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Effectiveness of vacuum impregnation brining of Manchego-type curd
International Dairy Journal 9 (1999) 143}148
E!ectiveness of vacuum impregnation brining of Manchego-type curd Ch. Gonzalez*, C. Fuentes, A. AndreH s, A. Chiralt, P. Fito Department of Food Technology, Universidad Polite& cnica de Valencia, 46021 Valencia, Spain Received 15 August 1998; accepted 21 March 1999
Abstract Salt gain in brining of calf rennet coagulated uncooked pressed curds (20% ewe's and 80% cow's milk), using conventional brine immersion and brine vacuum impregnation, was analysed and compared as a function of the curd size, the curd ageing time and the relative position of the curd sample to the plunger press. Vacuum impregnation implied an additional salt gain of 0.005 g NaCl per g of curd, due to hydrodynamic mechanism action, independently from the curd size. Additional di!usional salt gains seemed to be reached by promotion of the e!ective di!usion associated with brine penetration. The degree of compactness of the casein clusters in curd increased with the distance to the plunger press and the curd ageing time. This greatly a!ected the brine vacuum impregnation e!ectiveness related with hydrodynamic mechanisms in the great extra-micellar porosity. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Salt penetration in cheese during brining has been described by several authors (Geurts, Walstra & Mulder, 1974, 1980; Guinee & Fox, 1987; Gros & RuK egg, 1987) as a di!usion process where curd composition and structure play an important role. Geurts et al. (1974,1980) developed a mass transfer model to explain salt uptake in terms of fat, protein and water content of curd as well as its size and shape, pH and temperature. The NaCl pseudodi!usion coe$cient (DH) in the cheese aqueous phase was obtained and correlated with NaCl di!usion in aqueous solutions through di!erent coe$cients dependent on fat and protein matrix arrangement in curd. These coe$cients quantify the friction e!ects which reduce NaCl di!usion during cheese brining. The protein matrix was considered to have a &sieve e!ect', where the smallest pores determine the greatest resistance to the salt entrance (Geurts et al., 1974). Curd has a porous structure (Eino, Biggs, Irvine & Stanley, 1976; Stanley & Emmons, 1977) where air or whey may be entrapped and unevenly distributed depending on several factors such as pressing conditions. Vacuum treatments have been applied in line with pressing in cheese-making of large Cheddar cheese blocks
* Corresponding author.
(Irvine & Burnett, 1962; Reinbold, Hansen, Gale & Ernstrom, 1993) in order to reduce the air or whey pockets, thus avoiding cheese mechanical openings and increasing the evenness of moisture distribution. A brine vacuum impregnation (BVI) process (Chiralt & Fito, 1997) has been developed to reduce brining time, taking advantage of the salt uptake by hydrodynamic mechanisms (HDM) occurring in capillary pores by the action of pressure gradients. From the developed HDM model it is possible to predict the amount of liquid that can be introduced into a porous food when a vacuum impregnation (VI) operation is carried out. This operation consists of applying vacuum in a tank containing the porous product immersed in the required liquid phase till the mechanical equilibrium is reached in the system, afterwards restoring the atmospheric pressure for a determined time in a second step. In the "rst approach, HDM was modelled for rigid products with homogeneous pores of diameter D and length z, which were immersed in a liquid phase (Fito, 1994). The interior of the pore is assumed to be occupied by gas at an initial pressure p whereas in the liquid phase the external pressure is p 'p . When the system is at constant pressure, *p"p !p equals the capillary pres sure (p ), thus *p"p . The mathematical relationship (Eq. (1)) between the volume fraction of the product impregnated by the external liquid (X) and the applied *p was deduced at the mechanical equilibrium condition,
0958-6946/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 8 - 6 9 4 6 ( 9 9 ) 0 0 0 3 5 - 7
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Ch. Gonzalez et al. / International Dairy Journal 9 (1999) 143}148
assuming an isothermal compression of the internal gas. In Eq. (1), e is the product e!ective porosity and r the compression ratio, given by Eq. (2), where p is the pore internal pressure reached during the "rst VI step and p the external (atmospheric) pressure imposed in the system during the second VI process step. Capillary contribution to compression ratio could be neglected in VI processes if vacuum pressure during the "rst step was lower than 400 mbar for the usual food pore diameter range (Fito, 1994).
1 X"e 1! r
(1)
p #p r" (2) p Impregnation only due to capillary forces without pressure changes imposed on the system can also be obtained by Eqs. (1) and (2) by substituting p "p "p. In this case it can inferred that capillary penetration is promoted at low pressure, as compared with atmospheric conditions. Therefore, according to the HDM model, curd porosity will play a more important role in salt uptake in the BVI process than in the BI process although capillary entry will also occur in the latter to a very small extent. In this paper, we analysed salt uptake in uncooked pressed curds, produced by calf rennet coagulation of a mixture of 20% ewe's and 80% cow's milk, during brine immersion (BI) and brine vacuum impregnation (BVI) process and compared the di!erences related to curd structure.
2. Materials and methods 2.1. Cheese making Manchego-type cheeses, manufactured from uncooked pressed curd of ewe's (20%) and cow's (80%) milk coagulated by calf rennet, were supplied by an industrial manufacturer immediately after pressing (2 h at 0.010 kg cm\ on the cheese surface). Cheeses had a cylindrical shape (10 cm height, 20 cm diameter) and mean weight of 3900$ 150 g. They had a fat content of 0.291$0.003 g (g curd)\ and a protein content determined as 0.0345$ 0.0009 g nitrogen (g curd)\, with an initial moisture content of 0.442$0.008 g (g curd)\. 2.2. Salting experiments Three kinds of salting experiments were carried out at 103C using brine with a 0.244 NaCl mass fraction: (a) salting of cylindrical samples of di!erent diameters cut along cheese axis, (b) whole cheeses and (c) cylindrical samples taken from the top and bottom zones of cheese pieces.
