Field of the invention The present invention is related to a method for elimination of microorganisms present in milk, comprising sequentially the steps of physical and thermal treatment of the milk, with the milk flow being kept constant at least throughout the entire thermal treatment in order to assure low lactulose levels in the final product obtained by the combination of the physical and thermal treatments.

The state of the art and the problem solved by the present invention There are two state-of-the-art methods to eliminate microorganisms from milk: physical and thermal treatment.

The thermal treatment commonly used in the dairy industry consists in the method named UHT (Ultra High Temperature). In addition, there is also the conventional sterilization method.

The final product obtained through the UHT process is commercially named UHT Milk, Long-Life Milk or Ultrapasteurized Milk, and in the conventional method, it is commercially named Sterilized Milk. The final product from both methods may be stored at room temperature.

The UHT process consists in exposing the milk to high temperatures, usually ranging from 130° C to 150° C, for a short period of time, from 2 to 4 seconds. Then the milk is rapidly cooled and filled in aseptic packages.

The conventional sterilization method consists in exposing the product, previously heated and already filled in its container, to temperatures from 110° C to 120° C for a period of 15 to 40 minutes, using a type of equipment known as autoclave. The package used in this process may be glass, plastic or can.

The methods for elimination of microorganisms by thermal treatment present disadvantages particularly in relation to the high temperatures applied, which effectively eliminate the microorganisms but also cause slight (UHT process) or substantial (conventional sterilization method) changes in the organoleptic characteristics of the in natura milk, which are unpleasant to consumers many times.

One of the effective signs that the milk was subjected to a high level of process temperature is its lactulose content (lactulose is a compound formed in the Maillard reaction). Milk flavor change is directly proportional to the lactulose content found in its composition.

In UHT products available in the Brazilian market, for example, the average lactulose content is 230 mg/kg.

Regarding physical treatments to eliminate microorganisms, centrifugation and microfiltration are commonly employed. However, both of them are unable to eliminate the entire load of microorganisms present in milk.

Centrifugation consists in removing microorganisms from milk by applying centrifugal force to it. Such force separates milk components from microorganisms due to difference in their density.

Centrifugation is performed by using a piece of equipment known as centrifuge. Inside the equipment, milk is separated in two phases. The light phase occupies the central part of the machine and has a low microorganism concentration. The heavy phase, which is a bacterial concentrate, occupies the periphery of the equipment and is destined to be discharged.

The physically treated milk, that is, the light phase with a lower microorganism concentration, is conveyed to an outlet located on the top of the centrifuge.

Microfiltration is a process to separate microorganisms from milk components through membranes that are highly specific in terms of pore size and material parameters. Thus, separation is only possible due to a difference in size between milk proteins molecules and microorganisms contained therein.

In this process, milk should be previously skimmed and subsequently microfϊltered. The liquid retained by the membrane, named "microfiltration retentate", concentrates the larger size components, that is, the microorganisms. The liquid that passes through the membranes, the "microfiltered milk", contains the milk components. It should be pointed out the microfϊltration process may only be applied to skimmed milk. Therefore it is necessary to remove the fat content before the treatment, and then it can be reincorporated into the microfiltered skimmed milk. This process requires additional steps that end up impacting the processing costs.

Both after centrifugation and microfiltration, the remaining microorganisms in milk are able to deteriorate it if the milk is stored at room temperature, becoming improper for consumption.

The Brazilian patent application No. PI 0302282-0 filed on May 22, 2003, describes a method for removal and elimination of microorganisms present in milk, combining centrifugation and the thermal treatment by UHT to obtain milk with decreased lactulose content.

This decreased lactulose content is associated with the fact of being able to use lower temperatures at the thermal treatment to eliminate microorganisms, as compared to traditional systems known in the state of the art, since a substantial amount of microorganisms have already been removed at the centrifugation step.

Lower temperatures during thermal treatment bring several benefits. In terms of final product optimization, for instance, the lower lactulose content of the treated milk brings the flavor of the final product closer to that of in natura milk. Furthermore, the treated milk is whiter due to having a lower content of components formed by the Maillard reaction during thermal treatment.

Practical tests evidenced a reduction of up to 29% in the lactulose content when compared to the UHT treatment alone. In addition, due to the lower temperature in the UHT process, the scaling formation on the product pipe walls is decreased. Therefore the intervals between cleaning operations on the processing plant can be extended, reducing equipment downtime and minimizing cleaning costs, which enhances the annual productivity.

In terms of minimizing production costs, for example, the reduction of the thermal treatment temperature allows a reduction of steam consumption. In practical tests there was a reduction of up to 15% in relation to the system using only UHT treatment.

