Eggs Eggs • Source of high quality protein • Important ingredients in prepared foods. • Act as emulsifiers in mayonnaise, cream puffs, cheese schouffle and shortened cakes. • Function as a coating materials for foods, a thickening agent in soft pie filling. • A gelling agent in custard and structural material in shortened cake. • Whipped to a foam, egg serve as a means of incorporating air into meringues, divinity,candy,puffy omelets and sponge and angel cakes. Structure cont SHELL • the first line of defense 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 fibers GERMINAL DISC • appears as a slight depression on the surface of the yolk • the entry for the fertilization of the egg WHITE (ALBUMEN) • there are 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 • the fresher the egg the more prominent the chalazas • unnoticeable when the egg is cooked 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 color 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 • White (Albumen) • Albumen accounts for most of an egg's liquid weight, about 67%. • It consists of four opalescent layers of alternately thick and thin consistencies. • The white of a freshly laid egg has a pH between 7.6 and 7.9 and an opalescent (cloudy) appearance due to the presence of carbon dioxide. • As the egg ages the CO2 escapes which increases the pH. • Egg white also becomes thinner as an egg ages because its protein changes in character. That's why fresh eggs broken onto a plate sit up tall and firm while older ones tend to spread out. • The albumen of older eggs is more transparent than that of fresher eggs. • Fresh egg whites coagulate in the range 62° to 65°C, the temperatures decrease with increasing pH and hence age. This is why very fresh eggs require more time to cook than older eggs. • Yolk • The yolk (yellow portion) makes up about 33% of the liquid weight of the egg. • It contains all of the fat in the egg and slightly less than half of the protein. • With the exception of riboflavin and niacin, the yolk contains a higher proportion of the egg's vitamins than the white. • All of the egg's vitamins A, D and E are in the yolk. • Egg yolks are one of the few foods naturally containing vitamin D. • The yolk also contains more phosphorus, manganese, iron, iodine, copper, and calcium than the white, and it contains all of the zinc. The yolk of a large egg yields about 250 kJ of energy. • Egg yolks have a pH of about 6.0 which stays relatively constant as the egg ages as there is no CO2 loss. Coagulation occurs in the range 65° to 70°C. Quality of eggs • A freshly laid egg has a high proportion of thick white that resist spreading when the egg is broken from the shell. • The yolk will be upstanding and remains centered in the white. • Changes in an egg leading to a deterioration in quality may occur as soon as the egg is laid. • BLOOD SPOTS Blood or "meat" spots are occasionally found on an egg yolk. These tiny red or red-brown spots are not harmful. They are caused by the rupture of a blood vessel during formation of the egg. Blood spots do not indicate a fertilized egg. • . Deteriorative changes • 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 flattens. • The thinner white is no longer able to keep the yolk in the centre . • Candling • Candling is the only method of testing eggs for quality, internally and externally, without breaking them. It consists of inspecting an egg with a beam of light that makes the interior quality visible. A very simple form of candling is placing a candle in a dark room and positioning an egg in front of the flame and looking at the interior quality Grades of eggs • In order of decreasing quality, Grades of eggs grades are AA, A, and B. All ungraded eggs sold to consumers must meet B standards. “Restricted eggs” do not meet B standards; their disposition is regulated to prevent them from reaching consumers, although two types of restricted eggs, checks (the shell is cracked but the membrane beneath is not broken), and dirties, may be sold to factories equipped to process them properly. Sizes • U.S. sizes are defined by Size Weight of a dozen the weight of a dozen eggs eggs. (Not individual Jumbo 30 ounces eggs. An egg in a carton of Extra Large eggs need Extra Large 27 ounces not weigh at least 27/12 ounces, but the dozen Large 24 ounces must weigh at least 27 ounces.) Medium 21 ounces • Most recipes that call for eggs usually mean Large Small 18 ounces eggs. Peewee 15 ounces 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 or gelation • Coagulation – involves a random aggregation in which protein – protein integration predominates • Gelation – an orderly