AU2008306878B2

AU2008306878B2 - Method and plant to obtain milk with low sugar content

Info

Publication number: AU2008306878B2
Application number: AU2008306878A
Authority: AU
Inventors: Francesco Vernazza
Original assignee: Parmalat SpA
Current assignee: Parmalat SpA
Priority date: 2007-10-01
Filing date: 2008-10-01
Publication date: 2014-02-20
Anticipated expiration: 2028-10-01
Also published as: ITUD20070181A1; CA2701066C; WO2009043882A1; CA2701066A1; AU2008306878A1

Abstract

Method to obtain milk (LFM) with low sugar content starting from milk (M), comprising at least two steps: a first step in which the milk (M) is subjected to ultrafiltration (UF) so as to produce, at exit, an ultrafiltration permeate (UFP) and an ultrafiltration retentate (UFR); and a second step in which the ultrafiltration permeate (UFP) is subjected to nanofiltration (NF) so as to produce, at exit, a nanofiltration permeate (NFP) and a nanofiltration concentrate (NFC). The nanofiltration permeate (NFP) is continuously recircled directly to ultrafiltration (UF) so as to dilute the ultrafiltration retentate (UFR) and obtain the milk (LFM) from the latter.

Description

WO 2009/043882 PCT/EP2008/063165 "METHOD AND PLANT TO OBTAIN MILK WITH LOW SUGAR CONTENT" FIELD OF THE INVENTION 5 The present invention concerns a method, and the relative plant, to obtain milk with low sugar content, reducing or eliminating the lactose naturally present in the milk. BACKGROUND OF THE INVENTION It is known that lactose is a disaccharide naturally present in the composition 10 of milk, to about 5% in weight. Lactose may entail difficulties in digestion or other problems connected to its metabolization in some categories of consumers, for example people intolerant of lactose or following a course of antibiotics. Methods to reduce or eliminate the lactose present in milk are known, which 15 are based on enzymatic hydrolysis of the lactose by means of the enzyme galactosidase. The hydrolysis determines the conversion of more than 80% of the lactose into monosaccharides, or simple sugars, glucose and galactose. In known methods, the enzymatic hydrolysis is often combined with other 20 separation methods, such as membrane filtering, chromatography or others. These known methods are of the discontinuous type, where the various components are produced in different batches, and are then mixed or remixed in order to make the final product. However, as a result of this, these methods have disadvantages connected to 25 the need for intermediate storage, which increases the costs of management and control. Furthermore, the production time is not optimized, as serial passages are provided, obligatory for the various components. These known methods, furthermore, are very rigid, since the quantity to be 30 produced and the beginning and end of the production cycle are connected to the type of execution, that is, in batches. It is not therefore possible, with said known methods, to reply effectively to sudden needs to change production. Another disadvantage is that the simple sugars produced in hydrolysis cause a 2 taste that may be too sweet for some categories of consumers, and may be unadvisable in diets with low calorie consumption or for those suffering from disturbances in their sugar metabolism. Therefore, in general, in the milk industry, the need to eliminate, or at least 5 reduce, the content of sugar in milk, whether lactose or simple sugars, glucose and galactose, is particularly important. Purpose of the present invention is to perfect a method and make a relative plant to obtain milk with low sugar content, and particularly lactose, or substantially sugar-free and/or lactose-free, which is quick to execute, flexible to 10 manage, which also has limited costs and which at the same time allows to obtain milk which does not taste too sweet. The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages. 15 SUMMARY OF THE INVENTION The present invention is set forth and characterized in the independent claim, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea. The method according to the present invention can be used to obtain milk with 20 low sugar content, starting from milk. The initial milk may be raw milk or milk previously heat treated and possibly standardized to a content of fat comprised between the natural value and a substantially zero value. In accordance with the above purpose, the method according to the present invention comprises at least the following steps: 25 - a first step in which the milk is subjected to ultrafiltration in order to produce, at exit, an ultrafiltration permeate, with a content of dry matter based on milk salts and sugars, and an ultrafiltration retentate; and - a second step in which the ultrafiltration permeate is subjected to nanofiltration in order to produce, at exit, a nanofiltration permeate, and a nanofiltration 30 concentrate, wherein the nanofiltration permeate is continuously recircled directly to ultrafiltration so as to dilute the ultrafiltration retenate and obtain the milk from 3 the latter, and wherein in the second step the nanofiltration concentrate is collected continuously from nanofiltration. The nanofiltration permeate is recircled directly to ultrafiltration, so as to re integrate the milk salts and dilute the content of dry matter of the milk which is 5 subjected to ultrafiltration, consequently reducing the content of dry matter of the ultrafiltration retentate from which the milk with low sugar content is obtained. Advantageously, the dilution of the content of dry matter of the milk subjected to ultrafiltration is at the expense of the sugars. Thanks to the reduction in the content of dry matter of the ultrafiltration 10 retentate which, according to the main solution of the present