In this article we will discuss about:- 1. Milk as a Raw Material and Food 2. Milk Production 3. Composition 4. Physical Properties 5. Chemical Properties 6. Detection of Adulteration 7. Detection of Water.
Contents:
- Milk as a Raw Material and Food
- Production of Milk
- Composition of Milk
- Physical Properties of Milk
- Chemical Properties of Milk
- Detection of Adulteration in Milk
- Detection of Water in Milk
1. Milk as a Raw Material and Food:
Milk is the liquid excreted by the mammary glands of both human (women) and mammals, which are serving the newborn as food, within the context of dairy technology or science the following definition can be used.
Milk is the product obtained by simple or multiple milking of mammals that are kept for the purpose of milk production.
In majority of cases, the term is used for cow’s milk, which is produced as raw material in agricultural institutions and is treated as in dairies. Milk of other kinds of animals kept for milk production is labelled specifically (goat milk, sheep milk, buffalo milk and others) and the quality requirements are similar to those for cow’s milk.
Apart from the milk proper, a variety of products based on milk are of importance for feeding humans and animals. Milk based products can be divided into milk products and recombined milk products.
Milk products are made entirely from milk. In certain cases additives can be used during the manufacture, but they cannot replace milk component either partially or completely. Recombined milk products are products in which milk or milk components constitute are major part. Milk components cannot be replaced by additives.
2. Production of Milk:
Milk production in India is dominated by small and marginal farmers and by landless labourers who, in aggregate own about 70% of the National Milch Herd. The animal’s nutritional requirements are largely met by agricultural waste and by products.
India is now occupying first position regarding its milk production among the developing countries. The present production of milk in India is 133 million tonnes (According to Annual Report, 2012-13). The per capita availability of the milk has reached a level of 290 gm per day during the year 2011-12, which is more than the world average of 284 gm per day.
3. Composition of Milk:
Milk contains several substances which could be classified under two major heads, viz. liquid and solids. Water in milk forms the liquid base in which all solids are dissolved. Milk solids such as fat, protein, lactose and minerals, when all taken together, are termed as Total Solids (TS) in milk. These solids are further grouped into two categories namely fat and Solids Not Fat (SNF). All the constituents except fat are known as milk serum.
Milk is the most common balanced food prepared by nature for a newly born animal. However, by employing modern techniques the production potential of a milch animal is increased beyond the requirement of its young one so that human also benefit from the nature’s gift of balanced food. Commercialisation of milk and milk products necessitates the bulk handling of milk in dairy plants big or small, at the rural areas or urban areas by illiterate, semi-literate or illiterate people.
However, methods of such handling such as collecting, storing, pasteurising, packing, transporting etc., are to a great extend determined by the physical and chemical properties of the milk. The most common properties among them are explained for the benefit of the common people.
4. Physical Properties of Milk:
The physical properties of milk are freezing point, colour, specific gravity, boiling point, refractive index, electrical conductivity, coefficient of thermal expansion, thermal conductivity, velocity, surface tension and germicidal property.
These are briefly described as follows:
1. Taste and Odour:
To most of the people, normal milk tastes slightly sweet with its mild aromatic flavour. The sweet taste comes from the flavour and aroma principally from milk sugar and butter fat.
When milk is produced under uncleaned surroundings or from animals which are in their late lactation, the taste and aroma of milk are adversely affected. Feeding of certain weeds or fodder and infection of udder also cause abnormal flavour and taste in milk.
2. Colour:
Milk contains several pigments which impart colour to it. Cow’s milk is always slightly yellow in colour due to the presence of carotene.
The intensity of yellow colour in cow’s milk increases when they are fed with more green fodder. Buffalo milk is white due to the absence of carotene. White opaque colour of milk seen in reflected light is caused by the interference with the passage of light caused by the fat and the colloidally dispersed substances.
When milk fat is removed from the milk, then skimmed milk shows greenish tint which is due to the presence of riboflavin or lactochrome. Riboflavin has a tendency to convert all carotene into vitamin-A which is a colourless substance.
3. Specific Gravity:
The term Specific Gravity (SG), as applied to milk means the weight of given volume of milk compared with the weight of the same volume of water at the same temperature. Generally, 15.6° is the temperature at which SG of milk is determined. Lactometer is used to determine the SG of milk.
