BACKGROUND ART The heat treatment of food products for extended shelf life is a well-known and often employed method. The food product may, for example, be various dairy products such as milk, cream or yoghurt.

The heat treatment may be put into effect in a plurality of ways, directly or indirectly. Indirect methods are, for example, heating by means of different types of heat exchangers. There are two major types of direct methods, injection and infusion, where the heating takes place by means of steam. By employing a direct method, an extremely rapid heating will be obtained. A rapid heating is, today, to be sought for when, at the same time as a product with long shelf life is desirable, a product with improved flavour properties is equally desirable. The rapid heating permits heating to elevated temperature for brief periods of time, which reduces thermal stresses on the product and thereby reduces the risk of overtreatment.

The present invention relates to heating by means of infusion. Infusion implies that a finely-divided liquid is heated in a steam-filled cavity. The principle of heating a liquid, for example a liquid food, by injecting the liquid into a space filled with steam has been known since the latter part of the 19th century.

In principle, the infusor consists of a pressure vessel or autoclave with a conical bottom. In the upper region of the pressure vessel, there is an inlet for the product which is to be heat treated and there is also a device disposed there for dividing the product into small droplets which then fall through the pressure vessel.

In the bottom of the pressure vessel, the droplets are collected to form a liquid reservoir and finally depart from the pressure vessel through an outlet in the bottom.

The pressure vessel also has an inlet for steam which may be placed in its upper region, or alternatively in the lower region of the pressure vessel. Regardless of the

placing of the steam inlet, the hot water vapour or steam is to heat up the product droplets while on their way down through the pressure vessel.

All modern heat treatment of food products is intended, on the one hand, to heat the product to a certain predetermined temperature, and, on the other hand to keep the product at this temperature for a certain predetermined time interval. During this time interval, the product stays in a so-called holding tube. By selecting different temperatures and different stay times, products will be obtained which are aseptic or which have extended shelf life.

In heat treatment plants using infusion heating, a separate holding tube is normally employed which consists of a pipe section of a predetermined length in direct connection with the product outlet of the infusion vessel. However, such a holding tube suffers from the drawback of not being adjustable for different time intervals or for other variations in the heat treatment process.

Moreover, a conventional holding tube may cause the product to begin to boil because of the pressure drop which occurs when the product departs from the infusion vessel. This boiling may give rise to frothing which entails an increase in volume and thereby reduced control of how the product can be kept at a certain temperature for a certain time. In order to prevent the product from boiling in this situation, a so-called lobrotor pump may be placed between the infusion vessel and the holding tube and ensure that the product obtains a pressure increase. However, this is a costly solution but is, in certain countries, a requirement of the authorities in order to ensure that the product is kept at the correct temperature during the correct period of time.

A further problem inherent in conventional holding tubes in connection with infusion heating is that the liquid which is accumulated in the conical bottom region of the infusion vessel gives a high degree of uncertainty in the calculation of the stay time of the product, since the relatively large volume makes it impossible to know how long the product stays here, and to know whether all fractions of the product have been treated for the same period of time.

One solution to the problems inherent in the conventional holding tube in infusion heating is disclosed in Swedish Patent Specification SE 513537. Here, the holding tube is integrated in the conical bottom section of the infusion vessel in that a conically shaped body for the greater part fills out the space in the bottom section and that the remaining space in the bottom section constitutes a holding tube together

with a certain volume above the conical body. By adjusting the level above the conical body, this holding tube may be regulated for different time intervals and for variations in the production process. However, a holding tube according to the foregoing suffers from the drawback of being relatively costly to manufacture.

OBJECTS OF THE INVENTION One object of the present invention is to design a holding tube which is adjustable and which gives a reliable and defined stay volume for the product.

A further object of the present invention is that the design of the holding tube ensures that the product pressure is maintained and that, as a result, it is not necessary to utilise an interjacent costly pump.

