- Email: [email protected]
Syneresis of submerged single curd grains and curd rheology
International Dairy Journal 10 (2000) 489}496
Syneresis of submerged single curd grains and curd rheology Andrea Unger Grundelius *, Kristina Lodaite , Karin OG stergren , Marie Paulsson , Petr Dejmek Food Engineering, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden Food Technology, Kaunas University of Technology, Radvilenu pl. 19, Kaunas 3028, Lithuania Received 28 September 1999; accepted 15 June 2000
Abstract Syneresis of a single curd grain submerged in ultra"ltrated milk permeate was determined over time by progressive dilution of an added tracer, blue dextran. The in#uence of pH, rennet concentration and the size of the curd grain was investigated. Dynamic rheological measurements were performed with renneted milk under the same conditions. Analysis of variance showed that grain size was the most important factor in#uencing shrinkage of the grain in the initial stage of syneresis. Smaller curd grain size resulted in more intense syneresis. The in#uence of pH and rennet concentration on shrinkage were of lesser importance. At a later stage of syneresis, pH was shown to be the dominating factor with the shrinkage being more pronounced at a lower pH. Inclusion of the storage modulus of curd in the syneresis model did not improve the prediction. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Syneresis; Curd grain; Tracer method; Rheology
1. Introduction The expulsion of whey from coagulum (syneresis) is caused by the contraction of the curd due to the rearrangement of bonds between protein aggregates (Fox, 1987; Walstra & Jenness, 1984). Syneresis is a complex process involved in many of the steps in cheese making. Syneresis can occur spontaneously but can also be driven by external forces. The pressure of endogenous syneresis in contracting curd is small, of the order of 1 Pa (van Dijk, Walstra, & Geurts, 1979). If the curd is constrained, shrinkage is hindered. However, local syneresis can take place, leading to shrinkage in small regions in the curd while large pores are formed in adjacent regions (van Dijk, 1982; Walstra & Jenness, 1984). The rate of syneresis is determined by the pressure gradient developed in the network and by the #ow resistance through the gel network (Walstra, van Dijk, & Geurts, 1985). The permeability of the network determines the #ow resistance. All parameters involved in the syneresis process are dependent on time and the stage of the shrinkage of the network. Factors empirically known to a!ect syneresis
* Corresponding author. E-mail address: [email protected] (A. Unger Grundelius).
are mechanical treatment (cutting, stirring and pressing), pH and the composition of the milk, pretreatment of the milk, heating and cooling rates as well as the volume/ surface ratio of the curd (Patel, Lund, & Olson, 1972; Pearse & Mackinlay, 1989; Walstra et al., 1985; Walstra & Jenness, 1984) whereas the e!ect of the chymosin/ pepsin ratio of rennet is small (Andersson & AndreH n, 1990). Many studies on the relations between syneresis properties of casein gels and their rheological properties have been presented but reports of the e!ects of the storage modulus (G ) on the syneresis behaviour are not conclusive (Lelievre, 1977; van Dijk, 1982; Roefs, 1986; van den Bijgaart, 1988; Zoon, 1988). The analysis of van Vliet, van Dijk, Zoon and Walstra (1991) concludes that syneresis is not dependent on G , but increases with increasing tan d ("G /G ) and decreased fracture stress. The relation is indirect and there is an optimal rate of bond relaxation. Di!erent approaches have been used to determine syneresis as a function of time, such as the direct measurement of the shrinkage of the curd, the amount of whey expelled found by estimating the volume drained or by the progressive dilution of a tracer added to the whey, estimating the dry matter of the curd or measuring the density of the curd (Beeby, 1959; Lawrence, 1959; Lawrence & Hill, 1974; Zviedrans & Graham, 1981; Marshall, 1982; Pearse, Mackinlay, Hall, & Linklater,
0958-6946/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 8 - 6 9 4 6 ( 0 0 ) 0 0 0 8 0 - 7
490
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
1984; Nilsen & Abrahamsen, 1985; Walstra, van Dijk, & Geurts, 1987; von BuK eler, Jakob, & Puhan, 1997; Renault, Gastaldi, Lagaude, Cuq, & Tarodo De La Fuente, 1997). The parameters controlling syneresis under di!erent conditions are still unclear. Temperature, pH, rennet concentration and the overall composition of the curd will determine the strength of the curd and its permeability. The balance between the forces contracting the curd, the rheological properties of the curd network and the #ow resistance determines the rate of syneresis. The impact of the permeability on the shrinkage rate of the curd grain is highly dependent on grain size. By varying the size of the curd grain, the permeability in#uencing the syneresis can be distinguished from the e!ects of the syneresis pressure of the curd. In this study, targeted at soft-cheese production, we investigated the in#uence of pH, rennet concentration and grain size (volume/surface ratio) on the syneresis of a single curd grain and the rheological properties of curd made of reconstituted skim milk. Milk powder was chosen to avoid seasonal variations of milk and the powder was reconstituted according to a standardised preparation. Skim milk (not whole milk) powder was used in order to avoid the uncertainties caused by the damage of the milk fat globule membrane during drying. Native fat globules a!ect the gel network and may alter the #ow resistance. Seasonal variation in fat globule size distribution would complicate the evaluation of the results. Thus, we chose to work with skim milk powder without cream addition. Syneresis of a single curd grain was measured using progressive dilution of blue dextran (BD) in ultra"ltrated (UF) milk permeate. The dilution of the tracer was measured spectrophotometrically as a function of time.
2. Materials and methods 2.1. Preparation of skim milk The standard skim milk solution was prepared by mixing 9.1% (w/w) of low-heat skim milk powder (INRA, Technologie Laitie`re, Rennes, France) with Milli Q water (resistance 18 M) cm\) which had been deionized, distilled and passed through a Millipore Q puri"cation system (Millipore Corporation, Bedford, MA). The skim milk was stirred for 20 min and the pH was adjusted (6.4, 6.2 or 6.0) by adding 9% (v/v) lactic acid (BDH Laboratory Supplies, Poole, UK) at 203C. The skim milk was stored at 43C at a minimum time of 20 min. Skim milk powder that was held for 24 h at 333C produced the same curd grain syneresis behaviour as skim milk powder held 20 min at 43C. Before use, the skim milk was stirred for 5 min and the pH was checked and adjusted at 203C.