aggregation in which protein –solvent as well as protein to protein integration denotes • 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 600C – yolk begins near 650C • 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. Factors affecting foaming • 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. • Temperature • Presence of fat, salt or acid • Time of adding sugar. Methods of cooking eggs • Cooked in the shell • Poached egg • Fried Egg • Scrambled egg • Custards Soft pie filling 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 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 Tutorial 1. How can you test the quality of an egg without breaking it? Describe 2. How does heat affect egg protein? 3. Compare fresh egg with a stale egg 4. Why is the pH of egg yolk relatively stable? 5. Why does the air cell enlarge? 6. Which eggs cook fast? Give a reason 7. Egg are a source of high quality protein. What do you understand from ‘high quality protein’? 8. Give some functions of egg 9. How can you from ‘foam’ from eggs? 10. Why do we need to cook the eggs? 11. What are some of the health issues associated with eggs? 12. How are you going to prevent them? 13. What are some of the problems of cooking eggs? Plant proteins Plant proteins • Not much attention has been done on plant proteins then animal proteins. • Plant proteins can be obtained from leaves,cereals,oilseeds and nuts. leaf proteins denature at 500C and undergo surface denaturation at pH 4.5-6.0. • Wheat contains about 8-14% protein, rye -12%,barley -10% and rice -9%. Nutritional quality of vegetable proteins • Vegetable proteins are of lower quality because they lack one or more of the essential amino acids, either in quantity or in unfavorable ratios • The amino acid lacking is called a limiting amino acid • Wheat is most limited in lysine, corn in tryptophan, and soybean in methionine • Legumes are low in Cysteine and methionine. Advantages • Plant food are generally lower in fat, saturated fat and cholesterol and moderate in calories and protein • The quantities of folic acid, magnesium, fibre, antioxidants (vit. E, vit. A, and carotenoids are higher in plant foods • Having vegetarian diets lowers the risk of coronary diseases such as coronary heart disease,, diabetes. Disadvantages • Plant foods are bulky hence people with high calorie demands for example small children may not be able to get enough calories from plant foods. • Adequate amounts of vitamin D may be lacking in vegan diets if vitamin D fortified dairy foods are excluded. esp for young children and pregnant women • VitaminB12 is only present in animal foods • High content of plant foods increases amount of fibre, phytates and oxalates which binds to minerals in the intestine and limits their absorption leading to problems in calcium, iron and zinc absorption Wheat Protein • Wheat protein are unique among plant proteins and responsible for the bread making properties of wheat. • There are 4 main fractions of proteins in wheat. 1. Albumin-water soluble and coagulated by heat 2. Globulin-soluble in neutral salt solution 3. Gliadin – a protein soluble in 70% ethanol 4. Glutenin – a glutelin insoluble in alcohol but slightly soluble in dilute acid or alkali. • Gliadin and glutenin are the storage, or gluten forming, proteins of wheat. The formation of gluten takes place when flour is mixed with water. • The gluten is a coherent elastic mass, which holds together other bread components such as starch and gas bubbles, thus providing the basis for the crumb structure of bread. • Gluten proteins have a high content of glutamine but are low in the essential amino acids(lysine,methionine and tryptophan). • The insolubility of gluten proteins can be directly related to their amino acids composition. Soybean Proteins • Soya protein is a good source of all essential amino acids except methionine and tryptophan. • The high lysine content makes it good complement to cereal proteins, which are low in lysine. • Soya proteins have neither gliadine or glutenin, the unique proteins if wheat gluten. As a result, soya flour cannot be incorporated into bread without the use of special additives that improve loaf volume. • The soya proteins have a relatively high solubility in water or dilute salt solutions at pH values below or above the isoelectric point. This means they are classified as globulins. Tutorial 1. Why are vegetable proteins considered to be of lesser quality than animal proteins? 2. What are the advantages and disadvantages of vegetarianism? 