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This study deals with the syneresis of curd. Rennet gels are primarily considered; some comparisons with acid milk gels are given.After curdling the milk, the curd tends to shrink; in other words, the network of aggregated paracasein micelles (PCM) will be under stress. If the curd is cut or - as was the case in our expirements - a curd surface is wetted, syneresis starts. The rate at which the whey is expelled depends on the pressure gradient in the whey and on the permeability of the network.In Chapter 2 the materials and methods generally used are described. Unless mentioned otherwise, standard conditions were used in the experiments. By standard conditions is meant: reconstituted skim milk with the saw dry matter content as the original milk, to which 500 ppm rennet was added; the temperature during the whole experiment was kept at 30 °C; no CaCl 2 was added.The endogenous syneresis pressure ( Ps) appeared to be very low, about 1 Pa. In Chapter 3 two methods are described which give an order of magnitude of the stresses involved. Moreover, the weight of the network can cause an additional pressure. The maximum pressure caused by the weight ( Pg) at a level hc below the interface is (ρ curd -ρ curd ) ghc≈75hc Pa (hc in m).The permeability measurements are described in Chapter 4. Two methods were used; in both, the flow of whey through a vertical column of curd was measured as a function of head pressure. A problem is that the curd is deformed during the experiment. In the "tube" method, deformation is a function of the pressure gradient (d Pt /dx), the diameter of the tube holding the curd (d t ), and the rigidity of the gel. In the second method the "torsionflux" method, the deformation was adjustable. 'The tube method led to the following results.- The permeability is of the order of 10 -13m 2.- Permeability increases with time, which is ascribed to "microsyneresis", i.e. syneresis at local sites in the gel. The rate of increase is approximately constant.- The increase in permeability (d B /d t ) is higher for a higher pressure gradient or a wider tube; both lead to larger deformation of the curd.- The change of the permeability with time in the absence of deformation (d Be /d t ) was obtained by applying the head pressure at different times after addition of rennet. Shortly after clotting permeability increases fastest. Between 1 and 24 hd Be /d t was constant.- The permeability of curd made from ultrafiltered skim milk ( B ( i )) and its change with time (d B ( i )/d t ) were determined. Thisyielded the permeability as a function of concentration and time ( B ( i,t )).- The permeability also depends on temperature, CaCl 2 concentration, acidity, fat content and type of skim milk.- In acid milk geld permeability was of the same order of magnitude, but it hardly changed with time.The rheological behaviour of curd is discussed in Chapter 5. The dynamic measurments with the "Den Otter" rheometer show that the moduli G ' and G " kept increasing for a long time (~3 h) after rennet addition. From the dependence of G' and G" on the angular frequency it was deduced that G" is due to the relaxation of bonds and that the relaxation time is a few times 10 s.The instantaneous shear modulus ( G0 ) was determined as a function of protein concentration. The obtained relation can be explained in term of an only partly effective contribution of the casein to the network; this contribution being relatively smaller at lower concentrations. Also from the creep measurements it was concluded that the endogenous syneresis pressure was less than 10 Pa.If both permeability and pressure are known for all values of concentration (or relative remaining volume ( i )) and time ( t ), the syneresis can in principle be calculated. This is in the model described in Chapter 6, in which the equation of Darcy is combined with the equation of continuity. A numerical procedure is developed, for a one dimensional case; the syneresis of a thin slab.The pressure in the whey is the sum of the endogenous syneresis pressure ( Ps) and the pressure caused by the weight of the network ( Pg). For Ps( i ) and Pg( i ) some trial functions were considered.In Chapter 7 the syneresis of slabs is studied. The results of the experiments show that initiallyΓ= dlogΔH/dlog t is about 0.5. For t >0.5 hΓincreases to ~0.78.Γis independent of the original thickness of the slab ( H0 ) during a certain period (penetration period). The length of this period depends on H0.After one day H did not change any more and H∞ / H0 was about one third. The best fit between model calculations and experimental results was obtained if it was assumed that:- the permeability increases with time ( t ) and decreases with i , as was found in the experiments,- endogenous syneresis pressure (Ps) decreases only with shrinkage, - maximum gravitational pressure ( Pbg) is constant,- P0s= Pbg= 1 Pa ( H0 = 10 mm).P0swas found to be a function of time after renneting, at first increasing, then (after 1 - 2 h) decreasing. However, the introduction of such a relation in the model did not improve the fit to the experimental results. After all, the pressure cannot relax twice, both by shrinkage and by "ageing".The effects of several parameters (pH, temperature, Ca concentration, etc.) on milk clotting, gel permeability, syneresis and curd rigidity are interrelated. A survey is given in Table 7.2 and a tentative explanation is summarized in Table 7.3.In Chapter 8 it is shown that external pressure has a dramatic effect m the syneresis rate. Extrapolation to zero external pressure yields, again, an endogenous syneresis pressure of about 1 Pa. |