2.2. Preparation of curd grains Cylindrical polypropylene syringes (20, 10 and 5 mL) were used for the preparation of cheese curd grains. The needle end of the syringe was cut-o! squarely from syringes with diameters of 21, 16 or 13 mm and their openings were covered with para"lm. Skim milk samples were preheated at 333C for 15 min before renneting. Renneted skim milk samples were prepared at 333C by adding rennet (Batch 97255, Bovine Rennet SBI Gand-gassiot SOREDAB, La Boissie`re-Ecole, France) to a "nal rennet concentration of 210 lL L\ of milk or 105 lL L\. The rennet contained 520 mg chymosin L\ and the ratio chymosin/pepsin was 2 : 1. The syringes were "lled with skim milk (8.0, 3.0 and 1.5 mL) immediately after the addition of the rennet and kept in a water bath at 333C for 70 min. 2.3. Production of ultraxltrated milk permeate with addition of blue dextran Ultra"ltered milk permeate (UF milk permeate) was produced from commercial consumer milk (MinimjoK lk, Ska nemejerier, Sweden) with a fat content of less than 0.1% (w/w). The milk was slowly preheated to 503C for approximately 20 min and maintained at 503C for at least 20 min before ultra"ltration. As a preservative, 40 mg L\ phenoxymethylpenicillinkalium (Ka vepenin, Astra LaK kemedel, Lund, Sweden) was added. Ultra"ltration was carried out using a hollow "bre membrane (H5P10-43, Amicon Div., MA, USA) with an area of 0.45 m and a cut-o! of 10,000 MW. The set-up was sterilised with 200 ppm NaOCl (15% of active Cl ) at room temperature for 30 min and rinsed with deionised water. The ultra"ltration unit was heated with deionised water to 503C and drained on the retentate side just prior to the experiment. The milk was circulated with a volumetric concentration factor of two, as the permeate was removed. The inlet and outlet gauge pressures of the ultra"ltration unit were 1.5 and 0.4 bar respectively. The pH of the UF milk permeate was adjusted to the same pH as the skim milk samples by adding 9% (v/v) lactic acid at 203C. Blue dextran (BD) (Sigma Chemical CO, St. Louis, MO, USA) was added at a concentration of 0.2% (w/w) to the UF milk permeate. Di!usion and absorption of BD into the curd are prevented by the high molecular weight of BD (2,000,000 Da) (Zviedrans & Graham, 1981). The UF permeate solution, &blue whey', was stirred with a magnetic stirrer for 20 min and stored at 43C before being used in syneresis measurements. 2.4. Method for measuring syneresis The method for measuring syneresis is a further development of a tracer method devised by Zviedrans and Graham (1981) and von BuK eler et al. (1997). After 70 min
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
491
equation < "< (A /A !1), (1) R 5 R where < is the total volume (mL) of whey expelled R from one curd grain at time t when the sample was taken, < is the initial volume (mL) of &blue whey', A is the 5 initial absorbance of UF milk permeate with BD at 620 nm before syneresis and A is the absorbance of the R sample at 620 nm at time t. 2.5. Rheological measurements Fig. 1. Experimental set-up of the method.
of renneting, the curd grain was gently pushed out by a piston from the syringe and carefully placed on a plastic draining surface in thermostatically controlled &blue whey', avoiding exposure to air (Fig. 1). Submerging the curd grain in &blue whey' initiated the syneresis, corresponding to the &cutting' time in cheese making. The curd grain with a diameter of 21 mm was completely submerged in 17 mL\ &blue whey', while the grains of 16 and 13 mm diameter were submerged in 6 and 9.5 mL\, respectively. In order to improve the accuracy of the measurements for the smallest grains, three grains were used in these experiments. The temperature during the experiment was kept constant at 333C and the syneresis was measured for 100 min. To have homogeneous samples for the absorbance measurements, the &blue whey' was gently stirred during the syneresis with a magnetic stirrer. All experiments were carried out with continuous stirring at 32 rpm, causing no visible deformation of the grain. To judge the impact of continuous stirring on curd shrinkage, experiments were checked by stirring for only 30% of the time, resulting in no signi"cant di!erence in syneresis. A stirring rate higher than 100 rpm resulted in visible deformation of the grains. The impact on the curd resulted in the release of small gel particles and no reliable absorbance measurements could be made. Samples of diluted &blue whey' (0.8 mL) were analysed every 5 min. The absorbance of the sample was measured at 620 nm relative to UF milk permeate and transfered back with a pipette after each measurement. The absorbance was monitored using a Beckman DU-50 spectrophotometer (Beckman Instruments Inc, Irvine, CA). Syneresis experiments were carried out in triplicate for each condition. 2.4.1. Amount of whey expelled The amount of whey expelled from the curd grain is based on the measurement of the progressive dilution of BD in &blue whey' (Beeby, 1959; Zviedrans & Graham, 1981; von BuK eler et al., 1997). The total volume of whey expelled from one curd grain is calculated using the
The dynamic rheological behaviour of renneted skim milk was studied in a concentric cylinder cell (C 14) of the Bohlin VOR rheometer (Metric Analys, Stockholm, Sweden). A torsion bar of 0.23 g cm was used in the experiments and the strain was kept below 0.0103. Skim milk was prepared with a known pH (see above). The solution was preheated for 15 min at 333C. Rennet was added to give the desired concentration (see above) to a skim milk sample of 3 mL. The solution was mixed and poured into the cup of the rheometer. The addition of rennet corresponds to time zero. The gel formation was followed at a frequency of 0.5 Hz by measuring the storage modulus (G ), Pa, and the viscous modulus (G ), Pa, at intervals of 60 s during 170 min at 333C. Rheological measurements were performed at least in triplicate for each condition. 2.6. Statistical analysis Data from the syneresis and rheological measurements were evaluated statistically with ANOVA, analysis of variance, and linear regression (MINITAB Release 12., Statistical Software, Minitab Inc., PA, USA).
3. Results and discussion 3.1. Syneresis of curd grains The shrinkage (%) of one curd grain was calculated by the amount of whey (< ) expelled (Eq. (1)) divided by the initial volume of the curd grain. All data series were "tted to a three-parameter exponential equation describing the shrinkage process over time: y"y #a (1!e\@R), (2) where y is the shrinkage in percent, t is time and y corresponds to the initial expulsion of whey as the grain is being placed on the draining surface in the &blue whey'. y #a is the asymptote of the equation and b is a rate constant describing the expulsion. Mean values of the experimental data and the "tted data of curd shrinkage as a function of time are presented in Fig. 2. Error
492
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
Fig. 2. Shrinkage (%) of a curd grain as a function of time. Time zero corresponds to the time of &cutting', i.e. submerging the curd grain in liquid 70 min after rennet addition. Curd grain diameter (a) and (d) 13 mm; (b) and (e) 16 mm; (c) and (f ) 21 mm. Rennet concentrations ((a)}(c)) 210 lL L\ of milk; ((d)}(f )) 105 lL L\ of milk. Points with error bars show the mean value of the experimental data and the mean standard deviation of the method for curd produced at di!erent pH as a function of time; pH 6.0 (䢇), pH 6.2 (*) and pH 6.4 (䉲). Curve "tting of the experimental data; pH 6.0 (*), pH 6.2 (2) and pH 6.4 (} } }).
