Sugars Flashcards
Define:
- Sugar_
- Sugars
- Glycemic carbohydrate
- Total sugars
- Added sugars
- Sugar_ : describe sucrose
- Sugars : describes all mono- and di- saccharides occuring/added to foods.
- Glycemic carbohydrate : refers to glucose; available for metabolism
- Total sugars : intrinsic + added sugars
- Added sugars : naturally occurring mono and disaccharides added to food during processing or preservation (e.g., HFCS, sucrose, glucose)
What are sugar thresholds for total daily calories from sugar?
10%
(WHO)
Describe D-glucose.
- Abundant monosaccharide used for energy by cells; occurs naturally
- L-glucose is metabolically inert; synthesized for diabetics to provide sweetness
- Oxygen’s electronegativity is important for the sweetness of glucose
- Can isomerize; a reducing sugar
- Present as free glucose in honey, ripe fruits, and body fluids of vertebrates.
- Integrates with disaccharide or polysaccharide formations
- fructose + glucose = sucrose
- galactose + glucose = lactose
- glycogen, starch, cellulose
Which is sweeter; aldose or ketose sugars?
Ketose!
Why are sugars added to food?
- functional
- safety
- sensory
What is invert sugar?
Components of sucrose; prevents crystallization
Describe general properties of D-glucose.
- Has reducing properties
- Aldohexose sugar
- Referred to as dextrose monohydrate due to dextrorotary properties
- Formed by 4 asymmetrical chiral carbon atoms (C-2,3,4 and 5)
- Chemically reduced (by hydrogenation) to form sorbitol (sugar alcohol)
- Undergoes dextrarotary mutarotation in solvent
- Has both alpha and beta D-forms.
Describe D-fructose.
- Ketose sugar; exists as a 5-membered hemiketal ring (furanose)
- Will establish equilibrium with glucose; isomer of glucose
- More soluble than other monosaccharides
- Most intensely sweet
- Reducing sugar
- Ability to mutarotate
Which monosaccharide is the sweetest and most soluble and why?
D-fructose
Oxygen atom is fully insulated/isolated from other hydrogens that exist on neighbouring groups; electronegative charge is not compromised in any way
Describe D-Galactose.
- Component of milk sugar (lactose) and agar.
- beta-galactose and alpha/beta-glucose forms lactose (enzymatically hydrolyzed by beta-galactosidase)
- Almost identical to glucose except for position of C4 hydroxyl group.
- Reducing sugar
- Mutarotates
- Less sweet than glucose
- Not very water soluble relatively.
What is mutarotation?
Change in optical rotation of plane polarized light as a result of the reversible conversion of one isomeric form to another.
In aqueous solution, alpha and beta anomers quickly equilibrate
Glucose, fructose, galactose, and maltose: reducing sugars (free hydroxyl at anomeric carbon; also have ability to mutarotate)
Sucrose: no free hydroxyl at anomeric carbon (and no ability to mutarotate)
How are oligosaccharides classified?
- Oligosaccharides contain two or more sugar units joined by glycosidic bonds; few produced in nature, most produced by hydrolysis of polysaccharides into smaller units
- Disaccharides = sucrose, maltose, lactose, cellubiose, trehalose
- Trisaccharides = raffinose
- NOTE: sugar must be in monosaccharide form in order for metabolization for energy
What is the difference between maltose and cellubiose?
The glycosidic linkage; maltose is alpha-1,4; cellubiose is beta-1,4
Both are disaccharides composed of two glucose units.
Describe sucrose.
- Most abundant disaccharide C12H22O11
- Composed of α-D-glucopyranosyl (glucose) unit and a β-D-fructofuranosyl (fructose) unit linked head to head (reducing end to reducing end)
- Unusual linkage between anomeric C1 of glucose and anomeric C2 of fructose.
- Not a reducing sugar
- Sugar canes and sugar beets
- Concentration affects water activity and thus stability against microbial growth.
What prevents sucrose from hydrolization by common carbohydrate-cleaving enzymes (e.g., amylase), and from non-enzymatic reactions with amino acids or proteins? Why is this important?
The unavailability of the anomeric carbon
This is important because sucrose does not contribute to browning reactions. The glycosidic bond is not very strong and can be broken by the addition of heat and acid. As a consequence, under certain conditions, browning does occur, but only after sucrose has been hydrolyzed to its two components.
How is invert sugar formed? Why is invert sugar useful?
- Hydrolysis by acid or enzyme (e.g., invertase or sucrase) yields a mixture of individual glucose and fructose units (= invert sugar).
- Sucrose as invert sugar is used to suppress ‘bitter’ or ‘sour’ notes (i.e., basis of sucrose to ‘smooth’ flavour profiles)
- Using invert sugar prevents against the sugar recrystallization that would otherwise occur if sucrose was used in the necessary quantities to achieve the desired effect
- If using for fermentation, sucrose must first be hydrolyzed (e.g., zymase enzymatic activity) to allow invert sugar to undergo fermentation.
