Finding Science in Ice Cream

Brought to you by:
The Department of Food Science

As the hot weather approaches and students minds begin to drift from the rigors of the high school lab, a fun afternoon might be spent making ice cream and in so doing, introducing several aspects of the science and technology "behind the scenes". To suggest that there is no science in ice cream could not be further from the truth. I have made a career out of ice cream research which has taken me into aspects of physical and organic chemistry, microbiology, and chemical engineering to name but a few. Because all of you are from different disciplines and teach in different ways, I will give you enough background information and practice from which you can prepare your own experimental work. You can use the ice cream lab, for example, to demonstrate heat transfer in physics classes, freezing point depression phenomena and emulsions and foams in chemistry classes, or pasteurization and the food use of seaweeds(!) in biology classes. However you use the following information, even if it is for your own family picnic this summer, I hope you enjoy it!

The history of ice cream

Once upon a time, hundreds of years ago, Charles I of England hosted a sumptuous state banquet for many of his friends and family. The meal, consisting of many delicacies of the day, had been simply superb but the "coup de grace" was yet to come. After much preparation, the King's french chef had concocted an apparently new dish. It was cold and resembled fresh-fallen snow but was much creamier and sweeter than any other after-dinner dessert. The guests were delighted, as was Charles, who summoned the cook and asked him not to divulge the recipe for his frozen cream. The King wanted the delicacy to be served only at the Royal table and offered the cook 500 pounds a year to keep it that way. Sometime later, however, poor Charles fell into disfavour with his people and was beheaded in 1649. But by that time, the secret of the frozen cream remained a secret no more. The cook, named DeMirco, had not kept his promise.

This story is just one of many of the fascinating tales which surround the evolution of our country's most popular dessert, ice cream. It is likely that ice cream was not invented, but rather came to be over years of similar efforts. Indeed, the Roman Emperor Nero Claudius Caesar is said to have sent slaves to the mountains to bring snow and ice to cool and freeze the fruit drinks he was so fond of. Centuries later, the Italian Marco Polo returned from his famous journey to the Far East with a recipe for making water ices resembling modern day sherbets.

In 1774, a caterer named Phillip Lenzi announced in a New York newspaper that he had just arrived from London and would be offering for sale various confections, including ice cream. Dolly Madison, wife of U.S. President James Madison, served ice cream at her husband's Inaugural Ball in 1813. Commercial production was begun in North America in Baltimore, Maryland, 1851, by Mr. Jacob Fussell, now known as the father of the American ice cream industry. The first Canadian to start selling ice cream was Thomas Webb of Toronto, a confectioner, around 1850. William Neilson produced his first commercial batch of ice cream on Gladstone Ave. in Toronto in 1893, and his company produced ice cream at that location for close to 100 years. The ice cream division of Neilson was recently purchased by Ault Foods of London, Ont.

The composition of and ingredients in ice cream

Today's ice cream has the following composition : a) greater than 10% milkfat by legal definition, and usually between 10% and as high as 16% fat in some premium ice creams; b) between 9 and 12% milk solids-not-fat, the component which contains the proteins (caseins and whey proteins) and carbohydrates (lactose) found in milk; c) 12% to 16% sweeteners, usually a combination of sucrose and glucose-based corn syrup sweeteners; and d) 0.2% to 0.5% added stabilizers and emulsifiers, necessary components that unfortunately have unfamiliar sounding names that occupy three-quarters of the space of the ingredient listing and that will be described subsequently. The balance, usually 55% to 64%, is water which comes from the milk. Ice milk is very similar to the composition of ice cream but contains between 3% and 5% milkfat by definition. Light ice cream contains between 8% and 10% milkfat.

The ingredients used to supply this composition include: a) a concentrated source of the milkfat, usually cream or butter; b) a concentrated source of the milk solids-not-fat component, usually evaporated milk or milk powder; c) sugars including sucrose and "glucose solids", a product derived from the partial hydrolysis of the corn starch component in corn syrup; and d) milk.

The fat component adds richness of flavour, contributes to a smooth texture with creamy body and good meltdown, and adds lubrication to the palate as it is consumed. The milk solids-not-fat component also contributes to the flavour but more importantly improves the body and texture of the ice cream by offering some "chew resistance" and enhancing the ability of the ice cream to hold its air. The sugars give the product its characteristic sweetness and palatability and enhance the perception of various fruit flavours. In addition, the sugars, including the lactose from the milk components, contribute to a depressed freezing point so that the ice cream has some unfrozen water associated with it at very low temperatures typical of their serving temperatures, -15o to -18oC. Without this unfrozen water, the ice cream would be too hard to scoop.

