Process for preserving low-lactose or lactose-free milk product

Abstract

The invention relates to a process for the production of well-preserving lactose-free and low-lactose milk products. The process of the invention is characterized by separating the sugars and proteins in a raw material, thermally treating them in such a manner that the plasmin enzyme system and other proteolytic enzymes are inactivated, and combining the fractions and other preparation agents into a drink with a required composition and properties.

Claims

1. A process for the preparation of a well-preserving low-lactose, lactose-free or carbohydrate-free milk product, comprising the steps of: separating sugars and proteins in a milk raw material or milk and whey raw material mixture into a sugar fraction and a protein fraction, respectively, thermally treating at least the protein fraction at 60 to 70 C. for 2 to 10 hours to inactivate the natural plasmin enzyme systems and other harmful enzymes therein either during or after the separation into the sugar and protein fractions, heat treating the thermally treated protein fraction and the sugar fraction, and composing the well-preserving low-lactose, lactose-free or carbohydrate-free milk product with a desired composition and sweetness from the resultant heat-treated protein fraction.

2. The process as claimed in claim 1, wherein the separation in neutral conditions is performed chromatographically or using membrane techniques or by crystallization or by combining these techniques in one or more steps.

3. The process as claimed in claim 1, wherein the thermal treatment is performed during the separation.

4. The process as claimed in claim 1, wherein the separation is performed chromatographically at a temperature of 65 C. during 2 to 4 hours.

5. The process as claimed in claim 1, wherein the heat treatment is performed by pasteurizing, high pasteurizing, using ESL treatment or UHT treatment, preferably UHT treatment.

6. The process as claimed in claim 1, wherein the heat treatment is accomplished after or prior to combining the protein and sugar fractions into the milk product.

7. The process as claimed in claim 1, wherein the process is component preparation in which milk and whey components having different fat, protein and lactose contents are combined prior to aseptic packaging.

8. The process as claimed in claim 1, wherein the preparation process is either continuous or a batch process.

9. The process as claimed in claim 1, wherein a carbohydrate-free milk product is prepared.

10. The process as claimed in claim 1, wherein the separation in neutral conditions is performed chromatographically and with membrane techniques.

11. The process as claimed in claim 1, wherein the separation is performed during 2 to 6 hours.

12. The process as claimed in claim 1, wherein a carbohydrate-free, UHT or ESL milk product is prepared.

13. The process of claim 1, wherein the step of composing the well-preserving low-lactose, lactose-free or carbohydrate-free milk product further includes with composing the well-preserving low-lactose, lactose-free or carbohydrate-free milk product with the sugar fraction.

14. The process of claim 13, wherein lactose in the sugar fraction is hydrolyzed.

15. The process of claim 14, wherein the lactose is hydrolyzed after or prior to combining the protein and sugar fractions.

Description

BRIEF DESCRIPTION OF FIGURES

(1) FIG. 1: Plasmin enzyme system (modified Datta and Deeth, 2001).

(2) FIG. 2a: SDS-PAGE analysis on UHT milk samples after processing.

(3) FIG. 2b: SDS-PAGE analysis on UHT milk samples after 4-week storage.

(4) FIG. 2c: SDS-PAGE analysis on UHT milk samples after 12-week storage.

(5) FIG. 3: Development of furosine content in UHT milk as a function of the storage time.

(6) FIG. 4: Colour change in UHT milks as a function of the storage time when storing at 22 C.

(7) FIG. 5: Furosine content of UHT milks.

DETAILED DESCRIPTION OF THE INVENTION

(8) The present invention relates to a process for the preparation of a well-preserving low-lactose, lactose-free or carbohydrate-free milk product, comprising the steps of: separating the sugars and proteins of a milk raw material, or milk and whey raw material mixture into separate fractions, thermally treating at least the protein fraction to inactivate the natural plasmin enzyme system and other harmful enzymes therein, if desired, hydrolyzing lactose in the sugar fraction, heat treating said protein and sugar fractions, combining one or more of the fractions and other optional preparation agents into a milk product with the desired composition and sweetness.

