FREE Notes Food Chemistry - Knowledge clips Phenolics

Knowledge clips - Phenolic compounds
Introduction
 Functions
o Provide colour
o Can cause browning reactions (brown banana)
o Complex with saliva proteins (astringency)
o Give special taste when heated
o Anti-oxidative properties
 Basic skeleton
o Benzene ring (C6) and hydroxyl group (OH)
o Can be multiple OH groups.
o Can by aliphatic C-chains attached
o Can be more than 1 benzene group
 Other names: polyphenols or phenolics
 Examples
o Caffeic acid: benzene ring with two OH groups and aliphatic chain
o Anthocyanidin: two benzene rings that are linked
o Lignin: a lot of benzene rings
 Abundance
o Minor quantities are present in food: 1-2% of dry basis
o Exceptions: coffee, cocoa, tea
 Classification
o Monomeric phenolic compounds: only one benzene group =
hydroxybenzoic acids (phenolic acid), hydroxycinnamic acid
o Dimeric phenolic compounds: two benzene rings = stilbenes,
flavonoids, lignans
o Oligo/polymeric: more than 2 benzene rings = lignins, condensed
tannins, hydrolysable tannins.
 Can be present free or bound. Cranberry contains a lot of phenolics. A lot of
them are free, so not bound. Corn, wheat, oat and rice have more bound
phenolics.
 Most of phenolics intake is by fruit, non-alcoholic beverages and vegetables
 Mainly flavonoids, hydroxy cinnamic acids, proanthocyanidins (tannins) are
consumed.
 Apple, egg, nut, wheat, meat: phenolic compounds are present in plant food,
not in meat and egg!
 No free OH group = no phenolic compound. Tyrosine amino acid is actually
also a phenolic compound based on its structure.

Mono- and dimeric phenolic compounds
 Monomeric: hydroxycinnamic acids
o Benzene ring and unsaturated aliphatic chain and one or two OH
(caffeic or coumaric acid)
o Can also be esterified with methyl
o Benzene ring contains double bonds! Normal C6 ring not.
o Present as free molecules (apples), as ester in mono-oligosaccharides.
Bound to sugar unit or wheat arabinoxylans.

, o Binds to metal ions. Complex if formed, stable complex. Cook potatoes
after harvesting: no change in colour. After harvesting, iron in potato is
bound to citric acid (anionic form of citrate) = colourless complex.
Upon storage: there is no citrate left because it is all metabolised. Iron
complexes with the chlorogenate to form a black complex  cooking
darkening. Iron-chlorogenate complexes.
o Substrate for enzymatic oxidation. Polyphenol oxidase. Oxidises the OH
groups present in the phenolic compound into a quinone moiety.
Because of the oxidation, the molecule polymerizes into brown
pigments.
 Dimeric: flavonoids
o C6 (benzene ring), C3 (aliphatic chain), C6 structure (benzene ring)
o C3 can form a C-ring (pyran-C-ring) which is oxidized in between the
benzene rings. Now you have a structure of 3 rings: two benzene rings
interlinked with a C-ring.
o A and B ring are benzene rings and C ring is from the aliphatic chains.
o Subclasses
 Flavonols
 Flavones
 Anthocyanidins
 Flavanols
 Flavanones
 All subclasses differ in substitution and oxidation level of C ring.
Double bond or OH or ketone group present…





o Occur free or bound to sugars.
o Occur in many plants (protect against UV, give colour so insects are
attracted)
o Act as an antioxidant, give colour to food, substrate to enzymatic
oxidation.
o Free form: aglycon, not very water soluble. Mainly flavonols (catechins)
o Bound form: glycosylated, you add polar groups (sugar) so increased
solubility. For instance flavonols, flavones, anthocyanidins and flavanols
o Nomenclature.
 No sugar present: anthocyanidin!
 When saccharide is added: anthocyanin! Bigger molecule =
shorter name
 Glycoside: two sugar units attached