phenolics

do phenolics have a nutrional value

no

where are phenolics mainly present

mainly present in raw materials from plant origin

what do phenolics do in plants

Phenolics serve diverse biological functions in plants, such as protecting the plant against insects and UV-light, and attracting pollinators 

give the definition for phenolic compound

A molecule that contains at least one aromatic ring with one or more hydroxyl-groups

give examples of phenolic rich foods

Coffee 

Tea 

Cocoa 

Berries 

Grapes 

Wine and juices 

Herbs and spices (u would need to eat a lot tho)-(dried oregano highest) 

why is the term phenolic compounds or phenolics preffered over polyphenols

1. The prefix "poly" suggests multiple hydroxyl groups. However, many common phenolics found in nature, such as p-coumaric acid and p-hydroxybenzoic acid, are monophenols, possessing only one hydroxyl-group on an aromatic ring. 

    2. The term is inconsistent: some scientists restrict "polyphenols" only to the polymeric forms, while others use it broadly for monomeric, oligomeric, and polymeric forms. 

• Using "phenolic compounds" or "phenolics" helps to avoid this ambiguity. 

what are the 3 structural variations in phenolics

polarity

reactivity

size of conjugated system

what kind of polarity do phenolics usually have

Phenolics are typically medium polar 
→ limited water solubility 

what does a large conjugated system mean

Large conjugated system: aborb visible light and leads to observed colour 

how many double bonds needed to see colour

Rule of thumb: 
Conjugated system < 8 bonds = no colour 

>8 =colour

what are the 6 structural features typically present in phenolics

hydroxylation

methylation

glycosylation

carboxylation

extension of conjugated system

formation fused ring system

describe hydroxylation

adding OH group

increase in polarity

increases reactivity (higher susceptibility to oxidation/higher antioxidant capacity)

higher water solubility

higher oxidation

higher antioxidant act

what is o-diphenol susceptible to

The o-diphenol structure (1,2-dihydroxybenzene) is especially susceptible to oxidation. 

O- ortho position , 2 oh groups next to eachother on aromatic ring 

describe methylation

adding CH3 to the OH

reduces the polarity

reduces reactivity

reduces water solubility

redcues oxidation

reduces antioxidant act

describe glycosylation

adding a saccharide

increases polarity…. increases water solubilty

reduces reactivity….reduces oxidation, reuces antiocidant act

describe carboxylation

Adding COOH

increases polarity.., increases water solubility

lowers pka….lowers food ph

describe extension of conjugugated system

(e.g., via alkenes or fused rings)

Increases light absorbance. I

increases reactivity due to more possibilities for resonance structures. 

increased oxidation

increased antioxidant act

Leads to color if the conjugated system becomes long enough (typically ≥ eight conjugated double bonds). Important for antioxidant activity. 

what do phenolic compounds occur as in nature

glycosides

Phenolics as glycosides in plants 
→ better water-soluble and less reactive 

 

This combination of increased solubility and reduced reactivity makes glycosides easier for the plant to store 

 

what are the structures of the six main classes of monomeric phenolicsc

1. Simple phenolics

2. Hydroxybenzoic acids

3. Hydroxycinnamic acids

4. Hydroxycinnamyl alcohols

5. Stilbenoids

6.flavanoids

hydroxycinnamic acids and hydroxycinnamyl alcohols serve as

nature’s building blocks. 

Esterification-hydroxycinnamoyl esters 

Amidation- phenolamides 

Oxidative coupling-  form di-, oligo-, and polymeric phenolics, such as lignins and lignans 

Reduction- forming hydroxycinnamyl alcohols) 

monomeric phenolics can be converted to di-, oligo-, and polymeric phenolic compounds via

oxidative coupling

This process is often initiated by the enzyme polyphenoloxidase (PPO), which converts phenolics to highly reactive o-quinones.

what does the term flavonoids refer to

isoflavnoids (2nd )

flavanoids (3 rd)

may include chalcones

give 4 examples of main classes of di-, oligo-, and polymeric phenolic compounds

● Lignans 
● Lignins 
● Condensed tannins 
● Hydrolysable tannins 

how are lignans made

Hydroxycinnamic acids and/or hydroxycinnamyl alcohols. 

how is lignin made

Primarily hydroxycinnamyl alcohols (monolignols)

how are condensed tannins made

Flavanols

how are hydrolysabe tannins made

Gallic acid or derivatives (Glycosidic core)  + a central monosaccharide 

Explain how condensed tannins and hydrolysable tannins can influence food properties

Tannis: Oligo- and polymeric phenolic compounds that strongly interact with proteins 

 

The interaction of tannins with proteins is not restricted to animal skin proteins: All types of proteins, including food proteins, can interact with tannins. This interaction can influence food properties in various ways, including: 

