Ice cream is one of the great delights of summer especially its warmer days. It is consumed throughout the world and is viewed as a delicacy.
The Roman emperors would have slaves travel into the mountains to collect snow where it was flavoured in the cucina to make a basic ice-cream. The food developed very much in Europe and was taken to other parts of the world including the USA.
In the USA, in 1851, a wholesale ice cream factory was started in Baltimore, Maryland. In the USA nowadays, people consume something like 1.5 billion gallons of ice cream annually. The USA was producing something of the order of 1.53 billion gallons or 5.79 billion litres of the stuff in all sorts of guises in 2011 (USDA reports, 2011). It is the leader in this dessert with phenomenal growth expected through to 2050.
The number of products in the ‘better for you’ frozen desserts category has really expanded in the last few years. There were 136 high protein, low sugar ice cream launches in 2018 for example alone according to Mintel.
Halo Top overtook Ben and Jerry’s to become the highest grossing pint of ice cream in the USA.
Here, we describe in brief detail the structure and manufacture of this dessert. Numerous texts are available on the subject by Arbuckle (2013).
What is Ice Cream?
Ice cream is a frozen aerated mixture of water, milk solids and cream, usually sweetened with sugar. It can also be described as an aerated emulsion or a partly frozen foam containing 40 to 50% of air by volume. A foam is defined as an agglomeration of gas bubbles separated from each other by thin liquid films (Bikerman, 1953). The other description is a complex colloidal material containing fat globules and air bubbles which are dispersed in a partly frozen aqueous dispersion.
A 3rd description was provided by Scholten (2014) who pictured it as ice crystals, air cells and a network of coalesced fat droplets. These are all enclosed in a tick continuous phase that makes up the microstructure of ice cream.
The continuous phase is mainly ice crystals, dissolved and colloidal solids including proteins, salt, sugars and stabilizers. The fatty phase is always present in an emulsified form.
The product developer then adds other ingredients such as flavours, fruit pieces, colours, chocolate, stabilisers and emulsifiers (Berger, 1976).
Legal Definitions
Ice cream is best defined as a frozen blend of a sweetened cream mixture and air with various added flavourings.
Ice-Cream Formulations
Formulating an ice cream is a complex process when all the ingredients are considered.
Typical formulations for ice-cream might look like this which is a basic formulation:
| Ingredient | Percentage (wt/wt) |
| Milk fat | 10-12 |
| Milk solids non-fat (SNF)/nonfat milk solids (NMS) | 9.7-11 |
| Sucrose (Extra fine granulated cane sugar) | 12 |
| Corn Syrup Solids | 4 |
| Stabilizers | 0.3 |
| Total solids | 38 |
The rest is the balance of water.
An experimental formulation was put forward by Sherman (1965).
| Ingredient | Percentage (wt/wt) |
| Oil Phase: Vegetable fat | 10 |
| Oil Phase: Commercial-grade glyceryl monostearate | 0.53 |
| Water phase: Water | 60.6 |
| Water phase: Non-fat milk solids | 13.1 |
| Water phase: Sugar | 15.6 |
| Water phase: Vegetable gum | 0.18 |
Lactose produces a sandy texture in ice-cream if handled poorly.
The Addition Of Fat
Fat is a vital ingredient and without out produces poorly textured products. As we will see through legal content values, fat is 10 to 15% of a product’s formulation. It can be dairy or vegetable. Fats produce a creamy texture and it helps reduce melting by stabilising the air structure in the product.
Milk fat may be added in the form of whole milk, butter, cream, butter oils, ghee, anhydrous milk fat (AMF). Some ices use skimmed milk and the fat is provided by coconut oil.
In non-dairy applications, vegetable fat is used such as hydrogenated and refined or hardened coconut oil and palm kernel oil. The use of vegetable fat is carefully regulated in many countries to prevent fraudulent formulations.
The fat globules has an essential role to play in ice cream structure.
Fat Destabilization
Fat destabilization results in the following beneficial properties: dryness which implies shape retention, upon extrusion during the manufacturing stages which facilitates packaging and novelty molding, for example. Other benefits include a smooth, creamy texture in the frozen dessert, and resistance to melt-down or good stand-up properties which is necessary for soft serve operations (Bolliger et al., 2000a; Goff and Spagnuolo, 2001).
Types Of Ice Cream By Fat Content
In US law which is termed a Standard of Identity, the first feature is a typical ice cream must contain at least 10% milk fat and 20% total milk solids (TMS). This level of fat determines the type of product and label i.e. reduced-fat, light, low fat and nonfat. Milk solids includes protein, lactose and minerals and well as air.
