Food Biotechnology
Dr. Kamal E. M. Elkahlout
Food Biochemistry 4
Protein Foods
Eggs
• A hen requires 24-26 hours to produce an egg. Thirty
minutes later she starts all over again.
• If an egg is accidentally dropped on the floor, sprinkle it
heavily with salt for easy clean up.
• Egg yolks are one of the few foods that naturally
contain vitamin D.
• Yolk color depends on the diet of the hen. Natural
yellow orange
– Substances such as marigold petals may be added to
light-colored feeds to enhance colors.
Egg Functions
1. Flavor, color, nutrition
2. Emulsifying agent
3. Aids in thickening/structure
4. Binding/coating agent
5. Leavening agent
6. Interfering substance
Composition & Nutritional Value
1. One medium egg contains between 4-5
grams of fat
2. High cholesterol
• ~200 mg/egg
3. High in Complete Protein
4. Little to no CHO
5. High in vitamins & minerals
• Vitamins A,D,E,K, some B vitamins, selenium,
iodine, zinc, iron, copper
Cross Section of an Egg
SHELL
• The first line of defence against the entry of bacteria can be
brown or white; nutritional value of the egg is the same
• Composed mainly of calcium carbonate
• Approximately 8,000 to 10,000 tiny pores allow moisture and
gases in (O2) and out (CO2)
SHELL MEMBRANES
• There are two membranes on the inside of the shell One
membrane sticks to the shell and one surrounds the white
(albumen)
• The second line of defense against bacteria
• Composed of thin layers of protein fibres
GERMINAL DISC
• Appears as a slight depression on the surface of the yolk
• The entry for the fertilization of the egg
WHITE (ALBUMEN)
• Two layers: thin and thick albumen
• Mostly made of water, high quality protein and some minerals
• Represents 2/3 of the egg's weight (without shell)
• when a fresh egg is broken, the thick albumen stands up firmly
around the yolk
CHALAZA
• A pair of spiral bands that anchor the yolk in the centre of the
thick albumen
YOLK MEMBRANE (VITELLINE MEMBRANE)
• surrounds and holds the yolk
• the fresher the egg the stronger the membrane
YOLK
• the egg's major source of vitamins and minerals, including
protein and essential fatty acids
• represents 1/3 of the egg's weight (without shell)
• yolk colour ranges from light yellow to deep orange, depending
on the Xanthophyll content of the ration fed to the hen.
AIR CELL
• forms at the wide end of the egg as it cools after being laid
• the fresher the egg the smaller the air cell
Characteristics of Fresh/High
Quality Eggs
• Yolk is high & firm above the white
• Small yolk diameter
• Yolk is centered in white
• High ratio of thick to thin white
• High standing thick white
Which is the Highest Quality Egg
Deterioration Changes in Eggs
• Enlargement of air cell due to loss of moisture spoils
the appearance of an egg cooked in the shell.
• Quality – loss of CO2 ,permits the egg white to
become more alkaline
•
PH
of egg white may increase from about 7.6 (freshly
laid) to 9.0 -9.7 in few days.
• The white becomes thin and spreads when broken
• White eventually becomes yellow and even cloudy.
• The yolk flatens.
• The thinner white is no longer able to keep the yolk in
the centre .
Egg Candling
Measures quality or
freshness without
breaking
Characteristics of older,
low quality eggs:
• Larger air cell
• Yolk off center
• Speckled
appearance
Sizes
• U.S. sizes are defined by the
weight of a dozen eggs. (Not
individual eggs. An egg in a
carton of Extra Large eggs
need not weigh at least 27/12
ounces, but the dozen must
weigh at least 27 ounces.)
• Most recipes that call for eggs
usually mean Large eggs.
Size
Weight of a
dozen eggs
Jumbo
30 ounces
Extra Large
27 ounces
Large
24 ounces
Medium
21 ounces
Small
18 ounces
Peewee
15 ounces
Preparation of Eggs
Dry Heat
– Fried
– Scrambled
– omelets
Moist heat
– “Boiled” eggs
– Coddled eggs
prepared in a cup
– Poached eggs
– A variety of
custards
– Eggs that are
prepared using the
microwave
Effects of heat on Eggs protein
Eggs are useful in binding, thickening and gelling agents
because they contain proteins that are denatured by
heat leading to :
– Coagulation
– Gelation
Egg white
• changes from a viscous ,transparent sol to an opaque
,elastic solid .
