In this article we will discuss about the steps involved in manufacturing and storing ice cream. The steps are: 1. Selection of Ingredients 2. Figuring the Mix 3. Making the Mix 4. Pasteurizing the Mix 5. Homogenizing the Mix 6. Cooling and Ageing the Mix 7. Freezing the Mix 8. Packaging of Ice Cream 9. Hardening and Storage of Ice Cream.
Step # 1. Selection of Ingredients:
The selection of ice cream ingredients depends on:
(i) Availability of milk products;
(ii) Perishability of the products;
(iii) Convenience in handling;
(iv) Effect on flavour, body and texture of ice cream;
(v) Cost, and
(vi) Equipment available.
The selection of good ingredients is without doubt the most important factor in successful ice cream manufacture. A clean, fresh, creamy flavour in ice cream can be secured only by the use of products which have been carefully produced and handled.
Ice cream ingredients may be grouped into dairy and non-dairy products. Some dairy products supply the major source of fat, others the major source of milk-solids-not-fat and still others supply both fat and milk-solids-not-fat. The non-dairy products include sweetening agents, stabilizers and emulsifiers, flavours and colours, fruits and nuts, etc.
The common ingredients used in each category have been listed below:
I. Dairy Products:
Source of Fat:
(i) Sweet cream—this is the most desirable concentrated source of fat for use in a mix;
(ii) Frozen cream;
(iii) Plastic cream;
(iv) Unsalted butter;
(v) Butteroil.
Sources of Milk-Solids-Not-Fat:
(i) Skim milk;
(ii) Skim milk powder—this is most frequently used in the spray dried or flaked form;
(iii) Condensed skim milk (plain/sweetened);
(iv) Sweet cream buttermilk.
Sources of both Fat and Milk-Solids-Not-Fat:
(i) Whole milk;
(ii) Whole milk powder;
(iii) Condensed whole milk (plains/sweetened);
(iv) Evaporated milk.
II. Non-Dairy Products:
Sweetening Agents:
(i) Cane or beet sugar (sucrose) sweetest common;
(ii) Corn sugar (dextrose);
(iii) Corn syrup solids (dextrose + maltose);
(iv) Corn syrup;
(v) Invert sugar (glucose + fructose);
(vi) Saccharin.
Stabilizers:
(i) Gelatin—of animal source; orthodox vegetarians object to its use in ice cream;
(ii) Sodium alginate/Dariloid (modified form)—this is of vegetable origin and widely used;
(iii) Guar gum—this is of Indian origin;
(iv) Others.
Emulsifiers:
Mono-or di-glycerides of fat-forming fatty acids.
Flavours:
(i) Vanilla—this is the most popular flavour all over the world;
(ii) Chocolate;
(iii) Strawberry;
(iv) Pineapple;
(v) Lemon;
(vi) Banana;
(vii) Mango;
(viii) Orange, etc.
Colours:
(i) Yellow;
(ii) Green;
(iii) Pink, etc.
Egg Solids:
Yolk solids—improve whipping ability.
Fruits and Nuts:
(i) Apple;
(ii) Banana;
(iii) Mango;
(iv) Pineapple;
(v) Grape;
(vi) Almond;
(vii) Pistachio;
(viii) Cashewnut;
(ix) Walnut,
(x) Groundnut.
Step # 2. Figuring the Mix:
A know ledge of calculation of ice cream mix is helpful in properly balancing a mix, in establishing and maintaining uniform quality and in producing ice cream that conforms to legal standards. Ice cream mix may be divided into two groups, namely simple and complex.
Simple mixes require the least calculation and are made of ingredients each of which supplies one constituent. Complex mixes are more difficult to calculate. They include mixes where at least one constituent is obtained from two or more products. (Complex mixes require the use of the Pearson’s Square, the Serum Point or Algebra method.)
Before a mix can be calculated, it is necessary to:
(i) Decide upon the composition of the mix to be made;
(ii) Decide on the amount of mix to be made in the batch at one time. (Whatever the amount, it may be calculated on the basis of 100 kg., if so desired.)