The standard lactometers are calibrated to read correct when the temperature of milk is 15°C. The most common lactometer used for finding out the SG of milk are zeal and quevenne make. The SG of milk can also be determined by determining the weight of a known volume for which either pycnometer or a hydrostatic balance may be used.
However, the most common method of determining scale of lactometer is calibrated not in terms of volume but as a function either density or specific gravity.
Milk contains many constituents whose SG is greater than that of water. Hence, its SG should always be more than one. Fat is the only important constituent in milk which has SG less than one. The SG of individual constituents found in milk is approximately as follows water 1.0, fat 0.93, protein 1.346, lactose 1.066 and mineral salts 4.12.
The SG of milk is influenced by the proportion of its constituents. Fat, being the most variable ingredient of milk, has maximum influence on the SG of milk. Increase in the fat content of milk results in lowering of SG and decrease in the fat content of milk increases the specific gravity.
A SG variation of 1.0289 to 1.034 is observed in milk. Generally in cow’s milk it ranges from 1.028 to 1.030 and from 1.030 to 1.032 in buffaloes milk. The SG of skimmed milk should always be higher than the whole milk from which it is obtained. Skimmed milk from cow’s milk will have approximately the same SG of buffaloes skimmed milk, of which 1.036 may be the corresponding value.
It also varies with temperature. At higher, temperature, lactometer shows lower reading whereas, at lower temperature, it gives higher reading. The SG of milk is determined by using the following formula
Specific Gravity = 1 + (CLR / 1000)
Where in, (CLR = Corrected Lactometer Reading)
Addition of water or milk fat causes decrease in the specific gravity while addition of solids raises the specific gravity of milk.
Due to wide variation in SG of normal milk, it is not practically possible to detect the adulteration of milk unless SG values fall beyond the normal range. However, it is a quick test to detect the adulteration with water and other solids. Quvenne lactometer is designed to determine the SG of milk at 15.5°C, whereas the Watson lactometer is used at a temperature of 39° C when all the fat is in liquid form.
4. Boiling Point:
The boiling point of any material in the temperature pressure, the material is in equilibrium both as a liquid and as a gas. This is the temperature at which the liquid phase will vaporise and the gas phase will condense or liquify, according to the heat supply, the latent heat of vaporisation is involved at the liquid/gas equilibrium. Water boils at 100°C under normal atmospheric temperature and pressure.
The presence of dissolved substances increases the boiling point of a solution. Since, milk contains several dissolved substances, it has higher boiling temperature than that of water. Because there is a variation in the dissolved substances of milk, the boiling point of milk also varies. The boiling point of both cow and buffalo milk ranges from 100.2°C to 101°C with an average of 100.5°C.
5. Freezing Point:
The freezing point of a material is the temperature at a given pressure, a material is in equilibrium between solid and liquid state. This is the temperature at which the liquid phase may freeze or crystallise and the solid phase may melt or liquify. The latent heat of fusion is involved in all this solid liquid equilibrium.
Pure distilled water freezes at 0°C under normal atmospheric pressure. Milk freezes at a temperature slightly lower than that of water due to the soluble constituents such as lactose, soluble salts, etc., which lowers or depresses the freezing point. The freezing point of cow or buffalo milk ranges from – 0.535° to – 0.55°C with an average of – 0.545°C. The addition of 1% water to milk will raise the freezing point by 0.006°C.
Despite the variation in the normal freezing point of milk, it is one of the most constant physical properties of milk. It is therefore considered to be very useful in detecting adulteration of milk with water.
The lactose minerals and dissolved CO2 are the principal contributors, to the freezing point of milk. The fat, proteins and colloids do not contribute appreciably to the osmotic pressure and, hence, to the freezing point.
6. Refractive Index:
Refractive index is a number indicating the speed of light in a given medium, usually as the ratio of the speed of light in a vacuum, or air, to that in the given medium. The refractive index of milk is sometimes used to indicate adulteration, especially watering.
The refraction of light by a solution depends upon the individual molecular species present and their concentrations. The total refraction is sum of the individual refractions of the constituents present is solution. The refractive index of milk, then, is the refractive index of the solvent plus the indices of the solutes. The refractive index of the aqueous phase ranges from 1.3440 to 1.3480 for milk.
