I don't get it. Who are the participants in the blockchain and how do you ensure it cannot be tampered with i.e. could someone just go back to a previous block

I don't get it. Who are the participants in the blockchain and how do you ensure it cannot be tampered with i.e. could someone just go back to a previous block and rewrite history. Difficult admittedly. I don't understand what a consensus means in this implementation. Also, it sounds like the trust is completely in the hands of one entity i.e. the person who adds details about the cow and cut of meat. How is that trustworthy ? Either, the article doesn't explain how blockchain could fix a key problem in China's food industry, or it doesn't fix a key problem. In my opinion food safety has to be regulated by a central agency like a Government. A decentral system like Blockchain isn't what solves the problem. Regulations and process are.