NZ214093A

NZ214093A - Freezing a milk product

Info

Publication number: NZ214093A
Application number: NZ214093A
Authority: NZ
Inventors: Robert James Dickinson
Original assignee: Robert James Dickinson
Priority date: 1984-11-07
Filing date: 1985-11-06
Publication date: 1987-08-31
Also published as: GB8428089D0; GB2166939A; GB2166939B; GB8527510D0

Description

<div class="application article clearfix" id="description"> 2140 93 Priority Dato(s): ........... Complete Specification Filed.-^lUT^jS. Cfasc: B^SC^.[Qk ;;;;; ^::'oZ 31'absw- P.O. Journal, No: PATENTS FORM NO. 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION A "MKTHOD OF FREEZING.MILK PRODUCT" I I ■ ■ - I HI- !■ ■ "■ !■ — ■ I " ■ ■ ' ^ I, WB* ROBERT JAMES DICKINSON of Lower Longley Farm Holmfirth, Huddersfield, England HD7 IRS, a British citizen hereby declare the invention, for ttfiich l/-we-pray that a patent may be granted to me/a-57 and the method by vfaich it is to be performed, to be particularly described in ap&fl&yi /■ the following statements- _i_ 13 FEB 1986 t ^ ^followed by page 214Q9 3 METHOD OF FREEZING A MILK PRODUCT The present 'invention relates to an improved method of freezing a milk product such as cream, particularly cream 5 having a high fat content. Cream consists of an emulsion of fat globules in an aqueous liquid, each globule being surrounded by a phospholipid membrane. These membranes are susceptible to damage and if it occurs, the fat contained in the globules 10 flows out through the damaged membrane and becomes free fat. This is what happens when the cream is beaten or churned to become butter. When freezing cream and other milk products having this type of structure, such as yoghurt, it is desirable to 15 reduce this membrane damage as much as possible in order to minimise the concentration of free fat in the final thawed product. This ensures that the thawed cream retains the colour, texture and taste of fresh cream. The freezing process itself causes membrane damage, 20 and this is particularly troublesome when creams having a high fat content/ for instance more than 60%, are frozen. This is due to the fat globules being closer together than in lower fat content creams, and thus the membranes are more susceptible to damage during freezing. 25 Thus membrane damage may be caused by the formation of ice crystals in the cream which press the fat globules together. The ice crystals have sharp edges which tend to pierce the globule membranes, causing the fat to escape. This free fat results in an inferior colour, texture and 30 taste in the resultant thawed cream. When a milk product such as cream freezes, it can be thought of as following a solidification curve, which may be expressed as percentage solid fat verses temperature, as shown in Figure 1 (curve 1). The other curve 2 represents 35 the rate of solidification of the fat with respect to .<.""7;" /'k t? ?/'Y Z13 FEB 1986 ■ ' • *" ' V : " :i>0X 214093 temperature, which exhibits a marked peak between 0°C and 30°C. In currently known methods of freezing cream, the cream is cooled very rapidly, so that, in effect the cream 5 is taken along the peak 2a and beyond it very rapidly. This results in considerable membrane damage, the final product having, when thawed, an inferior flavour, colour and texture when compared to fresh cream. Methods of this type include drum freezing in which the cream is frozen 10 quickly in flakes and pello freezing, in which the cream is frozen quickly in small pellets. These methods also suffer from the disadvantage that the frozen cream has a large surface area and air can easily be trapped in the packaging, thus reducing the shelf life of the frozen 15 cream. Also, although these methods produce an acceptable final product in the case of the lower fat creams (less than 60% fat), they are not suitable for use with higher fat content creams (greater than 60%). The acceptability of the final thawed product is primarily determined by the 20 amount of free fat, which should be as small as possible. These conventional methods produce a product which, when thawed, has an unacceptably high amount of free fat. According to a first aspect of the present invention there is provided a method of freezing a milk product 25 consisting of fat in an aqueous component, the method comprising cooling the product from a first temperature at which the fat contained therein is substantially liquid to a second temperature above the freezing point of the aqueous component in a manner such that a substantial 30 portion of the fat has solidified throughout the volume of the product, and thereafter cooling the product rapidly to a third temperature below the freezing point of the aqueous component and at which the product as a whole is substantially solid. 35 It should be appreciated that the freezing point of s j- 21 93 the aqueous component may be a well defined temperature, as in the case of a product having few or no salts dissolved in the aqueous component, or alternatively may be a temperature range within which freezing of the aqueous 5 component occurs. The actual upper and lower limits of this range depend on the type of product being frozen, and in particular the types and concentrations of the salts contained in the product. In the case where there is a freezing range of the aqueous component, the second 10 temperature should lie outside this range, on the high temperature side. Thus, by cooling the cream slowly from a first temperature to a second temperature above the freezing point of water, a substantial portion of the fat *•» in the cream has solidified before any ice crystals have 15 formed in the cream. It is thought that this makes the globule membranes more resistant to damage. The cooling from the first temperature to the second temperature needs to be carried out