(iii) Choose from the available ingredients those that will give the desired quality characteristics and composition at the lowest cost;
(iv) Be familiar with the composition (i.e., the analysis) of ingredients to be used.
Since ice cream contains a higher percentage of fat than fluid milk, one of the mix ingredients must be high in fat. Cream, plastic cream, or butter is usually used for this purpose. Ice cream also contains more solids-not-fat than milk; and a source high in these solids, such as condensed skim milk, condensed whole milk or (preferably skim) milk powder is needed.
Fluid milk is usually used as the basic ingredient in ice cream mix. Calculation of the mix involves finding the required weights of these ingredients along with the necessary sugar and stabilizer.
The calculations are simplified by first finding the weights of the ingredients needed to make 100 kg. of the desired mix. With the algebraic method; symbols such as X, Y, Z are used to represent the weights of dairy ingredients required for a 100 kg. batch of mix.
These symbols are then used in writing the three equations that express the weights of fat, serum solids (solids-not-fat) and the total weight of dairy ingredients needed for 100 kg. of mix. The method is applicable to all types of mix problems.
A typical problem and its solution follows:
Problem:
Prepare an ice cream mix containing fat—10 per cent, serum solids—11 percent, sugar—14.5 percent and stabilizer—0.3 per cent. Given- whole milk testing 6.8 per cent fat and 9.6 percent serum solids, cream testing 40 per cent fat and 5.4 per cent serum solids, skim milk powder testing 0.5 per cent fat and 97 per cent serum solids, sugar and stabilizer (cent per cent dry matter).
Solution:
Step # 3. Making the Mix:
In order to make good ice cream, the milk products and other ingredients must first be selected and combined so as to produce the desired body and a delicately blended flavour. Obviously the selection of good, wholesome ingredients and calculation of a satisfactory composition precede the mixing of the ingredients in a vat, where they can be heated to facilitate dissolving, blending and pasteurizing.
The mixing process may range in scope from the small batch operation, where each ingredient is weighed or measured into the pasteurizing vat, to the large automatic continuous operation where many of the ingredients are metered into the batch.
Continuous mix-making procedures may be quite variable and some such operations may actually be modifications of the batch operation. (Modern electronic computers can be used to control the cost and quality of ice cream mixes by the mathematical technique known as linear programming.)
The order in which ingredients are added is as follows- all liquid ingredients are placed in the jacketed vat provided with a power stirrer, and the agitation and heating started at once. The dry ingredients, including skim milk powder, sugar and stabilizer (with a few exceptions), are added while the liquid material is agitated and before the temperature reaches 49°C/120°F.
Proper suspension to avoid lumpiness of the dry ingredients may be obtained by either mixing the dry material thoroughly with part of the sugar before slowly adding it to the liquid, or by sifting/slowly adding these substances to the liquid.
If gelatin is the stabilizer used, it is best added after it is thoroughly mixed with an equal volume of sugar, and before the liquid material reaches 49°C (120°F); alternatively, it can be sprinkled on the surface of the cold liquid and allowed to soak before the mixture is heated, or soaked in water and then the mixture heated to completely dissolve the gelatin, which is usually added to a warm (38- 49oC/100-120°F) mix.
If sodium alginate (Dariloid) is used, it should not be added until the temperature of the liquid material has reached at least 66°C (150°F). The dry Dariloid is not allowed to soak but is stirred up with cold water and immediately dumped into the hot mix.
When butter, plastic cream, frozen cream or other frozen products are used, they should be cut into fairly small pieces and added after time has been given to allow for complete melting before the pasteurizing temperature is reached. With a few exceptions, colouring and flavouring materials are added when the mix is frozen.
Step # 4. Pasteurizing the Mix:
Proper pasteurization of all ice cream mixes should be compulsory because this process destroys all pathogenic or disease-producing bacteria, thereby safeguarding the health of the consumer. Pasteurization has come to be considered as highly desirable and requires only slight additional expense, since the homogenization process can be best accomplished at the pasteurization temperature level.