(9) In the process of the invention, the separation of the raw material into sugar and protein fractions is performed chromatographically, using membrane techniques, or by crystallizing in a manner known per se. Different separation methods may also be combined as desired in one or more steps. The sugar and protein of the raw material are preferably separated chromatographically and using membrane processes.

(10) The sugar and protein fractions obtained in the raw material separation are treated thermally to inactivate their plasmin enzyme systems and other proteolytic enzymes. The thermal treatment can take place either during the separation or separately for each fraction after the separation. Sugar and protein separation is typically performed at a temperature of 3 to 80 C., preferably 60 to 70 C. and most preferably 65 C. The separation typically takes from 10 minutes to 10 hours, preferably 2 to 10 hours, more preferably 2 to 6 hours.

(11) Chromatographic separation in neutral (pH 6 to 7) conditions typically takes from 10 minutes to 10 hours, preferably 2 to 4 hours, and it is performed at a temperature of 3 to 70 C., preferably 60 to 70 C., especially 65 C.

(12) In an embodiment of the invention, when the sugar and protein separation is performed at lower temperature ranges, longer separation times are used, whereas when operating at higher separation temperatures, lower separation times are preferred. The separation can be accomplished for instance at 65 C. in 1 to 3 hours, at 70 C. in 0.5 to 1 hour, at 75 C. in 10 minutes to 0.5 hour, or at 80 C. in less than 10 minutes.

(13) The plasmin enzyme system of milk is described in general in FIG. 1. In raw milk, plasmin is in an enzymatically active form (thermally stable plasmin) and as an inactive precursor (thermally stable plasminogen). In raw milk, their weight ratio is 1:9 to 10. An activator (thermally stable plasminogen activator) and inhibitors (thermally labile plasminogen activator inhibitor and plasmin inhibitor) control the activity of plasmin. Plasmin associates with casein micelles. Thermal treatment of milk alters the natural ratio between the activators and inhibitors in favour of the activators. Heat treatment, such as UHT, promotes the transformation of plasminogen into plasmin, which causes proteolysis and precipitation of UHT milk especially during storage (Datta and Deeth, 2001). Decimal reduction time D that represents the inactivation of the plasmin enzyme is 55.6 min at 67.5 C. The half-life of plasmin is 35.3 min at 70 C., that of plasminogen is 33.3 min and that of the plasmin inhibitor is 34.5 min. [Driessen F. M., Inactivation of lipases & proteinases (indigenous & bacterial), Bulletin of IDF 238, 1989, 71-93] In the process of the invention, chromatographic separation removes the problematic proteolytic enzyme activities that cause, among other things, taste and structural problems in UHT milk products when stored at room temperature.

(14) An example of a particularly preferable embodiment of the invention that inhibits the plasmin enzyme system permanently is chromatographic separation in which inactivation occurs simultaneously with separation, when the residence time in the column at 65 C. is 2 to 3 hours. The inactivation is irreversible.

(15) The obtained sugar and protein fractions are combined into a product with a required composition and sweetness. Also, other preparation components can be introduced into the product. In the process of the invention, some of the milk salts may be lost, in which case these may be returned in this combination step as an RO concentrate (concentrate obtained by reverse osmosis) or powder, for instance. Part of the sugar fraction, or all of it, obtained from lactose hydrolysis may then also be added to the protein fraction. Other sweeteners, such as non-reducing sugar or an artificial sweetener, and vitamins may also be added to the milk product.

(16) If necessary, the lactose contained in the milk product combined as mentioned above may be hydrolyzed in a manner known per se. In an embodiment of the invention, the lactose is hydrolyzed prior to combining the protein and sugar fractions into a milk product.

