Process for increasing the capacity of systems for membrane filtration of milk or milk products

A process for increasing the capacity of plants for membrane filtration of milk or milk products, whereby prior to the membrane filtration, the milk or milk product is usually subjected to one or more pretreatments selected from the following: PA0 (a) heat treatment for bacteriological reasons, PA0 (b) fat standardization in view of the fat content of the product, PA0 (c) homogenization in view of the physical and organoleptic properties of the end product, and PA0 (d) pre-acidulation in order to influence the solubility of the ash components of the milk. According to this process, prior to the membrane filtration, said milk or milk product is additionally subjected to an enzymatic treatment with curdling enzyme under agitation of the milk or milk product to form an integrated dispersion of a coagel in whey. The resulting concentrate may be used as feeding additive or feeding starting material as well as a nutritive medium by microbial processes.

FIELD OF THE INVENTION 
The present invention relates to a process for increasing the capacity of 
systems for membrane filtration of milk or milk products, wherein, prior 
to the membrane filtration, the milk is usually subjected to one or more 
pre-treatment steps including one or more of the following steps: 
(a) heat treatment for bacteriological reasons, 
(b) fat standardization in view of the fat content of the product, 
(c) homogenization in view of the physical and organoleptic properties of 
the end product, and 
(d) pre-acidulation in order to influence the solubility of the ash 
components of the milk. 
More particularly, the invention relates to a process, whereby it is 
possible to obtain an increase in the capacity of a membrane filtration 
system by filtering enzymatically curdled milk or milk product. 
BACKGROUND ART 
Danish patent specification No. 129,556 discloses a process for membrane 
filtration of milk or milk products for the preparation of an aqueous 
concentrate with a high content of protein substances, said process being 
carried out without additions of enzymes or chemicals such as acids or 
bases. 
Journal of the Society of Dairy Technology, vol. 35, No. 2, April 1982, 
page 43 and the following pages, discloses ultrafiltration of non-curdled 
preacidulated milk. As it appears from the article on page 46, it is not 
of particularly great importance to the filtration capacity whether the 
filtration is carried out on pre-acidulated or normal milk. 
SUMMARY OF THE INVENTION 
It is accordingly one object of the present invention to provide an 
improved milk or milk product for introduction to membrane filtration. 
A further object of the invention is to subject milk or milk products to an 
enzymatic treatment with curdling enzyme prior to introduction to membrane 
filtration. 
Other objects and advantages of the present invention will become apparent 
as the description thereof proceeds. 
In satisfaction of the foregoing objects and advantages, there is provided 
by the present invention a process for increasing the capacity of membrane 
filtration systems for milk or milk products, whereby prior to the 
membrane filtration, the milk or the milk product is normally subjected to 
one or more of the following pre-treatment steps: 
(a) heat treatment; 
(b) fat standardization; 
(c) homogenization; and 
(d) pre-acidulation; 
the improved process comprising subjecting said milk or milk product, prior 
to the membrane filtration, to a further pretreatment in the form of an 
enzymatic treatment with curdling enzyme under agitation of the milk or 
milk product to form an integrated dispersion of a coagel in whey. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The characteristic feature of the process according to the present 
invention is that said milk or milk product, prior to the membrane 
filtration, is additionally subjected to an enzymatic treatment with a 
curdling enzyme under agitation of the milk or milk product to form an 
integrated dispersion of a coagel in whey. 
In this manner a surprisingly large capacity of the membrane filtration 
system is obtained so that a smaller system can be used for the same 
amount of milk compared to known techniques. In addition less expenses of 
upkeep of the membranes are involved, less energy is consumed, less room 
is necessary, and less investments are necessary. 
The concentrate obtained by the process according to the invention may also 
be further concentrated to a dry matter content of up to about 73% by 
weight without involving a tendency for visible, free fat to appear in the 
product. In comparison it can be mentioned that by the preparation of 
so-called Cheese-Base from a concentrate obtained by the traditional 
process, which for instance has been described in 1980, J. Dairy Sci. 
63:228-234 and the British patent application No. 8,122,195 filed on July 
17, 1981, a dry matter content of 60% by weight should be considered the 
upper limit, since a higher dry matter content causes the fat of the 
product to be damaged, which causes an undesired content of free fat in 
the Cheese-Base. 
