Answer by Rabbi Yosef Braun, member of the Crown Heights Beis Din:
Styrofoam cups comprise a lubricant containing zinc stearate which is derived from tallow (animal fat), a non-kosher product. This raises the question whether Styrofoam cups may be used for hot liquids, since, according to halachah, hot food is generally said to absorb the properties of the receptacle it’s in.
However, the consensus among poskim is that it’s permissible to use Styrofoam cups, and in fact there are no grounds for being machmir (stringent). Even though some of the reasons for permitting their use might not suffice on their own, the combination of the following factors determine the cups’ kosher status:
Due to the high temperature of the purification process, the tallow is, in effect, denatured (the characteristic properties are destroyed, thereby removing it from the category of food).
The non-kosher food element is not b’ein (visible), and the quantity of the non-kosher ingredient present in the cup is so negligible that it’s batel (halachically nullified and considered as if it doesn’t exist).
There is no problem when using the cup of ein mvatlin issur l’chatchillah (deliberately overwhelming something treif with a large quantity of kosher ingredients in order to nullify it), which is generally not permissible, because in this case ein kavanaso l’vatel (the intention is not to nullify it). The intention in this case is simply to use a cup, not to nullify the treif ingredient.
Zeh v’zeh gorem: When something is by necessity a product of both permitted and forbidden ingredients, there is room to permit it under certain circumstances (the details of which necessitate a separate discussion). The lubricant in this case can only be produced through a combination of various ingredients, kosher and non kosher.
It’s possible that it’s nosen ta’am lifgam (the taste added by the non-kosher ingredient is actually detrimental to the taste of the contents).
It’s not ben yomo (within 24 hours of the cup becoming treif), and therefore the remaining taste absorbed within the cup can no longer affect the taste of the drink in a positive way.
The fact that these cups are disposable and generally used only once is also grounds for leniency, for various reasons.
Since the cups are never used as a kli rishon (a vessel placed directly on the fire), only for irui mikli rishon (the liquid is poured into the cup from the urn), the amount of non- kosher ingredients in the cup is of little concern.
Due to these factors, and others, poskim are in agreement that it‘s perfectly acceptable to use Styrofoam cups for hot food.
Originally published on halacha2go.com