invention, constitutes the final product, the method according to the present invention allows to obtain milk with low sugar content and particularly low lactose content, or substantially lactose-free. Since it operates substantially continuously, the present invention needs no 15 intermediate storage, thus reducing plant costs, management costs and the cost of controlling the various steps. Furthermore, the production times are optimized, since there are no serial passages. The present invention allows an extremely elastic production since it allows to decide at any moment how much finished product to produce, when to start and 20 when to finish production. This gives considerable advantages in management and the possibility to reply effectively to sudden requirements to change production. The present invention allows to determine a desired sugar content, but in any case low, in the final product, so as to adapt to the different dietary requirements 25 of consumers. It is possible, for example, to obtain milk with low lactose content comprised between about 0.1% and 3% in weight, the so-called "low-lactose milk", and advantageous for consumers who need to keep active the enzymatic system to metabolize lactose. Furthermore, since enzymatic hydrolysis is not provided in the main solution, 30 but filtration, the present invention also allows to obtain, in a rapid manner, milk 4 in which, once the lactose has been reduced, other sugars are substantially not present. Advantageously, the first and the second steps are performed substantially concurrently, advantageously in time and place, and provide to feed the milk 5 continuously to ultrafiltration, and to collect the ultrafiltration retentate continuously, which represents the final product with reduced or zero sugar - A content, and the nanofiltration concentrate. Furthermore, it is provided to feed continuously a stream of water to nanofiltration, the flow rate of which is substantially equal to that of the 10 nanofiltration concentrate collected. An advantageous variant of the present invention provides that the content of dry matter of the ultrafiltration retentate is regulated at least by controlling the content of dry matter of the ultrafiltration permeate and the content of dry matter of the nanofiltration permeate. 15 Advantageously, the content of dry matter of the ultrafiltration retentate is also regulated by controlling the flow rates of the streams entering and exiting ultrafiltration and nanofiltration. In a variant, it is possible to combine the continuous method as described above with an enzymatic hydrolysis of the lactose, before or after the steps 20 described above, in order to hydrolyze the residual lactose. In this way, substantially lactose-free milk is obtained, with lactose comprised between about 0.1% and 0.5% in weight, for consumers completely intolerant of lactose. In another aspect, the invention provides a plant according to obtain milk with low sugar content starting from milk comprises at least an ultrafiltration unit able 25 to produce at exit an ultrafiltration permeate, whose content of dry matter is based on milk salts and sugars, and an ultrafiltration retentate, and a nanofiltration unit able to produce at exit a nanofiltration permeate whose content of dry matter is based on milk salts, and a nanofiltration concentrate with a content of dry matter based on milk sugars. The plant also comprises means to feed and collect liquid, 30 said means being able to feed continuously a stream of milk to the ultrafiltration unit, a stream of ultrafiltration permeate to the nanofiltration unit and a stream of 5 water to the nanofiltration unit and able to continuously collect a stream of the ultrafiltration retentate from the ultrafiltration unit and a stream of nanofiltration concentrate from the nanofiltration unit. The plant also comprises recircling means able to continuously recircle the 5 stream of nanofiltration permeate to the ultrafiltration unit, so as to reintegrate the milk salts and to dilute the content of dry matter of the milk subjected to ultrafiltration, thus reducing the content of dry matter of the ultrafiltration retentate and obtaining the milk from the latter. Means are provided for continuously collecting the nanofiltration concentrate from nanofiltration and 10 continuously eliminating the nanofiltration concentrate as a sub-product. A variant of the present invention also provides the enzymatic hydrolysis of the milk, before or after the ultrafiltration and nanofiltration operations. In the case of hydrolysis performed before, the subsequent first and second steps are performed on milk with low lactose content, but with a determinate content of 15 glucose and galactose which are in any case reduced or eliminated thanks to the ultrafiltration and nanofiltration and the relative recircling of the nanofiltration permeate according to the present invention. The milk obtained as final product can also be integrated, or not, with salts deriving from whey, or also possibly from milk, in order to balance the final taste 20 and savor. DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF EMBODIMENT With reference to the attached drawing, a method according to the present invention is used to process milk, indicated by the reference M, having a typical 25 initial composition of about 5% in weight of lactose, about 0.7% in weight of salts, milk proteins comprised between about 3% and 4% in weight, and also a fat content comprised between about 0% (skimmed milk) and 4% in weight. A first step of the method provides to feed the milk M continuously to an ultrafiltration unit UF in order to subject it to ultrafiltration, from which an 30 ultrafiltration permeate UFP is obtained, and an ultrafiltration retentate UFR.

