Food Chemistry and Functional Foods: Carbohydrates 2

Carbohydrates 2: Reactions and Browning

Reactions of Mono- and Disaccharides

Oxidation

The carbonyl group is key to oxidation and requires an open chain sugar structure.
Controlled Oxidation to Aldonic Acids:
- Process: Achieved using bromine water in buffered neutral or alkaline media.
- Specificity: Oxidation exclusively targets the lactol group.
- Naming: The acid name is derived by adding the suffix "-onic acid" to the stem name of the parent sugar (e.g., D-glucose
  ightarrow D-gluconic acid).
- Lactones: These are cyclic esters formed from aldonic acids.
  - gamma ($\gamma$)-lactones are generally more stable than delta ($\delta$)-lactones.
  - In the food industry, $\delta$-lactones (like D-glucono-$\delta$-lactone, GDL) are frequently used because they undergo slow hydrolysis in foods.
Uses of GDL (Glucono-$\delta$-lactone):
- Generally Recognized As Safe (GRAS) status.
- Leavening Agent: Functions in combination with sodium bicarbonate in refrigerated and frozen dough products.
- Acidulant: Used as a partial replacement for vinegar in salad dressings and as a protein coagulant in the production of cheese and tofu.
- Raw Fermented Sausages:
  - Reduces the amount of required nitrite and accelerates curing by fostering a faster conversion from nitrite to nitric oxide (NO).
  - Permitted in cured, comminuted meat products and in Genoa salami.
- Vegetable Products: Improves color stability and firmness in canned and frozen vegetable products.
- Metal Chelator: Helps inhibit melanosis (dark spots, particularly in seafood).
- Food Examples: Found in ingredients lists for products like pesto, pizza crust, silken tofu, raw fermented sausage, and chicken salad.
Uronic Acids in Nature and Food:
- Plant Cell Walls: Primary constituents of plant cell wall polysaccharides, mainly pectins (e.g., galacturonic and glucuronic acids).
- Gums: Components of gum polysaccharides that serve as gel-forming and thickening agents.
- Ascorbic Acid Formation: The biosynthesis of ascorbic acid in plants commences with the oxidation of glucose to glucuronic acid.
- Detoxification: Xenobiotic compounds are partially conjugated with glucuronic acid in the liver for detoxification.

Reduction

Process: Sugars are reduced to sugar alcohols through methods like reduction by NaBH$_4$ (laboratory scale) or catalytic hydrogenation (industrial scale).
Naming: Alcohol names are derived by replacing the "-ose" suffix of the sugar with "-itol" (e.g., D-glucose
ightarrow D-glucitol/sorbitol, D-mannose
ightarrow D-mannitol, D-fructose can also yield sorbitol and mannitol upon reduction).
Natural Occurrence:
- Sorbitol is found naturally in many fruits, such as pears, apples, and plums (e.g., apple juice contains 300 \text{ - } 800 \text{ mg/100 mL}).
- Berries, citrus fruits, pineapples, and bananas typically do not contain sorbitol, which is important for analytical evaluation of fruit products.
Properties of Sugar Alcohols:
- Sweetness Comparison (10% aqueous solution, relative to sucrose = 100):
  - D-fructose: 114
  - D-glucose: 63
  - D-mannose: 59
  - D-xylose: 67
  - D-sorbitol: 60
  - D-mannitol: 50
  - D-xylitol: 95
- Baked Goods: Their effect on gelatinization and gluten network formation is dependent on their solubility, which is a critical factor when replacing or reducing sucrose in recipes.
- Gastrointestinal Effects: Ingestion of large quantities can lead to flatulence and laxation.
- 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}
- FDA Labeling Requirement: For sorbitol and mannitol, the FDA mandates the statement: "Excess consumption may have a laxative effect" on food product labels if daily consumption of sorbitol/mannitol might exceed 50 \text{ g/20 g} respectively (21 CFR 184.1835).

