I reuse yeast collected from fermentation but have never had to make a starter with it. Another thing I have never done is wash the collected yeast. An article I saved long ago against washing yeast is pasted below. Another tip for reusing yeast pertains to pitching a new batch of wort on top of a fresh yeast cake is to only use 1/2 to 1/3 of that yeast. Over pitching can result when pitching onto a whole, fresh yeast cake. That method is probably not what you are doing but I thought I'd throw it out there. Sidenote: 1.017 when you are expecting 1.010 is not bad. I wouldn't even think of trying to "fix" that.
Now, the article written by Denny Conn with information from the man known as the "yeast whisperer" Mark Van Ditta...
DON’T WASH/RINSE YOUR YEAST!
Don't rinse your yeast (unless you use acid you're rinsing, not, washing.) Read this...FromMark Van Ditta
Basically, it has always been a bad move that has been difficult to kill because every newbie wants to
gain some street cred by publishing an article on yeast rinsing. Do you want to know where the practice
of rinsing yeast with boiled water originated within the amateur brewing community? Charlie Papazian
introduced it in the “New Complete Joy of Homebrewing” (it may even date back to an earlier
publication of that book). He also promoted the use of a secondary fermentation vessel as a way to
prevent autolysis. While using a secondary fermentation vessel to prevent autolysis has gone the way of
the do-do bird, amateur brewers still cling to yeast rinsing, a practice that is not based on science and
provides no microbiological advantage.
Brewing yeast strains are Crabtree positive. What that means is that whenever the medium gravity is
above the Crabtree threshold of 0.2% w/v (an S.G. of 1.0008), brewing yeast cultures will chose
fermentation over respiration even in the presence of O2. There is scientific evidence that brewing
cultures became Crabtree positive due to competitive pressure. You see, the main reason why we have
cell counts in the first place is that they are primarily a safeguard against a micro-organism other than the
pitched yeast culture owning the wort. From the time a yeast culture is pitched until the culture grows
large enough to reach high krausen, it is in competition for ownership of the wort with wild microflora
(boiled wort is not absolutely sterile and sanitization is not a synonym for sterilization). A yeast culture
owns a batch of wort by doing three things. First, it consumes all of the dissolved O2, shutting out
aerobic microflora. It then lowers the pH to around 4, which shuts out pH sensitive microflora. The pH
sensitive microflora include the pathogen Clostridium botulinum, which cannot replicate below a pH of
4.6. The final defense that a yeast culture mounts is the production of ethanol, which is toxic to all living
organism at a given level, even human beings (i.e., people die from alcohol poisoning every day).
When a brewer rinses yeast with boiled water, he/she removes the protective force field that a yeast
culture built for itself, basically opening it up to infection from house microflora while providing zero
microbiological advantage. A yeast culture does not need to be kept free from trub and hop particulate
matter. It is needs to be kept as free from wild microflora as possible because every time a culture is
pitched it is an opportunity for microflora other than the culture to replicate. This reality is what places
an upper limit on bottom-cropped yeast more so than any other reason when a yeast culture is not
serially overpitched.
Now, top-cropping an interesting take on cropping. While the top-cropped yeast should also be stored
under green beer, top-cropping naturally purifies a yeast culture because wild microflora do not floc to
the top, which means that top-cropped yeast can be re-pitched almost indefinitely as long as care is
taken to not infect the culture. The sad thing is that I have never heard of true top-cropping lager yeast.
If you need further evidence that yeast rinsing an amateur brewer fabrication that is not based on
microbiology, watch how a craft or industrial brewery bottom crops yeast. They either pump it out of
the cone into a yeast brink for temporary storage or into a fermentation vessel with fresh wort. I have
yet to see a professional brewery rinse yeast with water before repitching it.