5 a The present invention, as an optional passage, provides to standardize the content of fat upstream of the enzymatic hydrolysis, shown by block S, in order to take it to the desired percentage, by means of known techniques. According to the present invention, the lactose and the salts pass into the 5 ultrafiltration permeate UFP in equal proportion to that of the milk from which the UFP permeate derives, since the membranes of the ultrafiltration UF are not able to hold them back. The lactose and salts constitute the dry matter, indicated hereafter by the abbreviation d.m., of the ultrafiltration permeate UFP. On the contrary, the ultrafiltration membranes UF are able to hold back the insoluble 10 proteins, typically casein, which remain in the retentate UFR. In a second step, the ultrafiltration permeate UFP is fed continuously to a nanofiltration unit NF to be subjected to nanofiltration, in order to obtain a WO 2009/043882 PCT/EP2008/063165 -6 relative nanofiltration concentrate NFC which, under normal working conditions, is continuously collected, and a nanofiltration permeate NFP. The nanofiltration NF membranes separate a large part of the lactose and the soluble proteins which go to the concentrate NFC, while substantially all the salts 5 pass to the permeate NFP. The method according to the present invention is based on the concept of reducing to a desired value the value of d.m., and hence lactose, of the ultrafiltration retentate UFR, by diluting the value of d.m. of the stream which is subjected to ultrafiltration UF, performed by the nanofiltration permeate NFP. 10 The reduction in the d.m. value of the ultrafiltration permeate UFP is directly correlated to the reduction in the d.m. value of the retentate UFR. Therefore, controlling the d.m. value of the ultrafiltration permeate UFP is equivalent, based on suitable calculations and/or conversions, to controlling the d.m. value of the retentate UFR. 15 To this purpose, as we said, the nanofiltration permeate NFP is used to gradually dilute the stream that is subjected to ultrafiltration UF, and hence the two outlet streams, the permeate UFP and the retentate UFR, until, based on the control of some significant process parameters, the retentate UFR exiting from ultrafiltration has the desired properties for the milk as final product, indicated by 20 LFM. It is therefore a characteristic of the present invention that the nanofiltration permeate NFP is recircled directly to ultrafiltration UF, while the nanofiltration concentrate NFC is continuously eliminated as a sub-product. Recircling is allowed by at least a pipe D, which connects the nanofiltration 25 NF to the ultrafiltration UF and is provided with a recircling valve VFM, which will be described more fully later. Therefore, the stream actually subjected to ultrafiltration UF is given by the sum of the streams of milk M and the stream of the nanofiltration permeate NFP. In other words, under normal working conditions, before passing through the 30 membrane, the ultrafiltration retentate UFR is continuously taken from the ultrafiltration unit UF, re-diluted by the stream of nanofiltration permeate NFP, which makes up the final product LFM. Under normal working conditions, a stream of water W is continuously fed to WO 2009/043882 PCT/EP2008/063165 -7 nanofiltration NF, in order to respect the balance of material in its stationary state. Therefore, under normal working conditions, the stream actually subjected to nanofiltration N is given by the sum of the streams of water W and the stream of 5 ultrafiltration permeate UFP. Under normal working conditions, in the stationary state, there is no accumulation of material, and therefore the method according to the present invention provides to set the following ratios between the different flow rates: - the ratio between the flow rate of the recircled nanofiltration permeate NFP and 10 the flow rate of the ultrafiltration permeate UFP is equal to about 1; - the ratio between the flow rate of water W added to nanofiltration NF and the flow rate of the nanofiltration concentrate NFC extracted from nanofiltration is equal to about 1; - the ratio between the flow rate of milk M fed to ultrafiltration and the flow rate 15 of ultrafiltration retentate UFR removed from ultrafiltration, re-diluted by the nanofiltration permeate NFP so as to make the final product LFM, is equal to about 1. Furthermore, in order to have the desired dilution of the stream that is ultrafiltered, the flow rate of nanofiltration permeate NFP that is recircled is such 20 that its ratio with the flow rate of milk M fed to ultrafiltration UF is comprised between about 1 and 6, preferably between 3 and 5. The recircling of the nanofiltration permeate NFP is advantageous due to the desired dilution, since it is typically a stream with a high content of water with a low content of d.m., substantially consisting of milk salts. Therefore, the 25 recircling brings the necessary diluting water and does not bring lactose to the ultrafiltration UF, but only salts. Therefore, we have a continuous re-integration of the salts present in the ultrafiltration permeate UFP, although diluted, to the ultrafiltration unit UF, preventing or drastically reducing any external re integrations or additions of salts from other sources to the final product. 