Egg yolk
• increases in thickness as it is heated becoming either a
pasty or mealy solid depending on heat
Coagulation Temperature
White begins near 60°C
yolk begins near 65°C
Coagulation temperature is influenced by
pH of the dispersion
Presence of salts
How fast the temperature rises
Foam Formation
• Foams are used as angel food, sponge cakes, meringues,etc
• When the bubbles of air are incorporated in egg white (colloidal
dispersion of protein in water) with a wire whip or the blades of
a beater, individual proteins contribute aspects of its film –
forming potential.
• The presence of hydrophobic groups with an affinity for air and
hydrophilic groups that are soluble in water are essential in
lowering surface tension which enable the incorporation of air
possible and denaturation of surface protein molecules by heat
alter the native conformation and thus stabilize the foam.
Volume & Stability Factors
Sugar
– increases stability
– delays foam formation
– added at foamy or soft
peak stage
Acid
– increases stability
– doesn’t delay foam
formation
Fat
– decreases foam
Formation
Temperature
– room temperature
Utensils used
size of the bowl,
type of beater
the finer the wire the thinner
the blade, the smaller the
cells and the finer the foam.
Problems with cooking Eggs
• Boiled egg if overcooked a green ring is
formed around egg yolk due to the presence
of iron and sulfur.
• Can also happen if there is too much iron in
cooking water.-chill egg in cold water
• Cooking leads to the oxidation of cholesterolincreases risk of heart disease
Health issues
• Eggs have cholesterol-increases risk of heart diseases
• Contamination by salmonella a pathogenic bacteria-hence
commercially eggs should be washed with a sanitizing
solution after being laid
• Cook eggs to kill pathogenic bacteria
• Food allergy esp in infants due to egg albumin hence
introduction of egg white to infants is not recommended
• Raw egg whites contain a protein called conalbumin that
binds iron as well as avidin that binds biotin &can impair
function of other B vitamins
Milk
and
Milk Products
Milk
Milk should contain
 8.25% milk-solid-not-fat
 3.25% milk fat
Milk is ...
a true solution for salts, lactose, water-soluble
vitamins
milk is a colloidal dispersion for proteins and some
Ca phosphate
a dilute emulsion for fat globules
Milk
• pH of milk: 6.6 at 25oC
– when heated, pH decreases, H+ are liberated when Ca
phosphate precipitates
• Freezing Point: slightly lower than water
• Lower surface tension than water due to presence of
milk fat, protein, free fatty acids, & phospholipids:
easy to foam
• Components of milk varies with the feed, breed,
nutrition & the physiological condition of the cow
Nutritional Components
of Milk
•
•
•
•
Water: 87%, aw = 0.993
Proteins: caseins, whey proteins, enzymes
Lipids: mainly triglycerides
Carbohydrates: mainly lactose, also glucose,
galactose, & other saccharides
• Salts (<1%),
• Vitamins
Milk Proteins - Caseins
• Phosphoproteins, account for 80% of total
milk proteins
• Easily precipitated by adding acid to pH 5.24.6
Milk Proteins - Caseins
• Acid precipitates caseins
• Rennin casein: enzyme rennin coagulates caseins
• Caseinates: salts of caseins, highly soluble, used as
emulsifier, binder, thickening, foaming & gelling
Milk Proteins - Whey proteins
• Not precipitated by acid, but easily denatured
by heat (>60C)
• Four Fractions:
– lactoglobulins (50%),
– lactalbumins (25%),
– serum albumin
– immunoglobulins
• rich in sulfur-containing amino acids
Milk Proteins Whey proteins
• Use of whey protein products:
– Dried Whey: puddings, cakes, baked products
– Whey protein concentrates: binder in sausage (up
to 3.5%)
– Ultra Filtered Skim Milk : coffee whiteners
– Cheese Whey: animal feed
Milk Proteins - Enzymes
• Most of the enzymes are inactivated by
pasteurization
• alkaline phosphatase is an index of adequacy
of pasteurization (phosphatase test)
• Lipase may cause hydrolytic rancidity in dairy
products if it is not deactivated
Milk Lipids
• Triglycerides = 98% of total milk lipids
• Other lipids: phospholipids, free fatty acids, sterols,
carotenoids, fat-soluble vitamins
• Distinctive dairy flavor due to short-chain saturated fatty
acids (C4-C10) : butyric (C4:0), caproic (C6:0), caprylic
(C8:0), capric acids (C10:0)