The advantages of pasteurization are:
(i) It renders the mix completely free of pathogenic bacteria;
(ii) It dissolves and helps to blend the ingredients of the mix;
(iii) It improves flavour;
(iv) It improves keeping quality, and
(v) It produces a more uniform product.
Proper pasteurization consists in rapidly heating the mix to a definite temperature, holding it at that temperature for a definite minimum period of time and then rapidly cooling it to below 5°C (40°F).
The ISI specifications (IS: 2802, 1964) for pasteurization temperature-time combination for ice cream mix are as follows:
Batch method— 68.5°C (155°F) for not less than 30 min.
HTST method— 80.0°C (175°F) for not less than 25 sec.
There is a trend towards the higher temperature process. In the batch system, the mix is usually heated and held before going to the homogenizer and from there passes over a cooler. The heating and holding may be accomplished in the vat used for mixing the ingredients.
Step # 5. Homogenizing the Mix:
Homogenization of the ice cream mix is essential. The main purpose of homogenization is to make a permanent and uniform suspension of the fat by reducing the size of the fat globules to a very small diameter, preferably not more than 2 microns.
The advantages of homogenization are:
(i) It prevents fat separation during ageing;
(ii) Produces more uniform ice cream with a smoother texture;
(iii) Improves whipping ability;
(iv) Shortens ageing period;
(v) Decreases the risk of churning occurring in the freezer, and
(vi) Leads to the use of slightly less stabilizer.
Note:
Butter, butteroil, plastic cream and frozen cream can be used in the mix only when the latter is homogenized.
The mix is usually homogenized at temperatures from 63 to 77°C (145° to 170°F). At low temperatures, homogenization increases the formation of clumps of fat globules, as also the viscosity and the freezing time in batch freezers.
The pressure required for homogenization depends upon several factors: desired viscosity; composition of the mix; stability of the mix; temperature used; and construction of the homogenizing machine. A pressure of 2,500 to 3,000 psi with one valve, or 2,500 to 3,000 psi at the first stage and 500 psi at the second will usually give good results for an average mix (with 3 to 12 per cent fat).
Step # 6. Cooling and Ageing the Mix:
Cooling the mix immediately after homogenization to 0-5°C (32-40°F) is essential, after which it should be held in ageing tanks until used. Coolers of the surface or cabinet types are generally used for this purpose. Ageing the mix before freezing has been practised since the inception of the ice cream industry.
Ageing refers to holding the mix at a low temperature for a definite time before freezing. The ageing temperature should not exceed 5°C (40°F). The ageing time under average commercial conditions may be 3 to 4 hours, except for sodium alginate which requires no ageing.
Ageing produces the following results:
(i) It improves the body and texture of ice cream;
(ii) Improves whipping capacity of mix;
(iii) Increases maximum overrun;
(iv) Increases melting resistance.
Step # 7. Freezing the Mix:
When the mix has been properly aged, it is ready for the freezing process, which follows immediately. Freezing is one of the most important operations in the making of ice cream for upon it depends the quality, palatability and yield of the finished product.
The freezing process may, for convenience, be divided into two parts:
1. The mix, with the proper amount of colour and flavouring materials generally added at the freezer, is quickly frozen while being agitated to incorporate air in such a way as to produce and control the formation of small ice crystals—so necessary to give smoothness in body and texture, palatability and satisfactory overrun in the finished ice cream.
2. When the ice cream is partially frozen to a certain consistency, it is drawn from the freezer into packages and quickly transferred to cold storage rooms, where the freezing and hardening process is completed without agitation.
Note:
Although the general procedure of the freezing process is easily learnt, the correct handling of the details to produce a uniform product requires expert judgement and almost split second timing a technique acquired only through experience guided by continual and careful study.
Classification of Freezers:
Freezers may be classified as follows:
(i) Batch Freezer:
Horizontal, Direct Expansion (Freon or Ammonia).