Describe lactose.
- Milk sugar; not as sweet as glucose; disaccharide; recrystallizes easily due to distinct isomers alpha and beta.
- Alpha form is very susceptible to recrystallization
- β-D-galactose (1,4) α/β-D-glucose
- Lactose can exist as beta (hydroxyl on anomeric carbon points up) or alpha. (i.e., glucose may be an α or β pyranose)
- When lactose is hydrolyzed by β-D-galactosidase (lactase) sweetness increases and freezing point decreases.
Describe the hydrolysis of lactose.
- Intestinal villi secrete lactase (β-D-galactosidase)
- Hydrolyzes lactose into galactose and glucose
- Lactose intolerance: insufficient enzymatic activity to hydrolyze lactose; lactose adsorbs water (=bloating, discomfort, diarrhea)
- Bacteria in the large intestine ferment lactose (i.e., lactobaccilus have a competitive advantage to ferment lactose)
Differentiate between lactose intolerance and cow’s milk intolerance.
Lactose intolerance: lack lactase (i.e., digestive problem)
Cow’s milk intolerance: allergic reaction (i.e., immune response)
Describe maltose.
- Reducing sugar
- Two units of glucose joined with alpha 1,4 glycosidic linkage
- Easily digested (i.e., no similarity to lactose in terms of intolerance)
- Maltose occurs to a limited extent in sprouting grain
- Formed most often by partial hydrolysis of starch and glycogen
- In beer production, maltose is liberated by the action of malt (germinating barley) on starch
Describe the hydrolysis of maltose.
- Produced by hydrolysis of starch using beta-amylase (present in sprouting grains like barley but not saliva)
- Maltose can be broken down into monomers by hydrolysis of alpha-amylase (present in saliva) or by heating with a strong acid.
Describe raffinose and its hydrolysis.
- Raffinose is a trisaccharide found in beans, cabbage, broccoli, and whole grains
- Composed of galactose, fructose, and glucose.
- Hydrolyzed to D-galactose and sucrose by alpha-galactosidase (not found in humans!)
- Raffinose passes undigested through the stomach and upper intestine
- In the lower intestine, it is fermented by gas-producing bacteria (carbon dioxide, methane and/or hydrogen)
How is sugar reducing power determined?
- Benedicts reagent: reducing sugar turns solution green/orange red.
- Fehling’s solution: contains copper (II) ions (i.e., cupric ion); in presence of reducing sugar forms a brick red precipitate of copper (I) oxide (i.e., cuprous oxide) upon heating.
How is sugar reducing power determined?
- Benedicts reagent: reducing sugar turns solution green/orange red.
- Fehling’s solution (requires alkaline conditions so that reducing power is maximized): contains copper (II) ions (i.e., cupric ion); in presence of reducing sugar forms a brick red precipitate of copper (I) oxide (i.e., cuprous oxide) upon heating.
What is reducing power?
A reducing sugar in basic solution oxidizes itself to form an aldehyde or a ketone.
A reducing sugar acts as a reducing agent and itself becomes oxidized (i.e., it loses an electron)
Describe the importance of reducing power.
- Important for the initiation of the Maillard reaction
- Reducing sugars react with amino acids in a non-enzymatic browning reaction (initial stages occur more rapidly with fructose than glucose because it exists in a greater extent in its open-chain form)
- Contributes to changes in food palatability and other nutritional effects
Describe sugar solubility. [6]
- Aldose sugars (especially disaccharides) are prone to recrystallize from solution.
- Ketose sugars have greater solubility than aldose sugars (fructose candy are softer than glucose candy)
- Fructose has highest solubility; difficult to crystallize
- Simple sugars have high solubility in water
- Supersaturated sugar solutions are formed in water when enhanced with heating and agitation
- Lactose occurs in two crystalline forms (alpha and beta); alpha hydrate can be crystallized from supersaturated solution, beta form is much more soluble
What does the solubility of sugar depend on?
- The solubility of sugar depends on its crystal structure or the affinity to form crystals.
- Crystals are an important factor for texture and threshold detection of sweetness.
Compare solubilities of fructose, sucrose, glucose and lactose.
- Fructose is the most soluble.
- Lactose is the least soluble.
- Sucrose is more soluble than glucose at lower temperatures
- Glucose is more soluble than sucrose at higher temperatures
Describe halogenation of sugars.
- Hydroxyl group replaced with halogen by esterification
- Sweetness depends on type, number, and position of halogen substitutes.
- Sucralose (e.g., Splenda); indigestible due to beta position of glycosidic bond; 0 calories
- 250x sweeter than sucrose; chloride groups contribute to perceived sweetness due to increased overall electronegative charge of the molecule
- Recall AH-B theory.