• Fat content in milk decreases as cow ages
• Fat in feed does not appreciably affect fat content of the
milk
Milk Carbohydrates
• Mainly lactose (4.8%), small amount glucose
• Lactose is the source of lactic acid, formed by bacteria as
milk sours
• As milk is coagulated, lactose is in the whey
• Problems associated with lactose
– lactose intolerance
– crystals from condensed milk or ice cream
– lumping and caking of dried milk during storage
Milk Carbohydrates
• Use of lactose
– browning
– dispersing agent
– topping & icing
– carrier for flavor
– color ingredients
– body & viscosity
Vitamins
• Riboflavin
– water-soluble, light sensitive
– light yellowish & greenish color
• Carotene
– fat-soluble, yellowish color of milkfat
– skim milk is fortified with retinyl palmitate to replace
the carotene in milk fat
• Milk also provides thiamine & niacin
• Vitamin D is added to almost all milk
Minerals
• Rich in Ca and P
• trace elements: Fe, Mg, Mo, Ni, Zn
Pasteurization of milk
• Heating milk at a definite temp for a definite time
to destroy pathogens but not all bacteria
– 62oC for 30 min
– 70oC for 15 sec to 100oC for 0.01 sec (HTST)
– 138oC for 2 sec (UHT)
• to inactivate enzymes (lipase), control rancid &
oxidized flavor, and increase shelf life
Pasteurization of milk
Fermentation of Milk
 All fermented milk contains lactic acid
 Undesirable (spoiled products)
 Desirable (buttermilk, yogurt, cheese)
 streptococcus lactis initiates the process with lactobacilli
spp. continuing the fermentation
 Increased thickness due to the association of casein
micelles, often accompanied by ß-lactoglobulin.
Homogenization of Milk
• Forces milk through small openings and break up
the fat globules.
• The fat globule membrane is disrupted as new
surfaces are created
• Homogenized milk is whiter in color, more
viscous, more bland in flavor, and foam easily.
• Less heat stable (curdle more readily), form softer
curd
Kinds of milk
• Homogenization – the process of making a stable emulsion of
milk fat and milk serum by mechanical treatment and
rendering the mixture. Homogenized milk has a creamier
structure, bland flavor and a whiter appearance
•
Drying of Milk
• Dry milk products
– whole dry milk powder (WDM)
– nonfat dry milk (NFDM)
– dry butter milk
– dried whey
– dry cream
– malted milk powder
Evaporation &
Canning of Milk
• Evaporated milk
– sterilized canned milk that has been concentrated to
about half its original volume by evaporation under a
partial vacuum.
– 25% total milk solids including >7.5% milk fat
– high temp canning may give cooked flavor (methyl
sulfide)
– storage at high temp for long time may develop offcolor (Maillard reaction)
Evaporation &
Canning of Milk
• Sweetened condensed milk
– 15% sugar is added after concentration of whole milk
through evaporation
– total CHO concentrations of approx. 56% which is
sufficient to prevent spoilage by microorganisms
Milk Foams
• Milk can form gas-in-liquid foams because the milk
proteins have
– low surface tension - easier to spread the liquid proteins into
thin films
– low vapor pressure - reduces the evaporation
• In fluid milk, the protein concentration is too low to
produce a foam with any stability
• Foams can be formed in evaporated milk or dried milk
solids.
Effect of Heat on Milk Products
• Scorching
– Some of the whey proteins (-lactoglobulin and lactalbumin) denature and ppt to form a thin layer of protein
on the bottom.
– This protein gradually undergos the Maillard reaction with
lactose, leading to scorching.
• Scum formation
– denatured protein molecules join together
– evaporation of water from the surface (increasing the
concentration of casein and salts)
Effect of Heat on Milk Products
• Casien is quite resistant to ppt when heat is
applied, but severe heating can cause it to form
a curd at pH 7.
MEAT & POULTRY
• Meat is the flesh of animals
Composition
• Water content-Appro.75%
• About 15-20% protein mainly high quality
• Fat content ranges from 5-30%-varies with the type of
animal with the breed, feed and age, pigs convert high %
of their feed to fatty tissue
• Minerals eg Ca in bones, lean muscle rich in Fe and P.Liver
excellent source of Fe and Vit.A
• Vitamins-excellent source of niacin and riboflavin. Pork
rich in thiamin. meat is deficient in vit.C.