bars show the mean standard deviation of the experimental data. The six graphs illustrate data series as points at three pH values (6.4, 6.2 and 6.0) for speci"c grain sizes (21, 16 and 13 mm) and rennet concentrations (105 and 210 lL L\ of milk). The triplicate experiments show good agreement, the mean standard deviation being 1.0% (comparable to 5.4% in von BuK eler et al., 1997). E!ects of the process parameters investigated, namely pH, rennet concentration and curd grain size, on syneresis of the curd grains can be seen, even though, how the process parameters a!ect the syneresis is not obvious without further analysis. In Eq. (2) the three parameter estimates (y , a and b) are strongly correlated and therefore not suitable for further evaluation. To detect a di!erence in the factors determining curd grain shrinkage in the initial stage of syneresis compared to a later stage, two time points were chosen, 80 and 170 min after the addition of rennet corresponding to 10 and 100 min after &cutting'. For each experiment, the shrinkage values at 80 and 170 min were interpolated using Eq. (2) and the data were evaluated statistically by analysis of variance, ANOVA. The in#uences of grain size, pH, rennet concentration and their second-order cross e!ects were investigated (Table 1). Curd grain size was the most signi"cant factor in#uencing the initial stage of syneresis (80 min). The in#uences of pH and rennet concentration were statistically signi"cant, but, as the signi"cant cross term indicates, the
Table 1 In#uence of grain size, pH, rennet concentration at 80 and 170 min after rennet addition according to statistical analysis of variance (ANOVA) Process parameter (value)
DF
Shrinkage at 80 min
Grain size (13, 16, 21 mm) pH (6.0, 6.2, 6.4) Rennet conc. (105, 210 lL L\) Grain size;pH Grain size;rennet conc. pH;rennet conc. Mean residual error
170 min
F
P
2
22.76
0.000
4.90
0.015
2 1
10.93 1.70
0.000 0.204
40.22 3.26
0.000 0.082
4 2
2.78 1.03
0.047 0.369
23.05 2.47
0.000 0.104
2
4.78
0.017
12.05
0.000
$1.8% shrinkage
F
P
$1.5% shrinkage
DF"Degrees of freedom, F"F-statistic for the factor in question, P"Statistical signi"cance, the probability that e!ect of the factor is zero.
e!ects were not additive in the initial stage of syneresis. In the later stage of syneresis (170 min), pH was the most signi"cant factor (Table 1) but again, grain size and rennet concentration showed signi"cant non-additive e!ects.
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
493
Fig. 3. Shrinkage (%) of curd grains illustrated as surface models involving pH and grain size at the rennet concentration (a) and (b) 210 lL L\ of milk; (c) and (d) 105 lL L\ of milk. The shrinkage surface at 80 min ((a) and (c)) was obtained from a three-parameter model (Eq. (3a)); at 170 min ((b) and (d)) the shrinkage was from a two-parameter model (Eq. (4)).
All possible subsets of linear regressions were performed on the data to "nd a most parsimonious response model in terms of the in#uence of grain size, pH, rennet concentration and their cross e!ects at the chosen times after the initiation of syneresis. At 80 min the best models describing syneresis were two di!erent, three-parameter models (Eqs. (3a) and (3b)) with r of 0.64 and 0.63, respectively. shrinkage (%) at 80 min"17.8!0.535 (grain size) #0.329 (rennet concentration) !0.0518 (pH;rennet concentration), (3a)
shrinkage (%) at 80 min "17.7#0.280 (rennet concentration) !0.0854 (pH;grain size) !0.0438 (pH;rennet concentration).
(3b)
At 170 min the following two-parameter model (Eq. (4)) described the shrinkage su$ciently with r of 0.49; the model was not further improved by including the in#uence of grain size.
494
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
Fig. 4. Storage modulus G for skim milk gels at two rennet concentrations (105 and 210 lL L\ of milk) shown as a function of time at pH 6.0 (䢇); 6.2 (*); and 6.4 (䉲). Error bars show the mean standard deviation of the experimental data at the time of &cutting', 70 min.
shrinkage (%) at 170 min"32.9 #0.538 (rennet concentration) !0.0880 (pH;rennet concentration).
(4)
Models (3a) and (4) describing syneresis are shown as surfaces in Fig. 3 where shrinkage (%) is a function of grain diameter and pH for the di!erent rennet concentrations and time points. Under the same conditions, a smaller curd grain shrinks faster than a bigger grain in the initial stage of syneresis (80 min). This is consistent with the view that initially, whey is lost from the surface region of the grain, and thus the in#uence of the volume/surface ratio, being approximately equal to the grain diameter, dominates. Comparing the syneresis of curd grains at di!erent pH shows a more intense shrinkage at the lower pH, independent of time and rennet concentration. These results agree with the pH dependence of permeability for renneted milk gels where permeability of constrained gels produced at pH 6.0 or 6.4 di!er by a factor of two (van Dijk, 1982; van den Bijgaart, 1988; van Vliet, Roefs, Zoon, & Walstra, 1989). It is well known that lowering the pH enhances syneresis (Patel et al., 1972; Walstra & Jenness, 1984; Walstra et al., 1985; Pearse & Mackinlay, 1989). The e!ect of grain size was of much less importance in the later stage than in the initial stage of syneresis. In the later stage the curd is closer to a putative equilibrium, which should be independent of water transport kinetics. 3.2. Curd rheology The storage modulus (G ) and the viscous modulus (G ) of renneted skim milk were measured under the same
conditions (pH, rennet concentration, temperature and time) as the syneresis experiments. In Fig. 4 the storage modulus is shown as a function of time for skim milk gels prepared with di!erent rennet concentrations (105 and 210 lL L\ of milk) and pH values (6.0, 6.2 and 6.4). As expected, the storage modulus increased faster at lower pH and higher rennet concentration (van Dijk, 1982; Roefs, 1986; Zoon, 1988; van Vliet et al., 1989). The milk gels produced in our syneresis experiments had di!erent G values at &cutting' time (70 min after rennet addition). tan d ("G /G ) remained approximately constant (0.24}0.26) for all pH and rennet combinations at &cutting' time and later, in accordance with the results of Zoon (1988). 3.3. Rheology and syneresis van Vliet et al. (1991) reported that tan d in dynamic rheological measurements is a good parameter for predicting syneresis properties. However, in our case tan d was approximately equal for all experimental conditions. The one rheological variable that did vary was G . Storage modulus often correlates with yield strength, and yield strength is expected to a!ect syneresis pressure and permeability. Statistical analysis of variance (ANOVA) was performed to investigate the in#uence of G at 70 min and grain size on shrinkage (Table 2). G was in this case treated as a covariate. As expected, grain size was the most important factor in the initial stage of syneresis. The in#uence of the G on shrinkage of the curd grain was of low signi"cance at both 80 min and at 170 min after rennet addition. Since pH and rennet concentration can predict the storage modulus of the gel, the correlation of shrinkage with G is not surprising. However, inclusion of the storage modulus in the linear regression equations
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496 Table 2 In#uence of grain size and storage modulus at 70 min on curd grain shrinkage at times 80 and 170 min after rennet addition according to statistical analysis of variance (ANOVA) Process parameter (value)
DF Shrinkage at 80 min
G Grain size (13, 16, 21 mm) Mean residual error
1 2
495
which is funded by the Nordic Academy for advanced Study (NorFA).
