Food Chemistry and Functional Foods: Carbohydrates 2

Carbohydrates 2: Reactions of Mono- and Disaccharides & Browning Reactions

Reactions of Mono- and Disaccharides

Oxidation

Carbonyl group is key: The open-chain structure of sugars is necessary for these reactions.
Oxidation to Aldonic Acids (Controlled Oxidation)
- Mechanism: Exclusively involves the lactol group.
- Reagents: Achieved using bromine water ( $Br<em>2$ $H</em>2O$ ) in buffered neutral or alkaline media.
- Naming Convention: Acid name is obtained by adding the suffix "-onic acid" to the stem name of the parent sugar (e.g., D-glucose becomes D-gluconic acid).
- Lactones: The oxidation product can form lactones, which are cyclic esters. D-glucono-$\delta$-lactone (GDL) is commonly used in the food industry, undergoing slow hydrolysis in foods, and is generally more stable than the $\gamma$-lactone.
Use of Glucono-$\delta$-lactone (GDL, E575)
- GRAS Status: Generally Recognized As Safe (GRAS) by the FDA, making its use widespread in the food industry.
- Leavening Agent: Used in combination with sodium bicarbonate in refrigerated and frozen dough products.
- Acidulant: Helps regulate acidity. Partial replacement for vinegar in salad dressings.
- Protein Coagulant: Used in cheese and tofu production.
- Raw Fermented Sausages: Reduces the amount of nitrite required and accelerates curing by faster conversion from nitrite to nitric oxide (NO). Permitted in cured, comminuted meat products and Genoa salami.
- Color Stability and Firmness: Improves these qualities in canned and frozen vegetable products.
- Metal Chelator: Helps inhibit melanosis (dark spots in seafood) by chelating metal ions.
- Examples of GDL in products: Pesto alla Genovese, Pizza Crust, Silken Tofu, Genoa Salami, Chicken Salad.
Uronic Acids in Nature and Food
- Plant Cell Wall Polysaccharides: Primarily constituents of pectins (e.g., galacturonic acid, glucuronic acid).
- Gum Polysaccharides: Constituents of gums used as gel-forming and thickening agents.
- Ascorbic Acid Formation: Glucose is oxidized to glucuronic acid as an initial step in ascorbic acid (Vitamin C) formation in plants.
- Detoxification: Partially conjugated with glucuronic acid in the liver for the detoxification of xenobiotic compounds.

Reduction to Sugar Alcohols

Methods: Reduction can occur in the lab via NaBH4 or industrially via catalytic hydrogenation.
Naming Convention: Alcohol name is derived from the sugar by replacing "-ose" with "-itol" (e.g., D-glucose to D-glucitol, D-mannose to D-mannitol, D-fructose can also be reduced to sorbitol and mannitol).
Natural Occurrence: Sorbitol is naturally found in many fruits like pears, apples, and plums ( $300$ - $800\text{ mg/100 mL}$ in apple juice). Berries, citrus fruits, pineapples, or bananas do not contain sorbitol, which is important for analytical evaluation of fruit products.
Properties of Sugar Alcohols
- Sweetness: Variable relative sweetness compared to sugars:
  - Sucrose: $100$
  - D-fructose: $114$
  - D-glucose: $63$
  - D-mannose: $59$
  - D-xylose: $67$
  - D-sorbitol: $60$
  - D-mannitol: $50$
  - D-xylitol: $95$
- Solubility: In baked goods, their effect on gelatinization and gluten network formation depends on their solubility, which is crucial when replacing/reducing sucrose.
- Caloric Values:
  - Isomalt: $2.0 \text{ kcal/g}$
  - Lactitol: $2.0 \text{ kcal/g}$
  - Xylitol: $2.4 \text{ kcal/g}$
  - Maltitol: $2.1 \text{ kcal/g}$
  - Sorbitol: $2.6 \text{ kcal/g}$
  - Mannitol: $1.6 \text{ kcal/g}$
- Laxative Effect: Ingestion of large quantities can result in flatulence and laxation.
  - FDA Requirement: For sorbitol and mannitol, FDA requires the statement "Excess consumption may have a laxative effect" on food labels if consumption might exceed $50 \text{ g/day}$ for sorbitol or $20 \text{ g/day}$ for mannitol (21 CFR 184.1835).