(17) The process of the invention uses a technique known per se in the heat treatment of milk products, and in aseptic packaging. Examples of heat treatments to be used in the process of the invention are pasteurization, high pasteurization, or heating at a temperature lower than the pasteurization temperature for a sufficiently long time. Worth mentioning in particular are UHT treatment (e.g. milk at 138 C., 2 to 4 s), ESL treatment (e.g. milk at 130 C., 1 to 2 s), pasteurization (e.g. milk at 72 C., 15 s), or high pasteurization (95 C., 5 min). The heat treatment may be either direct (vapour to milk, milk to vapour) or indirect (tube heat exchanger, plate heat exchanger, scraped-surface heat exchanger). UHT treatment is preferably used in the process of the invention. In an embodiment of the invention, the heat treatment is accomplished after combining the protein and sugar fractions into a milk product. In another embodiment of the invention, the heat treatment is performed to sugar and protein fractions separately prior to combining them into a milk product.

(18) The milk raw material used in the process of the invention refers to milk, whey, and combinations of milk and whey. The milk raw material may also be supplemented with various fat, protein, or sugar fractions, etc. Milk is understood to comprise milk components having different fat, protein, and lactose contents. Combined milk may thus be for instance full-fat milk, low-fat milk, or skim milk, ultrafiltered milk, diafiltered milk, or recombined milk from milk powder, organic milk or a combination of these.

(19) The process of the present invention is suitable for the preparation of all types of milk products including those containing whey. A drink with the required composition, properties and sweetness is combined from various fractions and other preparation agents. The process is typically suitable for the preparation of low-lactose, lactose-free, or carbohydrate-free UHT milk. Specifically, the process is suitable for the preparation of lactose-free UHT milk. More specifically, a carbohydrate-free UHT milk product is prepared. In this application, the term lactose-free milk product refers to a milk product with a lactose content of less than 0.01%. A low-lactose milk product refers to a milk product with a lactose content of less than 1%. A carbohydrate-free milk product refers to a milk product with a carbohydrate content of less than 0.5%.

(20) The organoleptic properties of a milk product prepared according to the invention unexpectedly keep at room temperature even during a long storage. The nutritional value of a well-preserving milk product prepared with the process of the invention does not weaken even during storage. The process is easy to perform in production conditions without significant extra costs.

(21) The process of the invention is also suitable for modern component preparation in which milk components having different fat, protein, and lactose contents are combined in a known manner only just before aseptic packaging.

(22) The process of the invention may be applied to both batch and continuous production. Preferably, the process of the invention is performed as a batch process.

(23) The invention also relates to an especially well-preserving low-lactose, lactose-free or carbohydrate-free product containing milk and/or whey which is prepared by the process described above. In an embodiment of the invention, the product is prepared by combining one or more sugar and protein fractions obtained in the separation and other optional preparation components in the desired combination and ratio. An aspect of the invention provides a low-lactose milk product derived from the sugar and protein fractions so as to contain less than 1% lactose. Another aspect of the invention provides a lactose-free product derived from the sugar and protein fractions so as to contain less than 0.01% lactose. Still another aspect of the invention provides an essentially carbohydrate-free product. This is preferably formulated from the protein fraction obtained from the separation procedure.

(24) The following examples illustrate the invention but do not limit the invention to the embodiments mentioned.

EXAMPLE 1

Carbohydrate-free UHT Milk

(25) Description of Chromatographic Separation and Compositions of Fractions

(26) A column (height 9 m, diameter 3 m) was packaged with a strong cation-exchange resin (Finex Oy) with an average particle size of 0.4 mm and a structure similar to Duolite C 2404 F (Duolite International, SA, Rohm and Haas), for instance. The resin volume of the packed column tempered to a temperature of 65 C. was 30 000 L. To achieve ion balance, skim milk (pH 6.7, 25 000 L) was pumped through the column and flushed with water. Into the column, 4 000 L of skim milk concentrate (dry content 29%) was fed and eluted with demineralised water at a flow rate of 4 000 L/h. Sugar and protein fractions were collected. The fractions remained in the column at 65 C. for 3 hours, after which they were cooled to below 10 C.

(27) Table 1 shows the composition of the skim milk fractions after chromatographic separation. The separation was performed in such a manner that the protein fraction contained as little lactose as possible.