(a) Cylindrical curd samples (12 cm high) were extracted with sharp tubular borers along curd axial direction with three di!erent diameters (56.4, 30.4 and 19.5 mm). These samples (three of each size) were salted for 30 min using BI or BVI. In BVI, a pressure of 50 mbars (p ) was applied in the tank for 15 min (t ) and afterwards atmo spheric pressure (p ) was restored and samples remained immersed the rest of the time (t ). (b) Salting of whole cheeses (12 cm high, 20 cm diameter) was also carried out by BI and BVI procedures immediately and 1 day after pressing. For BI salting, 5 and 17 h treatments were considered. For BVI, t ranged from 0.5 to 4 h, whereas 0, 1.75 and 2 h were taken for t . (c) Cylindrical samples (12 cm high, 1 cm diameter) were taken from curd and, afterwards were cut in half, to separate the top and bottom according to the pressing direction. These samples were held immersed in the brine after application of a 15 min vacuum pulse, and salt and moisture analysed periodically until no changes in composition were observed (20 days). Final concentration was taken as the equilibrium value. 2.3. Analytical procedures Salt content was analysed in curd cylindrical samples as well as in whole cheese pieces before and after salting treatments. In whole pieces, the external zone (to a depth of 1.5 cm from the rind) and the internal zone were analysed separately and the overall content was estimated from these values and the weight fraction of each part. For the sodium chloride determination, samples were homogenised in distilled water at 9000 rpm in an Ultraturrax T25 (Janke & Kunkel, Staufen, Germany) for 5 min and centrifuged to remove any "ne debris present in the sample. An aliquot of a centrifuged sample was taken and titrated in a Chloride Analyser equipment (Sherwood Mod. 926, Cambridge, UK). Moisture content was quanti"ed by oven drying to constant weight at 1053C. Porosity (air volume fraction) of unsalted curd pieces was determined from their real and bulk densities, both measured by volume displacement with a pycnometer. For real density measurements, vacuum was applied in the pycnometer with the sample immersed in distilled water to promote the gas expulsion. Whey drainage capacity (WDC) of curd samples by vacuum action was determined in order to analyse the capability of curd to interchange whey for brine during BVI treatments. For this, weighed curd cylinders were enveloped with "lter paper and submitted to 50 mbar pressure for 15 min; afterwards, the atmospheric pressure was restored. The amount of whey released during vacuum step remained entrapped in the "lter paper and weight loss (g (g of initial curd)\) was taken as WDC. Ten replicates were carried out for porosity and WDC determination.
Ch. Gonzalez et al. / International Dairy Journal 9 (1999) 143}148
2.4. Cryo-SEM observations Microstructure of curd was examined in a cold-stage scanning electron microscope by cryoSEM (Schmidt, 1989). Two millimeter thick slices of curd were cut, placed in the holder, frozen in slush nitrogen and immediately introduced into the vacuum chamber of the microscope. The samples were fractured, etched, gold coated and viewed in a Cryo-scanning electron microscope JEOL JSM-5410 at!1503C. Samples taken from the upper and lower part of curd (in the sense of pressing direction), immediately and 1 day after pressing, were observed.
145
Table 1 Salt and moisture content (g (g cheese)\) reached in each cylinder 30 mv after salting by BI and BVI methods Treatment
BI
BVI
Radius (mm)
x
x , !
x
x , !
56.4 30.4 19.5
41.8$0.6 40$1 37.5$0.7
0.96$0.04 1.41$0.10 2.00$0.09
42.6$0.3 40.5$0.2 37.9$0.2
1.47$0.04 1.82$0.02 2.40$0.01
For de"nitions see text.
3. Results and discussion 3.1. Comparison of salt uptake in curd samples treated by BI and BVI Table 1 gives the mean values of salt (x ) and water , ! (x ) mass fractions of cylindrical samples as a function of their diameter for 30 min of salting by BI and BVI. To compare salting kinetics in BI and BVI and to quantify the salt uptake due to HDM, the salt gain (g NaCl (g initial curd)\) given by Eq. (3) was estimated and plotted as a function of the inverse of the sample radius (1/r). In Eq. (3) m and m are the initial and "nal sample mass and x and x are the mass fractions of salt in , ! , ! the initial curd and after the salting treatment, the x being 0.0013$0.0002 for the studied curds. , ! mx !mx , ! *M " , ! , ! m
(3)
A linear relationship between salt gain and 1/r was observed for both kinds of salting methods (equations in Fig. 1). The straight line slopes are related with the apparent di!usion coe$cients of salt in each sample taking into account a simpli"ed di!usional equation (only one term of the series solution) for short times in a "nite cylinder (Crank, 1975). Whereas a similar apparent di!usion can be inferred from Fig. 1 for both BI and BVI, an increase in salt gain, constant for all sample diameters, can be observed in BVI. This increment (*M ) must be attributed to the brine entry , ! &"+ throughout the curd pores due to the HDM action associated with the vacuum pulse and it was estimated as 0.0049$0.0007 g NaCl (g initial curd)\. Taking into account the curd density (1006$2 kg m\), the brine density (1178 kg m\) and concentration (y "0.244), , ! the salt increment implied a volume fraction of brine penetrated into the curd pores of X"0.019. The fact that this value is constant for samples with di!erent diameters indicates that the total curd porosity applies for the HDM in all cases and so the increase in sample diameter does not imply a decrease in the availability of more
Fig. 1. Salt gain in curd cylinders salted for 30 min by brine immersion (BI) and brine vacuum impregnation (BVI) as a function of cylinder radius.
