Download Report

Food Biotechnology
Dr. Kamal E. M. Elkahlout
Food Biochemistry 6
Chemical properties of
food
Pigments
– Natural substances in cells and tissues of plants
and animals that impart colour
• Contributions
– Colors help insect to get attracted to flowers
which helps in the pollination
– Color and appearance are major quality attributes of foods
• Because of our ability to easily perceive these factors as first evaluated by
consumers when purchasing food
• Consumers relates colors of food to quality
– Redness in meat: freshness
– Green pawpaw : immature
– Orange pawpaw : ripe
– Color also influence flavour perception
• Red drinks: strawberry, cherry flavor
• Green drinks: lime flavor
– Colors such as carotenoids, riboflavin, etc are
not only colors but are nutrients
• thus colors have multiple effects on consumers
– Many food pigments are unstable during
processing and storage, thus need careful
preparation
Categories of natural pigments
– 1. tetrapyrrole compounds
• include chlorophyll, blood pigments (myoglobin and
haemoglobin)
– 2. isoprenoid derivatives
• carotenoids
– 3. benzopyran derivates
• anothocyanins and flavonoids
– 4. artefacts
• melanoidins, caramels.
Tetrapyrrole Pigments
• Chlorophyll-major pigment in green plants and algae for
photosynthesis
• Occurs in different forms
• Chlorophyll a
• most abundant form in nature
• has a methyl group at the R position
• total resonance and the methyl (CH3) group result in a
pigment color of blue-green
– Chlorophyll b
• R group is an aldehyde (C=OH) group
• gives a yellowish green color
Structure of chlorophyll
Chlorophyll degradation
• In heated vegetables chlorophyll is degraded by
change in pH
• Acidic pH(3)-chlorophyll is unstable
• Basic pH(9)-chlorophyll is stable
• Mg2+ atom is easily displaced by 2 hydrogen ions
– result in formation of olive-brown pheophytin
a and b
• reaction is irreversable in aqueous solution
• Prolonged heating further degrade pheophytin into
Pyropheophytin which has an olive drab color
– To avoid degradation
• lessen the action of heat through the use of high
temperature short-time thermal treatments
• by raising the pH of the medium (by the use of
baking soda) which this results in the formation of
chlorophyllin that has an unrealistic bright color
and the texture of the vegetable is extremely
mushy
• Mushiness can be prevented by the addition of
calcium acetate or any other calcium salt whish
will prevent breakdown of the hemicellulose in the
alkaline medium
ISOPRENOID DERIVATIVES
• Carotenoids
– Pigments are responsible for
• most of yellow and orange colours
• Have antioxidant activity-prevent lipid perioxidation
Stability during processing
• Quite stable during storage and handling
• There is a little change in content during freezing
• Blanching influence levels due to inactivation of
lipoxygenase, which catalyse oxidative
decomposition of carotenoids and enhance
efficiency of extraction of pigments compared to
raw sample
• Very high tempt yield fragmentation products
that are volatile
• Air dehydration exposes to oxygen causing
extensive degradation
• Exposure to light aggravates degradation
Benzopyran derivates
– Flavonoids and anthocyanins
Flavonoids
• Polyphenolic compounds in fruits and vegetables
• In fresh plants present in the form of glycosides
• In processed foods present in aglycones form
Anthocyanins
Anthocyanidin
R1
R2
colour
Pelargonidin
H
H
Orange-red
Cyanidin
OH
H
red
Delphinidin
OH
OH
pink
Peonidin
OCH3
H
Bluish purple
Petunidin
OCH3
OH
purple
Malvidin
OCH3
OCH3
Reddish purple
Compounds
Food sources
Cyanidin
apple fruit skin, blackberry, elderberry fruits, red cabbage,
Peonidin
peony flowers, cherry, cranberry, sweet potato, plum, strawberry
fruit, pomegranate flowers
Pelargonidin
pelagonium flowers
Pelphinidin
delphinium flowers, hydrangea flowers, bluebell flowers
Petunidin
petunia flowers, cherries, berries, grape skins, cyclamen flowers
strawberry, rhubarb, black currant, corn, plum, raspberry, red onion
Grapes, pine bark
Malvidin
Stability
• Is relatively unstable
• Stable in acidic conditions
• Stability is affected by
– Substituents of the aglycone
• Pelargonidin, cyanidin or delphinidin are less
stable
• Petunidin and malvidin are more stable
– Temperature
– Oxygen conc
Artefacts
– melanoidins, caramels.
– These are products of a physical process resulting
in a chemicals process
– result of enzymatic or non-enzymatic browning
Enzyme Activity
• Enzymes are biological catalysts which is an agent
affecting the velocity of a chemical reaction
without appearing among the final products of the
reaction.
• They operate at a temperature of approximately
37°C and pH close to neutrality.
