Yeast is a living organism. It is logical to assume that its action may be inhibited or undermined if subjected to freezing temperatures of around -18°C. The reality, however, is somewhat different. Cold temperatures have two effects on the yeast cell: at temperatures below + 10°C, its metabolism is slowed, which explains why Lesaffre generally recommends, with regard to the storage of fresh yeast, that yeast bread be ideally kept at a temperature between + 2 and + 4°C.
At temperatures of between – 2 and – 3°C, the cold preserves the yeast intact, but has no impact on its vitality. It is therefore possible to freeze fresh yeast, but once defrosted, it must be used within 24 hours. Beyond this time, it undergoes autolysis.
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Observation under a microscope shows that when freezing a yeast cell, the first phenomenon to occur is the transformation of liquid water into solid water. The ice crystal formation process is accompanied by a migration of the salts and alcohol produced during fermentation to the outer level of the cell. This increases the osmotic pressure and results in the haemolysis of the most fragile cells. Hence the merits of controlling the start-up of fermentation as much as possible and avoiding the use of rapid yeasts.
Moreover, during negative cold temperature storage, yeast cells consume their sugar reserves (trehalose and glycogen). These are not then available in sufficient amounts to enable a rapid fermentation start-up following defrosting during final proofing. In order to guarantee good stability of its yeast on freezing, Lesaffre takes care to produce yeasts with a maximum amount of sugar and a low cell budding rate.