Structure of meat
• Cut meat consists of lean tissue which is mainly
protein with some bone and fatty tissue.
• lean meat consists of one or more muscle fibers that
is the basic structural unit of meat.
• Connective tissue provides support for the muscle
fibres, fat and bones of meat.
• Fat is deposited in the connective tissue within the
muscle and is known as marbling.
Color of meat
• Fresh meat-color differences in
color is due to the content of
myoglobin concentration in meat
(75%) remainder is due to
haemoglobin(25%)
• beef has more myoglobin.
Exercised muscles tend to be
deeper red eg heel of hound
• Sometimes there is color change
in meat-due to change in the
pigment myoglobin
• Molecules of myoglobin contain
the iron porphyrin as shown:
• Resonance of the conjugated double bonds in the
porphyrin ring gives rise to the color of meat
pigments.
• Myglobin like haemoglobin can unite temporarily &
reversibly with oxygen
• In live animal, the myoglobin takes O2 from
haemoglobin.When the animal dies, O2 supply is cut off.
Hence unoxygenated myoglobin in fresh beef is
purplish-red in color.
• Upon exposure to O2,oxymyoglobin is formed which is a
bright cherry red color
• At low levels of O2,the myoglobin- O2 complex
dissociates ,oxidising the Fe to ferric state resulting in
brownish red metamyglobin
Cured Meat Pigments
• When meat eg. bacon, ham, corned beef is cured myoglobin
unites with nitric oxide forming nitric oxide myoglobin giving
it a light pink color
• When meat is exposed to low heat during curing, part of the
nitric oxide myoglobin is changed to a stable complex (Fe still
in ferrous state) The pigment is now called nitric oxide
hemochrome.
Storing Meat
• Meat is highly perishable-refrigeration at 4°C or lower reduces
the growth of microorganisms on the cut surfaces and
maintains freshness.
• Meat should be frozen if they are not to be used within three
days
• Raw meats carry pathogens-surfaces they contact should be
washed thoroughly to prevent cross contamination
• Store :
– ground meat & variety meat-no longer than 2days
– Processed meats eg slices of ham, frankfurters & lunchen no longer
than 5 days in refrigerator
– Bacon smoked sausage and smoked ham up to 1 week
Cooking meat
• Destroys microorganisms that may have
contaminated the surface of meat-should be heated
to a temp of 70°C or until juice from meat is clearnot pink in color
• Affects tenderness
• Changes color
• Alters water holding capacity
• Develops flavor and characteristic aroma
Methods of cooking
• Meat can be cooked by either dry of moist
methods
• Dry methods –meat is cooked added water
and uncovered so that the moisture from the
meat can evaporate. meat is in contact with
hot air, a hot frying pan or hot fat from which
heat is conducted
• Methods include-roasting, broiling, panboiling
and frying
• Recommended for tender cuts of meat
• Moist cooking-meat is cooked in a covered utensil,
water is added or meat is cooked in the
steam/liquid released from the meat as the
proteins coagulate
• Braising (pot roasting &breading) and cooking in
water are moist methods of cooking
• Tough cuts of meat are ideal for moist cooking
Storage of cooked meat
• Leftover cooked meat should be cooled
promptly and refrigerated at 4°C if they are to
be used within 3-4 days
• Otherwise they should be frozen at-18°C
Effects of cooking on pigments and
color
• When meat is heated the myoglobin is first converted first to
oxymyoglobin (bright red color)
• With further heating, protein moiety of the pigment is
denatured, ferrous Fe is oxidized-meat color becomes grayish
brown due to denatured globin hemichrome
Effect of cooking on meat protein and
tenderness
• Connective tissue-cooking decreases
toughness of meat since the tensile strength
of meat comes from fibres of collagen that are
part of connective tissue
• Muscle fibres- proteins of muscle fibres are
denatured but are not solubilized.
• They lose their ability to associate with water
esp with high temperatures
• This results in shrinkage of fibres and firmness
of tissue
• Flavor-cooking decomposes one or more precursers in the
lean meat to give the basic cooked meat taste
Less tender cuts of meat have more extractives-more flavorful
Lactones and sulfur containing compounds eg sulfides,
mercaptans and cyclic cpds such as pyrazine make important
contributions to the flavor of cooked meats
• Initial fat content-meat cooked by pan frying and broiling
differ little in fat content. Fat melts, causing browning on the
surface and increase in flavour.
