Ice Cream and Frozen Treats

• Lecture 5 is about Ice Cream and Frozen Treats

What is Ice-Cream?

• Ice-cream is traditionally a frozen mixture of milk and/ or cream, sugar, optional egg yolks, and flavourings like fruits or nuts.
• Commercial versions contain emulsifiers, stabilizers, and gums.
• A foam emulsion of ice (water), fat, and air makes up the structure of ice cream.

Types of Ice-cream

• The two main types are sweetened frozen cream/milk and custard-based.
• Flavor and texture will differ based on type.
• All can be flavored in the same manner.

Making Ice-cream – Egg-less

• Milk and cream solidify when frozen.
• Adding sugar softens the mixture and lowers the freezing point.
• Ice-cream turns solid at a lower temperature.
• Some water remains partially liquid.
• Dissolved sugar molecules disrupt water molecules as they settle into ordered crystals, softening the ice cream.
• Ice crystals forming results in a concentrated cream (high in fat) solution.
• Water remains liquid, even at -18 °C, up to a fifth of ice cream's mass.
• A thick fluid of liquid water, milk fat, milk proteins, and sugar coats and sticks together millions of tiny ice-crystals.
• Churning prevents ice-cream from solidifying into an un-scoopable hard mass.
• Churning breaks down the ice crystals, making a smoother ice-cream as well as incorporating air into the mixture.
• Volume increase after churning is called overrun, and in budget ice-cream, half the volume may actually be air.

The Importance of Churning

• Incorporation of air into ice-cream increases its volume.
• Further weakens the ice crystal-cream matrix and makes it softer.

Balancing the Ingredients

• A well-balanced ice-cream has proportions of ingredients such that the result is creamy, smooth, firm but not rock hard, and slightly chewy but not very airy.
• Achieved balance follows: water content at 60%, sugar content around 15%, and milk fat at 10-20%.
• While flavourings get added as desired, using pureed fruit requires increases of the fat content to keep it from getting too hard.
• Fatty ingredients such as Nutella will increase the fat content significantly, making the ice-cream softer.
• The less water (milk), the easier it is to make small crystals.
• Too much sugar and milk solids (fats and proteins) gives a heavy, syrupy result.

Making Ice-cream – Custard-based

• It has a characteristic cooked-egg flavour.
• Due to the proteins and emulsifiers present egg yolks, ice crystals are kept small.
• Texture is smooth even at relatively low fat and high water levels.
• Mixture is cooked, which disperses the proteins and emulsifiers and kills any bacteria present, resulting in a thickened mixture.
• That mixture is then churned and frozen.

Ice Crystal Size – Ostwald Ripening

• Best stored at -18 °C to preserve smoothness (ie keeping the ice crystals as small as possible).

How to Store Ice-cream

• During storage, ice-cream fat can absorb odours from the freezer and be damaged and go rancid when dried out by the freezer air (sublimation of water).
• To prevent this, ensure that the ice-cream is well-covered and sealed, ideally by pressing cling film directly to the ice-cream's surface.

How to Serve Ice-cream

• Stored at -18 °C, ice-cream is hard to serve and eat.
• Should be allowed to warm up to -13 - -10 °C
• Taste buds are not numbed as much, giving it a tastier experience.
• More water will be in the liquid state, hence easier to eat.

Why Does Ice-cream Get Harder with Time?

• The two main factors affection ice-cream softness are ice crystal size and air content.
• Taking out a tub of ice-cream and letting it warm up to serve means smaller ice-crystals will have melted. In cooling, water re-freezes on fewer, larger crystals.
• Some air pockets may also be deflated or filled with liquid water, giving a denser ice-cream.

Brain Freeze

• Eating too much ice-cream, or ice-cream which is very cold, leads to "brain freeze".
• Brain-freeze occurs when the hard palate (top part of the mouth) becomes very cold due to the ice-cream.
• Blood vessels shrink to reduce blood flow and minimize heat loss.
• When ice-cream is swallowed, blood vessels dilate, with the nerves surrounding the palate sensing this as pain.
• This sensation transmits to the trigeminal nerve that runs from the forehead, behind the eyes and along the jaw.
• Even though brain-freeze is caused by intense cold, it actually happens once the mouth starts to warm up again.

Why is Bought Ice-cream Softer?

