Causes and consequences of milk variability

Topic 1. Foaming behaviour of milk proteins.

Foams are unstable and suffer from creaming, drainage, aggregation coalescence, and disproportionation. The role of surface and bulk components in multicomponent food systems on the formation and stability of foams is still not well understood. For example, it often happens that a certain batch of milk does not properly foam, while others do, even if they are processed in the same way. It is still unclear what causes this and how this must be solved.

Project description

Various studies have been performed on the formation and stability of dairy foams. The foamability of milk is strongly influenced by components that are naturally present in milk as well as by the production methods. The components that are naturally present in milk can influence the foamability and stability both positively and negatively. Frothed milk is mainly stabilized by the proteins at the air/liquid interface. It is known that β-casein is a good foam stabilizer. The presence of this protein in soluble form in milk will therefore have a beneficial effect on the foam formation. Fat and free fatty acids have a destabilizing effect because they are able to displace the proteins from the interface. The presence of soluble β-casein and free fatty acids in milk is dependent on factors related to the physiology of the cow, nutritional, and technological operations after milking. The cooling of the milk for example, results in increased solubility of β-casein. The pressure applied in the homogenization further affects the size of the fat globules. This parameter also has an effect on the foaming behavior of milk. Therefore, thermal history and homogenisation conditions are two important technological parameters that influence the foaming properties of milk.

Aim

Within this project we hope to increase the insights in the formation and stability of dairy foams and to find the key ingredient and process parameters that control it. This will allow industry to improve the production of milk and dairy aerated products of constant high quality.

For more information on this topic please contact Min Chen.

Topic 2. Growth of yoghurt bacteria in milk with different genetic variants.

k-Casein has different genetic variants which are AA, AB, BB, AE. These phenotypes results in differences in degree of glycosylation of k-casein. The AA phenotype is less glycosylated then the BB variant. The most predominantly found glycosylate is sialic acid (also called N-acetyl Neuraminic).  Sialic acid is negatively charged and will therefore cause steric, hydrophilic and electrostatic interaction. Differences in growth rate and the formation of metabolites may also be caused  by differences in the genetic variants of β-lactoglobulin.
Project description
Yoghurt will be prepared from milk samples with different genetic variants of κ casein or β-lactoglobulin. Normal yoghurt starter bacteria, consisting of Str. thermophiles and Lb. bulgaricus, will be used.  Yoghurts can also be made using probiotic bacteria in combination with the Str. thermophiles and Lb. bulgaricus. The break-down of caseins will be studied, and the formation of lactic acids, volatiles and other metabolites.

Aim

Define the relationship of the differences between genetic variant of caseins and β-lactoglobulin on the properties of yoghurt.

For more information on this topic please contact Elsa Antunes Fernandes.

Topic 3. Physical-chemical characteristics of genetic variants of milk proteins.

This research will focus on the influence of genetic variants of milk proteins.
In the Milk Genomics Project relations have been studied between concentrations of genetic variants in milk and a start has been made with studying the characteristics of different genetic variants. This project is a continuation of the Milk Genomics project and consists of many projects can be done by MSc thesis students. Projects are often in collaboration with NIZO, the Animal Breeding and Genetics Group of WUR and Food Physics.

Project description

Project have been defined on the following topics.

• The influence of different genetic variants of β-kactoglobulin on the activity of endogenous milk enzymes such as plasmin and lipase.
• The formation of aggregates upon heating in samples with different genetic variants of β-lactoglobulin and κ-casein at different pH values.
• The size of casein micelles with the variant B of κ-casein is on average smaller than micelles with the A variant. This may influence the     concentrations of caseins and calcium in the milk serum.
• Heat stability may differ between milks with different genetic variants.
• Differences in break-down products after hydrolysis of different genetic protein variants after incubation with plasmin, pepsine or chymosin.
• The effect genetic variants on the viscosity  after concentration of milk.
• Other projects can easily be defined.

All experiments are done with milk samples of individual cows.

Aim

Determination Physical-chemical characteristics of genetic variants of milk protein.