References 170 min
F
P
F
P
4.85 15.90
0.034 0.000
4.17 0.11
0.048 0.900
$2.4% shrinkage $3.8% shrinkage
DF"Degrees of freedom, F"F-statistic for the factor in question, P"Statistical signi"cance, the probability that e!ect of the factor is zero.
did not improve the prediction of shrinkage. Possibly, properties at lower deformation rates and larger deformations are more relevant. In addition, the curd is constrained in the rheometer, whereas it shrinks globally during syneresis.
4. Conclusions Measurements on individual grains showed that the syringe method of preparation of single grains was satisfactorily reproducible and there was little variability between individual grains. Generally, our "ndings agree well with expectations. The grain size was the most important factor determining the shrinkage of the curd grain in the initial stage of the syneresis, but in the later stage no signi"cant e!ect could be found. The in#uence of pH on the syneresis was the dominating factor a!ecting the shrinkage in the later phase of syneresis and a lower pH resulted in a more intense syneresis. The storage modulus of the curd at &cutting' time did not predict syneresis well.
Acknowledgements Financial support has been received from EU-FAIR Concerted Action project (TIROS), the Nordic research network &Milk Proteins*Structure and Functional Properties', NorFA, Norway and the Nordic Council of Ministers via the Swedish Institute. This paper was presented at the workshop on &Milk proteins*Structure and Functional Properties' held in Naantali, Finland, 28}30 November 1998. The workshop was organised as part of activities of the Nordic network on Milk Proteins,
Andersson, H., & AndreH n, A. (1990). In#uence of chromatographically pure bovine chymosin and pepsin A on cheese curd syneresis. Journal of Dairy Research, 57, 119}124. Beeby, R. (1959). A method for following the syneresis of the rennet coagulum in milk. The Australian Journal of Dairy Technology, April}June, 77}79. Fox, P.H. (1987). Cheese: Chemistry, physics and microbiology. General aspects. vol. 1. London: Elsevier Applied Science. Lawrence, A. J. (1959). Syneresis of rennet curd. Part 1*e!ect of time and temperature. The Australian Journal of Dairy Technology, October}December, 166}169. Lawrence, A. J., Hill, R. D. (1974). A method for measuring the syneresis of cheese-curd. Proceeding of the 19th International Dairy Congress 1E (pp. 204}205). Lelievre, J. (1977). Rigidity modulus as a factor in#uening the syneresis of renneted milk gels. Journal of Dairy Research, 44, 611}614. Marshall, R. J. (1982). An improved method for measurement of the syneresis of curd formed by rennet action on milk. Journal of Dairy Research, 49, 329}336. Nilsen, K. O., & Abrahamsen, R. K. (1985). Di$culties in measuring the syneresis of goat milk rennet curd by dilution of an added tracer. Journal of Dairy Research, 52, 209}212. Patel, M. C., Lund, D. B., & Olson, N. F. (1972). Factors a!ecting syneresis of renneted milk gels. Journal of Dairy Science, 55(7), 913}918. Pearse, M. J., & Mackinlay, A. G. (1989). Biochemical aspects of syneresis: A review. Journal of Dairy Science, 72, 1401}1407. Pearse, M. J., Mackinlay, A. G., Hall, R. J., & Linklater, P. M. (1984). A microassay for the syneresis of cheese curd. Journal of Dairy Science, 51, 131}139. Renault, C., Gastaldi, E., Lagaude, A., Cuq, J. L., & Tarodo De La Fuente, B. (1997). Mechanisms of syneresis in rennet curd without mechanical treatment. Journal of Food Science, 62(5), 907}910. Roefs (1986). A study of gels formed in the cold. Ph.D. thesis, Wageningen Agricultural University, The Netherlands. van den Bijgaart, H. J. C. M. (1988). Syneresis of rennet-induced milk gels as in-uenced by cheesemaking parameters. Ph.D. thesis, Wageningen Agricultural University, The Netherlands. van Dijk, H. J. M. (1982). Syneresis of curd. Ph.D. thesis, Wageningen Agricultural University, The Netherlands. van Dijk, H. J. M., Walstra, P., & Geurts, T. J. (1979). Preliminary note on syneresis pressure in rennet curd. Netherlands Milk and Dairy Journal, 33, 60}61. van Vliet, T., Roefs, S. P. F. M., Zoon, P., & Walstra, P. (1989). Rheological properties of casein gels. Journal of Dairy Science, 56, 529}534. van Vliet, T., van Dijk, H. J. M., Zoon, P., & Walstra, P. (1991). Relation between syneresis and rheological properties of particle gels. Colloid and Polymer Science, 269, 620}627. von BuK eler, T., Jakob, E., & Puhan, Z. (1997). Bestimmungsmethode fuK r die SynaK rese von Labgallerten. Milchwissenschaft, 52(3), 131}133. Walstra, P., & Jenness, R. (1984). Dairy chemistry and physics. USA: Wiley. Walstra, P., van Dijk, H. J. M., & Geurts, T. J. (1985). The syneresis of curd. 1. General considerations and literature review. Netherlands Milk and Dairy Journal, 39, 209}246.
496
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
Walstra, P., van Dijk, H. J. M., Geurts, T. J. (1987). The syneresis of curd. In P. H. Fox, Cheese: Chemistry, physics and microbiology, vol. 1, General aspects (pp. 135}177). London: Elsevier Applied Science. Zoon, P. (1988). Rheological properties of rennet-induced skim milk gels. Ph.D. thesis, Wageningen Agricultural University, The Netherlands.
Zviedrans, P., & Graham, E. R. B. (1981). An improved tracer method for measuring the syneresis of rennet curd. The Australian Journal of Dairy Technology, 36(3), 117}120.