Freezing point depression of a solution is a colligative property associated with the number of dissolved molecules. The lower the molecular weight, the greater the ability of a molecule to depress the freezing point. Thus monosaccharides such as fructose or glucose produce a much softer ice cream than disaccharides such as sucrose. This limits the amount and type of sugar which one can successfully incorporate into the formulation.

The stabilizers are a group of compounds, usually polysaccharides, that are responsible for adding viscosity to the unfrozen portion of the water and thus holding this water so that it cannot migrate within the product. This results in an ice cream that is firmer to the chew. Without the stabilizers, the ice cream would become coarse and icy very quickly due to the migration of this free water and the growth of existing ice crystals. The smaller the ice crystals in the ice cream, the less detectable they are to the tongue. Especially in the distribution channels of today's marketplace, the supermarkets, the trunks of cars, and so on, ice cream has many opportunities to warm up, partially melt some of the ice, and then refreeze as the temperature is once again lowered. This process is known as heat shock and every time it happens, the ice cream becomes more icy tasting. Stabilizers help to prevent this.

Gelatin, a protein of animal origin, was used almost exclusively in the ice cream industry as a stabilizer but has gradually been replaced with polysaccharides of plant origin due to their increased effectiveness and reduced cost. The stabilizers in use today include: a) carboxymethyl cellulose, derived from the bulky components of plant material; b) locust bean gum which is derived from the beans of exotic trees grown mostly in Africa (Note: locust bean gum is a synonym for carob bean gum, the beans of which were used centuries ago for weighing precious metals, a system still in use today, the word carob and Karat having similar derivation) ; c) guar gum, from the guar bush, a member of the legume family grown in India for centuries and now grown to a limited extent in Texas; d) carrageenan, an extract of Irish Moss or red algae, originally harvested from the coast of Ireland, near the village of Carragheen; or e) sodium alginate, an extract of another seaweed, brown kelp. Often, two or more of these stabilizers are used in combination to lend synergistic properties to each other and improve their overall effectiveness.

The emulsifiers are a group of compounds in ice cream which aid in developing the appropriate fat structure and air distribution necessary for the smooth eating and good meltdown characteristics desired in ice cream. Emulsifiers are characterized by having a molecular structure which allows part of the molecule to be readily solubilized in a polar compound such as water, and another part of the molecule to be more readily solubilized in non-polar solvents such as fats. As a result, emulsifiers reside at the interface between fat and water, and lower the free energy or tension associated with two immiscible liquids in contact with each other. Their action will be more fully explained in the section below on emulsions and foams.

The original ice cream emulsifier was egg yolk, which was used in most of the original recipes. Today, two emulsifiers predominate most ice cream formulations: a) mono- and di-glycerides , derived from the partial hydrolysis of fats or oils of animal or vegetable origin; and b) Polysorbate 80, a product consisting of a glucose molecule bound to a fatty acid, oleic acid. Both of these compounds have hydrophobic regions ( the "fat loving" part), the fatty acids, and hydrophilic regions ( the "water loving" part), either glycerol or glucose. All of the compounds mentioned above are either fats or carbohydrates, important components in most of the foods we eat and need.

Together, the stabilizers and emulsifiers make up less than one half percent by weight of our ice cream. They are all compounds which have been exhaustively tested for safety and have received the "generally recognized as safe" or GRAS status.

The manufacturing process

Ingredients are chosen by the manufacturer on the basis of desired quality, availability, and cost. The ingredients are blended together and produce what is known as the "ice cream mix". The mix is first pasteurized. Pasteurization is a process which is designed to kill all of the possible pathogens (disease causing organisms) that may be present. Organisms such as Mycobacterium tuberculosis, Salmonella, Staphylococcus, Listeria, and others that cause human disease can be found associated with farm animals and thus raw milk products must be pasteurized. In addition to this very important function, pasteurization also reduces the number of spoilage organisms such as psychrotrophs, and helps to "cook" the mix. The mix is also homogenized which forms the fat emulsion by breaking down or reducing the size of the fat globules found in milk or cream to less than 1 µm. Homogenization helps to produce a smooth product when frozen. The mix is then aged for at least four hours and usually overnight. This allows time for the fat to cool down and crystallize, and for the proteins and polysaccharides to fully hydrate.