For more information on this topic please contact Hein van Valenberg.

Topic 4. Influence of fatty acid composition and stereochemistry of triacylglycerols on physical properties of milk fat.

Selective breeding and feeding can make a significant contribution in increasing the unsaturated fraction. It has been reported that a change in the fatty acid composition leads to a change in the physical properties (polymorphic transitions and melting behavior) of milk fat; from these reports, it can be expected that fatty acid modifications will lead to a rearrangement of the TAG molecule which will also influence the physical properties of milk fat.  A shift in the stereochemistry of TAGs may also have an effect in the digestion and absorption of fatty acids, especially in new born babies.

Project description

Bovine MF is considered one of the most complex naturally occurring fats. It contains around 400 different individual FAs that can differ on length (C4-C22), level of saturation and type of double bond (cis or trans). Just 12 of them are present in amounts higher than 1 mol%, consequently they are depicted as major FA. If only the 12 major FA are taken into account, the possible FA arrangements in the TAG is 1728.Physical and chemical properties of the fat vary with the presence and arrangement of FAs in the TAG; crystallization and melting point of MF are two properties affected by the variations in TAG molecule. During crystallization, the variation in the configuration of the TAG affects the chain packing of the individual molecules, therefore the polymorphic behaviour. Crystallization affects the consistency of high-fat milk products; it influences the occurrence and rate of partial coalescence in oil-in-water emulsions, as a result churning and whipping processes also alter; crystallization also affects the mouth feel of butter-like products. Therefore, crystallization is an important characteristic in dairy technology.

Aim

The objective will be to find the causes of variation of the sn-position within the TAG molecule and the effect in the crystallization; this conclusion could be reached by analyzing MF with diverse profiles. With this knowledge, bovine MF could be modified for different needs, for instance as a source for infant formulas.

For more information on this topic please contact Daylan Tzompa Sosa.

Topic 5. The influence of shortening the dry period on processing characteristics of milk.

Dairy cows are commonly subjected to a dry period –a period without milking the cow- of approximately 8 weeks between two lactations. This dry period length results in a high milk production in the next lactation. A shorter dry period results in a lower peak production, which is beneficial for the health status of the cow. The project focuses on the influence of a shortened dry period on composition and processing quality of the milk. Obtaining high quality milk is a prerequisite for applying a shorter dry period. Besides that, changes in milk composition can help explaining the health status of the cow.

Project description

Variations in casein composition result in different processing characteristics of milk. Previous results indicate that casein composition changes in late lactation when the dry period is shortened. It is not known how these changes influence properties of casein micelles, how they are related to other components in milk, and eventually to the processability of milk. Processes that are of interest when casein composition changes are renneting (cheese making), acid gelation and heat treatment. Techniques that can be used in this project for characterisation of the milk are a.o. RP-HPLC and casein micelle size measurements. Product characteristics can be analysed with texture analysis or rheology measurements.

Aim

Evaluate the influence of shortening the dry period of the cow on processing characteristics of milk.

For more information on this topic please contact Kasper Hettinga or Ruben de Vries.

Topic 6. Changes of salt composition of late lactation milk in continuously milked cows.

Commonly, a period without milking the cow (‘dry period’) of 60 days is applied to dairy cows prior to calving. Omitting this dry period (‘continuous milking’) is hypothesised to be beneficial for cow health. This means that milk is obtained during the last 60 days before calving, which is normally not the case. 

Project description

Processing properties of late lactation milk of continuously milked cows have not been studied before. From previous work it is known that protein contents increase and lactose contents decrease during the last weeks of lactation when no dry period is applied. Salt concentrations have not been studied in this milk. These may change in very late lactation due to e.g. changes in milk yields and mammary cell structures. Changes in salt concentrations may influence processability of milk, and therefore are important to consider before continuous milking can be applied.

Aim

Analyse salt composition of milk samples of continuously milked cows at different time points in late lactation.

For more information on this topic please contact Kasper Hettinga or Ruben de Vries.