Use of enzymes in food industry
-malted barley in brewing
- yeast in the production of alcoholic beverages
- In bread and cheese making
Factors Affecting the Rate of Enzyme Catalyzed Reactions
• Effect of Enzyme Concentration –the rate of
reaction depends on the concentration of enzyme
• Effect of Substrate Concentration -reaction speed is
proportional to substrate concentration only at low
concentrations of the substrate and is independent of it
at high concentration.
• Effect of Temperature -Enzyme activity proceeds very
slowly at low temperatures
• As the temperature increases, the rate of the enzymecatalyzed reaction increases
• since enzymes are proteins, thermal denaturation of the
apoenzyme sets in as temperature increases, resulting in
inactivation of the enzyme.For most enzymes optimum
activity is between 30-40°C
• Effect of pH -The activity of an enzyme depends on the
pH of the reaction medium
• Most enzymes are active in the pH range from 4.5 to 8.0.
There are exceptions: pepsin has a pH optimum of 1.8
and arginase around 10.
• At extremes of acidity and alkalinity the denaturation of
the protein takes place with a concomitant loss of enzyme
activity
Flavor and aromatic compounds in food
• Appearance of a food is important, but it is the flavor that
ultimately determines the quality and acceptability of
foods. No matter how safe, nutritious, inexpensive and
colorful a food may be, if the flavor is undesirable it is
rejected .
• is a sensory phenomenon depending upon taste, odour
or aroma, appearance, temperature—sensation of heat
and cold, and texture or “mouthfeel” affecting the sense
of touch
The Sensation of Taste
The sense of taste refers to the ability of the taste organs to
perceive and recognize
The four basic tastes → sweet, sour, salty and bitter
Substances with Taste
• Saltiness is produced by inorganic salts.
– Sodium chloride is the only compound that gives a
pure salty taste.
– Other salts have additional tastes, e.g. some
iodides and bromides are bitter and some
inorganic salts of lead and beryllium are sweet.
• Sweetness is stimulated by a number of
organic compounds. Aliphatic hydroxy
compounds, such as carbohydrates, are sweet
substances. Diverse compounds, such as
saccharin, peptides and cyclamates, are also
sweet.
• Sourness is caused by proton donors. This does
not, however, entirely account for sourness; at
the same pH, acetic acid tastes more sour than
hydrochloric acid. Aliphatic long chain fatty
acids are more sour than short chain acids.
• One group of bitter substances are alkaloids like
quinine, strychnine and nicotine.
• Caffeine, a constituent of coffee and tea, is
bitter. Phenolic compounds like tannin and
some flavonoid compounds contribute to
bitterness and astringency.
The Smell Sensation
• Odour or smell plays a prominent role in
most flavours. One can experience
thousands of odour sensations.
• Like taste, smell is recognized by receptor
cells called olfactory cells
pH and acidity
• organic and inorganic acids occur in foods and serve a variety
of roles
• contribution of sour taste
• acids have the power of intensifying and modifying the taste
perception of other flavor agents
• Acids and acid salts are used as chemical leavening agents to
impart a characteristic porous, cellular structure to baked
foods.
• Acids act as microbial inhibitors in food preservation (e.g.,
benzoic acid).
• They also help achieve the sterilization of fruits and
vegetables under less severe thermal conditions.
• Acids are also important in the setting of pectin gels, in
defoaming and bringing about emulsification.
• Acids induce coagulation of milk proteins in the production
of cheese and other dairy products.
Alkaline agents
• are used to neutralize excess acid in the production of butter
from fermented milk. The reduction of acidity improves churning
efficiency and retards the development of oxidative off-flavours.
• substances like sodium carbonate and bicarbonate are used,
along with acids, for the production of carbon dioxide during
baking.
• are also used for the enhancement of colour and flavour of
foods. For example, treatment of ripe olives with dilute sodium
hydroxide solution aids the removal of the bitter principal and
development of dark colour.
• Use of sodium bicarbonate during cocoa processing produces
dark chocolate.
• Strong bases are employed for peeling a number of fruits and
vegetables.
Tutorial
1. What are the most frequently employed techniques of enzyme inhibition in food?
2.
Explain using graph the effect of Enzyme Concentration?
3.
Discuss the 4 major flavor sensations perceived by tongue
4. What are some of the functions of Acids and Bases in Foods?
5. How does pH change affect chlorophyll?
6. As a food technologist what will you do to preserve the color of garden peas and maintain its texture as well?
7. What is metmyoglobin? When is it formed?
8. What conditions degrade carotenoids?
9. How does temperature affect the property of enzymes?
10. Pigments in fruits and vegetables contribute to appreciation of food. Food preservation and processing try to
maintain as close as possible the original color of the raw materials. Name the pigments that you have
studied and describe how they are degraded during food processing