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Milk fat is a mixture of different fatty-acid esters called triglycerides, which are composed of an alcohol called glycerol and various fatty acids. In saturated fatty acids, the carbon atoms are linked together in a chain by single bonds, while in unsaturated fatty acids there are one or more double bonds in the hydrocarbon chain, see Figure 2. Each glycerol molecule can bind three fatty-acid molecules, and as the three need not necessarily be of the same kind, the number of different glycerides in milk is extremely large, see Figure 2.
Table 2. The first three are solid and the last is liquid at room temperature. As the quoted figures indicate, the relative amounts of the different fatty acids can vary considerably. This variation affects the hardness of the fat.
Fat with a high content of high-melting fatty acids, such as palmitic acid, will be hard; on the other hand, fat with a high content of low-melting oleic acid makes soft butter. Determining the quantities of individual fatty acids is a matter of purely scientific interest. For practical purposes, it is sufficient to determine one or more constants or indices which provide certain information concerning the composition of the fat.
Fatty acids with the same numbers of C and H atoms but with different numbers of single and double bonds have completely different characteristics. The most important and most widely used method of indicating their specific characteristics is to measure the iodine value IV of the fat. The iodine value states the percentage of iodine that the fat can bind. Iodine is taken up by the double bonds of the unsaturated fatty acids.
Since oleic acid is by far the most abundant of the unsaturated fatty acids, which are liquid at room temperature, the iodine value is largely a measure of the oleic-acid content and thereby of the softness of the fat. The iodine value of butterfat normally varies between 24 and The variations are determined by what the cows eat. Green pasture in the summer promotes a high content of oleic acid, so that summer milk fat is soft high iodine value. Certain fodder concentrates, such as sunflower cake and linseed cake, also produce soft fat, while coconut and palm oil cake and root vegetable tops produce hard fat.
It is therefore possible to influence the consistency of milk fat by choosing a suitable diet for the cows. The iodine value is a measure of the oleic acid content of the fat. The amount of different fatty acids in fat also affects the way it refracts light. It is therefore common practice to determine the refractive index of fat, which can then be used to calculate the iodine value.
This is a quick method of assessing the hardness of the fat. Instead of analysing the iodine value or refractive index, the ratio of saturated fat to unsaturated fat can be determined by pulsed NMR. A conversion factor can be used to transform the NMR value into a corresponding iodine value if desired. The NMR method can also be utilized to find out the degree of fat crystallization as a function of the time of crystallization.
The NMR value of butterfat normally varies between 30 and During the crystallization process, the fat globules are in a very sensitive state and are easily damaged and opened up — even by moderate mechanical treatment. Electron microscope studies have shown that fat crystallizes in monomolecular spheres, see Figure 2. At the same time fractionation takes place, so that the triglycerides with the highest melting points form the outer spheres.
Because crystallized fat has a lower specific volume than liquid fat, tensions arise inside the globules, making them particularly unstable and susceptible to breakage during the crystallization period. The result is that liquid fat is released into the milk serum, causing formation of lumps where the free fat glues the unbroken globules together the same phenomenon that occurs in butter production.
It is important to bear this important property of milk fat in mind in production of cream for various purposes. The crystallization curve is based on analysis made by the NMR method. Proteins are an essential part of our diet. The proteins we eat are broken down into simpler compounds in the digestive system and in the liver. The great majority of the chemical reactions that occur in the organism are controlled by certain active proteins, the enzymes.
Proteins are giant molecules built up of smaller units called amino acids, Figure 2. A protein molecule consists of one or more interlinked chains of amino acids, where the amino acids are arranged in a specific order. A protein molecule usually contains around — linked amino acids, but both smaller and much larger numbers are known to constitute a protein molecule. R in the figure stands for organic material bound to the central carbon atom.
The amino acids in Figure 2. The amino acids belong to a group of chemical compounds which can emit hydrogen ions in alkaline solutions and absorb hydrogen ions in acid solutions. Such compounds are called amphotery electrolytes or ampholytes. Proteins are built from 20 amino acids.
An important fact with regard to nutrition is that eight nine for infants of the 20 amino acids cannot be synthesized by the human organism. As they are necessary for maintaining a proper metabolism, they have to be supplied with the food. They are called essential amino acids , and all of them are present in milk protein.
