Food Biotechnology Dr. Kamal E. M. Elkahlout Food Biochemistry
4 Protein Foods
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Eggs
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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.
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
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Cross Section of an Egg
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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
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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
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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
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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
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Which is the Highest Quality Egg
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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 CO 2,permits the egg white to become more
alkaline P H 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.
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Egg Candling Measures quality or freshness without breaking
Characteristics of older, low quality eggs: Larger air cell Yolk
off center Speckled appearance
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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 Jumbo30 ounces Extra Large27
ounces Large24 ounces Medium21 ounces Small18 ounces Peewee15
ounces
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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
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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
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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 60C yolk begins near 65C
Coagulation temperature is influenced by pH of the dispersion
Presence of salts How fast the temperature rises
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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.
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Volume & Stability Factors Sugar increases stability delays
foam formation added at foamy or soft peak stage Acid increases
stability doesnt 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.
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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
cholesterol- increases risk of heart disease
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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
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Milk and Milk Products
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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
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Milk pH of milk: 6.6 at 25 o C 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
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Nutritional Components of Milk Water: 87%, a w = 0.993
Proteins: caseins, whey proteins, enzymes Lipids: mainly
triglycerides Carbohydrates: mainly lactose, also glucose,
galactose, & other saccharides Salts (
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
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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
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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
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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
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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
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Milk Carbohydrates Use of lactose browning dispersing agent
topping & icing carrier for flavor color ingredients body &
viscosity
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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
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Minerals Rich in Ca and P trace elements: Fe, Mg, Mo, Ni,
Zn
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Pasteurization of milk Heating milk at a definite temp for a
definite time to destroy pathogens but not all bacteria 62 o C for
30 min 70 o C for 15 sec to 100 o C for 0.01 sec (HTST) 138 o C for
2 sec (UHT) to inactivate enzymes (lipase), control rancid &
oxidized flavor, and increase shelf life
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Pasteurization of milk
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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
-lactoglobulin.
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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
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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
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 off- color (Maillard reaction)
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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
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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.
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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)
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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.
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MEAT & POULTRY
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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.
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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.
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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:
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Resonance of the conjugated double bonds in the porphyrin ring
gives rise to the color of meat pigments.
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Myglobin like haemoglobin can unite temporarily &
reversibly with oxygen In live animal, the myoglobin takes O 2 from
haemoglobin.When the animal dies, O 2 supply is cut off. Hence
unoxygenated myoglobin in fresh beef is purplish-red in color. Upon
exposure to O 2, oxymyoglobin is formed which is a bright cherry
red color At low levels of O 2,the myoglobin- O 2 complex
dissociates,oxidising the Fe to ferric state resulting in brownish
red metamyglobin
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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.
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Storing Meat Meat is highly perishable-refrigeration at 4C 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
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Cooking meat Destroys microorganisms that may have contaminated
the surface of meat-should be heated to a temp of 70C or until
juice from meat is clear- not pink in color Affects tenderness
Changes color Alters water holding capacity Develops flavor and
characteristic aroma
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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
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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
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Storage of cooked meat Leftover cooked meat should be cooled
promptly and refrigerated at 4C if they are to be used within 3-4
days Otherwise they should be frozen at-18C
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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
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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
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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
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Poultry Includes meat obtained from ducks, geese, guineas,
pigeons, turkeys and chickens
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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