• Commercial ice-cream has one more ingredient, stabilizers.
• Stabilisers used are nearly all of plant origin: alginates and carrageenans (from seaweed), locust bean (carob) and guar gum (tree seeds), pectin (fruit), and carboxymethyl cellulose (cotton).
• Others include gelatin (animal origin) and xanthan gum (bacterial origin).
• Stabilisers/gums can produce smoothness, reduce meltdown rate, prevent shrinkage and migration, mask ice crystals, and facilitate controlled air incorporation to produce a stable foam.
• Commercial ice-cream is frozen quickly.
• Crystal size depends heavily on how fast the ice is formed – the longer the freezing time, the larger the crystals.
• Blast freezing enables microcrystals to form, giving a smooth ice-cream.
• Done on a smaller scale using liquid nitrogen or dry ice.

The Physics and Effect of Rapid Freezing

• Blast freezing occurs at temperatures of -30 to -40 °C (artisan ice-cream is not frozen down to these temperatures).
• Cold air is continuously blown over the product to rapidly lower temperature.
• When the temperature of a product is brought down below the freezing point, water molecules start freezing through spontaneous nucleation
• Number of nuclei depends on temperature.
• Also, if temperature is brought down slowly, the uncrystallised water can crystallize around nuclei, resulting in large crystals.
• If temperature is brought down quickly, more nuclei form quicker and uncrystallised water will not be able move around as much, resulting in smaller crystals.

The physics and Effect of Rapid Freezing

• "high temperature" /slow temperature causes fewer crystal nuclei, more water movement, and thus large crystals.
• "low temperature"/rapid temperature causes many crystal nuclei, little water movement, and small crystals.
• Blowing cold air over the product also helps with rapid freezing.
• Heat transfer is effected through conduction, convection, and radiation.
• Convection is the main mechanism.
• Conduction is very slow, as air is a very poor heat conductor.
• Radiation occurs even in a vacuum and is most effective when there is a large temperature difference between the object and air.
• Static air will form a layer of warm air around the object, hence the temperature difference between the object and the air around will be smaller, meaning a smaller rate of heat transfer.
• The higher the air speed, the higher is the convective heat transfer coefficient, which means that the rate of heat transfer increases.

Low-Fat Ice-cream

• Creaminess comes from milkfat
• Low-fat ice-cream doesn't have as much, or any, milkfat, making it very icy.
• Gums are added to compensate, increasing viscosity and lending a more chewy rather than hard texture.

Other Iced Foods

• Sorbet is a fruit-based, aerated sugar syrup that contains neither fat nor milk.
• Sherbet is similar to a sorbet, but contains some milk or cream.
• Frozen yoghurt is yoghurt with live bacteria which is churned and frozen.
• Water ice (ice lollies) is frozen sugar syrup with flavour and colour.

Sorbet

• Many variations exist (watermelon, orange, peach, melon), with the most famous probably being lemon.
• Can be eaten at any time of day, or during any part of the meal.
• Sometimes used as a palate cleanser between courses.
• Make a simple syrup of sugar and water, add pureed fruit (and some lemon juice if using very sweet fruits), strain if necessary, churn and freeze.
• Same principles apply as for ice-cream apply where too little sugar and the result is hard and icy; too much sugar and the result is soft and syrupy.
• The smaller the ice crystal size, the smoother it feels on the tongue.

Granita

• Prepared in a very similar manner, however usually has a coarser texture, or is actually shaved off a large block.
• Adding alcohol to the mix before freezing (such as Limoncello) will depress the freezing point of the water further and may actually result in not freezing, so only add a small amount of alcohol, if any.

Sherbet

• Essentially a sorbet with added milk or cream
• Gives a whitish, opaque final product.
• The presence of the milkfat adds a creaminess to the frozen result.

Frozen Yoghurt

• Essentially plain or flavoured yoghurt which is churned and frozen.
• Yoghurt usually doesn't contain added sugars, and can be fat-free.
• Given the lack of sugar and possibly fat, frozen yoghurt turns into one solid mass if frozen to very low temperatures, even if churned. Therefore it should be served at warmer temperatures (usually around -8 °C as for soft serve ice-cream) to allow for the larger proportion of water to be liquid rather than ice crystals.
• Incorporating more air during churning simplifies is also helps to make it softer and provides an interesting contrast to ice-cream with its typical sour taste.

Ice Lollies

• Frozen sugar syrup with added colour and flavouring, in contrast to a sorbet which contains any fruit.
• It is not churned and freezes into a solid mass.
• Has a higher thermal capacity than ice-cream, therefore feels colder in the mouth (actually they absorb more heat faster hence your mouth feels colder).