Syneresis of submerged single curd grains and curd rheology Andrea Unger Grundelius *, Kristina Lodaite , Karin OG stergren , Marie Paulsson , Petr Dejmek Food Engineering, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden Food Technology, Kaunas University of Technology, Radvilenu pl. 19, Kaunas 3028, Lithuania Received 28 September 1999; accepted 15 June 2000
Abstract Syneresis of a single curd grain submerged in ultra"ltrated milk permeate was determined over time by progressive dilution of an added tracer, blue dextran. The in#uence of pH, rennet concentration and the size of the curd grain was investigated. Dynamic rheological measurements were performed with renneted milk under the same conditions. Analysis of variance showed that grain size was the most important factor in#uencing shrinkage of the grain in the initial stage of syneresis. Smaller curd grain size resulted in more intense syneresis. The in#uence of pH and rennet concentration on shrinkage were of lesser importance. At a later stage of syneresis, pH was shown to be the dominating factor with the shrinkage being more pronounced at a lower pH. Inclusion of the storage modulus of curd in the syneresis model did not improve the prediction. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Syneresis; Curd grain; Tracer method; Rheology
1. Introduction The expulsion of whey from coagulum (syneresis) is caused by the contraction of the curd due to the rearrangement of bonds between protein aggregates (Fox, 1987; Walstra & Jenness, 1984). Syneresis is a complex process involved in many of the steps in cheese making. Syneresis can occur spontaneously but can also be driven by external forces. The pressure of endogenous syneresis in contracting curd is small, of the order of 1 Pa (van Dijk, Walstra, & Geurts, 1979). If the curd is constrained, shrinkage is hindered. However, local syneresis can take place, leading to shrinkage in small regions in the curd while large pores are formed in adjacent regions (van Dijk, 1982; Walstra & Jenness, 1984). The rate of syneresis is determined by the pressure gradient developed in the network and by the #ow resistance through the gel network (Walstra, van Dijk, & Geurts, 1985). The permeability of the network determines the #ow resistance. All parameters involved in the syneresis process are dependent on time and the stage of the shrinkage of the network. Factors empirically known to a!ect syneresis
* Corresponding author. E-mail address: [email protected] (A. Unger Grundelius).
are mechanical treatment (cutting, stirring and pressing), pH and the composition of the milk, pretreatment of the milk, heating and cooling rates as well as the volume/ surface ratio of the curd (Patel, Lund, & Olson, 1972; Pearse & Mackinlay, 1989; Walstra et al., 1985; Walstra & Jenness, 1984) whereas the e!ect of the chymosin/ pepsin ratio of rennet is small (Andersson & AndreH n, 1990). Many studies on the relations between syneresis properties of casein gels and their rheological properties have been presented but reports of the e!ects of the storage modulus (G ) on the syneresis behaviour are not conclusive (Lelievre, 1977; van Dijk, 1982; Roefs, 1986; van den Bijgaart, 1988; Zoon, 1988). The analysis of van Vliet, van Dijk, Zoon and Walstra (1991) concludes that syneresis is not dependent on G , but increases with increasing tan d ("G /G ) and decreased fracture stress. The relation is indirect and there is an optimal rate of bond relaxation. Di!erent approaches have been used to determine syneresis as a function of time, such as the direct measurement of the shrinkage of the curd, the amount of whey expelled found by estimating the volume drained or by the progressive dilution of a tracer added to the whey, estimating the dry matter of the curd or measuring the density of the curd (Beeby, 1959; Lawrence, 1959; Lawrence & Hill, 1974; Zviedrans & Graham, 1981; Marshall, 1982; Pearse, Mackinlay, Hall, & Linklater,
0958-6946/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 8 - 6 9 4 6 ( 0 0 ) 0 0 0 8 0 - 7
490
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
1984; Nilsen & Abrahamsen, 1985; Walstra, van Dijk, & Geurts, 1987; von BuK eler, Jakob, & Puhan, 1997; Renault, Gastaldi, Lagaude, Cuq, & Tarodo De La Fuente, 1997). The parameters controlling syneresis under di!erent conditions are still unclear. Temperature, pH, rennet concentration and the overall composition of the curd will determine the strength of the curd and its permeability. The balance between the forces contracting the curd, the rheological properties of the curd network and the #ow resistance determines the rate of syneresis. The impact of the permeability on the shrinkage rate of the curd grain is highly dependent on grain size. By varying the size of the curd grain, the permeability in#uencing the syneresis can be distinguished from the e!ects of the syneresis pressure of the curd. In this study, targeted at soft-cheese production, we investigated the in#uence of pH, rennet concentration and grain size (volume/surface ratio) on the syneresis of a single curd grain and the rheological properties of curd made of reconstituted skim milk. Milk powder was chosen to avoid seasonal variations of milk and the powder was reconstituted according to a standardised preparation. Skim milk (not whole milk) powder was used in order to avoid the uncertainties caused by the damage of the milk fat globule membrane during drying. Native fat globules a!ect the gel network and may alter the #ow resistance. Seasonal variation in fat globule size distribution would complicate the evaluation of the results. Thus, we chose to work with skim milk powder without cream addition. Syneresis of a single curd grain was measured using progressive dilution of blue dextran (BD) in ultra"ltrated (UF) milk permeate. The dilution of the tracer was measured spectrophotometrically as a function of time.
2. Materials and methods 2.1. Preparation of skim milk The standard skim milk solution was prepared by mixing 9.1% (w/w) of low-heat skim milk powder (INRA, Technologie Laitie`re, Rennes, France) with Milli Q water (resistance 18 M) cm\) which had been deionized, distilled and passed through a Millipore Q puri"cation system (Millipore Corporation, Bedford, MA). The skim milk was stirred for 20 min and the pH was adjusted (6.4, 6.2 or 6.0) by adding 9% (v/v) lactic acid (BDH Laboratory Supplies, Poole, UK) at 203C. The skim milk was stored at 43C at a minimum time of 20 min. Skim milk powder that was held for 24 h at 333C produced the same curd grain syneresis behaviour as skim milk powder held 20 min at 43C. Before use, the skim milk was stirred for 5 min and the pH was checked and adjusted at 203C.
