Effect of Heat on Milk

I) Effect on salt system: The heat-induced changes in the milk salt system can be covered under three cateogories:
  •  Readily reversible shift in salt balance by changes in temperature
  •  Irreversible shift in salt balance.
Variations in temperature and concentration adversely affect salt balance. Calcium phosphate is less soluble at high temperature than at low temperature. Thus, the concentration of soluble calcium and phosphate is decreased during heating. Dissolved or soluble calcium and phosphate during heating is transferred to the colloidal state.

This transfer action occurs on the colloidal micelles of caseinate phosphate. This transfer of soluble calcium and phosphate causes extensive changes in the structure of the micelles produced by heat treatment. Dissolved calcium and phosphate tend to revert to the original system but it is not completely transferred to the original structure after heat treatment. At the same time aggregation of the caseinate-phosphate micelles may occur (reversibly or irreversibly).

II) Effect on Acidity: During heat treatment CO 2 is removed from the milk system. This causes a decerase in acidity of milk. The effect is through the release of H + ions. This process is affected by the insolubilization of calcium and phosphate.

3Ca+++2HPO 4- → Ca 3 (PO 4 ) 2 +2 H +

On the basis of available data, heat treatment lead to an increase in the dissolved citrate in milk.

III) Effect on the milk proteins: The heat-induced changes in milk are of great practical importance to the dairy industry. During denaturation the original three-dimensional structure changes. Denaturation consists of non-proteolytic changes in the structure of protein. Amongst the heat induced changes caused by denaturation of whey proteins are:
  •  Development of cooked flavour
  •  Development of anti-oxygenic properties
  •  Impairment of clotting properties
  •  Imparting of soft curd characteristic to milk
  •  Prevention of age-thicking in evaporated milk
  •  Improvement in the baking quality for non-fat dry milk in the bakery industry
These changes are related to whey proteins. The whey proteins are present to the extent of 0.6 to 0.7% in milk. Beta-lactoglobulin is the major whey protein of milk accounting for 50 percent of the total whey proteins. The observed changes in milk are: release of H 2 S production, of cooked flavour, development of anti-oxygenic properties and lowering of curd tension. All these changes are related to whey proteins.

a) Heat denaturation of whey proteins: Heat denaturation of whey proteins occur between 68 0 C to 80 0 C. Heat denaturation starts from 68 0 C onwards when milk is heated for 30 minutes or 71 0 C for 15 minutes. The denaturation of whey proteins occurs at a higher temperature than pasteurization. The order of denaturation of whey proteins are immunoglobulin, blood serum albumin,beta-lactoglobulin while alpha-lactalbumin is the most heat resistant whey protein.

b) Changes associated with whey protein denaturation: Above 75 0 C –SH groups are released from whey protein, which are highly reducing in nature.These groups are susceptible for oxidation. The activation of-SH groups accompanies by an important phenomenon of anti -oxygenic property of heat-induced changes in whey protein. Sulphahydryal (-SH) groups are powerful reducing agent. The ability of these groups to bind oxygen results in anti-oxygenic property. As a result it lowers the oxidation-reduction potential of milk, which shows the activation of these groups. Formation and activation of -SH also results in the liberation of volatile sulphides. These volatiles also include H 2 S. The release of H 2 S is one of the most important component responsible for cooked flavour of milk. Cysteine amino acid containing maximum number of -SH group is responsible for producing H 2 S. Whey proteins are a rich source of cysteine and are a main cause of cooked flavour. Beta-lactoglobulin is very rich in -SH group.

Another important change resulting, as a function of heat denaturation of whey proteins is the soft curd forming property of milk. It is accompanied by two important changes in curd. These are the development of a soft curd characteristic in the curd and partial loss of clotting property in cheese manufacture. These are related to changes in the flocculation of serum protein particles. The impairment of milk clotting property seems to be due to interaction of casein with whey protein(beta-lactoglobulin). The denatured whey proteins bind with casein and thus affect its lotting property.Milk contains a factor, which affect the loaf volume of bread when milk is added during bread making. As a result volume of bread is depressed and slackens dough is produced. This defect can be overcome by heating milk. This is supported by the role of added skim milk powder to dough during bread making, which contain heat denatured whey proteins. Heat denaturation of whey proteins in skim milk powder is thus used as an index of baking quality.

There is loss of creaming property and increase in whitening of milk due to denauration of whey protein. Loss of creaming property has been attributed to the interactions between whey proteins notably immunoglobulins which interact with proteins of fat globules. This interaction affect the creaming ability. Cream layer formed in such milk is shallow and indistinct from normal milk. Reflectance or improvement in whitening has been attributed to a heat denatured state of milk proteins just before browning. At this stage flocculation of whey protein occur, along with aggregation of casein and conversion of soluble calcium to insoluble salt.

c) Destabilization of caseinate system: Caseinate-phosphate particles in milk exist in a precarious equilibrium with soluble Ca ++ and Mg ++ , dissolved salts and whey proteins. Slight changes occurring as a result of heating or changes in ionic environment through pH will alter this equilibrium. Casein binds Ca ++ and Mg ++ ions very strongly. Casein is stabilized in the system by charge it carries.
Heating causes pH changes which affect this process. The caseinate particles are very sensitive to changes in pH. Casein start precipitating below pH 6.0 and micelles precipitation starts at pH 5.2 to 5.3 where they still contain Ca ++ and Mg ++ attached to them. The manufacture of cottage cheese is based on the phenomenon of caseinate system by heat and acidity. During this process the destabilization of the caseinate particles leads to the formations of a smooth gel occupying the entire volume originally occupied by the milk. In this system a three-dimensional type network is formed that entraps the liquid along with gel structure formation or a network and a semi-solid system is formed. On applying heat to this system at cooking stage of the process, the caseinate particles become more closely knit together, water is expelled, and the clot shrinks. A desirable product is obtained by judicious use of pH and proper heat treatment.