On the other hand, the centrifuge makes a discharge at every 15 to 20 minutes to remove residues from the centrifugation process. During the discharge, the milk flow which is sent to the thermal treatment may reduce, causing an increased exposure time of the milk to the high temperature within the thermal treatment equipment. This fact will cause an increase in the milk lactulose content, changing the organoleptic characteristics of the final product obtained by combining physical and thermal treatments.

In order to assure the organoleptic characteristics of the final product during the continuous process, the method according to the present invention includes a subsequent step to the physical treatment to remove microorganisms, which consists in keeping the milk flow constant at least during the thermal treatment.

A buffer tank installed after the centrifuge is used to guarantee a constant flow during the thermal treatment, maintaining milk characteristics during the continuous operation of the plant.

The maintenance of a constant flow during the thermal treatment step is also important from the operational point of view. Sudden changes in flow rate may cause the temperature to fall in the UHT process, jeopardizing the effectiveness of the microorganism elimination system and causing stops to clean the plant.

Summary of the invention The present invention describes a method for eliminating microorganisms from milk by means of a continuous milk flow through the physical and thermal treatment steps, thereby obtaining milk with lower lactulose content and hence better organoleptic characteristics, in addition to production process gains.

The method comprises the following: a) supplying milk to a treatment plant; b) preheating the milk; c) physically treating the milk in order to remove microorganisms; d) keeping the milk flow constant after step c); e) heating the milk; f) subjecting the milk to a thermal treatment in order to eliminate microorganisms; g) homogenizing the milk; h) cooling the milk; and i) aseptically filling the treated milk.

Brief description of the drawings Figure 1 shows a diagram of the equipment from the treatment plant where the method of the present invention is carried out.

Figure 2 shows a flow diagram of the main steps from the method of the present invention.

Detailed description of the invention The method according to the present invention is intended to guarantee a reduction of microorganisms present in milk associated with a low lactulose content of the treated milk obtained by a continuous flow during physical and thermal treatments by using lower temperatures in the thermal treatment. The method comprises the following steps:

Step A consists in supplying milk to a treatment plant. A diagram of such plant is shown in Figure 1.

In Step B, the milk is preheated, preferably at a temperature between 50° C and 60° C in order to facilitate the physical removal to be performed in the subsequent step C, which consists in physically treating the milk to remove microorganisms.

Physical treatment of the milk is preferably carried out by centrifugation, which is performed in equipment known as centrifuge.

In Step E, the milk is heated, preferably up to a temperature around 80° C, and then it is sent for thermal treatment in Step F for further elimination of microorganisms.

The preferred thermal treatment of the present invention is the UHT process, whereby the milk is kept at a temperature from 138° C to 142° C for a time interval of 2 to 4 seconds.

During the thermal treatment, water steam injection is normally used to heat the milk to the desired temperature. In this case, the milk subsequently passes through an expansion chamber, Step J, in order to eliminate the water originated form the steam.

After the thermal treatment, the milk is homogenized in Step G, then it is cooled, preferably to room temperature, in Step H, and it is finally aseptically packaged in Step I.

The use of a physical treatment prior to the thermal treatment allows the operation to be performed at lower temperatures in the thermal treatment step, thus reducing the lactulose content of the treated milk. As previously mentioned, the reduction in the milk flow rate, which can occur in the physical treatment step during the discharge of the centrifuge, may cause the milk to remain excessively longer in the thermal treatment step, normally more than 4 seconds. This fact could potentially compromise the flavor and color of the final product.

In order to avoid such flow reduction, the method from the present invention provides a Step D, whereby the milk flow is kept constant after the physical treatment step.

In the preferred system from the present invention, a buffer tank is installed downstream from the centrifuge in the treatment plant. The buffer tank is kept filled to supply milk during the discharge of the centrifuge in case there is a flow reduction. The milk stored in the tank is sufficient to guarantee a constant flow to the subsequent process steps.

The physical and thermal treatment steps occur preferably in a single, hermetic and continuous processing.

In the preferred system from the present invention, the equipment from the treatment plant items are interlinked through a programmable logic control (PLC) units.

The test results relative to the milk lactulose content achieved by implementation of the method according to the present invention are shown in the following table. It is observed a reduction of up to 29% of lactulose content in relation to the treatment method that uses only the UHT process:

Although the present invention has been described based on what is considered more practical and preferred, it should be understood that the invention is not limited to the systems described herein, and it intends to embody any modifications and dispositions introduced hereinto.