Drainage of curd

Cheese making starts with transformation of the liquid milk into a gel by proteolytic enzymes and/or acid producing bacteria. The gel is cut into pieces. The protein matrix contracts, by which whey is expelled from the pieces, this process is called syneresis. The process of whey expulsion is enhanced by stirring and usually heating. Finally fairly rigid curd grains and a large amount of whey are obtained. The subsequent separation of whey and curd grains is called drainage of curd. For most types of cheese, the obtained curd mass is subsequently pressed, salted and ripened. This study is mainly aimed at the drainage in case of Gouda cheese, but some aspects may be applicable to other cheese types as well.Drainage of curd not only separates (most of) the whey from the curd grains, but it also leads to the formation of a coherent mass. The drainage is not an isolated process, but its outcome depends heavily on the preceding curd preparation and the subsequent shaping and pressing. The study of the drainage process therefore requires a well-controlled curd preparation. In our conditions large-scale curd production was often not possible, but scaling down of the curd making process led to curd grains that had only little resemblance to those produced on factory scale. A new way of curd preparation was therefore developed. It enabled the production of almost identical cube shaped curd grains. The basic features of the new apparatus were: the gel was cut in cubes by two wire grids, and the cubes were stirred by subsequently pumping an amount of whey and large air bubbles through holes in the bottom of the cheese vat. The delicate curd pieces were hardly broken up by the resulting gentle stirring. As the used air is toxic to starter bacteria curd was prepared without starter.The curd grains were characterized by measuring their density in thermostatted sodium chloride solutions of various concentrations. The density can be recalculated to a moisture content or relative remaining volume, i.e. the volume of curd grains as a fraction of its volume before syneresis. The density measurement could, in the case of uniform curd grains, replace the various empirical methods currently applied to determine the moment to begin with drainage. However, in case of factory made curd other factors may also be of importance, e.g. pH, temperature and the amount of curd fines.The uniform curd grains were used to study the expression and deformation of a single curd grain in a uni-axial compression setup. The compression led to an instantaneous (elastic) and a slower viscous compression of the curd grain. Release of the stress within a few seconds after the start of the compression led to a clearly visible recovery; this was not the case after 15 minutes of compression. The elastic and viscous compression increased with the exerted stress ( pm ) and initial relative remaining volume of the curd grain (i 0 ). The extent of compression increased with time ( t ). The relative remaining volume at time t of a single curd grain ( it ) could be expressed as:where ( ioo ). is the relative remaining volume at infinity and k is a constant (4.10 -5Pa -1.s -½). The expression also led to horizontal broadening of the curd grain. The broadening increased with the exerted pressure and decreased slightly with decreasing initial remaining volume of the curd grain. At maximum, the broadening increased the original cross-sectional area to about 1.6 times. The extent of compression and volume decrease of the curd grain appeared to be closely correlated. The rheological properties of the paracaseinate strands probably become soon after the start the rate-determining factor at expression. The resistance to flow of whey may be a rate limiting factor at the start of the expression. It was observed in CSLM photographs that the surface layer of the cube shaped curd grains showed cracks. In the outer layer of curd grains obtained from factory made batches no cracks were observed, but, after application of a stress, cracks were formed.The fracture stress of fused curd grains could be studied at constant macroscopic fusion area with a newly developed technique. The fracture stress was positively correlated with the fusion time and the exerted pressure. The initial remaining volume also showed a positive correlation with the fracture stress, within the small test range. The temperature also had an effect upon the fracture stress: the highest fracture stress was obtained at about 35 °C and higher or lower temperatures both led to somewhat lower values.The sedimentation rate of curd grains may vary due to differences in apparent weight and density, but the fact that small curd grains are usually found at the bottom of a settled layer of curd grains can not be attributed hereto. Small curd grains can, however, fill the gaps between adjacent larger curd grains. The cube shaped curd grains rotated when freely settling. Anisometric curd grains sank with their longest axes perpendicular to direction of sedimentation, regardless of their initial position.Due to preferential orientation of the curd grains and the deformation of curd grains the curd bed has to be considered an anisotropic medium. The flow of whey occurs mainly through the interconnected openings between the curd grains, and the permeability of the curd bed in the horizontal direction will be greater than that in the vertical direction.Various types of drainage equipment are used to perform drainage, depending on the scale of production, the variety of cheese produced and the cost/benefit ratio. Batch drainage systems