Continuous cheese-making process utilizing an immobilized rennet enzyme reactor

Cheese is made continuously by flowing milk through a spiral flow path of a reactor containing an immobilized enzyme such as rennet which coagulates the milk. The spiral flow path is formed by a spirally wound microporous sheet containing the immobilized enzyme. Adjacent surfaces of the sheet are spaced apart with a spacer which can be a plurality of microporous ribs or a sheet in open net form.

This invention relates to an improved method for forming cheese in a 
continuous manner. More particularly, this invention relates to a 
continuous process for making cheese starting at low and up to high 
temperatures, e.g. up to 40.degree. C., or a combination of the two in 
steps to cause clotting and cheese curd formation. Still further, this 
invention relates to a method whereby repeat treatment of partially or 
fully reacted milk is utilized in a cheese-making process including 
reutilization of any residual unreacted values which may further be 
exhausted of any valuable constituents thereof. 
BACKGROUND OF THE INVENTION 
In a typical cheese-making operation, milk is inoculated with an 
appropriate culture often called a starting culture or bacterial starter. 
The bacterial starter causes acidity to develop in the cheese as a result 
of lactic acid formation. The various bacterial additions also produce 
various cheese types. Thereafter the thus inoculated milk is contacted by 
stirring with an appropriate amount of enzyme rennet used to coagulate the 
casein in the milk to produce the cheese curds. While coagulation takes 
place, the vessel contents are not stirred. This batch reaction may be 
conducted in another tank although the same tank may be utilized. 
Thereafter stirring is resumed; curd formation takes place, and whey is 
separated from the curds. Thereafter a salt wash may be used to separate 
further the whey from the cheese. Whey, of course, is not affected by 
rennet. 
Typically the feedstock is at a proper pH, e.g. about 6-6.5, and 
temperature and thus produces the controlled quality of cheese sought to 
be obtained. The present invention, however, does not relate to acid 
precipitation such as at a pH of 4 to 5, e.g. for cottage cheese. 
As it is evident from the above description, cheese-making is essentially a 
batch process wherein different vessels may also be used to produce 
different reactions. These operations are described in literature such as 
Van Slyke and Price, "Cheese", Orange Judd Pub. Co., New York, NY (1949). 
In an endeavor to reduce the large scale batch vessels or to speed up the 
production of cheese, a number of batch or semi-batch operations have been 
proposed. 
One of the proposed methods requires an enzyme deposited on flat surfaces. 
The substrate, that is the bacterially inoculated material, is flowed past 
the enzyme. Thus the substrate is being treated in a continuous manner. 
These surface deposited enzymes, of course, have imposed a great limitation 
on the flow rates, especially since the enzyme deposition has been by 
adsorption on a surface when these enzymes have been sought to be 
deposited on the surface. 
Additionally, long pipe reactors have been proposed such as where rennet 
and milk are mixed in the pipe reactor, and then dumped in a vessel, or a 
long pipe reactor for a flow method in which the reaction would take place 
on the walls of the pipe containing the adsorbed enzymes. 
According to the last method the bacterially inoculated material flows 
through the pipes at low velocities so as not to strip, by turbulent flow, 
the enzymes deposited on the wall surfaces. As it is well known, at low 
velocities the laminar flow conditions obtain and the velocity profile in 
a pipe reactor often resembles substantially a parabola. As the reaction 
rates are different based on the velocity profile as well as the 
turbulence or lack thereof, the production quality or yield often 
suffered. 
Additionally, various enzymes deposited on filter surfaces likewise have 
been sought to be immobilized by other enzyme carriers which were retained 
on the filter material, e.g. due to mesh constraints or filter material 
type for filter leaves of the apparatus. The fluid flow had been sought to 
be such as to cause the reaction to be continuously conducted. 
However, one of the great shortcomings of the prior art batch, wall, or 
leaf filter type of reactors has been the inexact rennet to milk ratio 
caused by improper mixing, or clogging and/or stripping of these 
surface-deposited materials which all have affected the fairly sensitive 
rennet to milk ratios, thus adversely affecting the necessary quality, 
control and yields. 