• Effect on nutritive value-retention of B vitamins when meat is
cooked is good, however there is greater loss of thiamine
Poultry
• Includes meat obtained from ducks, geese,
guineas, pigeons, turkeys and chickens
Market class of poultry
• Different classes of poultry are marketed on the basis
of their age that influences tenderness& fat content
& dictates the cooking methods.
• Birds <6 weeks-have no hair on skin and little
subcutaneous fat weigh <1kg
• Broiler-fryer birds-6-8 weeks weigh 1-1.5 kg
• Roasters-2.5-5 months-weigh 1-2kg
• Capon -<8 months-weigh2-4kg
• Hens->1year-weigh 1-2.5kg
Preparing poultry for market
• Birds are killed by cutting the jugular vein
• Dip in hot water(52-54°C loosens feathers without damaging
the cuticle of the skin
• Feathers are removed using machine
• Evisceration-abdominal cavity is slit &entails removed
• Head, feet &oil gland are removed
• Bird is chilled-to prevent bacterial growth
• Tenderness of the meat depends on the way meat is cooledprompt cooling in ice water-meat is tender; slow cooling in
air–meat is tough (pH of meat is low 6.3 due to accumulation
of lactic acid)
Inspection and Grading
• The following characteristics are considered when
grading poultry for quality:
- Shape and meatiness
- Distribution of fat
- General appearance of bird
Breaks in skin, bruises and feathers lower the grade
Quality grades for poultry are A,B and C
Quality considerations in poultry selection
• Skin-moist but not sticky, unmarked, light cream in
color,slightly blue at the thigh area
• Flesh-pink, no bruising
• Bones-unbroken; flexible breastbone.
• Pin feathers-none
• Smell-freash and clean
• Temperature-refrigerated at 0-2°C
Common Chicken Cuts
Breast
Popular way to buy chicken
Ideal size for single portion
Can be bought with or without skin
Meat is usually lean
If skin is removed, the meat can dry out during cooking
Wing
Dark meat
Fairly inexpensive cut
Quite fatty contains a large amount of bone
Ideal for barbecuing
Leg
Darker meat
Contains more fat than breast
Cheaper way to buy chicken but bone in drumstick increases weight so less meat per kilo than breast.
Ideal for roasting and frying
Handling raw poultry
• Raw poultry spoils quickly-should be held in the
coldest part of the refrigerator and cooked within 2-3
days
• Frozen poultry should be thawed in the refrigerator
or in changes of cold water
• Once thawed it should be cooked promptly
• Surface contacted by poultry –wash with warm water
and detergent. Cutting boards should be rinsed with
sanitizer.
• this is to prevent cross contamination of other
foods-as incidence of Salmonella is high in poultry
Composition and structure
• Comparable to red meat in composition and
nutritive value
• Has high quality proteins
• Good source of B vitamins, Iron and
phosphorous. Dark meat is rich in riboflavin.
Light meat higher in niacin.
• Fat content of meat varies-depends on age,
feed color of meat. Skin has more fat.
• The muscles are made of muscle fibres and
connective tissue.
Cooking poultry
• Cooking method depends on the basis of tenderness. Young
poultry can be cooked by broiling, frying, roasting
• Stuffing poultry before roasting-not recommended-stuffing
raises temp. slowly –microorganisms (salmonella and
staphylococcus) multiply.
• Microwaving –not recommended as heat is not distributed
evenly. Pathogens such as Clostridium perfringens, salmonella
and staphylococcus aureus survive microwave cooking
• Mature birds-moist methods allow time to tenderise meat
such as stewing and braising.
• Poultry is cooked when its juice is free of pink color
Cooking losses
• Cooking has little effect on vitamin B retention
-90% retention of riboflavin
-80% retention of thiamin
Leftover cooked poultry-cool promptly and
refrigerate at 4°C if it is to be eaten within 2-3
days. Otherwise it should be frozen and held
at -18°C
Frozen poultry
• Once thawed-do not refreeze
• When young chicken are cooked, discoloration
in meat next to larger bone may occurfreezing and thawing release haemoglobin
from the red cells in the bone marrow as the
bones of young chicken is porous and heating
denatures the pigment-discoloration
• Discoloration is not a problem in older birdsbones are dense.
Flavor of cooked poultry
• Volatile carbonyls such as hydrogen sulfidegive cooked chicken its flavor
• Otherwise the sulfur compounds present will
give only meaty of beef like aroma