ICE CREAM

No one knows exactly when ice cream was first produced. Ancient manuscripts tell us that the Chinese liked a frozen product made by mixing fruit juices with snow – what we now call water ice. This technique later spread to ancient Greece and Rome, where the wealthy in particular were partial to frozen desserts.
After disappearing for several centuries, ice cream in various forms reappeared in Italy in the Middle Ages, most probably as a result of Marco Polo returning to Italy in 1295 after some 17 years in China, where he had acquired a liking for a frozen dessert based on milk. From Italy, ice cream spread through Europe during the 17th century, long remaining a luxury product for the royal courts. Industrial ice cream production began at the end of the 19th century when the first mechanical refrigerators were pioneered.

Categories of ice cream and related products

Ice cream and related products can be divided into a number of categories. As legislation varies from one country to another, the following should be regarded as a guideline only.
The fat content of ice cream typically determines the category to which it belongs. In some countries fat content has to exceed 9% to "qualify” for the ice cream category. Below this level, the product is typically called milk ice, whereas ice cream with more than 12 to 13% fat is often categorized as either luxury or premium.
The fat can be either of animal or vegetable origin. If the latter, legislation in a number of countries dictates that the product cannot then be called ice cream, but must be labelled, for example, non-dairy ice cream or frozen dessert.

Categories of related products

Sorbet is the term used for a frozen, typically fruit juice-based product with a certain amount of overrun. The mix passes through a continuous freezer where air is incorporated. Sorbet products are characterized by fresh eating properties and do not contain fat or milk solids-non-fat (MSNF). Due to the higher viscosity from the freezers, fruit pieces and other inclusions can also be added to the sorbet product before filling.
In order to obtain a final product with more body, ice cream producers also produce sherbet that contains a small amount of fat and/or MSNF. Sherbet still retains the fresh eating properties associated with sorbet.
Frozen yoghurt gained enormous popularity in the US during the 1980s due to its relatively low fat and calorie content. Weight and cholesterol watchers were delighted. Typically, a frozen yoghurt is a blend of standard ice cream mix and yoghurt with live bacteria, yoghurt ice cream tends to have a fresher taste than standard ice cream. Today also Greek yoghurt is popular due to the high protein content.
Water ice is a blend of sugar, fruit concentrates, stabilizers, flavour and colour. The finished mix is pasteurized and mostly filled into moulds (or pockets) on a rotary or in-line moulding machine. Freezing takes place in the pockets, which pass through a cold brine (salt solution) freezing zone. When the water ice is frozen , it is extracted from the pocket. Water ice sticks are typical children’s products and great varieties in colour and flavour are quite common. Combinations together with an ice cream core also make the products appealing for adults.
The development of extrusion technology has created a new category known as extruded water ice. Basically, a water ice containing a whipping agent is pumped to the continuous freezer, where air incorporation and a significant part of the freezing of water takes place in the continuous freezer cylinder before extrusion. The final product typically contains 20 to 30% air, is very fresh and has a clean flavour release.

Ice cream terminology

Depending on the filling method, ice cream products fall into one of the following categories:

Moulded

Ice cream or water ice mix is filled into moulds and frozen to produce stick novelties. After extraction, the products can be dipped in chocolate or other coatings.

Filled

Ice cream is filled into cups, cones or containers/tubs, often combined with more flavours and may be decorated with chocolate, cream, ripple and dry materials.

Extruded

Ice cream is typically extruded onto a tray by means of extrusion with a cutter. A wide variety of products can be produced including stick novelties, sandwiches, desserts, ball-top cones and so on. Extrusion technology provides the possibility to work with ice cream drawn at much lower temperature where the viscosity of the ice cream is high due to more water being frozen into ice crystals. This generates ice creams that are both smoother due to smaller ice crystals and creamier due to higher churning of fat compared to moulding and filling technologies. The higher viscosity also offers the possibility to work with detailed forms and flavours, decorations and coatings.

Preparing the ice cream mix

The basic steps of the ice cream process are shown in Figure 19.1.

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Fig. 19.1

The ice cream process.

Reception and storage of raw materials

The manner in which raw materials and ingredients are received varies from one factory to another depending on its facilities and capacity.
Dry products are usually delivered in bags. Bulk materials such as sugar and milk powder can be delivered in containers and blown into storage silos using compressed air.
Liquid products are often delivered in tankers. Milk products are stored below 5°C during storage, while sweetened condensed milk, glucose and vegetable fat must be heated to a relatively high temperature (30 to 50°C) to keep the viscosity low enough for pumping. Milk fat is delivered in the form of anhydrous milk fat (AMF) or blocks of frozen butter, which are melted prior to use and pumped into storage tanks, where a temperature of 35 to 40°C must be maintained.