The type and the order of the amino acids in the protein molecule determine the nature of the protein. Any change of amino acids regarding type or place in the molecular chain may result in a protein with different properties. As the possible number of combinations of 20 amino acids in a chain containing — amino acids is very large the number of proteins with different properties is also very large. As mentioned before, amino acids contain both a slightly basic amino group —NH 2 and a slightly acid carboxyl group —COOH.
These groups are connected to a side chain, R. If the side chain is polar, the water-attracting properties of the basic and acid groups, in addition to the polar side chain, will normally dominate and the whole amino acid will attract water and dissolve readily in water.
Such an amino acid is named hydrophilic water-loving. If on the other hand the side chain is of hydrocarbon which does not contain hydrophilic radicals, the properties of the hydrocarbon chain will dominate. A long hydrocarbon chain repels water and makes the amino acid less soluble or compatible with water. Such an amino acid is called hydrophobic water-repellent. If there are certain radicals such as hydroxyl —OH or amino groups —NH 2 in the hydrocarbon chain, its hydrophobic properties will be modified towards more hydrophilic.
If hydrophobic amino acids are predominant in one part of a protein molecule, that part will have hydrophobic properties. An aggregation of hydrophilic amino acids in another part of the molecule will, likewise, give that part hydrophilic properties. A protein molecule may therefore be either hydrophilic, hydrophobic, intermediate or locally hydrophilic and hydrophobic. Some milk proteins demonstrate very great differences within the molecules with regard to water compatibility, and some very important properties of the proteins depend on such differences.
Hydroxyl groups in the chains of some amino acids in casein may be esterified with phosphoric acid. Such groups enable casein to bind calcium ions or colloidal calcium hydroxyphosphate, forming strong bridges between or within the molecules. The side chains of some amino acids in milk proteins are charged, which is determined by the pH of the milk. When the pH of milk is changed by addition of an acid or a base, the charge distribution of the proteins is also changed.
The electrical status of the milk proteins and the resulting properties are illustrated in Figures 2. The protein molecules remain separated because identical charges repel each other. If hydrogen ions are added, Figure 2.
At a pH value where the positive charge of the protein is equal to the negative charge, i. The protein molecules no longer repel each other, but the positive charges on one molecule link up with negative charges on the neighbouring molecules and large protein clusters are formed. The protein is then precipitated from the solution. The pH at which this happens is called the isoelectric point of the protein. In the presence of an excess of hydrogen ions, the molecules acquire a net positive charge as shown in Figure 2.
Then they repel each other once more and therefore remain in solution. If, on the other hand, a strong alkaline solution NaOH is added, all proteins acquire negative charges and dissolve. Milk contains hundreds of types of protein, most of them in very small amounts. The proteins can be classified in various ways according to their chemical or physical properties and their biological functions.
The old way of grouping milk proteins into casein, albumin and globulin has given way to a more adequate classification system. Minor protein groups have been excluded for the sake of simplicity. Whey protein is a term often used as a synonym for milk-serum proteins, but it should be reserved for the proteins in whey from the cheese making process.
In addition to milk-serum proteins, whey protein also contains fragments of casein molecules. Some of the milk-serum proteins are also present in whey in lower concentrations than in the original milk. This is due to heat denaturation during pasteurization of the milk prior to cheese-making.
The three main groups of proteins in milk are distinguished by their widely different behaviour and form of existence. The caseins are easily precipitated from milk in a variety of ways, while the serum proteins usually remain in solution. The fat-globule membrane proteins adhere, as the name implies, to the surface of the fat globules and are only released by mechanical action, e. Casein is a mixture of several components Table 2. Genetic variants of a protein differ from each other only by a few amino acids.
The caseins self-associate and form large clusters called micelles. The micelles are built up of hundreds and thousands of individual casein protein molecules and vary in size from 50 to nm. Since the micelles are of colloidal dimensions they are capable of scattering light and the white colour of skim milk is largely due to light scattering by the casein micelles.
Rollema H. Elsevier Science Publications Ltd. Casein micelles have important consequences for the properties of milk. They determine to a large extent the physical stability of milk products during heating and storage, are essential in cheese making and determine rheological properties of fermented and concentrated dairy products.
Casein micelles are fairly dense aggregates with small regions of calcium phosphate, which links the micelles together, giving the micelles an open, porous structure. Removal of calcium phosphate CCP — colloidal calcium phosphate , e. Disintegration also occurs when pH becomes greater than 9.