The concentrate prepared according to the invention is particularly suited 
for the production of Cheese-Base. The enzymatic curdling implies that a 
concentrate prepared according to the invention does not possess quite the 
same characteristic properties as known by common ultrafiltered 
concentrates. When a non-acidulated milk product has been used as starting 
material by the process according to the invention, the concentrate 
obtained according to the invention is sweet. The concentrate may either 
be acidulated to a desired pH and subsequently be evaporated to a desired 
dry matter content or starter may be added immediately before an 
evaporation whereafter the concentrate is evaporated to a desired dry 
matter content. The acidulation to a desired pH is carried out by means of 
the starter added immediately before an evaporation. 
In comparison with the traditionally ultrafiltered concentrate, the 
additionally concentrated product obtained on the basis of the concentrate 
prepared according to the invention may have a slightly grainy 
consistency, which, however, decreases essentially after the evaporation. 
The above grainy consistency in the retentate and consequently also in the 
end product may be avoided or removed by subjecting the retentate to a 
suitable additional treatment, such as for instance pumping through a 
homogenization nozzle or other weak homogenization. 
A preferred embodiment of the process according to the invention is 
characterized in that the enzymatic reaction is carried out in portions 
during a constant stirring. As a result, a mixture of small grains and 
whey is formed. By carrying out the enzymatic reaction in small portions 
it is possible to ensure that the membrane filtration system is fed with a 
homogeneous material so as to thereby obtain a homogeneous end product. 
According to a second preferred embodiment of the process according to the 
invention, the curdling is carried out in a continuous system during 
constant stirring to form a mixture of small grains and whey. Rennet 
curdling during stirring has never previously been connected with a 
following cheesing process. Previously it was always necessary to carry 
out the curdling of milk for the production of cheese calmly or without 
stirring in order to ensure achievement of as homogeneous a coagulatum as 
possible. Curdling during stirring usually causes formation of an unstable 
coagulum, which in the traditional production of cheese, causes an 
increased loss of "fines" (small cheese grains/cassein grains), which are 
drained off together with the whey. In the present invention, the 
formation of these small coagel grains (fines) is desired as it has been 
discovered that these small grains are the actual causes of obtaining the 
surprisingly increased or larger capacity of the membrane filtration 
system since these grains decompose the secondary membrane usually formed 
or deposited on the filtration membrane. 
When the enzymatic reaction causing curdling takes place in a continuous 
system, it is advantageous that the reaction comprise two steps, viz. a 
first step at a low temperature, e.g. below about 10.degree. C., usually 
5.degree.-0.degree. C., with a standing time of up to about 6 hours, and a 
second step which is the curdling phase and which is carried out at a 
temperature above the curdling temperature, e.g. above 25.degree. C., 
usually 30.degree.-50.degree. C., dependent on the membrane filtration 
temperature. The reaction time at 30.degree. C. is over a period of about 
75 seconds and at 50.degree. C. almost instantaneous, provided the 
temperature is 2.degree. C. for about 5 hours in the first phase. By 
operating with differentiated temperatures during the enzymatic reaction, 
the actual curdling time and consequently the volume of the necessary 
system for the continuous curdling are drastically reduced. The 
temperature of the second step corresponds advantageously to the 
operational temperature of the membrane filtration system in such a manner 
that the product resulting from the curdling may enter the membrane 
filtration system directly. 
In the process according to the invention, the curdling phase takes place 
in the second step during a constant turbulent flow. Curdling during 
vigorous stirring or the like is inapplicable to the traditional cheesing 
technique. 
By the process according to the invention rennet is most suitable as the 
curdling enzyme although other curdling enzymes can be used. As examples 
of suitable rennet enzymes, both animal and microbial rennet enzymes can 
be mentioned. In the following scheme a number of applicable rennet 
enzymes are mentioned. 
______________________________________ 
Country of 
Trade Name Type Producer Production 
______________________________________ 
Hannilase Microbial Chr. Hansens 
Denmark 
Laboratorium 
Standard Animal Chr. Hansens 
" 
Laboratorium 
Stabo " Chr. Hansens 
" 
Laboratorium 
Stamix " Chr. Hansens 
" 
Laboratorium 
Fifty-fifty " Chr. Hansens 
" 
Laboratorium 
Rennilase Microbial Novo " 
Marzyme-I " Marshall U.S.A. 
Marzyme-II " " " 
New Marzyme " " " 
Animal rennant 
Animal " " 
______________________________________ 
By the tests described in the following Examples Chr. Hansen Standard was 
used as rennet which possesses a strength of 15,000 Soxhlet degrees. When 
products having lower strength are used, longer reaction periods are 
necessary. 
According to the invention it is possible to treat all types of milk or 
milk products, which include natural animal milk or fat standardized milk 
of the same origin including treated milk such as pasteurized milk as well 
as recombined milk including recombined milk both containing animal and 
vegetable fat, which, if desired, may be treated as stated above. 