(28) TABLE-US-00001 TABLE 1 Compositions of fractions after chromatographic separation according to example 1 Skim milk concentrate Skim milk (29%) (initial Protein Lactose (initial situation) fraction fraction situation) Volume (L) 12 500 5 600 7 600 4 000 Ash (%) 0.7 1.1 0.6 2.2 Protein (%) 3.5 6.0 1.4 10.2 Lactose (%) 4.7 0.05 8.7 16.5 Dry matter 9.3 7.4 10.5 28.9 (%)
The Preparation of (Carbohydrate-free) UHT Milk of the Invention

(29) The carbohydrate-free UHT milk of the invention was prepared of a protein fraction (688 L) and water (312 L) and a non-reducing sweetener sucralose (Splenda, 0.0027%). The solution was heat treated (direct UHT 146 C., 4 s) and packaged aseptically. A carbohydrate-free UHT milk product was obtained.

(30) The shelf life of UHT milk prepared in accordance with the invention was monitored for 6 months by determining the progress of proteolysis in tyrosine equivalents and using SDS-PAGE, by determining the furosine content as a marker of the Maillard reaction and the organoleptic properties (structure as the formation of flakes, sediment and precipitation, taste and colour). A comparison was made with regular skim and lactose-hydrolyzed (hydrolysis prior to heat treatment) UHT milk (direct UHT 146 C., 4 s).

(31) Proteolysis

(32) Table 2 shows the progress of proteolysis in carbohydrate-free UHT milk prepared in the manner described in the invention and stored at 22 C. for 36 weeks in comparison with the proteolysis of normal lactose-hydrolyzed milk (=low-lactose milk).

(33) TABLE-US-00002 TABLE 2 Tyrosine equivalent content as a measure of proteolysis during storage Tyrosine equivalent content (mg/L) Normal lactose-hydrolyzed Carbohydrate-free UHT Time (weeks) UHT milk (reference) milk (invention) 0 95 51 4 298 71 8 454 88 12 609 93 36 1100 115

(34) Tyrosine content in tyrosine equivalents describes the progress of proteolysis. In the carbohydrate-free UHT milk of the invention, there is thus clearly less proteolysis than in the reference milk.

(35) FIGS. 2a to 2c show an SDS-PAGE analysis of proteolysis changes after storage of 0, 4, and 12 weeks. The carbohydrate-free milk has kept nearly as well as the original (comparison with centrifuged raw milk and/or UHT milks at 0 weeks), whereas in the lactose-hydrolyzed milk the casein bands have weakened a great deal and the gamma casein band strengthened. This refers to plasmin enzyme activity [Datta N. and Deeth H. C., Lebensm.-Wiss. Technol. 36 173-182 (2003)]. In milks prepared with the process of the invention, the activity of the plasmin enzyme system is inhibited.

(36) Maillard Browning Reactions

(37) The development of Maillard browning reactions was monitored by determining the furosine content (IDF standard 193/ISO standard 18329, 2004). FIG. 3 shows the development of furosine content describing the Maillard reaction in UHT milk prepared in accordance with the invention and in normal lactose-hydrolyzed UHT milk. The least amount of furosine and Maillard browning reactions were generated during storage in sweetened carbohydrate-free milk.

(38) Organoleptic Properties

(39) During storage, the carbohydrate-free UHT milk prepared in accordance with the invention showed no formation of flakes, sediments or precipitates. During storage, no taste or smell foreign to normal UHT milk was detected.

(40) FIG. 4 shows the change of colour in milk as a function of the storage time at 22 C. It is possible to avoid completely the change of colour in the carbohydrate-free milk prepared in accordance with the invention, whereas in a normal lactose-hydrolyzed product, Maillard browning is strong. During storage at higher temperatures, the difference between the milks increased even more.