The US Code of Federal Regulations (CFR) has explicitly defined ice-cream as such in Part 135 of the Standards Of Identity. The relevant sections are 21 CFR 135.110 for ice cream, 21 CFR 135.115 for goat’s milk ice cream, and 21CFR 135.140 for sherbet.
The other key aspect of the regulation on Standard of Identity is the completed ice-cream must weigh at least 4.5lb/gal (449 kg/m3)and contain at least 1.6 lb of food solids per gallon.
These limits also establish the maximum overrun, which is the increase in volume from aeration resulting from whipping at about 100%. The average mix often weighs about 9 lb/gal (1,080 g/L).
Milk Solids Non Fat (MSNF)
MSNF is protein, mineral salts and lactose which are derived from various milk sources. This ingredient has considerable nutritional value. It also helps stabilise the ice structure because of its emulsifying and water-binding benefits. It also has a positive effect on air distribution in the product during the freezing process which adds to improved body and creaminess.
The MSNF is always formulated in relation to water content. The optimal amount is 17 parts MSNF to 100 parts of water.
It is calculated as:
%MSNF = 17(100-other solids in percent)/117
The Use Of Whey
Whey is the protein fraction of milk. It is used heavily in frozen desserts because it is relatively cheap and a good source of milk solids. The USA allows 25% of the MSNF to be substituted with whey solids.
Whey comes in many forms ranging from whey protein concentrates (WPCs), milk protein concentrates (MPCs), milk powder, caseinates and whey powders with lower protein levels than skim milk powder. The latter is also used widely.
Milk Protein Concentrate
Milk protein concentrate is a source of milk solids nonfat (MSNF) which happens to be mainly protein. It is high in protein but low in lactose. They help maintain the casein: whey ratio of milk (Novak, 1992).
The Addition Of Sugar
Sugar is used to increase the solids content of a product and reduce freezing point, to improve texture by making it firmer and provide an appropriate level of sweetness. Most frozen ices contain between 12% and 20% sugar. That usually makes sugar the second largest ingredient by weight behind milk on the ingredient list.
Sugar in this context covers a multitude of saccharide ingredients. These range from glucose and fructose through to sucrose (granulated sugar), invert sugar, dextrose, lactose (milk sugar), honey, and starch derived sugars such as glucose-fructose syrup, glucose syrup (corn syrup) and HFGS (high fructose glucose syrup). Sugar is usually dissolved into the mixture otherwise if it remained in a granular form, it would ruin the sensory appeal by making it gritty.
One of its main functions is to lower the freezing point so that it is easier to handle. Careful selection of sugar makes it easier to handle especially when scooping or pouring.
Then it comes to texture, the type of sugar added affects the texture of the ice-cream. It is one of the major contributors to this aspect and is treated with the same importance as the use of fat.
Sucrose was for many years the only sweetener used in ice-cream. It is still the standard by which other sugars and sweeteners are judged both in this industry and in others such as the beverages. Sucrose is usually purchased as a granulated solid or as a liquid. The liquid version contains 67% sucrose solids. Blends of sucrose with corn sweeteners are commonplace.
Typical addition levels of sucrose are 12%w/w. Sucrose crystallization is sometimes associated with ‘whitespot’ which occurs most often with water ices. When water disappears from the matrix the sucrose concentration rises to a point where it becomes insoluble. It is possible for supersaturated solutions to form where the solute concentration increases beyond the point at which it would normally precipitate out of solution.
Glucose syrup (corn syrup) is commonly used because it reduces if not prevents crystallization of any other sugars in the mix. They are supplied as liquids with 20% water or as spray-dried powders. Sucrose with medium- or high-conversion corn syrup solids is often used in a formulation.
Other sugars considered include grape or raisin molasses. Sugarcane molasses is a rich source of sucrose (about 55%), fructose, proteins and minerals (Soukoulis & Tzia, 2018).
Honey has a medium scale glycemic index (GI) of 55 (Foster-Powell et al., 2002) in comparison to sucrose (GI = 100). It has been claimed to promote the reduction of blood glucose levels in diabetics (Busserolles et al., 2002), as well as a marked reduction of plasmatic triglycerides (Al-Waili, 2004).
Lactose also lowers the freezing point of ice cream but increases the sanidiness texture.
Sweeteners are used to replace sugar in low-sugar or sugar-free formulations. aspartame, acesulfame K and even stevia have been used. The texture must be provided by maltodextrins, lactitol, sugar alcohols and polydextrose if anything like a sugar-free variant is to be produced successfully with the right texture.