- 250x sweeter than sucrose; chloride groups contribute to perceived sweetness due to increased overall electronegative charge of the molecule
Describe reactions of oxidation and reduction of sugars.
- Carbs have hydroxyl groups available for reaction
- Simple monosaccharides and other low MW compounds also have reactive carbonyl.
- Oxidizing agents oxidize aldehyde or carbonyl groups (e.g., glucose oxidase)
- Reducing agents can reduce carbonyl groups of an aldehyde or ketone to an alcohol (i.e., hydrogenation).
When might oxidation reactions be employed?
To remove glucose from a food matrix for prevention of MR.
Oxidation of D-glucose (an aldehyde) to D-gluconic acid (a carboxylic acid) is catalyzed by glucose oxidase (specific for glucose).
May be used to measure glucose in foods and biological materials such as blood.
D-gluconic acid is present in fruit juices and honey naturally.
Describe the reduction of carbohydrates.
- Addition of hydrogen to the double bond between the oxygen atom and the carbon atom of the carbonyl of the aldose or ketose (= hydrogenation)
- Aldehyde to alcohol
- Alditols and hexitols are produced (=sugar alcohols)
What is produced by hydrogenation of:
D-glucose
D-mannose
D-fructose
D-xylose
Sorbitol or D-glucitol
D-mannitol
D-mannitol
Xylitol
How are sugar alcohols formed?
- Derived by treating sugars with hydrogen iodide under pressure (catalytic hydrogenation) in the presence of a metal catalyst (i.e., nickel) or an active metal in water.
- A reduction of the carbonyl group to an alcoholic hydroxyl group of simple sugars results
- The carbonyl group (aldehyde or ketone reducing sugar) has been reduced to a primary or secondary hydroxyl group
Compare sugar alcohols to sucrose. [8]
- Not as sweet
- Less caloric
- Not metabolized by oral bacteria; do not contribute to tooth decay
- Does not brown or caramelize when heated
- Commonly used for replacing sucrose in foodstuffs
- Often used in combination with high intensity sweeteners to counter the low sweetness.
- Incomplete absorption into bloodstream from small intestines; smaller change in blood glucose
- No reducing capacity; do not react in MR
What is the first step of the Maillard reaction and when does it occur?
The chemical reaction is initiated with heat and involves reducing sugars conjugating with free amino acid or a free amino group of an amino acid.
The reactive carbonyl group of the sugar reacts with the nucleophilic amino group.
This process is accelerated in an alkaline environment as the amino groups are deprotonated and hence have increased nucleophilicity.
Foods processed at high temperatures provide the energy required for non-enzymatic browning.
Describe the initial stage of the Maillard reaction. [3]
- Irreversible
- pH must be alkaline to initiate (deprotonated amino groups have increased nucleophilicity)
- a condensation reaction between reducing sugar and amino group and production of an unstable intermediate called a Schiff base which undergoes further transformation into an N-substituted-amino-deoxy-ketose (Amadori compounds are aldose products)
- Heyn’s compounds are ketose products
Discuss temperature and water activity as it pertains to the MR.
- Both impact rate of reaction.
- Temperature is the most important
- Water activity is an important cofactor.
What are the steps of caramelization of sucrose? [8]
Heat at 160C
- Equilibration of anomeric and ring forms
- Sucrose inversion to fructose and glucose
- Condensation (enables intermolecular bonding within the sugar)
- Intramolecular bonding
- Isomerization of aldoses to ketoses (increases sweetness)
- Dehydration reactions
- Fragmentation reactions
- Unsaturated polymer formation
What must a substance contain in order to elicit a sweet taste sensation according to AH-B theory?
- A hydrogen bond donor (AH): electrophilic
- A hydrogen bond acceptor (B): nucleophilic
- They are separated by 2.5-4 A.
Describe AH-B-X Theory.
- 3rd component involves van der waal (dispersion) or hydrophobic interactions (important in sweeteners and some a.a.)
- Designated site X functions as lipophilic
- Facilitates increased affinity with AH-B glycophore at receptor site (increases taste potency by increasing duration of transient hydrogen-bonds)
- The hydrophobic component from the ligand attaches to the lipophilic regions of the taste receptor (overall geometric spacing = optimized)
- Lay-out of compound-tastebud interaction produces differing degrees of sweetness.
What facilitates the tremendous sweetness of artificial sweeteners?
- Natural sugars have AH-B components
- Artificial sweeteners have AH-B-X components
- One exception is sucralose, which is halogenated with chloride, and has a very active AH-B component.
Give an examples of a non-caloric sweetener.
- Acesulfame Potassium
- Often used with aspartame due to lack of depth in flavour
- 200x sweeter than sucrose
- Does not contribute to dental caries
- Sucralose (e.g., Splenda)
- Chlorinated molecule
- 3-OH groups of sucrose replaced by Cl.
- 600x sweeter than sucrose
- Does not contribute to dental caries
- Resists non-enzymatic browning
