GB2166939A - Method of freezing a milk product - Google Patents

Abstract

A method of freezing a milk product comprises cooking the product relatively slowly from a first temperature t1 at which it is liquid to a second temperature t2. The second temperature t2 is chosen to lie on the high temperature side of the freezing point, or of the freezing range of the aqueous component of the product, and at this temperature t2 a substantial portion of the fat is solid before any ice crystals have formed in the product, thus minimising membrane damage. The product is then cooled rapidly from the second temperature t2 to a third temperature t2 at which the product as a whole is solid. <IMAGE>

Description

SPECIFICATION Method of freezing a milk product The present invention relates to an improved method of freezing a milk product such as cream, particularly cream having a high fat content.

Cream consists of an emulsion offat 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 theglobulesflowsoutthroughthedamagedmembrane and becomes free fat. This is what happpens 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 desirableto reduce this membrane damage as much as possible in orderto minimise the concentration of freefat in the final thawed product. This ensures that the thawed cream retains the colour, texture and taste offresh cream.

The freezing process itself causes membrane damage, and this is particularlytroublesome 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.

Thus membranedamage may be caused bythe 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 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 the rate of solidification ofthe fat with respect to temperature, which exhibits a marked peak between 0 C and 30"C.

In currently known methods offreezing cream, the cream is cooled very rapidly, so that, in effect the cream is taken along the peak2a and beyond it very rapidly. This results in considerable membrane damage, the final product having, when thawed, an inferiorflavour, colour and texture when compared to fresh cream. Methods of this type include drum freezing in which the cream is frozen 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 cream.Also, although these methods produce an acceptable final product in the case of the lowerfat creams (less than 60% fat), they are not suitable for use with higherfat content creams (greaterthan 60%). The acceptability ofthe final thawed product is primarily determined by the 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 offree fat.

According to a first aspect ofthe present invention there is provided a method of freezing a milk product consisting offat in an aqueous component, the method comprising cooling the productfrom afirst 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 throughoutthe 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.

It should be appreciated thatthefreezing point of 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 ofthe aqueous component occurs. The actual upperand lower limits ofthis range depend on the type of product being frozen, and in particularthe types and concentrations ofthe salts contained in the product. In the case where there is a freezing range of the aqueous component, the second temperature should lie outside this range, on the high temperature side.Thus, by cooling the cream slowlyfrom a first temperaturetoa second temperatu re above the freezing pointofwater,a substantial portion ofthefat in the cream has solidified before any ice crystals have formed in the cream. it is thought that this makes the globule membranes more resistantto damage. The cooling fromthefirsttemperatureto 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 a temperature below the freezing point ofthe aqueous component.This could be carried out in a number of differentways. An example of one way in which this may be achieved is to put the product in a controlled temperature environment at or close to the desired second temperatu re so that the product is cooled relatively slowly from the first to the second temperature. Then, by rapidly cooling the cream from the second tempernature to the third temperature until it is actually solid to thetouch,theformation of ice crystals is minimised, since the latter is time dependant.

Preferably the second temperature is in the range -50Cto 100C.

More preferably the second temperature is between Oto 5"C.

The most preferred val ueforthe second temperature is 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 firstto thesecondtemperature is less than 20"C per hour.

The most prefered rate of cooling from the first to the second temperature is2.6'C per hour.

The first stage may take place over a period of 18 hours, oralternatively it maytake place in a shorter period when a blast cooler is used.

This method has been found to be ideal forfreezing any milk productwhich includesfat in globules such as creams, yoghurts and milks. The method is very effectiveforfreezing all types of cream with any level offat contained therein. Although, as previously described there already exists several methods of freezing creams, none have yet been found acceptable for freezing creams having morethan 60% fatwithin them. The present invention is particularly appropriate, but not limited to, creams having more than 60% fat content In the United Kingdom, all milk products for consumption in the home markethaveto be heat treated in accordance with strict government regulations. Previously cream has been made, heattreated and cooled, and when cooled, placed in its final packaging.However, this allows an opportunity for contamination ofthe cream by bacteria.

In the case of high fat content creams, the method according to the present invention preferably includes the additional step of placing the cream into its final packaging whilst hot, and subsequently cooling the packaged productfrom thefirsttem peratureto the second temperature, and thereafterto the third temperature. This means that the hot cream serves to steriliseorpasturise its packaging and is removed from sources of contamination directly after its heat treatment.

Preferably, in the case of high fat content creams, the cream is packaged hot at a first temperature of greaterthan 35"C. There is no real upper limitto the temperature atwhich the cream may be packaged, but this would ultimately be limited by the handling ofthe cream and the properties ofthe packaging material.

More preferably, the cream is packed at a tempera ture of 48.9 C.

Preferably the packaging forthe cream comprises a polythene bag, which may be easily pasturised.

The filled polythene bags, after having being cooled to the second temperatu re, are then placed in a plate freezerwhich includes a hydraulic ram to compress the freezing plate onto the bag to maintain perfect contactwith the bag. This isto reduce the thickness of the slab of cream as much as possible to reduce the time takento freeze the cream and to prevent formation of an insulating layer, butto preventthe slabfrom becoming so brittle that it snaps.

Thus the cream or milk productwhen 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 the packaging ofthe cream or within the innerstructure ofthe 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 lowfree-fat content and therefore the taste, texture and colouroffresh cream.