(ii) Continuous Freezer:
Horizontal, Direct Expansion (Freon or Ammonia).
(iii) Soft Serve Freezer:
Batch and Automatic Continuous Freezers of the Direct Expansion type.
Importance of Rapid Freezing:
Rapid freezing is essential for a smooth product because ice crystals that are formed quickly are smaller than those formed slowly. Therefore, it is desirable to freeze and draw the mix from the freezer in as short a time as possible. A continuous freezer accomplishes this in a few seconds, while batch freezers take 6 to 10 minutes, depending on several factors.
Also, since freezing continues after the ice cream is placed in the hardening rooms, the ice crystals formed during the hardening period are larger because they are formed more slowly than in the freezer. For this reason, it is desirable to freeze the ice cream as stiffly as possible and yet have it liquid enough to be able to draw it out of the freezer.
Factors Influencing Freezing Time:
I. Mechanical:
(i) Type and make of freezer;
(ii) Condition of freezer wall and blades;
(iii) Speed of dasher;
(iv) Temperature of refrigerant;
(v) Velocity of refrigerant passing around freezing chamber;
(vi) Overrun desired;
(vii) Temperature at which ice cream is drawn;
(viii) Rate at which freezer is unloaded.
II. Character of Mix:
(i) Composition of the mix;
(ii) Freezing point of the mix;
(iii) Acidity content of ingredients;
(iv) Kind of ingredients, particularly those carrying fat;
(v) Methods by which the mix is processed;
(vi) Kind and amount of flavouring materials added.
Note:
The influence of type of freezer on the freezing time and temperature is shown in Table 6.3.
Changes which Take Place during the Freezing Process:
The function of the freezing process is to freeze a portion of the water of the mix and to incorporate air into the mix. This involves: lowering the temperature of the mix from ageing temperature to the freezing point, freezing a portion of the water in the mix, incorporating air into the mix, and cooling the ice cream from the temperature at which it is drawn from the freezer to hardening-room temperature.
The temperature of the mix which is put into the freezer drops very rapidly while the sensible heat is being removed and before any ice crystals are formed. This process takes less than a minute or two. Meanwhile, the rapid agitation reduces the viscosity by partially destroying the gel structure and by breaking up the fat- globule clusters. The gel structure may restore itself partially during the hardening process in the hardening room. Also, the rapid agitation hastens incorporation of air into the mix.
When the freezing point is reached, the liquid water changes to ice crystals which appear in the mix. These ice crystals are practically pure water in a solid form, and thus the sugar as well as the other solutes become more concentrated in the remaining liquid water.
Increasing the concentration of these solutes slightly depresses the freezing point of the liquid portion, so that the temperature must be lowered before any more ice crystals will form.
Thus, in freezing ice cream, the freezing point is continually being lowered by the formation of ice crystals; and while the temperature drops, more ice crystals are formed, increasing the concentration of sugar and other solutes in the remaining liquid water until the concentration is so great that further freezing will not occur. Thus all the water does not freeze even after long periods in the hardening room.
Table 6.4 shows the approximate percentage of water frozen in the mix at various drawing temperatures.
The first phase of the freezing process accounts for the freezing of 33 to 67 per cent of the water, depending on the drawing temperature. The second phase, i.e. the hardening process, may then account for freezing another 23 to 57 per cent, depending on the drawing temperature.
Batch Freezing:
The Batch Freezer consists mainly of a freezing chamber and a dasher. The dasher consists of two parts, viz., scraper blades and the beater. The dasher performs the following functions: aids in transmission of refrigeration by keeping the mix in continuous contact with the freezer walls, scrapes freezer walls free of ice crystals; beats in air and continually pushes mix forward, which is essential for unloading the freezer. (It is important that the dasher be in proper alignment, and the blades sharp.)