Yet a further object of the present invention is that the holding tube be more economical to manufacture than prior art adjustable holding tubes for infusion heating.

SOLUTION These and other objects have been attained according to the present invention in that the holding tube of the type described by way of introduction has been given the characterising feature that the first, adjustable section consists of an inclined conduit of a predetermined length in which is disposed a device operative to maintain a defined liquid level in the conduit.

Preferred embodiments of the present invention have further been given the characterising features as set forth in the appended subclaims.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING One preferred embodiment of the present invention will now be described in greater detail hereinbelow, with reference to the accompanying Drawing, in which: Fig. 1 is a side elevation of the present invention placed in a part of a heat treatment plant.

The Drawing shows only those parts and details essential to an understanding of the present invention, and the remaining parts of the heat treatment plant-well known to a person skilled in the art-have been omitted.

DESCRIPTION OF PREFERRED EMBODIMENT Fig. 1 shows a part of a heat treatment plant for infusion heating. An infusor-l consists of a pressure vessel 2 and an inlet 3 for product located in the upper region of the pressure vessel 2. At the product inlet 3, there is also disposed a device 4 for finely dividing the liquiform product into small droplets. The device 4 for finely dividing the product may, for example, consist of a distribution chamber which, in its lower wall, has a large number of holes or gaps. The pressure vessel 2 has a conical bottom section 5 with an outlet 6 for the product.

The pressure vessel 2 moreover displays an inlet 7 for steam. In the preferred embodiment, the steam inlet 7 is placed in the lower region of the pressure vessel 2 and is disposed so that the steam dissipates in the pressure vessel 2 through a concentric distribution chamber 8 and a concentric distribution gap 9. Alternatively, the steam inlet may be placed in the upper region of the pressure vessel 2 in association with the product inlet 3.

As shown in Fig. 1, the conical bottom section 5 of the pressure vessel 2 may be formed with a dual wall 10. By supplying coolant between the dual wall 10 and the conical bottom section 5, depositions of coagulated product may be minimised without appreciable cooling effect on the product outflow out of the pressure vessel 2.

The holding tube according to the present invention is placed immediately after the pressure vessel 2 and includes a first section 11 which consists of an inclining conduit of a certain predetermined length. The inclination may be from a straight, vertical conduit with a slope of 90° from the horizontal plane to a slope of approx. 12°. The preferred inclination from the horizontal plane is 20-25°. The inclination should be uniform throughout the entire holding tube section 11 in order that the product have the same flow rate, but in order to save space, the holding tube is suitably designed alternatingly with straight conduit sections and pipe bends as shown in the Drawing. On the Drawing figure, the first section 11 of the holding tube consists of the part components 11 la, lib, lie and lid.

The holding tube according to the present invention moreover consists of a second section 12 which comprises a conduit of a certain predetermined length. In the preferred embodiment, the second section 12 of the holding tube consists of the part components 12a and 12b.

The holding tube is further provided with a level indicator 13 of conventional type. The level indicator 13 may employ ultrasound, floats, lasers or other suitable methods and it actually consists of height meters 14,15 at two different levels. The measurement points 14,15 lie above and below, respectively, the levels where the level indicator 13 is operative. The value of the level indicator 13 is registered by a regulator 16 of conventional type which is disposed to control a conventional, aseptic regulator valve 17.

The Drawing figure also shows a vacuum vessel 18 placed immediately after the regulator valve 17. The vacuum vessel 18 has a tangential inlet 19 for the heat treated product and a product outlet 20 placed in the lower region of the vacuum vessel. The outlet 20 of the vacuum vessel 18 is normally connected to a pressure- elevating pump 21.

The product which is to be heat treated in a plant for which the holding tube according to the present invention is intended enters the infusor 1 through the product inlet 3. The product is liquiform and may, for example, consist of a dairy product such as milk. In the device 4 for this purpose, the incoming product is finely divided into small droplets. The small droplets fall through the infusion vessel 2.