3. How may an adequate diet be planned if only vegetable proteins are available? Seafood Sea food Proteins • Excellent source of proteins • Different forms of protein sources in sea – Fish – Shell fish – Crustaceans e.g crabs,crayfish,lobsters,prawns,shrimps. – Mollusks e.g. bivalves – clams,mussels,scallops,snails,octopus,squid,periwi nkle. • Composition of fish varies: -low fat fish- eg cod, rockfish contain less than 2.5% fat -medium fat fish-contain 2.5-5% fat eg yellow fin tuna, mullet swordfish, and bluefish -high fat fish-contain more than 5% fat eg salmon, mackerel, albacore tuna, blue fin tuna, sardines, herrings, anchovies, trout. Fat content of fish varies with its location and color of muscle. Red muscle in the belly flap area has maximum amount of oil followed by the flesh near the head. Sections from the white muscle tail is low in fat content. • The protein content of fish is approximately 15-20% • Two types of muscle in vertebrate fish • Lateral muscle make up bulk of edible part, which is usually colorless except for salmon which is pink due to carotenoid - astaxanthin. • Outside the lateral muscle is a small superficial muscle that fans out on each side of lateral line. It is dark reddish brown, rich in myoglobin. • The muscle of fish consist of muscle fibers and connective tissues. Mycofibrils are embedded in the sarcoplasm of the fibers defined by the sacrolemma. • Myofibrils appear to be made up of overlapping thick (myosin) and thin (actin) filaments. • The contractile proteins – actin and myosin make up of about 2/3 of total protein in muscle. • The connective tissue is more abundant in the skin, fins and skeleton than muscle. Nutritive value • The protein of fish is of excellent quality • Fish contains both saturated and unsaturated fat, however the total fat content of raw fish is less than that of raw meat and poultry. • Fish contains special fatty acids called omega 3 fatty acids which is associated with decreased incidence of heart disease. • Fatty fish contain omega 3 fatty acid in highest concentrations. • Shrimps are higher in cholesterol than other seafood, meat or chicken but are low in fat. • Low in carbohydrate • Vitamin content of fish varies according to the fat content. High fat fish are good sources of vitamin A and D • Most fish are good sources of thiamin, riboflavin, niacin vitamins B6 and B12 • However raw fish contains an enzyme thiaminase which breaks thiamin thus it is not absorbed. Cooking denatures and deactivated this enzyme. • Fish particularly shellfish are good sources of minerals • Fish canned with bones are good sources of calcium and phosphorous • Ocean fish are good source of iodine • Oysters and clams are good sources of iron, zinc and copper. • Tuna is a good source of selenium which protects against mercury and cadmium toxicity. • However there is very little salt in the flesh of fish except shark meat. The salt content of fish is slightly higher than meat. Postmortem changes • Fish muscle undergoes rigor mortis similar to meat tissue. • The limp tissue becomes rigid and with time the muscle relax again • Time required for rigor mortis depends on the species, physical condition and temperature. • Poor physical condition depletes muscle glycogen eg if fish struggled before death, spawning, stress. High amount of glycogen will keep the fish in rigor longer- good quality fish. • Cooling the fish quickly to 0° will delay the onset of rigor. • If fish undergoes rigor at high temp. and the concentration of glycogen is high then gaping can occur. Gaping results from tears in the connective tissue due to strong muscle contraction during rigor. • Fish should be filleted after rigor mortis has ended. If filleted before rigor-muscle will shorten and toughen later. If frozen pre rigor the fillets will go into rigor and slowly relax during freezing. If cooked before the muscle relax, it will become tough and distorted when cooked. Fishy Odor • Fishy odor is due to amines • Amines are liquid at room temperature and not water soluble. • Tirmethylamine oxide (TMO) is a natural amine found in fish muscle. • As bacteria grow, they reduce this cpd to trimethylamine • During cold /frozen storage TMO can be degraded to dimethylamine and formaldehyde. • Bacteria also produce sulfur cpds and ammonia-unplesant odors. • Lemon juice –improves the odor and flavor-amines to amine salts Buying fish • Eyes should be bright, clear and bulging • Scales -should be shiny and clinging tightly to the skin • Gills-reddish pink, surface free of dirt or slime • Flesh-firm to the touch with no traces of browning or drying around the edges • No fishy odor. Storage of fish • Fish and shellfish are highly perishable, therefore icing or refrigeration is essential