Note:
The temperature of the refrigerant is rather important and may vary from —23 to —29°C (—10 to — 20°F) to ensure rapid formation of ice crystals, which results in a smoother ice cream. However, the freezing should be slow and ought to permit incorporation of the desired amount of air, since this affects the body and texture of ice cream as well.
The freezing procedure in the batch freezer consists of:
(i) Preparation of the Freezer:
The freezer parts should be inspected to ensure that they are clean and dry. Then they should be assembled in accordance with the instructions from the manufacturer. The operator’s hands should be clean.
After the freezer is assembled, it should be sanitized either by running hot water at 82°C/180°F followed by a cold water rinse; or by a cold solution of a chemical sanitizing agent (such as chlorine solution containing 100 to 200 ppm available chlorine). After the addition of hot water, cold water or chlorine solution, the dasher should be revolved a few times and the particular liquid drained out.
(ii) Adding the Mix:
It is always desirable that the temperature of the mix be below 5°C (40°F) when it goes into the freezer. Colour and flavour should be accurately measured, and poured into the mix. The total volume of the mix, flavour and colour should be about half the total capacity of the freezing chamber.
The flavour and colour must be so added as to be uniformly distributed. Acid, fruits, nuts, etc., should be added only after some ice crystals have formed. The freezer should be operated uniformly as to speed, refrigerant, temperature, etc.
(iii) Freezing and Incorporation of Air:
The mix should now run into the freezer, the dasher should be started and then the refrigerant turned into the freezing chamber. This sequence must be preserved to avoid damage to the machine.
To prevent the scraper blades from becoming blunt and wearing out rapidly, the dasher must never be operated when there is no mix in the freezer; neither should the refrigerant be turned on unless the dasher is in motion. This order of operation and these precautions apply to all freezers, regardless of size, type, or installation.
(iv) Drawing of Ice Cream:
When ice cream is drawn from the freezer, it should be sufficiently stiff to ‘ribbon’ or almost hold its shape, and yet soft enough to ‘settle’ or lose its shape within a minute or two. The container or package into which the ice cream is drawn should be cooled sufficiently to prevent the ice cream melting.
This melting may cause a coarse and icy texture around the edge of the package and also loss of overrun. Formation of air- pockets within the container should also be avoided. The freezer should be emptied rapidly to prevent wide fluctuations in overrun in the packages. Therefore, avoid filling small packages directly from the batch freezer.
Note:
When the freezer is not going to be used for 2 hours or more, it should be taken apart, cleaned and sanitized.
Continuous Freezing:
Continuous freezers are of two types in the market today:
1. One of these has two pumps just behind the freezer barrel. One pump pulls the mix from the supply tank and pumps it to the second one. This second pump operates at a speed approximately twice as great as that of the first pump. This has the effect of creating a partial vacuum between the two pumps.
The valve device in the piping between the pumps allows for air to be sucked in. The amount of air incorporated can be regulated very accurately, and once it is in the system, it cannot get out provided the machine is operating normally. The mix and air are pumped into the freezing barrel by the second pump. As the mix freezes, semi-frozen ice cream is forced out from the front of the machine.
2. The second type operates in a very similar fashion as the first. However, here the two pumps are at the front. One pump pushes the mix into the rear end of the freezer and the other helps to pull out the semi-frozen ice cream from the freezer. There is a separate air pump that forces air directly into the freezer barrel. This second kind of continuous freezer is just as versatile as the first one.
Some important advantages of the continuous freezing method are:
(i) Less stabilizer is needed;
(ii) A shorter ageing time is possible;
(iii) Less flavouring material is needed;
(iv) Smoother ice cream is obtained;
(v) Tendency towards development of sandiness decreases;
(vi) A more uniform yield is obtained with less variation between packages.
Some disadvantages are:
(i) Great care must be taken in handling the parts of the machine which lit with very small clearance;
(ii) Operators and mechanics must have special experience and training in order to avoid operational difficulties and possible damage to the equipment.
(iii) It is difficult to prevent excessive overrun;
(iv) There is a greater tendency for ice cream to shrink in volume after hardening:
(v) Initial cost of the equipment is high.
