In this scenario where people are desperate to do a food riot, a power outage has occurred for multiple consecutive days if not at least a week. In which case, something truly apocalyptic is happening, in which case I doubt people would be civilized enough to bargain for food for useless cash or shavings of gold, in that scenario resources are the only useful 'cash'. In this scenario, it's likely it'd be some sort of national emergency and I'd imagine most people have enough food in their pantries to tide them over until the government provides food or restores power.
Modern power infrastructure is incredibly resilient and outages are usually localized to regions and power is restored within hours. I understand the idea of using cash for privacy reasons or keeping few hundred just in case, but keeping with cash just in case a food riot happens is wholly unconvincing.