2.2. Preparation of curd grains Cylindrical polypropylene syringes (20, 10 and 5 mL) were used for the preparation of cheese curd grains. The needle end of the syringe was cut-o! squarely from syringes with diameters of 21, 16 or 13 mm and their openings were covered with para"lm. Skim milk samples were preheated at 333C for 15 min before renneting. Renneted skim milk samples were prepared at 333C by adding rennet (Batch 97255, Bovine Rennet SBI Gand-gassiot SOREDAB, La Boissie`re-Ecole, France) to a "nal rennet concentration of 210 lL L\ of milk or 105 lL L\. The rennet contained 520 mg chymosin L\ and the ratio chymosin/pepsin was 2 : 1. The syringes were "lled with skim milk (8.0, 3.0 and 1.5 mL) immediately after the addition of the rennet and kept in a water bath at 333C for 70 min. 2.3. Production of ultraxltrated milk permeate with addition of blue dextran Ultra"ltered milk permeate (UF milk permeate) was produced from commercial consumer milk (MinimjoK lk, Ska nemejerier, Sweden) with a fat content of less than 0.1% (w/w). The milk was slowly preheated to 503C for approximately 20 min and maintained at 503C for at least 20 min before ultra"ltration. As a preservative, 40 mg L\ phenoxymethylpenicillinkalium (Ka vepenin, Astra LaK kemedel, Lund, Sweden) was added. Ultra"ltration was carried out using a hollow "bre membrane (H5P10-43, Amicon Div., MA, USA) with an area of 0.45 m and a cut-o! of 10,000 MW. The set-up was sterilised with 200 ppm NaOCl (15% of active Cl ) at room temperature for 30 min and rinsed with deionised water. The ultra"ltration unit was heated with deionised water to 503C and drained on the retentate side just prior to the experiment. The milk was circulated with a volumetric concentration factor of two, as the permeate was removed. The inlet and outlet gauge pressures of the ultra"ltration unit were 1.5 and 0.4 bar respectively. The pH of the UF milk permeate was adjusted to the same pH as the skim milk samples by adding 9% (v/v) lactic acid at 203C. Blue dextran (BD) (Sigma Chemical CO, St. Louis, MO, USA) was added at a concentration of 0.2% (w/w) to the UF milk permeate. Di!usion and absorption of BD into the curd are prevented by the high molecular weight of BD (2,000,000 Da) (Zviedrans & Graham, 1981). The UF permeate solution, &blue whey', was stirred with a magnetic stirrer for 20 min and stored at 43C before being used in syneresis measurements. 2.4. Method for measuring syneresis The method for measuring syneresis is a further development of a tracer method devised by Zviedrans and Graham (1981) and von BuK eler et al. (1997). After 70 min
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
491
equation < "< (A /A !1), (1) R 5 R where < is the total volume (mL) of whey expelled R from one curd grain at time t when the sample was taken, < is the initial volume (mL) of &blue whey', A is the 5 initial absorbance of UF milk permeate with BD at 620 nm before syneresis and A is the absorbance of the R sample at 620 nm at time t. 2.5. Rheological measurements Fig. 1. Experimental set-up of the method.
of renneting, the curd grain was gently pushed out by a piston from the syringe and carefully placed on a plastic draining surface in thermostatically controlled &blue whey', avoiding exposure to air (Fig. 1). Submerging the curd grain in &blue whey' initiated the syneresis, corresponding to the &cutting' time in cheese making. The curd grain with a diameter of 21 mm was completely submerged in 17 mL\ &blue whey', while the grains of 16 and 13 mm diameter were submerged in 6 and 9.5 mL\, respectively. In order to improve the accuracy of the measurements for the smallest grains, three grains were used in these experiments. The temperature during the experiment was kept constant at 333C and the syneresis was measured for 100 min. To have homogeneous samples for the absorbance measurements, the &blue whey' was gently stirred during the syneresis with a magnetic stirrer. All experiments were carried out with continuous stirring at 32 rpm, causing no visible deformation of the grain. To judge the impact of continuous stirring on curd shrinkage, experiments were checked by stirring for only 30% of the time, resulting in no signi"cant di!erence in syneresis. A stirring rate higher than 100 rpm resulted in visible deformation of the grains. The impact on the curd resulted in the release of small gel particles and no reliable absorbance measurements could be made. Samples of diluted &blue whey' (0.8 mL) were analysed every 5 min. The absorbance of the sample was measured at 620 nm relative to UF milk permeate and transfered back with a pipette after each measurement. The absorbance was monitored using a Beckman DU-50 spectrophotometer (Beckman Instruments Inc, Irvine, CA). Syneresis experiments were carried out in triplicate for each condition. 2.4.1. Amount of whey expelled The amount of whey expelled from the curd grain is based on the measurement of the progressive dilution of BD in &blue whey' (Beeby, 1959; Zviedrans & Graham, 1981; von BuK eler et al., 1997). The total volume of whey expelled from one curd grain is calculated using the
The dynamic rheological behaviour of renneted skim milk was studied in a concentric cylinder cell (C 14) of the Bohlin VOR rheometer (Metric Analys, Stockholm, Sweden). A torsion bar of 0.23 g cm was used in the experiments and the strain was kept below 0.0103. Skim milk was prepared with a known pH (see above). The solution was preheated for 15 min at 333C. Rennet was added to give the desired concentration (see above) to a skim milk sample of 3 mL. The solution was mixed and poured into the cup of the rheometer. The addition of rennet corresponds to time zero. The gel formation was followed at a frequency of 0.5 Hz by measuring the storage modulus (G ), Pa, and the viscous modulus (G ), Pa, at intervals of 60 s during 170 min at 333C. Rheological measurements were performed at least in triplicate for each condition. 2.6. Statistical analysis Data from the syneresis and rheological measurements were evaluated statistically with ANOVA, analysis of variance, and linear regression (MINITAB Release 12., Statistical Software, Minitab Inc., PA, USA).
3. Results and discussion 3.1. Syneresis of curd grains The shrinkage (%) of one curd grain was calculated by the amount of whey (< ) expelled (Eq. (1)) divided by the initial volume of the curd grain. All data series were "tted to a three-parameter exponential equation describing the shrinkage process over time: y"y #a (1!e\@R), (2) where y is the shrinkage in percent, t is time and y corresponds to the initial expulsion of whey as the grain is being placed on the draining surface in the &blue whey'. y #a is the asymptote of the equation and b is a rate constant describing the expulsion. Mean values of the experimental data and the "tted data of curd shrinkage as a function of time are presented in Fig. 2. Error
492
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
Fig. 2. Shrinkage (%) of a curd grain as a function of time. Time zero corresponds to the time of &cutting', i.e. submerging the curd grain in liquid 70 min after rennet addition. Curd grain diameter (a) and (d) 13 mm; (b) and (e) 16 mm; (c) and (f ) 21 mm. Rennet concentrations ((a)}(c)) 210 lL L\ of milk; ((d)}(f )) 105 lL L\ of milk. Points with error bars show the mean value of the experimental data and the mean standard deviation of the method for curd produced at di!erent pH as a function of time; pH 6.0 (䢇), pH 6.2 (*) and pH 6.4 (䉲). Curve "tting of the experimental data; pH 6.0 (*), pH 6.2 (2) and pH 6.4 (} } }).