Note:
The processing procedures in the manufacture of ice cream have become fully mechanized in developed countries and various degrees of automation of these processes are now common.
Overrun in Ice Cream:
Overrun is usually defined as the volume of ice cream obtained in excess of the volume of the mix. It is usually expressed as a percentage. This increased volume is composed mainly of the air incorporated during the freezing process.
The amount of air which is incorporated depends upon the composition of the mix and the way it is processed; and is regulated so as to give that percentage of overrun or yield which will give the proper body, texture and palatability necessary to a good quality product. Too much air will produce a snowy, fluffy, unpalatable ice cream while too little, a soggy, heavy product.
In order to secure uniform overrun and yield, the following points should receive attention:
(i) Uniformity in refrigerant temperature and rate of flow of refrigerant;
(ii) The use of overrun testers;
(iii) Uniform make, etc., of freezer for the operator;
(iv) Not too many freezers per operator;
(v) Hopper system for filling containers if batch freezers are used.
The control of overrun is very important and should be as nearly constant as possible from batch to batch and from day to day. The correct overrun percentage depends upon the kind and composition of product and freezing equipment. The desirable percentage overrun in different ice creams is given in Table 6.5.
There are two basic or fundamental methods for calculating percentage overrun, viz., by volume and by weight:
Step # 8. Packaging of Ice Cream:
When ice cream is drawn from the freezer, it is usually collected in containers which give it the desired shape or size for convenient handling during the hardening and marketing processes. The chief requirements for packages of ice-cream are- protection against contamination; an attractive appearance; ease of opening and reclosure; and ease of disposal. Protection against moisture loss and temperature fluctuations is also desirable.
Broadly speaking, ice cream packages are of two types, viz., multi-service and single-service:
(i) Multi-Service:
This type of container is seldom used in modern operations; a tinned-steel can is an example. They are cleaned, sanitized and used repeatedly.
(ii) Single Service:
This type of package has met with increasing favour, especially during recent years. The quantity filled may be for bulk or retail sale. Most bulk ice cream is packaged in fibre board cartons coated with wax or polythene-wax blends for protection against moisture and oxygen.
The most recent trend for such factory- filled carry-home packages is the all-plastic cylindrical container with a recloseable lid. Retail ice cream may be packaged in cups, stick or bars. Cups may be of paper or cardboard, treated as above to make them impervious to moisture. The recent trend is towards multi- packs (such as six packs) in polythene bags, multi-bars in foil cartons, etc.
Step # 9. Hardening and Storage of Ice Cream:
When ice cream is drawn from the freezer and put into the container to be placed in the hardening room, it has a semi-fluid consistency not stiff enough to hold its shape. The freezing process is, therefore, continued without agitation during hardening until the temperature of the ice cream reaches — 18°C (0°F) or below. Here, as in the freezer, quick hardening is desirable, since slow hardening favours large ice crystals and coarseness. Most operators allow a hardening time of 12 hours.
The factors affecting hardening time are:
(i) Size and shape of the ice cream package;
(ii) Speed of (cool) air circulation;
(iii) Temperature of cooling air;
(iv) Section of the hardening room;
(v) Temperature of ice cream drawn from the freezer;
(vi) Composition of mix, and
(vii) Percentage overrun in the ice cream being hardened.
The different hardening systems are given in Table 6.6.
After the ice cream is hardened, it may be immediately marketed, or it may be stored for a week or two at the most. The operation of storage rooms is the same as for the hardening room with two exceptions: the temperature should be maintained uniformly at a point between -23°C to -18°C (-10°F to G°F) and the packages should be piled very close, to delay changes in ice cream temperature.
Some precautions to be observed in the operation of hardening/storage rooms are:
(i) Provide facilities for calling for aid, should the operator accidentally be trapped inside;
(ii) Keep both an axe and a sledge hammer in a definite place just inside the door;
(iii) Avoid fluctuations in temperature.