Cold tolerance and brewing
The exact mechanisms controlling cold tolerance in the lager yeast are still unknown. The uptake of the a-glucosides maltose and maltotriose is known to be sensitive to temperature (Vidgren et al., 2010) but different cryotolerances of the yeast groups are not necessarily related to differences in the sensitivity of maltose transporters. Differential maltotriose utilization may explain why Frohberg strains have historically been considered more industrially significant. The lack of active maltotriose transport in Saaz strains suggests that the transporters responsible for uptake are absent or non-functional.
Cold tolerance and membrane fluidity
The principal fatty acyl chains in the plasma membrane of S. cerevisiae are oleic acid (18:1) and palmitoleic acid (16:1), with trace amounts of palmitic acid and stearic acid also present. Membrane fluidity is largely determined by the packing of these molecules. Lowering of the temperature leads to a more ordered membrane structure and hence a reduction in fluidity (Shinitsky, 1984). The membrane is essentially modified from a liquid crystalline form to a gel state (Thieringer et al., 1998). This transformation alters various functions of membrane-bound proteins, such as the import and export of metabolites and proteins across the plasma membrane. The mechanisms by which microorganisms tolerate changes in membrane fluidity has been well characterized and is termed ‘homeoviscous adaptation' (Sinensky, 1974). Depending on the microorganism, this process involves increasing proportions of unsaturated fatty acids and/or cis double bonds into lipids, chain shortening, and methyl branching (Shaw and Ingraham, 1967; Sinensky, 1974; Shinitsky, 1984; McElhaney, 1982; Russell, 1989). Another physiological effect of lowering the temperature is the reduction in the hydrophobic interactions between the carbon skeleton of the polypeptide and the side chains of amino acids, exposing non-polar regions to water and risking protein denaturation (Gounot and Russell, 1999).