Following mix processing, the mix is drawn into a flavour tank where any liquid flavours, fruit purees, or colours are added. The mix then enters the dynamic freezing process which both freezes a portion of the water and whips air into the frozen mix. The "barrel" freezer is a scraped-surface, tubular heat exchanger, which is jacketed with a boiling refrigerant such as ammonia or freon. Mix is pumped through this freezer and is drawn off the other end in a matter of 30 seconds, (or 10 to 15 minutes in the case of batch freezers) with about 50% of its water frozen. There are rotating blades inside the barrel that keep the ice scraped off the surface of the freezer and also dashers inside the machine which help to whip the mix and incorporate air. Ice cream contains a considerable quantity of air, up to half of its volume. This gives the product its characteristic lightness. Without air, ice cream would be similar to a frozen ice cube.

As the ice cream is drawn with about half of its water frozen, particulate matter such as fruits, nuts, candy, cookies, or whatever you like, is added to the semi-frozen slurry which has a consistency similar to soft-serve ice cream. In fact, almost the only thing which differentiates hard frozen ice cream from soft-serve, is the fact that soft serve is drawn into cones at this point in the process rather than into packages for subsequent hardening. After the particulates have been added, the ice cream is packaged and is placed into a blast freezer at -30o to -40oC where most of the remainder of the water is frozen. Below about -25oC, ice cream is stable for indefinite periods without danger of ice crystal growth; however, above this temperature, ice crystal growth is possible and the rate of crystal growth is dependant upon the temperature of storage. This limits the shelf life of the ice cream.

Salt and ice

Making ice cream at home requires the use of an ice cream machine. The "homemade" or hand-crank freezer used was the forerunner to today's modern equipment. Many people enjoy fond memories of hot summer days spent preparing the ice cream mix, loading the bucket with ice and salt, and cranking the freezer for a half hour until it was considered too stiff to continue or until one's hunger got the best of them. All of the various steps in making ice cream via the bucket are similar to the commercial processing stages. The mix is prepared and pasteurized, aged, dynamically whipped and frozen in a freezer equipped with blades and dashers, and then hardened prior to consumption. Ice and salt are used, however, rather than the ammonia or Freon jacket in the commercial freezer above.

The concept of melting ice with salt is not new to anyone in this latitude. Indeed, our roads, driveways, and sidewalks are kept bare in the winter by such a process. As salt is applied to ice, the ice crystal sructure is broken due to the depressed freezing point of the resulting brine solution. As the salt continues to dissolve more ice melts to accommodate this concentrated salt solution with its very low melting point. At the same time, both the heat of solution of the dissolving salt, and the latent heat of fusion of the melting ice are adsorbed from the ice itself, thereby lowering the temperature of the salt, ice and brine mixture. The temperature of this mixture can be controlled by the amount and ratio of salt and ice present. The lowest temperature which can be achieved with a sodium chloride brine is -21oC, at a concentration of 23% salt. Higher concentrations result in salt crystallization.

This brine, in turn, is adsorbing heat from the freezing ice cream inside the can, and thus ice and salt need to be continually added to keep the ice temperature low enough to freeze the ice cream. (Bear in mind that the freezing temperature of the ice cream is depressed below 0o due to the presence of dissolved sugars.) This process is a lesson in heat transfer in itself!

The structure of ice cream - emulsions and foams

An emulsion is defined as liquid droplets dispersed in another immiscible liquid. The dispersed phase droplet size ranges from 0.1-10 µm. Important oil-in-water food emulsions, ones in which oil or fat is the dispersed phase and water is the continuous phase, include milk, cream, ice cream, salad dressings, cake batters, flavour emulsions, meat emulsions, and cream liqueurs. Examples of food water-in-oil emulsions are butter or margarine. Emulsions are inherently unstable because free energy is associated with the interface between the two phases. As the interfacial area increases, either through a decrease in particle size or the addition of more dispersed phase material, i.e. higher fat, more energy is needed to keep the emulsion from coalescing. Some molecules act as surface active agents (called surfactants or emulsifiers) and can reduce this energy needed to keep these phases apart.

A foam is defined as a gas dispersed in a liquid where the gas bubbles are the discrete phase. There are many food foams including whipped creams, ice cream, carbonated soft drinks, mousses, meringues, and the head of a beer. A foam is likewise unstable and needs a stabilizing agent to form the gas bubble membrane.

Ice cream is both an emulsion and a foam. The milkfat exists in tiny globules that have been formed by the homogenizer. There are many proteins which act as emulsifiers and give the fat emulsion its needed stability. The emulsifiers discussed above in the Ingredients section which are added to ice cream actually reduce the stability of this fat emulsion because they replace proteins on the fat surface. When the mix is subjected to the whipping action of the barrel freezer, the fat emulsion begins to partially break down and the fat globules begin to flocculate. The air bubbles which are being beaten into the mix are stabilized by this partially coalesced fat. If emulsifiers were not added, the fat globules would have so much ability to resist this coalescing due to the proteins being adsorbed to the fat globule that the air bubbles would not be properly stabilized and the ice cream would not have the same smooth texture (due to this fat structure) that it has.

