•Oxidation occurs at the double bonds of the unsaturated fatty acids. The presence of iron and copper salts accelerates the onset of auto-oxidation and the development of a metallic flavor. Also, the presence of dissolved oxygen and exposure to light promotes oxidation. Oxidation of milk fat may limit the shelf life of whole milk powders, but is not usually a serious problem. Oxidation is catalyzed by light, so if milk is exposed to light for too long a period of time, off-flavors will occur. Fat oxidation can be counteracted by microorganisms in milk such as lactic acid bacteria because they consume oxygen thereby reducing oxidation. Also, pasteurization helps to reduce oxidation because reducing compounds such as sulfhydryl groups are formed when milk Is heated. The metallic oxidation off-flavor is more common in the winter than in the summer because of differences in the cows diet. Summer feeds are higher in Vitamins A and C. These vitamins increase the amount of reducing substances in milk.
Oxidation of protein –
•When exposed to light, the amino acid methionine is degraded to methional by riboflavin and vitamin C. Methional causes the “sunlight flavor” in milk. However, methionine is present in milk as part of the milk proteins. So fragmentation of the proteins must occur to release the free amino acid methionine. Factors that relate to sunlight flavor development are:Intensity of the light, especially light from flouresecent tubes, Duration of exposure,
Homogenized milk is more sensitive that non-homogenized milk, Nature of the package – opaque packages such as plastic and paper give good protection.
•Hydrolysis is the liberation of free fatty acids from the glycerol backbone. This reaction requires the presence of a lipase, which is an enzyme that catalyzes the hydrolysis of ester bonds of lipids. A large amount lipoprotein lipase is present naturally in milk, but fortunately fat globules with an intact milk fat globule membrane are not susceptible to hydrolysis by the enzyme. Also, lipoprotein lipase is inactivated at temperatures required for pasteurization.
•Spoilage bacteria provide a heat stable lipase, but the spoilage bacteria must exceed normal levels, and routine bacterial control should prevent such an occurrence.
Lipolysis –
•The breakdown of fat into glycerol and free fatty acids is called lipolysis.
•Fat that has been lipolysed tastes rancid and smells rancid. This effect is due to the presence of low molecular free fatty acids such as butyric and caproic acids.
•Lipolysis is enhanced by lipases and high storage temperatures. Lipases can only act if the fat globules have been damaged so that the fat is exposed. The fat glubules can be damaged by pumping, stirring, or splashing the milk. Therefore, unnecessary agitation of unpasteurized milk should be avoided to prevent the damage of the fat globules.
•High temperature pasteurization inactivates the lipases.
Milk Proteins
Casein
Casein Micelle and Submicelle
http://www.foodsci.uoguelph.ca/deicon/casein.gif
Additional information to better understand the pictures:
– Casein micelle is composed of millions of casein submicelles that are clumped together
– Casein micelle almost visible under microscope
– protein structure: different lines within structures represent different peptide structures that are well characterized (e.g. alpha helix, beta sheet)
•There are two distinct types of proteins in milk, casein and whey. Caseins make up over 80% of the total protein content and they can be further divided into five groups – the alphas1, alphas2, beta, gamma, and kappa caseins. Caseins do not have an organized structure, thus they cannot be denatured by heating.
•The casein molecules form polymers containing several identical and different molecules. Some molecules have hydrophillic regions and some have hydrophobic regions. The polymers are made up of many individual molecules and form into casein micelles. They are also formed into a colloidal solution, which gives skim milk its whitish-blue color.
•The alpha and beta caseins account for the calcium sensitivity of milk protein. Casein coagulates when calcium is added to casein solutions. This reaction is the key to cheese making and to the formation of acidic gels such as yogurt. Casein also coagulates when rennet is added. Rennet is a proteolytic enzyme that is found in the stomachs of calves.
•The amino acids in casein have hydrophobic and hydrophilic regions, allowing the caseins to act as highly effective surface active agents. In other words, caseins act as stabilizers of foams and emulsions.
•If an acid is added to milk or if acid producing bacteria are allowed to grow in milk, the casein micelle will change in two ways.
•First, colloidal calcium hydroxyphosphate, present in the casein micelle, will dissolve and form ionized calcium. This ionized calcium will penetrate the micelle structure and create strong internal calcium bonds.
•Second, the pH of the solution will approach the isoelectric points of the individual casein molecules.
•So you’re probably thinking great – what does this mean? What happens is this – the casein micelles get bigger because they aggregate with one another.
•The making of cottage cheese is a great example of this principle. Alton Brown from the Food Network has a recipe for cottage cheese. You can see it at this website: http://www.foodnetwork.com/food/recipes/recipe/0,,FOOD_9936_36973,00.ht ml
•Also, the other night I did a silly thing making dinner. I tried to make a lower fat lemony alfreado sauce using 2% milk instead of heavy cream. So the lemon lowered the pH of the milk and the casein in the milk coagulated and I made cottage cheese instead!
Milk Proteins
Whey
Alpha-lactalbumin
Beta-lactalbumin
3-D structure of beta- lactoglobulin
http://en.wikipedia.org/wiki/Beta- lactoglobulin
Whey –
•Whey is the milk serum proteins. When casein is precipitated out of skim milk, whey is left over. Casein are the curds and the liquid left over is the whey.
•Whey proteins are globular proteins which may be denatured when heated above temperature above 65oC. The major components of whey are beta lactoglobulin and alpha lactalbumin. Whey is the liquid remaining after milk has been curdled and strained during the manufacture of cheese. It is used to produce ricotta and brown cheeses and is an additive in many processed foods, such as breads, crackers, pastries, and animal feed.
•Many of the important nutrients that are comprise milk are partitioned into whey. It contains many important nutrients, such as lactose, water soluble vitamins, and most of the minerals, with the exceptions of considerable calcium and phosphorus which go with the casein into the cheese curd. The amino acid composition of whey is very close to that which is considered as the optimum amino acid value.
Alpha-lactalbumin
•Alpha-lactalbumin is considered the typical whey protein. It is present in milk from all mammals and it plays an important role in the synthesis of lactose in the udder.
Beta-lactalbumin
•This protein is found only in ungulates and is the major whey protein component of milk from cows.