30 In particular, the parameters that are controlled in the method according to the present invention are the d.m. values of the following streams: - ultrafiltration permeate UFP; - nanofiltration permeate NFP; WO 2009/043882 PCT/EP2008/063165 - nanofiltration concentrate NFC. The parameters are measured, for example, by measuring the brix. In the attached drawing the blocks relating to three dry matter sensors are indicated by DMS, able to measure the content of dry matter of the various 5 streams concerned. The flow rates set for the various streams must guarantee that the control parameters described above, under normal working conditions, are maintained in the relative ranges of desired values. In particular, if the d.m. value of the nanofiltration permeate NFC diverges from its optimum value, the flow rates of 10 additional water W to nanofiltration and/or of the NFC concentrate must be suitably varied. In order to measure and regulate all the flow rates valves are provided, with a flow measurer, or flow rate measurer, associated or for example integrated; the valves are indicated schematically in the attached drawing by the reference VFM 15 for each stream. Both the flow measurer valves VFM and the dry matter sensors DMS detect in line the relative signals of quantities measured and send them to a control unit of the electronic type, indicated by CU in the attached drawing, of the remote type or disposed near the processing plant, to which they are connected, as shown by 20 dashes in the attached drawing. The unit CU processes the signals received and, according to one or more computer programs loaded into its memory, or interfaced therewith, regulates and commands the opening and/or closing, or modulation, of the valves VFM and, in general, consequently commands and controls the whole course of the milk 25 processing treatment which, under normal working conditions, is thus completely automated. We shall now describe the short start-up step that leads to the establishment of dynamic balance in the method according to the invention, until the stationary state is reached under normal working conditions, where the ultrafiltration 30 retentate UFR is in the desired condition, particularly with a low d.m. value and therefore with a low content of lactose and/or other sugars. For example, we shall now illustrate the case where the flow rate of milk M is equal to 100 liters per hour, and the flow rate of ultrafiltration permeate UFP is WO 2009/043882 PCT/EP2008/063165 -9 such that its ratio with the flow rate of milk M fed is equal to 4. Consequently, the flow rate of permeate UFP and the flow rate of permeate NFP are equal to 400 liters per hour each. Initially the permeate UFP has a d.m. value comprised between about 5.6% 5 and 5.8%, substantially equal to that of the soluble dry matter of the milk M. Furthermore, at the beginning, the concentrate NFC has a d.m. value comprised between about 5.6% and 5.8%, whereas the relative permeate NFP has a d.m. value comprised between about 0.3% and 0.7%. The permeate NFP is recircled to ultrafiltration UF in order to dilute the 10 stream that is actually ultra-filtered, until the permeate UFP has a d.m. value comprised in a range of desired values between about 0.3% and 3.7%, advantageously between about 1.5% and 2%. Consequently, under normal working conditions in stationary state, the ultrafiltration retentate UFR also has a soluble d.m. value much lower than the 15 initial starting value, comprised in a range of desired values between about 0.3% and 3.7%, advantageously between about 1.5% and 2%. The total d.m. value of the diluted ultrafiltration retentate, which makes up the final product LFM, under normal working conditions is equal to said soluble d.m. value of the UFR retentate, plus the percentage of proteins and possible fats present in the original 20 milk, which are considered insoluble and in any case remain in the stream of the ultrafiltration retentate UFR. Correspondingly, the d.m. value of the concentrate NFC increases, preferably to a range of desired values between about 8% and 30%, preferably between about 9% and 12% under normal working conditions in stationary state. 25 The d.m. value of the permeate NFP, on the contrary, is a parameter which, in optimum conditions, must remain substantially constant in a range comprised between about 0.3% and 0.7% and is determined by the separating capacity of the nanofiltration NF membrane. When the d.m. value of the ultrafiltration permeate UFP settles at a 30 determinate value, comprised in the desired range between about 1.5% and 2% and indicative of a low d.m. value of the retentate UFR, this means that the stationary state has been reached. Consequently, the streams entering M and W and emerging LFM (ultrafiltration retentate UFR diluted by the nanofiltration WO 2009/043882 PCT/EP2008/063165 - 10 permeate NFP) and NFC are activated, continuously and with determinate flow rates, so as to continuously produce the milk LFM as final product and the lactose concentrate. To give an example, let us consider a soluble d.m. value of the milk M equal 5 to about 5.7% and a d.m. value of the nanofiltration permeate NFP of about 0.6%. From these example values, reported in the following Table 1, we have the following values of residual lactose of the milk LFM, as the ratio R between the stream of ultrafiltration permeate UFP (equal to the stream of nanofiltration 10 permeate NFP) and the stream of original retentate milk M varies: Table 1 R (UFP/M) Residual lactose (%) 1 2.5 2 1.7 4 1.0 It is clear from