Raw materials and ingredients

The ingredients used in ice cream production are:

• Fat
• Milk solids non-fat (MSNF)
• Sugar/non-sugar sweetener
• Emulsifiers/stabilizers
• Flavours
• Colours
• Other ingredients

Fat

Fat makes up about 10 to 15% of an ice cream mix and may be milk or vegetable fat. The fat gives creaminess and improves melting resistance by stabilising the air cell structure of the ice cream.
Milk fat is used in the form of whole milk, cream, butter or anhydrous milk fat (AMF). Milk fat can be replaced by vegetable fat, where refined or hydrogenated (hardened) coconut oil and palm kernel oil are most commonly used. The use of vegetable fat in ice cream is regulated by legislation in many countries.

Milk solids-non-fat (MSNF)

MSNF consist of proteins, lactose and mineral salts derived from whole milk, skim milk, condensed milk, milk powders and/or whey powder. In addition to its high nutritional value, MSNF helps to stabilize the structure of ice cream due to its water-binding and emulsifying effect. The same effect also has a positive influence on air distribution in the ice cream during the freezing process, leading to improved body and creaminess.
In a well-balanced recipe, the quantity of MSNF should always be in proportion to the water content. The optimal level is 17 parts MSNF to 100 parts water:

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Formula 19.1

Sugar

Sugar is added to increase the solids content of the ice cream and give it the level of sweetness consumers prefer. Ice cream mix normally contains between 12 to 20% sugar. Sugar is the common description for the saccharides, including the monosaccharides (i.e. glucose/dextrose and fructose), disaccharides (i.e. sucrose and lactose (milk sugar)) and starch derivates (i.e. glucose syrup, glucose/fructose syrup and high fructose syrup).
The consistency of the ice cream can also be adjusted by selecting different types of sugar. This makes it possible to produce ice cream that is easy to scoop.
In the production of sugar-free ice cream, sweeteners are used to replace sugar. Aspartame, acesulfame K and sucralose are the most commonly used sweeteners in ice cream and are applied in conjuction with a bulking agent such as malto-dextrin, poly-dextrose, sorbitol, lactitol, glycerol or other sugar alcohols.

Emulsifiers and stabilizers

Emulsifiers and stabilizers are typically used as combined products at dosages of 0.5% in the ice cream mix. Traditionally, these products were produced by dry blending, but today integrated products are preferred due to the improved dispersion and high storage stability.

Emulsifiers

Emulsifiers are substances that assist emulsification by reducing surface tension between two phases. There exists several different emulsifier types utilized in ice cream production, but the far most dominant and probably most effective emulsifier is the mono-/diglyceride of fatty acids. The mono-/diglyceride is often derived from a a vegetable fat (triglyceride) where fatty acid chains have been removed, creating a molecule that has a lipophilic end (fat loving) and a hydrophilic part (water loving). The mono-/diglyceride has two main functions during ice cream processing, where it assists the dairy protein displacement from the fat surface membrane, in order to improve the churning during the freezing process. The mono-/diglycerides also seed the crystallization of fat, which is essential for avoiding overchurning of the fat during the freezing process. Egg yolk is a well-known emulsifier, but is expensive and less effective than the most commonly used types.

Stabilizers

A stabilizer is a substance that has the ability to bind water when dispersed in a liquid phase. This is called hydration and means the stabilizer forms a matrix that prevents the water molecules from moving freely. Most of the stabilizers utilized for ice cream are large molecules derived from seeds, wood or algae/seaweed. Stabilizers are used in ice cream production to increase the viscosity of the mix and create body and texture. They also control the growth of ice crystals and improve melting resistance.

Flavours

Flavours are a very important factor in the customer’s choice of ice cream and can be added at the mixing stage or after pasteurization. The most popular flavours are vanilla, chocolate and strawberry.
In the EU, flavours are classified in three groups: natural, nature-identical and artificial. Nature-identical flavours are the most commonly used.

Colours

Natural or artificial colours are added to the mix to give the ice cream an attractive appearance. Local legislation exists in most countries regarding the use of colours in food.

