BACKGROUND ART A general problem for storage of dairy products is the susceptibility of these to attack by microorganisms, particularly where refrigeration is unavailable.

Sterilised concentrated milk is known and valued for its long keeping properties. It is diluted for use in place of fresh milk. Conventional production process includes processes in which concentrated milk is placed in containers and sterilised by external heating. In another process a concentrate is UHT sterilised and then packed aseptically into containers. These production processes rely heavily on the use of stabilisers to prevent coagulation and deterioration of the product during the sterilisation stages and in storage.

US Patent 4,921,717 describes a multi step process in which a sterilised, concentrated milk process is produced by a process comprising the steps of : (a) partially concentrating milk or a milk product, (b) sterilising the partially concentrated milk product, and (c) further concentrating the sterilised milk product of step (b) under sterile conditions.

The object of the present invention is to produce an improved sterile milk product and/or an improved process for preparing a sterile milk product and/or to provide the public with a useful choice.

The invention as a result of the applicant's discovery that relatively brief UHT treatments of high concentrated milk concentrates can be used to produce sterile milk concentrates and powders which are both free of bacterial spores, and have minimal damage to milk proteins.

DISCLOSURE OF THE INVENTION In one aspect the invention provides a method of preparing a sterile, stable milk concentrate comprising : (a) providing a milk concentrate with a total solid contents of at least 40% (w/w), (b) ultra-heat treating the concentrate at a temperature in the range 120°C to 155°C for 0.1-10 seconds and cooling.

In another aspect the invention provides a method of preparing a powder comprising the steps of : (a) providing a whole milk concentrate or milk protein concentrate with a total solid contents of at least 40°'0 (w/v).

(b) ultra-heat treating the concentrate at a temperature in the range 120°C to 155°C for 0.1-10 seconds and cooling. and (c) drying the product of step (b) to obtain a powder.

The milk concentrate used as a starting material may be produced by standard evaporation methods. Alternativelv the concentrate may be prepared from reconstituted dried milk and milk fat. A further alternative is that the milk concentrate is a protein-concentrate.

Ultrafiltration may be used as an alternative for concentrating the whole milk/full milk protein.

Preferably the milk concentrate is a whole milk concentrate. However. the term"milk concentrate"includes other concentrates of milk in which the proportions of the constituents of the milk have been altered.

Another preferred type of milk concentrate is a skim milk concentrate.

Preferably the concentrate contains at least 47%. or more preferably at least 55% (w/w) total solids.

The UHT treatment is preferably carried out by direct steam injection. Alternatively it mav be carried out by direct steam infusion. Indirect heating is generally less preferred.

Preferably the milk concentrate is at a temperature in the range 45° to 75 °C immediately before the commencement of the UHT treatment.

The temperature and duration of the UHT treatment mav be varied. At higher temperatures a shorter duration will generally be used to minimise damage to the milk proteins.

A preferred range is 130-150°C for duration of 0.2 to 5. 0 seconds. More preferred is the range 135°C-145°C for 0. 2 to 2. 0 seconds.

Where the end product is a powder. conventional drying methods may be used. Spray- drying is the currently preferred method.

Stabilisers such as phosphate salts may optionally be included in the processes and products of the invention.

In another aspect the invention provides a milk concentrate having a total solids content of at least 40%. preferably at least 47%. more preferably at least 55%. wherein the product has minimal protein damage due to limitation of the sterilisation step to heating at 120 to 155'four 0.1 to 10 seconds, preferably 130 to 150° for 0.2 to 5 seconds. more preferably 135°C to 145°C for 0. to 2 seconds. wherein any spores are present at a level less than lOcfu/g. preferably 0.

In yet a further aspect the invention provides a sterile milk powder prepared from a concentrate of the previous aspect of the invention.

In this specification the term"sterile"is used when bacterial spores are undetected at a detection limit of 10 cfu/g.

EXAMPLES The following examples further illustrate practice in the invention.

Example 1-Preparatioo1 of a sterile whole silk powder A whole milk concentrate having total solids content of 50% was heated from an initial temperature of 70°C to 140°C and held for various residence times (0.2.0.5 and 1.7 seconds). Heat treatment was performed by direct steam injection (DSI) which was placed directly before the atomiser. The heated milk concentrate was injected via the atomiser directly into the drying chamber. The heated milk concentrate can also be flashed down to <65 °C prior to atomisation and drying through the use of a standard vacuum vessel.

Before the heat treatment, spores of Bacillus siibtilis were added to obtain a total concentration of S x 104 cfu/ml. The total numbers of spores present in the milk powder manufactured from the heat treated milk concentrate using the three different residence times at 140°C were determined. Surprisingly, at all three residence times the bacterial spores were undetected in the milk powder using a detection limit of < 10 cfu/g. The heat treatment had minimal effect on the functionality of the milk powder.

Example 2-Preparation of a sterile milk concentrate In this example milk concentrate having a total solids content of up to 60% was heat treated at 140°C and held for various residence times (0.9s 0.5 and 1.7 seconds). Milk concentrate was heated from 65 °C to 140°C bv direct steam injection (DSI). The product was cooled instantlv to <65°C in a flash vessel.

Before the heat treatment spores of Bacillits cereals were added to obtain a total concentration of 1 x 106 cfu/ml. After heat treatment Bacillus cerelzs was undetected at a level of <10 cfu/ml.

Surprisingly this product. although thicker than standard milk concentrate, was a stable fluid. did not thicken. did not svnerese and had a smooth texture. The product was easily reconstituted in water.

The above examples illustrate practice in the invention. It will be appreciated by those skilled in the art that the invention can be carried out with numerous modifications and variations. For example. a total milk solids content of the milk concentrate. the heat treatment temperature and duration may be varied. Likewise, the practice of the invention, the microorganisms and spores destroyed will vary.