One of my friends told me that if you brew a lambic with lambic yeast (e.g. the WLP one) you shouldn't be re-using that carboy for anything else.
I thought the whole purpose of cleaning was no yeast stays behind? Can anyone confirm this?
Yeast wild or tame die just as easily in cleaning chemicals as most bacteria, they are not special super organisms, oxidation and reduction destroy their cell membranes just like any other cell. Including ours so always follow instructions when using chemicals and wear hand and eye protection.
Regardless of the material your FV is made from if your sanitation and cleaning are sufficient to prevent contamination normally then you should be absolutely fine. One added precaution I would take is to ensure all pipes and fittings are dismantled and placed for 15min in water at or above 90C.
If your fittings and flexible tubing are sterilised, and you wash down your FV, then sterilise with bleach or caustic, then rinse, then use a no-rinse sanitiser you will have no problems.
If cross contamination occurs, first re-examine your sanitation processes, then inspect your FV, fittings, seals, and tubes and potentially replace the old if scratched or pitted, as if Brett or Pedio can survive from one brew to the next so can lacto and Acetabacter and it is only a matter of time before you have spoiled beer.
I speak from experience, I had issues in the past and my sanitation was not good enough, I changed my process since that time I have had no issues, we are talking over 4 years now Wild and clean fermentations on the same kit, no cross contamination.
I use Caustic, Phosphoric and Peracetic Acid to clean all my vessels, be they stainless, glass or plastic. If you do not have access to these then Caustic, Bleach, a water rinse, StarSan works very well.
Don't use bleach on stainless unless it is 316L or it will damage it. Strong caustic with prolonged contact time or heat will etch glass. Never use above a 3% solution. Always add chemicals to water not water to chemicals when diluting. Always wear goggles and gloves when handling chemicals, and long sleeves and long trousers.
If it is a glass carboy, it can be cleaned so nothing stays behind. If the carboy is made from plastic, you shouldn't use it for clean beer after a funky one. Plastic easy scratches and in the scratches the yeast and mostly bacteria will survive. To make a lambic you use a mixed fermentation, with yeast and bacteria. Not harmful bacteria, but Lactobacillus and maybe Pediococcus, that produce the funky flavours we love so much.
But there is also highly scratch resistant plastics used for fermenters, i.E. The Catalyst. I have no idea if it is safe to reuse after a mixed fermentation.
There is certainly a lot of anxiety (and misconception) about separating "wild" beer from "clean" in terms of fermentors, location and cleaning regime. As far as I can tell, the majority of this anxiety is rooted in the traditional wine making processes, where barrels would get infected with brettanomyces or resilient bacteria, necessitating replacement of those wooden barrels.
In my opinion, in terms of beer production -be it at home or in a professional environment- the main concern is mixing a small cell count of a fairly resilient, highly attenuative yeast or a bacterium into your beer. These small cell counts can come from poorly sanitized fermenters, from bottling equipment, from o-rings or any number of other places. After the primary fermentation of your wort with a regular beer yeast is done, these yeasts or bacteria will continue fermenting for extended times and thus lead to bottle bombs, sours or accidental brett beers.
If your cleaning regime is able to kill all yeast and bacteria in your fermentor, in your bottling equipment and in your bottles, I don't see an issue with brewing clean beer following a wild batch. I have successfully used my plastic fermentors to produce classic styles with a high amount of residual sugar immediately following batches of sours by using high quality caustic cleaners, followed by alkaline cleaners followed by disinfecting agents.
Your friend was right: you shouldn't reuse the fermentation vessels after a wild yeast fermentation. Wild yeast will usually remain in any scratches and irregularities in your plastic fermenter's inner surface.
It's often easier and cheaper to just buy an extra fermenter that will be used for wild fermentations only, but if you must reuse the one you already have (e.g. you're using one of the fancy expensive ones like the FastFerment), you can sterilise it using sodium hydroxide.
Be aware though that sodium hydroxide is corrosive and can cause nasty burns. Always handle with caution and remember to wear gloves!
You should be able to obtain sodium hydroxide granulate rather easily. Prepare a 3% solution in warm water and fill the fermenter. Leave for 30 minutes. Afterwards, dispose of the liquid, rinse thoroughly with water and use an acid to neutralise the remains of the sodium hydroxide (Starsan is fine, though you can use pretty much anything, e.g. citric acid).