bars show the mean standard deviation of the experimental data. The six graphs illustrate data series as points at three pH values (6.4, 6.2 and 6.0) for speci"c grain sizes (21, 16 and 13 mm) and rennet concentrations (105 and 210 lL L\ of milk). The triplicate experiments show good agreement, the mean standard deviation being 1.0% (comparable to 5.4% in von BuK eler et al., 1997). E!ects of the process parameters investigated, namely pH, rennet concentration and curd grain size, on syneresis of the curd grains can be seen, even though, how the process parameters a!ect the syneresis is not obvious without further analysis. In Eq. (2) the three parameter estimates (y , a and b) are strongly correlated and therefore not suitable for further evaluation. To detect a di!erence in the factors determining curd grain shrinkage in the initial stage of syneresis compared to a later stage, two time points were chosen, 80 and 170 min after the addition of rennet corresponding to 10 and 100 min after &cutting'. For each experiment, the shrinkage values at 80 and 170 min were interpolated using Eq. (2) and the data were evaluated statistically by analysis of variance, ANOVA. The in#uences of grain size, pH, rennet concentration and their second-order cross e!ects were investigated (Table 1). Curd grain size was the most signi"cant factor in#uencing the initial stage of syneresis (80 min). The in#uences of pH and rennet concentration were statistically signi"cant, but, as the signi"cant cross term indicates, the
Table 1 In#uence of grain size, pH, rennet concentration at 80 and 170 min after rennet addition according to statistical analysis of variance (ANOVA) Process parameter (value)
DF
Shrinkage at 80 min
Grain size (13, 16, 21 mm) pH (6.0, 6.2, 6.4) Rennet conc. (105, 210 lL L\) Grain size;pH Grain size;rennet conc. pH;rennet conc. Mean residual error
170 min
F
P
2
22.76
0.000
4.90
0.015
2 1
10.93 1.70
0.000 0.204
40.22 3.26
0.000 0.082
4 2
2.78 1.03
0.047 0.369
23.05 2.47
0.000 0.104
2
4.78
0.017
12.05
0.000
$1.8% shrinkage
F
P
$1.5% shrinkage
DF"Degrees of freedom, F"F-statistic for the factor in question, P"Statistical signi"cance, the probability that e!ect of the factor is zero.
e!ects were not additive in the initial stage of syneresis. In the later stage of syneresis (170 min), pH was the most signi"cant factor (Table 1) but again, grain size and rennet concentration showed signi"cant non-additive e!ects.
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
493
Fig. 3. Shrinkage (%) of curd grains illustrated as surface models involving pH and grain size at the rennet concentration (a) and (b) 210 lL L\ of milk; (c) and (d) 105 lL L\ of milk. The shrinkage surface at 80 min ((a) and (c)) was obtained from a three-parameter model (Eq. (3a)); at 170 min ((b) and (d)) the shrinkage was from a two-parameter model (Eq. (4)).
All possible subsets of linear regressions were performed on the data to "nd a most parsimonious response model in terms of the in#uence of grain size, pH, rennet concentration and their cross e!ects at the chosen times after the initiation of syneresis. At 80 min the best models describing syneresis were two di!erent, three-parameter models (Eqs. (3a) and (3b)) with r of 0.64 and 0.63, respectively. shrinkage (%) at 80 min"17.8!0.535 (grain size) #0.329 (rennet concentration) !0.0518 (pH;rennet concentration), (3a)
shrinkage (%) at 80 min "17.7#0.280 (rennet concentration) !0.0854 (pH;grain size) !0.0438 (pH;rennet concentration).
(3b)
At 170 min the following two-parameter model (Eq. (4)) described the shrinkage su$ciently with r of 0.49; the model was not further improved by including the in#uence of grain size.
494
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
Fig. 4. Storage modulus G for skim milk gels at two rennet concentrations (105 and 210 lL L\ of milk) shown as a function of time at pH 6.0 (䢇); 6.2 (*); and 6.4 (䉲). Error bars show the mean standard deviation of the experimental data at the time of &cutting', 70 min.
shrinkage (%) at 170 min"32.9 #0.538 (rennet concentration) !0.0880 (pH;rennet concentration).
(4)
Models (3a) and (4) describing syneresis are shown as surfaces in Fig. 3 where shrinkage (%) is a function of grain diameter and pH for the di!erent rennet concentrations and time points. Under the same conditions, a smaller curd grain shrinks faster than a bigger grain in the initial stage of syneresis (80 min). This is consistent with the view that initially, whey is lost from the surface region of the grain, and thus the in#uence of the volume/surface ratio, being approximately equal to the grain diameter, dominates. Comparing the syneresis of curd grains at di!erent pH shows a more intense shrinkage at the lower pH, independent of time and rennet concentration. These results agree with the pH dependence of permeability for renneted milk gels where permeability of constrained gels produced at pH 6.0 or 6.4 di!er by a factor of two (van Dijk, 1982; van den Bijgaart, 1988; van Vliet, Roefs, Zoon, & Walstra, 1989). It is well known that lowering the pH enhances syneresis (Patel et al., 1972; Walstra & Jenness, 1984; Walstra et al., 1985; Pearse & Mackinlay, 1989). The e!ect of grain size was of much less importance in the later stage than in the initial stage of syneresis. In the later stage the curd is closer to a putative equilibrium, which should be independent of water transport kinetics. 3.2. Curd rheology The storage modulus (G ) and the viscous modulus (G ) of renneted skim milk were measured under the same
conditions (pH, rennet concentration, temperature and time) as the syneresis experiments. In Fig. 4 the storage modulus is shown as a function of time for skim milk gels prepared with di!erent rennet concentrations (105 and 210 lL L\ of milk) and pH values (6.0, 6.2 and 6.4). As expected, the storage modulus increased faster at lower pH and higher rennet concentration (van Dijk, 1982; Roefs, 1986; Zoon, 1988; van Vliet et al., 1989). The milk gels produced in our syneresis experiments had di!erent G values at &cutting' time (70 min after rennet addition). tan d ("G /G ) remained approximately constant (0.24}0.26) for all pH and rennet combinations at &cutting' time and later, in accordance with the results of Zoon (1988). 3.3. Rheology and syneresis van Vliet et al. (1991) reported that tan d in dynamic rheological measurements is a good parameter for predicting syneresis properties. However, in our case tan d was approximately equal for all experimental conditions. The one rheological variable that did vary was G . Storage modulus often correlates with yield strength, and yield strength is expected to a!ect syneresis pressure and permeability. Statistical analysis of variance (ANOVA) was performed to investigate the in#uence of G at 70 min and grain size on shrinkage (Table 2). G was in this case treated as a covariate. As expected, grain size was the most important factor in the initial stage of syneresis. The in#uence of the G on shrinkage of the curd grain was of low signi"cance at both 80 min and at 170 min after rennet addition. Since pH and rennet concentration can predict the storage modulus of the gel, the correlation of shrinkage with G is not surprising. However, inclusion of the storage modulus in the linear regression equations
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496 Table 2 In#uence of grain size and storage modulus at 70 min on curd grain shrinkage at times 80 and 170 min after rennet addition according to statistical analysis of variance (ANOVA) Process parameter (value)
DF Shrinkage at 80 min
G Grain size (13, 16, 21 mm) Mean residual error
1 2
495
which is funded by the Nordic Academy for advanced Study (NorFA).
