Does freezing kill the live culture in yogurt?
The culture in the yogurt does not die by freezing, it becomes dormant instead. When the yogurt is
thawed, the dormant culture becomes active again.
The bacteria is most active and multiplies rapidly at 115° F. That's why
the incubation temperature is kept at 115° F when making yogurt. The bacteria starts to die over
115° F. At 125° F, the bacteria is destroyed.
Below 90° F, the
bacteria starts to become dormant. The dormancy means decreased rate
of bacteria multiplication. The typical temperature of refrigerator is 41° F. When the yogurt is
stored in the refrigerator, the culture remains dormant with a greatly reduced rate of bacteria multiplication.
Below 32° F, the yogurt starts to freeze, the culture becomes completely dormant with no bacteria
multiplication. The typical temperature in the freezer compartment of a house-refrigerator is 25° F. In the
freezer, the yogurt is completely dormant with no bacteria multiplication.
When the yogurt is defrosted (thawed), the dormant bacteria becomes active again, and the bacteria starts to
multiply. The freezing/thawing exerts stress on the walls of bacteria cells, destroying bacteria with weaker
cell walls, it comprises about 2% of the total bacteria.
The taste and texture of thawed yogurt
The thawed yogurt has 98% of the active bacteria prior to freezing. The taste is same unchanged. The
texture becomes runny.
How to freeze and thaw yogurt at home?
1. Cool the yogurt in the refrigerator for about 2 hours before freezing. This reduces the thermal shock on
the bacteria cells.
2. Whip the yogurt. This will minimize appearance change when the yogurt will be thawed
3. Pour one cup of yogurt in a quart size zip-lock bag.
Let the air out and then seal the bag. Freeze the bag in a flat position so that the frozen yogurt will look
thin rectangular slab.
4. When needed, thaw the yogurt in the refrigerator tominimze stress on bacteria cells.