METHOD OF PRODUCING LOW-LACTOSE AND LACTOSE-FREE SOUR MILK PRODUCTS

FIELD OF THE INVENTION

[0001] The invention relates to a method of producing low-lactose and lactose-free sour milk products, such as yogurt, fermented milk, viili (a fermented milk product), fermented cream, sour cream, quark, cottage cheese, and feta cheese.

BACKGROUND OF THE INVENTION

[0002] The production of low-lactose milk products is generally known. Several methods exist for removing lactose from milk. A method generally known in the field is the conventional enzymatic process of lactose degradation that comprises a step of adding mould- or yeast-based lactase into milk in such a manner that more than 80% of lactose is degraded into monosaccharides, i.e. glucose and galactose. The problem is then the too sweet a taste of milk, which is caused by the monosaccharides.

[0003] Methods for removing lactose from milk raw material by membrane techniques, in particular, are also known. Four basic types of membrane filtering methods are generally used: reverse osmosis (RO), nanofiltra- tion (NF), ultrafiltration (UF), and microfiltration (MF). Of these, UF is mainly suitable for separating lactose from milk. Reverse osmosis is generally applied to concentration, ultra- and microfiltration to fractionation, and nanofiltration to both concentration and fractionation. A lactose removal method based on a membrane technique is disclosed in WO publication 00/45643, for instance. The problem in the method, as in membrane techniques in general, is that during ultrafiltration not only lactose but also some of the salts are removed from the milk, the salts being significant to the taste of the milk and milk products produced from it.

[0004] Fl publication 115,752 discloses a method, in which a milk product is ultrafiltered, nanofiltered, and concentrated through reverse osmosis, after which the salts removed during ultrafiltration are returned to the UF retentate. The thus obtained residual lactose of the low-lactose milk product is hydrolyzed with a lactase enzyme into monosaccharides, thus producing a substantially lactose-free milk product. With this method, lactose can be removed from milk without affecting the organoleptic properties of the milk product to be prepared.

[0005] Lactose can also be separated from milk specifically by chromatography. For instance, EP publication 226,035 B1 describes a lactose separation method, in which milk is fractionated in such a manner that the lactose fraction is separated and the salts are in the protein fraction or protein-fat fraction. The advantage of the method is that all agents essential for the taste remain in the milk. However, chromatographic lactose separation is a slow and complex process, and is not directly suitable for conventional dairies without expensive equipment investments.

[0006] Further, it is known to use milk after lactose removal as raw material in the production of dairy products. Recent studies have concentrated on membrane filtration of milk and on the use of such filtered milk in the production of dairy products, such as cheese, ice cream and yogurt. Fl publication 115,752 discloses a method in which lactose-free milk is used to prepare lactose-free cream by adjusting the fat content of the product to be as desired prior to lactose hydrolysis. Lactose-free milk can correspondingly be made into fermented milk by adding fermented milk souring agent into the milk and letting the milk sour. With small changes in the production parameters, the method has been found to be suitable for the production of nearly all possible lactose- free milk products by means of conventional production technology. A common feature with the method for the production of lactose-free products described in the publication is that prior to ultrafiltration, milk is standardized to a desired fat content and pasteurized by heating it to a temperature of 60 to 90 ⁰ C.

[0007] PCT application WO 2004/019693, Dunker et al., describes a method for milk separation into individual components with membrane techniques (ultrafiltration, nanofiltration, and reverse osmosis) and the recombination of the components into milk products, such as ice cream, set-type or mixed yogurts and milk drinks. A low-lactose example product, such as ice cream or yogurt, consists of fat (2%), protein (4.5%), minerals (0.8%), and lactose (2%). The product is composed of milk fat (5.01 percentage by volume), UF retentate (32.1 percentage by volume), RO retentate (15.7 percentage by volume), and water (47.19 percentage by volume). Yogurt can be a set-type, mixed, fruit, or drinkable yogurt. The composed milk raw material can be skimmed milk, low-fat milk, whole milk, lactose-free milk, milk concentrate, milk powder, organic milk, or a combination thereof.