At the same time, hot steam is fed into the infusion vessel 2 through the steam inlet 7 and via the concentric distribution chamber 8 which discharges in the concentric inlet gap 9, the steam rapidly dissipates in the infusion vessel 2. The falling product droplets meet the steam and are quickly heated to the desired temperature. The heated product falls down towards the conical bottom section 5 of the infusion vessel 2 and is recollected in the product outlet 6.

The heated product thereafter reaches the holding tube which, according to the invention, consists of two sections 11 and 12. In the holding tube, the product is to stay at a desired temperature and for a desired time interval. The second, fixed section 12 should be totally filled with product in order that the holding tube perform the desired function.

The first, adjustable section 11 of the holding tube is filled with product up to a defined liquid level 22. The defined liquid level 22 is located between a minimum level 23 and a maximum level 24 and is adjustable between these two. The minimum level 23 is located in the interface between the fixed section 12 of the holding tube and the adjustable section 11. The maximum level 24 is located approximately at the product outlet 6 out of the infusion vessel 2. The diameter of the conduit in the

adjustable section 11 of the holding tube is selected so that the product flow rate below the liquid level 22, at maximum flow, does not exceed 0.45 m/s.

The defined liquid level 22 is measured using some form of conventional level indicator 13. The level indicator 13 may, for example, utilise measurements of the pressure in two points 14,15. The upper point 14 should be placed above the maximum level 24 and is suitably placed in the infusion vessel 2. The lower point 15 should be placed below the minimum level 23 and it is ideally placed in the part component 12a of the fixed holding tube section 12.

The value of the level indicator 13 is registered by the conventional regulator 16 which controls the regulator valve 17. The regulator valve 17 may consist of a conventional, aseptic throttle valve which opens or closes so that the defined liquid level 22 is constantly maintained. The level indicator 13, the regulator 16 and the regulator valve 17 together constitute an apparatus 25 for maintaining the defined liquid level 22.

After the regulator valve 17, the product which has now been heat treated at the desired temperature and during the desired time interval continues into the vacuum vessel 18. The product, which includes water and water vapour, enters into the vacuum vessel 18 through the tangential inlet 19. The water which had been supplied to the product is boiled off and the ready-treated product departs from the vacuum vessel 18 through the outlet 20. Via a pressure-elevating centrifugal pump 21, the product is conveyed further for additional treatment or packing in consumer packages. Packing of the product may take place in filling machines for aseptic single-use disposable packages.

The defined liquid level 22 is a norm value which is set in relation to further equipment in the heat treatment plant, such as the number of filling machines which the plant is to supply with product. The norm value which is to be selected also determines the desired degree of heat treatment, such as treatment into aseptic product or into a product possessing extended shelf life.

As the holding tube is shown in Fig. 1, the fixed section 12 constitutes approx. 40% of the holding tube and the adjustable section 11 constitutes 60%. This relationship is particularly suitable for a heat treatment plant which supplies two filling machines with product. If one filling machine drops out of production, the holding tube may rapidly be adjusted so as to supply only the remaining filling machine with product, without down time and with a minimum of product recycling.

The holding tube according to the present invention may naturally be designed so as to have a different split between the fixed section 12 and the adjustable section 11.

The liquid column which is formed in the adjustable section 11 and which is delimited by the defined level 22 has, above the surface, air-mixed steam which exercises a static pressure on the liquid surface. The static pressure is greater than the pressure at which the liquid can boil. The holding tube according to the present invention thereby possesses integral protection ensuring that the product does not begin to boil. This entails that it is not necessary to introduce into the plant a pressure-elevating lobrotor pump which may be required in conventional holding tubes.

As will have been apparent from the foregoing description, the present invention realises a holding tube intended to be used for infusion heating which is simple and relatively economical to manufacture. The holding tube is adjustable for variations in production or for various stay times. The design of the holding tube affords integrated protection preventing boiling of the product when it departs from the infusion vessel.