This summary is based on the lectures 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
reader (available at WUR) and ohter learning material of this course have not been implemented in
this summary.
Lecture 1 – Introduction
Composition casein micelles (100nm): caseins (αs1, β, αs2, κ), Ca, Pi, citrate and water. Calcium
phosphate nanocluster hold protein together. Protein sticking out which protects against
aggregation. pI is around 4.6. Composition fat globules (10um): triglycerides, fatty acids.
Encapsulated by membrane (double) which consists of proteins and phospholipids. Milk also contains
non-protein nitrogen: amino acids, peptides, urea.
Cheese: aggregation of casein micelles. κ-casein is cleaved off by chymosin, induces aggregation. Also
add a starter for pH decrease, protein breakdown which is important for taste and consistency in
structure. Yoghurt: aggregation of casein micelles caused by pH decrease. Can be fast or slow
(starter).
Methods of analysis can be precise, accurate, both or
neither. Freezing point depression is both accurate and
precise, thus very reliable.
Lecture 2 – Composition and structure
Fat globules are very large and are covered by a double layer of phospholipids having a pI of 3.6. Are
considered as emulsions, 4% of dry matter, 0.04% of volume, 0.1-10um and have a surface area of
700 cm2/ml milk. Casein micelles are considered to be fine dispersions, 2.8% of the dry matter but
0.1% volume fraction, 200-300 nm and 40.000 cm2/ml milk. pI casein micelles = 4.6. They do not sink
due to their size being very small, Brownian movement and a lot of water present. Globular proteins
are 0.6% of the dry matter, a very small part of the volume, 3-6nm and have a surface area of 50.000
cm2/ml milk, thus the number is the highest
From highest to lowest:
- Dry matter content (%): fat, casein, serum
- Volume fraction: casein, fat, serum
- Particle diameter: fat, casein, serum
- Number per ml: serum, casein, fat
- Surface area (cm2/ml): serum, casein, fat
Protein increases and fat decreases density. The density of milk is thus closely related to the
composition. Titratable acidity is determined by titrating to 8,3 due to the use of fenoftaline in the
old method. Milk has higher buffering capacity compared to whey. At pH 5.5, the buffering capacity
is high which has to do with the calcium phosphate. Fat content does not influence TA, however,
there is a smaller volume of milk thus the TA will be smaller. Vice versa for skimmed milk. Increase of
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Summary Lectures – Dairy Chemistry and Physics
, TA by growth of lactic acid bacteria. TA is not a good measure for spoilage since it is suppressed at a
low pH.
Important for composition milk: feed (forage and concentrates). During digestion several processes
take place: rumination, microbial breakdown which forms new metabolites: acetate, butyrate etc.. If
more fat in milk is wanted, more fibres should be fed. Fat can be changed by feed, protein only in
some extent but lactose cannot be changed. The amount of protein is more dependent on genetics,
heritability, A2 produces more protein.
Lecture 3 – Composition and structure 2, Microbiology
dB/bpH is the quantity of acid/base needed to change pH by a certain value. Freezing point
depression is used to check if water was added to the milk. I is the van ‘t Hoff factor which is the
number of individual molecules of a solute. The value of freezing point is very stable due to the
osmolality being equal to the cow’s blood. Viscosity. Milk is Newtonian, raw milk and cream are non-
Newtonian fluids. At low temperatures, the size of a casein micelle will increase, leading to a higher
volume fraction thus increasing the viscosity. Immunoglobulin IgG attaches to fat globules, forming a
network with water in between, which leads to an increase in volume fraction. Start stirring, the
bonds break and viscosity goes down, only at low temperatures. Due to inward attraction and
hydrogen bonds of water, fat globules are shaped in a sphere. The membrane makes that the surface
tension goes down.
Try to keep bacteria in the lag phase, cooling. It is important to keep low levels of microorganisms in
farm milk, which is obtained by a low contamination level, cooling at 4 degrees and short storage
time (max 3 days). Bacteria from equipment are very bad since these have already past the lag phase
due to adaptation to the environment. Inhibitors of bacterial growth are less important than cooling
and contamination level. Natural inhibitors (immunoglobulins, lactoferrin, lysozyme and
lactoperoxidase-thiocyanate-H2O2-system), antibiotics and disinfectants.
Bacteria that grow fast in milk are psychrotrophs (produce heat stable proteases and lipases, long
shelf life problems) at low temperature and LAB at ambient temperature. Both originate from the
milk environment. Consequences: bacterial enzymes cause problem in milk products with a long shelf
life, production of acids renders caseins lees stable which causes problems during production and
loss of ingredients.
Spoilage microorganisms:
1. Psychrotrophs: milk environment, mainly Pseudomonas, growth at low temperature, do not
survive thermalisation, produce heat stable proteases and lipases causing off-flavours in milk
products with a long shelf life (UHT milk).
2. Bacillus Cereus: come from the milk environment, feed, spores survive pasteurisation, can
germinate and grow at low temperature. Determines the shelf life of pasteurised milk.
Spores can be removed by microfiltration of bactofugation.
3. Other Bacilli: bacteria with high heat resistance (that even can survive sterilisation).
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Summary Lectures – Dairy Chemistry and Physics