the above that the method according to the present invention allows to reduce to a desired value the content of dry matter, and hence of 15 lactose, of the stream of finished product LFM. As far as the thermodynamic process parameters are concerned, both ultrafiltration UF and nanofiltration NF are performed in the range of temperature allowed by the membrane processes used, for example by means of a suitable pre-heating of the milk M. The range of temperature is of the usual type, and 20 known in the state of the art, for example comprised between about 5oC and 50'C, or can also be at higher temperatures, for example about 60'C, 70'C, 80'C or 90'C, or lower, for example about 0 0 C, 1C, 2'C, 3C or 4C. The working pressure of the ultrafiltration and nanofiltration units UF and NF is that recommended by the membrane producers, that is, a few bar, for example 25 comprised between about 1 bar and 4 bar for ultrafiltration, and a few bar and several tens of bar, for example between about 5 bar and 40 bar for nanofiltration. According to needs, the ultrafiltration and nanofiltration operations can be provided in a single phase or in several cascade phases, with possible recircling WO 2009/043882 PCT/EP2008/063165 - 11 of the streams, or a combination of these solutions. It is clear that modifications and/or additions of parts may be made to the method to obtain milk with low sugar content as described heretofore, without departing from the field and scope of the present invention. 5 For example, it is possible to subject the milk M to enzymatic hydrolysis by means of lactase in order to reduce or eliminate the content of lactose and use the method and plant as described heretofore to reduce the quantity of simple sugars thus formed. The hydrolysis is advantageously performed before ultrafiltration UF. 10 Subsequently, the hydrolyzed milk HM is fed normally to ultrafiltration UF and the process according to the invention proceeds, in order to reduce the glucose and galactose. The enzymatic hydrolysis, according to a variant not shown, can be performed on the final product LFM, or on the retentate UFR, so as to reduce the content of 15 residual lactose. The milk can be subjected, as we said, to an operation to standardize the fat, block S, by means of centrifugal separation so as to separate the cream from the milk, in order to reduce the fat content and obtain skimmed or partly skimmed milk. This variant is advantageous since it implies a reduced deposit of fats, the 20 so-called fouling effect, on the membranes used in filtering. Advantageously, moreover, in this way a milk is obtained that has reduced calorie content both in terms of sugar and also in terms of fat. This milk, with low sugar content and low fat content, is suitable for consumers following a diet that provides a limited quantity of these nutrients. 25 Moreover, the milk can also be subjected to heat treatment to guarantee conservation. The heat treatment can be performed on the milk LFM downstream of the membrane filtering operations, as indicated by block HT. Furthermore, a heat treatment can also be carried out before the membrane filtering operations. 30 For example, the milk can be pasteurized, at a temperature comprised between about 60'C and 90'C, for example at about 72'C for 15 seconds, ultra pasteurized using the ESL method for example at a temperature of about 130'C for about 1 - 2 seconds, or subjected to U.H.T. treatment (Ultra High 12 Temperature) at a temperature of about 145'C for about 2 - 4 seconds, and homogenized at a pressure comprised between about 100 and 300 bar. In this way milk with low sugar content is obtained, and optionally also with low fat content, with the desired properties of hygiene and preservation. 5 Another variant provides to apply the teachings of the international patent application WO-A-2004/110158, in the name of the present Applicant, so as to obtain milk both with a reduced sugar content and also of the ESL type, that is, long life, but which has not been subjected to too strong heat treatments or for too long a time, such as for example the U.H.T. pasteurization treatment, which 10 could denature the nutrients originally present in the milk and worsen the original organoleptic properties. In this variant, the initial milk is subjected to skimming and centrifugal clarification in order to separate the fat and to kill about 70% - 90% of bacterial content and reduce the content of other microbes and pathogenic substances. 15 The milk then follows the main method according to the invention to reduce the sugars. Subsequently, the skimmed milk, with its reduced sugar content, as obtained by the steps of the method according to the main solution, is optionally subjected to heating to 50 0 C - 60 0 C, and then subjected to micro-filtering, in one or more 20 stages, in order to obtain milk with a low bacterial and microbe content. This milk is finally standardized to the desired fat content and subjected to low-temperature pasteurization. In this way, milk is obtained with a long shelf life. It is also clear that, although the present invention has been described with 25 reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of method to obtain milk with low sugar content, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby. Any reference to publications cited in this specification is not an admission that 30 the disclosures constitute common general knowledge in Australia.