[0008] PCT application WO 00/45643, Lange M., describes applying ultrafiltration and diafiltration for removing lactose from milk, after which the

retentate is hydrolyzed enzymatically. The obtained lactose-free milk can be used to make other dairy products, such as cream, yogurt, ice cream, butter, and cheese. A problem with the described method is the high solution volume of the permeate created during diafiltration, the processing of which causes high costs when using the method. PCT application WO 2005/074693, Lange M., also describes the use of a membrane technique in the production of lactose-free milk. The method typically dilutes the ultrafiltration retentate with water, thus producing a lactose content of approximately 3.0%. The residual lactose is hydrolyzed enzymatically. A lactose content of less than 0.02% is proposed as the definition of lactose-free milk. Milk produced according to the method is suitable for use in the production of milk products, such as yogurt and cheese.

[0009] A problem with known enzymatic lactose removal methods in general is the changing of the organoleptic properties of milk or milk products produced from it. When using filtration methods, the creation of side fractions, such as permeates, or lactose fractions is often a problem.

[0010] It is typical of the production of sour milk products that a souring agent is added to strongly heat-treated milk, and when souring the milk, the souring agent provides it with the typical structure and taste properties of the product. The typical creation of the thick structure of yogurts is also affected by the increase in milk dry solid matter and lactose content during production. The souring process is usually quite long and is shortest in yogurts, approximately 2 to 7 hours, and longest in sour creams, approximately 24 hours. When making lactose-hydrolyzed products, the aim is to utilise this time efficiently so the lactase enzyme is often added together with the souring agent. The approximately 20 to 45°C temperature used in souring is then also advantageous for the lactase enzyme that is able to work nearly at its optimum temperature.

[0011] When making completely lactose-free products, in which the requirement for residual lactose content is less than 0.01 %, a larger quantity of the lactase enzyme needs to be added than in low-lactose products. Unexpected problems are, however, encountered during lactase hydrolysis. The taste of the sour milk product is not pure, but often has a musty-flat, chemical- or medicine-type flavour defect especially towards the end of the sales period, the flavour defect being partly due to side activities of commercial enzyme preparations. In yogurts, the sales period is typically approximately 3 weeks.

The sales period of other sour milk products is 2 to 5 weeks. For instance, the protease activity of GODO-YNL yeast lactase (Godo Shusei Company, Japan) is 7 PU/g and its lactase activity 53,000 U/g. Protease activity may cause flavour defects in a milk product due to the production of bitter-tasting peptides, for instance.

[0012] The stores and consumers require longer and longer sales periods for sour milk products. It is thus desirable to provide natural methods, with which the preservability of the taste properties of sour milk products can be improved and consequently the sales period of the products lengthened.

[0013] Microbiological problems often also occur in the production of sour milk products, when lactase hydrolysis is done using conventional prior art. Enzymes can have foreign microbes that can grow during souring.

[0014] A method has now been unexpectedly invented for making low-lactose or lactose-free sour milk products being free from flavour defects without any specific additional costs. The method of the invention also provides a more reliable decrease in lactose content and minimizes risks related to microbiological problems.

BRIEF DESCRIPTION OF THE INVENTION

[0015] The invention provides a new method for making low-lactose and lactose-free sour milk products, which is characterised by what is stated in the independent claim. Preferred embodiments of the invention are disclosed in the dependent claims. The method of the invention avoids flavour defects caused by excess use of the lactase enzyme and by side activities typical of commercial enzyme preparations. With the method of the invention, the preservation of the taste properties of low-lactose and lactose-free sour milk products can be improved, which results in longer sales periods for products.

[0016] With the method of the invention, it is also possible to avoid hygiene risks in the production of sour milk products, which are related to commercial enzyme preparations.

[0017] In addition, the invention provides a method that is simple, economical, industrially feasible in a grand scale, and does not cause any extra costs. It was unexpectedly found that the dosage of the costly enzyme can be reduced in comparison with that used in the current hydrolysis methods, when the hydrolysis is done in two steps under mainly neutral conditions in accordance with the invention. In the prior art, hydrolysis is done during sour-

ing, whereby pH decreases to a range disadvantageous for the lactase enzyme.