in such a way as to ensure that at no time during the cooling does any part of the product reach 20 a temperature below the freezing point of the aqueous component. This could be carried out in a number of different ways. An example of one way in which this may be achieved is to put the product in a controlled temperature enviroment at or close to the desired second temperature so 25 that the product is cooled relatively slowly from the first to the second temperature. Then, by rapidly cooling the cream from the second temperature to the third temperature until it is actually solid to the touch, the formation of ice crystals is minimised, since the latter is time 30 dependant. Preferably the second temperature is in the range -5°C to 10°C. More preferably the second temperature is between 0 to 5°C. 35 The most preferred value for the second temperature is 4 214093 between 1 and 2°C, for instance 1.5°C. Preferably the rate of cooling from the first to the second temperature is less than 50°C per hour. More preferably, the rate of cooling from the first to 5 the second temperature is less than 20°C per hour. The most prefered rate of cooling from the first to the second temperature is 2.6°C per hour. The first stage may take place over a period of 18 hours, or alternatively it may take place in a shorter 10 period when a blast cooler is used. This method has been found to be ideal for freezing any milk product which includes fat in globules such as creams, yoghurts and milks. The method is very effective for freezing all types of cream with any level of fat 15 contained therein. Although, as previously described there already exists several methods of freezing creams, none have yet been found acceptable for freezing creams having more than 60% fat within them. The present invention is particularly appropriate, but not limited to, creams having 20 more than 60% fat content. In the United Kingdom, all milk products for consumption in the home market have to be heat treated in accordance with strict government regulations. Previously cream has been made, heat treated and cooled, and when 25 cooled, placed in its final packaging. However, this allows an opportunity for contamination of the cream by bacteria. In the case of high fat content creams, the method according to the present invention preferably includes the 30 additional step of placing the cream into its final packaging whilst hot, and subsequently cooling the packaged product from the first temperature to the second temperature, and thereafter to the third temperature. This means that the hot cream serves to sterilise or pasturise 35 its packaging and is removed from sources of contamination - - s ■ - "" / • ■ , ■ **'■/ . • 2140 9 3 directly after its heat treatment. Preferably, in the case of high fat content creams, the cream is packaged hot at a first temperature of greater than 35°C. There is no real upper limit to the temperature 5 at which the cream may be packaged, but this would ultimately be limited by the handling of the cream and the properties of the packaging material. More preferably, the cream is packed at a temperature ^ of 48.9°C. 10 Preferably the packaging for the cream comprises a -polythene bag, which may be easily pasturised. The filled polythene bags, after having being cooled to the second temperature, are then placed in a plate D freezer which includes a hydraulic ram to compress the 15 freezing plate onto the bag to maintain perfect contact with the bag. This is to reduce the thickness of the slab of cream as much as possible to reduce the time taken to freeze the cream and to prevent formation of an insulating layer, but to prevent the slab from becoming so brittle 20 that it snaps. Thus the cream or milk product when frozen has a long storage life since at each stage of preparation the contamination by bacteria has been kept to the minimum. Also substantially no air entrapment occurs either within 25 the packaging of the cream or within the inner structure of the cream. Thus oxidation of the cream cannot occur. Also, since membrane damage has been reduced to a minimum, the resultant cream, when thawed, has a low free-fat content and therefore the taste, texture and colour of 30 fresh cream. If such a high fat content cream is to be frozen in this way it is also desirable to produce the cream with as t ^ many fat membranes intact as possible. Care has therefore to be taken in the production of the cream. 35 It is found that if high fat content cream is made ,13FEB1986^ directly from milk less damage occurs to the fat globules than if the cream is produced in two steps, i.e. by making cream and then concentrating the fat in the cream. This is because in the latter case the milk undergoes many more 5 processes and goes through more machines and there is therefore much more opportunity for damage to the internal structure of the cream to occur. The one-step method of making cream is preferably by using a hermetic separator i/""s) such as the Alfa-Laval MRPX 418HG-74C. This one-step 10 method also minimises contamination and air entrapment in the cream. This is important since it extends the shelf life of the cream. The frozen high fat content cream has proved to be ir~N! extremely useful in the form of single, whipping or double 15 cream, and also as an ingredient in many products containing fats such as ice cream, yoghurt, and cream cheese. According to a second aspect of the present invention there is provided a frozen cream having more than 60% total 20 fat content and having less than 10% free fat content as measured by the Sudan III Dyebinding Method. Preferably, the frozen cream has more than 65% total fat content. More preferably, the frozen cream has 69% total fat 25 content. The method of the present invention may be equally used with creams with lower fat content, and in this case o the cream need not be packaged whilst hot, as lower fat creams will still flow at lower temperatures. 