for highly – quality products. • Spoilage occurs rapidly in fish and bacterial spoilage does not begin until the fish has gone into and passed out of rigor Tutorial • What happens to the fish when it is overcooked? • What are the characteristics of fresh fish? • Why does a fish spoil more quickly than other fresh foods? • What contributes to the fishy odor in fish and describe how this compound changes due to improper storage • Which fatty acids are present in fish but not in meat? What is the importance of these fatty acids? Poultry Poultry • Includes ducks ,geese,guineas,turkey and chicken. • Different classes of poultry are marketed on the basis of age. • Age influences tenderness and fat content and so dictates the cooking methods that are appropriate. – Signs of youth – Smooth leg skin – Supple wing joint – Pliable keel – Breast bone – Presence of pin feathers – No air on the skin – Little subcutaneous fat. • The weight of the chicken varies with age. Preparing poultry for market • Marketed ready to eat. i.e. heat, feet and entrails are removed. • Birds are fasted eight hours prior to slaughter. • Killed by a method that minimizes struggle. • Birds are scaled – dipped in hot water to remove feathers • Evisceration follows scalding and picking. • Chilling follows – prompt cooling is essential to control growth of bacteria . • Trisodium phosphate solution as a spray or dip to follow chilling is an optional step to reduce but not eliminate bacterial contamination on the birds. Chilling • Also important to control rigor mortis • The time of onset of rigor, its duration and the tenderness of the meat once rigor has passed are influenced by the way the bird is cooled. • To avoid meat toughness prompt cooling from body temperature to 150C before the pH is lowered to 6.3 by accumulation of lactic acid is essential. • Poultry goes in and out of rigor more rapidly then other meats. • Aging longer then 24 hours results in no additional tenderizing. Inspection and grading • All ready to eat poultry are inspected for quality and well as wholesome • Characteristics in assigning grades – Shape – Meatiness – Distribution of fat – General appearance of the bird. – Breaks in the skin, bruises and pin feathers lower the grade. • Quality grades for poultry are A,B,C. Cuts of poultry Handling raw poultry • Should be held in the coldest part of a refrigerator and cooked within two or three days. Commercially it should be stored in the freezer. • Incidence of Salmonella has been found to be high in raw poultry this is mostly due to cross contamination. • If poultry is frozen it should be wrapped in moisture proof film to avoid freezer burn (when skin of chicken dries and discolors) • Frozen poultry can be defrosted in two ways: -defrosted in the refrigerator -defrosted in cold water-water to be changed every 30 minutes Defrosting at room temperature provides suitable conditions for bacterial multiplication. Nutritive value and composition • Proteins supplied by poultry are complete proteins- contain all the amino acids that are essential for building body tissues. • Good source of B vitamins-thiamin, riboflavin and niacin. • Fat content in young birds is low. ducks and goose have more fat than chicken and turkey. • Levels of iron and zinc are less than those of red meats-due to less myoglobin content of its muscle. 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 tenderize 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 occur- freezing and thawing release hemoglobin 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 birds-bones are dense. Flavor of cooked poultry • Volatile carbonyls such as hydrogen sulfide- give cooked chicken its flavor • Otherwise the sulfur compounds present will give only meaty of beef like aroma Tutorial • Give reasons why poultry should not be thawed at room temperature. • How can you thaw poultry to avoid multiplication of microorganisms? • Why is the mineral content of poultry low? • What is the purpose of trisodium phosphate solution? • When grading a chicken what characteristics will you look at? • What is some common food borne illness spread due to improper handling of poultry? How can you prevent them? • What are some effects of cooking frozen poultry? • Does cooking poultry have any effect on its nutrient content? Explain Meat • 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 fibers, fat and bones of meat. • Fat is deposited in the connective tissue within the muscle and is known as marbling. Different levels of fat marbling in meat 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. • Myoglobin like hemoglobin 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 ,oxidizing the Fe to ferric state resulting in brownish red metmyoglobin 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 clear-not 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 fibers and firmness of tissue • Flavor-cooking decomposes one or more precursors 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. • Effect on nutritive value-retention of B vitamins when meat is cooked is good, however there is greater loss of thiamine Tutorial 1. What gives the meat its color? Why does its color change upon exposure to oxygen? 