GB2182539A - A process for preparing a fermented dairy product - Google Patents

Abstract

In a process for preparing a fermented dairy product, in which milk is subjected to ultrafiltration and the resulting retentate is fermented, the retentate has a protein content of 5-8%, a fat content of 0.5-5%, and a content of nitrogen not exceeding 7 mg/kg, and is acidulated to a pH of 4.4-4.7, and the resulting product is then subjected to such shear forces as to produce a homogeneous mass with a firmness of 10-25, expressed in mm depth of penetration per 10 sec of a conical body having an apex angle of 90 DEG and a mass of 54.8g.

Description

SPECIFICATION A process for preparing a fermented dairy product This invention relates to a processforpreparing a fermented dairy product, in which milk is subjected to ultrafiitration and the resulting retentate is fermented.

Asimilar process is disclosed in Netherland patent application 7306505. In that application there is described that curd- a fresh ch eese -- ca n be prepared by concentrating milk to a volume suitable for the preparation ofcurd, using ultrafiltration, and subsequently acidulating it in the conventional manner.

Although, in this way, a high-grade food is obtained, practice has shown that the prior process does not lead to the preparation of a product, which, as regards flavour and consistency, meets consumer demands under all conditions.

For itto fully satisfy consumer demands, a fermented milk product must be 'freshly sourish' in taste, and otherwise be organoleptically acceptable. One disadvantage of the product prepared using the prior process is that it is not 'smooth', that is to say, it makes a somewhat chalky impression on the consumer.

Moreover, the organoleptical characteristics of such a product are not always found acceptable, which applies in particularto the somewhat acrid and bitter note in its flavour (cf Dairy Industries International 49 (1984) no. 12, page 11.

Accordingly, there is a need for a fermented milk product which, on the one hand is considerably 'firmer' than yogurt and per unit volume has a higher nutritional value, in particular as regards its protein composition, but on the other hand, does not have the above disadvantages, and in particularthe 'feel' of for exam ple curd in the mouth, which is less appreciated by some consumers.

In preparation of such a product, a number of problems are encountered. It has been found that an unpleasant 'feel' is correlated to some extent with a non-homogeneous structure, which is partly caused byundissolved gases present in the product, in particular gaseous carbon dioxide.Furthermore, it has been found that these gases cannot be removed after fermentation without detracting from the structure and firmness of such a product It has now been found that when a retentate is acidulated in which the ratio ofthe protein to the fat is maintained at a specific value, while during the ultrafiltration productive ofthe retentate care is taken thatthe content nitrogen gas in themixtureto be fermented is no more than 7 mg/kg, the problems described above are avoided.

According to the present invention, a retentate with a protein content of 5-8%, a fat content of 0.5-5%, and a content of nitrogen gas of no more than 7 mg/kg is acidulated to a pH of 4.4-4.7, whereafterthe resulting product is subjected to such shear forces as to produce a homogeneous mass with a firmness of 10-25, expressed in mm depth of penetration per 10 secofaconical body having an apex angle of 90" and a mass of 54.8 g.

Without wishing to be bound by any theory, we are assuming that, by using the process according to the invention, owing to the selected maximum content of nitrogen gas in the mixtureto be acidulated,together with the fat'protein ratio therein, the carbon dioxide formed during the acidulation is largely dissolved in the mixture and consequently is not present as a free gas.

Using the process according to the present invention, it is of importance that it is ensured that during the entire process of preparation, in particular during and after ultrafiltration and during the pumping ofthe milk and the retentate, the inclusion of air is avoided.

When the starting product is milk into which,for example, during winning and/or shipment, an amount of air has been beaten, this can generally be reduced to an acceptable level by means of a thermal treatment.

The process according to the invention makes it possible to adjustthe desired end structure ofthe productwithin certain limits, because the ready product does not require to be degased. Yet a beautiful 'smooth' homogeneous structure is obtained.

To produce a retentate having the desired fat/ protein balance, it is in principle possible to use any milktype,for example, whole milkorskim milkasthe starting product. If necessary, as much fat or cream or decreamed milk can be added to the retentate that the desired ratio is adjusted.

The gaseous, free nitrogen is generally mainly present as air nitrogen. Measuring the amount of free nitrogen presentin thestarting milkor in the mixture to be acidulated can therefore suitably be effected using a known measuring method, for example, by measuring the free oxygen content using a known oxygen gauge.

For subjecting the acidulated product to sufficient shearforces, in principle any known method, and in particularthose generally used for homogenization, is suitable. Suitable methods are,forexample,the use of a grinding stirrer (ultra Turrase), a turbo-stirrer, a colloid mill, a homogenizer, etc.

The invention is illustrated in and by the following examples.

Example 1 801 of skim milk, after being pasteurized at 750C for 15 sec, was subjected to an ultrafiltration treatment using a Pasilac pilot-plant ultrafiltration unit (with a diaphragm surface area of 4 m2) at a temperature of 48"C and a pressure drop across the diaphragm of 0.38 MPa. 481 retentatewas obtained, which was mixed with 2.51 cream with 40% fat which resulted in a product having a protein content of 5.5% and a fat content of 2.0%. This product was subsequently pasteurized for 5 minutes at 85"C and homogenized with a Rannie homogenizer at 65"C and at a pressure of 20 MPa.

