Food BiotechnologyDr. 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, strawberry, rhubarb, black currant, corn, plum, raspberry, red

onion Peonidin peony flowers, cherry, cranberry, sweet

potato, plum, strawberry fruit, pomegranate flowers

Pelargonidin pelagonium flowers

Pelphinidin delphinium flowers, hydrangea flowers, bluebell flowers

Petunidin

Malvidin

petunia flowers, cherries, berries, grape skins, cyclamen flowers

Grapes, pine bark

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 enzyme- catalyzed 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.

Tutorial1. 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