The freezing point determination is more reliable than the refractive index for determining adulteration by water.
7. Electrical Conductivity:
Electrical conductivity is the property of a substance to transfer an electrical charge. In a pure solution, the conductivity is a function of the ionic concentration.
In a heterogeneous system, such as what exists in milk, the fat and the colloidally dispersed substances obstruct the ions in their migration and decrease the conductivity. The electrical conductivity is due to the presence of sodium, potassium and chlorides in milk.
The electrical conductivity increases with increase in temperature. The average value at 300°C is 3.67 for buffalo milk, and 4.67 million/cm for cow milk. The conductivity test is not a satisfactory method for detection of adulteration of milk by water but can be used to detect added neutralisers and abnormal milk.
8. Specific Heat:
The specific heat is defined as the amount of heat needed to raise 1gm of the substance through 1°C. It usually ranges between 0.920 and 0.930 for the milk. It changes with temperature. Milk has highest specific heat (0.938) at 15°C.
9. Coefficient of Thermal Expansion:
Milk expands when heated and contracts when cooled. The ratio of increase in volume per degree of increase in temperature is known as Coefficient of Expansion.
10. Thermal Conductivity:
The thermal conductivity is the quantity of heat transferred in unit time through a layer of unit thickness and unit cross-sectional area when the temperature difference between the two surfaces of the layer is 1 Kelvin (1 Kelvin = 1 Celsius). It is an important property which determines how fast milk is cooled or heated. Increasing temperature increases milk thermal conductivity. The higher the fat content the lower is the thermal conductivity. Thermal conductivity of milk at 30°C is 13.5 x 104 cal cm4 S4 C4.
11. Viscosity:
The viscosity of a substance refers to its resistance of flow. It is a measure of the friction between molecules as they slide past one another.
The viscosity of a heterogeneous substance such as milk at a given temperature depends upon its composition and the physical state of its colloidally dispersed substances, including milk fat.
The net viscosity temperature relation, is however, temperature coefficient. At normal temperature, viscosity of milk varies from 1.5 to 2 centipoises. The viscosity of milk is determined by viscosimeter. The viscosity of milk is always higher than the viscosity of water. This is due to the presence of dissolved solids in the milk. Homogenisation increases the viscosity of milk.
12. Surface Tension:
Surface tension is due to the force of attraction between molecules. These forces of attraction coverage to the centre of molecules from all direction.
The surface tension of milk at 20°C is 54.5 dynes/cm. It decreases as the temperature rises and at 60°C, it is about 40 to 45 dynes/cm. The presence of fat lowers the surface tension. Whole milk has a slightly lower surface tension than skimmed milk and that of cream is still lower. Milk and cream on aging undergo a slight decrease in surface tension.
The general fact that the surface tension of milk is definitely lower than that of water is of interest, and raises the question as to which milk constituent produces this result. Unquestionably the proteins are primarily responsible for the lower surface tension. Surface tension of milk varies with temperature and total solids. Development of acidity and churning lower the surface tension of milk.
13. Germicidal Property:
Freshly drawn milk exhibits inherent quality to retard the growth of microorganisms in milk. This property is called germicidal property of milk. It lasts for a very short period and is considered to be of no practical importance. This property of milk is attributed to the presence of lactomin which is effective against certain organisms while several other organisms continue to grow in milk. This property of milk is lost on heating.
5. Chemical Properties of Milk:
Freshly drawn milk normally shows an amphoteric reaction, i.e., it turns red litmus blue, and blue litmus red. This property in milk is due to the presence of different amino acids. Amino acids consist of NH22+ as well as COOH– groups which show alkaline as well as acidic reactions.
However, most common chemical properties are briefly explained here:
1. Acidity:
When normal fresh milk is titrated with an alkali solution using phenolphthalein as an indicator, it appears acidic though fresh milk does not contain any acid. This acidic nature of freshly drawn milk is due to the presence of phosphates, proteins, citrates and dissolved carbon dioxide. This acidity in milk is called natural acidity.
The natural acidity in milk may range from 0.13 to 0.21%. This is always expressed as per cent lactic acid. Natural acidity in milk is considered important from the heat stability point of view.