The internal structure of a casein micelle has been under debate for a long time and is still not fully understood. There are three main models proposed: the nanocluster model, the dual binding model and the sub-micelle model. There is, however, consensus around several characteristics. The micelles are roughly spherical particles with an average diameter of about nm but with a large spread in size. If the hairy layer is removed e. The forces holding the micelle together are hydrophobic interactions between protein groups, cross-links between peptide chains by the nanoclusters and ionic bonds.
The nanocluster model Figure 2. The dual-binding model Figure 2. The sub-micelle model Morr ; Slattery and Evard ; Walstra suggest that the casein micelle is built up of smaller micelles, sub-micelles some nm in diameter, which are linked together by calcium phosphate clusters see figure 2. A casein micelle structure is not fixed, but dynamic.
A casein micelle and its surrounding keep exchanging components. It responds to changes in the micellar environment, temperature, pH and pressure. In an intact micelle there is a surplus of negative charges, so they repel each other. If the hydrophilic sites are removed, water will start to leave the structure. This gives the attracting forces room to act. New bonds are formed, one of the salt type, where calcium is active, and the second of the hydrophobic type.
These bonds will then enhance the expulsion of water and the structure will finally collapse into a dense curd. The explanation of this phenomenon is that b - casein is the most hydrophobic casein, and that the hydrophobic interactions are weakened when the temperature is lowered.
Micelles appear to disintegrate and the voluminosity of the casein micelles increases. The loss of CCP causes a weaker attraction between individual casein molecules. These changes make the milk less suitable for cheese making, as they result in longer renneting time and a softer curd. The graph in Figure 2. On increasing the temperature, the micelles shrink somewhat and the amount of CCP increases.
When serum proteins are present during heating , the serum proteins become associated with casein micelles during their heat denaturation and they largely become bound to the micelle surface. Most of these associations cannot be reversed by cooling. One characteristic property of casein is its ability to precipitate. Due to the complex nature of the casein molecules, and that of the micelles formed from them, precipitation can be caused by many different agents.
It should be observed that there is a great difference between the optimum precipitation conditions for casein in micellar and non-micellar form, e. The following description refers mainly to precipitation of micellar casein. The pH will drop if an acid is added to milk or if acid-producing bacteria are allowed to grow in milk. This will change the environment of the casein micelles in two ways. The course of events is illustrated in Figure 2. Firstly colloidal calcium hydroxyphosphate, present in the casein micelle, will dissolve and form ionized calcium, which will penetrate the micelle structure and create strong internal calcium bonds.
Secondly the pH of the solution will approach the isoelectric points of the individual casein species. Both methods of action initiate a change within the micelles, starting with growth of the micelles through aggregation and ending with a more or less dense coagulum. Depending on the final value of the pH, this coagulum will either contain casein in a salt form, or casein in its isoelectric state, or both.
The isoelectric points of the casein components depend on the ions of other kinds present in the solution. Theoretical values, valid under certain conditions, are pH 5. In salt solutions, similar to the condition of milk, the range for optimum precipitation is pH 4.
A practical value for precipitation of casein from milk is pH 4. If a large excess of sodium hydroxide is added to the precipitated iso-electric casein, the redissolved casein will be converted into sodium caseinate , partly dissociated into ions. The pH of cultured milk products is usually in the range of 3.
In the manufacture of casein from skim milk by the addition of sulphuric or hydrochloric acid, the pH chosen is often 4. From an enzymatic point of view the bond between amino acids phenylalanine and methionine is easily accessible to many proteolytic enzymes.
Some proteolytic enzymes will attack this bond and split the chain. The soluble amino end contains amino acids to , which are dominated by polar amino acids and the carbohydrate, which gives this sequence hydrophilic properties. The formation of the curd is due to the sudden removal of the hydrophilic macropeptides and the consequent imbalance in intermolecular forces.
Bonds between hydrophobic sites start to develop and are enforced by calcium bonds that develop as the water molecules in the micelles start to leave the structure. This process is usually referred to as the phase of coagulation and syneresis. There is also a tertiary phase of rennet action, where the rennet attacks the casein components in a more general way. This occurs during cheese ripening.
The durations of the three phases are determined mainly by pH and temperature. In addition the secondary phase is strongly affected by the calcium ion concentration and by the condition of micelles with regard to absence or presence of denatured milk serum proteins on the surfaces of the micelles.
Whey protein is the name commonly applied to milk serum proteins. If the casein is removed from skim milk by a precipitation method, such as the addition of mineral acid, a group of proteins remains in solution that are called milk serum proteins. As long as they are not denatured by heat, they are not precipitated at their isoelectric points. When milk is heated, some of the whey proteins denature and form complexes with casein, thereby decreasing the ability of the casein to be attacked by rennet and to bind calcium.