Throughoutthe procedure described above, it was carefullyensuredthattherecould be no inclusion or drawing ofair; the resu It was that at the moment of acidulation the retentate had a content of nitrogen gas of 5.1 mg/kg, which was calculated from an analysis of the oxygen gas content (2.97 mg/kg, calculated with an oxygen gauge of the Orbisphere Model 2714, a diaphragm detector with a sensorwhich consists of a polarographic element which causes the generation of an electrical current proportional to the amount of oxygen) taking accou nt of the known ratio of oxygen to nitrogen in air of 20.9,/78.1,and starting from a maximum solubility at 20"C and atmospheric pressure of 44 mg/kg for oxygen and 19 mg/kg for nitrogen.

This retentate was inoculated with 1.5% of a BD starter consisting of a mixture ofthe bacterial strains Streptococcus lactis, Streptococcus crem oris, Streptococcus diacetylactis and Leuconostoc cremoris.

Afterfermentation for 18 hours at 22 C the pH was 4.62, whereafter the product was stirred with a bladed stirrerfor2 minutes ata speed of 1500 rpm and a peripheral velocity of 9.8 m/sec.

The product thus treated was poured into 500 g plasticjars and cooled. The resulting product was beautifullyfirm and fully smooth in structure, had a pure fresh taste and an attractive, smooth, non-chalky feel in the mouth.

The firmness of the product was measured at 5"C, using a penetrometer according to Sommer and Runge as described,forexample, in the thesis 'Verdikken en geleren' (thickening and gelating) by H.

Beltman, Mededelingen Landbouwhogeschool Wageningen (Communications Agricultural University,Wageningen) 75-2 (1975), in which a cone having an apex angle of 90d and a mass of 54.8 g penetrates into the product for 10 sec. The distance covered was 22.4 mm. Even when kept in refrigeration for 2 weeks, the product still exhibited fully the above described excellent characteristics: the depth of penetration was 22.2 mm.

Example 11 10,000 litre milk with a fat content of 2.0% was pasteurized for 5 minutes at 900C and subsequently homogenized at 55"C and at a pressure of 18 MPa.

This milk was subjected to ultrafiltration at50 C, using a RhônePoulencPleiadetype ultrafiltration unit, diaphragm type IRIS 3038.4,5001 retentate was obtained with a fat content of 4.4% and a protein content of 7.6%.to This retentate was pasteurized for 30 sec at 85"C and inoculated with 1% of a BD starter (see Example I).

During the procedure much attention was paid to the piping being well closed, couplings being closed free from air,the inclusion of a irdudng pumping being avoided and the liquid being caused to flow into tanks and the like in the absence of air. The result was a retentate which atthe moment of acidulation had a nitrogen gas content of 6.5 mg/kg (determined as described in Example I).

Afterfermentation for 16 hours at 240C a pH of 4.58 was reached, whereafterthe product was pumped through ahomogenizer,wherethe productwas subjected to a shear velocity gradient of about 10,000 s-l, and subsequently pumped through a coolerto a fillerwherethe product was filled into plastics jars of 2509.

The productwas beautifully firm with a depth of penetration of 19.8 mm, measured bythe penetro meter(see Example I). The product was entirely smooth in structure, pure and freshly-creamy in taste, and its feel in the mouth was attractive, smooth, and not chalky.

After being stored in refrigeration for 2 weeks, the product was still fully unchanged in charactistics: the depth of penetration was 19.9 mm.

Comparative example The procedure of Example II was repeated in full, except that in this case no particular attention was paid to the inclusion of air during the procedure. This resulted in a nitrogen gas content of the retentate at the moment of acidulation of 15.1 mg/kg.

The end product th us obtained was not smooth in structure which was both visually perceivable and also felt in the mouth.

This non-attractive feel in the mouth imparted generally an unpleasant taste sensation to the product.

In an attemptto obtain a smooth productafterall,a portion of the fermented product was subjected to prolonged intensive stirring. After stirring for about 3 hours,the result was that indeed the product had obtained a somewhat smoother structure, butthe firmness of the product thus treated was very poor, which manifested itself in the depth of penetration which no longer ways measurable in a reproductable manner.

Claims (3)

1. A process of preparing a fermented dairy product, which comprises subjecting milk to ultrafiltration and fermenting the resulting retentate, characterised in that a retentate having a protein content of 5-8%, a fat content of 0.5-5% and a nitrogen content not exceeding 7 mg/kg is acidulated to a pH in the range from 4.4-4.7 and the resulting product is then subjected to shearforcesso asto produce a homogeneous mass having a firmness of 10-25, expressed in mm depth of penetration per 10 sec of a conical body having an apex angle of 90" and a mass of 54.8 g.

2. A process of preparing a fermented dairy product substantially as described in Example I or Example II herein.

3. A fermented dairy product prepared in accordance with the process of claim 1 or 2.