone and –OH group

Explain how sugar alcohols are formed, identify and name sugar alcohols commonly used in food, and discuss food applications.

• D-Sorbitol, general humectant

• D-mannitol, nonsticky coating on candies, sugar free chocolate, pressed mints, cough drops, candies

• Xylitol, naturally found in fruits (strawberries, rasberries), noncariogenic

• Erythritol, more recently approved

Some disadvantages of sugar alcohols:

• Sometimes lingering bitter aftertaste

• Laxative effect when consumed at high levels in the diet

• They do no undergo caramelization browning reactions, carbonyl-amine reactions

Define disaccharides and oligosaccharides, and give examples.

Disaccharides:

• 2 saccharide units linked by glycosidic bond

• Typical formula: (C₁₂H₂₂O₁₁)

• Sucrose, Maltose, and

Oligosaccharides:

• 3-10 saccharide units linked by glycosidic bonds

  • Trisaccharides – 3 units, raffinose

  • Tetrasaccharides – 4 units, stachyose

  • Pentasaccharide – 5 units, verbascose

Explain and use the nomenclature for naming.

Know the structures and glycosidic linkages for the common di- and oligo-saccharides presented.

Be able to draw the formation of a glycosidic bond.

Define reducing sugar and identify reducing and non- reducing sugars.

• Reducing sugars: sugars that have a free anomeric carbon (hemiacetal) and have a positive Fehling’s test

  • Ex: almost all monosaccharides, maltose, and lactose

• Non-reducing sugars: no anomeric carbon

  • Ex: sucrose

Describe the Fehling’s test and what it is used to measure.

aldehyde + Fehling A+B (Cu²⁺) +heat + alkali → carboxylate + Cu₂O (brown ppt)

Define cyclodextrin and enzymes used to create them, and identify the three forms, their structural cavity size and common food uses.

• Cyclic oligosaccharides with 1->4 linked α-D-glucopyranosyl units

• Formed from partially hydrolyzed starch by action of cyclodextrin glycotransferase (CGTase)

• Cyclodextrins may consist of 6 (α, 0.56nm), 7(β, 0.7nm), and 8 (γ, 0.88nm) glucose units.

• γ cyclodextrin is most soluble

• All three have FDA approval for food use → GRAS status

Applications:

• Stabilization of unstable compounds (i.e flavor compounds susceptible to oxidation)

• Increase physical stability of food ingredients (i.e. controlled release of flavors)

• Selective extraction/removal of cholesterol

• Complex undesirable compounds (i.e bitter flavors)

Identify sources of starch, including cereal grains, roots, and tubers.

• Starch: D glucose monomers with α-1,4-glycosidic bonds, branching points occurring with β-1,6-glycosidic bonds.

• Consists of two major types of polysaccharides of α-D-glucopyranose: Amylose & Amylopectin

• Synthesized in amyloplast in form of water-insoluble granules

• Sources:

  • Corn (Maize), Potato, Cassava (Tapioca), Wheat, & Rice

Describe the structure and composition of starch, including amylose and amylopectin, how they contribute to starch properties.

• Amylose:

  • essentially linear, but creates helices with OH hydrogen bonding like proteins, α-1,4-glycosidic bonds.

  • contributes to gelling and film-forming properties. Cooked paste → cool (set back or retrogradation) → gel (syneresis); higher the amylose content, more gelling capability

• Amylopectin:

  • β-1,6-glycosidic bonds every branch point at 24, 30 residues.

  • is the primary component responsible for crystalline structure, swelling power, and viscosity development; molecular branching inhibits re-association, thus retrogradation-

• In aqueous solutions amylose forms a long helix, amylopectin form short helices

• Starches and their amylose content: Normal corn 20-28 %, Cassava 17-30 %, Potato 20-30 %, Wheat 17-27 %, Rice 16-17 %, Waxy corn 0 %, Waxy rice 0-7 %, High-amylose corn 40-85 %

Semicrystalline structure of starch granules:

• Branch chains of amylopectin form double helices and contribute to the crystalline structure → not soluble in water

• Amylose is present in the amorphous regions of starch granules

Discuss gelatinization, including suspension, imbibition, birefringence, gelation, gel, pasting, viscosity.

Optical Birefringence:

• when viewing crystalline uncooked starch with a Polarized Light Microscope, the light refracts in two directions exhibiting a Maltese Cross.

Suspension:

• starch in water, without heat, precipitates into a paste

Discuss gelation, retrogradation, and syneresis of starches.

Explain why starch is hydrolyzed, what is DE? How is it used?