Browning Reactions

Definition: Reactions that produce brown pigments and flavors.
Types:
- Enzymatic Browning: Involves phenolics reacting with enzymes.
- Non-enzymatic Browning: Does not involve enzymes; includes Maillard browning and Caramelization.

Maillard Reaction / Maillard Browning

Significance: Most widely practiced chemical reaction in the world, responsible for many desirable food flavors and colors.
Historical Context: Named after French physician and chemist Louis Camille Maillard, who described it in $1912$ . Brewers and maltsters were aware of similar reactions even earlier (e.g., Ling's description in $1908$ regarding malt kilning).
Reagents: Reducing sugar + Amine.
Products: Brown pigments (melanoidins) + Flavor compounds (pyrazines, furanones, furans, aldehydes, pyranones, thiophens).
- Flavor Examples:
  - Pyrazines: Meaty, burnt
  - Furanones: Sweet, caramel
  - Furans: Cooked, roasted
  - Aldehydes: Pungent, spicy, cocoa, fruity
  - Pyranones: Cooked, roasted
  - Thiophens: Meaty, roasted
Desirable Aspects
- Flavor and Color Formation: Essential for products like malt, bread, soy sauce, roasted/grilled meats, milk chocolate, coffee, caramel.
  - Example visual: Comparing uncooked vs. browned bread illustrates appealing color/flavor from Maillard reaction.
- Formation of Reducing Compounds (Reductones): Contribute to food stability and inhibit oxidation.
  - Example: Darker beers made with highly roasted malts have more reducing potential due to higher reductone content, leading to longer shelf-life stability.
Undesirable Aspects
- Color and Aroma Alteration: Can occur during storage or processing.
  - Examples: Dehydrated potatoes, egg powder, corn starch, dried foods (milk, fruits).
  - Specific example: 2-acetyl-1-pyrroline in ultra-high-temperature/pasteurized milk causes an off-putting aftertaste.
- Reduction in Nutritional Value: Amino acids (especially lysine) can be rendered unavailable, and cross-linkage of proteins can reduce digestibility.
- Toxicity: Formation of toxic compounds.
  - Acrylamide: A key concern.
  - Heterocyclic Aromatic Amines (HAAs)
  - 5-hydroxymethylfurfural (HMF)

Maillard Reaction Stages (Network of Reactions)

I. Initial Stage (Products colorless)
- A) Sugar-amine Condensation: Nucleophilic attack of the amine on the reducing sugar's carbonyl group forms an imine (Schiff base), then an N-substituted glycosylamine (addition compound). $-H_2O$
- B) Amadori / Heyns Rearrangement: An acid-catalyzed reaction.
  - Amadori Rearrangement: Aldose-amine (N-substituted glycosylamine) rearranges to a 1-amino-1-deoxy-2-ketose (Amadori compound) via an eneaminol intermediate (Aldose $\rightarrow$ Ketose amine).
  - Heyns Rearrangement: Ketose-amine rearranges to an Aldose-amine (Ketose $\rightarrow$ Aldose amine).
II. Intermediate Stage (Products colorless or yellow)
- C) Sugar Dehydration: Leads to aroma formation.
- D) Sugar Fragmentation: Also contributes to aroma formation.
- E) Amino Acid Degradation (Strecker Degradation): Amino acids react with dicarbonyl compounds to form Strecker aldehydes, which contribute significantly to aroma.
  - Selected Examples of Amino Acids $\rightarrow$ Strecker Aldehydes and Odors:
    - Valine $\rightarrow$ 2-Methylpropanal (Green, overripe fruit)
    - Leucine $\rightarrow$ 3-Methylbutanal (Malty, fruity, toasted bread)
    - Isoleucine $\rightarrow$ 2-Methylbutanal (Fruity, sweet, roasted)
    - Phenylalanine $\rightarrow$ Phenylacetaldehyde (Green, floral, hyacinths)
    - Methionine $\rightarrow$ Methional, methanethiol, 2-propenal (Vegetable-like aromas)
  - Flavor Compounds from Intermediate Stage (Simplified Schematic):
    - Furanones (sweet, caramel, burnt)
    - Pyranones (cooked, roasted)
    - Thiazoles (nutty, green, toasted)
    - Pyrroles (cereal-like, nutty, maple-like, sweet)
    - Thiophenes (meaty, roasted)
    - Furans (meaty, burnt, caramel-like)
    - Alkylpyridines (bitter, burnt, pungent)
    - Pyrazines (cooked, roasted, nutty, sweet)
    - Oxazoles (chocolate, green, sweet)
    - Imidazoles (nutty, bitter)
III. Final Stage (Products highly colored)
- F) Aldol Condensation
- G) Aldehyde-amine Condensation and Formation of Heterocyclic Nitrogen Compounds: Leads to melanoidins (insoluble, high molecular weight), which are responsible for color formation.
- H) Free Radical Breakdown of Intermediates

