MILK COMPOSITION, NUTRITIVE VALUE, ATWATER VALUES, HOMOGENISATION AND CREAM FORMATION

ABSTRACT

This experiment was carried out based on the 5 kinds of milk to analyze the composition of milk, its nutritive value, the Atwater energy value, the effect of homogenisation on fat globule size and the effect of temperature on creamline formation. The analysis was performed by accomplishing the organoleptic test, the measurement of pH and the refractometer value, the comparison and the appearance of fat globules size of homogenised milks under microscope and the heating and cooling process to measure the creamline formation. The organoleptic test was carried out in terms of its smell, appearance, taste, viscosity and the leftover amount in emptied cup, the pH meters was used to record the pH values of the different milk samples and refractometer was use to record the soluble solids content of milks, the microscope was used to analyze the appearance of fat globules for homogenised milk and the heating process was performed under different temperature, 60C, 70C and 80C respectively, and the cooling process was performed by placing in the refrigerator for 24 hours. From organoleptic test, the results support the evidence that the level of creaminess between full cream milk and skim milk in relate with the contribution of compounds from fat globules membrane, the effect of homogenisation make the great difference in viscosity between each milk. The correlation between the pH value and refractometer record was found out from table 1.2. From table 1.3, the result showed that the homogenised milk can form thinner creamline than unhomogenised milk.

INTRODUCTION

Milk contains proteins, fat, water, lactose, inorganic salts, and other minor organic material such as phospholipids, enzymes, hormones, vitamins, amino acids and etc. Milk is primarily water – 87.4 percent and the rest 13 percent consists of lactose, proteins, fat and minerals. The high concentration of water gives milk a near-neutral pH of 6.6 at room temperature but several conditions and processes can affect the pH value. The viscosity is low, about twice that of water, which means that milk can readily be mixed, even by convection currents resulting from small temperature fluctuations. Lactose is the primary carbohydrate found in milk. When bacteria in milk metabolize lactose, lactic acid is produced. The protein in milk is a complete protein which contains all the essential amino acids in adequate quantities necessary to support growth and the maintenance of life. Milk contains cholesterol, an average of 33mg in a cup of whole milk, 18mg in reduced-fat milk, and 4mg in fat-free milk. Milk contains vitamins A and D, riboflavin (B₂), and tryptophan, an amino acid important in the formation of the B vitamin niacin. Milk is low in vitamins C and E. Milk exposed to ultraviolet light loses riboflavin, so it is packaged in cardboard or opaque plastic containers to prevent the degradation of this vitamin by light. The major mineral in milk is calcium with 1 cup containing 300mg of the nutrient.

The synthesis of milk components occurs for the greater part in the secretory cells of the mammary gland. At the basal end precursors of milk components are taken up from the blood, and at the apical end milk components are secreted into the lumen. Proteins are formed in the endoplasmic reticulum and transported to the Golgi vesicles, in which most of the soluble milk components are collected. The vesicles grow in size while being transported through the cell and then open up to release their contents in the lumen. Most are synthesized in the cell. How the major constituents are structurally arranged in milk is necessary to predict how milk processing conditions affect flavor, texture, and the keeping qualities of dairy products. The gross composition of milk varies according to the species of mammal, the breed which affect the level of fat and protein and the stage of lactation.

The fat in milk is emulsified with a membrane material consisting primarily of proteins, phospholipids, enzymes and the minor components are glycoprotein, phospholipids, carotenoids and sterols. The membrane serves to protect the fat from undesirable oxidation. Fat globules will still cream unless being accentuated by the agglutinin reaction, which results in clustered-fat-globule that rise rapidly to the surface to form a cream layer. Approximately 98 percent of the fat in milk exists in the form of triacylglyceride which synthesized in the cytoplasm, forming small globules. Milk fat is entirely solidified at -40C and has melted completely at 40C and 50 percent of solid, despite the apparent solid appearance at this temperature.

Milk and cream are naturally occurring oil-in-water emulsions but the fat globules are more complicated than emulsion droplets. In particular, the surface layer or membrane of fat globule is not absorption of one single substance but consists of many components; its structure is complicated. The dry mass of the membrane is about 2.5% of that of the fat. A small part of the lipids of milk is found outside the fat globules. A small part of the lipids of milk is found outside the fat globules. At temperatures below 35C, part of the fat in the globules can crystallize.