13 The term 'comprise' and variants of the term such as 'comprises' or 'comprising' are used herein to denote the inclusion of a stated integer or stated integers but not to exclude any other integer or any other integers, unless in the context or usage an exclusive interpretation of the term is required. 5

Claims

1. Method to obtain milk with low sugar content starting from milk, comprising at least the following steps: - a first step in which the milk is subjected to ultrafiltration so as to produce, at 5 exit, an ultrafiltration permeate and an ultrafiltration retentate ; and - a second step in which the ultrafiltration permeate is subjected to nanofiltration so as to produce, at exit, a nanofiltration permeate and a nanofiltration concentrate; wherein the nanofiltration permeate is continuously recircled directly to 10 ultrafiltration so as to dilute the ultrafiltration retentate and obtain the milk from the latter; and wherein in the second step the nanofiltration concentrate is collected continuously from nanofiltration.

2. Method in accordance with claim 1, wherein the first step and the second step 15 are performed substantially concurrently.

3. Method in accordance with claim 1 or 2, wherein in the first step the milk is fed continuously to ultrafiltration.

4. Method in accordane with any one of the preceding claims, wherein in the first step the ultrafiltration retentate is collected continuously from ultrafiltration, to constitute the milk with low sugar content. 20

5. Method in accordance with any one of the preceding claims, wherein in the second step a stream of water is fed continuously to nanofiltration.

6. Method in accordance with claim 5, wherein the ratio between the flow rate of the stream of water and the flow rate of the nanofiltration concentrate is equal to about 1. 25

7. Method in accordance with any one of the preceding claims, wherein the content of residual lactose in the milk with low sugar content is comprised between about 0.1% and 3%.

8. Method in accordance with any one of the preceding claims, wherein the ratio between the flow rate of milk and the flow rate of milk with low sugar content is 30 equal to about 1. 15

9. Method in accordance with any one of the preceding claims, wherein the ratio between the flow rate of ultrafiltration permeate and the flow rate of nanofiltration permeate is equal to about 1.

