Aparentemente mesmo fervendo não se resolve completamente o problema. A solução proposta é trocar todas as semanas:
http://www.techtimes.com/articles/212267/20170804/kitchen-sponges-are-host-to-millions-of-bacteria.htm
Citando:
""Sanitation by boiling or microwave treatment has been shown to significantly reduce the bacterial load of kitchen sponges and can, therefore, be regarded as a reasonable hygiene measure. However, our data showed that regularly sanitized sponges (as indicated by their users) did not contain less bacteria than uncleaned ones," the researcher reveal.
Not only did some of the bacteria survive the sanitation process, the researchers found that those "sanitation resistant" strains were able to quickly infest the sponge again and could even increase the number of disease-causing bacteria.
The researchers admit that further analyses are needed to substantiate their findings, but they still advise against DIY sanitation of kitchen sponges because it may actually do more harm than good. Their short-term advice is to simply replace kitchen sponges on a regular, weekly basis."