However, in most dairy-based food products except butter (water-in-oil emulsion), water is the predominant phase and the fat phase is emulsified. The water and fat can be separated out into the two phase by covering the surface of the emulsion globules with some kind of surface-active component. For fat to be broken up into small micrometer-size globules, energy must be supplied to the food product. This process is called homogenization which prevents the separation of water and fat known as creaming. By and large, the more energy supplied by the homogenizer, the smaller the fat globules formed, with a greater stability against creaming. By homogenizing milk, fat globules are physically broken down into many smaller globules to prevent the fat from grouping together and floating to the top of the liquid. Milk homogenization on dairy products has no effect on nutrient content however it makes the fat globules become smaller as pressure increases, increases lipolysis of milk, less oxidized off-flavors in milk and inactivation of the agglutinin properties which causes clustering of fat globules.

Casein micelles consist of water, protein, and salts. The protein is casein which can be concentrated and separated from milk, for instance, by membrane filtration. Casein also aggregates and forms a gel when the pH of the milk is lowered to about 4.6. Moreover, water can be removed from milk by evaporation. Altogether, a range of liquid milk products of various compositions can be made (e.g. Whole milk, Low-fat milk and Skim-milk). The flavor of fresh milk is fairly bland. The lactose produces some sweetness and the salts some saltiness. Several small molecules present in very small quantities also contribute to flavor. The fat globules are responsible for the creaminess of whole milk. Since milk is rich in nutrients, many microorganisms, especially bacteria, can grow in milk. Not all bacteria that need sugar can grow in milk, some being unable to metabolize lactose. Milk is poor in iron, which is an essential nutrient for several bacteria, and contains some antibacterial factors, such as immunoglobulins and some enzyme systems. On top of that, milk contains too much oxygen for strictly anaerobic bacteria. Altogether, the growth of several bacteria is more or less restricted in raw milk, but several others can proliferate, especially at high ambient temperatures. For the nutritional value of milk, it contains virtually all nutrients mostly in significant quantities but it is poor in iron and the vitamin C content is not high. Though it lacks dietary fibre, it contains no antinutritional factors.

METHODS

To test for the composition of milk, organoleptic test was carried out on the five kinds of milk which are unhomogenised full cream milk, unhomogenised low-fat milk, goat milk, homogenised full cream milk, and homogenised skim milk. Measuring pH level was performed by using pH meters. In order to record the soluble solids content of milks, the refractometer was used. To make the comparison of the fat globule between homogenised and unhomogenised milk, a microscope equipped with an eyepiece scale and a stage micrometer was used to estimate the range and mean diameter of the globules. The effect of temperature on cream line formation was tested on 3x100ml of milk covered the mouth with aluminium foil under three different temperature in terms of 60’C, 70’C and 80’C water bath and were placed in a refrigerator for 24 hours to get the cream layer of the milk and recorded the volume of the cream-layer on the top of each milk. The energy content of average composition of different mammalian species given in the table was calculated by means of their carbohydrates, protein and fat by using Atwater energy values.

RESULTS

The organoleptic test was carried out on the unhomogenised full cream milk, unhomogenised low-fat milk, goat milk, homogenised full cream milk, and homogenised skim milk, and recorded the data in terms of the smell of the milk, the appearance, the taste, the level of viscosity and the amount left in the cup as soon as the milk was emptied. The following table 1.1 represent the data from the organoleptic test.

Table 1.1. Organoleptic Tests for Different kind of Milks

	Unhomogenised Full cream Milk	Unhomogenised Low-fat Milk	Homogenised Full cream Milk	Homogenised Skim Milk	Goat Milk
Smell	Slightly milky	No smell	Milky	No smell	No smell
Appearance	Off-white	Pale white	White	Pale white	White
Taste	Full flavor	Tasty	Rich flavor	Light	Unpleasant mouthfeel after tasting
Viscosity	Creamy	Light	Thick	Lightly thick	Watery
Leftover amount in emptied cup	Very thin layer	Barely any left	Thin layer	Nothing	Thin layer

Secondly, the pH values of the different milk samples was obtained by using the pH meters and the soluble solids content of the milk was obtained by using the refractometer. The following table shows the pH values and the refractometer record of the soluble solids content of the milks.