Moreover, only a substrate of a high degree of purity could be employed to 
produce the desired or sought-after enzymatic conversion at sacrifices in 
reaction rates and flow rates. As a general proposition, all substrate 
materials which carried particulates such as proteins, fats, coagulants, 
etc., would cause to blind, i.e. clog, the reaction surfaces. Moreover, 
these particulates will also cause to strip the enzyme from the reaction 
surfaces. Hence, substantially non-uniform, noncontrollable reactions 
would occur. As a result, only partially reacted products could be 
obtained which then thereafter had to be further treated or mixed to be 
normalized as to content, quality, etc., to obtain the sought-after 
product. 
In order to remove the disadvantages of enzyme stripping and low flow 
rates, various other approaches have been used such as pulsating reactors 
which supposedly attempt to overcome the disadvantages of the prior art 
apparatus limitations, yet rely on the batch process advantages of 
quiescent coagulation followed by pulsation. 
Other attempts have been made to fluidize a reaction using fluid bed 
principles. For example, the enzyme is deposited on the fluidized 
heterogeneous phase with a substrate forming the liquid phase being 
reacted upon by the fluidized particles. As it is known from fluid bed 
mechanics, considerable abrasion exists between the fluidized particles 
which results in the loss of enzyme. Thus fixed and fluid bed reactors 
have been suggested with various attempts made to improve the mass 
transfer, the rates and/or interface restrictions in the prior art 
reactors or processes. 
Needless to say, these complications have introduced numerous problems such 
that the continuous cheese-making process has been a long sought-after 
goal. Hence the present process has as an objective the production of 
cheese at substantially improved rates, with high quality, in a consistent 
and controlled manner at either low or high temperatures, with a tolerance 
for a high percentage of particulates in the substrate, excellent rennet 
to milk ratios, repeatable, controlled precise exposure to rennet, with 
substantial elimination of operator error, and also if desired a cheese 
without rennet being present. 
THE PRIOR ART 
In evaluating the present invention, the applicant is aware of the 
following art: 
U.S. Pat. No. 3,796,175 granted Oct. 30, 1973 to Berdelle-Hilge; 
U.S. Pat. No. 3,884,641 granted May 20, 1975 to Kraffczyk et al.; 
U.S. Pat. No. 3,945,310 granted Mar. 23, 1976 to Stenne; 
U.S. Pat. No. 4,016,293 granted Apr. 5, 1977 to Coughlin et al.; 
U.S. Pat. No. 4.048,018 granted Sept. 13, 1977 to Coughlin et al.; 
U.S. Pat. No. 4,102,746 granted Jul. 25, 1978 to Goldberg; 
U.S. Pat. No. 4,169,014 granted Sept. 25, 1979 to Goldberg; 
U.S. Pat. No. 4,292,409 granted Sept. 29, 1981 to Cremonesi; 
U.S. Pat. No. 4,416,993 granted Nov. 22, 1983 to McKeown. 
BRIEF DESCRIPTION OF THE PRESENT INVENTION 
In accordance with the present invention, it has now been found that rate 
quality, flow, and mixing constraints imposed by an enzyme deposited on a 
flat surface or suspended particle may be substantially avoided by 
employing a microporous sheet upon and in which the enzyme is immobilized. 
Thus rennet is not in the final product. At the same time, microporosity 
is maintained in the sheet while the advantages of a flow-by reactor are 
maintained and, more importantly, the high particulate fluids may also be 
treated at great advantage. The reactor design and the advantages thereof 
have been disclosed in a companion application Ser. No. 595,954 filed Apr. 
2, 1984, now U.S. Pat. No. 4,689,302. 
However, the benefits of the present process accrue as a result of the 
highly controllable flow characteristics, the precise milk to rennet 
ratios, the enzyme orientation or disposition, the enzyme immobilization, 
the quality control, the porosity considerations, and the flow 
distribution which is obtained by the process steps herein. Other 
advantages are, for example, yields which are highly controllable; quality 
which is repeatable; the freedom to make changes including saving milk (as 
rennet is not present until contacted with the immobilized enzyme); the 
starter culture may be easily metered in a continuous manner; renneting 
can be done independent of coagulation; renneting at low temperature can 
be followed by coagulation elsewhere; off line coagulation may be done 
later; renneting and coagulation may be done in a continuous tube; 
renneting could also be done at almost coagulation temperature; the pipe 
reactor can be shut down and reactivated without microbial contamination 
(following proper precautions), etc. 
