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Syneresis (also spelled 'synæresis' or 'synaeresis'), in chemistry, is the extraction or expulsion of a liquid from a gel, as when serum drains from a contracting clot of blood. Another example of syneresis is the collection of whey on the surface of yogurt. Syneresis can also be observed when the amount of diluent in a swollen polymer exceeds the solubility limit as the temperature changes. A household example of this is the counter intuitive expulsion of water from dry gelatin when the temperature increases. Syneresis has also been proposed as the mechanism of formation of the amorphous silicate composing the frustule of diatoms.
In the processing of dairy milk, for example during cheese making, syneresis is the formation of the curd due to the sudden removal of the hydrophilic macropeptides, which causes an imbalance in intermolecular forces. Bonds between hydrophobic sites start to develop and are enforced by calcium bonds which form as the water molecules in the micelles start to leave the structure. This process is usually referred to as the phase of coagulation and syneresis. The splitting of the bond between residues 105 and 106 in the κ-casein molecule is often called the primary phase of the rennet action, while the phase of coagulation and syneresis is referred to as the secondary phase.
In cooking, syneresis is the sudden release of moisture contained within protein molecules, usually caused by excessive heat, which over-hardens the protect shell. Moisture inside expands upon heating. The hard protein shell pops, expelling the moisture.
This process is what changes juicy rare steak to dry steak when well-done. It creates weeping in scrambled eggs, with dry protein curd swimming in released moisture. It causes emulsified sauces, such as hollandaise, to "break". It creates unsightly moisture pockets within baked custard dishes such as flan or crème brûlée.
In dentistry, syneresis is the expulsion of water or other liquid molecules from dental impression materials (alginate for example) after an impression has been taken. Due to this process, the impression shrinks a little and therefore its size is no longer accurate. For this reason, many dental impression companies strongly recommend to pour the dental cast as soon as possible to prevent distortion of the dimension of the teeth and objects in the impression.
The opposite process of syneresis is imbibition, meaning, a material that absorbs water molecules from the surrounding. Alginate is also an example of imbibition since if soaked in water, it will absorb it.