Table 1.2. pH value and the Refractometer record

	Unhomogenised Full cream Milk	Unhomogenised Low-fat Milk	Homogenised Full cream Milk	Homogenised Skim Milk	Goat Milk
pH value	6.67	6.71	6.63	6.72	6.57
Refractometer	10	9.8	12	10.5	9.9

Thirdly, the samples milk were tested to analyze the effect of temperature on cream line formation under the 3 different temperatures water bath, 60C, 70C, and 80C respectively, until each milk reaches the required temperature and place in a refrigerator for 24 hours to get the creamline formation and the result of the volume for the cream layers of different sample milks at different temperatures are mentioned below.

Table 1.3. The volume of Cream Layers at Different Temperatures

Temperature Difference	60C	70C	80C
Unhomogenised Full cream Milk	54C	64C	80C
Volume of Creamlayer	1ml	2ml	No result
Unhomogenised Low-fat Milk	47.3C	62C	67C
Volume of Creamlayer	2ml	2.5ml	3ml
Homogenised Full cream Milk	53C	67C	73C
Volume of Creamlayer	1.5 – 2 cm	1 cm	1cm
Homogenised Skim Milk	54C	63C	71C
Volume of Creamlayer	-	-	-
Goat Milk	53C	64C	70C
Volume of Creamlayer	1.5ml	2ml	1.5ml

DISCUSSION

According to the recorded data from table 1.1, full cream milk and skim milk differ considerably in flavor as the fat globules are largely responsible for the ‘creamy’ flavor. It is often assumed that compounds from the fat globule membrane contribute to the creaminess. It is quite likely that the physical presence of the fat globules plays an important part because creaminess can also be enhanced by other small spherical particles. Naturally, cream tastes much creamier than milk. Increasing the pH of milk also increased its viscosity, presumably owing to additional swelling of casein micelles.

Formation of homogenisation clusters can greatly increase the viscosity of a product such as cream. Homogenised milk has thicker viscosity than unhomogenised milk because the fat globules that are now partly covered with casein participate in the aggregation of the casein micelles. When milk is homogenised, the natural membrane on the globules is disrupted but remains adsorbed. Homogenised milk fat globules generally show a high stability toward partial coalescence because they are so small. Moreover, surface layers that consist fully of plasma protein yield a very high stability to the globules. During homogenisation, when a partly denuded fat globule collides with another globule that has been covered with casein micelles, such a micelle can also reach the surface of the former globule. As a result, both fat globules are connected by a bridge and form a homogenisation cluster.

Homogenization is a process of size reduction of the fat globules to prevent fractionation of the cream and skim milk by density difference. A combination of a high-pressure pump and special valves provides high shear. Intense turbulence during homogenisation markedly reduces fat globule size, with a consequent four- to sixfold increase in surface area. Because the amount of available original membrane material is insufficient to cover this area, plasma proteins absorb onto the fat globule surface; the new membranes in homogenised milk consist of MFGM material, caseins, casein micelles, and whey proteins.

Casein micelles absorb preferentially over the whey proteins, and the adsorbed casein micelles partly spread on the fat surface, possibly as submicelles. Although the whey proteins make up a much smaller proportion about 5%, they cover a disproportionately greater area about 25% of the fat globule surface. The relative content of original membrane material in the new membrane depends on the increase in surface area, and thus on the intensity of homogenisation. The homogenized fat globules act as large casein micelles and will participate in any reaction of caseins, such as renneting, acid precipitation, and heat coagulation.

After homogenisation, clusters of fat globules in which casein micelles are shared by globules may be found. Two stage homogenisation may partly prevent the occurrence of homogenization clusters. Homogenisation retards creaming because the fat globules are smaller. Lipase is activated by homogenisation because of transfer of casein-associated lipase to the surface of fat globules, increase in fat surface area, and dissociation of casein micelles.

The heat treatments cause a number of important chemical and physical reactions in milk proteins, and many reactions involve nonprotein constituent of milk as well. When full cream milk is heated above 60C, fat globule membrane proteins denature, resulting in exposure of reactive thiol groups, As a consequence, thiol-disulfide interchange reactions may occur between different membrane proteins, and why proteins could also participate in these reactions. Both α-lactalbumin and β-lactoglobulin have been shown to bind via intermolecular disulfide bridges to the surface of milk fat globule on heating milk at 65-85C. Whey proteins in full cream milk have more affinity for the MFGM than for the casein micelle surface. Phospholipids migrate from the fat globules to the aqueous phase during heating, to some extent.