[0018] The analysis of residual lactose in sour milk products produced with conventional prior art and the implementation of the analytics under production conditions are problematic and expensive. However, the analysis of the residual lactose of a product made according to the invention under production conditions is effortless and inexpensive, because the analysis can be made of a neutral product. The analysis of a neutral product is possible, because according to the invention, an essential part of the lactose already degrades at the pre-hydrolysis stage. An analysis at this stage thus provides a reliable preliminary result on the actual final residual lactose content.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention relates to a method of producing a low-lactose or lactose-free sour milk product, and the method is characterised by: a) performing lactase pre-hydrolysis on milk raw material, b) inactivating the lactase enzyme and side activities by heat treating the pre-hydrolyzed milk raw material obtained in step a) prior to souring the product, and c) hydrolyzing the residual lactose simultaneously with the souring of the product.

[0020] The method of the present invention also minimizes the microbiological problems in the production of sour milk products made using the prior art by performing high-pasteurization or pasteurization or by heating, before souring, at a lower temperature for a sufficient time to inactivate the enzyme preparation and side activities.

[0021] The method of the present invention is suitable for producing sour milk products of all types, typically yogurt, fermented milk, viili, fermented cream, sour milk, and quark, cottage cheese and feta cheese.

[0022] It was also unexpectedly found that it is easy and inexpensive to analyze the residual lactose of a product produced according to the invention under production conditions and from a neutral product.

[0023] A sour milk product produced with the method of the invention is lactose-free or low-lactose, preferably lactose-free. In this application, a lactose-free sour milk product refers to a sour milk product with a lactose con-

tent of less than 0.01%. A low-lactose sour milk product refers to a sour milk product with a lactose content of less than 1%.

[0024] The milk raw material used in the method of the invention refers to milk or a lactose-containing fraction of milk. Herein, milk is intended to comprise milk components having various fat, protein, and lactose contents. Milk can thus be whole milk or skimmed milk, ultrafiltered milk, diafiltered milk, or milk reconstituted from milk powder, for instance.

[0025] According to the method of the invention, in step a), the milk raw material is pre-hydrolyzed with lactase [e.g. 9,700 units of Maxilact L2000 lactase/l of milk (from DSM, Holland), approximately 51 to 99%, preferably 90 to 99% of the entire used enzyme amount] before heat treatment and addition of a souring agent (as in the preparation of yogurt) or a souring agent and rennet (as in the preparation of quark or cottage cheese). The lactase unit illustrates the activity that under specific conditions (37°C, skimmed milk, 10 min) hydrolyzes 1 μmol lactose/min. The unit of enzyme activity is thus μmol/min g.

[0026] Several different lactase enzymes (β-D-galactosedases) suitable for use in the method of the invention are available commercially. These include those produced with Kluyveromyces fagilis, such as HA-lactase (Chr. Hansen A/S, Denmark), or those produced with Kluyveromyces lactis, such as Validase (Valley Research Inc., USA), Maxilact L2000 lactase (DSM, Holland), and Godo YNL (Godo Shusei Company, Japan). An example of mould-bases lactase preparations is GLL cone, lactase (Biocon Japan Ltd, Japan) produced with Aspergillus oryzae. Optimal hydrolysis conditions depend on each enzyme and can be obtained from the producers of commercial enzymes.

[0027] Pre-hydrolysis under neutral (pH 6 to 7) conditions takes 1 to 36 hours, preferably 2 to 24 hours, and is performed at a temperature of 3 to 70 ⁰ C, preferably 6 to 45°C.

[0028] After the pre-hydrolysis, the lactase is inactivated in step b) with heat treatment, for example. Heat treatment is performed in a manner known in the art. Examples of heat treatments used in the method of the invention are pasteurization, high-pasteurization, or heating at a temperature lower than that used for pasteurization for a sufficiently long time. Especially the following are worth mentioning: UHT treatment (e.g. milk 138°C, 2 to 4 s), ESL treatment (e.g. milk 130 ⁰ C, 1 to 2 s), pasteurization (e.g. milk 72°C, 15 s), or high-pasteurization (95°C, 5 min). An example of heat treatment of the inven-

tion is high-pasteurization in the production of yogurt (93°C, 5 min, example 2). On the other hand, lactase and the side activities of a lactase preparation are also inactivated sufficiently in the production of cottage cheese according to example 14 already at a temperature of 51 to 62°C, when the heating time of a cheese granule is sufficiently long, 1.5 to 3 hours. With the method of the invention, the inactivation of lactase performed after pre-hydrolysis removes the side activities found problematic in commercial products and causing taste problems, among other things, in the production of sour milk products and the final product.