The Use Of Stevia
Stevia is used as a sugar substitute in a number of foods. It is about 250 to 300 times sweeter than sucrose. It is primarily based on Rebaudioside A which is even sweeter. Alizadeh et al., (2014 a &b) produced ice-cream using mixes of sucrose and stevia. When stevia replaced sucrose, the mix viscosity was higher, a higher overrun and melting rate.
Sugar alcohols form when sugars are reacted with hydrogen in the presence of a catalyst. These include erythritol.
Emulsifiers
We had earlier described ice cream as a mixture of ice crystals with air cells and coalesced fat droplets enclosed in a thick continuous phase. It is apparent that the ice quality of any ice cream depends on its composition. It is a very complex system which can easily become unstable without the use of emulsifiers and stabilisers which are food additives.
The emulsifiers help keep the milk fat evenly dispersed throughout the product. They are essential when the product is frozen and stored. A fair and equitable distribution of fat helps stabilise the air incorporated into the ice and enhances the smoothness.
Emulsifiers include egg yolk, lecithins and mono- and diglycerides.
Emulsifiers help in creating an even more creamy feel and texture. To create a thicker mouthfeel and body, gums may be added. This concoction is literally frozen into whatever shape we like.
The types of stabilizers tried include:
- chia seed gum/mucilage (Campos et al., 2016; Feizi et al., 2021)
- modified wheat protein
- red bean
- milk protein concentrate and reduced-calcium milk protein concentrate.
The level of added chia seed mucilage is approximately 0.2%w/w (Feizi et al., 2021).
Types Of Ice Cream
Premium Ice cream
Premium Ice Cream generally has between 11% and 15% fat, a total solids content of between 38 and 40% and 60% to 90% of overrun, which is the air that is pumped into the ice cream.
This creates a denser, heavier, creamier, richer and more caloric product than regular ice cream, and is reflected in the price.
Super Premium ice cream
This has even more butterfat— greater than 14%, with some having up to 18% and more—and less overrun, from as low as 25% up to 50%. The total solids content is above 40%. Premium and super premium ice creams come in more complex flavors in addition to the basic ones. The super premium ice cream producers category includes smaller companies that make particular gourmet flavors.
In addition to lower overrun and greater butterfat, the third way in which a super premium ice cream can be made richer is by using an egg custard base, which is known as French or French-style ice cream
Regular Ice Cream
This cream is less dense: it contains 10% to 11% butterfat and more air with a 90% to 100% overrun. It is usually sold in standard flavors, since the addition of rarer flavours adds to the cost increase. Some consumers prefer the texture and reduced degree of richness, and prefer it in milkshakes where the subtlety of the richer ice cream can be lost.
Economy Ice cream contains exactly 10% butterfat – the minimum USDA standard, a legal minimum in terms of total solids of 36%, and 95% to 100% overrun but the legal maximum could be up to 120%. It is made in basic flavors.
Light Ice Cream means that there is either 50% less fat or 33% fewer calories than the company’s standard ice cream. Read the labels carefully: the “light” ice creams of a super-premium brand often have more calories than the “regular” ice cream of other brands.
Low fat Ice Cream has 25% less fat than the company’s regular ice cream. Similar to the light ice cream analogy above, it can contain more calories than a regular ice cream of another brand.
A typical low fat ice cream formulation could contain 2% milk fat with 11.5% serum solids. The fat source was cream and serum solids based on cream, milk and nonfat dry milk, 11.5% sucrose, 6.5% 36 DE (dextrose equivalent) corn syrup solids, 2.5% 10 DE maltodextrin with 0.3% stabiliser blend (Baer et al., 1997).
The Physical Chemistry Of Ice Cream
In a pure physical chemistry perspective ice cream is a multiphase frozen product. These phases comprise ice crystals, fat globules, and air cells which are distributed throughout an unfrozen serum phase (Goff and Hartel, 2013).
In terms of fat content it contains between 8 to 15% fat (Li et al., 1997). This fat has the most important role of giving ice-cream its most desirable properties which is a smooth, creamy and very soft feel to its texture when in the mouth. Good flavour is the other but sensory studies show that it is nothing if the texture is not right (Guinard et al. 1997).
The most widely accepted model is of a solid foam with a network of coagulated fat (Sherman, 1965). The fat globules are coagulated when ice cream freezes at a low enough temperature. Plenty of agitation and whipping during manufacture distribute the solidified fat particles throughout the liquid phase between air cells and ice crystals.
The fat globules are held in this solid emulsion by strong London-van der Waals’ attractive forces.
Much of the physical structure has been determined using rheology experiments.