This fat structure which exists in ice cream is the same type of structure which exists in whipped cream. When you whip a bowl of heavy cream, it soon starts to become stiff and dry appearing and takes on a smooth texture. This results from the formation of this partially coalesced fat structure stabilizing the air bubbles. If it is whipped too far, the fat will begin to churn and butter particles will form. The same thing will happen in ice cream which has been whipped too much.

Also adding structure to the ice cream is the formation of the ice crystals. Water freezes out of a solution in its pure form as ice. In a sugar solution such as ice cream, the initial freezing point of the solution is lower than 0oC due to these dissolved sugars. As ice crystallization begins and water freezes out in its pure form, the concentration of the remaining solution of sugar is increased due to water removal and hence the freezing point is further lowered. This process, known as freeze concentration, continues to very low temperatures. Even at the typical ice cream serving temperature of -16oC, only about 72% of the water is frozen. The rest remains as a very concentrated sugar solution. This helps to give ice cream its ability to be scooped and chewed at freezer temperatures. The air content also contributes to this ability as mentioned above in discussing freezing.

Thus the structure of ice cream can be described as a partly frozen foam with ice crystals and air bubbles occupying a majority of the space. The tiny fat globules, some of them flocculated and surrounding the air bubbles also form a dispersed phase. Proteins and emulsifiers are in turn surrounding the fat globules. The continuous phase consists of a very concentrated, unfrozen solution of sugars.

One gram of ice cream of typical composition contains 1.5 x 1012 fat globules of average diameter 1 µm that have a surface area of greater than 1 square meter (in a gram!), 8 x 106 air bubbles of average diameter 70 µm with a surface area of 0.1 sq. m., and 8 x 106 ice crystals of average diameter 50 µm with a surface area of another 0.1 sq. m. The importance of surface chemistry becomes obvious!

I hope you have enjoyed this overview of ice cream processing and chemistry and have gained some useful insights into the field of Food Science, and that from this overview you might be able to have some fun with your students and pass along to them some of our enthusiasm for this field. In today's world of rapidly expanding technology, evident even on the grocery store shelf, we need students who are willing to learn and apply new and existing technologies to the stable, exciting, vital, and profitable food industry.

Appendix I: Making homemade ice cream

Homemade ice cream has not lost any of its good, old-fashioned appeal. Now there is a delicious homemade ice cream to meet every need: regular, low calorie, non-cooked and one using a non-dairy liquid coffee whitener. Everyone can make a homemade ice cream to suit their need with convenience afforded by ingredients from the supermarket.

Ice cream fills a useful place in homes throughout the country. It is a favourite for desserts or snacks incorporating an array of many flavour variations. Ice cream contains many nutrients. With the recipes provided, all should be able to enjoy some type of this tempting food. If the regular recipe does not suit your needs, there is the low calorie recipe which contains less than 3% fat for both a cost and calorie saving. The recipe using coffee whitener is significantly less costly than the regular and does not contain milk fat should that be your limitation. So let's mix up a batch of ice cream for anyone and everyone to enjoy!

Ingredients Used
The main constituents of ice cream are fat, milk solids-not-fat (skim-milk powder), sugar, gelatin (or other suitable stabilizer), egg and flavouring.

A variety of milk products can be used: cream, whole milk, condensed milk and instant skim-milk powder. The recipes stated below proved satisfactory using whipping cream (32-35% fat), table cream (18% fat) and whole milk. The fat gives the product richness, smoothness and flavour. Skim-milk powder is used to increase the solids content of the ice cream and give it more body. It is also an important source of protein which will improve the ice cream nutritionally. Use good quality, fresh powder to avoid imparting a stale flavour to the ice cream.

Liquid coffee whitener (usually purchased frozen) is a cream substitute in one of the recipes. It will yield a slightly different flavour which is still very acceptable. The texture of the ice cream is very creamy. Liquid coffee whitener offers the convenience of being stored frozen in your freezer and is readily available if a quick decision is made to make ice cream.

Sugar is a common ingredient to use as a sweetener. It increases the palatability and improves the body and texture.

The next ingredient, gelatin (or similar substance) assists in absorbing some of the free water in the ice cream mix and helps prevent the formation of large crystals in the ice cream.

It also gives substance or a less watery taste when the ice cream is consumed. The eggs are added to make the fat and water more miscible and also to improve the whipping ability which gives the ice cream greater resistance to melting.

Although vanilla is the flavour added to all of the mixes listed below, you may add flavours to suit you taste.

