Sour Starter

I think I ran into some problems with a yeast starter.
I had a pack of Wyeast ESB yeast, manufacturing date is Jan 09. (It isn't new , but it worked in the past).
After popping the inner pocket, it barely inflated in 48 hours.
I made a starter with 1 liter of water, 100g light LME and some nutrient.
I moved it to a sanitized soda bottle, aerated , added the yeast and sealed with a flamed peace of aluminum foil.
Now, three days after pitching, there seems to be a nice layer of yeast on the bottom. However, there seems to be a couple of (possible) problems:

First, I noticed some brown areas in the yeast deposit. The bottle I used is made of green plastic, so it's a bit hard to say what the exact color is.
It looks a bit like brown-Hefe, but that's mix of protein, yeast and hop resins, so I'll be surprised if it forms in a LME (usually low on protein) starter with no hops. I've never seen this before.

Second, I've read that before pitching a starter you should taste the starter liquid for off-flavors. I poured some out and it tastes quite sour.
I'm not sure as of the nature of this sourness, I'm not to good with distinguishing between lactic, acetic, acetaldehyde, etc.
other than this sourness, I couldn't point out any other off-flavors.
I have noticed acidity in starters in the past...

what do you guys think?
I can't buy another liquid pack, so I'm considering buying s-04 instead.

The brown areas are most likely protein from the LME-- I get some all the time in my starters. The sourness is normal-- fermenting wort without hops leads to a sour tasting "beer". The guys at Basic Brewing have a podcast in which they run into this.

Unless the starter smells foul, then I'd say it's probably just fine. Starters are meant for growing yeast, not making beer, so sourness shouldn't be a concern. Chill the starter so the yeast flocculate, decent the beer off of it, then pitch your yeast slurry.

-Steve

I've ended up pitching it.
I hope it was the correct choice.

thanks.