Acrylamide Formation through Maillard Chemistry

Mechanism: Decarboxylation of asparagine through Strecker degradation.
Toxicity Concerns:
- Toxic to the nervous and reproductive system at certain doses.
- Group 2A probable carcinogen in humans, posing a risk for various types of cancer.
Real-World Incidents:
- Sweden Tunnel (1997): Sealant material with high levels of acrylamide monomer contaminated groundwater, leading to fish death and paralysis in cows (acrylamide adducts with hemoglobin). Workers experienced paralysis and tingling in hands and feet.
- Food Discovery: Acrylamide adducts with hemoglobin were found in rodents fed fried food, leading to the discovery that food is a major source of acrylamide.
Sources of Acrylamide: Water, cosmetics, cigarette smoke, and food (e.g., fried foods).
Factors Affecting Acrylamide Formation:
- pH
- Temperature / Time
- aw / Moisture Content
- Reducing Sugar / Amino Acid Concentration (especially asparagine).

Melanoidins

Formation: Complex polymeric compounds formed in the final stages of the Maillard reaction.
Characteristics: Insoluble, high molecular weight.
Color Intensity: Directly related to the degree of polymerization.

Factors Affecting the Maillard Reaction

Nature and Concentration of Reactants: Especially reducing sugars, proteins, peptides, and amino acids. An increase in these leads to a higher reaction rate.
pH: Alkaline pH increases the open-chain form of hexose sugars and enhances the nucleophilicity of the amino group, thus accelerating the reaction.
aw / Moisture Content: The reaction rate is generally highest at intermediate water activity values (e.g., $a_w \approx 0.6-0.7$ ).
Presence of Additional Substances: Metals, antioxidants, or other compounds can influence the reaction.
Time x Temperature: Higher temperatures and longer reaction times generally lead to more extensive browning.
Storage Conditions

Maple Syrup as a Case Study for Maillard Reaction Factors

Ojibwe Tradition: American Indians have made syrup and sugar from tree sap long before Europeans arrived.
Color Differences: Maple syrup color ranges from very light to dark, which is a key grading criterion.
Factors Responsible for Color Differences:
- pH of the sap: Lower pH (more acidic) generally leads to lighter syrup.
- Concentration of reducing sugars of the sap
- Concentration of amino acids of the sap
- Temperature and time of the evaporation process: Longer/higher heat exposure leads to darker syrup.
- All of the above interplay to determine the final color and flavor profile.
Sap Composition Over Time: Early in the season (e.g., March), sap tends to have lower reducing sugar and amino acid content and may be more acidic. Later in the season (e.g., April), these concentrations might increase, impacting Maillard reactions and caramelization.
Ideal Sap for Light Syrup: To produce light-colored maple syrup, ideal sap would have lower concentrations of reducing sugars and amino acids. The best time to collect such sap is typically at the beginning of the season.
Maple Syrup Grading: Grades are based on USDA standards, with color determined by the transmittance of light at $560 \text{ nm}$ .

Caramelization

Definition: The heating of carbohydrates (particularly sucrose and reducing sugars) in the absence of nitrogenous compounds, promoting a complex group of reactions.
Facilitators: Small amounts of acids and some salts can facilitate the reaction.
Similarity to Maillard: Although it does not involve amino acids and proteins, caramelization is similar to non-enzymatic browning in producing complex polymeric compounds.
Conditions for Occurrence:
- High Temperatures: Typically around $~200^<br>\circ C$
- Low Water Content / High Sugar Concentration
Mechanisms: At high temperatures, sugar reactions are accelerated, involving hydrolysis/isomerization, water elimination, and oxidation.
Products: Formation of enediols and dicarbonyls, leading to caramel flavors and pigments (polymeric compounds formed from unsaturated cyclic compounds, both five- and six-membered rings).
Key Difference from Maillard: Does not require amines or amino acids; all sugars react irrespective of their reducing capacity, under appropriate conditions.

