“Instant cell death” for yeast occurs at around 143 degrees F. Between about 115 degrees F and 143 degrees F, the damage to yeast depends principally on the time at which the yeast is held at the elevated temperature. If flour and other dry ingredients are promptly added to the water/yeast mixture, the dry ingredients will buffer the yeast and reduce further damage. This discussion as to cake yeast and IDY should be academic (except for VCM applications) since the damage to these forms of yeast can be avoided by simply adding them to the flour and other dry ingredients, as Tom regularly suggests. When cake yeast and IDY are blended in with the flour, especially if done a short time before using (to initiate hydration in the case of IDY), the flour buffers the yeast and, as a result, the tolerable water temperature can be as high as 130 degrees F. I have gone as high as 135 degrees F in experiments with IDY without noticeable damage to the yeast.
ADY is another story since it has to be rehydrated in warm water, at around 100-115 degrees F, and for about 10-15 minutes. Consequently, it is important that the water temperature be within the above range. Material deviation outside of this range, on either the low side or the high side, can result in degraded yeast performance.
Another point to keep in mind is that modern strains of yeast are hardier and resist temperature better than their counterparts of years past. Nonetheless, it is still prudent not to shock yeast with direct contact with excessively cold water or overly warm water or let them sit too long in water along with salt and sugar.
PN