Manufacturing Process
All ingredients are blended and mixed in a specific mixing tank according to specific formula. Ingredients are weighed in, then blended together to produce the ‘ice cream mix’. rapid and vigorous agitation is needed to incorporate all the powders together. Very high speed shear mixers are employed for this purpose. The blending operation depends on the type of pasteurisation process that follows.
Pasteurisation
The mixture is pasteurised usually in a scraped surface heat exchanger or plate pasteuriser if a the operation is continuous and HTST (High Temperature Short Time). Typical operating conditions for pasteurisation are 68.3°C (155°F) for 30 minutes or 80°C (175°F) for 25 seconds. Pasteurisation conditions are a little more severe compared to milk because ice-cream is more viscous.
The intention is to completely destroy harmful bacteria or pathogens and reduce spoilage organisms such as psychrotrophs which are able to survive in cold temperatures, to an acceptable level. It is usually the main control point in a HACCP plan. Additionally, some ingredients improve their hydration and solubility during this critical heating phase.
Batch pasteurisers are used for smaller operations. Some manufacturers believe that a batch pasteurisation alters whey protein denaturation which creates a different mouthfeel in the ice-cream.
Homogenisation
Homogenization is fundamental to the smooth texture that characterizes ice cream. It also prevents fats in the formulation from churning during the later freezing stage. The mixture is cooled to about 5°C (40°F) and aged for approximately 12 hours to allow the fat to partially crystallize. the step also allows stabilizers and proteins to solubilize so that the fat particles remain small enough and entrap any air.
The heated mixture is homogenised at a pressure of between 2,000 and 2,500 pounds per square inch which reduces the milk fat in the mixture to smaller globules. Some manufacturers will use a two-step homogenisation. This also keeps the globules in suspension with a more uniform distribution throughout the liquid phase of the system. The milk fat globules are now less than 2 micron diameter which is a size that helps them remain dispersed and keeps the emulsion stable.
The benefits of homogenisation include a greater surface area of the fat globule, improved viscosity and stability. A key aspect of mouthfeel and sensory pleasure comes from the smooth and creamy taste, and texture of these fat globules. The process also blends the emulsifiers and stabilisers better and helps them maintain fat globule stability.
Cooling (Aging)
The mixture is rapidly cooled to just above freezing (about 5°C /40°F), held for on average 4 hours and then frozen. Aging the mixture cools it down before freezing, allows the milk fat to partially crystallize and the gives the proteins stabilizers time to hydrate. This improves the whipping properties of the mix.
The fat in the globules remains in the liquid state at 4.5ºC because of supercooling. The rapid reduction of temperature in the freezer, alongside the vigorous whipping action weakens the monoglyceride-protein film around the globules and eventually ruptures it. This released fat solidifies.
The breakdown of emulsifier films on freezing is due to dehydration by loss of water to the ice phase (Cole et al., 1959).
Freezing
Freezing is a key aspect of the process because it maintains that smooth texture. It’s also the point at which most structural changes occur. A typical freezer is a barrel freezer. During the freezing process, the mix is aerated by ‘dashers’ which are revolving blades in the freezer unit. This churning prevents large water crystal formation. Ice forms at the barrel surface and is scraped off by the dasher blades whilst large amounts of air are being incorporated as tiny air cells through a form of whipping. Air injection may be applied or though this is not always necessary. The whole freezing process takes about 30 seconds.
The whipping action produced by air prevents the formation of a solid mass of just frozen material which would be like an ice-cube. The volume of the ice-cream increases from between 60 and 100% because of the amount of incorporated air.
Shear forces during the freezing process can create partially-coalesced fat globule clusters that provide structural integrity and stabilize the air cells (Goff et al., 1999).
At this point, the ice-cream is a mixture of ice crystals, entrained air, sugar, water and milk solids. Ice crystal size is affected by formulation and processing conditions such as draw temperature, throughput rate, freezer type, and storage conditions.
The mix exits the freezing barrel where it is pumped through a second ingredient tank or feeder so that other ingredients are added. Examples are flavourings, fruits, nuts, chocolate chips, sprinkles etc. In some systems, a swirl or variegate is produced.
A portion, roughly 50% of the serum (water) phase remains unfrozen due to freeze concentration of the solute. The quantities and sizes of these microstructural components governs the behaviour and sensory properties of the final ice cream. Soft serve ice cream is generated at this point in the freezing process.
Premium ice creams have less overrun (approximately 80%) and are more dense than regular types.
The frozen ice which is now at 20°F is packaged quickly into moulds such as packets, cups, cones and the like.