are mostly used in small scale production units, whereas continuous drainage vessels are commonly used in bulk cheese production. Both types have been realized in several, somewhat varying constructions and also the mode of operation varies from plant to plant. It was tried to study the effects of various designs upon the drainage by the construction of small scale drainage vessels. Scaling down of the often applied batch drainage vessels could not be done adequately, as the leakage of whey between the vessel wall and the compressed curd column contributed significantly to the flow of whey. Vertical perforated cylindrical pipes were used as a model for the continuous drainage vessels. Obvious parameters are drainage time and temperature, whey discharge conditions, and duration and magnitude of the exerted pressure. Other relevant parameters are the pressure loss due to the friction of the wall and the leakage of whey between the wall of the vessel and the curd. The packing near the wall is less dense due to steric exclusion. The friction at the wall alters the orientation of the adjacent curd grains: the curd grains are turned to an acute angle to the direction of movement of curd column.Experiments with colored curd grains in a Casomatic drainage pipe showed that curd grains were slightly mixed at the top. The flow of curd grains through the pipe can be considered as a plug flow with a parabolic flow superimposed on ft. A trail of colored curd grains at the rind of the subsequent blocks was observed, these curd grains probably got stuck at the filter grids near the whey outlets.The compaction of a curd/whey mixture in the vertical drainage columns initially increased when the exerted pressure and/or the initial remaining volume of the curd grains increased. However, above a certain external pressure, called threshold level, the transport of whey out of the column becomes rate determining; the compression then is accompanied by a fast increase in liquid pressure. The threshold level is primarily determined by the curd properties, but the relations strongly depend on thedrainage vessel's properties. Whey discharge conditions, the contact area between curd and the wall, the exerted stress upon the curd grains and the drainage time are mainly determined by the drainage vessel. The packing of the curd and the extent of fusion are likely to be relevant, but were not extensively studied. The whey content and the firmness of the curd grains and the presence of curd fines had significant effects.Visual inspection of cross-sections of curd blocks made in a Casomatic showed pores in various sizes and shapes, the largest ones being about 5 mm. The curd blocks lost a considerable amount of whey after the drainage. This drip whey originated largely from the inside of the curd grains.The porosity of a curd column could be measured with a newly developed porosity meter. The working principle of the apparatus is a moving optical fibre that penetrates the curd bed. The difference in scattering properties of curd grains and whey in the pores allows discrimination between either. The porosity of a curd bed so estimated, showed a large standard deviation, as the number of pores was rather small. The porosity generally decreased in time, which was due to decrease in the number and in the (apparent) size of the pores. Significant effects of the initial relative remaining volume upon the porosity were not detected. The compaction of a curd column at an external pressure level below the threshold value resulted from the decrease of porosity as well as from the whey expulsion from the curd grains. In this case, the porosity did not differ significantly between the central part and the outer regions. A higher external pressure led to a faster decrease of the porosity. At an external pressure above the threshold level the porosity of the outer region became lower than that of the central part.The permeability of a curd block decreases over several decades during the drainage process. Initially the permeability is very high. The permeability is mainly determined by the interconnected pores, whereby the largest pores contribute most. The pores can become disconnected due to reorientation and deformation of the curd grains. Application of pressures above the threshold level finally do not result in further expressed curd columns, but cause jamming. The high pressure causes a substantial reorientation and deformation, hence, the permeability decreases very rapidly. The flow of whey from the central region becomes insufficient to keep up with the fast whey expulsion from the outer layer, causing a further decrease of the porosity in the outer region. This, in turn, retards further transport towards the outer layer, causing a rapid sealing near the rind.A simple computer model was made. The expression of a single cube shaped curd grain was used as a model for the expression of a batch of these curd grains. It appeared that the model gave reasonable results at low external pressures. At high external pressures the model was inadequate, as the transport of whey from the pores between the curd grains becomes a rate determining factor, and this factor is not taken into account in the model. In the intermediate pressure range the model predicted a too fast expression of the curd grains, although the overall liquid pressure was below the detection limits. The latter does not rule out the existence of isolated pores, hence the liquid flow may locally be retarded. The deformation of curd grains in a curd bed may also be retarded due to fusion and geometrical constraints