internal pores. If it is assumed that no deformations occur due to the pressure changes, the HDM model (Eqs. (1) and (2), Fito, 1994) allows us to estimate the e!ective porosity (e ) of this kind of curd available to the HDM action as 0.020. This porosity value may correspond to the air volume in the cheese but also to whey pockets that may be replaced by brine. When vacuum was applied to curd immersed in brine, small spots of occluded gas expanded thereby causing the whey to be expelled from pores. Afterwards, when the atmospheric pressure is restored, the residual gas is compressed and (if the brine in the tank is well stirred) the external brine penetrates into the curd pores allowing a faster salt uptake and a di!erent salt distribution from that reached during BI (Chiralt & Fito, 1997). Table 2 shows the values of the experimental porosity (volume fraction of air in curd) for the upper and lower parts of the curd, determined immediately and 1 day after pressing. A signi"cantly greater volume fraction of air was observed in the lower part of curd immediately after pressing, but di!erences disappeared throughout time, in line with the biochemical and structural changes occurring during the protein matrix ageing (Knoop & Peters, 1975a,b). Likewise, the whey drainage capacity (WDC)
146
Ch. Gonzalez et al. / International Dairy Journal 9 (1999) 143}148
Table 2 Porosity, whey drainage capacity (WDC) and water, salt and weight gains in the upper and lower part of curd Sample
Upper Lower a
Porosity;10 t"0 (d)
t"1 (d)
0.18$0.16 0.33$0.15 (0.05
0.19$0.14 0.20$0.11 n.s.
WDC;100
*M g (g curd)\
*M , ! g (g curd)\
*M g (g curd)\
4.3$0.2 4.9$0.2 (0.05
!0.122$0.007 !0.130$0.009 (0.05
0.055$0.001 0.070$0.002 (0.01
!0.077$0.007 !0.080$0.006 n.s.
After 20 days brining, applying a 15 min vacuum (50 mbar) pulse at the beginning of treatment. Statistical signi"cance of di!erences between upper and lower position in cheese (n.s.: not signi"cant). t"Time after pressing. Table 3 Salt and moisture contents (g (g cheese)\) reached in each salting treatment for whole curds Salting treatment
x
x , !
BVI-0 (1.75-1.75) BVI-0 (1.75-1.75) BVI-1 (3-1.75) BVI-1 (0.5-1.75) BVI-0 (4-0) BVI-0 (2-2) BI-0 (0-17) BI-1 (0-5)
0.471$0.002 0.464$0.002 0.439$0.002 0.449$0.004 0.432$0.004 0.453$0.003 0.445$0.001 0.444$0.002
0.0151$0.0002 0.0160$0.0002 0.0082$0.0002 0.0061$0.0002 0.0170$0.0002 0.0180$0.0003 0.0135$0.0017 0.0083$0.0002
Digit (0, 1) refers to time (day) between pressing and salting; numbers in brackets indicate values of salting times t and t (h); BVI"brine vaccum impregnation; BI"brine immersion.
during vacuum treatments is also re#ected in Table 2. A greater WDC was also detected in the lower part of curd, probably due to the greater air volume and its subsequent higher expansion. This will imply a higher capacity for the whey brine exchange during BVI in the lower part of curd and so, a di!erent e!ectiveness of vacuum impregnation in the di!erent parts of the cheese. This behaviour will be principally due to the curd viscoelastic response during pressing and the subsequent higher compactness of the cheese zones nearer to the plunger press. Salt gain, water loss and weight loss of curd for 20 days salting (with an initial 15 min vacuum pulse) treatments are also given in Table 2. The greater e$ciency of salting, even at long salting times in the lower part of the curd can be clearly appreciated, which indicates the important role of curd structure in salting behaviour. Table 3 shows the salt gain and the moisture content (g (g cheese)\) reached in each salting treatment for whole cheese pieces salted immediately and 1 day after pressing. Fig. 2 shows the salt gains of whole cheeses salted by each procedure (BI and BVI) as a function of the square root of the total salting time. The overall salt gain and that which occurred in the more external zone (1.5 cm depth from rind) and in the remaining internal one were plotted to observe the di!erent in#uence of the vacuum treatment at di!erent depths in the cheese. In
Fig. 2. Salt gain in whole curds salted by brine immersion (BI) and brine vacuum impregnation (BVI) as a function of total salting time, in (a) whole piece, (b) external part and (c) internal part. Digits 0 and 1 in legend refer to ageing days of curd. t"salting time.