[0029] According to step c) of the method of the invention, the residual lactose or lactose resulting from milk component standardization is hy- drolyzed by adding a small amount of lactase [e.g. 300 units of Maxilact L2000 lactase/l (DSM, Holland), approximately 1 to 49%, preferably 1 to 10% of the entire used enzyme amount] simultaneously with the souring agent. In this application, the residual lactose refers to the remaining lactose after the pre- hydrolysis of step a) of the method of the invention and the lactose resulting from milk component standardization, such as the addition of cream after pasteurization, in the milk product. Milk component standardization is a concept known to a person skilled in the art.

[0030] The sour milk products produced with the method of the invention are made using a technique generally used with these sour milk products with respect to the souring agents, rennets, souring conditions, such as temperature, time, and mixing, and heat treatments.

[0031] The souring is done by adding a biological souring agent [e.g. bulk starter or DVS (direct-to-vat) starter], chemical souring agent, or organic or inorganic acids with or without the addition of rennet. The souring can be done alternatively in a tank before the product is packaged, or immediately after packaging in a consumer or food service package.

[0032] The method of the invention is also suitable for modern component production, in which milk components having various fat, protein and lactose contents are combined in a known manner only just before packaging.

[0033] The method of the invention can also be applied to both batch and continuous production. The method of the invention is preferably done in a batch process.

[0034] The following examples illustrate the invention, but do not limit it to the described embodiments.

Comparison example 1

[0035] To produce yogurt, 100 I of milk containing 1.5% fat was used. The milk was heated to 45°C, 2 kg of fat-free milk powder was added, and the mixture mixed until the powder was completely dissolved. The milk was heat treated at 93°C for 5 minutes, after which it was cooled to 41 ⁰ C for souring. To the milk, 9,000 units/I of milk of Maxilact L2000 enzyme (DSM, Holland) was added as well as 1 ,000 units/I of GLL cone, lactase (Biocon Japan Ltd, Japan), which operates at a sour pH range, to produce a lactose-free product, and 3 I of yogurt bulk starter (3% of the milk amount). The milk was let sour for 5 hours, during which its pH dropped to 4.5. After this, the product was cooled to 15°C while mixing. The product was packaged in cups, closed with lids, and transferred to a 5 ⁰ C storeroom for storage. The obtained product contained 0.01% lactose. The obtained yogurt was tasted after a week. The taste was of a slightly sweet yogurt with a clearly detectable metallic flavour defect. The side taste could not easily be covered with the addition of jam or flavour.

Example 2

[0036] To produce yogurt, 100 I of milk containing 1.5% fat was used as in example 1. The milk was heated to 40 ⁰ C, 2 kg of fat-free milk powder was added and the mixture was mixed until the powder was completely dissolved. After this, 9,700 units/I of milk of Maxilact L2000 enzyme (DSM, Holland) was added and the mixture was let hydrolyze for 2.5 hours while slowly mixing it at 40 ⁰ C (blade mixer, 10 rpm). After this, the milk was heat treated at 93°C for 5 minutes and cooled to 41 ⁰ C. To the milk, 3 I of yogurt bulk starter (3% of the milk amount) and 300 units/I of Maxilact L2000 lactase were added. The milk was mixed and let sour at 41 ⁰ C for 4.5 hours. During this time, its pH dropped to 4.5. After this, the product was cooled to below 15°C and packaged into cups as in example 1. The product was stored at 5°C. The yogurt was tasted after a week, and the taste was of a fresh, slightly sweet yogurt. No flavour defect could be detected. The residual lactose content was <0.01 %.

[0037] Drinkable yogurt was also produced as described above, but it was mixed more strongly, whereby the viscosity of the yogurt became lower. Otherwise the properties of the product corresponded to those above and no flavour defect was detectable.

Example 3

[0038] Yogurt was produced as described in example 2, but 2 kg of fat-free milk powder was added to 10 ⁰ C milk and mixed until the powder was completely dissolved. After this, 9,700 units/I of Maxilact L2000 lactase (DSM, Holland) was added to the milk and let hydrolyze at 10 ⁰ C while slowly mixing it for 24 hours. The properties of the yogurt produced according to this example were as in the yogurt of example 2. No flavour defect was detectable. The residual lactose content was <0.01%.