2. Why does cured meat have a light pink color? 3. How can you prevent the growth of microorganisms in meat? 4. What is the effect of cooking on the pigments in meat? 5. How does cooking affect meat quality? Meat proteins? Dairy Milk • Is the secretion of the mammary glands of mammals Composition Milk fat • is in the form of triglycerides, 95-95% -66% saturated -30%unsaturated -4% polyunsaturated Milk has high no. of short chain fatty acids- butyric, caproic Remaining lipids are phospholipids(0.9-1%) and sterols(0.22-0.41%), minute quantities of free fatty acids and fat soluble vitamins (A,D.E and K) • Carbohydrates-primary carbohydrate is lactose.(4.8%) • Lactose is not very sweet as sucrose. • In the souring of milk, lactic acid bacteria convert lactose to lactic acid eg in yoghurt and cheese production. • Proteins-are found in two forms: • Casein-predominant protein in milk (2.7% of total milk) is 80% of the total proteins present in milk. It has all essential amino acids. They are separated from milk by acidification (pH 4.6)They form curd in milk. • Whey- is 20% of the total proteins in milk. It has higher nutritional value than casein. Separated from milk by heat.It has the lactalbumin and lactaglobulin and they do not coagulate. • Minerals-primary mineral in milk is calcium, phosphorous, potassium, chloride, and sulfur Nutritive value • Proteins in milk are of high quality-complete proteins • Lactose in milk promotes the growth of favorable microorganisms in the intestine that produce some B vitamins. • Also lactose increases the absorption of the minerals calcium, phosphorous, magnesium and zinc. • Iron and copper are found in low concentrations in milk. • Vitamins • Whole milk is an excellent source of vitamin A but it does not provide large amount of vitamin D unless fortified. • Milk is an excellent source of riboflavin, however when exposed to ultraviolet light, milk losses riboflavin quickly • Thiamin and niacin are found in milk in fair quantities. • Milk provides considerable amounts of tryptophan that is a precursor for niacin • Small amount of ascorbic is present in raw milk. Sanitary controls • Purpose • To insure a safe milk, free of disease producing bacteria, toxic substances and foreign flavors • It also helps to produce milk that has low bacterial count, food flavor, satisfactory keeping qualities and high nutritive value. • Pasteurization-the aim of pasteurization of milk is to get rid of any disease producing bacteria. Some nutrients are also destroyed eg, thiamin, vitamin B12,proteins and calcium. • Extends storage life of milk. • HTST(High Temperature Short Time) – heating milk to 72C for 15 seconds. The holding method is when milk is heated to 63C for 30 minutes. • UHT(Ultra High Temperature) – treating milk at 138C for 2 seconds and then cooling rapidly. • LTLT-low temperature longer time.63° for 30 minutes or more. 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 • • Vitamin A/D fortified milk-milk that is fortified with vitamin A/D • Skim milk-it contains less than less than 0.5% fat • Low fat milk-has some of its fat removed. • Flavored milk-milk can have a variety of flavoring agents-chocolate, strawberry, cocoa, coffee. • Condensed milk is made by evaporating fresh milk sweetened with sucrose or dextrose to a point when it contains not less than 28% total milk solids and 8% milk fat • Evaporated milk-is made by evaporating considerable amount of water so that the finished product contains no less than 7.5% by weight of milk fat and 25.5% of total milk solids. • Dry milk-dried whole milk contains a maximum moisture content of 5% and 26-39% milk fat. Non fat dry milk is made from pasteurized skim milk. It contains 5% moisture and 1.5% milk fat unless otherwise indicated Yogurt • Is a product made from milk and S thermophilus and Lactobacillus bulgaricus bacteria • Made from milk that is pasteurized or ultrapasteurised. • The cooled milk is inoculated with a mixture of bacteria and incubated for several hours at 42-46°C • During fermentation lactose is converted to lactic acid and the pH decreases. • Some lactose is also converted to acetaldehyde that gives yogurt tartness. • Diacetyl is produced from citrate that gives yogurt a buttery flavor • Fermentation is stopped when yogurt has developed desired acidity and viscosity by chilling the containers or by heating. • Flavors and stabilizers (gelatin and pectin) are then added . Tutorial 1. What are the major constituents of milk? 2. Differentiate between casein and whey protein? 