Ice Cream and Frozen Treats

• Lecture 5 is about Ice Cream and Frozen Treats

What is Ice-Cream?

• Traditionally a frozen mixture of milk and/or cream, sugar, (egg yolks), flavourings and/or fruits/nuts
• Commercial versions include emulsifiers, stabilisers and gums
• Key components are foam emulsion of ice (water), fat and air

Types of ice-cream

• Two main types exist: Sweetened frozen cream/milk and custard-based
• Flavor and texture will differ between the types
• All can be flavoured in the same manner

Making ice-cream – egg-less

• Milk and cream are rock solid when frozen
• Adding sugar makes it softer and lowers its freezing point
• Ice-cream turns solid at a lower temperature, with some water remaining partially liquid
• Dissolved sugar molecules intervene with water molecules as they settle into ordered crystals and make mixture softer
• When the mixture freezes, ice crystals form, giving ice-cream its solidity
• The size of the ice crystals determines the smoothness of the ice-cream; smaller crystals result in a smoother feel, while large ice crystals cause graininess
• Ice crystal formation results in a concentrated cream/high-fat solution that is still emulsified
• Even with sugar, up to a fifth of the water remains liquid at -18°C
• The liquid water, milk fat, milk proteins, and sugar create a very thick fluid
• This fluid coats the ice-crystals and sticks them together, but not too strongly
• Churning prevents ice-cream from solidifying into an un-scoopable mass
• Churning breaks ice crystals into smaller pieces and incorporates air into the mixture

The Importance of Churning

• Incorporating air is crucial, increasing volume and further weakening the ice crystals-cream matrix, making it softer
• Overrun is increase in volume after churning; budget ice-creams can reach 100% overrun (half the volume in air)

Balancing the Ingredients

• A well-balanced ice-cream has proportions that result in a creamy, smooth, firm (but not rock-hard), and slightly chewy (not very airy) texture
• Less water (milk) makes it easier to form small crystals and achieve a smooth texture
• Too much sugar and milk solids (fats and proteins) can result in a heavy, syrupy consistency
• A good balance involves 60% water content, 15% sugar content, and 10-20% milk fat
• Other flavorings, such as vanilla, cocoa powder, Nutella, or pureed fruit, are added as desired
• If using pureed fruit (mostly water), the fat content may need to be increased to prevent ice-cream from becoming too hard
• Similarly, using fatty ingredients like Nutella will increase fat content and make ice-cream softer

Making Ice-Cream – Custard-Based

• Has a characteristic cooked-egg flavor
• Egg yolks' proteins and emulsifiers keep ice crystals small, resulting in a smooth texture even at low fat and high water levels
• Unlike egg-less ice-cream, custard-based mixtures are cooked to disperse proteins and emulsifiers and kill any bacteria present
• Cooking results in a thickened mixture that is then churned and frozen

Ice-cream Close Up

• Ice cream contains liquid, ice crystals and air cells

Ice Crystal Size – Ostwald Ripening

• Showing ice crystals at Different temperatures

How to Store Ice-Cream

• Best stored at -18°C to preserve smoothness by keeping ice crystals small
• During storage, the fat can absorb odors, and sublimation of water can cause it to become rancid
• Ensure ice-cream is well covered and sealed, ideally by pressing cling film directly to its surface

How to Serve Ice-Cream

• Typically stored at -18°C, too hard to serve and eat, should be allowed to warm up to -13°C to -10°C
• Taste buds are not as numbed, allowing for a tastier experience
• More water will be in a liquid state, making it softer and easier to eat

Why does Ice-cream get Harder with Time?

• Factors are ice crystal size and air content
• Warming ice-cream slightly for serving melts smaller ice-crystals
• Cooling ice-cream again re-freezes water on fewer, larger crystals
• Some air pockets deflate or fill with water, causing denser ice-cream

Brain-Freeze

• Brain-freeze can occur from eating too much or very cold ice-cream
• Occurs when the hard palate (top of the mouth) becomes very cold
• Causes diameter of blood vessels to shrink to reduce blood flow and heat loss
• Swallowing ice-cream causes blood vessels to dilate, with surrounding nerves sensing this as pain
• The pain sensation transmits this to the trigeminal nerve, running from forehead, behind eyes, and along jaw
• Even though caused by intense cold, happens once the mouth warms up

Why is bought ice-cream softer

• Commercial ice-cream contains stabilisers, which homemade ice-cream usually lacks
• Stabilisers are typically of plant origin, including alginates, carrageenans, locust bean, guar gum, pectin, and sodium carboxymethyl cellulose
• Others include gelatin (animal origin) and xanthan gum (bacterial origin)

Why is Bought Ice-Cream Softer?

