Lecture: major reactions in food
All macronutrients can react in major reactions, proteins, lipids and carbs. In addition, the compound
water is also very important. Food reactions are important for the quality of the food:
- Taste
- Appearance
- Toxicity
- Nutritional value
- Structure
Imagine you are a chef
- How do you store the food?
- Prepare the food?
- How do you preserve the food?
1. Enzymatic browning
Enzymatic browning = is a chemical process that occurs in fruit and vegetables and catalysed by
polyphenoloxidase. It can lead to a brown colour. Why do fruit and vegetables turn brown?
Melanins produced during the browning process form a barrier and have antimicrobial properties
which prevents the spread of infections and bruisng in plant tissues. This production of melanins is
catalysed by polyphenoloxidases, they oxidise polyphenols. Polyphenols are antioxidants of the plant
and have multiple phenolic rings in the structure. Polyphenoloxidases are a class of enzymes
originally discovered in mushrooms, they reside in chloroplasts of plants or plastids and they can also
exist freely in the cytoplasm of plants. Polyphenoloxidase plays an important in the resistant of the
plant against microbial and virus infections and adverse climate conditions.
There are different classes of polyphenols:
- Anthocyans (colours in fruit)
- Flavonoids (catecins, tannins in tea and wine)
- Non-flavonoids components (gallic acid in tea leaves)
Why are polyphenols important?
- Antioxidants of the plants
- Taste: adding tannin of red wine to a steak this can soften the protein in the steak and makes
it more pleasant to eat.
- Colour
What happens during polyphenol oxidation?
- In the presence of oxygen in air the enzyme phenoloxidase catalyses the first steps of
biochemical conversion of pholyphenols (tyrosine) to produce quinones.
- Finally this leads to melanins that give the brown colour.
,How to prevent browning?
- Reversible: browning can still happen if you stop it
o Cooling
o Freezing: decreasing availability of water due to crystallization. After melting the
browning can occur again.
o Adding chemicals (anti-browning agents)
o High pressure processing (3000-8000 bars): polypehnoloxidase can also be
deactivated by exposing to high pressure. Integrity of porous products can be
affected and the air limited in food matrix is exposed to expansion and compression
which can disturb the food tissue and create an operation inappropriate for food and
vegetables. So, it cannot be done for fruit and vegetables.
- Irreversible: destroying polyphenoloxidase
o Heating (70-95 c): disadvantage is destruction of heat sensitive vitamins and loss of
delicate texture and aroma.
Blanching (70-100 c): increase temperature for a short time and this inactive
enzyme but prevents the loss of vitamins and texture effects.
Heat shock (45 – 70 c – 5 min): high temperature short time.
How do enzymes work?
Enzymes are proteins that catalyse a reaction. A substrate binds to the active site of the enzyme.
Apart from the substrate sometimes also cofactors are needed, they can be non-protein such as iron
or copper, they are used by the enzymes.
- Apoenzyme = enzyme without cofactor
- Holoenzyme = whole enzyme with cofactor, it enables the enzyme for binding its substrate in
its active site
The substrates are converted and the products are released. The active site can be used again.
, Inhibiting an enzyme can by adding an inhibitor
- Competitive inhibitor: binds to active site of enzyme. Competitor inhibitors of
polyphenoloxidase are union solution, lemon juice or wine.
- Non-competitive inhibitor: binds to other not active sites of enzyme which leads to
conformational changes in the enzyme and on the active site the substrate cannot be bind
anymore.
- Chelators: can chelate or scavenge away the cofactors and prevents the enzyme of working.
- Antioxidants: can react with oxygen which is necessary for polyphenoloxidase to work or
they can inhibit the formation the quinones.
Temperature leads to inhibition because due to heating the main bonds for the 3d structure (mainly
non-covalent) will be broken. Due to the loss of the structure the substrate cannot be recognized
anymore by the active site, it is called denaturation. This can also be done with pH.
