Chemistry of Milk : Chemistry of Milk Chemistry of Milk : Chemistry of Milk The source for illustrations in this presentation are from:
DAIRY PROCESSING HANDBOOK
Tetra Pak Processing Systems AB
S-221 86 Lund, Sweden
Text – Gosta Bylund, M.Sc. (Dairy Techn.)
Editor: Teknotext AB
Illustrations: Origrit AB
Cover: Torkel Dohmers
Printer: LB Grafiska AB
Printed in 1995 Chemistry of Milk : Chemistry of Milk Milk Components
A. Water 87.0%
B. Fat 4.0%
C. Proteins 3.5% Average
D. Lactose 4.7%
E. Minerals 0.8% Chemistry of Milk : Chemistry of Milk F. Other Substances in Trace Amounts
5. Gases Chemistry of Milk : Chemistry of Milk Basic Chemical Concepts
1. An atom is the smallest building block of all matter in nature and cannot be divided chemically.
2. A substance in which all the atoms are the same is called an element.
3. Most natural occurring substances are composed of several elements. i.e.: H2O = Water Chemistry of Milk : Chemistry of Milk Atoms cont.
4. Are comprised of:
a. Protons – Positively charged particles
b. Neutrons – Neutrally charged particles
c. Electrons – Negatively charged particles
5. An Atom will have an overall neutral because it will have equal numbers Protons + and Electrons - Chemistry of Milk : Chemistry of Milk Chemistry of Milk : Chemistry of Milk B. Ions
1. Ions are atoms that have lost or gained additional electrons and are therefore no longer electrically neutral.
a. Cations (positively charge ions) have lost electrons
b. Anions (negatively charged ions) have gained electrons
2. Positive and negative ions will always be present together in liquid solutions or in solid form as salts. Chemistry of Milk : Chemistry of Milk C. Molecules
1. Atoms can combine into larger units which are called molecules
2. Molecules can then form:
a. Solids – Sand SiO2
b. Liquids – Water H2O
c. Gases – Hydrogen H2
3. Molecules that consist primarily of Carbon, Hydrogen and Oxygen atoms are referred to as organic. Chemistry of Milk : Chemistry of Milk 4. Lactic Acid is an example of an organic compound and has the formula C3H6O3. Chemistry of Milk : Chemistry of Milk Basic Physical/Chemical Properties of Cows’ Milk
Average Emulsion Colloidal True
Composition Type Solution/ Solution
% Oil/Water Suspension
Fat 4.0 X
Proteins 3.5 X
Lactose 4.7 X
Ash 0.8 X Chemistry of Milk : Chemistry of Milk A. Emulsion is a suspension of droplets of one liquid in another.
1. Milk is an emulsion of fat in water
2. Butter is an emulsion of water in fat
B. Colloidal Solution is when matter exists in a state of division intermediate to true solution (e.g. sugar in water) and suspension (e.g. chalk in water) it is said to be in colloidal solution or colloidal suspension. Chemistry of Milk : Chemistry of Milk 1. Characteristics of a colloid are:
a. Small particle size
b. Electrical charge and
c. Affinity of the particles for water molecules
2. In milk the whey proteins are in colloidal solution and the casein in colloidal suspension. Chemistry of Milk : Chemistry of Milk C. True Solutions Matter which, when mixed with water or other liquids, forms true solutions divided into:
1. Non-ionic solutions. When lactose is dissolved in water, no important changes occur in the molecular structure of the lactose.
2. Ionic solutions. When common salt is dissolved in water cations (Na+) and anions (Cl-) are dispersed in the water forming an electrolyte. Chemistry of Milk : Chemistry of Milk D. Acidity of Solutions
1. When an acid (e.g. hydrochloric acid, HCl) is mixed with water it releases hydrogen ions (protons) with a positive charge (H+). These quickly attach themselves to water molecules, forming hydronium (H3O+) ions.
2. When a base (a metal oxide or hydroxide) is added to water, it forms a basic or alkaline solution. When the base dissolves it releases hydroxide (OH-) ions. Chemistry of Milk : Chemistry of Milk Acidity of Solutions cont.
a. A solution that contain equal numbers of hydroxide and hydronium ions is neutral
b. A solution that contains more hydroxide ions that hydronium ions is alkaline
c. A solution that contains more hydronium ions than hydroxide ions is acid Chemistry of Milk : Chemistry of Milk E. pH
1. The acidity of a solution is determined as the concentration of hydronium ions.
2. The symbol pH is used to denote the hydronium ion concentration.
3. Mathematically pH is defined as the negative logarithm to the base 10 of the hydronium ion concentration expressed in molarity, I.e. pH = -log(H+) Chemistry of Milk : Chemistry of Milk pH cont.
pH > 7 - Alkaline Solution
pH = 7 - Neutral Solution
pH < 7 - Acid Solution
1. Mix an acid with an alkali the hydronium and hydroxide ions react with each other to form water.
2. If mixed equally with the same number of hydronium ions and hydroxide ions the solution will be neutral.
H3O+ + OH- H2O + H2O Chemistry of Milk : Chemistry of Milk G. Diffusion: is the migration or movement of particles from an area of high concentration to an area of lower concentration.