[0039] Drinkable yogurt was also produced as described above, but it was mixed more strongly, whereby the viscosity of the yogurt became lower. Otherwise the properties of the product corresponded to those above and no flavour defect was detectable.

Example 4

[0040] Yogurt was produced as described in example 2, but without adding milk powder. The addition of milk powder was replaced by hydrocol- loids, such as pectin, starch, modified starch, gelatine, or guarkum. Here, 10% less lactase enzyme is required than in example 2.

Example 5

[0041] To produce fermented cream, sour cream, and fermented milk, a lactase enzyme (Godo YNL 9,700 units/I or Maxilact L2000 9,700 units/I) was added to milk with a standardized fat content (0 to 42%). Lactose was allowed to degrade for 2.5 hours at 40 ⁰ C. Pasteurization was done at 93°C for 5 minutes, after which bulk starter (3%) and a lactase enzyme (300 units/I) were added. The product was packaged at 15°C and let sour in the package for 24 hours, after which it was placed in a cold storage (<6°C) to await distribution.

[0042] Fermented cream, sour cream, and fermented milk were produced as described above, but the product was packaged at 25°C and let sour in the package for 15 hours, after which the packages were transferred to a cold storage (<6°C) to await distribution.

[0043] Alternatively, the souring can be done in a tank at 15 ⁰ C for 24 hours or at 25°C for 12 hours, after which the product is packaged and let cool in a cold storage (<6°C).

[0044] The products did not have a detectable flavour defect. The residual lactose content was <0.01%.

Example 6

[0045] Fermented cream, sour cream, and fermented milk were produced from pasteurized milk with a standardized fat content (0 to 42%), to which a lactase enzyme was added as described in example 5, but the lactose was allowed degrade for 24 hours at 6 ⁰ C.

[0046] The products did not have a detectable flavour defect. The residual lactose content was <0.01%.

Example 7

[0047] Production as in examples 5 and 6, but in the beginning or at a later stage, a stabilization agent or a mixture of stabilization agents (e.g. starch, National Starch, dosage 0.6%) was added to the milk to ensure the preservation of the structure of the product in the pasteurization or UHT treatment before packaging. UHT treatment lengthens the shelf life of the product, and post-pasteurized lactose-free sour cream, such as fermented cream or sour cream, is known as a long life sour cream product.

[0048] The products did not have a detectable flavour defect. The residual lactose content was <0.01 %.

Example 8

[0049] Fermented cream, sour cream, and fermented milk were produced as in examples 5 and 6, but DVS starter (0.02%) was used instead of bulk starter.

[0050] The products did not have a detectable flavour defect. The residual lactose content was <0.01%.

Example 9

[0051] In the production of lactose-free yogurts, soured creams, such as fermented cream and sour cream, and fermented milks, based on modern component production, the milk components having different fat contents were only combined immediately before souring or before packaging. These milk components are also used in dairies to produce other milk products.

[0052] A lactase enzyme (Godo YNL 9,000 units/I of milk, or Maxi- lact L2000 9,000 units/I of milk) was added to pasteurized skimmed milk, and lactose was allowed degrade for 2.5 hours at 40 ⁰ C. The hydrolyzed skimmed milk was heat treated by pasteurization at 93 ⁰ C for 5 minutes, after which bulk starter (3%) or DVS starter (0.02%) was added as well as a necessary amount of pasteurized cream depending on the required fat content of the final product. If the required fat content of the final product was 12%, 30% of 38% cream was used.

[0053] The components were well mixed, after which a lactase enzyme (Godo YNL 1 ,000 units/I, or Maxilact L2000 1 ,000 units/I) was added to split the amount of lactose obtained from the cream, in this case approximately 0.9 g/l.

[0054] The product was packaged at 15 ⁰ C and let sour in the package for 24 hours, after which the packages were transferred to a cold storage (<6°C) to await distribution.

[0055] Alternatively, the product was packaged at 25°C and let sour in the package for 15 hours, after which the packages were transferred to a cold storage (<6°C) to await distribution.

