We are considering buying a mechanized dishwasher — but I am confused about a Shabbos-related issue. I often see families that are shomrei torah u-mitzvos wetting and brushing down their dirty dishes on Shabbos before placing them in the machine to be washed after Shabbos. Isn’t this forbidden because one is preparing on Shabbos for after Shabbos (hachonoh)?
If the residual food left on the plates has not yet hardened, you may soak or moisten the remnants. Although this will make the plates easier to clean after Shabbos, it is not forbidden as preparation for the next day. As long as the food remaining on the plate is still moist and cleaning it now would be relatively easy, you are merely doing something that will allow the food on the plate to retain its moisture until Shabbos is over and the plate can be cleaned. Only when the food has already hardened, and you moisten it by soaking or some other method in order to make the cleaning process easier or more effective later on, is that procedure an act of preparation that should wait until Shabbos is over. ((שמירת שבת כהלכתה פי”ב ה”ג. )
Furthermore, if by leaving it as is it will likely attract ants or other pests, then you can even soften hardened food. That is not preparation for the next day, but doing something to help you on Shabbos. Similarly, you can wash silver eating utensils. Often, if silver is not cleaned right away, it will ruin. Something that can only be done on Shabbos is considered a need for Shabbos. Thus, you are not preparing for the next day; you are doing something for today’s (Shabbos) needs. ((שמירת שבת כהלכתה פי”ב ה”ב וסק”ח. ))