1. The diffusion process will continue until an entire solution of homogeneous (the same concentration throughout).
2. Rate of diffusion depends upon:
a. Particle velocity
c. Size of particles
d. Concentration differences within a solution Chemistry of Milk : Chemistry of Milk H. Reverse Osmosis (RO)
1. When a pressure higher than the osmotic pressure is applied to the sugar solution, water molecules diffuse and the solution becomes more concentrated.
2. This process is used commercially to concentrate solutions.
1. Is used to separate large particles from small ones in solution, i.e.: proteins from salts. Chemistry of Milk : Chemistry of Milk Basic Physical/Chemical Properties of Cows’ Milk
Main Constituent Limits of Variation Mean Value
Moisture (Water) 85.5% - 89.5% 87.5%
Total Solids 10.5% - 14.5% 13.0%
1. Fat 2.5% - 6.0% 3.9%
2. Proteins 2.9% - 5.0% 3.4%
3. Lactose 3.6% - 5.5% 4.8%
4. Ash (Minerals) 0.6% - 0.9% 0.8% Chemistry of Milk : Chemistry of Milk Milk Fat
Milk and cream (milk fat) are examples of fat-in water (or oil-in-water) emulsions.
Milk Fat exists as small globules or droplets dispersed in the milk serum. Chemistry of Milk : Chemistry of Milk Chemistry of Milk : Chemistry of Milk Fat Cont.
3. Fat globules range in size from 0.1 to 20 µm (1 µm = 0.001mm with an average size of 3 – 4 µm.
4. There may be up to 15 billion globules per ml of milk.
5. Chemical Structure of Milk Fat
a. Milk fat is a mixture of difference fatty-acid esters called triglycerides, which are composed of an alcohol called glycerol and various fatty acids.
b. Fatty acids make up about 90% of milk fat. Chemistry of Milk : Chemistry of Milk c. Saturated fatty acids the carbon atoms are linked by single bonds.
d. Unsaturated fatty acids the carbon atoms are linked by one or more double bonds.
6. Melting Point of Fat
a. The melting points of the fatty acids vary considerably from –7.9 C to 62.6 C.
b. Therefore milk fat with a greater content of high- melting point fatty acids will be harder and fat with a high content of low-melting point fatty acids will be softer, at room temperature. Chemistry of Milk : Chemistry of Milk 7. Iodine Value
a. Iodine Value is an indicator of the relative softness or firmness of a fat as a percentage of Iodine bonded to a fat sample.
b. Iodine is taken up by the double bonds of the fatty acids.
c. Since the unsaturated fats, primarily Oleic acid, are the lower melting point fats the Iodine value is primarily a measure of the relative softness of a fat.
d. Iodine value of butter fat varies between 24 – 46 with 32 – 37 being ideal for butter manufacturing. Chemistry of Milk : Chemistry of Milk 8. Refractive Index
a. The different fatty acids in fat also affects the way it refracts light. Therefore it is a common practice to determine the refractive index of fat, which can then be used to calculate the Iodine Value.
b. The Refractive index of fat normally ranges from 40 – 46.
9. Nuclear Magnetic Resonance (NMR)
a. Is another way to determine the relative softness / hardness of milk fat. This value varies between 30 – 41. Chemistry of Milk : Chemistry of Milk 10. Fat Crystallization
a. Milk fat crystallization is an exothermic reaction, which means that the chemical reaction is accompanied by evolution of heat.
b. During this process the fat globules are very unstable and may rupture releasing liquid fat into the milk serum.
c. The crystallization process is important in the production of cream for various purposes. Chemistry of Milk : Chemistry of Milk Proteins in Milk
a. Amino Acids are the referred to as the ‘Building Blocks of Protein’
b. Amino Acids will always contain Carbon, Hydrogen, Oxygen, Nitrogen, and either/or Sulfur, Potassium, and/or Phosphorus
c. A single Amino Acid molecule will generally consist of an amino group (NH2) and a carboxyl (COOH) group bound to the same carbon atom Chemistry of Milk : Chemistry of Milk Chemistry of Milk : Chemistry of Milk d. Essential Amino acids are those which cannot be synthesized by humans and therefore must be added to the diet.
e. Milk contains all of the Essential Amino Acids
f. They are: PVT TIM HALL
Threonine Valine Chemistry of Milk : Chemistry of Milk g. Amino Acids emit hydronium ions in alkaline solutions and absorb hydronium ions in acid solutions hence the term amphotery electrolytes or ampholytes.
h. Therefore Amino Acids can appear in three states:
1) Negatively charged in alkaline solutions
2) Neutral at equal + and – charges
3) Positively charge in acid solutions
i. Most proteins will be a combination of 100 – 200 of approximately 18 different amino acids Chemistry of Milk : Chemistry of Milk 2. Classes of Milk Proteins
There are literally 100’s of types of milk proteins, most in very small amounts.
