Since the legal questions regarding proprietary info are somewhat in my specialty, just a little info:
Patents: Publicly disclosed information. Protects the mixture from being used (without permission) by other companies for the length of the patent. Patents do expire. The implied exchange is a temporary monopoly on the product in exchange for that public disclosure. And since its public, other companies have the option of tinkering with the mixture, and if they are able to make an advancement that "wouldn't be obvious to one reasonably skilled in the art", they can then patent that advancement.
Trade Secrets: Company does not disclose information to public, patent office, or anyone else. Protection lasts indefinitely, so long as the company makes a reasonable attempt to keep the secret. Many employees may not know the information, and those who do have signed agreements with the employer to not disclose. If any outside work is done, the company can obtain a non-disclosure agreement (NDA) first. Other companies cannot obtain info via industrial espionage.
The protection given to a trade secret is in if it is improperly disclosed (by an employee or outside party under an NDA) or discovered (industrial espionage), the company is entitled to damages. However, the danger is that other companies CAN attempt to reverse engineer the process or independently discover it on their own, and if they discover it, there is no protection.
In most states, the interest of protecting trade secrets outweighs other legal interests, such as health, safety, environment, etc. So companies are within their legal right to deny the information even if the DEP, etc. orders them to disclose. One of those areas where "legal" rights and "moral" rights may not be in harmony. Of course, one would hope in some situations companies would do the morally right thing to protect public perception of their brand, even if not legally required.