These advantages are obtained without the prior art disadvantages such as 
clogging or blinding of the reactor surfaces. Still further, the great 
benefit vis-a-vis for example the filter type reactors, has been realized 
by the ability to process particulate containing substrates which 
contrariwise to the prior art shortcomings, do not substantially strip, 
abrade, or remove the immobilized enzymes. Rather, the present process, as 
practiced, improves the flow characteristics, the fluid distribution, and 
at the same time does not allow the particulates to grow to such size as 
to cause macro-curd formation. 
In accordance with the present invention, it has also been found that the 
fluid distribution aids immeasurably to the production of a high quality 
product by the immobilized rennet, as precise milk to rennet ratios may be 
computed, including obtaining precise time, temperature, and space 
velocity conditions. 
While the exact mechanisms responsible for the reaction are not known, it 
is believed that the fluid flow properties interact with the microporous, 
reticulated three-dimensional matrix in which the immobilized rennet is 
held. It is postulated, without being bound by this theory, that the 
individual liquid "cell" residence time in the reactor is beneficially 
improved by the microporosity, yet all the flow reactor, but more 
precisely, flow-by reactor process characteristics are retained. 
It is believed that the flow characteristics are especially beneficial 
because the partially reacted or even reacted and not yet fully coagulated 
fluid does not clog the surfaces, but rather strips these without 
stripping the rennet. As a further benefit, a flow reactor configuration, 
as further explained herein and which forms one of the process 
embodiments, has been improved by the rib structure being also of a 
microporous nature and carrying the immobilized enzyme rennet. A mesh 
material likewise may be of a microporous nature carrying the immobilized 
enzyme rennet and disposed in a reaction zone. Use of the mesh device 
forms another embodiment herein. These flow division or distribution means 
further aid and yet not impede the reaction causing thus the enzyme 
initiated reaction to continue in an advantageous manner with controlled 
turbulence further benefiting the process. 
Turning now to the specific method whereby the immobilized rennet is 
obtained, in previous U.S. Patents such as No. 4,102,746 and No. 
4,169,014, the method of immobilization has been disclosed. These patents 
are incorporated by reference herein and need not be discussed in greater 
detail. However, the immobilization technique employed in these patents is 
useful for the employment of the present enzymes so that the 
immobilization in the three-dimensionally reticulated microporous 
structure can be obtained in a similar manner with excellent results. 
Based on the above disclosure and based on the studies as conducted, it is 
apparent that it is not merely the enzyme-reactor surface causative effect 
that plays a role, but also that the three-dimensional structure 
contributes to the overall reaction in the fluid flow-by, acting through 
the in-depth deposited enzyme. In other words, there is a benefit which is 
far greater than attributable to the surface catalyzed reactions. The 
results go beyond the expected results which are merely obtained from the 
surface catalyzed reactions and are gained by employing in the reaction 
three-dimensionally immobilized rennet which somehow has an ability to 
influence the reaction conditions in the flow-by reactor, yet without any 
substantial drawbacks of the surface deposited enzymes, e.g. stripping, 
abrading, clogging, etc. 
Based on these considerations and based on the various alternatives which 
are possible to employ, the further benefits accrue as a result of the 
following advantages. One, there is an improved ability to exhaust the 
substrate constituents which may be usefully converted by the enzyme. The 
typical discharge of useful components in whey, e.g. when making cheddar 
cheese, in accordance with the prior art, is about 89.4% of whey, of which 
water is 83.1%, of which the residue, 6.3% on dry weight basis, are 
useful. In accordance with the present process, by being able to react the 
particulate-containing substrate from which coagulated curd material has 
been substantially removed, the results have been improved such that 
substantially entirely the components convertible to cheese by the action 
of rennet may be converted. Of the still useful material left in the whey, 
this may, of course, be treated in a different manner to convert it to 
different products, including cheeses made from whey, e.g. Ricotta cheese. 