(41) Nutritional Quality

(42) Nutritional quality was estimated from an amount of nutritionally unavailable lysine calculated on the basis of the furosine content in the manner described by Evangelisti et al. (Evangelisti, F., Calcagno, C., Nardi, S., Zunin, P., J. Dairy Res. 66: 1999:237-243). The nutritional quality of the carbohydrate-free milk prepared with the process of the invention was good.

EXAMPLE 2

Preparation of Well-preserving Low-lactose UHT Milk

(43) Lactose-hydrolyzed UHT Milk of the Invention

(44) UHT milk raw material was combined from protein fraction (62% by volume) and lactose fraction (38% by volume) described in table 3.

(45) Before combining the fractions, 0.10% lactase (Godo YNL2) was added to the lactose fraction. The hydrolysis temperature was 37 C. and duration 4 hours. The protein fraction was heat treated (UHT 146 C., 4 s) and pumped into a sterile tank after cooling to 10 C. (milk base). The lactase treated lactose fraction was run through the same UHT process and added to the above milk base.

(46) TABLE-US-00003 TABLE 3 Chromatographic separation of milk performed with milk protein collected as thoroughly as possible to protein fraction Skim milk Skim milk concentrate (initial Protein Lactose (20%) situation) fraction fraction (initial situation Volume (L) 12500 7930 5300 4000 Ash (%) 0.75 1.21 0.17 2.5 Protein (%) 3.53 5.35 0.1 10.9 Lactose (%) 4.65 0.77 11.5 17.9 Dry matter (%) 9.28 7.63 11.9 28.9

(47) The combined solution was packaged aseptically. A low-lactose UHT milk product was obtained with very little Maillard browning during preparation and with an inactivated milk plasmin enzyme system. At room temperature, the milk kept clearly better than conventionally prepared lactose-hydrolyzed milk.

EXAMPLE 3

Low-lactose UHT Milk Product

(48) Skim milk was ultrafiltered at 50 C. with concentration factor (CF) 4 to obtain an UF retentate, the composition of which is shown in table 4. The UF permeate obtained from ultrafiltration was nanofiltered at 10 C. with CF 4. The compositions of the obtained fractions are shown in table 5.

(49) TABLE-US-00004 TABLE 4 Compositions of fractions at first filtration Skim milk (initial UF NF NF situation) retentate 1 retentate 1 permeate 1 Volume (L) 1000 250 187.5 562.5 Ash (%) 0.75 1.58 1.11 0.25 Protein (%) 3.53 13.72 0.4 0 Lactose (%) 4.65 4.6 17.1 0 Dry matter (%) 9.28 19.8 19.6 0.27

(50) NF permeate 1 and UF retentate 1 were combined and ultrafiltered with CF 3.6 at 50 C. UF permeate 2 was further nanofiltered at 10 C. with CF 12. The compositions of the fractions are shown in table 5.

(51) TABLE-US-00005 TABLE 5 Composition of fractions at second filtration Feed UF retentate 2 NF retentate 2 NF permeate 2 Volume (L) 812.5 225.7 48.9 537.9 Ash (%) 0.66 1.30 0.86 0.17 Protein (%) 4.22 14.65 0 0 Lactose (%) 1.48 1.64 16.8 0 Dry matter 6.28 18.5 18.9 0.19 (%)

(52) UF retentate 2 and NF permeate 2 obtained at the second filtration were combined (763 liters, protein 4.33%, lactose 0.52%, ash 0.50%, and dry matter 5.6%) and heat treated (direct UHT 146 C., 4 s). The milk base was cooled to 65 C. and transferred to a sterile tank for 2 hours. After this, the milk base was cooled to 5 C.

(53) An amount of 107 liters of water was added to 129 liters of NF retentate 1 from the first filtration. Lactase (Godo YNL2) 0.10% was added to the mixture. The hydrolysis temperature was 37 C. and duration 4 hours. The solution was heat treated (direct UHT 146 C., 4 s) and mixed in a sterile tank with a milk base, the preparation of which is described in the previous paragraph.