 
1. The formation of insoluble particles, which leads to turbidity of products (see §4.8); 
2. The inhibition of enzymatic reactions by the inactivation of enzymes; 
3. As a result of (2), diminished digestibility of vegetable raw materials, due to inhibition of digestive 
enzymes; 
4. Astringency, a dry, puckering mouthfeel 

where does phenolic compounds antioxidant activity originate from

Their antioxidant activity originates from their ability to act as reducing agents (e.g. by scavenging radicals) and as metal chelators (i.e. they can bind metals, preventing those metals form acting as pro-oxidants).

consumption of antioxidants has beneficial effects for?

consumption of antioxidants has beneficial effects for human health, for instance, by lowering the risk for cardiovascular disease and cancer 

what are the 2 mechanims phenolics exibit antioxidant activity

Reducing oxidised compounds
→ e.g. by radical scavenging
Chelating metals
= binding metal ions

Phenolic compounds are able to act as antioxidants due to their main structural feature

hydroxyl-group(s)
attached to an aromatic ring

describe radical scavenging

are resonance stabilised radicals more or less reactive

Resonance stabilised radicals are much less reactive than non-stabilised radicals

what are desirable structural features for radical scavenging

is chelation direct or indirect antioxidant acitivty

inidrect

are bound metal ions less or more likely to participate in redox reactions

Bound metal ions are less likely to participate in redox reactions, thus metal
chelation effectively leads to a more stable food product

what is a strong metal binding site for flavanoids

o-diphenol moiety on the B-ring of flavonoids is one of the most important structural features, as it
is a strong metal binding site

what are desirable structural features for metal chelation

what enzyme catalyses oxidation of phenolic compounds

PPO

Polyphenoloxidases

▪ Phenolics as substrates
▪ Oxygen as electron acceptor
▪ Two copper ions in active site

why does oxidation only take place in damged cells

In intact plant tissue, enzymatic oxidation by PPO does not take place because phenolic compounds and PPO are not in contact with each other, they are present in different compartments in the plant cells.

If the plant tissue is 
damaged during harvesting or processing (e.g. cutting), enzymatic oxidation can take place because PPO, phenolics, and oxygen (from air) all come into contact with each other.

what does PPO do

o-diphenol moieties importance in oxidation. 

Why important for oxidation: o-diphenols are excellent PPO substrates and are easily oxidized to o-quinones 

in which step is PPO involved in

It is important to remember that only the first step (i.e. the initial enzymatic reaction) is in fact an enzymatic 
reaction that is catalysed by PPO. The formed o-quinones are highly reactive and will quickly react in spontaneous 
follow-up reactions, without further involvement of PPO 

what kind of follow up reactions are there for o-diphenol

1.Quinones react with phenolics: Formation of phenolic dimers and phenolic oligomers= orange brown pigments ,Interaction with proteins. formation of phenolic polymers= insoluble brown pigments 

2. quinones react with other food molecules: changes in flavour ,colour and appearance

give an example of the effect of o-quinone in food

Enzymatic browning

what is a quinone

A quinone is a class of organic compounds derived from aromatic rings (like benzene) with two carbonyl (C=O) groups in a conjugated cyclic structure 

role of cresolase and catecholase activity in oxidation by PPOs

cresolase activity

Some PPOs can convert monophenols to o-diphenols
via hydroxylation. In this reaction, the PPO utilises ½ mole of O2 to perform addition of a hydroxyl-group to the
aromatic ring at the position ortho to the first hydroxyl-group

do all PPOs have cresolase activity

However, not all PPOs have cresolase activity, some only possess 
catecholase activity, which is discussed in the next section. PPOs that don’t have cresolase activity cannot 
oxidise monophenols to o-quinones 

do all PPO have catecheoase activity

All PPOs possess catecholase activity, which is the main reaction catalysed by PPOs 

describe speed of catecholase and cresolase

Cresolase typicsally slow

while catecholase fast

monophenols brown much 
more slowly than o-diphenols, because?

monophenols brown much 
more slowly than o-diphenols, because hydroxylation is the rate limiting step 

 

which factors influence the formation of o-quinones by PPO. 

what are the 2 types of phenolic protein reactions

covalent and non covalent

describe covalent and non covalent protein phenolic formation

Covalent----> irreversable: formation of protein phenolic conjugates 

 

Non covalent---> reversible: formation of protein phenolic complexes 

Explain how covalent protein-phenolic conjugates are formed

reaction of electron-deficient o-quinones with nucleophilic 
groups.  