As another benefit, the rate of coagulation of casein can be carefully 
controlled by raising the temperature. This characteristic may be used in 
conjunction with or made dependent upon the flow rate. Thus the 
coagulation condition and the subsequent coagulation may be appropriately 
controlled without affecting the rennet, the quality or consistency of the 
cheese, etc. The temperature effect upon rennet under these conditions is 
considerably better monitored and the enzyme is considerably less 
susceptible to damage, because the conditions are essentially adiabatic 
due to the temperature maintenance being capable of very careful control. 
Also staged reactors with different flow and/or temperature conditions may 
be used. 
One of the principal factors which has been found to affect the product and 
process has been the spacing of the sheets. For example, the range from 
0.050 to 0.003 inches has been found to be useful with 0.040 to 0.005 
being the preferred range for the critical spacing distance. At the lower 
end of the spacing range, the pressure drop becomes too great; at the 
higher end the renneting reaction is less susceptible to precise control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A type of microporous sheet 30 is shown and described in U.S. Letters Pat. 
No. 4,102,746 issued July 25, 1978 in the name of Bruce S. Goldberg and 
assigned to the assignee of the instant invention. In addition, other 
microporous plastic and rubber sheets can be used, e.g. as shown in U.S. 
Pat. No. 4,226,926, also assigned to the assignee of the instant 
invention. 
Proteinaceous preparations, for example enzymes, can be immobilized upon 
the silica fragments on the surface of the sheeting and in the pores of 
the sheeting employing the techniques described in U.S. Pat. No. 4,102,746 
and using with material such as described in U.S. Pat. No. 4,226,926. 
Other suitable techniques are also known in the prior art. 
Because of the pore 32 size being in the range of 0.01 microns to 100 
microns, the presence of fat globules, protein, cheese solids and any 
materials which can coagulate to form globules can quickly seal the pores, 
and reduce or completely cut off any flow through the reactor. Since fat 
globules are larger than the pore size, either alone or with a number 
combined, these globules soon coat the surface penetration and prevent any 
of the feedstock from entering into the pores. These fat globules must be 
removed by filtration of the feedstock before it is introduced into the 
reactor system or periodically the feedstock stream must be stopped and 
the reactor system backwashed by the use of suitable material, such as 
water, and causing it to flow reversibly through the reactor system, that 
is from surface to surface. 
Turning now to FIG. 1, there is shown a spiral reactor 50 constructed to 
achieve the results of the present process. The reactor 50 comprises a 
porous core 52 of appropriate diameter and volume such that it can accept 
the feedstock introduced into the reactor 50 or to receive the converted 
feedstock to conduct it from the reactor 50. The spiral reactor 50 is 
substantially made up of microporous sheets 54 having a proteinaceous 
preparation immobilized on or in the pores of the sheets 54. However, the 
sheets 54 are modified to include a plurality of longitudinal ribs 56 on 
one surface thereon as is shown in FIGS. 1a and 2. The ribs 56 contact the 
adjacent convolutions of the spiral to keep them separated and to 
establish a series of passages 58 therebetween in conjunction with the 
spiral convolutions themselves. The height h (see FIG. 2) is so chosen 
that a desired flow-through reactor 50 is attained while insuring proper 
conversion of the feedstock. The height h of the ribs 56 has been found to 
be in the range of 0.003" to 0.050" and most particularly about 0.005" to 
0.040". The height will, of course, have to be altered depending upon the 
feedstock employed. The total space available for feedstock flow is 
identified as the void volume and measured in cubic centimeters. 
Alternatively, a spacing sheet 60 as shown in FIGS. 3 and 3a may be 
employed instead of the raised ribs 56. The spacing sheet 60 is made of a 
flexible plastic or rubber material and in an open fish net format to 
assure light weight and flexibility. The spacing sheet may be made of a 
microporous material with an enzyme immobilized thereon. Spacing sheet 60 
is simply laid atop a microporous sheet 70 as shown in FIG. 3a. Even 
though the surfaces of the spacing sheet 60 are substantially flat, the 
surface of the sheeting will rest on a number of peaks on the surface of 
sheet 70, providing, along with the open net format of the sheet 60, 
sufficient passages to permit the feedstock to pass the spacing sheet 60. 
