Oligosaccharides

What is a glycosidic bond?

A glycosidic bond is a type of ether bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate.

What is a glycoside?

A substance containing a glycosidic bond.

What is an O-glycosidic bond?

A glycosidic bond between an anomeric carbon and an oxygen atom.

What is an N-glycosidic bond?

A glycosidic bond between an anomeric carbon and a nitrogen atom

What is an oligosaccharide?

A carbohydrate composed of 2–10 monosaccharides linked by glycosidic bonds.

Where are oligosaccharides commonly found?

They occur widely in plants and can also be produced synthetically or via microbial fermentation and enzymatic processes.

What monosaccharides make up sucrose?

Glucose + fructose.

What are common food sources of sucrose?

Sugarcane, fruits, vegetables.

What monosaccharides make up trehalose?

Glucose + glucose.

What are common sources of trehalose?

Mushrooms, yeast, shellfish.

What monosaccharides make up raffinose?

Glucose + fructose + galactose.

What are common sources of raffinose?

Vegetables, whole grains, beans, cabbage, Brussels sprouts.

What monosaccharides make up stachyose?

Glucose + fructose + galactose + galactose.

What are common sources of stachyose?

Green beans, soybeans.

What monosaccharides make up verbascose?

Glucose + fructose + galactose + galactose + galactose.

What are common sources of verbascose?

Seeds of legumes.

What is a disaccharide?

Two chemically bonded monosaccharides.

How are disaccharides formed?

By a glycosidic link between the reducing group of one saccharide and the hydroxyl group of another.

What are the two classifications of disaccharides?

Reducing and non-reducing.

What is the structure of sucrose?

α-D-glucopyranosyl β-D-fructofuranose.

What is lactose chemically?

4-O-β-D-galactopyranosyl-D-glucopyranose.

What is maltose chemically?

4-O-α-D-glucopyranosyl-D-glucopyranose.

What makes a disaccharide reducing?

It has a free anomeric carbon that can open to the aldehyde form.

What makes a disaccharide non-reducing?

Both anomeric carbons are involved in the glycosidic linkage.

Name four reducing disaccharides.

Maltose, cellobiose, isomaltose, gentiobiose.

Why is trehalose non-reducing?

It has an α-1,1 linkage between both anomeric carbons.

Why is sucrose non-reducing?

Both anomeric carbons (C1 of glucose and C2 of fructose) are involved in the α(1→2)β linkage.

Why is maltose reducing?

Only one anomeric carbon participates in the α(1→4) bond, leaving the other free.

Why does maltose have half the reducing power of glucose?

Because only one of its two anomeric carbons is free.

Which sugars have strong reducing power?

Aldoses with a free aldehyde group.

What reaction allows aldehydes to act as reducing agents?

Oxidation-reduction reactions that convert aldehydes to carboxylic acids.

What does Fehling’s test detect?

Reducing sugars.

What happens in Fehling’s test?

Cu²⁺ is reduced to Cu⁺ (Cu₂O precipitate).

Why does Fehling’s reagent only react with open-chain sugars?

Because only the open-chain form contains a free aldehyde group.

Why does the reaction continue even though only ~1% is open chain?

Ring forms continuously open to re-establish equilibrium.

What happens when the anomeric OH is bonded?

The sugar loses reducing power because the ring cannot open.

Why is fructose reducing even though it is a ketose?

It can isomerize into aldoses under alkaline conditions.

What intermediate forms during fructose isomerization?

An enediol intermediate.

Into which aldoses can fructose convert?

Glucose and mannose.

Why does this conversion allow a positive Fehling’s test?

Because the resulting aldose contains an oxidizable aldehyde group.

Can the enediol itself reduce Cu²⁺?

Yes, it is a strong reducing agent.

Why are dehydration reactions important in food science?

They form the basis of caramelization and Maillard reactions.

Under what conditions do dehydration reactions increase?

Above 130°C or under very acidic conditions (pH < 2.2).

What happens as water evaporates from heated sugar solutions?

Internal dehydration and oxidative fission occur.

What are the two ways water can be eliminated from sugars?

Between two molecules or within one molecule.

What happens when two sugar molecules lose water?

Random polymerization occurs.

What happens during internal dehydration?

Highly unsaturated ring structures form.

What happens to sucrose in acid?

It splits into glucose and fructose.

What compound forms from dehydration of fructose?

Hydroxymethylfurfural (HMF).

What happens to glucose under acidic conditions?

It can isomerize and dehydrate to contribute to HMF formation.

What eventually forms after extensive dehydration?

Brown caramel pigments.

What are the two types of browning reactions?

Oxidative and non-oxidative.

What enzyme causes oxidative browning?

Polyphenol oxidase (PPO).

What do quinones form?

Brown polymeric pigments.

What are the two types of non-oxidative browning?

Caramelization and Maillard reaction.

What triggers caramelization?

Heating concentrated sugar solutions.

What determines caramel flavor vs caramel color?

Sugar type and reaction conditions.

What sugar is typically used for caramel flavor?

Sucrose.

What sugar is typically used for caramel color?

Glucose syrup.

What reacts in the Maillard reaction?

Reducing sugars and primary amines.

What amino acid side chain is especially reactive?

The ε-amino group of lysine.

What is the first step of Maillard reaction?

Formation of a Schiff base.

What rearrangement forms the Amadori compound?

Amadori rearrangement.

Why is the Amadori compound important?

It is a key intermediate in flavor and color development.

What is Strecker degradation?

A reaction between amino acids and dicarbonyl compounds.

What are the main products of Strecker degradation?

CO₂, ammonia, formaldehyde, pyrazines, Strecker aldehydes.

What is a Strecker aldehyde?

An aroma-active aldehyde formed from amino acid degradation.

What forms from condensation of two aminoketones?

Pyrazine derivatives.

What aromas are associated with pyrazines?

Roasted, nutty aromas.

Which sugar is the sweetest?

Fructose.

Which sugar is the least sweet?

Lactose

What happens to sweetness as temperature increases?

Sweetness decreases.

Why can sweetness perception vary?

It is subjective and influenced by other components.

What is the synergistic effect of sugars?

Mixtures taste sweeter than individual sugars alone.

Which sugar has the lowest solubility?

Lactose (~20%).

Which sugar has the highest solubility?

Fructose (~85%).

Why is sucrose good for syrups?

It is highly soluble over a wide temperature range.

Why can lactose crystallize in dairy products?

It has low solubility.

How can sugar crystallization be prevented?

By mixing two different sugars.

How do sugars preserve food?

They lower water activity by binding water.

How do sugars assist in meat curing?

Reducing sugars prevent oxidation of meat pigments.

What are sugar alcohols used for?

Bulking agents, viscosity enhancers, and water-binding agents.

Why does sucrose not reduce Fehling’s solution?

It has no free anomeric carbon.

Does sucrose undergo mutarotation?

No.

Why is sucrose easily hydrolyzed in acid?

It has a carbonyl-to-carbonyl linkage.

What enzyme hydrolyzes lactose?

β-D-galactosidase (lactase).

Why are lactose-intolerant individuals symptomatic?

Lactose is fermented in the lower GI tract.

What is average lactose content in cow’s milk?

~4.8%.

What is lactose content in human milk?

~7.0%

What linkage is present in maltose?

α(1→4).

Is maltose fermentable?

Yes, its fermentable

Does maltose show mutarotation?

Yes, it shows mutarotation

What is isomaltose formed from?

Breakdown of starch and dextran.

Where is gentiobiose found?

As a glycoside in amygdalin (stone fruits).

Where is trehalose found naturally?

Yeast.