 

Typical nucleophilic groups that may be present in food molecules like proteins and peptides are amine 
(–NH2) or thiol (–SH) groups. Amine and thiol groups are found in proteins and peptides as parts of the side 
chains of amino acid residues (e.g. cysteine has a thiol group, lysine has an amine group). 

are thiols are amines stringer nucleophiles

Thiols are much stronger nucleophiles than amines. Thus thiols are much more reactive towards o-quinones, so they react much faster 

Upon further oxidation of the protein-bound o-diphenol

protein-bound o-quinones are formed. This can lead to reactions that form various types of cross-links between proteins 

 

protein-bound o-quinone reacts with a non-oxidised phenolic, this can potentially lead to

browning of the protein via 
similar reactions to those that occur in enzymatic browning of phenolics 

Explain the interactions that lead to formation of non-covalent protein-phenolic complexes.

for protein phenolic complexes what factors does it depend on

1.protein structure and characteristics: type of side chains,he accessibility of reactive amino acid side chains 

 

2.Phenolic compound structure and characteristics: including the hydrophobicity, solubility, andmolecular size.  

3.The pH of the food product or ingredient determines the charge of the proteins and the phenolics. 

 

do monomeric phenolic compounds form string protein phenol complexes

Monomeric phenolic compounds generally do not form strong protein-phenolic 
complexes or stable cross-links between proteins.  

do polymeric phenolic compounds that consist 
of more than 10 to 15 monomeric subunits form strong phenolic protein complexes

polymeric phenolic compounds that consist 
of more than 10 to 15 monomeric subunits can also not form stable complexes with proteins anymore, due to the fact that these molecules are poorly water-soluble, too large, and too rigid to approach proteins in solution

protein-phenolic complexation is maximal around

In general, protein-phenolic complexation is maximal around the iso-electric 
point of the proteins. 

for protein phenolic complexes what promotes ionic bonds vs hydrophobic

at neutral or alkaline pH carboxylic acid groups are dissociated and thus negatively 
charged, enabling formation ionic bonds.  

 

Phenolic compounds that are not charged are more hydrophobic, which promotes hydrophobic interactions.  

phenolic/protein ratio influences aggregation and precipitation. 

 

the methods that can be used to prevent or reduce protein-phenolic interactions

how can u remove phenolic compounds

Principle 

• Mainly di- and oligomeric phenolics form strong complexes with proteins. 

• Proline-rich proteins (e.g. gelatin) bind phenolics via hydrophobic interactions, caused by stacking of aromatic rings of phenolics with pyrrolidine rings of proline residues. 

Practice 

• Gelatin is added to bind phenolics, causing aggregation and precipitation, which is then removed by filtration or sedimentation. 

• Alternatively, protein-analogues are used, most commonly polyvinylpolypyrrolidone (PVPP). 

• PVPP contains pyrrolidone side chains that resemble proline, giving it a high capacity to bind phenolics. 

Result 

• Removal of oligomeric phenolics strongly reduces later protein–phenolic complex formation during processing, storage, or consumption. 

describe hydrolysis of proline rich proteins

Principle 

• Enzymatic hydrolysis reduces protein molecular size and disrupts hydrophobic regions. 

Practice 

• Proteases are used, especially those that cleave near proline residues. 

• These proteases are particularly effective for proline-rich proteins, while having limited effects on other proteins. 

Result 

• Smaller proteins with disrupted hydrophobic regions have a lower tendency to interact with multiple phenolics, reducing cross-linking, aggregation, and precipitation. 

 
 

describe Controlling oxidation of phenolics

Principle 

• Covalent protein–phenolic interactions mainly occur via o-quinones. 

• Preventing phenolic oxidation limits quinone formation. 

Practice 

• Control PPO activity by: 

• Adding antioxidants 

• Using conditions that inhibit or inactivate PPO 

Result 

• Reduced o-quinone formation leads to fewer follow-up reactions and thus fewer covalent protein–phenolic interactions. 

Effect of phenolic compounds on appearance

formation of a haze, i.e. turbidity 

 

What are the methods that can be used to control and inhibit enzymatic browning

1.eliminate oxygen

2.lowering pH

3.heat induced denaturation PPO

4.cooling

5. adding chelating agents

6. adding sulfite

7. adding thiol compounds

8. removal phenolic compounds

what is oxidative coupling

oxidative coupling is formation of a brown colour, therefore, the sequence of reactions initiated by PPO is also known as enzymatic browning 

explain the mechanism of oxidative coupling

Route C. Coupled oxidation, the dimer is oxidised by an o-quinone. In this process this o-quinone is 
reduced to an o-diphenol while simultaneously forming the dimer o-quinone. The dimer o- 
quinone can then react with a non-oxidised phenolic

with increasing size of coupling products the water solubility….

