To be brutally honest, I wouldn’t employ that ‘microbiologist’ in my lab. Some glaring mistakes there, frankly. Freezing (and thawing) is so much more stressful on yeast cells (especially using glycerol, not just at a low conc and without incubation time to replace enough H2O molecules in and around membranes, cell and organelles
) that they really need QC on an agar plate with healthy-looking colonies being selected for propagation. Mutation is going to occur at a much higher rate when freezing compared with ‘cosy’ refrigeration temperature. The most vulnerable bits of DNA are the mitochondrial genomes (hello petit!) and the strands of DNA exposed on the surface of chromosomes. Then there’s the impact of freezing on the mRNA, the transcriptome and proteome, the ‘memory’ of the cells. I routinely QC yeast on plates when they come out of the freezer, after an overnight culture. I can say yeast kept in small aliquots in the fridge have noticeably fewer suspect-looking colonies on agar plates. Sure, much longer term, freezing is probably better, but I’m really not confined that time frame is relevant for most home brewers for something like yeast. There are clearly much simpler alternatives that are more reliable and more accessible to home brewers generally, imho.
As far as pressure cookers go, definitely use one if you have one, but don’t worry if you din’t. Double boil. ‘Simmer’ temp with lid on really. After the first boil, let it cook then leave it overnight. Any viable spores present ‘germinate’. Boil again to kill them. This is wort, it’s highly unlikely there’s going to be nasty thermophilic bugs in there. Focus on sterility when aliquoting and propagating 10-50ml cultures. After that, just follow good home brew practices. It’s not a problem to start treating it a little bit like food. It’s kind of what it is at this point, right? Best thing we can do really is pitch sufficient healthy yeast to get ethanol production going as soon as, to protect the wort from infection. That cooled kettle wort ain’t sterilised. It’s only been sanitised. Like most things at this stage, if we’re thorough.