Fig. 2a}c, no signi"cant di!erences in salting behaviour for cheeses salted by BI and by BVI 1 day after pressing were observed, which indicates that the HDM action was
Ch. Gonzalez et al. / International Dairy Journal 9 (1999) 143}148
not e!ective if vacuum was not applied immediately after pressing. In all these cases a linear relationship between salt gain and the square root of salting time was clearly observed which seems to indicate that the di!usional mechanisms are principally responsible for salt uptake. The "tted linear equations and their R values appear in the plots. Nevertheless, a great in#uence of vacuum treatment was observed in the salt entry when BVI was carried out immediately after the pressing step. These results are coherent with the porosity collapse in the curd throughout the development and ageing of the protein matrix after coagulation. Taking into account the linear models *M vs. t , , ! the di!usional contribution to *M after the total , ! salting time in BVI-0 experiments was predicted and compared with the actual values of *M . The values of , ! *M were estimated as 0.0090$0.0007 and , ! &"+ 0.0107$0.0003 g NaCl (g curd)\, for treatments of 210
147
and 240 min salting time, respectively. These values are higher than those obtained in the small cylinders. However, di!erences could be due to a promotion of the e!ective di!usion coupled with the curd pore "lling, since salting times for whole cheeses were greater than 30 min. and therefore di!usional contribution to salt uptake reached higher levels. The promotion of di!usion mechanisms coupled with HDM is coherent with an increase in the volume that is active to di!usion as air was replaced by liquid phase, increasing the liquid fraction in the product where di!usion occurs. Comparison of the *M values for external and internal parts of the , ! &"+ cheese also point to the same hypothesis. Salt gains in the internal zone for BI and BVI-1 samples were very small, which indicates that salt concentration pro"le throughout the cheese promoted by di!usion mechanisms will be very abrupt, as was previously observed (AndreH s, Panizzlo, Camacho, Chiralt & Fito, 1997), whereas
Fig. 3. Cryo-SEM micrographs in the upper (a and c) and lower (b and d) parts of curd immediately (a and b) and 1 day after (c and d) pressing. FG: fat globules, PM: protein matrix.
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Ch. Gonzalez et al. / International Dairy Journal 9 (1999) 143}148
greater internal salt gains were reached in the BVI-0 treatments. The value of *M was 0.0067$ , ! &"+ 0.0014 g NaCl (g curd)\ in the internal part (in the same order as in small cylinders salted for 30 min) whereas this was 0.012$0.002 in the external zone where di!usion mechanism acted in a greater extent. These results seem to indicate that the HDM action was extended to the whole cheese volume, including the internal zone where a very small di!usional contribution to salt gain was observed, at the same time as this mechanism action promotes di!usion in the more external cheese zone. Nevertheless, vacuum treatment is only e!ective in freshly pressed cheese. 3.2. Ewect of pressing and ageing on curd microstructure Fig. 3 shows the cryoSEM micrographs of the upper and lower part of curd immediately and 1 day after pressing at the same magni"cation level. Fat globules (FG) and the developing protein matrix (PM) can be observed in all cases as in other rennet coagulated curds (Schmidt, 1989; KalaH b, 1993). Nevertheless, the interconnected clusters of casein micelles appear with a di!erent aspect depending on their location in curd (related to the distance from the plunger press) and on the curd ageing. The micrographs re#ect a greater compactness of protein matrix or lower mean free path of casein micelles in line with the pressing e!ectiveness (greater in the upper part of curd) and the ripening process. The minor compression of the lower part of the curd seems to delay the casein transformations and the establishment of the protein matrix continuity associated with the ripening changes. In this sense, microstructure of the lower part of curd 1 day after pressing is very close to that at the upper part of curd immediately after pressing. These observations are coherent with the di!erent behaviour during BI and BVI of the di!erent zones of curd and the reduction of the salt uptake capacity by BVI after 1 day of curd ageing. 4. Conclusions The faster cheese salting by BVI was greatly a!ected by the curd microstructure. Di!erences in the e!ective porosity along the cheese axis (as a function of distance from plunger press) seemed to lead to a heterogeneous salt uptake in same direction, especially in BVI process. To increase the e!ectiveness of this salting method, neither hard pressing conditions nor curd ageing are recommended to maintain a free reasonably path unhindered by casein micelles.
Acknowledgements The authors thank the Comision Interministerial de Ciencia y TecnologmH a (CICYT) for the "nancial support of this work.