Milk proteins used to be grouped according into three major categories:
1) Casein 2) Albumin 3) Globulin
Today they are grouped into the major groups:
1) Casein – 79.5% 2) Whey Proteins – 19.3%
3) Globule Membrane Proteins – 1.2% Chemistry of Milk : Chemistry of Milk 1) Casein
a) Casein is the most prominent protein found in milk and there are three primary types:
* ά-casein 38.6%
* κ-casein, and 30.8%
* β-casein 10.1%
b) Casein Micelles
* Are groups of casein submicelle’s bonded together to form somewhat of a network that allows curd to form in cheese making. Chemistry of Milk : Chemistry of Milk c. Precipitation of Casein
1) Precipitation by Acid
* Added acid or acid from acid-producing bacteria will cause precipitation (coagulation) of the milk.
* The range for optimum precipitation is pH 4.5 to 4.9. Ideal pH for Casein precipitation is 4.7
2) Precipitation by Enzymes
* κ-casein are easily split by proteolytic enzymes
* κ-casein consists of 169 amino acids
* The enzyme splits the molecule at A.A. 105-106 Chemistry of Milk : Chemistry of Milk Precipitation by Enzymes cont.
* κ-casein A.A.’s 106 to 169 are soluble amino acids and are released into the whey in cheese making
* The remaining part of the κ-casein, A.A.’s 1 to 105 are insoluble and remains in the curd together with ά-casein and β-casein
* This splitting of the 105-106 bond in the κ-casein molecule referred to as the primary phase of the rennet action
* The Phase of coagulation and syneresis follows. Chemistry of Milk : Chemistry of Milk 2. Whey Proteins
a. Whey protein is the name commonly applied to milk serum proteins and consist primarily of:
1) ά-lactalbumin - 3.7% of milk protein
2) β-lactoglobulin - 9.8% of milk protein
b. They are very high quality proteins with an Amino Acid profile that is regarded as a biological optimum.
c. Responsible for the cooked flavor and odor of milk and milk products Chemistry of Milk : Chemistry of Milk d. Immunoglobulins
1) Immune related proteins
3. Membrane Proteins
a) Membrane proteins are a group of proteins that form a protective layer around fat globules to stabilize the emulsion.
4. Denatured Proteins
a) Biological function is lost due to:
1) Heat 2) Acidity 3) Alkali 4) Radiation 5) Violent Agitation Chemistry of Milk : Chemistry of Milk Enzymes in Milk
1. Enzymes are a group of proteins produced by living organisms.
2. They trigger chemical reactions and affect the course and speed of such reactions
3. Enzymes do this without being consumed.
4. They are referred to as ‘Biocatalysts’
5. Enzymes action is very specific; each type of enzyme catalyses only one type of reaction
6. Temperature and pH strongly influence enzyme action Chemistry of Milk : Chemistry of Milk 7. Primary Enzymes found in milk are:
d. Lipase Chemistry of Milk : Chemistry of Milk Lactose (Milk Sugar)
1. Lactose is a disaccharide (sugar) found only in milk.
2. It is a carbohydrate made up of two molecules of simple sugars (glucose – galactose)
3. Lactose content of milk varies between 3.6% to 5.5%
4. Lactose can be attacked by lactic acid bacteria which produce an enzyme call lactase which splits the lactose molecule into glucose and galactose which then forms into lactic acid through a fermentation process. This is what has happened to sour milk Chemistry of Milk : Chemistry of Milk Vitamins in Milk
1. Vitamins are organic substances which occur in very small concentrations in both plants and animals
2. Milk contains many vitamins. However the primary vitamins found in milk are:
a. Vitamin A Fat Soluble
b. Vitamin B1 Water Soluble
c. Vitamin B2 Water Soluble
d. Vitamin C Water Soluble
e. Vitamin D Fat Soluble Chemistry of Milk : Chemistry of Milk Minerals and Salts in Milk
1. The total concentration of Minerals in milk is less than 1%
2. Mineral salts occur in solution in milk serum or in casein compounds.
3. Primary mineral salts include calcium, sodium, potassium and magnesium Chemistry of Milk : Chemistry of Milk G. Other Constituents of Milk
1. Somatic Cells (White Blood Cells/Leucocytes)
2. Gases – Carbon Dioxide, Nitrogen and Oxygen