With increasing size of the coupling products, their water solubility 
decreases. Trimers and oligomers of 6-8 phenolic monomers as building blocks are brown pigments that are typically still water-soluble to some extent. Very large oligomers (> 6-8 monomeric subunits) and polymers are 
typically not water-soluble at all, they are insoluble brown pigments. 

anthocyanins and anthocyanidins, the presence of colour is a result of

anthocyanins and anthocyanidins, the presence of colour is a result of the conjugated system that connects 
the A-ring and the B-ring, via the C-ring 

8 double bonds

whats difference between anthocyanin or anthocyanidin

If R4 at C3 is a saccharide, then the structure is an anthocyanin (i.e. a glycoside), whereas if R4 is H, then it is an 
anthocyanidin (i.e. an aglycon) 

what colour is anthocyanina nd anthocyadinin responsible for

responsible for the red, 
violet, or blue colour of popular fruits, like blue berries and raspberries, and some vegetables, like red cabbage 

what colour is anthocyanina nd anthocyadinin for acidic , slightly acidic and alkaline

Acidic pH- positive charge = red 

Slightly acid – neutral = violet 

Alkaline- negative charge = deep blue  

 

what colour is anthocyanina nd anthocyadinin if water is added.

Water addition breaks conjugation in the C-ring, causing loss of colour

is flavor and aroma volatile or non volatile

Flavour = taste (non-volatile) & aroma (volatile) 

whats a bitter fruit example

Isoflavanoids known to be bitter 

grapefruit, which contains bitter flavanone glycosides 

what affects bitterness

the linkage position of the saccharide unit in flavanone glycosides affects their 
bitterness 

1→2 = bitter 
1→6 = not bitte 

what is astringency

Astringency: Caused by precipitation of proline-rich proteins 
in saliva due to protein- phenolic interactions 

where is astringency most relavant in

most relevant in food products such as wine and tea that are rich in
oligomeric phenolic compounds

astringency increases with?

Astringency increases with the strength of protein–phenolic binding 

describe the astrigency mechansim

1. Phenolic compounds (especially tannins) interact with salivary proteins 

2. These proteins are often proline-rich salivary proteins 

3. Phenolics form strong non-covalent complexes with proteins 

4. This can lead to: 

• Aggregation 

• Cross-linking 

• Precipitation of salivary proteins 

5. Precipitation reduces saliva’s lubricating function 

6. Result → dry and rough mouthfeel (astringency) 

Influence of degree of polymerisation (DP) on astringency

Effect of DP on protein binding 

• Low DP (monomers, dimers) 

• Weak protein binding 

• Low astringency 

• Medium DP (oligomers, DP ≈ 5–7) 

• Strong protein binding 

• Efficient cross-linking of proteins 

• Maximum astringency 

• High DP (large polymers) 

• Reduced solubility or flexibility 

• Less effective interaction with proteins 

• Decreased astringency 

Effect of phenolic compounds on aroma 

phenolics can affect the aroma of foods by acting as aroma compounds 
themselves or via o-quinones participating in Strecker degradation to form amino acid- 
derived aldehyde aromas. 

Why phenolics are challenging to analyse

Phenolics show high structural diversity

what can be used to detect phenolics

UV-Vis spectroscopy and the Folin-Ciocalteu assay can be used to detect 
phenolics 

describe UV–Vis spectrophotometry for phenolics 

Principle 

• UV–Vis spectrophotometry measures absorption of light in the range ~200–700 nm 

• Phenolics always contain at least one aromatic ring 

• Aromatic rings absorb light in the UV region 

Why it works for phenolics 

• Aromatic rings → UV absorption 

• Larger conjugated systems → absorption shifts toward the visible region 

• Therefore: 

• Simple phenolics → mainly UV absorption 

• Highly conjugated phenolics → may absorb visible light (colour) 

Use 

• Direct detection of phenolics in solution 

• Measurement of absorbance at specific wavelengths 

• Basis for many colorimetric assays 

describe Folin–Ciocalteu assay

• Folin–Ciocalteu reagent contains two metals 

• Phenolic compounds: 

• Are oxidised 

• Reduce the metals in the reagent 

• Reduced metals form a bright blue colour 

• Colour intensity is proportional to phenolic concentration 

📌 This makes it a colorimetric assay, measured by UV–Vis spectrophotometry 

Quantification 

• A calibration curve is prepared with a known phenolic 

• Most commonly gallic acid 

• Results expressed as: 

• Gallic Acid Equivalents (GAE) 

whats advantage and disadvantage of

Folin–Ciocalteu assay

Advantages 

• Simple 

• Fast 

• Low-cost 

• Does not require advanced equipment 

• Widely used in research and industry 

 
 

Main disadvantage (VERY IMPORTANT FOR EXAM) 

• The assay is not specific for phenolics 

• The reagent reacts with any reducing compound, including: 

• Reducing sugars 

• Ascorbic acid 

• Therefore: 

• TPC values may be overestimated 

• Accuracy depends on sample composition 

what is the main phenolic compound in tea

flavan-3-ols (catechins) are the main phenolic compounds in tea leaves and green 
tea. 

Explain the role of fermentation by PPO in the formation of colour in black tea production