References AndreH s, A., Panizzolo, L., Camacho, M. M., Chiralt, A., & Fito, P. (1997). In R. Jowitt, Engineering and Food at ICEF 7 (pp. A133}A136). She$eld: She$eld Academic Press. Chiralt, A., & Fito, P. (1997). Salting of Manchego-type cheese by vacuum impregnation. In P. Fito, G. Barbosa, & E. Ortega, Food Engineering 2000 (pp. 215}230). New York: Chapman and Hall. Crank, J. (1975). ¹he Mathematics of Di+usion. London: Oxford University Press. Eino, M. F., Biggs, D. A., Irvine, D. M., & Stanley, D. M. (1976). A comparison of microstructures of Cheddar cheese curd manufactured with calf rennet bovine pepsin and porcine pepsin. Journal of Dairy Research, 43, 113}115. Fito, P. (1994). Modelling of vacuum osmotic dehydration of foods. In P. Fito, A. Mulet, & B. McKenna, Water in foods. Fundamental aspects and their signixcance in relation to processing of foods (pp. 313}328). London: Elsevier Applied Science. Geurts, T. J., Walstra, P., & Mulder, H. (1974). Transport of salt and water during salting of cheese. 1. Analysis of the processes involved. Netherlands Milk and Dairy Journal, 28, 102}129. Geurts, T.J., Walstra, P., & Mulder, H. (1980). Transport of salt and water during salting of cheese. 2. Quantities of salt taken up and the moisture lost. Netherlands Milk and Dairy Journal, 34, 229}254. Guinee, T. P., & Fox, P. F. (1987). Salt in cheese: physical, chemical and biological aspects. In P. F. Fox, Cheese: chemistry, physics and microbiology, vol. 1 (pp. 251}297). London: Elsevier Applied Science. Gros, J. B., & RuK egg, M. (1987). Determination of the apparent di!usion coe$cient of sodium chloride in model foods and cheese. In R. Jowitt, F. Escher, B. Kent, M. McKenna, & B. Roques, Physical properties of foods*2 (pp. 71}104). London: Elsevier Applied Science. Irvine, R. O., & Burnett, K. A. (1962). E!ects of vacuum treating on the textured qualities of Cheddar cheese. Canadian Dairy and Ice Cream Journal, 41, 24}28. KalaH b, M. (1993). Practical aspects of electron microscopy in dairy research. Food Structure, 12, 95}114. Knoop, A. M., & Peters, K. H. (1975a). Formation of the protein matrix during coagulation of milk by rennet and acid. Kieler Milchwirtschaftliche Forschungsberichite, 27, 227}248. Knoop, A. M., & Peters, K. H. (1975b). Structural changes of rennet curds during ageing. Kieler Milchwirtschaftliche Forschungsberichite, 27, 315}330. Reinbold, R. S., Hansen, C. L., Gale, C. M., & Ernstrom, C. A. (1993). Pressure and temperature during vacuum treatments of 290-kilogram stirred-curd Cheddar cheese blocks. Journal of Dairy Science, 76, 909}913. Stanley, D. W., & Emmons, D. B. (1977). Cheddar cheese made from bovine pepsin, II. Texture microstructure-composition relationships. Canadian Institute of Food Science and Technology Journal, 10, 78}84. Schmidt, D. G. (1989). Electron microscopy of milk and milk products: problems and possibilities. Food Microstructure, 1, 151}165.
E!ectiveness of vacuum impregnation brining of Manchego-type curd Ch. Gonzalez*, C. Fuentes, A. AndreH s, A. Chiralt, P. Fito Department of Food Technology, Universidad Polite& cnica de Valencia, 46021 Valencia, Spain Received 15 August 1998; accepted 21 March 1999
Abstract Salt gain in brining of calf rennet coagulated uncooked pressed curds (20% ewe's and 80% cow's milk), using conventional brine immersion and brine vacuum impregnation, was analysed and compared as a function of the curd size, the curd ageing time and the relative position of the curd sample to the plunger press. Vacuum impregnation implied an additional salt gain of 0.005 g NaCl per g of curd, due to hydrodynamic mechanism action, independently from the curd size. Additional di!usional salt gains seemed to be reached by promotion of the e!ective di!usion associated with brine penetration. The degree of compactness of the casein clusters in curd increased with the distance to the plunger press and the curd ageing time. This greatly a!ected the brine vacuum impregnation e!ectiveness related with hydrodynamic mechanisms in the great extra-micellar porosity. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Salt penetration in cheese during brining has been described by several authors (Geurts, Walstra & Mulder, 1974, 1980; Guinee & Fox, 1987; Gros & RuK egg, 1987) as a di!usion process where curd composition and structure play an important role. Geurts et al. (1974,1980) developed a mass transfer model to explain salt uptake in terms of fat, protein and water content of curd as well as its size and shape, pH and temperature. The NaCl pseudodi!usion coe$cient (DH) in the cheese aqueous phase was obtained and correlated with NaCl di!usion in aqueous solutions through di!erent coe$cients dependent on fat and protein matrix arrangement in curd. These coe$cients quantify the friction e!ects which reduce NaCl di!usion during cheese brining. The protein matrix was considered to have a &sieve e!ect', where the smallest pores determine the greatest resistance to the salt entrance (Geurts et al., 1974). Curd has a porous structure (Eino, Biggs, Irvine & Stanley, 1976; Stanley & Emmons, 1977) where air or whey may be entrapped and unevenly distributed depending on several factors such as pressing conditions. Vacuum treatments have been applied in line with pressing in cheese-making of large Cheddar cheese blocks
* Corresponding author.
(Irvine & Burnett, 1962; Reinbold, Hansen, Gale & Ernstrom, 1993) in order to reduce the air or whey pockets, thus avoiding cheese mechanical openings and increasing the evenness of moisture distribution. A brine vacuum impregnation (BVI) process (Chiralt & Fito, 1997) has been developed to reduce brining time, taking advantage of the salt uptake by hydrodynamic mechanisms (HDM) occurring in capillary pores by the action of pressure gradients. From the developed HDM model it is possible to predict the amount of liquid that can be introduced into a porous food when a vacuum impregnation (VI) operation is carried out. This operation consists of applying vacuum in a tank containing the porous product immersed in the required liquid phase till the mechanical equilibrium is reached in the system, afterwards restoring the atmospheric pressure for a determined time in a second step. In the "rst approach, HDM was modelled for rigid products with homogeneous pores of diameter D and length z, which were immersed in a liquid phase (Fito, 1994). The interior of the pore is assumed to be occupied by gas at an initial pressure p whereas in the liquid phase the external pressure is p 'p . When the system is at constant pressure, *p"p !p equals the capillary pres sure (p ), thus *p"p . The mathematical relationship (Eq. (1)) between the volume fraction of the product impregnated by the external liquid (X) and the applied *p was deduced at the mechanical equilibrium condition,
0958-6946/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 8 - 6 9 4 6 ( 9 9 ) 0 0 0 3 5 - 7
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Ch. Gonzalez et al. / International Dairy Journal 9 (1999) 143}148
assuming an isothermal compression of the internal gas. In Eq. (1), e is the product e!ective porosity and r the compression ratio, given by Eq. (2), where p is the pore internal pressure reached during the "rst VI step and p the external (atmospheric) pressure imposed in the system during the second VI process step. Capillary contribution to compression ratio could be neglected in VI processes if vacuum pressure during the "rst step was lower than 400 mbar for the usual food pore diameter range (Fito, 1994).
