Milk Science

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Facts About Foam

MS 4.09 Chapter 4 Recap and Glossary


  • Drainage is the process of milk percolating through a layer of foam. Drainage rates are faster when foams are made of bubbles with larger diameters.
  • Rheology in foam science is the measure of a foam’s firmness. For latte art purposes, we need foams to be firm to create a silky texture, but we also need a low yield stress to allow bubbles to slide past each other for ease of pouring.
  • Lipolysis is a process wherein triglycerides are damaged and break apart into free fatty acids. The process of lipolysis causes the free fatty acids, which are surface active, to displace the stabilizing surfactant whey proteins from the bubble films, causing foams to rapidly break up.
  • Ostwald ripening: Gases in air bubbles having relatively higher air pressure diffuse into neighbouring bubbles having relatively lower air pressure, causing them to inflate.


Adsorption  The process through which atoms, ions, or molecules spread across the surface of a material to form a film

Aeration level  The proportional increase in the volume of milk expressed as a percentage after the introduction of entrained air; volume gains added to the milk as a result of condensed H20 also contribute to this volume increase.

Amphiphilic  Quality of a molecule which has hydrophobic and hydrophilic components

Closed-cell foam  A foam wherein each cell occupies a distinctly separate pocket, surrounded by solid material

Drainage  The separation of the liquid and air fractions of a foam, promoted by gravity and capillary action

Hydrolysis  The chemical breakdown of a molecule caused by an interaction with water

Hydrophilic  The tendency of a substance to mix with or dissolve into water

Hydrophobic  The tendency of a substance to repel or resist dissolution with water

Ionics  Compounds that are joined together by ionic bonding and are soluble in water; non-ionics are compounds formed of covalent bonds that can’t be dissolved in water.

Lamella  The film coating the outside of a bubble

Liquid fraction  The proportion of the total volume of a foam which is occupied by fluid

Melanoidins  The complex brown-coloured compounds formed through the Maillard reaction;