Here are the answers to your questions:

The brown-colored compounds formed during the Maillard reaction are called melanoidins.
The factors that affect the Maillard reaction include:
- Nature and Concentration of Reactants: Especially reducing sugars, proteins, peptides, and amino acids. An increase in these leads to a higher reaction rate.
- pH: Alkaline pH increases the open-chain form of hexose sugars and enhances the nucleophilicity of the amino group, thus accelerating the reaction.
- aw / Moisture Content: The reaction rate is generally highest at intermediate water activity values (e.g., $a_w \approx 0.6-0.7$ ).
- Presence of Additional Substances: Metals, antioxidants, or other compounds can influence the reaction.
- Time x Temperature: Higher temperatures and longer reaction times generally lead to more extensive browning.
- Storage Conditions
Based on the factors affecting acrylamide formation, its production in foods like potato chips can be reduced by controlling:
- pH
- Temperature / Time (e.g., lower temperatures or shorter cooking times)
- aw / Moisture Content
- Reducing Sugar / Amino Acid Concentration (especially asparagine, e.g., by selecting raw materials with lower concentrations or through processing steps to reduce these components).
In the Maillard reaction, the Intermediate Stage (Stages C, D, and E) produces the most flavor compounds through sugar dehydration, sugar fragmentation, and amino acid degradation (Strecker Degradation).
Caramelization is the heating of carbohydrates (particularly sucrose and reducing sugars) in the absence of nitrogenous compounds, promoting a complex group of reactions that produce caramel flavors and pigments. It differs from the Maillard reaction in that it does not require amines or amino acids; it involves strictly sugar chemistry triggered by high temperatures and low water content, whereas the Maillard reaction involves reducing sugars reacting with amines.
Yes, non-reducing sugars can undergo caramelization. The note states that in caramelization, "all sugars react irrespective of their reducing capacity, under appropriate conditions."
An alkaline pH accelerates the Maillard reaction by increasing the open-chain form of hexose sugars and enhancing the nucleophilicity of the amino group.
The provided notes do not contain information differentiating between amylose and amylopectin, nor do they specify which is the predominant form in sticky rice.

Here are the answers to your questions:

The brown-colored compounds formed during the Maillard reaction are called melanoidins.
The factors that affect the Maillard reaction include:
- Nature and Concentration of Reactants: Especially reducing sugars, proteins, peptides, and amino acids. An increase in these leads to a higher reaction rate.
- pH: Alkaline pH increases the open-chain form of hexose sugars and enhances the nucleophilicity of the amino group, thus accelerating the reaction.
- aw / Moisture Content: The reaction rate is generally highest at intermediate water activity values (e.g., $a_w \approx 0.6-0.7$ ).
- Presence of Additional Substances: Metals, antioxidants, or other compounds can influence the reaction.
- Time x Temperature: Higher temperatures and longer reaction times generally lead to more extensive browning.
- Storage Conditions
Based on the factors affecting acrylamide formation, its production in foods like potato chips can be reduced by controlling:
- pH
- Temperature / Time (e.g., lower temperatures or shorter cooking times)
- aw / Moisture Content
- Reducing Sugar / Amino Acid Concentration (especially asparagine, e.g., by selecting raw materials with lower concentrations or through processing steps to reduce these components).
In the Maillard reaction, the Intermediate Stage (Stages C, D, and E) produces the most flavor compounds through sugar dehydration, sugar fragmentation, and amino acid degradation (Strecker Degradation).
Caramelization is the heating of carbohydrates (particularly sucrose and reducing sugars) in the absence of nitrogenous compounds, promoting a complex group of reactions that produce caramel flavors and pigments. It differs from the Maillard reaction in that it does not require amines or amino acids; it involves strictly sugar chemistry triggered by high temperatures and low water content, whereas the Maillard reaction involves reducing sugars reacting with amines.
Yes, non-reducing sugars can undergo caramelization. The note states that in caramelization, "all sugars react irrespective of their reducing capacity, under appropriate conditions."
An alkaline pH accelerates the Maillard reaction by increasing the open-chain form of hexose sugars and enhancing the nucleophilicity of the amino group.
The provided notes do not contain information differentiating between amylose and amylopectin, nor do they specify which is the predominant form in sticky rice.