Based on the table 1.2, the pH value of unhomogenised milks are 6.67 in full cream and 6.71 in low fat with the refractometer record of 10 and 9.8 respectively. Consequently, in unhomogenised milk, the lower the pH value is, the higher the refractometer record represents. pH determines the conformation of proteins, the activity of enzymes, the dissociation of acids present in milk, etc. The pH of milk depends strongly on temperature. The dissociation of most of the ionizable groups depends on temperature but the dependence varies widely. The acidity of milk can also decrease somewhat by heat treatment due to loss of CO₂, but at high temperatures greater than 100C acidity slowly increases due to the formation of acids.

According to the data from table 1.3, the unhomogenised low-fat milk formed the thickest cream layer with 2.5ml and 3ml which were waterbath before cooling in the fridge at 70C and 80C. Based on the recorded date, the extent of craming seems to greatly depend on the temperature. Besides, the data showed that the homogenised milk formed thinner cream layer than the unhomogenised milk due to the inactivation of the cold agglutinin which unable the fat globules unable to flocculate in the cold.

When full cream milk is homogenised before heating, the fat globule behave differently during and after heating from natural milk fat globules. The membranes of the homogenised globules have a different composition, and the size of the globules is smaller. The subsequent heating of homogenised milk caused complex formation between whey proteins and the casein adsorbed on the fat globule surface. Homogenised fat globules act as large casein micelles because of the casein in their new membranes and thus participate in any reaction of the caseins, as stated earlier. When full cream milk is heated before homogenisation, the whey proteins are denatured if the temperature is greater than 70C and interact with both the κ-casein of casein micelles at the natural pH and the native fat globule membrane. During subsequent homogenisation, the micelle complex of casein and whey protein will absorb on to the newly created fat surfaces. Competition between the caseins and whey proteins for the fat surface will be small or even absent. The two treatments, homogenisation and heat treatment, cause major variations in the quality of processed dairy products such as evaporated and sweetened condensed milk. When skim milk is homogenised under normal milk homogenisation condition (e.g., 15 Mpa), there is no change in the casein micelle size, but the micelles can be disrupted at high homogenisation pressures.

The full cream milk contains not less than 3.25 percent milk fat and 8.25 percent solids-not-fat whereas low-fat milk contains 0.5, 1.5, or 2.0 milk fat with not less than 8.25 percent solids-not-fat and skim or non-fat milk contains less than 0.5 percent milk fat with not less than 8.25 percent solids-not-fat.

Table 1.4. Average Composition of Some Mammalian Milk

Species	Fat	Protein	Lactose	Ash	Water
	%	%	%	%	%
Goat	4.09	3.71	4.20	0.79	87.81
Horse	1.59	2.00	6.14	0.41	89.86
Sheep	8.28	5.44	4.78	0.90	80.60
Seal	54.00	12.00	None	0.53	34.00
Cow	3.90	3.40	4.80	0.72	87.10
Woman	3.80	1.20	7.00	0.21	87.60

Based on the data from above table, the nutrient content between cow milk and human milk is highly similar in comparison with other’s milk. Cow milk should be the substitute milk in the place of human milk for infant due to its similar nutrient content and its wide availability around everywhere.

REFRENCES

Richard K. Robinson, 2002, Homogenisation and changes in fat globules, Dairy Microbiology, pp. 39-44.

Amy Brown, 2008, Chapter 9 in Milk, Understanding Food Principles and Preparation (Third Edition), pp. 187-194.

Pieter Walstra, Jan T.M. Wouters, Tom J. Geurts, 2006, Chapter 3 in Collodal Coloidal Particles of Milk, Chapter 4 in Milk Properties and Chapter 5 in Homogenisation, Dairy Science And Technology ( Second Edition).

Robert G. Jensen, 1995, Chapter 2 in Structure of Milk, Handbook of Milk Composition, pp. 14-16.

Gregory D. Miller, Judith K. Jarvis, Lois D. McBean, 2007, Chapter 1 in The Importance of Milk and Milk Products in the Diet, Handbook Of Dairy Foods And Nutrition (Third Edition), pp. 1-12.