P. Walstra

Dairy technology: principles of milk properties and processes

This book covers the chemistry, physics, and microbiology of milk; the main unit operations applied in the manufacture of milk products; and procedures to ensure consumer safety, product quality, and efficient processing. The final section is devoted to describing the processes and interactions unique to the manufacture and maturation of cheese.

Relation between syneresis and rheological properties of particle gels

The relation between the tendency to exhibit syneresis of rennet (pH 6.65) and acid (pH 4.6) skim-milk gels and the rheological properties of these gels is discussed. Based on the syneresis model for milk gels of Van Dijk and Walstra [1, 4, 14, 19] it is reasoned that the average lifetime of the protein-protein bonds in the casein strands forming the gel network, the fracture force of these strands, and their flexibility are the main mechanical properties of importance. It is argued that the ratio of the loss modulus to the storage modulus (tan δ) as a function of the time scale of the measurement in dynamic experiments is a good measure of the first property. Results are given for rennet skim-milk gels at 25, 30, and 40°C and for an acid skim-milk gel at 30°C. Moreover, the stress strain relation and fracture behavior of both gels were assessed in creep measurements as a function of the duration of the applied stress. Rennet skim-milk gels have a lower fracture stress σf and therefore probably a lower fracture force per strand and a higher tan δ over long time periods than acid gels, while tan δ increases with temperature. As predicted, low fracture stresses and high tan δ correlated with an increased tendency to exhibit syneresis.

Properties of acid casein gels made by acidification with glucono-δ-lactone. 1. Rheological properties

The effects of gelation temperature (20, 30 or 40 °C), assay temperature, concentration of glucono-δ-lactone (GDL) added, and NaCl concentration on the rheological properties of acid casein gels were studied at small and large deformations. Gels prepared at a high incubation temperature had very low storage moduli (G′), whereas those made at a low incubation temperature had extremely high G′ values. A higher concentration of GDL resulted in faster gelation but slightly lower G′ values of aged gels. Addition of NaCl resulted in longer gelation times and a slower rate of increase of G′. Cooling of gels prepared at 30 or 40 °C resulted in an increase in G′. However, for gels formed at 20 °C, G′ decreased initially on cooling but returned to its original value on holding at 5 °C. The loss tangent (tan δ) of gels formed at 20 or 30 °C was independent of frequency; however, for gels formed at 40 °C, tan δ was lower at low frequencies. As gels were cooled to 5 °C, tan δ increased. Fracture stress (σfr) of gels formed at 20 °C was much greater than that of the gels formed at higher temperatures. Heating gels to temperatures higher than the gelation temperature resulted in a decrease in σfr. At low gelation temperatures, young gels had very high (σfr values.

Effects of structural rearrangements on the rheology of rennet-induced casein particle gels.

During ageing of casein or skim milk gels, structural changes take place that affect gel parameters, such as pore size and storage modulus. These changes can be explained in terms of rearrangements of the gel network at various length scales. In this paper, rheological experiments on rennet-induced casein gels and a general model on rearrangements are presented. The results of experiments (e.g. microscopy, permeametry) and computer simulations, the model, and recent literature on casein gels and other types of particle gels are compared to each other. Experiments presented include measurements of storage and loss moduli and maximum linear strain of the casein gels. Parameters varied were pH (5.3 and 6.65) and temperature (25 and 30 degrees C). In addition, the casein volume fraction (5-9 vol.%) was varied, which enables application of fractal scaling models. For rennet-induced casein gels, it is demonstrated that at the lower pH, all types of rearrangements proceed significantly faster. The rearrangements include: an increase in the size of compact building blocks; partial disappearance of fractal structure; and the formation of straightened strands, some of which eventually break. All of these rearrangements seem to be a consequence of particle fusion. There are indications of universality of the relation between particle fusion and gel syneresis for gels composed of viscoelastic particles.

Partial coalescence in oil-in-water emulsions 2. Influence of the properties of the fat

Abstract The influence of the properties of the fat in oil-in-water (o/w) emulsions on partial coalescence was investigated. As a surfactant either sodium dodecyl sulphate (SDS) or a protein was used. The presence of crystals in the fat globules of an o/w emulsion can tremendously enhance the instability of the emulsion. It was observed that in emulsions exhibiting partial coalescence a continuous fat network is present throughout the globules. The formation of such a network depends on the size of the crystals and of the globule: the larger the crystals and/or the smaller the globule the higher the solid fat content needed for a continuous fat network to be formed. Consequently, in most cases partial coalescence considerably increased with increasing solid fat content. Above a certain solid fat content, the rate of partial coalescence decreased again. The optimum solid fat fraction for partial coalescence depended both on the properties of the fat and on the velocity gradient to which the emulsion was subjected: the higher the velocity gradient, the lower the solid fat content at which the partial coalescence rate started to decrease again.

On the fractal nature of the structure of acid casein gels.