The resulting concentrate may be used as feeding additive or feed starting 
material and may also be used as a nutritive medium in microbial 
processes. The process according to the invention is particularly suitable 
in connection with a Cheese-Base production. Cheese-Base prepared on the 
basis of the concentrate according to the invention may be used as 
starting material for the preparation of process cheese. 
The milk or milk product to be treated is usually subjected to one or more 
pre-treatments selected from the following: 
(a) heat treatment for bacteriological reasons, 
(b) fat standardization in view of the fat content of the product, 
(c) homogenization in view of the physical and organoleptic properties of 
the end product, and 
(d) pre-acidulation in order to influence the solubility of the ash 
components of the milk. 
All these treatments are known per se and require no further explanations. 
It should, however, be mentioned in connection with pre-treatment (d) that 
the pre-acidulation must, of course, only be carried out to such a pH that 
no acid curdling takes place since it is not possible to carry out an 
enzymatic curdling after an acid curdling. 
The milk product pre-treated in the manner according to the invention is 
subsequently subjected to an enzymatic treatment with a curdling enzyme 
under agitation of the milk or milk product. The treatment is 
advantageously carried out with stirring and may be carried out either in 
portions or continuously. As mentioned it is possible, if desired, to keep 
the temperature of the curdled product at a temperature corresponding to 
the operational temperature of the membrane filtration system used for the 
following membrane filtration. 
As suitable membrane filtration systems, conventional ultrafiltering 
systems can be mentioned, optionally combined with equipment for 
diafiltration.

The present invention will be illustrated below by means of Examples, of 
which one Example is a comparative Example. 
EXAMPLE I 
Preparation of Retentate for the Preparation of Cheese-Base 
Milk was pasteurized at 72.degree. C. for 15 sec. and cooled to 40.degree. 
C. and subsequently placed in a tank with vigorous stirring. 30 ml of 
rennet, Chr. Hansen Standard, were added per 100 kg of milk. During a 
period of about 25 minutes the curdling was terminated and by means of a 
centrifugal pump, the curdled milk was transferred to the feeding tank 
(1000l) of an ultrafiltration system, said tank being provided with a 
propeller-driven stirrer. The ultrafiltration was carried out at 
40.degree.-44.degree. C., a mean pressure of 4-5 bar by means of DDS 
ultrafiltration system type 37 equipment with GR-51-PP-membranes, whereby 
a flux of 110 l/m.sup.2 /h was obtained at a dry matter content of 40% by 
weight in the concentrate. 
The results obtained are stated in the following Table. 
EXAMPLE II 
(Comparative Example) 
Milk was pasteurized at 72.degree. C. for 15 seconds and cooled to 
50.degree. C. In a system corresponding to the system of Example I an 
ultrafiltration was carried out at 50.degree. C., a mean pressure of 4-5 
bar, and with GR-61-pp-membranes, whereby a flux of 10-12 l/m.sup.2 /h was 
obtained at a dry matter content of 40% by weight in the concentrate. 
The results obtained are stated in the following Table. 
TABLE 
______________________________________ 
Example I Example II 
Concen- Per- Concen- 
Component Milk trate meate trate Permeate 
______________________________________ 
Fat (1) 3.10 18.14 0.00 17.05 0.00 
Protein (2), 
3.39 17.22 0.52 17.72 0.21 
calculated 
as N % 
Dry matter (3) 
11.94 40.06 6.2 39.92 5.3 
Consumption 5.9 5.5 
(kg milk/kg 
concentrate) 
Flux 110 10-12 
l/m.sup.2 /h 
at 40% TS 
______________________________________ 
Explanation of the Table: 
(1) According to Gerber. 
(2) Kjeldahl N multiplied with 6.38. 
(3) (a) Milk: 4 h at 105.degree. C. 
(b) UFconcenttrate: about 2 g + 5 ml distilled H.sub.2 O, dried for 16 h 
at 105.degree. C. 
The use of the GR-51-PP-membrane in Example II provides corresponding data 
as obtained when using the GR-61-PP-membrane, viz. 10-12 l/m.sup.2 /h. 
As will be noted from the Table, a substantial increase in capacity of a 
membrane filtration system is obtained. This is shown by the surprisingly 
higher flux obtained according to the invention. 
By the process of the invention, it is also possible to membrane filter up 
to a very high dry matter content without the viscosity being impeded. 
The increase of the capacity obtained by the process according to the 
invention is not only surprising, but also surprisingly great since the 
capacity obtained is even greater than the capacity known by membrane 
filtration of pure whey.