Preparation of the Ice Cream Mix
The mix (unfrozen ice cream) needs to be pasteurized, which should be done in a double boiler to prevent scorching.

Place the liquid ingredients (milk, cream or coffee whitener) in the upper section of the double boiler. Beat in the eggs and the skim-milk powder. Mix the gelatin with the sugar and add to the liquid with constant mixing. While stirring, heat to about 70%C. Place the container in cold water and cool as rapidly as possible to below 18%C.
Aging the Mix
The ice cream mix is best if it is aged (stored in the refrigerator) overnight. This improves the whipping qualities of the mix and the body and texture of the ice cream. If time does not permit overnight aging, let the mix stand in the refrigerator for at least four hours. After the aging process is completed, remove the mix from the refrigerator and stir in the flavouring.
Freezing the Mix
The freezing procedure has a two-fold purpose, the removal of heat from the mix and the incorporation of air into the mix. Heat is removed by conduction through the metal to the salt water brine surrounding the freezing can. This transfer of heat depends upon the temperature of the brine, the speed of the dasher and how well the dasher scrapes the cold mix from the surface of the freezer can. The dasher speed and surface contact are important to achieve complete removal of the frozen ice cream from the wall of the freezer can. A brine made from 500 grams of salt and 5 kilograms of crushed ice (one pail full) makes a good freezing mixture.

Before starting to freeze the ice cream, make sure all parts of the freezer coming in contact with the ice cream are clean and have been scalded. Let the can cool before pouring in the mix. Place the empty can in the freezer bucket and insert the dasher ensuring both the can and the dasher are centred. Pour the cold, aged mix into the freezer can. The can should not be filled over two-thirds full to allow sufficient room for air incorporation.

The recipes listed below will fill the freezer can (5 quart U.S.) to just below the fill line. Attach the motor or crank mechanism, depending on whether your freezer is the electric or hand-cranked style, and latch down securely. Plug in the motor or start turning the crank. Immediately begin adding crushed ice around the can sprinkling it generously with salt. Try to add the salt and ice in the same one to ten proportion to get the proper brine temperature. After the bucket is filled with ice to the overflow hole, pour a little water over the ice to aid in the melting process.

Freeze the mix for 20 to 30 minutes. If the electric motor stalls, immediately unplug it. Remove the motor or crank and take the dasher out of the ice cream. The ice cream will be softly frozen. Scrape the ice cream from the dasher and either scoop into suitable containers or pack in the freezer can. Immediately place the ice cream in the deep freeze to harden.

If freezer facilities are not available, the ice cream can be left in the can, the lid plugged with a cork and placed back into the bucket. Repack the freezer with more ice and salt, cover with a heavy towel and set in a cool place to harden until serving time. This will require further addition of ice and salt depending on the length of time the ice cream is being held. The yield from the recipes listed below should be three to four litres.

Regular Vanilla Ice Cream

Table cream 2 l
Instant skim-milk powder 350 ml
Sugar 450 ml
Gelatin one 7 g pkg.
Egg one med or large
Vanilla 10 ml
Calories per 100 g 230

Low Calorie Vanilla Ice Cream

Whole milk 2 l
Instant skim-milk powder 500 ml
Sugar 350 ml
Gelatin one 7 g pkg.
Egg one med or large
Vanilla 10 ml
Calories per 100 g 125

Milk Substitute Vanilla Ice Cream

Coffee whitener 2 kg
Instant skim-milk powder 350 ml
Sugar 500 ml
Gelatin one 7 g pkg.
Egg one med or large
Vanilla 10 ml
Calories per 100 g 210

Hints for making good ice cream

1. If the ice cream is very soft, the brine is not cold enough. More salt should be added to reduce the brine temperature.

2. If the ice cream is coarse and ice in less than 20 minutes, the brine has become to too cold too quickly. Too much salt has been used.

3. Make the ice cream mix the day before it is frozen to get a smoother product and a higher yield.

4. Electric freezing takes longer than hand operated.

5. Use crushed ice for freezing.

6. Freeze at least 3 hours before the ice cream is to be served.

7. Be sure dasher is properly centred in the freezer can.

8. Add liquid flavours before freezing but if you want to add fruits or nuts, add them after freezing and before hardening.

9. Use a wire whip to blend ingredients for best results.

10. Clean the salt off all the metal parts of the freezer to prevent corrosion.

For further information about Finding Science in Ice Cream:

Professor Douglas Goff, Ph.D.
Department of Food Science
University of Guelph
Guelph, Ontario
N1G 2W1
tel: (519)824-4120 ext. 3878
fax: (519)824-6631