If such a high fat content cream is to be frozen in this way it is also desirableto producethe cream with as manyfatmembranes intact as possible. Care has therefore to be taken in the production of the cream.

It is found that if high fat content cream is made 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 processes and goes through more machines and there is therefore much more opportunity for damage to the internal structure ofthe creamtooccur.Theone-step method of making cream is preferably by using a hermetic separator such as the Alfa-Laval MRPX 418HG-74C. This onestep method also minimises contamination and air entrapment in the cream. This is important since it extends the shelf life ofthe cream.

The frozen high fat content cream has proved to be extremely useful in the form of single, whipping or double cream, and also as an ingredient in many products containing fats such as ice cream, yoghurt, and cream cheese.

According to a second aspect ofthe present invention there is provided a frozen cream having more than 60% total fat content and having less than 10% free fat content as measured bythe Sudan Ill Dyebinding Method.

Preferably, the frozen cream has more than 65% total fat content.

More preferably, the frozen cream has 69% total fat content.

The method of the present invention may be equally used with creams with lower fat content, and in this case the cream need not be packaged whilst hot, as lowerfat creams will still flow at lower temperatures.

An example of a method of manufactu ring and freezing cream having 69% fat in accordance with the invention will now be described with reference to the accompanying drawings, in which :- Figure lisa schematic graph of a solidification curve of cream; and Figure2 isaflowdiagram ofthe method.

Referring to Figures 1 and 2, the steps ofthe method ofthe present invention will now be described in more detail.

Step 1. Milk is stored in a bulktankwhich is mounted on load cells so thatthe weight of milk required to product each kilogram ofthe cream can be accurately determined. The fat contained in the milk is approximately 3.8% and the rate offlow of the milkforthe bulktank is approximately 5500 gallons per hour (5.78 litres per second).

Step 2. The milk is passed to a heat exchangerwhere it is warmedto 48.9"C.

Step 3. The milk is passed to the separator and skimmed milk is returned as a by-product. The separator is a hermetic separator, the Alfa-Laval MRPX 41 8HG-74C. This separates the milk into a high percentage fat content cream and skimmed milk. The hermetic separatortreats the milkvery gently which minimizes damage to the fat globules. Also in this separator there is less risk of any air becoming mixed with the milk which reduces the risk of oxidation ofthe fat and breakdown ofthe 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 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 Waukosha positive displacementpumpwherethe outlet pressure does not exceed 40 psi.

Step 5. At the heat exchanger, pasteurisation ofthe cream, occurs. The heat exchanger used is an Alfa-Laval P13 RCF. Within this the cream is heated to a pasteurisation temperature of 75.5 C and held at that temperature for 30 seconds. It is then cooled to 48.8"C.

The flow rate ofthe cream through the heat exchanger is chosen to avoid two possible dangers. If the flow rate is too high then theflow is too turbulent and the globules of fat are damaged. However, if the rate of flow between the hot plates ofthe heat exchanger is too low then it burns onto the pates ofthe heat exchanger and is damaged.

Step 6. The cream then flows to a buffertankto balance the flow between the heat exchanger and the filler. The tank is designed to hold the minimum possible amount of cream for a minimum amount of time so thatthe possibility of contamination is minimised. The maximum buffertankcontents are 20 gallons (75.7 litres).

Step 7.At 48.9"C (tem peratu re tl in Figure 1) the 69% fat cream is liquid and is hotfilled 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 upto40 pints (18.93 litres) of 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 cylinderthen injecting it into a bag. The ramfilleris very precise but since it is a positive displacement pump it minimizes damage to the fatglobules.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 betaken into a refrigerator.

Step 8. The bags of heavy cream are thentaken into the refrigerator where the temperature is held at 1.5 C (temperaturet2 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 ofthe cream. The period of pre-cooling is as long as possible, andthotimetakenforthis 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 35"C and 55"C, preferably48.9 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 M K5 J u nio r Horizontal Freezer.

Each rack of the freezer is loaded with th ree slabs making a total 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 fromthefreezerand put in a cardboard carton. Each carton holds 5 slabs. The width of each slab is only around 3.5 cm, the other dimensions being appproxmately 68cm x 47.5 cm.The polythene allows a very good heattransfer and the dimensions of the slab allow rapid freezing. The slab area is intended to be such that the 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 impracticle. The freezing must be as rapid as possible since it minimises the time available for ice crystals to growwhich wouid 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 required to be used.

Sincethe cream has been created and handled so gently 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.

Claims (9)

1. A method offreezing a milk product consisting of fat in an aqueous component, the method comprising cooling the productfrom a firsttemperaturo at which the fat contained therein is substantially liquid to a second temperature abovethefreezing point of the aqueous component in a mannersuchthata substantial portion of theft has solidified throughout the volume ofthe product, and thereafter cooling the productrapidlyto athirdtemperature belowthe freezing point ofthe aqueous component and at which the product as a whole is substantially solid.

2. A method according to Claim 1 in which the second temperature is in the range-S0Cto 10 C.

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

4. A method according to anyofthe preceding claims, 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 35"C.

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

7. Afrozencroam having morethan 60%totalfat content and having less than 10% free fat content as measured by the Sudan Ill Dyebinding Method.

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

9. A method according to Claim 1 and substantiallyas herein described.