1 X"e 1! r
(1)
p #p r" (2) p Impregnation only due to capillary forces without pressure changes imposed on the system can also be obtained by Eqs. (1) and (2) by substituting p "p "p. In this case it can inferred that capillary penetration is promoted at low pressure, as compared with atmospheric conditions. Therefore, according to the HDM model, curd porosity will play a more important role in salt uptake in the BVI process than in the BI process although capillary entry will also occur in the latter to a very small extent. In this paper, we analysed salt uptake in uncooked pressed curds, produced by calf rennet coagulation of a mixture of 20% ewe's and 80% cow's milk, during brine immersion (BI) and brine vacuum impregnation (BVI) process and compared the di!erences related to curd structure.
2. Materials and methods 2.1. Cheese making Manchego-type cheeses, manufactured from uncooked pressed curd of ewe's (20%) and cow's (80%) milk coagulated by calf rennet, were supplied by an industrial manufacturer immediately after pressing (2 h at 0.010 kg cm\ on the cheese surface). Cheeses had a cylindrical shape (10 cm height, 20 cm diameter) and mean weight of 3900$ 150 g. They had a fat content of 0.291$0.003 g (g curd)\ and a protein content determined as 0.0345$ 0.0009 g nitrogen (g curd)\, with an initial moisture content of 0.442$0.008 g (g curd)\. 2.2. Salting experiments Three kinds of salting experiments were carried out at 103C using brine with a 0.244 NaCl mass fraction: (a) salting of cylindrical samples of di!erent diameters cut along cheese axis, (b) whole cheeses and (c) cylindrical samples taken from the top and bottom zones of cheese pieces.
(a) Cylindrical curd samples (12 cm high) were extracted with sharp tubular borers along curd axial direction with three di!erent diameters (56.4, 30.4 and 19.5 mm). These samples (three of each size) were salted for 30 min using BI or BVI. In BVI, a pressure of 50 mbars (p ) was applied in the tank for 15 min (t ) and afterwards atmo spheric pressure (p ) was restored and samples remained immersed the rest of the time (t ). (b) Salting of whole cheeses (12 cm high, 20 cm diameter) was also carried out by BI and BVI procedures immediately and 1 day after pressing. For BI salting, 5 and 17 h treatments were considered. For BVI, t ranged from 0.5 to 4 h, whereas 0, 1.75 and 2 h were taken for t . (c) Cylindrical samples (12 cm high, 1 cm diameter) were taken from curd and, afterwards were cut in half, to separate the top and bottom according to the pressing direction. These samples were held immersed in the brine after application of a 15 min vacuum pulse, and salt and moisture analysed periodically until no changes in composition were observed (20 days). Final concentration was taken as the equilibrium value. 2.3. Analytical procedures Salt content was analysed in curd cylindrical samples as well as in whole cheese pieces before and after salting treatments. In whole pieces, the external zone (to a depth of 1.5 cm from the rind) and the internal zone were analysed separately and the overall content was estimated from these values and the weight fraction of each part. For the sodium chloride determination, samples were homogenised in distilled water at 9000 rpm in an Ultraturrax T25 (Janke & Kunkel, Staufen, Germany) for 5 min and centrifuged to remove any "ne debris present in the sample. An aliquot of a centrifuged sample was taken and titrated in a Chloride Analyser equipment (Sherwood Mod. 926, Cambridge, UK). Moisture content was quanti"ed by oven drying to constant weight at 1053C. Porosity (air volume fraction) of unsalted curd pieces was determined from their real and bulk densities, both measured by volume displacement with a pycnometer. For real density measurements, vacuum was applied in the pycnometer with the sample immersed in distilled water to promote the gas expulsion. Whey drainage capacity (WDC) of curd samples by vacuum action was determined in order to analyse the capability of curd to interchange whey for brine during BVI treatments. For this, weighed curd cylinders were enveloped with "lter paper and submitted to 50 mbar pressure for 15 min; afterwards, the atmospheric pressure was restored. The amount of whey released during vacuum step remained entrapped in the "lter paper and weight loss (g (g of initial curd)\) was taken as WDC. Ten replicates were carried out for porosity and WDC determination.
Ch. Gonzalez et al. / International Dairy Journal 9 (1999) 143}148
2.4. Cryo-SEM observations Microstructure of curd was examined in a cold-stage scanning electron microscope by cryoSEM (Schmidt, 1989). Two millimeter thick slices of curd were cut, placed in the holder, frozen in slush nitrogen and immediately introduced into the vacuum chamber of the microscope. The samples were fractured, etched, gold coated and viewed in a Cryo-scanning electron microscope JEOL JSM-5410 at!1503C. Samples taken from the upper and lower part of curd (in the sense of pressing direction), immediately and 1 day after pressing, were observed.
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Table 1 Salt and moisture content (g (g cheese)\) reached in each cylinder 30 mv after salting by BI and BVI methods Treatment
BI
BVI
Radius (mm)
x
x , !
x
x , !