When milk is kept under atmospheric conditions for sometime, an increase in acidity in milk is noticed. This increase in acidity known as developed acidity is due to the production of lactic acid as a result of microbial activity.
The microorganisms which gain entry into the milk after it leaves the udder of the animal convert lactose into lactic acid. If the growth of microorganisms is not checked, then the production of lactic acid continues and when sufficient amount of lactic acid is produced, milk gets curdled under atmospheric conditions.
2. pH:
Normal fresh milk has a hydrogen ion concentration of approximately pH 6.5 to 6.7, which indicates that the milk is slightly acidic. If the animal is suffering from udder disease mainly mastitis, then the milk obtained from such animal may have pH above 7 suggesting that it is alkaline. Such milk sample will have abnormal odour, taste and poor heat stability. If the pH value is less than 6.6, then it is considered to be added with colostrum or bacterial deterioration has taken place in milk.
3. Buffering Action:
Fresh milk acts as a complex buffer because it contains several substances which contribute to this property of milk. Certain constituents viz., carbon dioxide, proteins, phosphates, citrates and a number of minor constituents are responsible for buffering capacity of milk. This property of milk is considered important from the curdling and heat stability point of view.
4. Oxidation Reduction Potential:
In case of organic materials, oxidation is defined as the uptake of oxygen or loss of hydrogen, in the same manner, reduction may be defined as a process of loosing oxygen or gaining hydrogen. However, in a broader sense, the processes of oxidation and reduction are not necessarily limited to the gain or loss of oxygen and hydrogen.
In ionic systems, it can be demonstrated that phenomenon may involve loss or gain of electrons. In practice, the potential difference created by platinum electrode in a solution of an oxidant or reductant is measured by completing a circuit through calomel half-cell and a potentiometer.
The voltage measured under these conditions reflects the oxidising or reducing capacity of the solution. This potential is called the oxidation reduction potential or redox potential. It is designated by the symbol ‘Eh’. It normally falls within the range of +0.2 to +0.3 volt. Certain factors such as heat treatment, bacterial activity and contamination with trace metal, particularly with copper, affect the ‘Eh’ of milk.
Methylene blue reduction test of milk is based on available oxygen by the microorganisms. By this test, we can test the effect of metals used in dairy industry and also the level of microbial activity.
6. Detection of Adulteration in Milk:
The following tests may be done for the detection of adulteration in the milk sample:
(i) Detection of Sucrose – Add 0.1 g of resorcinol powder in 10 mL of milk. If sucrose is mixed, the milk will give red colour.
(ii) Detection of Starch – Add little iodine solution in milk. Milk containing starch will show violet colour.
(iii) Detection of Skimmed Milk Powder – If colour of milk by addition of nitric acid turns light violet, it indicates mixing of skimmed milk powder otherwise normal milk shows yellow colour.
(iv) Detection of Gelatin – Add picric acid and shake well and observe the precipitation which in case is of yellow colour, indicates mixing good amount of gelatin and if it shows light brown colour, it indicates mixing in very little quantity.
(v) Detection of Total Solids – This adulteration can be determined by following Richmond’s formula –
TS = G/4 + 1.2F + 0.14
TS = total solids percentage in milk
G = correct lactometer reacting of milk at 60° F
F = fat percentage in milk
7. Detection of Water in Milk:
The water adulteration in milk can be detected by following tests:
(i) Detection by Vieth Ratio:
Vieth ratio in milk is 13:9:2 for the presence of lactose : protein : mineral matter and if it gets altered, indicates the adulteration.
(ii) Nitrate Test:
Water from ponds, rivers, wells and hand pump contains nitrate, therefore, if milk is adulterated with such water, this test is positive. Treated tap water normally does not contain nitrate. This test is not useful in case, milk is adulterated with tap water.
(iii) Freezing Point Test:
The freezing point of normal milk is – 0.544°C. In case milk is adulterated with water, the fat and total solids in milk are reduced causing a rise in freezing point. This can be determined by use of standardised apparatus Hortvet cryoscope.
Per cent of water added in milk = 100 [(T – T1) / T]
Where, T and T1 are freezing points of pure and adulterated milk respectively.