Hardening
The packaged ice-cream is cooled as rapidly as possible to a holding temperature of less than -25°C(-13°F). Rapid cooling will promote quick freezing of water and create small ice crystals. Storage at -25°C(-13°F) will help to stabilize the ice crystals and maintain product quality. It is kept in a hardening room where the sub-zero temperatures allow the product to maintain a frozen temperature before further packaging, storage and distribution.
Every year, the Ice Cream Technology Conference of the International Dairy Foods Association honours the best frozen dessert innovations in the dairy industry.
Overrun
Overrun is a term used heavily in frozen food parlance for the degree of expansion in volume before and after the freezing process. It means the percentage amount of air contained in a frozen product.
All frozen products contain air. Without it the food would be hard, dense and lacking in a desirable smooth texture.
Air increases the volume of the finished product. Too much overrun and the value and quality is reduced and may well make it illegal by definition.
One litre of liquid ice cream mixture prior to freezing weights about 1 kilogram. The density is around 1kg/l. On freezing, the mixture expands from about 1 litre to 1.4 litres because air is incorporated into the mixture during the freezing process. It is one of the reasons why ice cream is sold by volume not weight.
Different frozen products have different percentages of air:
A scoop ice cream contains 50 to 60% air, a scoop gelato contains much lower levels of between 25 and 30%. A soft serve ice cream dispensed from a gravity machine will be higher at 30 to 35%. A pump machine delivers soft serve ice cream at between 60 and 80% air
The formula used to calculate the overrun of a frozen product is:
Weight of liquid mix minus the weight of frozen product divided by the weight of frozen product, times by 100 = % Overrun.
Crystallization of Fats
The physical structure of ice-cream dictates its quality. As we have described it earlier, the structure is made up of air bubbles, ice crystals, fat globules and an unfrozen serum phase. Of all the structural components, the ice crystals have the most important role in storage stability and texture.
Thermodynamically, there is a natural tendency for ice to undergo recrystallization which involves a decrease in the total number of crystals and mean increase in crystal size (Goff, 2005).
Crystallization of fat also occurs during ageing, creating a highly intricate structure of needle-like crystals within the globule. Triglycerides with a high melting point crystallize first, and continue to be surrounded by liquid oil of those triglycerides with lower melting points.
Analysis
The ice cream mix is usually the point at which most significant assessment occurs. It is worth checking pH, viscosity of the ice cream mix, overrun, fat globule size distribution, destabilised fat index, meltdown rate and hardness of ice cream.
Typical measures for the ideal ice-cream would be an overrun of 103%, 41 +/- 5.3% destabilised fat index, 40.7 ± 9 μm fat globule size (d4,3), low meltdown rate (0.9 ± 0.1 g min−1) and hardness of 40± 7 N.
Relating Texture To Physical Performance
The best texture for an ice-cream is based on a number of factors which have best been summarized by P. Sherman at T. Wall & Sons in Gloucester UK (1965).
- Ideally, a small globule size of between 1 and 2 microns diameter with a narrow size distribution in the mix. The globule size needs to be small enough so that the globules cannot form aggregates. That means the rate of coalescence ought to be greater than at the stage when globules form aggregates before coalescence.
- Emulsifiers are used to produce a substantial reduction in interfacial tension because homogenization cannot produce the required effects.
- Protein denaturation at the fat-water interface proceeds at a rate which quickly establishes an effective barrier to too rapid growth in globule size. Emulsifiers are used with fat to disperse them in aqueous media. The fat not only disperses as small globules (micron size) but an opposing tendency is established whereby the globules diffuse toward one another and coalesce. The rate of the latter process must be reduced to a very low level.
- Freezing promotes the production and stabilizing of small fat particles. These fat globules are distributed throughout the aqueous medium between air cells. Small fat particles and small particle aggregates offer better air retention and whipping properties (Keeney and Josephson, 1962).
- Solubility of the emulsifier in the oil phase is critical. Unsaturated emulsifiers contain fatty acids which remain soluble to lower temperatures than saturated fatty acids (Stistrup & Andrease, 1962).
- The protective monoglyceride-protein film around the globules may remain intact to lower temperatures with unsaturated emulsifiers than with the conventional saturated monoglycerides. These could influence the rate of film rupture during freezing and the rates of fat solidification and coagulation.
- The process of ice-cream manufacture is critical. It is extruded from the freezer at a relatively low shear rate approximating to ‘plug flow’. The inner part of the ribbon flows as a single unit, and only the surface layers are sheared.
In summary, ice cream is a delightful product for anyone but it needs care and attention in its manufacture.
References
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