Curious why your cold curd fizzes or bubbles when you stir it? Learn about the trapped gases that are released from the fermented mixture.
1 Answer
The bubbles in curd after refrigeration are usually due to the release of dissolved carbon dioxide (CO2) produced by the continued, albeit slowed, fermentation process of the bacteria present in the curd. Stirring agitates the curd, allowing the trapped CO2 to escape as bubbles.
Detailed Explanation:
Curd, also known as yogurt, is a dairy product produced by bacterial fermentation of milk. Specific bacteria, primarily *Lactobacillus* species, convert lactose (milk sugar) into lactic acid. This lactic acid lowers the pH of the milk, causing the milk proteins (casein) to coagulate and form the characteristic thick texture of curd.
Even when refrigerated, the bacteria in curd don't completely stop their metabolic activity. They continue to ferment lactose, albeit at a much slower rate due to the lower temperature. One of the byproducts of this fermentation is carbon dioxide (CO2). This CO2 gets dissolved within the curd matrix.
When you stir the curd, you are essentially disrupting the curd's structure and agitating the liquid. This agitation allows the dissolved CO2 to come out of solution and form bubbles. The same principle applies to opening a carbonated beverage – shaking it releases the dissolved CO2 as bubbles. The amount of bubbling depends on factors like the age of the curd, the specific bacterial cultures present, and the storage temperature. Older curd, or curd stored at slightly warmer temperatures, will generally produce more CO2 and thus more bubbles when stirred.
Pro Tip:
If you prefer a smoother curd with fewer bubbles, try letting it sit at room temperature for a short period (15-30 minutes) before stirring. This allows some of the dissolved CO2 to escape naturally before you agitate it.