Acid casein gels are formed by aggregation of casein particles in an acid environment. Two different types of gels were formed and studied by rheological measurements at small (< 2%) and large deformations (like creep measurements). Gel type 1 was made by quiescent warming of a sodium caseinate dispersion of pH 4.6 from 4 to 30°C and gel type 2 was made by acidification of a sodium caseinate dispersion with glucono-δ-lactone (GDL) at 30°C. For small deformations, the dynamic moduli of type 1 gels were much higher than those of type 2 gels. The relative difference between the moduli of the two gel types was greater at lower casein concentrations, probably caused by a difference in the way the big aggregates of casein particles are linked together. The difference supposedly stems from the difference in voluminosity of casein particles in the cold (3.5–4.5 ml g−1) and at 30°C (about 2.5 ml g−1). For gel type 1 the gelation occurred at a high voluminosity; after the gel was formed the voluminosity of the casein particles decreased due to temperature increase, and this led to a stretching of the strands in the gel. The structure of gel type 2 did not change significantly after the gel was formed. The proposed explanation agreed with the results of creep measurements, showing the deformation at which fracture occurred to range between 0.5 and 0.6 for type 1 and between 1 and 1.2 for type 2 gels. Little difference was observed between the shear stresses at which fracture occurred for either type of gel and the relative difference was independent of the casein concentration. The rheological behaviour of both gels could be explained quantitatively using the concept of fractal geometry.

Gelation and retrogradation of concentrated starch systems. 1. Gelation.

Abstract Small deformation properties of potato and wheat starch suspensions were studied during heating and cooling at rest by a small amplitude dynamic rheological test method. Starch concentrations used were 10 to 30% w/w. The temperature to which the suspensions were heated varied from 65 to 90°C. During heating the moduli of the starch sytems at first increase and subsequently decrease. On prolonged heating the moduli depend greatly on heating temperature: the higher the temperature, the lower the moduli. Properties of concentrated starch systems during heating are related to swelling of granules, melting of crystallites, separation of amy lose from amylopectin and disentanglement of non-covalent bonds. The differences between the rheological properties of potato and wheat starch suspensions are discussed, and compared with those of chemically cross-linked potato starches. Our hypothesis for the explanation of the observed phenomena is that physical entanglements or chemical cross-links, naturally or artificially present in starch granules, reduce the swelling capacity of the granules and increase the stiffness of the swollen granules; as a consequence, they affect the rheological properties of concentrated starch suspensions.

Properties of acid casein gels made by acidification with glucono-δ-lactone. 2. Syneresis, permeability and microstructural properties

Syneresis of casein gels made by acidification with glucono-δ-lactone (GDL) was studied in relation to gel structure as derived from permeametry and confocal scanning laser microscopy (CSLM). Gels made at 40 °C exhibited ‘spontaneous syneresis’ with wheying-off almost immediately after gelation, while those formed at 30 °C exhibited little syneresis and only after it was initiated by wetting the surface. Syneresis decreased with a reduction in the pH of gels (increasing amount of GDL added). Gels cooled to 5 °C (before initiating syneresis) and low pH gels exhibited ‘negative syneresis’, i.e. an increase in the height of the gel after wetting. Addition of NaCl had little effect on syneresis, except at pH values > 4.6, where gels with added NaCl exhibited stronger syneresis than those made without added NaCl. Higher gelation temperatures resulted in a far greater permeability coefficient (B), indicating the presence of large pores in these gels. Gels formed at low temperatures had a very low B. Addition of NaCl, at all gelation temperatures, markedly reduced B. Confocal scanning micrographs showed that gels made at high gelation temperatures had large pores, many > 20 μm. At low gelation temperatures, the pores were small, mostly < 5 μm. Fractal aggregation theory was used to explain some of the results, especially the rearrangement of aggregated particles at an early stage of the gelation process, i.e. over relatively short distances. It was concluded that gelation temperature had a large effect on this rearrangement. At low temperatures (e.g. 20 °C), rearrangement did not occur, whereas it was already extensive at 30 °C, implying that the ‘building blocks’ of the fractal gel consisted of dense aggregates of, say, 25 casein particles. This resulted in increased permeability. It was also concluded that the syneresis occurring at 30 °C is primarily due to consolidation of the gel network under its own weight, which is soon counteracted by the stress induced by the deformation of this network. Gels formed at higher temperatures may be unstable and show spontaneous syneresis, presumably because some of the strands in the gel network were weak enough to break. No conclusive explanation could be given for the effects of NaCl concentration and pH on the properties of acid casein gels.

Rheological properties of casein gels.

The rheological properties of rennet-induced skim milk gels at 30 °C are compared with casein gels formed by acidification to pH 4·6 at 2 °C and subsequent heating to 30 °C. Both types of gels are viscoelastic. However, the gels formed by rennet action are relatively more viscous over time scales longer than 1 s. This implies that on average the protein-protein bonds in these gels are relatively more mobile than in the acid casein gels. It is thought that this is primarily due to differences in the structure of the casein particles and in the type of the main interaction forces in and between the casein molecules and particles.

Water in Casein Gels; How to Get it Out or Keep it In

Abstract An important problem related to food preparation starting from milk gels is to remove just the required amount of water in the case of cheese production or to prevent syneresis of liquid in most other cases. About 90% of the water present in milk gels is mechanically enclosed between the casein strands forming the network and most of the other water is mechanically enclosed in the casein particles (strands) forming the network. In this respect the presence or absence of so-called bound water is of no importance. It is the structure of the casein aggregates which determines the ease of removal of water from the gel. The effects of pH and temperature on the structure of the casein aggregates as analysed by rheometry, permeametry and NMR and the consequences for the ease of removal of most of the moisture from the gel are discussed.