The reactor 50 is completed as shown in FIG. 4 by encapsulating the entire 
unit in a suitable housing 80. A pipe 82 is provided to gain access to the 
core 52 and plenum 84 is to provide access to the free end of the reactor. 
Although it is preferable to introduce the feedstock under suitable 
pressure into the core 52 of the reactor 50 and remove the converted 
feedstock from the free end of the reactor 50, the opposite flow pattern 
can also be used, namely introduce the feedstock to the free end and 
remove the converted feedstock from the core 52. 
The microporous sheeting together with a spacing layer, whether ribs 56 or 
spacing sheet 60, are wound upon core 52 to form the spiral configuration 
shown in FIG. 1. As the feedstock flows along the passage 58, it is 
sufficiently exposed to enzyme to convert the feedstock to a high degree. 
The spiral reactor 50 may be formed of a stack of individual microporous 
sheets 54, which are about 0.020 inches thick. It may also be formed by 
winding on itself a 90 inch length of 0.020 inch thick microporous 
material to produce a layer 1 inch thick and having a width of 3 inches. 
Considering that both sides of the sheets 54 are available, a total 
surface area of 270 square inches, reduced by the areas of contact of the 
sheet 54 edges with the housing 80, about 240 square inches of surface is 
effective. Employing ribs 56 having a height h of 0.010 inches and winding 
the sheets 54 on a porous core 52, 1 inch in diameter and 3 inches long, 
results in a reactor 50 having an outside diameter of 2 inches and a void 
volume or flow path volume of between 30 and 40 cubic centimeters. When 
potted, that is with the housing 80 in place, the outside diameter is 23/4 
inches thick. 
With the enzyme lactase immobilized on the sheets 54, as set out in U.S. 
Pat. No. 4,169,014 identified above, and employing skim milk adjusted to a 
pH of 5.1 at a flow rate of 10 milliliters per minute at 40.degree. C. 
with a residence time of 3 to 4 minutes, reactor 50 is capable of 
hydrolyzing 90% of the lactose in the skim milk. 
If a reactor 50 is constructed using a spacing sheet 60 of a fish net 
format having open areas from 50-90% but preferred in the range of 70-80% 
of the area of a continuous sheet of the same dimensions and a thickness 
of 0.010 inches, the available void volume will be about 50 cubic 
centimeters. Using the same feedstock under the same conditions set out in 
the previous example, approximately the same percentage of lactose in the 
skim milk will be hydrolyzed. It has been found that in actual practice 
conversions of the feedstock have reached values as high as 90%. The flow 
path through the reactor 50 is assumed generally to be laminar, but in 
practice it has been found to have a random flow, the flow causes eddies, 
and in some cases may also pass through the sheet itself. 
Various coagulation or clotting time studies at various temperatures and 
residence times have been illustrated in the drawings. These will be 
described in greater detail in the examples herein. In brief, the flow 
rate of the feed at the given temperature is indicated from which the 
contact or residence time is calculated based on the reactor volume. 
Thereafter the actual clotting or coagulation time is correlated again at 
the particular temperature used. These data will be described and 
explained below in the Examples as these relate to FIGS. 5, 6 and 7. 
These and others benefits will be further pointed out herein below with 
reference to the specific embodiments wherein the examples illustrate 
merely the various aspects of the invention and are not intended to limit 
the broader scope of the invention. 
EXAMPLE 1 
A calf-rennet milk clotting enzyme was immobilized on a spiral reactor as 
shown in FIG. 1 for testing on continuous coagulation of skim milk. The 
milk-clotting apparatus was set up for the coagulation time study and the 
parameters of temperature and residence time were studied. 
The calf-rennet clotting enzyme was immobilized on the spiral reactor by 
using the standard immobilization technique described in U.S. Patents such 
as No. 4,102,746 and No. 4,169,014. 
Using the milk-clotting enzyme immobilized spiral reactor, continuous 
coagulation on skim milk was achieved. Skim milk was pumped through the 
rennet immobilized reactor at various flow rates (residence times) and was 
coagulated at 30.degree. C. The obtained data were depicted in FIGS. 5 and 
6. 
