crystallization of ice in frozen desserts

CRYSTALLIZATION OF ICE IN FROZEN DESSERTS

ICE CREAM Ice cream is a colloid, a type of emulsion. An emulsion is a combination of

two substances that don't normally mix together. Instead, one of the substances is dispersed throughout the other.

In ice cream, molecules of fat are suspended in a water-sugar-ice structure along with air bubbles.

The presence of air means that ice cream is also technically a foam.

SHERBET Frozen dessert usually flavoured with fruit, made from water, sugar,

flavourings, and milk or cream.

Egg white or gelatin may be added to ensure a fine texture.

TERMINOLOGIES Colligative property

Freezing mixture

Ice crystals

Interfering substance

Milk fat or butter fat

Milk solids

Non-fat solids

Nucleation

nuclei

Overrun

Rate of freezing

Stabilizer

Viscosity

OBJECTIVES To identify the water content of

ice cream and sherbet mixtures and relate this to freezing rate

To understand the effect of sugar concentration on the depression freezing point of water in ice cream and sherbets.

To understand the effect of a mixture of salt and ice which surrounds a container of mixtures of ice cream and sherbets in the freezing process.

To appreciate the significance of agitation during freezing, its effect on the structure of the frozen mixture, and overrun and its role in breaking up large ice crystals to form a smooth-textured product.

To appreciate the significance of stabilizers in maintaining the size of crystals and the firm body of the frozen product.

FREEZER

FREEZING: SALT AND SUGAR Influence the freezing temperature of water.

Addition of a non-volatile solute like salt and sugar the freezing point drops in proportion to the amount of dissolved substances and for an ionisable one, in proportion to the dissociated ions.

BASIC COMPONENTS

ICE CRYSTALS Created when the water-content

in the base starts to freeze

Gives solidity and body.

The size of the ice crystals largely determines how fine, or grainy, the ice cream eventually turns out.

The main objective (apart from the freezing itself) is therefore to keep the size of the ice crystals down as much as possible.

FAT Often in the form of butter (milk) fat.

Adds richness, stabilises the base mix, improves density and the smoothness of texture and generally increase flavours

SWEETENERS Sugars, honey or syrups Adds sweetness but also improves texture and body.

Lowers the freezing point of the mix, ensuring that the ice cream does not freeze rock-solid

AIR The invisible (and cheapest) ingredient in ice cream.

The tiny air cells whipped into the base mix are largely responsible for the general consistency of ice cream, and greatly affect texture and volume.

“Over-run” is the technical term used to indicate how much air an ice cream holds; since air is free and increase the volume, non-premium commercial ice creams could well have an overrun sometimes even exceeding 100 %.

OTHER SOLIDS: PROTEIN (ETC) Usually so-called non-fat milk solids, such as proteins and mineral salts,

and flavourings such as cookie-crumbles.

Contribute to the body, texture and smoothness.

More solids means less “free-roaming” water in the ice cream – which in turn usually means less unwanted, large ice crystals in the ice cream.

Too little solids, the ice cream often tends to become unpleasantly icy.

Too much solids, however, may bring about an unpleasantly sandy-like sensation.

EMULSIFIERS  added to the ice cream base – one or more ingredients that help the other

“unwilling” ingredients to combine.

Egg yolk

STABILIZERS Improve the structure and texture by keeping down the growth-rate of the

ice crystals of the ice cream

Reduce the melt-down speed of the ice cream.

Gelatin, egg whites

THE GOOD QUALITIES A well frozen appearanceAbsence of large ice particlesA pleasing blend of banana with lemon or lime rind, a subtle acid taste which complements the bland banana flavor

SOURCES Introduction to Food Preparation by Guzman, M.P. and Fojas de Luna, M.V.

http://www.icecreamnation.org

Chemisrty: The Central Science by Brown and Le May