56.4 30.4 19.5
41.8$0.6 40$1 37.5$0.7
0.96$0.04 1.41$0.10 2.00$0.09
42.6$0.3 40.5$0.2 37.9$0.2
1.47$0.04 1.82$0.02 2.40$0.01
For de"nitions see text.
3. Results and discussion 3.1. Comparison of salt uptake in curd samples treated by BI and BVI Table 1 gives the mean values of salt (x ) and water , ! (x ) mass fractions of cylindrical samples as a function of their diameter for 30 min of salting by BI and BVI. To compare salting kinetics in BI and BVI and to quantify the salt uptake due to HDM, the salt gain (g NaCl (g initial curd)\) given by Eq. (3) was estimated and plotted as a function of the inverse of the sample radius (1/r). In Eq. (3) m and m are the initial and "nal sample mass and x and x are the mass fractions of salt in , ! , ! the initial curd and after the salting treatment, the x being 0.0013$0.0002 for the studied curds. , ! mx !mx , ! *M " , ! , ! m
(3)
A linear relationship between salt gain and 1/r was observed for both kinds of salting methods (equations in Fig. 1). The straight line slopes are related with the apparent di!usion coe$cients of salt in each sample taking into account a simpli"ed di!usional equation (only one term of the series solution) for short times in a "nite cylinder (Crank, 1975). Whereas a similar apparent di!usion can be inferred from Fig. 1 for both BI and BVI, an increase in salt gain, constant for all sample diameters, can be observed in BVI. This increment (*M ) must be attributed to the brine entry , ! &"+ throughout the curd pores due to the HDM action associated with the vacuum pulse and it was estimated as 0.0049$0.0007 g NaCl (g initial curd)\. Taking into account the curd density (1006$2 kg m\), the brine density (1178 kg m\) and concentration (y "0.244), , ! the salt increment implied a volume fraction of brine penetrated into the curd pores of X"0.019. The fact that this value is constant for samples with di!erent diameters indicates that the total curd porosity applies for the HDM in all cases and so the increase in sample diameter does not imply a decrease in the availability of more
Fig. 1. Salt gain in curd cylinders salted for 30 min by brine immersion (BI) and brine vacuum impregnation (BVI) as a function of cylinder radius.
internal pores. If it is assumed that no deformations occur due to the pressure changes, the HDM model (Eqs. (1) and (2), Fito, 1994) allows us to estimate the e!ective porosity (e ) of this kind of curd available to the HDM action as 0.020. This porosity value may correspond to the air volume in the cheese but also to whey pockets that may be replaced by brine. When vacuum was applied to curd immersed in brine, small spots of occluded gas expanded thereby causing the whey to be expelled from pores. Afterwards, when the atmospheric pressure is restored, the residual gas is compressed and (if the brine in the tank is well stirred) the external brine penetrates into the curd pores allowing a faster salt uptake and a di!erent salt distribution from that reached during BI (Chiralt & Fito, 1997). Table 2 shows the values of the experimental porosity (volume fraction of air in curd) for the upper and lower parts of the curd, determined immediately and 1 day after pressing. A signi"cantly greater volume fraction of air was observed in the lower part of curd immediately after pressing, but di!erences disappeared throughout time, in line with the biochemical and structural changes occurring during the protein matrix ageing (Knoop & Peters, 1975a,b). Likewise, the whey drainage capacity (WDC)
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Ch. Gonzalez et al. / International Dairy Journal 9 (1999) 143}148
Table 2 Porosity, whey drainage capacity (WDC) and water, salt and weight gains in the upper and lower part of curd Sample
Upper Lower a
Porosity;10 t"0 (d)
t"1 (d)
0.18$0.16 0.33$0.15 (0.05
0.19$0.14 0.20$0.11 n.s.
WDC;100
*M g (g curd)\
*M , ! g (g curd)\
*M g (g curd)\
4.3$0.2 4.9$0.2 (0.05
!0.122$0.007 !0.130$0.009 (0.05
0.055$0.001 0.070$0.002 (0.01
!0.077$0.007 !0.080$0.006 n.s.
After 20 days brining, applying a 15 min vacuum (50 mbar) pulse at the beginning of treatment. Statistical signi"cance of di!erences between upper and lower position in cheese (n.s.: not signi"cant). t"Time after pressing. Table 3 Salt and moisture contents (g (g cheese)\) reached in each salting treatment for whole curds Salting treatment
x
x , !
BVI-0 (1.75-1.75) BVI-0 (1.75-1.75) BVI-1 (3-1.75) BVI-1 (0.5-1.75) BVI-0 (4-0) BVI-0 (2-2) BI-0 (0-17) BI-1 (0-5)
0.471$0.002 0.464$0.002 0.439$0.002 0.449$0.004 0.432$0.004 0.453$0.003 0.445$0.001 0.444$0.002
0.0151$0.0002 0.0160$0.0002 0.0082$0.0002 0.0061$0.0002 0.0170$0.0002 0.0180$0.0003 0.0135$0.0017 0.0083$0.0002
Digit (0, 1) refers to time (day) between pressing and salting; numbers in brackets indicate values of salting times t and t (h); BVI"brine vaccum impregnation; BI"brine immersion.