Curd from milk heated to a high temperature will not release whey as ordinary cheese curd does, due to the smaller number of casein bridges within and between the casein molecules. Their amino acid composition is very close to that which is regarded as a biological optimum. Whey protein derivatives are widely used in the food industry. It is present in milk from all mammals and plays a significant part in the synthesis of lactose in the udder.
At high temperatures, sulphurous compounds such as hydrogen sulphide are gradually released. Fox and P. McSweeney, Dairy Chemistry and Biochemistry, This protein group is extremely heterogeneous, and few of its members have been studied in detail, Figure 2.
Immunoglobulins are antibodies synthesized in response to stimulation by specific antigens. They are specifically present in blood. In this way, bacteria can also be flocculated on fat globules and accumulate in the cream layer. When microorganisms are flocculated, their growth and action can be significantly inhibited. The agglutination reaction is specific with respect to a particular antigen. The proteins involved are called cryoglobulins.
The agglutinins are inactivated by heat treatment and their ability to flocculate particles disappears. Because of that, agglutination does not occur in pasteurized milk. In the future, many substances of importance will probably be isolated on a commercial scale from milk serum or whey. Lactoferrin and lactoperoxidase are substances of possible use in the pharmaceutical and food industries, and are now isolated from whey by a commercial process.
Lactoferrin is also an inhibitor of bacteria including B. The inhibition is caused by removal of iron from their serum. Membrane proteins are a group of proteins that form a protective layer around fat globules to stabilize the emulsion, Figure 2.
Their consistency ranges from soft and jelly-like, in some of the membrane proteins, to rather tough and firm in others. Some of the proteins contain lipid residues and are called lipoproteins. The lipids and the hydrophobic amino acids of those proteins make the molecules direct their hydrophobic sites towards the fat surface, while the less hydrophobic parts are oriented towards the water.
Weak hydrophobic membrane proteins attack these protein layers in the same way, forming a gradient of hydrophobia from fat surface to water. The gradient of hydrophobia in such a membrane makes it an ideal place for adsorption for molecules of all degrees of hydrophobia. Phospholipids and lipolytic enzymes in particular are adsorbed within the membrane structure.
No reactions occur between the enzymes and their substrate as long as the structure is intact, but as soon as the structure is destroyed the enzymes have an opportunity to find their substrate and start reactions. An example of enzymatic reaction is the lipolytic liberation of fatty acids when milk has been pumped cold with a faulty pump, or after homogenization of cold milk without immediate pasteurization afterwards.
As long as proteins exist in an environment with a temperature and pH within their limits of tolerance, they retain their biological functions. But if they are heated to temperatures above a certain maximum their structure is altered. They are said to be denatured, Figure 2. The same thing happens if proteins are exposed to acids or bases, to radiation or to high pressure.
The proteins are denatured and lose their original solubility. When proteins are denatured, their biological activity ceases. Enzymes, a class of proteins whose function is to catalyse reactions, lose this ability when denatured.
The reason is that certain bonds in the molecule are broken, changing the structure of the protein. After a weak denaturation, proteins can sometimes revert to their original state, with restoration of their biological functions. In many cases, however, denaturation is irreversible.
The proteins in a boiled egg, for example, cannot be restored to the raw state. Milk contains a large number of substances which can act either as weak acids or as weak bases, e. In chemistry, such a system is called a buffer solution because, within certain limits, the pH value remains constant when acids or bases are added. This effect can be explained by the characteristic qualities of the proteins. The pH value, however, is hardly affected at all, as the increase in the concentration of free hydrogen ions is very small.
Because of this, the pH value remains more or less constant, see figure 2. The more base that is added, the greater the number of hydrogen ions released. Other milk constituents also have this ability to bind or release ions, and the pH value therefore changes very slowly when acids or bases are added.
Almost all of the buffering capacity is utilized in milk that is already acidic due to long storage at high temperatures. In such a case it takes only a small addition of acid to change the pH value. If an alkali is added to acid the pH of the solution rises immediately — there is no buffering action. If an alkali is added to milk the pH changes very slowly — there is a considerable buffering action in milk.
Enzymes are proteins having the ability to trigger chemical reactions and to affect the course and speed of such reactions. Enzymes do this without being consumed. They are therefore sometimes called biocatalysts. The functioning of an enzyme is illustrated in Figure 2.
