The gist is that per capita they have been going down for a long time. Check out his charts.
Walmart moving into groceries in a big way has probably had a measurable effect on the $ per capita. The stuff they stock is going to be at lower price points relative to grocers.
I think to some degree brand name processed foods are pricing themselves out. The shrinkage of package size has been very noticeable, with potato chips, for example. Where I would buy this product occasionally, I now just walk past that aisle. Whatever taste temptation is destroyed by the price.