during vacuum treatments is also re#ected in Table 2. A greater WDC was also detected in the lower part of curd, probably due to the greater air volume and its subsequent higher expansion. This will imply a higher capacity for the whey brine exchange during BVI in the lower part of curd and so, a di!erent e!ectiveness of vacuum impregnation in the di!erent parts of the cheese. This behaviour will be principally due to the curd viscoelastic response during pressing and the subsequent higher compactness of the cheese zones nearer to the plunger press. Salt gain, water loss and weight loss of curd for 20 days salting (with an initial 15 min vacuum pulse) treatments are also given in Table 2. The greater e$ciency of salting, even at long salting times in the lower part of the curd can be clearly appreciated, which indicates the important role of curd structure in salting behaviour. Table 3 shows the salt gain and the moisture content (g (g cheese)\) reached in each salting treatment for whole cheese pieces salted immediately and 1 day after pressing. Fig. 2 shows the salt gains of whole cheeses salted by each procedure (BI and BVI) as a function of the square root of the total salting time. The overall salt gain and that which occurred in the more external zone (1.5 cm depth from rind) and in the remaining internal one were plotted to observe the di!erent in#uence of the vacuum treatment at di!erent depths in the cheese. In
Fig. 2. Salt gain in whole curds salted by brine immersion (BI) and brine vacuum impregnation (BVI) as a function of total salting time, in (a) whole piece, (b) external part and (c) internal part. Digits 0 and 1 in legend refer to ageing days of curd. t"salting time.
Fig. 2a}c, no signi"cant di!erences in salting behaviour for cheeses salted by BI and by BVI 1 day after pressing were observed, which indicates that the HDM action was
Ch. Gonzalez et al. / International Dairy Journal 9 (1999) 143}148
not e!ective if vacuum was not applied immediately after pressing. In all these cases a linear relationship between salt gain and the square root of salting time was clearly observed which seems to indicate that the di!usional mechanisms are principally responsible for salt uptake. The "tted linear equations and their R values appear in the plots. Nevertheless, a great in#uence of vacuum treatment was observed in the salt entry when BVI was carried out immediately after the pressing step. These results are coherent with the porosity collapse in the curd throughout the development and ageing of the protein matrix after coagulation. Taking into account the linear models *M vs. t , , ! the di!usional contribution to *M after the total , ! salting time in BVI-0 experiments was predicted and compared with the actual values of *M . The values of , ! *M were estimated as 0.0090$0.0007 and , ! &"+ 0.0107$0.0003 g NaCl (g curd)\, for treatments of 210
147
and 240 min salting time, respectively. These values are higher than those obtained in the small cylinders. However, di!erences could be due to a promotion of the e!ective di!usion coupled with the curd pore "lling, since salting times for whole cheeses were greater than 30 min. and therefore di!usional contribution to salt uptake reached higher levels. The promotion of di!usion mechanisms coupled with HDM is coherent with an increase in the volume that is active to di!usion as air was replaced by liquid phase, increasing the liquid fraction in the product where di!usion occurs. Comparison of the *M values for external and internal parts of the , ! &"+ cheese also point to the same hypothesis. Salt gains in the internal zone for BI and BVI-1 samples were very small, which indicates that salt concentration pro"le throughout the cheese promoted by di!usion mechanisms will be very abrupt, as was previously observed (AndreH s, Panizzlo, Camacho, Chiralt & Fito, 1997), whereas
Fig. 3. Cryo-SEM micrographs in the upper (a and c) and lower (b and d) parts of curd immediately (a and b) and 1 day after (c and d) pressing. FG: fat globules, PM: protein matrix.
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Ch. Gonzalez et al. / International Dairy Journal 9 (1999) 143}148
greater internal salt gains were reached in the BVI-0 treatments. The value of *M was 0.0067$ , ! &"+ 0.0014 g NaCl (g curd)\ in the internal part (in the same order as in small cylinders salted for 30 min) whereas this was 0.012$0.002 in the external zone where di!usion mechanism acted in a greater extent. These results seem to indicate that the HDM action was extended to the whole cheese volume, including the internal zone where a very small di!usional contribution to salt gain was observed, at the same time as this mechanism action promotes di!usion in the more external cheese zone. Nevertheless, vacuum treatment is only e!ective in freshly pressed cheese. 3.2. Ewect of pressing and ageing on curd microstructure Fig. 3 shows the cryoSEM micrographs of the upper and lower part of curd immediately and 1 day after pressing at the same magni"cation level. Fat globules (FG) and the developing protein matrix (PM) can be observed in all cases as in other rennet coagulated curds (Schmidt, 1989; KalaH b, 1993). Nevertheless, the interconnected clusters of casein micelles appear with a di!erent aspect depending on their location in curd (related to the distance from the plunger press) and on the curd ageing. The micrographs re#ect a greater compactness of protein matrix or lower mean free path of casein micelles in line with the pressing e!ectiveness (greater in the upper part of curd) and the ripening process. The minor compression of the lower part of the curd seems to delay the casein transformations and the establishment of the protein matrix continuity associated with the ripening changes. In this sense, microstructure of the lower part of curd 1 day after pressing is very close to that at the upper part of curd immediately after pressing. These observations are coherent with the di!erent behaviour during BI and BVI of the di!erent zones of curd and the reduction of the salt uptake capacity by BVI after 1 day of curd ageing. 4. Conclusions The faster cheese salting by BVI was greatly a!ected by the curd microstructure. Di!erences in the e!ective porosity along the cheese axis (as a function of distance from plunger press) seemed to lead to a heterogeneous salt uptake in same direction, especially in BVI process. To increase the e!ectiveness of this salting method, neither hard pressing conditions nor curd ageing are recommended to maintain a free reasonably path unhindered by casein micelles.
Acknowledgements The authors thank the Comision Interministerial de Ciencia y TecnologmH a (CICYT) for the "nancial support of this work.
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