Define pectic substances, including protopectin, pectinic acid, and pectic acid, what they are, how they are different

Pectic substances (overall):

• found in and between the primary cell wall

• linear, some branched, polymer of D-galacturonic acid joined by α-1,4 glycosidic linkages

  • [α-D-galactose (CH₂OH) → α-D-galacturonic acid (COOH) → pectic substances (COOCH₃)]

Protopectin:

• >80% methylated (esterified) galacturonic acid polymer (COOCH₃)

• immature fruit

• insoluble in water and non-gel forming

Pectinic acid/Pectin:

• partially demethylated galacturonic acid

• ripe fruit

• soluble in water and gel forming

Pectic acid:

• a short-chain derivative of pectin (<10% methylated)

• overripe (mushy fruit)

• non-gel forming

Know how pectin is obtained and fruit ripening enzymes

Commercial purified pectin [available either granular or liquid (sol)]

• apple pomace (cores and skin, 10-15% pectin, dry basis)

• albedo of citrus fruits (inner white of the peel, 20-30% dry basis)

(sugar beet has non-gelling pectins)

Extraction:

• pH = 1.5-3

• temperature = 70-90°C

Enzymatic process:

• Protopectin ——-→ pectinic acid ——→ pectic acid

protopectinase pectinase

• pectin(methly)esterases

  • cause demethylation (complete demethylation yields pectic acid), decreasing gel formation

• protopectinases

  • hydrolyzes protopectin forming lower MW pectin molecules

Define and discuss the classification of pectin according to the degree of, low and high methoxyl pectins.

High methoxyl pectins

• 50-80% methylation

• just under-ripe to just-ripe

• requires low pH (2-3.5) and sugar to gel

• crosslinks between hydroxyl and carboxyl groups

• shear thinning behavior (viscosity decreases with increasing shear rates)

Low methoxyl pectins

• <50% methylation

• just-rip to a lil over-ripe

• Ca²⁺ is necessary to form gel (works best at pH 2.8-6.5 and sets at 50-70 C)

• ionic crosslinks between divalent Ca²⁺ and carboxyl (-) charged groups

Low methoxyl amidated pectins

• reacting methyl ester groups with ammonia → carboxamide groups (15%–25%)

• increased sensitivity to calcium cations (often don’t require added Ca²⁺ (beyond that present in tap water) to induce gel formation

Identify and discuss the contributions of the variables that assist in pectin gel formation, including sugar, acid, pectin, and water in high methoxyl pectins, and calcium or divalent ions in low methoxyl pectins.

Gel formation:

• Pectin is dispersible in water and forms a sol (COOH ionize in water → COO^- and the water hydrogen bonds forming a stable sol. Water is held in a 3-D network of pectin molecules.

• Junction zones: specific regions where two molecules (galacturonic acid) of pectin align and H-bond

• For a standard gel, pectin concentration in final product must be 0.5% - 1.0%.

Sugar:

• decreases water activity and moisture content

• binds water to pectin preventing free water from leaving the gel

• [sugar]: 65% (HMP), 33-55% (LMP)

Acid:

HM pectins:

LM pectins:

LM pectin is used to make low-sugar jams, jellies, and marmalades.

Discuss pectin quality measurements: grading and % SAG

Grading:

• sugar carrying capacity

• ratio of sucrose to pectin to yield a specific rigidity

  • 100 grade pectin: Requires 100 grams of sugar to set one gram of pectin

  • 150 grade pectin: One unit weight of pectin will gel with 150 times its weight in sugar

%SAG (standard acid in glass):

• measures how much a gel sags after it's inverted (more sag, more tender the gel)

• % sag = (depth in container - depth on plate) - (depth in container) X 100

• Ridgelimeter measures the inverted height of gel at time = 0, The difference after 2 mins is measured. squishy hehe

Be able to explain how gums are used in food, why use food gums over starch, proteins to obtain similar functionality.

Be able to list and give examples of the five categories of gums.

Gums: water soluble or dispersible polysaccharides of gelling ability. Hydrocolloids include proteins and gums.

1. Seaweed Polysaccharides / Extracts

• Agar Agar (agar)

• Alginates

• Carrageenans: linear polymer from seweed

2. Plant Exudates

• Gum Arabic (Acacia Gum)

• Gum Tragacanth

3. Seed Gums

• Guar Gum

• Locust Bean Gum

4. Biosynthetic Gums

• Xanthan Gum

• Dextran

• Gellan Gum

5. Chemically Modified Carbohydrates (cellulose)

• Microcrystalline Cellulose

• Carboxymethyl Cellulose (aka: Cellulose Gum)

• Methylcellulose

• Hydroxypropyl Methylcellulose

Indicate gelling versus non-gelling gums

Be able to give specific examples of functions of individual gums

Carrageenan:

• linear polymer from seweed

Be able to discuss how the structure of the gums affects their functional properties

Nutritive Sweeteners

Nonnutritive Sweeteners (Approval?)

Applications and limitations in food systems

Compare relative sweetness to sucrose

Natural vs artificial sweeteners