Other ingredients

Many moulded and extruded ice cream products are coated with chocolate. Two types of chocolate coatings are used: real chocolate and chocolate compound. The cocoa mass and butter are replaced with a blend of cocoa powder and vegetable fat in the chocolate compound.
Ripples (sauces) are incorporated in ice cream for taste and appearance. They can also be applied for pencil filling and top decoration.
Dry ingredients are either added through an ingredient doser or as top decoration matter on cones, cups and bars. A great variety of products are used: chocolate, nuts, dried fruit pieces, candies, cookies, Smarties, caramel pieces, etc.
The mix composition and resulting ice cream are illustrated in Figure 19.2.

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Fig. 19.2

From ice cream mix to ice cream.

Mixing

The tank-stored raw materials are heated and blended to form a homogenous mix that is pasteurized and homogenized. Large production plants often have two mix tanks for each flavour with a volume corresponding to the hourly capacity of the pasteurizer, in order to maintain a continuous flow to the freezers. The dry ingredients, especially the milk powder, are generally added via a mixing unit, through which water is circulated, creating an ejector effect that sucks the powder into the flow. Before returning to the tank, the mix is normally heated to 50 to 60°C to facilitate dissolution. Liquid ingredients such as milk, cream, liquid sugar, etc. are measured into the mix tank.

Homogenization and pasteurization

In large-scale production the ice cream mix flows through a filter to a balance tank. From there it is pumped to a plate heat exchanger, where it is pre-heated to 73-75°C. After homogenization at 14 to 20 MPa (140-200 bar), the mix is returned to the plate heat exchanger and pasteurized at 83 to 85 °C for about 15 seconds. The pasteurized mix is then cooled to 5°C and transferred to an ageing tank.
The purpose of pasteurization is to destroy bacteria and dissolve additives and ingredients.
The homogenization process results in uniformly small fat globules which improves the whipping property and texture of the ice cream mix.

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Fig. 19.3

Principle of a continuous ice cream freezer.

Ageing

The mix must be aged for at least 4 hours at a temperature of 2 to 5°C with continuous gentle agitation. Ageing allows the milk proteins and water to interact and the liquid fat to crystallize. This results in better air incorporation and improved melting resistance.

Ice cream processing and packaging

Continuous freezing and ingredient feeding

Continuous freezing

The continuous freezer, Figure 19.4, has two major functions:

• To whip a controlled amount of air into the mix
• To freeze a significant part of the water content in the mix into a large number of small ice crystals

The ice cream mix is metered into the freezing cylinder by a gear pump. At the same time, a constant airflow is fed into the cylinder and whipped into the mix by a dasher.
Figure 19.3 shows the interior of the freezer cylinder with the dasher and beater. The refrigerant surrounding the cylinder generates the freezing process. The layer of frozen mix on the inside cylinder wall is continuously scraped off by the rotating dasher knife, and a second gear pump drives the ice cream forward either to an ingredient feeder or a filling machine.
The output temperature is -8 to -3°C depending on the type of ice cream product, where 30 to 55% of the water is frozen into ice crystals depending on the composition of the mix formulation.
The increase in volume following the incorporation of air in the mix is called overrun, and is normally 80 to 100%, corresponding to 0.8 to 1 litre of air per litre of mix.
Figure 19.4 shows the front of the freezer with, from bottom, mixing pump, cream pump and control panel.
The ice cream bar in figure 19.5 shows the texture of the frozen ice cream. Here the large air bubbles are covered with the crystallized fat globules that supports the air bubble structure. The unfrozen phase surrounding the air bubbles contains a freeze-concentrated mix of sugars, dairy solids and stabilizers. The unfrozen phase also contains the ice crystals.

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Fig. 19.4

Continuous ice cream freezer, automatically controlled.

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Fig. 19.5

Texture of an ice cream bar.

Ingredient dosing

The function of the ingredient doser, Figure 19.6, is to add ingredients continuously and accurately to the ice cream. The pump is designed to ensure the ingredients are gently fed into the ice cream flow from the freezer.

A wide range of ingredients can be accommodated by the feeder:

• Dry ingredients (e.g. nuts, cookies, chocolate)
• Soft ingredients (e.g. pieces of fruit, cookie dough, marzipan)
• Liquid ingredients (e.g. marmelade, jam, caramel)

The ingredient doser is designed to handle all three kinds of ingredient. Dosing accuracy is controlled by the use of ingredient-weighing cells under the ingredient hopper.

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Fig. 19.6

Ingredient doser for granulated products.