Browning Reactions

Browning reactions are broadly categorized:
- Enzymatic Browning: Involves phenolics; does not involve carbohydrates.
- Maillard Browning: A nonenzymatic reaction involving reducing sugars and amines at moderate to high temperatures, producing brown pigments and flavors.
- Caramelization: A nonenzymatic reaction involving only sugars at high temperatures, yielding brown pigments and flavors.

Maillard Reaction / Maillard Browning

Significance: Widely recognized as the most frequently practiced chemical reaction globally.
Historical Context: Named after Louis Camille Maillard who first described it in 1912. However, brewers and maltsters were aware of similar reactions as early as the 1900s, prior to Maillard's formal demonstration (e.g., Ling, 1908).
Reactants: A reducing sugar and an amine.
Products: A complex array of flavor compounds (e.g., pyrazines, furanones, furans, aldehydes, pyranones, thiophenes) and brown color compounds (melanoidins).
Desirable Aspects:
- Flavor and Color: Essential for the characteristic tastes and colors of many foods, including malt, bread, soy sauce, roasted/grilled meats, milk chocolate, coffee, and caramel.
- Formation of Reducing Compounds: Contributes to food stability by inhibiting oxidation, leading to a longer shelf-life. Darker beers, for example, have higher reductone content due to more highly roasted malts, enhancing their stability.
Undesirable Aspects:
- Color and Aroma Alteration: Can cause undesirable changes during food storage and processing, affecting products like dehydrated potatoes, egg powder, corn starch, dried milk, and fruits. An example is 2-acetyl-1-pyrroline in ultra-high-temperature/pasteurized milk, which imparts an off-putting aftertaste.
- Reduction in Nutritional Value: Leads to the loss of essential amino acids (e.g., lysine) and decreased protein digestibility due to cross-linkage of proteins.
- Toxicity: Can form toxic compounds such as acrylamide, heterocyclic aromatic amines, and 5-hydroxymethylfurfural.
Stages of the Maillard Reaction (according to Nursten, 2005): The Maillard reaction is a network of various intertwined reactions, not a single pathway.
- I. Initial Stage (products are colorless):
  - A) Sugar–Amine Condensation: Involves a nucleophilic attack from the amine on the reducing sugar's carbonyl group, forming an addition compound, which then dehydrates to an imine (Schiff base) and subsequently forms an N-substituted glycosylamine.
  - B) Amadori / Heyns Rearrangement: An acid-catalyzed reaction where the N-substituted glycosylamine rearranges. Aldose sugars yield an Amadori compound (1-Amino-1-deoxy-2-ketose), while ketose sugars yield a Heyns compound (a ketose
    ightarrow aldose amine).
- II. Intermediate Stage (products can be colorless or yellow, aroma formation occurs):
  - C) Sugar Dehydration: Sugars lose water.
  - D) Sugar Fragmentation: Sugars break down into smaller molecules.
  - E) Amino Acid Degradation (Strecker Degradation): Amino acids react to form specific aroma compounds (Strecker aldehydes). Examples include:
    - Valine
      ightarrow 2-Methylpropanal (green, overripe fruit odor).
    - Leucine
      ightarrow 3-Methylbutanal (malty, fruity, toasted bread odor).
    - Isoleucine
      ightarrow 2-Methylbutanal (fruity, sweet, roasted odor).
    - Phenylalanine
      ightarrow Phenylacetaldehyde (green, floral, hyacinths odor).
    - Methionine
      ightarrow Methional, methanethiol, 2-propenal (vegetable-like aromas).
    - This stage also leads to the formation of furanones (sweet, caramel, burnt), pyranones (cooked, roasted), furans (meaty, burnt, caramel-like), pyrroles (cereal-like, nutty, maple-like, vanillin-like, warm/spicy, fruity/jam-like), thiophenes (meaty, roasted), pyrazines (cooked, roasted, nutty, sweet, bitter), oxazoles (chocolate, green, toasted), imidazoles (nutty, bitter), thiazoles, and acylpyridines (cracker-like, cereal).
- III. Final Stage (products are highly colored):
