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 Componentsof Milk • Water: 87%, aw = 0.993 • Proteins: caseins, whey proteins, enzymes • Lipids: mainly triglycerides • Carbohydrates: mainly lactose, also glucose, galactose, & other saccharides • Salts (<1%), • Trace elements, • Vitamins
Milk Proteins - Caseins • Phosphoproteins, account for 80% of total milk proteins • Four groups: alfas1-, alfas1, Beta-, & Kappa-caseins • Present in milk as micelles, stabilized by Kappa-caseins & Ca phosphate • Easily precipitated by adding acid to pH 5.2-4.6
Milk Proteins - Caseins • Acid precipitates caseins • Rennin casein: enzyme rennin coagulates caseins, when Kappa-casein is destroyed by rennin (enzyme from the stomach lining of calves), other caseins react with Ca to form a coagulum • 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 • Two forms of lactose: -monohydrate & anhydrous- • 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 - lack -D-galactoside galactohydrolase • 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
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
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 off-color (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.