All macronutrients can react in major reactions, proteins, lipids and carbs. In addition, the compound
water is also very important. Food reactions are important for the quality of the food:
- Taste
- Appearance
- Toxicity
- Nutritional value
- Structure
Imagine you are a chef
- How do you store the food?
- Prepare the food?
- How do you preserve the food?
1. Enzymatic browning
Enzymatic browning = is a chemical process that occurs in fruit and vegetables and catalysed by
polyphenoloxidase. It can lead to a brown colour. Why do fruit and vegetables turn brown?
Melanins produced during the browning process form a barrier and have antimicrobial properties
which prevents the spread of infections and bruisng in plant tissues. This production of melanins is
catalysed by polyphenoloxidases, they oxidise polyphenols. Polyphenols are antioxidants of the plant
and have multiple phenolic rings in the structure. Polyphenoloxidases are a class of enzymes
originally discovered in mushrooms, they reside in chloroplasts of plants or plastids and they can also
exist freely in the cytoplasm of plants. Polyphenoloxidase plays an important in the resistant of the
plant against microbial and virus infections and adverse climate conditions.
There are different classes of polyphenols:
- Anthocyans (colours in fruit)
- Flavonoids (catecins, tannins in tea and wine)
- Non-flavonoids components (gallic acid in tea leaves)
Why are polyphenols important?
- Antioxidants of the plants
- Taste: adding tannin of red wine to a steak this can soften the protein in the steak and makes
it more pleasant to eat.
- Colour
What happens during polyphenol oxidation?
- In the presence of oxygen in air the enzyme phenoloxidase catalyses the first steps of
biochemical conversion of pholyphenols (tyrosine) to produce quinones.
- Finally this leads to melanins that give the brown colour.
,How to prevent browning?
- Reversible: browning can still happen if you stop it
o Cooling
o Freezing: decreasing availability of water due to crystallization. After melting the
browning can occur again.
o Adding chemicals (anti-browning agents)
o High pressure processing (3000-8000 bars): polypehnoloxidase can also be
deactivated by exposing to high pressure. Integrity of porous products can be
affected and the air limited in food matrix is exposed to expansion and compression
which can disturb the food tissue and create an operation inappropriate for food and
vegetables. So, it cannot be done for fruit and vegetables.
- Irreversible: destroying polyphenoloxidase
o Heating (70-95 c): disadvantage is destruction of heat sensitive vitamins and loss of
delicate texture and aroma.
Blanching (70-100 c): increase temperature for a short time and this inactive
enzyme but prevents the loss of vitamins and texture effects.
Heat shock (45 – 70 c – 5 min): high temperature short time.
How do enzymes work?
Enzymes are proteins that catalyse a reaction. A substrate binds to the active site of the enzyme.
Apart from the substrate sometimes also cofactors are needed, they can be non-protein such as iron
or copper, they are used by the enzymes.
- Apoenzyme = enzyme without cofactor
- Holoenzyme = whole enzyme with cofactor, it enables the enzyme for binding its substrate in
its active site
The substrates are converted and the products are released. The active site can be used again.
, Inhibiting an enzyme can by adding an inhibitor
- Competitive inhibitor: binds to active site of enzyme. Competitor inhibitors of
polyphenoloxidase are union solution, lemon juice or wine.
- Non-competitive inhibitor: binds to other not active sites of enzyme which leads to
conformational changes in the enzyme and on the active site the substrate cannot be bind
anymore.
- Chelators: can chelate or scavenge away the cofactors and prevents the enzyme of working.
- Antioxidants: can react with oxygen which is necessary for polyphenoloxidase to work or
they can inhibit the formation the quinones.
Temperature leads to inhibition because due to heating the main bonds for the 3d structure (mainly
non-covalent) will be broken. Due to the loss of the structure the substrate cannot be recognized
anymore by the active site, it is called denaturation. This can also be done with pH.