• Stabilizers/gums
• Provide smoothness in texture during eating
• Reduce the rate of meltdown, which helps prevent shrinkage and moisture migration during storage
• Masks the detection of ice crystals when eating and facilitate air incorporation to help produce a stable foam

The Physics and Effect of Rapid Freezing

• Commercial ice-cream is frozen much more quickly
• Ice crystal size depends on how fast the ice forms, longer freezing times cause larger crystals
• Blast freezing enables microcrystals to form, giving a very smooth ice-cream
• This can also be done on a smaller scale using liquid nitrogen or dry ice (frozen solid CO₂)
• Blast freezing occurs at temperatures of -30 to -40 °C, and cold air is used to rapidly lower temperature

The Physics and Effect of Rapid Freezing

• When temperature is brought down below freezing, water molecules freeze through spontaneous nucleation
• The number of nuclei depends on how low the temperature gets
• If temperature is brought down slowly, uncrystallized water can move and crystallize around existing nuclei, creating large crystals
• Fast cooling creates smaller crystals because more nuclei form quicker and uncrystallized water cannot move around

The Physics and Effect of Rapid Freezing

• Slow temperature decrease results in: Few crystal nuclei, more water movements and large crystals
• Rapid temperature decrease results in: Many crystal nuclei, little water movement and small crystals
• Blowing cold air over the freezing product helps with rapid freezing
• Heat transfer happens through conduction, convection and radiation, main mechanisms is convection

The Physics and Effect of Rapid Freezing

• Conduction is slow with air due to air being a poor heat conductor
• Radiation: Occurs in vacuum, irrespective of air circulation. Most effective when there is large temp difference between object and air
• Convection: Is the primary heat transfer mechanism
• Static air forms a layer of warm air around the object, reducing temperature difference
• High air speed increases convective heat transfer

The Physics and Effect of Rapid Freezing (combined)

• Combines conduction, radiation and convection as well as stating that the colder the air temperature and the higher the air speed (respectively), the more effective the heat transfer

Low-Fat Ice-Cream

• Creaminess comes from milkfat
• Low-fat ice-cream can be icy as fat is the matrix
• Gums may be added in low-fat ice-cream to compensate and increase the viscosity of the mixture, provide a more chewy texture

Other Iced Foods

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

More on Sorbet

• Variations exist (watermelon, orange, peach, melon), with lemon being probably the most popular
• Can be eaten any time and sometimes used as a palate cleanser between courses

More on Sorbet

• Very simple to prepare; involves making and churning sugar and water syrup with fruit

Principles as for Ice-Cream Apply to Sorbets

• Too little sugar makes it hard and icy, too much sugar makes it soft and syrupy
• The smaller the ice crystals, the smoother it is on the tongue

Granita

• Prepared similarly to sorbet but has a coarser texture
• Adding alcohol (such as Limoncello) depresses the freezing point
• This may result in trouble freezing and so if adding alcohol, add only a small amount

Sherbet

• A sorbet with added milk or cream
• Gives a whitish, opaque final product
• Milkfat adds creaminess to the frozen result

Frozen Yoghurt

• Plain or flavored yoghurt is churned and frozen
• Yoghurt does not contain any added sugars, and can be fat-free (standard Greek yoghurt has around 10% fat)
• Without sugar (and possibly fat), it turns into one solid mass if frozen to very low temperatures, even if churned
• Should be served at warmer temperatures to allow more liquid water rather than ice crystals
• Incorporating more air during churning also helps to soften
• Provides an interesting, sour-tasting contrast to ice-cream

Ice Lollies

• Frozen sugar syrup with added colour and flavouring
• Do not contain fruit like sorbet
• Is not churned and freezes into a solid mass
• Have a higher thermal capacity than ice-cream, causes the mouth to feel colder