Residence time (flow rate), temperature and pH of skim milk were the major 
factors that affect the coagulation time. 
After six hours of continuous operation as shown in FIG. 6 at low feed 
temperature (6.degree. C.) and low flow rate (20 ml/min at 1.5 min. 
residence or contact time) a slight loss of activity was observed, 
possibly due to the deposition of protein on the surface. Higher 
feed-temperatures and higher flow rates help to minimize substantially 
entirely this problem. Both microbial and calf rennet were tried. Protease 
enzymes such as pepsin or alkaline protease are also useful. 
For this example, a commercially obtainable calf rennet clotting enzyme was 
used. It contains approximately 95% chymosin and 5% pepsin. 
A spiral reactor, 3" high.times.1" ID.times.21/4" OD, as described 
previously, was made and immobilized with rennet enzyme using the standard 
immobilization process mentioned above. Skim milk was used as the 
substrate. 
A clotting test unit consists of a 30.degree. C. water bath, a test vessel 
in which the treated substrate is held and which is rotated by an electric 
motor. Rotation is at about 4-10 RPM. 
The continuous coagulation system was run as follows. A vessel for milk, a 
cooling coil, and the above described reactor were enclosed in a water 
bath which was then controlled at about 6.degree.-8.degree. C. and 
14.degree. C. and 19.degree. C., or at any other preselected temperature. 
The reactor was first flushed with a pH 6.5 buffer (0.5 gm sodium acetate 
per liter), followed with skim milk at the desired temperature, with 
various flow rates to determine the effect of both temperature and 
residence time on the coagulation time being thereby observed. Skim milk 
was also pumped through the reactor at a constant flow rate for a period 
of 6-8 hours to see if any change on activity occurs. 
The reactor was run each day for approximately six hours at one desired 
temperature. The reactor was then cleaned up with (a) 6.5 pH buffer (30 
min) and followed with (b) 6.5 pH buffer containing 0.05% hydrogen 
peroxide (30 min) and stored in refrigerator for the next day trial. 
Clotting time tests were established as follows. Ten milliliters of the 
treated skim milk effluent were collected from the reactor in the test 
vessel. The vessel was rotated at 4-10 RPM in 30.degree. C. water to 
determine both initial (flake on wall) and completed clotting time 
(gel-like on the whole surface of the 10 ml). However, it was found that 
many factors, such as size of coagulum, effluent feed temperature, heat 
transfer rate, etc., affect the initial coagulation time results. The 
completed coagulation time for 10 ml sample was used for this study. 
FIG. 5 presented a series of clotting tests on the rennet reactor skim milk 
samples which were run with various flow rates at different temperatures. 
Activity is expressed as clotting time (min.) which is the time for a 
sample of skim milk (10 ml) to clot completely under 30.degree. C. The 
results indicated that a continuous coagulation process is achieved. At 
6.degree.-8.degree. C. milk feed temperature, it took 30 min. and 45 min. 
to clot the treated milk with a flow rate of 20 ml/min. (1.5 min. 
residence, i.e. constant time) and 30 ml/min. (1. min residence time) 
respectively. If the feed temperature was increased to 19.degree. C., the 
clotting time was decreased from 30 min to 20 min and 45 min. to 30 min. 
with same above flow rates. Thus, the clotting time can be controlled by 
regulating both flow rate and feed temperature. 
As shown in FIG. 6, skim milk (6.degree.-8.degree. C. temperature) was 
continuously pumped through the reactor with a constant flow rate of 20 
ml/min. (1.5 min. residence time). The effluent was then collected at 
various operating hours for the clotting test. The results indicated that 
the clotting time increased from 20 min. at first hour to 30 min. after 
six hours of continuous operation. This may indicate that some protein 
contamination deposits on the surface of the reactor which causes a slight 
loss in activity. 
EXAMPLE 2 
Typically in the manufacture of cheese milk, either skim or whole milk is 
first pasteurized and then cooled to 32.degree. C. (about 88.degree. F.). 
