Technical Field

The present invention relates to a method of heat-treat¬ ing liquid milk product, which after packing is distri¬ buted and stored at no higher than conventional cold chain temperatures, and whereby the content of psychro¬ trophic, aerobic spores in the product is reduced by the heated product being held at a sufficiently high holding temperature for a predetermined holding period.

Background Art

Va.rious methods of heat-treating unpasteurized milk are known. Generally speaking, an increased holding tempera¬ ture of the milk causes an increased reduction of the bacterial content and accordingly, other things being equal, a longer keeping ability of the milk during suc- ceeding storage. However, the increased holding tempera¬ ture un ortunately results in an increased degree of chemical change and consequently in increased deteriora¬ tion of the sensory properties of the heat-treated milk.

The chemical change causing a deterioration of the sen¬ sory properties, such as the development of a so-called "cooked" taste, is of an extent which inter alia is re¬ flected in the extent of the denaturation of whey pro¬ teins, said denaturation more or less following paral- lelly to the development of the cooked taste. The whey protein denaturation can be easily demonstrated by che¬ mical analysis.

When unpasteurized milk is low-pasteurized, the heated milk is held at a holding temperature of about 75°C for at least 15 sec. and the thus heat-treated milk posses¬ ses particularly good sensory properties. The extent of the whey protein denaturation by such a method is about

18 to 20%. No reduction occurs, however, of psychro¬ trophic, aerobic spores, with the result that usually the low-pasteurized milk has a keeping ability of only 2 to 3 days when stored at 10°C.

When unpasteurized milk is high-pasteurized, the milk is held at a holding temperature of about 85 to 90°C for about 15 sec. with the result that the heat-treated milk has sensory properties somewhat comparable with those of the low-pasteurized milk. Such a method involves a whey protein denaturation of about 30 to 35%. However, this increased holding temperature surprisingly often in¬ volves an increased conversion of psychrotrophic, aerob¬ ic spores into bacteria and increased growth of these bacteria, resulting in a shorter keeping ability, espe¬ cially at a storing temperature of about 10°C. There¬ fore, a particularly reserved attitude applies to heat- treating unpasteurized milk at increased holding tempe¬ ratures to product treated milk to be distributed and stored at only the conventional cold chain temperature.

As a consequence, the conventional alternative is to sterilize the unpasteurized milk, so that it can be dis¬ tributed and stored at ambient temperatures or cold chain temperatures, indifferently. This sterilization is normally achieved by an ultra-high-temperature heat treatment, viz. UHT heat treatment. The definition of UHT treatment is according to IDF (International Dairy Federation) D-Document 191, 1989, a heat-treatment for killing all the micro-organisms capable of propagating in milk and milk products, and whereby the temperature is increased above 100°C so that an Fo value of at least 3 is obtained. This Fo value is obtained at 140°C for 2 to 3 sec., but in practice it is common to use a strong- er heat-treatment of 143°C for 3 to 4 sec. in order re¬ liably to ensure a satisfactory killing effect on the very heat-resistant thermophilic spores. The UHT-treated milk possesses a particularly long keeping ability of

more than 4 weeks at a storing temperature of 10°C or more, provided that the packing is carried out under so- called aseptic conditions, viz. sterile conditions, which is the most expensive of the packing methods. The extent of whey protein denaturation by such a heat- treatment is, however, particularly high and at least 55 to 80%, and the heat-treated milk possesses considerably deteriorated sensory properties compared with low- pasteurized milk and particularly suffers from a domi- nating "cooked" taste.

US-A-4, 161,909 and AU-B-610,233 disclose UHT-treatment systems in which direct heating by steam infusion is used, the unsterilized milk being sprayed into a steam atmosphere.

6B-A-1, 187 , 766 discloses an UHT-treatment in which milk, for example homogenized, pasteurized cows' milk, is ste¬ rilized by being heat-treated at at least 150°C for a fraction of one second, preferably between 180°C and 220°C for between 0.4 and 0.1 sec. For example, the heating of a column of milk by microwave radiation dur¬ ing the passage of the milk through a ceramic wave guide to 193°C and holding at this temperature for 0.15 sec. is disclosed as being sufficient to sterilize the milk without inducing any noticeable flavour, colour or tex¬ ture changes. However, in practice, at such high tem¬ perature the control of the holding time period at such low value is highly critical and any slight failure to maintain the necessary low value leads almost immediate¬ ly either to failure to sterilize if the time period is too short or very poor sensory properties if the time period is excessive.

NO-B-143,416 discloses a sterilization method for milk or similar protein-containing liquid foods, especially those with a high micro-organism content, in such a man¬ ner as to avoid the product acquiring a "cooked" taste.

