So. The serialization is not what you need to worry about, it is the fact that you'll have to be writing to disk. (slow)
This will however solve your problem of too much memory be consumed. At the same time, you can start caching searches through this, and possibly lowering the amount of queries being sent out to the remote site.
Your goal through this approach would be to keep as much as you can in the db without assigning it to a variable first. This is where I feel your breaking down, and I've had another person agree with me. In php, 8mb of raw data does not translate to 18 times the memory. You said you have over 190mb of PHP code? That's a lot of places where you can be duplicating these variables out unnecessarily.