The party warlock (Pact of the Tome) has recently acquired the Purify Food and Drink spell as a ritual in his Book of Ancient Secrets. The party has just destroyed a hobgoblin base. Facing a return overland journey of several days, they have found the food supplies of the base. The total stores are more than the party would choose to take with them, but they are of dubious provenance. The warlock would like to use Purify on some of the collected rations. At this point in time he has the opportunity to recast it as needed, but this may not always be the case.
The PHB presents food in units of individual items (eg, loaf of bread, hunk of cheese in Expenses), and in units of persons it could feed, cost, and weight (Adventuring Gear, p. 150; 1 ration (food for one person for one day) at 5sp, 2 lb).
The Purify Spell targets none of these units, but rather works on all non-magical food "within a 5-foot-radius sphere". Nowhere else as far as I know references food as a volume.
I track food for the characters using feeding units, weight, and sometimes price, all of which are easily converted into one another using the PHB listing.
It is likely that the warlock will continue to use his new spell. I have no interest at all in continually converting food from volume to weight. I would rather have a rewrite of the spell so that it specifically works on a limited person amount or weight.
An answer need not include a realistic conversion of food volume to weight, taking into account dried vs. fresh rations and quasi-medieval packaging, although estimates obtained from such a conversion may inform a good answer.
Rather, I am more interested in an answer supported by a comparison to similar utility spells that takes into account spell level, level progression, possibly character class, and other effects (creating food out of nothing vs. restoring food that was once healthful, spells that heal as well vs. those that provide only nourishment). A "good subjective" answer will demonstrate knowledge of similar spells and experience in applying such revisions.
I am interested in what would be an appropriate amount of food by weight or ration for a first level spell to purify, rather than calculating how much it could be, given the original volume specifications of the spell.