I think the substance u have write is curd ....
Now ur answer is here :
Brass is a metal ( we all know it very well ) ....
Curd contains lactic acid ...
If we put curd in the brass vessels , the lactic acid present in it reacts with the metal nd forms toxic substances which results in food poisoning nd sometimes causes human death also ...if any other substance is also putted in place of curd which contains any type of acid then it will also forms toxic substances....
So generally we uses vessels made up.of aluminium, copper nd silver ...because they do not react with acids ...
And one more information , copper does not react with acids ....

Why cute and sour substances should not be kept in brass and copper vessels?
Cute and sour substances should not be kept in brass and copper vessels due to the following reasons:

Chemical Reaction:
When cute or sour substances come in contact with brass or copper, a chemical reaction takes place. This reaction can lead to the formation of toxic compounds that can be harmful if ingested.

Leaching of Metals:
Brass and copper vessels have the potential to leach metals into the food or liquids stored in them. This can lead to metal poisoning over time, especially if acidic substances are stored for extended periods.

Health Risks:
The leaching of metals from brass and copper vessels can pose serious health risks, such as copper toxicity. Symptoms of copper toxicity include nausea, vomiting, and even organ damage in severe cases.

Impaired Taste:
Storing cute or sour substances in brass or copper vessels can also affect the taste of the food or liquids. The metallic taste from the leached metals can alter the flavor and quality of the stored items.

Alternative Options:
To avoid these risks, it is recommended to store cute and sour substances in safe containers such as glass, stainless steel, or ceramic vessels. These materials do not react with acidic substances and are considered safe for food storage.
In conclusion, it is important to be mindful of the materials used for storing food and beverages, especially when dealing with cute and sour substances. Avoiding brass and copper vessels for such purposes can help prevent potential health hazards associated with metal leaching and chemical reactions.