I maintain a large yeast bank (>150 strains) and have written a number of articles on banking in my blog (link in my sig). Without a picture it is hard to say what is happening in your starter, but it doesn't sound like an infection - infections in starters have similar characteristics (aromas, appearance, etc) to infections in the fermenter. Stressed yeast, precipitation from wort, etc, may account for it. Do you have a microscope that you could use to take a look at the sample? That would show you pretty quickly if there's something odd in there.
Leithoa provided some links for cell freezing/thawing protocols; unfortunately, those were (all but the BYO article) for mammalian cells and are not applicable to freezing/thawing yeast. When freezing yeast you want to freeze using between 15% and ~40% glycerol in a low-density word (YPD media is best, 1.020 OG wort is the home-brewers best option). If using a home freezer, aim for 30-40%; if freezing colder 15-20% is the ideal range. Previous studies with yeast have observed viability losses using less than 10% and more than 40% glycerol. Pure glycerol is highly stressful to yeasts and should not be used. Whites recommendation of ascorbic acid is based on how yeasts were preserved in the 1950's & 60's; academic labs no longer bother with that as the benefit - if it even exists - is so small as to be unmeasurable (it is more meaningful for refrigerator temps, where yeast continue to metabolize and produce oxygen radicals). Far more important is a consistent freezer temperature (e.g. use a non-frost-free freezer or use a temperature buffer in frost-free) plus use as cold a temp as you can manage.
The dose of glycerol is important - and contrary to what people think, is not there to prevent freezing; glycerol stocks in a -80C freezer are rock hard. Glycerol does lower the freezing point - a 15-20% glycerol solution will only partially freeze in a home freezer, but its main function is to act as a kosmotrope - a fancy way of saying it ensures that salts distribute evenly between the frozen and liquid phases. If you don't do this, salts get ever more concentrated in the ever smaller liquid phase, eventually killing the yeast through creating a hyperosmotic environment. Through the same chemical mechanism glycerol also protects the yeasts surface proteins, but since it doesn't get into the cell, the cytosol may still freeze, again depending on temperature and yeast condition. Glycerol however is toxic to yeast at higher doses, so there is no additive benefit to exceeding recommended amounts.
A good way to improve yeast stablity during freezing is to allow it to ferment out the pre-freezing wort completely, and then place that "spent" culture in the fridge for 2-3 days. This will induce a series of changes in the yeast which help them enter and survive dormancy - including laying down glycogen stores (an energy source key to them surviving thawing) and the production of trehalose (a sugar which protects from the effects of freezing). Once this is done, decant the wort and replace with your freezing mixture (also at refrigerator temps) - you then want to move the tube to the freezer immediately to prevent the yeast from reactivating and undoing all of the pro-dormancy things you've induced in the yeast. Quick-freezxing is not required, and would be overkill for home use; simply placing the tube in your freezer is sufficient for home use.
In terms of thawing, the rate doesn't matter much. If you thaw slowly (i.e. place the tube on some ice and let it thaw that way) you'll have slightly better recovery than if you warm quickly, but the faster onset of growth with a quick thaw will make up that difference pretty quickly. In my case I transfer 10-50ul of frozen yeast (using a loop) into 6ml of 1.040 wort room-temp wort; 24-48 hours later the yeast have fermented out the small tube of wort and are ready to transfer to a 250-500ml starter.
Bryan