(54) This produced a low-lactose UHT milk drink that contained 3.45% protein, 2.6% carbohydrates (lactose <0.5%), ash 0.52%, and dry matter 7.4%.

(55) The milk drink prepared with the process of the invention was stored at room temperature (22 C.) for 12 weeks. The progress of the Maillard reaction that causes Maillard browning in a milk drink was monitored by analyzing the furosine content of the milk at 4-week intervals (FIG. 5). A comparison was made with normal skim and lactose-hydrolyzed (hydrolysis prior to heat treatment) UHT milk (direct UHT 142 C., 4 s). Proteolysis was similar to that of the carbohydrate-free milk shown in table 2.

(56) The results of FIG. 5 show that the UHT milk drink prepared with the process of the invention during heat treatment shows much less Maillard browning than normal lactose-hydrolyzed comparison milk. Therefore, the Maillard reaction product content during storage was also much lower. In addition, the taste of the UHT milk prepared in accordance with the invention was after 12 weeks found to be clearly better than that of the normal lactose-hydrolyzed milk. No off-taste or precipitation was detected.

EXAMPLE 4

Low-lactose UHT Milk Product

(57) Skim milk was processed with a membrane technique in two steps as in example 3. Unlike example 3, 188 liters of NF retentate 1 was taken from the first filtration and 49 liters of NF retentate 2 was taken from the second filtration. Differing from example 3, water was not added to the mixture. Lactase (Godo YNL2) 0.10% was added to the mixture. The hydrolysis temperature was 37 C. and duration 4 hours. The solution was heat treated (direct UHT 146 C., 4 s) and mixed in a sterile tank with milk base prepared in the manner described in example 3.

(58) This way, a low-lactose UHT milk drink was produced containing protein 3.5%, carbohydrates 4.7% (lactose <0.5%), ash 0.7%, and dry matter 9.3%.

(59) A UHT milk drink shows less Maillard browning during heat treatment than normal lactose-hydrolyzed comparison milk (FIG. 5). Therefore, the Maillard reaction product content is also clearly lower during storage. In addition, after 12 weeks, UHT milk prepared in accordance with the invention tasted clearly better than the normal lactose-hydrolyzed comparison milk. No off-taste or precipitation was detected.

EXAMPLE 5

Low-lactose UHT Milk Product with Whey

(60) Skim milk (900 L) and whey (100 L) were ultrafiltered and nanofiltered as described in example 3. The composition of the obtained fractions is shown in table 6.

(61) TABLE-US-00006 TABLE 6 Compositions of fractions at first filtration Skim milk and whey (initial UF NF NF situation) retentate 1 retentate 1 permeate 1 Volume (L) 1000 250 187.5 562.5 Ash (%) 0.75 1.55 1.09 0.22 Protein (%) 3.19 12.3 0.35 0 Lactose (%) 4.61 4.7 17.1 0 Dry matter (%) 8.46 19.1 19.4 0.30

(62) NF permeate 1 and UF retentate 1 were combined and ultrafiltered and UF permeate 2 was further nanofiltered as in example 3. The compositions of the fractions are shown in table 7.

(63) TABLE-US-00007 TABLE 7 Compositions of fractions at second filtration Feed UF retentate 2 NF retentate 2 NF permeate 2 Volume (L) 812.5 225.7 48.9 537.9 Ash (%) 0.66 1.32 0.79 0.16 Protein (%) 3.80 13.7 0 0 Lactose (%) 1.50 1.64 16.2 0 Dry matter 6.10 17.1 18.1 0.19 (%)

(64) UF retentate 2 and NF permeate 2 obtained at second filtration were combined (763 L, protein 4.05%, lactose 0.52%, ash 0.50%, and dry matter 5.2%) and heat treated, and the obtained whey milk base was cooled, processed in a sterile tank and cooled as described in example 3.

(65) An amount of 129 liters of NF retentate 1 obtained at first filtration and 107 liters of water were mixed. Lactase was added to the mixture that was then hydrolyzed, the solution was heat treated and mixed in a sterile tank with the whey milk base as described in example 3.