30 An example of a method of manufacturing and freezing cream having 69% fat in accordance with the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a schematic graph of a solidification 35 curve of cream; and, 214093 Figure 2 is a flow diagram of the method. Referring to Figures 1 and 2, the steps of the method of the present invention will now be described in more detail. 5 Step 1. Milk is stored in a bulk tank which is mounted on load cells so that the weight of milk required to product each kilogram of the cream can be accurately determined. The fat contained in the milk is approximately 3.8% and the rate of flow of the milk for the bulk tank is 10 approximately 5500 gallons per hour (5.78 litres per second). Step 2. The milk is passed to a heat exchanger where it is warmed to 48.9°C. o Step 3. The milk is passed to the separator and 15 skimmed milk is returned as a by-product. The separator is a hermetic separator, the Alfa-Laval MRPX 418HG-74C. This separates the milk into a high percentage fat content cream and skimmed milk. The hermetic separator treats the milk very gently which minimizes damage to the fat globules. 20 Also in this separator there is less risk of any air becoming mixed with the milk which reduces the risk of oxidation of the fat and breakdown of the globules. The 69% fat cream is then passed to a standardiser at a flow rate of approximately 312.5 gallons per hour (0.33 litres 25 per second). Step 4. The 69% fat cream is now passed to a standardisation vat where samples are regularly taken to ensure that the fat level is correct. The cream is then pumped to a heat exchanger by a Waukesha positive 30 displacement pump where the outlet pressure does not exceed 40 psi. Step 5. At the heat exchanger, pasteurisation of the ,*■*? cream, occurs. The heat exchanger used is an Alfa-Laval P13 RCF. Within this the cream is heated to a 35 pasteurisation temperature of 75.5°C and held at that in I1SFEB19&6 . 2140 93 8 temperature for 30 seconds. It is then cooled to 48.8°C. The flow rate of the cream through the heat exchanger is chosen to avoid two possible dangers. If the flow rate is too high then the flow is too turbulent and the globules of 5 fat are damaged. However, if the rate of flow between the hot plates of the heat exchanger is too low then it burns onto the pates of the heat exchanger and is damaged. Step 6. The cream then flows to a buffer tank to balance the flow between the heat exchanger and the filler. 10 The tank is designed to hold the minimum possible amount of cream for a minimum amount of time so that the possibility of contamination is minimised. The maximum buffer tank contents are 20 gallons (75.7 litres). 15 69% fat cream is liquid and is hot filled into polythene bags using a Bexuda precise volume liquid filler. The polythene bags are 2-ply which reduces the possibility of a leak by 50%. The polythene bags are "Liqui-Box" bags which are designed to hold up to 40 pints (18.93 litres) of 20 cream. However, in this method only 10 kilogrammes of cream is placed in each bag, so that each bag is approximately 53% full. The Bexuda filler works by a moving ram sucking the cream into a cylinder then injecting it into a bag. The ram filler is very precise but since it 25 is a positive displacement pump it minimises damage to the fat globules. After filling any trapped air is squeezed out of the bag and a cap applied to the opening in the bag and the bag is lain on a pallet ready to be taken into a refridgerator. 30 Step 8. The bags of heavy cream are then taken into the refrigerator where the temperature is held at 1.5°C (temperature t2 in Figure 1). Each bag is lain on a rack and the cream is cooled to a refrigerator temperature of 1.5°C. This cooling procedure completes the pasteurisation 35 of the cream. The period of pre-cooling is as long as Step 7. At 48.9°C (temperature tl in Figure 1) the o 2140 93 possible, and the time taken for this pre-cooling is around 18 to 19 hours. The cream is then plastic at around 1.5°C. In the case of cream containing around 69% fat, the cream is placed into the bags at a temperature of between 5 35°C and 55°C, preferably 48.9°C. Then the cream is precooled to just above 0°C, preferably 1.5°C. Step 9. The bags of cream are then placed into a Jackstone Froster MK5 Junior Horizontal Freezer. Each rack of the freezer is loaded with three slabs making a total 10 loading 150 kilogrammes. The plate which operates at a temperature of -40°C are closed by a hydraulic ram which ensures perfect contact between the cream and the plates. After approximately 20 minutes the slabs are solid and can be removed from the freezer and put in a cardboard carton. 15 Each carton holds 5 slabs. The width of each slab is only around 3.5 cm, the other dimensions being approximately 68 cm x 47.5 cm. The polythene allows a very good heat transfer and the dimensions of the slab allow rapid freezing. The slab area is intended to be such that the 20 surface area to volume ratio and hence the unit heat transfer rate are as high as possible without designing a slab so thin that it is too brittle and impractical. The freezing must be as rapid as possible since it minimises the time available for ice crystals to grow which would 25 damage the fat globules. Step 10. The cartons are then transfered to a deep freeze where the cream can be stored at -23°C until it is o required to be used. Since the cream has been created and handled so gently 30 and frozen so carefully the free fat content is kept to a minimum so that the product retains all the qualities of cream while having a life of at least 6 months. « ' * - ' "V ' ■' "L';f ■ ■•; f 'Y ; G 21 93 10 </div>

