TL;DR: I try to come up with a 'sensible' scheme to peg currency to produce (taking into account some concerns voiced so far) and find a few of it's limitations.
Defining the Currency
1 groin (portmanteu of "grain" and "coin") buys the calories and protein to feed a hard working, fit adult for a day, all in plant-based, easily stored form - most likely grains + lentils or something. Most likly definition is x calories, y% as protein. Protein content and caloric value are not too hard to measure. There may be a fee for analysis etc. at the grank (granary/bank) when you bring your produce. Expect a large body of codes on quality and quality control for stuff allowed into the granks.
That means that one will suffer malnutrition if subsisting solely on the groin-backing foods (possibly little fat, hardly any vitamins).
Enforcing
The state, warlord or whoever collects a head tax in groin - meaning everyone must either get groin (for produce) from the central grank or work for payment in groin. This ensures that the grank is always well stocked.
Cash crops
There will be, in any given climate/region, one combination of produce that is most groin-efficient - bringing most groin/hectare and year. On the other hand, people like to eat a variety (and likely die on a groin-only diet) so we will also see people growing vegetables and other less groin-efficient foods. These, as well as meat and dairy products will be traded in groin but not via the granks. The actual daily expenditurey for food will be several groins for everyone but the most destitute.
Complications
A specific grank will store a variety of grains and beans and so on. 130
The baker will have to go through these steps to make a bread:
* Buy an assortment of grains and beans and so on for his groin, the assortment represents what's stored in the grank at this time
* Sell everything except one very specif type of grain because our baker needs wheat with this exact glutene content for his bread, and no barley or soy beans or whatever else is in the mix of the day
* Have the grain custom milled for the specific recipe
Maybe the grank will have a produce-trading house attached so step one and two can happen virtually. You could imagine trading 'virtual grains' - that are still stored in the grank but at the moment don't back any groins. The size and handling of the virtual grains market will influence how much trust people put into the granks.
Nothing lasts forever, so the grank will sell food that sits on it's shelves the longest or is most perishable otherwise. This gives the grank master quite some economical power if ther's any leeway.
Limitations
There will be only as much currency floating about as the grank stores foodstuff. A society on the brink of starvation will have hardly any liquidity. OTOH, to have a lot of capital floating about you need huge granks. How huge? I'm too lazy to do proper research right now, but for an estimate find these numbers:
* How much of a given currency is floating about? (X)
* How much is a groin approx.? (y)
* How many people are there using the currency exclusivly? (n)
Then X/(y * n) is the number of days of food storage you need to back your currency. My guess is that at one point the amount of food storage becomes ridiculous, so nothing resembling late capitalism as we know it will be possible with such a scheme. Not having researched how long grains keep, I guess about 10 years at most - that's maximum ~3600 groins per person in circulation - remember, each groin has to be backed!
Conclusion
I could imagine a post-apocalyptic warlord coming up wit such a scheme when consolidating his litle empire: It encourages food storage at the granks which gives the warlord an additional lever over his population. Maybe our warlord is a goldbug but there's not enough gold (or similar) in reach to back a currency, and neither warlord nor subjects trust fiat currency?
So we have a motive to start such a system, if our society continues on a capitalist trajectory the <PERSON> and granks will end at some point and in between we have a few weird effects. Now write your story. 130