I was simply trying to make the point that slants require some upkeep...a large library can be a time-consuming task. Slants will dry out (the way I make them anyway), and agar colonies will lose viability with time. But, many brewers use them . Yeast will lose viability if not stored at low enough temperatures (i.e. cryo-preservation, or -80 glycerol stocks)...refrigerator temps will prolong the viability of the yeast on a slant or plate, but not indefinately.
I am not suggesting that a slant will not be viable to some extent for more than three months...some claim six months or more without replating - I think that would be under optimal conditions (probably not sealed from gas exchange??). However, I readily admit that I have never tried to make a starter from a three-month old slant that was sealed in the refrigerator. Washing a slant that is beyond optimal storage life, or taking a large scrape of colonies from a slant will probably yield a viable starter...it may take a little longer than starting with a slant in optimal health.
You can seal a tube with parafilm or something similar (plastic wrap and/or tape) in the fridge...but note that yeast require oxygen for non-fermentive growth...if the tube is sealed completely, eventually the oxygen will be used completely (causing a slight vacuum??...hmmm, will have to think about that more...update:yeah, well thought about it more and wish I had the excuse of being drunk when I wrote it). The length of time involved before oxygen is depleted, is some function of the airspace above the slant, temperature, sugar concentration of the agar, and the concentration of yeast solution used to make the slant (other environmental conditions will also have some effect - pH, altitude, etc.)...I don't know exact parameters of this function - but I am sure someone has studied it.
Once the yeast in the slant utilizes all of the oxygen, it will convert to fermentive growth. If the tube does not "breath" - or probably a more appropriate term would be "exchange gas" - the tube will build up CO2 under fermentive growth, and kill the cells. The time required for all this process to take place is specific to the character of each slant.
I have never made slants and sealed them completely from gas exchange, so I am no help regarding storage length in a "sealed" situation...sorry.
True that the best method is glycerol stocks at -80. I have -80 glycerol stocks that are over twenty years old (actually passed off from someone else to me) and still viable. An even better method is storage under liquid nitrogen if you know someone that keeps a dewar - but this is generally an extreme approach for personal storage at home.
I suggested glycerol stocks in the freezer, because most folks have a freezer...it is pretty simple, and in my experience requires much less maintenance than a slant library. I should have noted that most refrigerator/freezer combinations are of the "auto defrost" type. The auto defrost cycle of a freezer is essentially a warming/cooling cycle...this will severely limit the life of your glycerol stock (by 1/2 in worst cases...I have measured this). You can still use an auto-defrost freezer for your glycerol stocks, just put them into a styrofoam container (i.e. a cold-shipping styrofoam box) before putting them in the freezer. The styrofoam will act as a buffer against the freeze-thaw cycle caused by auto-defrost.
BTW: smack packs make great glycerol stocks...one smack pack can be used to make around 20 glycerol stocks...cheapens up the costs of your beer.:rockin:
I did not intend to discourage use of slants, just to put out the idea that freezer stocks last longer and require less maintenance, IMHO.
Good luck with your library
PikledBill
There are times in my life when i really wish I had a "takeback" button...geesh