The product is pre-heated, for example in a plate heat- exchanger, to between 70°C and 90°C, preferably 85°C, to kill the pathogenic micro-organisms and also to cause the bacterial spores to absorb moisture, and then cen- trifuged in a second treatment step at for example 55°C for 10 to 20 sec. so that the majority of the present spores and other micro-organisms are separated out. In a third treatment step, the product is held at a tempera¬ ture of 35°C to 40°C for 10 to 20 min. to cause the spores to germinate and thus make them easier to kill, and finally heated, for example by steam injection, for a short time to a temperature of about 110°C to 140°C, preferably 130°C, for 1 to 3 sec, in order to render inactive the ^" remaining micro-organisms. This method is time—consuming and complicated and thus impractically expensive, especially for unpasteurized milk with a re¬ latively low micro-organism content.

Description of the Invention

The present invention is directed towards significantly increasing the keeping ability of liquid milk product which is to be distributed and stored in a non-steri¬ lized form at only temperatures no higher than conven- tional cold chain temperatures, without seriously chang¬ ing the sensory properties, in particular the taste, of the product, in comparison with those of low-pasteurized milk. A particularly pressing demand exists for such a product.

According to the present invention, there is provided a method of heat—treating liquid milk product, which after packing is distributed and stored at no higher than con¬ ventional cold chain temperatures, and whereby the con- tent of psychrotrophic, aerobic spores in the product is reduced by the heated product being held at a suffi¬ ciently high holding temperature for a predetermined holding period, characterised by producing a non-sterile

product by heat-treating the product by direct heating and by then holding the heated product at a holding tem¬ perature in the range of 130°C to 145°C for a holding period of no more than 1 sec.

The resulting heat-treated product surprisingly posses¬ ses a novel combination of good sensory properties and longer keeping ability. The good sensory properties are due to a negligible degree of chemical change for the heat-treatment temperature of 130°C to 145°C, and the sensory properties can compete with the properties ob¬ tained by low-pasteurization of liquid milk product.

We have found that the present invention results in an extent of whey protein denaturation of between about 30 and 40% and a keeping ability of about two weeks at 10°C with so-called pure filling during packing. The direct heating may in a particularly advantageous manner be carried out by way of a steam infusion method, where the product is injected into a steam atmosphere and held at a temperature of between 133°C and 145°C for less than 1 sec, particularly more than 0.25 sec., especially be¬ tween 0.5 and 1 sec. In the light of the use of tempera¬ tures between 133°C and 145°C, accuracy of the holding time is not highly critical, especially if holding times between 0.5 and 1 sec. are employed. The heated product may be held at a holding temperature of 135°C to 143°C.

The liquid milk product may be unpasteurized milk or cream, low-pasteurized milk or cream, or unpasteurized or low-pasteurized, fat-reduced milk.

Best Mode for Carrying Out the Invention

An example of the present invention will now be describ¬ ed.

Unpasteurized fresh milk is subjected to a heat-treat-

ment, after which the milk is packed in cartons and then distributed and stored in a cold chain at temperatures preferably below 6°C, but with temperature fluctuations up to 10°C being liable to occur. The heat-treatment comprises direct heating by a so-called steam infusion method where the milk is injected into a steam at¬ mosphere. An apparatus for subjecting the milk to such heat-treatment is for instance described in AU-B- 610,233. The condensed steam and the milk is quickly removed from this apparatus by means of vacuum by said steam and milk flowing into a vacuum chamber, in which the water deriving from the condensed steam is removed. The pressure drop of the condensed steam causes an in¬ stantaneous drop in temperature. The total holding pe- riod at the holding temperature is less than 1 sec. The holding temperature is in the range 133°C to 145°C, pre¬ ferably 135°C to 143°C. We have found that a reduction of at least 10 ^s times is obtainable in the content of psychrotrophic, aerobic spores in the milk with the ef- feet that the desired longer keeping ability of the milk is obtained during the succeeding storage and distribu¬ tion.

In order to compare the extent of whey protein denatura- tion (and of free sulphydryl groups consequently formed) and keeping ability during the succeeding storage at about 10 ^C C, test were carried out with various heat treatment methods using unpasteurized fresh milk from the same batch. The tests and their results are shown in Table 1.

Table 1

For comparison purposes, data have been included in the Table for a sterilized milk having a long keeping abili¬ ty and produced in the same apparatus by a so-called UHT direct heating method.

As appears clearly from the Table, the whey protein de¬ naturation by the new method is between about 30% and 40%, i.e. similar to the values obtained with high-pa¬ steurization and further the heat-treated milk has maintained particularly good sensory properties as well as keeping ability as long as 1.5 to 2 weeks.

It should furthermore be noted that as far as the keep¬ ing ability depends highly on the type of filling i.e. packing of the product. The following filling methods are known:

Conventional filling: Affected by the surrounding air.

Pure filling: Protected by air filters and sterilization of the carton.

Aseptic filling: Completely sterile filling.

The following keeping abilities were obtained with a milk produced by the present method.

Table 2

It should be noted that many modifications can be car¬ ried out without thereby deviating from the scope of the invention. For example a steam injection method, in which the steam is injected into the flow of milk, may instead be used.