The calcium caseinate phosphate micelles are readily precipitable by addition of various salts such as ammonium sulphate and urea. Heating hastens the process.This is the basis of producing various fractions of casein. The effects of heat and divalent cations are important from the view point of rennet action and heat. In this phenomenon ionic concentration and heat play an important role in the stability of casein micelles. Phosphate and citrate ordinarily exert an opposite effect over Ca ++ and Mg ++ because they form undissociated complexes with Ca ++ and Mg ++ .

Some milk apparently are stabilized by added calcium and destabilized by ions such as phosphate and citrate that sequester calcium. Observations of this type are the basis of the well-known salt balance theory first suggested by Sommer and Hart(1926). This theory holds that optimum stability depends on a certain ratio of calcium and magnesium ions to those of phosphate and citrate. The concept has been of great practical utility in developing practical procedures for controlling the stability of evaporated milk during heat sterilization. In practice evaporated milk to be sterilized is treated, as a series of samples on a pilot scale with graded level of phosphate or Ca the later being rarely if ever necessary. The samples are then sterilized and after cooling the minimum level of added salt that imparted satisfactory stability is noted and used to stabilize the lot of milk to be sterilized.

IV. Forewarming process and heat stability: Before sterilization in the preparation of evaporated milk forewarming of milk provides heat stability to milk. Generally,heating milk at 95 0 C for 10 minutes provide heat stability to milk. It has been shown that a high temperature short time process of heat treatment provides a better heat stability. However, it may be stated that this phenomenon of heat stability is complex and depends upon other factors such as quality of milk, storage temperature of milk, etc.

V. Browning of milk: Browning reactions in milk and milk products are the manifestation of heat induced processing of milk. Browning reaction occur due to changes related with pH, storage conditions, moisture content, relative humidity and temperature of processing and storage of milk and milk products. Browning reaction is absent in pasteurized milk but is evident in highly heated sterilized milk on storage. Browning reaction occurs in two forms on heating. The two types of browning in relation to heating are 

(a) amino sugar or Maillard browning and 

(b) non-amino browning or caramelization. 

a) Amino Sugar or Maillard browning: Two components are responsible for this browning reaction. They are milk protein particularly casein and lactose present in milk and milk products. Phosphate salts and whey proteins make minor contribution in browning reaction. Browning reaction is complex.

The reaction occurs between aldehyde groups (-CHO) of sugars and amino groups (-NH 2 ) of amino acids. They together start the browning reaction which ultimately lead to the formation of brown pigment melanoidin.

b) Caramelization or non-amino browning: Caramelization or browning may be defined as the heat decomposition of sugar as a function of pH and buffers in the absence of amino compounds. It requires a relatively high order of heat energy. On the other hand, Maillard type browning requires a relatively low order of energy for its initiation and exhibit autocatalytic qualities once it has started. Caramelization is desirable in milk based products such as caramelized flavour which is desirable and liked.

c) Changes related to browning: Along with browning many complex reactions also occur with the formation of various compounds. In addition,fluorescent and reducing substances, various sugar fragments and flavour compounds are formed. Many of these are detected before browning starts.

These changes have great practical utility. Notable amongst these is the development of flavour especially caramelized flavour. Following changes related to browning can occur:
Compound formation: A large number of lactose degradationcompounds are formed. These include furfuryl alcohol, furfuryl aldehyde,
maltol, acetol, acetaldehyde, acetic, formic and pyruvic acid, NH 3 , H 2 S and CO 2
 Reducing substances: Heated and dried milk contain’s a complex reducing system involving –SH compounds, ascorbic acid and substances
associated with browning reaction. Heating concentrated milk for a similar period has a significant effect on browning reaction.

d) Factors affecting browning of milk: The principle factors responsible for browning in milk are:

i)pH: A pH above 6.8 favours browning reaction. This defect is predominant in evaporated milk where pH of milk plays an important role. Due to variations in pH and protein concentration in differentmilks browning is affected due to these variations. This is due to release of protons during heating. As the pH is raised above pH 6.6 browning reaction occurs at a faster rate.

ii) Storage and temperature: Higher temperature and prolonged storage period favours browning. These changes are favoured in the presence of increased humidity and moisture. Colour intensity increases with storage time and is highest at a storage temperature of 40 0 C.

iii) Total solids concentration: During concentration of milk total solids concentration increases. As the total solids concentration in milk increases the browning reaction also gains momentum. Lactose plays a major part of total solids concentration along with casein. The interaction results in increased browning.

iv) Heat treatment: Heating milk as a pre-heat treatment between 85- 100 0 C for 30 minutes or more favours browning. It is one of the most important factors of browning. Reducing the heating time such as with HTST process will reduce the browning of milk products.

v) Oxygen: Oxygen favours browning as it reacts with –SH groups released during heating. Presence of oxygen destroys these reducing
groups. Problem can be reduced by replacing O 2 with N 2 while storing heated and dried milk products.

e) Prevention of Browning: Browning can be prevented to a great extent by storing milk and milk products at low temperatures and short period of storage. In dried products moisture should be below 5%. Also N 2 packing helps in reducing browning due to replacement of oxygen. Strong and long duration heating should be avoided.

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