  - F) Aldol Condensation: Reaction of aldehydes or ketones.
  - G) Aldehyde–Amine Condensation and Formation of Heterocyclic Nitrogen Compounds: Leads to the formation of melanoidins, which are insoluble, high molecular weight, highly colored compounds.
  - H) Free Radical Breakdown of Intermediates.
Acrylamide Formation (a focus on toxicity):
- Mechanism: Formed through the decarboxylation of asparagine via Strecker degradation.
- Toxicity: Considered toxic to the nervous and reproductive systems at certain doses. It is classified as a Group 2A probable carcinogen in humans, posing a risk for various types of cancer.
- Environmental and Health Impact (Swedish Tunnel Case, 1997): A tunnel construction in Sweden using sealants with high levels of acrylamide monomer led to groundwater contamination, causing fish deaths and paralysis in cows. Workers experienced paralysis and tingling in hands and feet. Studies revealed that even a control population exhibited acrylamide adducts with hemoglobin, indicating widespread exposure.
- Sources of Acrylamide: Water, cosmetics, cigarette smoke, and food.
- Factors Affecting Acrylamide Formation:
  - pH
  - Temperature/Time
  - Water activity (a_W)/Moisture content
  - Concentration of reducing sugars/amino acids
Melanoidins:
- The degree of polymerization of these compounds directly correlates with the intensity of the color produced.
- They are complex polymers formed from sugar fragmentation and dehydro derivatives.
Factors Affecting the Maillard Reaction:
- pH: Alkaline pH conditions (e.g., pH 7.2 vs. 5.1 for maple sap) lead to an increase in the open-chain form of hexose sugars and enhance the nucleophilicity of the amino group, thereby accelerating the Maillard reaction and increasing color formation. Acidic pH inhibits the reaction.
- Nature and Concentration of Reactants: The types and concentrations of reducing sugars, proteins, peptides, and amino acids available significantly influence the reaction rate and products.
- Water Activity (a_W): The reaction rate is optimal at intermediate water activity levels.
- Presence of Additional Substances: Certain substances can promote or inhibit the reaction.
- Time \times Temperature: Higher temperatures and longer reaction times generally accelerate the Maillard reaction and lead to more intense browning.
- Storage Conditions: Factors like light exposure and atmosphere can also play a role.
- Maple Syrup Case Study: The color differences in maple syrups are influenced by all these factors. A darker syrup (e.g., Syrup A) typically results from higher sap pH, higher concentrations of reducing sugars and amino acids, and/or higher temperature and longer time during the evaporation process. To produce a lighter-colored maple syrup, the ideal sap chemical composition would be characterized by a lower pH, and lower reducing sugar and amino acid content. The best time to collect such sap is usually at the beginning of the sap flow season (e.g., early March to April).

Caramelization

Description: This is a complex group of reactions that occur when carbohydrates, particularly sucrose and reducing sugars, are heated in the absence of nitrogenous compounds.
Facilitators: Small amounts of acids and certain salts can accelerate the reaction.
Similarities to Maillard Reaction: Like non-enzymatic browning, caramelization produces a complex mixture of polymeric compounds derived from unsaturated cyclic compounds (five- and six-membered rings).
Key Difference from Maillard Reaction: Caramelization does not involve amino acids or proteins. All sugars, not just reducing sugars, can undergo caramelization.
Conditions: Occurs at high temperatures (around 200^{\circ}\text{C}) and under conditions of low water content or high sugar concentration.
Reactions Involved: At high temperatures, sugar reactions are accelerated, including:
- Hydrolysis/Isomerization
- Water elimination
- Oxidation
Formation of Intermediates: Enediols and dicarbonyls are formed.
Products: Caramel flavors and pigments are the final products.