Claims

<div class="application article clearfix printTableText" id="claims"> CLAIMS ^ WHAT -tfWE CLAIM IS:-

1. A method of freezing a milk product consisting of fat in an aqueous component, the method comprising cooling 5 the product from a first temperature at which the fat contained therein is substantially liquid to a second temperature above the freezing point of the aqueous component in a manner such that a substantial portion of the fat has solidified throughout the volume of the 10 product, and thereafter cooling the product rapidly to a third temperature below the freezing point of the aqueous component and at which the product as a whole is substantially solid.

2. A method according to Claim 1 in which the second 15 temperature is in the range -5°C to 10°C.

3. A method according to Claim 1 or Claim 2 in which the rate of cooling from the first to the second temperature is less than 50°C per hour.

4. A method according to any of the preceding claims, 20 in which the milk product comprises a high fat content cream, the method including the additional step of placing the cream into its final packaging whilst hot.

5. A method according to Claim 4 in which the cream is packaged hot at a first temperature of greater than 25 35°C.

6. A method according to Claim 4 or Claim 5 in which the packaging for the cream comprises a polythene bag.

7. A frozen cream having more than 60% total fat content and having less than 10% free fat content as 30 measured by the Sudan III Dyebinding Method.

8. A method of freezing a milk product substantially as herein described with reference to the accompanying drawings.

9. A method according to Claim 1 and substantially as 35 herein described. - I' I P </div>