The action of enzymes is specific; each type of enzyme catalyses only one type of reaction. Two factors which strongly influence enzymatic action are temperature and pH. At these temperatures the enzymes are more or less completely denatured inactivated. The temperature of inactivation varies from one type of enzyme to another — a fact which has been widely utilized for the purpose of determining the degree of pasteurization of milk.
Enzymes also have their optimum pH ranges; some function best in acid solutions, others in an alkaline environment. The former are normal constituents of milk and are called original enzymes. The latter, bacterial enzymes , vary in type and abundance according to the nature and size of the bacterial population. Several of the enzymes in milk are utilized for quality testing and control.
Among the more important ones are peroxidase, catalase, phosphatase and lipase. The molecule splits. The enzyme is now free to attack and split another molecule in the same way. Peroxidase transfers oxygen from hydrogen peroxide H 2 O 2 to other readily oxidizable substances. Catalase splits hydrogen peroxide into water and free oxygen. By determining the amount of oxygen that the enzyme can release in milk, it is possible to estimate the catalase content of the milk and learn whether or not the milk has come from an animal with a healthy udder.
Milk from diseased udders has a high catalase content, while fresh milk from a healthy udder contains only an insignificant amount. There are, however, many bacteria that produce this kind of enzyme. Phosphatase has the property of being able to split certain phosphoric-acid esters into phosphoric acid and the corresponding alcohols.
The presence of phosphatase in milk can be detected by adding a phosphoric-acid ester and a reagent that changes colour when it reacts with the liberated alcohol. A change in colour reveals that the milk contains phosphatase. The routine test used in dairies is called the phosphatase test according to Scharer. The phosphatase test should preferably be performed immediately after heat treatment.
The analysis should be carried out the same day, otherwise a phenomenon known as reactivation may occur, i. Cream is particularly susceptible in this respect. Lipase splits fat into glycerol and free fatty acids, see figure 2. Excess free fatty acids in milk and milk products result in a rancid taste.
The action of this enzyme seems, in most cases, to be very weak, though the milk from certain cows may show strong lipase activity. The quantity of lipase in milk is believed to increase towards the end of the lactation cycle. Lipase is, to a great extent, inactivated by pasteurization, but higher temperatures are required for total inactivation. Many microorganisms produce lipase. This can cause serious problems, as the enzyme is very resistant to heat.
Lactose is a sugar found only in milk; it belongs to the group of organic chemical compounds called carbohydrates. Carbohydrates are the most important energy source in our diet. Bread and potatoes, for example, are rich in carbohydrates, and provide a reservoir of nourishment. They break down into high-energy compounds that can take part in all biochemical reactions, where they provide the necessary energy.
Carbohydrates also supply material for the synthesis of some important chemical compounds in the body. They are present in muscles as muscle glycogen and in the liver as liver glycogen. Glycogen is an example of a carbohydrate with a very large molecular weight.
Other examples are starch and cellulose. Such composite carbohydrates are called polysaccharides and have giant molecules made up of many glucose molecules. In glycogen and starch the molecules are often branched, while in cellulose they are in the form of long, straight chains. The molecules of sucrose ordinary cane or beet sugar consist of two simple sugars monosaccharides , fructose and glucose.
Lactose milk sugar is a disaccharide, with a molecule containing the monosaccharides glucose and galactose. Lactose is transported into the bacterial cell where enzymes attack the lactose, splitting it into glucose and galactose. Other enzymes from the lactic-acid bacteria then attack the glucose and galactose, which are converted via complicated intermediary reactions into mainly lactic acid.
The enzymes involved in these reactions act in a certain order. This is what happens when milk goes sour; lactose is fermented to lactic acid. Other microorganisms in the milk generate other breakdown pro-ducts. If milk is heated to a high temperature, and is kept at that temperature, it turns brown and acquires a caramel taste. This process is called caramelization and is the result of a chemical reaction between lactose and proteins called the Maillard reaction.
Maillard reactions are initiated by heat treatment and continue during storage of the product. The reaction kinetics is directly dependent on factors such as heat load and storage temperature. Lactose is water soluble, occurring as a molecular solution in milk.
In cheese making, most of the lactose remains dissolved in the whey. Evaporation of whey in the manufacture of whey cheese increases the lactose concentration further. Lactose is not as sweet as other sugars; it is about 30 times less sweet than cane sugar, for example. Vitamins are organic substances that occur in very small concentrations in both plants and animals.
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