[0056] The products did not have a detectable flavour defect. The residual lactose content was <0.01 %.

Example 10

[0057] Lactose-free yogurt, soured creams, such as fermented cream and sour cream, and fermented milks were produced with a technique based on modern component production as described in example 9, but the lactose was let degrade for 24 hours at 6°C.

[0058] The product was packaged at 15°C and let sour in the package for 24 hours, after which the packages were transferred to a cold storage (<6°C) to await distribution.

[0059] Alternatively, the product was packaged at 25°C and let sour in the package for 15 hours, after which the packages were transferred to a cold storage (<6°C) to await distribution.

[0060] The products did not have a detectable flavour defect. The residual lactose content was <0.01%.

Example 11

[0061] Lactose-free yogurt, soured creams, such as fermented cream and sour cream, and fermented milks were produced with a technique based on modern component production as described in examples 9 and 10, but the souring was done in a tank at 25°C for 12 hours, after which the packages were transferred to a cold storage (<6°C) to await distribution.

[0062] The products did not have a detectable flavour defect. The residual lactose content was <0.01%.

Example 12

[0063] In the production of flavoured and unflavoured quark with a fat content of >0.5%, a lactase enzyme (Godo YNL 9,000 units/I milk, or Maxi- lact L2000 9,000 units/I of milk) was added to pasteurized skimmed milk, and the mixture was pasteurized at 93°C for 5 minutes. Bulk starter and possibly a small amount of rennet were added to the milk and let sour to a pH value of 4.4 to 4.6 at 35°C for 15 hours.

[0064] The precipitate was mixed and directed to heat treatment (55 to 65°C), where the treatment time was at most 3 minutes. After the heat treatment, the precipitate was directed to a quark separator, in which whey separated from curd (quark). A static or dynamic mixer was used to mix cream and a lactase enzyme (Godo YNL 1 ,000 units/I milk, or Maxilact L2000 1 ,000 units/I of milk) with the obtained quark. The required fat content of the desired final product was adjusted with cream. If the required fat content of the final product was 6%, 15% of 38% cream was used. A possible flavouring berry/fruit supplement was then added and the product was packaged at 10 to 15°C and transferred to a cold storage to cool (<6°C) to await distribution.

[0065] The products did not have a detectable flavour defect. The residual lactose content was <0.01%.

Example 13

[0066] Flavoured and unflavoured quark (>0.5% fat) was produced as described in example 12, but the souring was done at 15°C for 25 hours.

[0067] The products did not have a detectable flavour defect. The residual lactose content was <0.01 %.

Example 14

[0068] In the production of cottage cheese, the curd of cottage cheese was produced from skimmed milk. Some of the skimmed milk can be replaced with ultrafiltered or diafiltered milk or milk reconstituted from milk powder. Kettle milk is soured with bulk starter and a small rennet addition (e.g. 31 to 33°C, 1 to 3 ml/1 ,000 I). The curd is cut when the precipitate is sufficiently hard (e.g. pH 4.6 to 4.9). The curd is heated and slowly stirred in whey (final temperature 51 to 62 ⁰ C, heating time 1.5 to 3 hours, for instance). The temperature and time is sufficient to inactivate enzymes. After this, the granulates are washed with cold water (e.g. 4 to 8°C) before drying and combining with a dressing.

[0069] In the production of lactose-free cottage cheese, a lactase enzyme (Godo YNL 2,000 units/I milk or Maxilact L2000 2,000 units/I of milk) is added to the kettle milk simultaneously with souring agents. The enzyme dosage depends on the activity of the enzyme. If ultrafiltered milk is used as raw material, the lactase enzyme treatment can also be done before ultrafiltering.

[0070] In the production of the dressing for the cottage cheese, soured cream (e.g. 5 to 10% fat) or skimmed milk is used. All supplements are added to the dressing: salt and preservative (potassium sorbate), for instance. A lactose-free cottage cheese dressing is made by adding a lactase enzyme (e.g. Godo YNL or Maxilact L2000) simultaneously with a souring agent or by letting cream/skimmed milk stand overnight in cold with a lactase enzyme (e.g. 12 to 24 hours, 4 to 8°C). The enzyme dosage depends on the addition time (e.g. 2,000 to 20,000 units/I).