10. Method in accordance with any one of the preceding claims, wherein the ratio 5 between the flow rate of nanofiltration permeate and the flow rate of milk is comprised between about 1 and 6.

11. Method in accordance with any one of the preceding claims, wherein the ratio between the flow rate of nanofilatration permeate and the flow rate of milk is comprised between about 3 and 5. 10

12. Method in accordance with any one of the preceding claims, wherein the content of dry matter of the ultrafiltration retentate is controlled by regulating the flow rates of the streams entering and exiting ultrafiltration and nanofiltration.

13. Method in accordance with any one of the preceding claims, wherein the content of dry matter of the ultrafiltration retentate is controlled according to the 15 monitoring of the content of dry matter of the ultrafiltration permeate, the content of dry matter of the nanofiltration permeate and the content of dry matter of the nanofiltration concentrate.

14. Method in accordance with claim 13, wherein the content of soluble dry matter of the ultrafiltration retentate is comprised between about 0.3% and 3.7%. 20

15. Method in accordance with any one of claim 13 or 14, wherein the content of soluble dry matter of the ultrafiltration retentate is comprised between about 1.5% and 2%.

16. Method in accordance with any one of claims 14, 15 or 16, wherein the content of dry matter of the ultrafiltration permeate (UFP) is comprised between 25 about 1.5% and 2%.

17. Method in accordance with any one of any claims 14 to 17, wherein the content of dry matter of the nanofiltration permeate is comprised between about 0.3% and 0.7%.

18. Method in accordance with any one of the preceding claims, wherein the 30 content of dry matter of the nanofiltration concentrate is comprised between about 9% and 1 2 %. 16

19. Method in accordance with any one of the preceding claims, wherein the method comprises an enzymatic hydrolysis of the milk or the milk with low sugar content.

20. Method in accordance with any one of the preceding claims, wherein the 5 method comprises an operation to reduce the content of fats in the milk.

21. Method in accordance with any one of the preceding claims, wherein the method comprises a step of heat treatment able to guarantee the preservation of the milk with low sugar content.

22. Method in accordance with any one of the preceding claims, wherein the 10 method comprises a step of centrifuge clarification of the milk.

23. Method in accordance with any one of the preceding claims, wherein the method comprises a step of micro-filtering of the milk with low sugar content.

24. Plant to obtain milk with low sugar content starting from milk, comprising at least: 15 - an ultrafiltration unit able to produce, at exit, an ultrafiltration permeate and an ultrafiltration retentate; - a nanofiltration unit able to produce, at exit, a nanofiltration permeate and a nanofiltration concentrate; - means to feed and collect liquid, able to feed continuously a stream of milk to 20 the ultrafiltration unit, a stream of ultrafiltration permeate to the nanofiltration unit and a stream of water to the nanofiltration unit, and able to collect continuously a stream of ultrafiltration retentate from the ultrafiltration unit and a stream of nanofiltration concentrate from the nanofiltration unit, wherein the plant also comprises recircling means able to continuously recircle 25 the stream of nanofiltration permeate directly to the ultrafiltration unit, in order to dilute the ultrafiltration retentate and obtain the milk from the latter, and means are provided for continuously collecting the nanofiltration concentre from nanofiltration and continuously eliminating the nanofiltration concentrate as a sub-product. 30

25. Plant in accordance with claim 24, wherein the plant comprises dry matter sensors able to detect the content of dry matter of the stream of ultrafiltration 17 permeate, the nanofiltration permeate and the nanofiltration concentrate, and generate a relative indicative signal.

26. Plant in accordance with any one of claim 24 or 25, wherein the plant comprises valve means able to selectively intercept at least part of the streams of 5 milk, water, ultrafiltration permeate, ultrafiltration retentate, nanofiltration permeate and nanofiltration concentrate.

27. Plant in accordance with claim 26, wherein the valve means is associated with means to measure the flow, able to generate a signal indicating the flow.

28. Plant in accordance with claim 27, wherein the plant comprises an electronic 10 control unit able to receive and process the signals transmitted by the dry matter sensors and by the valve means, so as to command and control automatically the functioning of the ultrafiltration unit and the nanofiltration unit.