3. What is the lactose content in milk? what happens when lactose is attacked by lactic acid bacteria? 4. Define: – Pasteurization – Homogenization 5. Distinguish between UHT and HTST pasteurization? 6. What happens during the production of yogurt? Cheese Classification • Very hard ripened cheese-made form low fat cows milk. Low in moisture content. Has sharp flavor so minimum curing time is 6 months eg parmesan cheese. • Hard ripened cheese-made from pasteurized milk and subject to action of lactic acid bacteria which bring about the proper acidity of the mixture for curdling . eg cheddar, gouda, edam. • Semisoft ripened cheese-have moderate moisture content-35- 45%.They are manufactured in a similar manner as hard cheese but the curd may not be heated, cut eg blue veined cheese. • Soft ripened cheese-eg cottage cheese, cream. They are made from cream, or from a mixture of milk, skim milk and concentrated skim milk. Nutritive value • Good source of high quality protein • Fat content depends on the type of cheese-cream cheese has more fat. • Cheese made from whole milk are good sources of vitamin A and riboflavin. However it may contain high amounts of sodium as salt is used during manufacture • Cheese made by acid coagulation retain only 25- 50% of the original calcium content as calcium ion precipitates when curd is formed. Cheese manufacture • Type of milk-homogenized milk is used for soft cheese because the breakdown of milk fat by mold enzymes is accelerated by increased surface area of the fat. • It is not suitable for the production of hard cheese as it creates a brittle texture and is rubbery. • White cheese is made from milk that is treated with 0.002% benzoyl peroxide. • Curd formation • Formation of curd depends on the agent used for coagulation, the temperature of milk and the presence of salts. • Acid coagulated cheese is coagulated by lactic acid bacteria. This curd is soft and gel like. eg cottage cheese, cream cheese. • Coagulation by enzyme rennin. This produces a tough rubbery curd with low moisture content eg cheddar cheese • Enzyme coagulation-reduces the time required • Temperature of milk during coagulation affects the texture and physical characteristics • Presence of salts influences curd formation. Up to 0.02 % calcium chloride is added to milk to improve curd strength and accelerate coagulation Curd treatment • Once the curd is formed, it is: • Cut-to increase surface area and permit loss of moisture, ensure uniformity in moisture, firmness and elasticity. • Heated-to increase expulsion of whey, allow lactic acid development, reduce undesirable microorganisms, and produce firm rubbery body • Whey is removed • Salt is then added ranging from 1%(cottage) to 5%(parmesan)This retards growth of microorganisms and lactic acid bacteria and improves flavor, texture and appearance • Finally curd is pressed to remove traces of whey, reduce the openness of curd and complete the manipulation. Ripening and curing • Ripening –when physical and chemical changes occur to the curd during curing • Curing- is the methods and environmental conditions eg length of time, temperature, humidity and sanitation. • Cheese may be ripened between 2-24°C • During ripening: -Proteins are hydrolyzed into proteoses and peptones and then into peptides and amino acids-softer cheese. -Fats are hydrolyzed into acetate and volatile fatty acids that contribute to sharp flavor of aged cheese - The carbohydrates in cheese are broken down into lactic acid which contributed flavor and increases its shelf life Some cheese are inoculated with molds that change its character and flavor. Some of these may produce eyes in the cheese (holes) Care and storage • Most cheese spoil quickly so they must be placed in a covered container and refrigerated until consumed. • It is best to store cheese in its original wrapper. Tutorial 1. Why is cheese a good substitute for milk? 2. What contributes to increased shelf life of cheese? 3. What contributes to the sharp flavor of aged cheese? 4. What is the function of salt in the production of cheese? 5. Which cheese has less calcium? 6. What is the purpose of cutting the curd? 7. What is the purpose of heating the curd?