(66) This way, a low-lactose UHT whey milk drink was obtained containing protein 3.14%, carbohydrates 2.6% (lactose <0.5%), ash 0.48%, and dry matter 6.9%.

(67) The UHT whey milk drink shows less Maillard browning during heat treatment than normal lactose-hydrolyzed milk. Therefore, the Maillard reaction product content during storage was also clearly lower. No off-taste or precipitation was detected in the UHT whey milk after 12-week storage.

EXAMPLE 6

The Inactivation of Plasmin and Plasminogen

(68) The effect of a short-term 3- to 70-minute thermal treatment on the plasmin enzyme system was examined by heating pasteurized skim milk (25 ml) at a temperature of 65 to 95 C. The treatment time was 3, 5, 7, 9, 20, 50, and 70 minutes and the corresponding temperatures were 95, 90, 85, 80, 75, 70, and 65 C. After thermal treatment, the skim milk was immediately cooled to below 5 C., and the relative plasmin and plasminogen activities were determined.

(69) Determining the activity of plasmin and plasminogen: 6 ml of pasteurized skim milk was centrifuged (25 C., 1 h, ca. 60 000 g), and the casein at the bottom of the test tube was dissolved into 3 ml of 0.05 Tris-HCl 0.1 M EDTA buffer (pH 7.4). Into the cuvette (1 ml) was added 600 l of buffer (0.05 Tris-HCl 0.1 M EDTA buffer, pH 7.4), 300 l of 2 mM D-Val-Leu-Lys-PNA.2HCl (peptide dissolved in 5% dimethylformamide), and 100 l of the above-mentioned dissolved casein. Samples were incubated at 30 C. Absorbance (405 nm) was measured at time instant 0 and after 3 hours against the buffer.

(70) The plasminogen activity was determined by adding into the cuvette 5 l of urokinase (100 PU) and by measuring absorbance (405 nm) after incubating for 3 hours. The total incubation period was thus 6 hours.

(71) Relative plasmin activity was calculated by comparing the absorbance changes of the samples during the 3-hour incubation with the comparison sample, i.e. normal pasteurized milk. Relative plasminogen activity was calculated by subtracting from the absorbance of the latter 3-hour incubation the absorbance after the addition of urokinase and then the absorbance change (plasmin activity) of the first 3 hours of the corresponding sample and the 0.0059 absorbance change caused by urokinase.

(72) Results: Short-term 20-minute thermal treatment at 75 C. and 9-minute thermal treatment at 80 C. effectively inactivated the plasmin and plasminogen activities of the plasmin system (table 8).

(73) TABLE-US-00008 TABLE 8 Relative plasmin and plasminogen activities of thermally treated pasteurized skim milk Temperature Time Plasmin activity Plasminogen activity C. Min % % Comparison 100 100 (no treatment) 65 70 100 82 70 50 77 67 75 20 4 2 80 9 <1 <1 85 7 <1 <1 90 5 <1 <1 95 3 <1 1

EXAMPLE 7

UHT Milk Product

(74) Skim milk was treated with a membrane technique in two steps as in example 3. Differing from example 3, the UF retentate 2 and NF permeate 2 obtained during the second filtration were combined to form the milk base that was divided into two parts. One part was heated to 80 C. and the milk base was kept at this temperature for 10 minutes, and the second part was correspondingly heated to 75 C. and the milk base was kept at this temperature for 20 minutes. Immediately after this, the milk bases were heat treated (direct UHT 146 C., 4 s), and cooled to 5 C. A low-lactose UHT milk product was prepared from these milk bases as described in example 3.

(75) The Maillard browning of the UHT milk products was at the same level as in the UHT milk drink of example 3. The milk drinks prepared with the process of the invention were stored at room temperature (22 C.) for 12 weeks. Normal fat-free lactose-hydrolyzed UHT milk was used for comparison. During the storage time, contrary to the comparison milk, the milk drinks prepared with the process of the invention did not develop a bitter off-taste indicating proteolytic activity.