A file is not a text file by its extension (.txt), but by its content. A quick test could be to open it in an editor like Notepad/TextPad and see if all the chracters are readable.
Serialization in Java does not print just the values of simple properties of the given object. It has to deal with a (possibly) complex object-graph. In other words, the object you're trying to write to a file may have references to other objects, and they in-turn, may have more such references. Hence, Java writes them on the file in a special way which makes it easy to read back (deserialize) the entire object graph from the file. This 'special' way is obviously not something a text-editor like Notepad can read.
What you want, is probably, a simple PrintStream (instead of an ObjectOutputStream) chained to a FileOutputStream. Using this PrintStream, write the property values one by one on the file.
If you're expecting serialization to produce some specific output different from what it actually does produce, then your expectation is wrong. And if you're expecting to learn something by examining the result of serializing something, that's probably wrong too. There is nothing useful to be learned from that.
So... can you go back and explain why you are doing this?