Thereafter a starter culture and rennet are added. In the present example 
and as shown in FIG. 7, the clotting (i.e. coagulation time) has been 
illustrated as a function of substrate temperature and clotting 
temperature in the apparatus as described in Example 1. However, the 
advantages of the high temperature operations, e.g. 30.degree. C. and 
37.degree. C., are evident as less cooling (or conversely heating) is 
necessary. 
The procedure was as follows: 
The calf rennet milk clotting enzyme was immobilized in the spiral reactor 
described above (size 3" long.times.21/4 O.D.) and run at 30.degree. C. 
and 37.degree. C. reactor temperature using both store bought pasteurized 
skim and whole milk as feedstocks. Coagulation tests were done at 
30.degree. C. and 40.degree. C. using a 10 milliliter coagulation test 
procedure described in Example 1. From the data shown in FIG. 7, the 
following apply. 
At 30.degree. C. and 37.degree. C., with a residence time of 0.75 min. and 
1.5 min., coagulation did not take place inside the rennet immobilized 
reactor. It did occur in the external clotting test tube at 30.degree. C. 
The coagulation took place inside the reactor at 30.degree. C. if the skim 
milk was passed through the reactor system twice. This proves that by 
adjusting the residence time (or reactor size), it is possible to control 
the desired coagulation time. However, part of the reaction may have been 
initiated in the heating coil before the reactor, the material was enzyme 
treated already on the first pass. 
Further, the clotting test at 40.degree. C. gives a much faster clotting 
rate (approximately 30%) than at 30.degree. C. The whole milk gave 
approximately 20% longer clotting time as compared to skim milk. 
Based on operating conditions, milk can be processed through the reactor at 
30.degree. and 37.degree. C. Coagulation or clotting external to or 
internal to the reactor was observed. The immobilized calf-rennet enzyme 
used in the reactor was approximately six months old. 
FIG. 7 thus presents a series of test results at various reactor and feed 
temperatures and the resulting clotting test results at different 
temperatures. As mentioned before, skim milk and whole milk were first 
heated to 30.degree. C. and 37.degree. C. and passed through the reactor 
at the same respective temperatures. No coagulation and pressure drop 
increase in the reactor was observed. If the first pass treated milk is 
stored overnight in the refrigerator and then reheated and repassed 
through the reactor, the cogulation took place inside the housing and 
reactor, which indicates that the residence time is still a major factor 
affecting the clotting rate. 
A reactor system that fits into a commercial process for continuous 
coagulation of 30.degree. C. or 37.degree. C. is practical. As shown in 
FIG. 7, the clotting test at 40.degree. C. gave a much faster clot rate 
than at 30.degree. C. For example, with a reactor residence time of 1 
minute, it takes 26 minutes clot time at 40.degree. C. as compared to 38 
minutes at 30.degree. C. Whole milk gave slightly longer clot time as 
compared to skim milk. 
Further, it was concluded that at 30.degree. and 37.degree. C. reactor 
temperatures, breakdown of casein occurs. Not to clot or coagulate in the 
reactor but external to it is practical. By adjusting the residence time, 
one may cause clotting or coagulation to occur in the immobilized enzyme 
reactor. 
As the reactor can tolerate particulates, it is evident that not only 
cheese can be obtained by casein coagulation, but protein coagulation in 
the whey may also be practiced. Appropriate whey protein enzymes are 
available for immobilization from commercial sources. 
Still further, immobilization of rennet allows its use even after prolonged 
storage (under appropriate conditions). The treated substrate is rennet 
free and rennet thereafter does not continue to cause further reactions in 
the product. Precise control, e.g. residence time, flow rate, temperature, 
are possible. As milk is free from rennet up to the time it flows through 
the reactor, the unreacted milk flow may be stopped and diverted, thus 
allowing swing capacity for different cheeses and also for storage of 
unreacted milk. Moreover, starter culture may be added shortly before the 
milk is introduced in the reactors. Hence not only rennet under or over 
addition is avoided, but also starter contact and addition is easily and 
precisely controlled. Various renneting contact times at various 
temperatures at various coagulation temperatures introduce also great 
flexibility in cheese making processes.