This summary is based on the reader available for student who follow(ed) the course Dairy Chemistry
and Physics (FQD-33306) and was made in October 2018. The information provided in the lectures
(available at WUR) and ohter learning material of this course have not been implemented in this
summary.
Chapter 1 – Composition, structure and physical properties
Composition: Major and minor components.
Protein: 80% casein, 20% whey and
enzymes. Mineral substances: K, Na, Ca, Mg,
Cl and phosphate. Citrate is principle acid.
Milk – fat globules = milk plasma.
Milk – fat globules – casein micelles = milk
serum
pH = 6.7 aw = 0.993
All particles have Brownian motion; electrostatic charge, being negative at the pH of milk. In raw
milk; fat globules are the largest followed by casein micelles and serum protein.
Composition Fat globules have a membrane, part of fat present is liquid, other part is crystallized.
Small part of the lipids is outside the globules. Casein micelles consist of casein, water and salts.
Negative charge at pH of milk, binds cations (calcium and magnesium). Colloidal calcium phosphate
(CCP), contains small amount of citrate. Open structure, containing much water, making the volume
large. Due to the size, light is scattered, causing the white colour. Also homogenized milk fat can
scatter light. Renneting or acidification (pH 4.6). Fast acidification (HCl), caseins precipitates in almost
pure form, slow acidification (bacteria) forms gel. Serum proteins are retained in UF, subject to heat
denaturation (80 ⁰C), smaller size, globular protein. Lipoprotein particles (milk microsomes), consist
of rests of mammary secretory cell membranes. Cells (e.g. leukocytes) are always present in milk,
containing enzymes such as catalase.
Physical aspects Polar components dissolve well in milk, high dielectric constant. Dissolved
components cause the milk to have a certain osmotic pressure and freezing point depression. Density
of milk depends on temperature, composition and temperature history. The density increases if the
solids-non-fat content increases and decreases when fat content increases. Density of fat depends on
the ratio of liquid fat (lower density) to solid fat (higher density). This ratio depends on temperature
and temperature history. Milk is usually preheated to 40 ⁰C to melt the fat and cooled to room
temperature to minimize the effects of thermal history.
pH of fresh milk is 6.7, lower in colostrum, higher in case of mastitis. Titratable acidity, mainly
determined by phosphates and proteins is measure of the buffering capacity of milk above pH 6.7.
Usually expressed in ⁰N = mmol NaOH/L. TA decreases by heat treatment due to loss of CO2. At very
high temperatures it increases due to formation of acids. TA of cream lower since fat hardly
contributes, higher fat content means lower protein content. Used to measure the amount of lactic
acid (highly dissociated at pH 5.5) formed. Buffering capacity (dB/dpH) relation between quantity of
acid or base added and the corresponding change in pH. Buffering components are soluble
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Summary Reader – Dairy Chemistry and Physics
, phosphate, colloidal calcium phosphate, citrate, bicarbonate and proteins. CCP dissolves on
acidification (below 5.6), formation of phosphate ions that combine with H+ to form HPO42- and
H2PO4-, resulting in buffering. The buffering effect at pH 7 arises from the formation of calcium-
phosphate. pH more meaningful; determines protein conformation, activity of enzymes and
dissociation of acids. Undissociated acids cause rancid taste and inhibit activity of micro-organisms.
Freezing point (depression usually 0.52 ⁰C) together with boiling point, osmotic pressure etc. are
colligative properties: governed by the number rather than kind or weight of particles, mainly
determined by salts and lactose. Change in salt is compensated by the lactose and vice versa.
Osmotic pressure is defined as the tendency of a solution to take in water by osmosis, equal to cow’s
blood. Quite constant so freezing point is also constant.
Viscosity: skim milk and whole milk are Newtonian fluids. Raw milk non-Newtonian under cold
conditions due to agglutination of fat globules. Flocculation gives shear rate thinning. Cream (> 40%)
is non-Newtonian, flow is impaired due to packing. Homogenisation of milk little effect on viscosity,
of cream increases viscosity. Viscosity mainly determined by the volume fraction of particles.
Maximum packing volume becomes more important at higher particle volume fractions. 1. Casein
micelles contribute to the viscosity. 2. More fat, higher viscosity. 3. Lower temperature; part of β-
casein becomes dissociated, increase in viscosity. Viscosity is lower at higher temperature. 4. Higher
pH gives swelling of casein micelles, higher viscosity. pH = pI, aggregation, increase in viscosity. 5.
Aggregation of fat globules (flocculation or partial coalescence) give increase in volume fraction
dispersed phase, increasing viscosity. 6. Concentration, when volume fraction approaches maximum
volume fraction the viscosity is large.
Interfacial attraction minimizes interfacial area. Force that causes this is known as interfacial tension,
related to the energy that is needed to deform the surface area of an interface. Surface tension of
milk decreases with increasing fat content and with increasing temperature (cohesive forces
decrease).
Chapter 2 – Lactose
Milk contains about 4.6% lactose, major contribution to colligative properties: osmotic pressure,
freezing point depression and boiling point elevation. Crystallization is used to separate lactose from
whey. Applications in several foods but also in pharmaceutical industry for tablets. Some lactose
derivates can be pharmaceutically applied ac prebiotic to promote gut health.
Chemical properties Disaccharide of D-galactose and D-glucose, 6-ring (5x C, 1x O). Glucose has
equilibrium with open ring structure. Open ring enables aldehyde group to be reactive, reducing
sugar. At high temperatures and at low pH values, the concentration of open-chain form increases up
to 10% (0,1% is normal), reactivity is enhanced.
- Important chemical reactions of lactose: a) hydrolysis: glycosidic linkage between monosaccharides
can be cleaved, only at high temperature or low pH. Can also be hydrolysed by enzyme lactase, which
is highly specific for the β-1,4 linkage. b) oxidation (aldehyde to carboxyl) or reduction (aldehyde to
alcohol) of the aldehyde group after ring opening.
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Summary Reader – Dairy Chemistry and Physics