1. Funnel for ingredient
2. Ice cream feed
3. Lamella pump
4. In-line mixer

Filling lines

A filling machine fills ice cream, sorbet and water ice directly from the freezer into cups, cones and containers of varying design, shape and size.
Filling takes place by means of a time-lapse filler, a volumetric filler or an extrusion filler. In the case of extrusion filling, a cutting mechanism is provided.
Decoration with various ingredients is possible, including nuts, fruits, chocolate, jams or gum balls.
Lids are put on the packs before leaving the machine, after which they are passed through a hardening tunnel where final freezing to -20°C product core temperature takes place.
Before or after hardening, the products can be manually or automatically packed in cartons or bundles. Plastic tubes or cardboard cartons can be filled manually by means of a can-filling unit equipped to supply single or twin flavours.

Moulded stick novelty lines

Ice cream or water ice bars are made in special machines, also known as stick novelty freezers or moulding lines, where the ice cream or water ice is moulded in pockets. The ice cream is supplied directly from the continuous freezer at a temperature of approx. -3°C. The filled moulds are conveyed through a brine solution with a temperature of -40°C, which freezes the ice cream or water ice solution through the mould wall.
Sticks are inserted before the moulds are completely frozen.
The frozen products are removed from the moulds by passing them through a warm brine solution which melts the surfaces of the products and enables them to be removed automatically by an extractor unit. After extraction the products may be dipped in chocolate and coated with nuts or other dry ingredients before being transferred to the wrapping machine. Since the products are fully frozen, they can be taken straight to the cold store after wrapping and cartoning.
A variety of shaped products can be produced on stick novelty freezers as well as products with one, two or three flavours and products with an ice cream core covered by a water ice shell.
Figure 19.7 shows a moulded stick novelty freezer for manufacturing ice cream and water ice bars.

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Fig. 19.7

Rotary ice cream moulder.

Extrusion lines – tray tunnel systems

Extruded premium stick products are among the most classic products on a tray tunnel system. Indeed, the combination of an extrusion temperature of -8 to -6°C, hardening to approx. -20°C product core temperature and enrobing in real chocolate has produced one of the most successful products of recent years.
Extruded stick products are only one of the product types which can be produced on a tray tunnel system. Using different filling and handling equipment, a wide range of products can be produced, such as sandwich products, ball-top cones, filled wafer cups, ice cream cakes, ice cream logs and bite-size products.
The basic tray tunnel process is illustrated in Table 19.2.
Extruded ice cream products are normally produced on a tray tunnel extruder. The ice cream can be extruded directly onto trays in a variety of shapes and sizes, into a cup or cone or onto a sandwich wafer. An extrusion unit for large desserts is shown in Figure 19.8.
After decoration, the products are carried on the trays through a hardening tunnel, illustrated in Figure 19.9, where they are frozen to -20°C product core temperature. The products are then removed from the trays ready for wrapping and packing in cartons, either manually or automatically. Such a system is continuous. Depending on the capacity of the extruder and product type, 5.000 to 43.000 units can be produced an hour.

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Fig. 19.8

A tray tunnel extruder

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Fig. 19.9

Hardening tunnel

Wrapping and packaging

Cups, containers, etc. are either bundled or packed in cartons. Hand-held products like stick novelties, cones and bars are wrapped in a single or multi-lane wrapping machine before being packed in cartons. The design of the wrapping and packaging section of an ice cream processing line depends on the type of product and the capacity. Varying degrees of manual and automatic operation can be employed.

Hardening and cold storage

The manufacture of ice cream is not complete until it has been thoroughly hardened at a core temperature of around -20°C. For products made on an extrusion line or a stick novelty freezer, the hardening operation is included in the process. However, products packed immediately after freezing must be transferred to a hardening tunnel. The faster the hardening, the better the texture. After hardening, the products are transferred to the cold store, where they are stored on shelves or pallet racks at a temperature of -25°C. The storage life of ice cream depends on the type of product, the packaging, and maintenance of a constant low temperature. The storage period ranges from 0 to 9 months.

Examples of production plants

The illustrated plant layout, Figure 19.10, gives an idea of the product flow in ice cream production. This plant is capable of producing 5.000 to 10.000 litres of ice cream an hour.

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Fig.19.10

Ice cream plant for production of 5.000–10.000 l/h of various types of ice cream. 

1. Mix preparation
2. Ageing tanks
3. Continuous freezers
4. Bulk filling
5. Cone filling
6. Moulding
7. Extrusion
8. Cartoning
9. Storage