References 170 min
F
P
F
P
4.85 15.90
0.034 0.000
4.17 0.11
0.048 0.900
$2.4% shrinkage $3.8% shrinkage
DF"Degrees of freedom, F"F-statistic for the factor in question, P"Statistical signi"cance, the probability that e!ect of the factor is zero.
did not improve the prediction of shrinkage. Possibly, properties at lower deformation rates and larger deformations are more relevant. In addition, the curd is constrained in the rheometer, whereas it shrinks globally during syneresis.
4. Conclusions Measurements on individual grains showed that the syringe method of preparation of single grains was satisfactorily reproducible and there was little variability between individual grains. Generally, our "ndings agree well with expectations. The grain size was the most important factor determining the shrinkage of the curd grain in the initial stage of the syneresis, but in the later stage no signi"cant e!ect could be found. The in#uence of pH on the syneresis was the dominating factor a!ecting the shrinkage in the later phase of syneresis and a lower pH resulted in a more intense syneresis. The storage modulus of the curd at &cutting' time did not predict syneresis well.
Acknowledgements Financial support has been received from EU-FAIR Concerted Action project (TIROS), the Nordic research network &Milk Proteins*Structure and Functional Properties', NorFA, Norway and the Nordic Council of Ministers via the Swedish Institute. This paper was presented at the workshop on &Milk proteins*Structure and Functional Properties' held in Naantali, Finland, 28}30 November 1998. The workshop was organised as part of activities of the Nordic network on Milk Proteins,
Andersson, H., & AndreH n, A. (1990). In#uence of chromatographically pure bovine chymosin and pepsin A on cheese curd syneresis. Journal of Dairy Research, 57, 119}124. Beeby, R. (1959). A method for following the syneresis of the rennet coagulum in milk. The Australian Journal of Dairy Technology, April}June, 77}79. Fox, P.H. (1987). Cheese: Chemistry, physics and microbiology. General aspects. vol. 1. London: Elsevier Applied Science. Lawrence, A. J. (1959). Syneresis of rennet curd. Part 1*e!ect of time and temperature. The Australian Journal of Dairy Technology, October}December, 166}169. Lawrence, A. J., Hill, R. D. (1974). A method for measuring the syneresis of cheese-curd. Proceeding of the 19th International Dairy Congress 1E (pp. 204}205). Lelievre, J. (1977). Rigidity modulus as a factor in#uening the syneresis of renneted milk gels. Journal of Dairy Research, 44, 611}614. Marshall, R. J. (1982). An improved method for measurement of the syneresis of curd formed by rennet action on milk. Journal of Dairy Research, 49, 329}336. Nilsen, K. O., & Abrahamsen, R. K. (1985). Di$culties in measuring the syneresis of goat milk rennet curd by dilution of an added tracer. Journal of Dairy Research, 52, 209}212. Patel, M. C., Lund, D. B., & Olson, N. F. (1972). Factors a!ecting syneresis of renneted milk gels. Journal of Dairy Science, 55(7), 913}918. Pearse, M. J., & Mackinlay, A. G. (1989). Biochemical aspects of syneresis: A review. Journal of Dairy Science, 72, 1401}1407. Pearse, M. J., Mackinlay, A. G., Hall, R. J., & Linklater, P. M. (1984). A microassay for the syneresis of cheese curd. Journal of Dairy Science, 51, 131}139. Renault, C., Gastaldi, E., Lagaude, A., Cuq, J. L., & Tarodo De La Fuente, B. (1997). Mechanisms of syneresis in rennet curd without mechanical treatment. Journal of Food Science, 62(5), 907}910. Roefs (1986). A study of gels formed in the cold. Ph.D. thesis, Wageningen Agricultural University, The Netherlands. van den Bijgaart, H. J. C. M. (1988). Syneresis of rennet-induced milk gels as in-uenced by cheesemaking parameters. Ph.D. thesis, Wageningen Agricultural University, The Netherlands. van Dijk, H. J. M. (1982). Syneresis of curd. Ph.D. thesis, Wageningen Agricultural University, The Netherlands. van Dijk, H. J. M., Walstra, P., & Geurts, T. J. (1979). Preliminary note on syneresis pressure in rennet curd. Netherlands Milk and Dairy Journal, 33, 60}61. van Vliet, T., Roefs, S. P. F. M., Zoon, P., & Walstra, P. (1989). Rheological properties of casein gels. Journal of Dairy Science, 56, 529}534. van Vliet, T., van Dijk, H. J. M., Zoon, P., & Walstra, P. (1991). Relation between syneresis and rheological properties of particle gels. Colloid and Polymer Science, 269, 620}627. von BuK eler, T., Jakob, E., & Puhan, Z. (1997). Bestimmungsmethode fuK r die SynaK rese von Labgallerten. Milchwissenschaft, 52(3), 131}133. Walstra, P., & Jenness, R. (1984). Dairy chemistry and physics. USA: Wiley. Walstra, P., van Dijk, H. J. M., & Geurts, T. J. (1985). The syneresis of curd. 1. General considerations and literature review. Netherlands Milk and Dairy Journal, 39, 209}246.
496
A. Unger Grundelius et al. / International Dairy Journal 10 (2000) 489}496
Walstra, P., van Dijk, H. J. M., Geurts, T. J. (1987). The syneresis of curd. In P. H. Fox, Cheese: Chemistry, physics and microbiology, vol. 1, General aspects (pp. 135}177). London: Elsevier Applied Science. Zoon, P. (1988). Rheological properties of rennet-induced skim milk gels. Ph.D. thesis, Wageningen Agricultural University, The Netherlands.
Zviedrans, P., & Graham, E. R. B. (1981). An improved tracer method for measuring the syneresis of rennet curd. The Australian Journal of Dairy Technology, 36(3), 117}120.